stunthanger.com
General control line discussion => Open Forum => Topic started by: frank williams on February 21, 2019, 12:42:34 PM
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For those who haven't seen Paul Walker's stab's leading edge, to say that its pointy is an under-statement. Its just downright extremely sharp pointed. I don't see his planes on a regular basis, only once a year at the Nats, but I always try to take note of the subtle design features that I think are unique.
Anyway, I wondered if there was any technical data on a pointy leading edged airfoil, and I found this. This is data for a 0012 airfoil in both the normal (blunt end forward) and reverse (pointy end forward) orientations. Right off I'll admit that the Reynolds number for this data is lower that what we normally have for a stunt ship stabilizer, but it better than nothing. This data gets generated for special vertical axis wind turbine analysis that see this full range of flow.
The data shows that in the reverse configuration, the lift curve is very linear through zero alpha. The normal configuration is not. The normal, blunt end forward, suffers from a nonlinear loop around the zero angle of attack range. If this was on a stuntship stab, we might see a hunting type feel in level flight. The slope of the curve does stabilize after a few degrees, but the damage has been done. This effect is probably due to a type of flow that is said to have laminar bubbles on the surface. once again, this is not at exactly our Reynolds number, but ....
So maybe Paul has something here. I think he says that it grooves better.
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So I'm guessing those "Green Box Noblers" and Ambroid "Ares" kits got the stab correct....
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So I'm guessing those "Green Box Noblers" and Ambroid "Ares" kits got the stab correct....
In their day, maybe. Things have changed, a lot! I can tell you first hand the LE of his stabs are now "really sharp" as in pointy.
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The problem with sharp LEs is once the AOA changes, the airflow over the surface is blacked out and stalls. This happens with any airfoil, but with a sharp LE it happens more abruptly. So it's efficient for speed, but not for maneuvers.
Kind of the same as tip stalling when the tip loses flow over one side of it's surface.
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It'd be interesting to see a sketch of just how pointy (Mike, a picture of your hand with missing fingers would be sufficient).
The problem with sharp LEs is once the AOA changes, the airflow over the surface is blacked out and stalls. This happens with any airfoil, but with a sharp LE it happens more abruptly. So it's efficient for speed, but not for maneuvers.
Kind of the same as tip stalling when the tip loses flow over one side of it's surface.
"Efficiency" and "Stunt" do not go together. We aren't trying to be efficient. We are trying to carve pretty pictures in the air with a pretty airplane.
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Well I was convinced. May need to carry some bandaids in the flight box.
Dave
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We are trying to carve pretty pictures in the air with a pretty airplane.
And, there is much more thrust to weight than with most prop driven full scale aircraft. This covers many sins. Our c/l planes don't need to glide efficiently like a sailplane, they just need to be controllable enough for a smooth landing.
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The problem with sharp LEs is once the AOA changes, the airflow over the surface is blacked out and stalls. This happens with any airfoil, but with a sharp LE it happens more abruptly. So it's efficient for speed, but not for maneuvers.
Kind of the same as tip stalling when the tip loses flow over one side of it's surface.
David ... that's not what the data shows ..... the performance is as good as or better than the forward direction.
Nobler, Ares .... yep pretty decent
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For those who haven't seen Paul Walker's stab's leading edge, to say that its pointy is an under-statement. Its just downright extremely sharp pointed. I don't see his planes on a regular basis, only once a year at the Nats, but I always try to take note of the subtle design features that I think are unique.
Anyway, I wondered if there was any technical data on a pointy leading edged airfoil, and I found this. This is data for a 0012 airfoil in both the normal (blunt end forward) and reverse (pointy end forward) orientations. Right off I'll admit that the Reynolds number for this data is lower that what we normally have for a stunt ship stabilizer, but it better than nothing. This data gets generated for special vertical axis wind turbine analysis that see this full range of flow.
The data shows that in the reverse configuration, the lift curve is very linear through zero alpha. The normal configuration is not. The normal, blunt end forward, suffers from a nonlinear loop around the zero angle of attack range. If this was on a stuntship stab, we might see a hunting type feel in level flight. The slope of the curve does stabilize after a few degrees, but the damage has been done. This effect is probably due to a type of flow that is said to have laminar bubbles on the surface. once again, this is not at exactly our Reynolds number, but ....
So maybe Paul has something here. I think he says that it grooves better.
I've seen data for the 0012 airfoil at perverse angles of attack that was probably at higher Reynolds numbers. It had helicopter blades in mind. I don't remember where I saw it.
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Well I was convinced. May need to carry some bandaids in the flight box.
Dave
Dave is this a new plane? ... have you flown it? ..... what are you comments on the sharp stab?
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Yes Frank new plane. Actually two new ones ready to rub out and a third (take apart) under construction. None of these have been flown yet but I'm hopeful I like the stab. Someone pointed Paul's out to me at the last Nats and I ran a finger down the edge-SHARP! Thought I'd try it. I feared the angle of attack could be more critical so these stabs are removable/adjustable. I can shim the stab incidence at the flying field if need be.
Dave
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That airplane looks good.
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David ... that's not what the data shows ..... the performance is as good as or better than the forward direction.
Nobler, Ares .... yep pretty decent
My first two full body planes were a Nobler and Ares (from plans). I have put a 1/16" diameter LE on everything I have designed ever since. Didn't know any better. Amazing what we rediscover every 50 years or so.
Ken
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That airplane looks good.
Thanks Howard.
Dave
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David ... that's not what the data shows ..... the performance is as good as or better than the forward direction.
Nobler, Ares .... yep pretty decent
Thats in level flight. Once AOA changes airflow loses laminar effect on the surface earlier than that of a rounder LE. Also lose lift from the lack of turbulator effects from the flow off the LE.
Sharp LEs are great, don't get me wrong. But all airfoils have pros and cons. It'll all depends on the application. The sharp vs round LE is similar to a thin vs thick airfoil.
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Stunt stabilizer-elevator combinations are weird critters. Around zero angle of attack, pretty much any airfoil works, but the trick is to keep the laminar-turbulent transition from moving around, which causes a limit cycle. Frank and Igor Burger wrote this up somewhere here. Most folks use a sharp LE these days. I have a clear vinyl lattice thing made my Mike Haverly that looks cooler (the lattice, not Mike).
At high pitch rates, such as in sharp corners, the flow is probably separated early regardless of LE radius. The sharp LE doesn't seem to affect turning ability. Danged if I know how the stab could get much lift in this condition:
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Here's a decent article on the subject. It explains the different applications of thin and sharp foils vs round thicker foils without getting too involved.
https://aviation.stackexchange.com/questions/58444/why-are-the-leading-edges-of-wings-not-always-made-as-sharp-as-possible
I had a really good essay from Michael Selig with reynold numbers and photos of his foils in a wind tunnel. I'll try to find my old text books and I'll post it. But they are buried deep somewhere. ::) It'll take time.
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Thats in level flight. Once AOA changes airflow loses laminar effect on the surface earlier than that of a rounder LE. Also lose lift from the lack of turbulator effects from the flow off the LE.
Sharp LEs are great, don't get me wrong. But all airfoils have pros and cons. It'll all depends on the application. The sharp vs round LE is similar to a thin vs thick airfoil.
I have a personal motto in my engineering efforts: "reality trumps theory, every time". Paul Walker and Igor Burger win contests that I'm not even good enough to qualify for. Franks' picture shows a more linear relationship of lift vs. -- yes -- AOA for a pointy LE. So, theory's great.
But reality is better.
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But saying those guys flying their planes and winning because of the LEs of their models isn't reality. There's a lot more variables to consider of why the same guys win at high levels of aviation than one particular mean.
But if it works for you, by all means go for it.
Airfoils are not "Theoretical" they are indeed fact and proven.
This is actually a simple concept. Google stall effects of a wing. The LE plays the biggest roll in that. It's all about keeping the airflow over the wings surface. Really simple. A sharp LE causes a shadow and you lose that laminar flow, and that is a stall.
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This is actually a simple concept. Google stall effects of a wing. The LE plays the biggest roll in that. It's all about keeping the airflow over the wings surface. Really simple. A sharp LE causes a shadow and you lose that laminar flow, and that is a stall.
OK, but how, exactly, is that detrimental to a good stunt flight?
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The photo of Dave's plane shows a stab that has a pointed LE but then it transitions to what looks like a flat section up to the hinge line. Frank's attachment shows an airfoil that although it has been reversed, it has an airfoil shape and not flat. For the aerodynamics engineers, is this difference of any significance?
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The photo of Dave's plane shows a stab that has a pointed LE but then it transitions to what looks like a flat section up to the hinge line. Frank's attachment shows an airfoil that although it has been reversed, it has an airfoil shape and not flat. For the aerodynamics engineers, is this difference of any significance?
Yes!
The (at least) three steps to enlightenment in this case are to run a Walker-style stab airfoil through some airfoil simulator, both with and without elevator deflection, then (if you want to be thorough) back-stop those results with wind tunnel simulations because the airfoil simulators won't have been designed to cope well with sharp leading edges, and then, finally, to sit and think about it up to -- but not over -- the point where your head explodes.
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The photo of Dave's plane shows a stab that has a pointed LE but then it transitions to what looks like a flat section up to the hinge line. Frank's attachment shows an airfoil that although it has been reversed, it has an airfoil shape and not flat. For the aerodynamics engineers, is this difference of any significance?
Yes indeed it is significant.
When deciding an airfoil, the designer is going to choose a shape that follows the air shape as much as possible, Many other variables are thrown in, but that is pretty much the goal, wing shape follow airflow. Thats all your trying to do. Reversing the shape reverses the efficiency. The high point towards the rear creates more drag since that is where you're trying to merge the air back together. This is why airfoils are longer after the highest point, and thin at the end. That transition needs to have the least amount of turbulence. But it doesn't matter too much because with a sharp LE and the high point so far aft, the wing will lose laminar effect so quickly, they'll be no air to consider.
The reversed foil will have a slicing effect in the wind. When you're doing the dishes, fill the sink. Then take a knife and move it through the water. You'll notice the knife wants to take its own direction if you twist it slightly. The faster you go the more force the knife has. The knife is actually stalling through the water.
If your out driving your car, roll down the window and fly your hand like a wing. angle your hand down. It moves abruptly. Same thing as the knife. Typically we don't want that. We want a subtle transition and the flying as much as possible. In reality a wing is always in a partial stall since we do lose some laminar air flow over the surface. Different air foils dictate where those spots will be.
This isn't always so with all applications. For instance a fighter jet is very thin, and has a very sharp LE. The designers don't really care much about lift as much as drag, so they choose this style. When a jet pulls up hard and points the nose up vertical but still maintains the same path for a moment, the whole plane stalls at this time. But that doesn't matter since it has enough thrust to pull it where it needs to go.
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Interesting, I hadn't heard of this 'til now, it's mostly Hillbilly planes in these parts. ;D
I'm curious, is Howard currently flying with the pointy L/E?
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Yes!
The (at least) three steps to enlightenment in this case are to run a Walker-style stab airfoil through some airfoil simulator, both with and without elevator deflection, then (if you want to be thorough) back-stop those results with wind tunnel simulations because the airfoil simulators won't have been designed to cope well with sharp leading edges, and then, finally, to sit and think about it up to -- but not over -- the point where your head explodes.
I Love That. n~
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It'd be interesting to see a sketch of just how pointy (Mike, a picture of your hand with missing fingers would be sufficient).
"Efficiency" and "Stunt" do not go together. We aren't trying to be efficient. We are trying to carve pretty pictures in the air with a pretty airplane.
All airfoils are based on an efficiency coefficient. Not necessarily a glide ratio, but drag vs lift vs speed vs AOA.
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OK, but how, exactly, is that detrimental to a good stunt flight?
It's not correct in the first place (or at best, moot), so, don't worry about it, you don't need to concern yourself with stalls on the stab, the issue us more how to keep spot where the flow separates and/or goes turbulent as stable as possible. If it stalls, you have already lost.
There's no such thing as a "shadow", there is the normal force/pressure, and there is where it separates (if at all). Keeping the normal pressure as far aft as you can and as stable as you can is the goal. Howard's comment earlier is probably most on point, and the blunt airfoils tend to separate near the LE, the ultimate in blunt being a flat stab with a round LE, which loses all the normal pressure right where the radius ends, slight changes at very low AoA tends to make it separate on the top, then the bottom, then the top again, causing Howard's limit cycle and inconsistency right around zero. You can do it that way and have success, but in many cases you end up with turbulators to get the flow to stay attached.
The relatively pointy (it doesn't have to be as pointy as shown) airfoil just needs to have a positive gradient over a sufficient fraction of the chord to avoid really sharp pressure gradients until you can't avoid it (the hinge line) and even then that needs to be smooth and benign as possible.
This is a previous discussion on the same topic:
http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=69453&mesg_id=69453&listing_type=search#69459
Brett
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Actually the term "shadow" is used widely in aeronautical engineering. It's where the surface of an object is blocked from the airflow by itself.
Stalling is an import degree on all surfaces of a plane especially over the emennage. Stalling the stab will result in loss of control of the elevator, since stalling is a loss of airflow.
The airflow over round LE are not all over the place as the planes transitions in AOA. On the contrary. It is actually the reason for a round or blunt LE.
The shape of a LE is to keep the air flowing over as much of the surface behind it as possible. That's all it really does.
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I'm surprised that nobody has even mentioned the big thing that disturbs the air before the LE of the stab even gets a crack at it. Especially in a square corner.
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Actually the term "shadow" is used widely in aeronautical engineering. It's where the surface of an object is blocked from the airflow by itself.
Stalling is an import degree on all surfaces of a plane especially over the emennage. Stalling the stab will result in loss of control of the elevator, since stalling is a loss of airflow.
The airflow over round LE are not all over the place as the planes transitions in AOA. On the contrary. It is actually the reason for a round or blunt LE.
The shape of a LE is to keep the air flowing over as much of the surface behind it as possible. That's all it really does.
There you go Paul, better get out the wood rasp!
Brett
P.S. look at Frank's CFD, figure B. See that big bubble? Now, move that to 1/4" from the LE and make it bigger and rounder - that's what happens with a flat stab with a round LE (i.e. the ultimate in "blunt") . Change the AoA even a little to the other side, the bubble on top goes away, and a different one forms the other side. That's what causes the limit cycle Howard was discussing and *just about everyone who has ever done it that way has found in practice*. That's why most of these airplanes end up with turbulators right near the LE.
Of course, you can build it any way you want, but you might at least consider *how a 12-time national champion and world champion (not to mentioned a retired Senior Fellow from Boeing) thinks it ought to be done*, instead of dismissing it out of hand. Same with Frank and Howard, both engineers with relevant experience at the highest professional levels.
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There you go Paul, better get out the wood rasp!
It better be a carbide rasp. Paul went to extreme measures to make a sharp stab.
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The photo of Dave's plane shows a stab that has a pointed LE but then it transitions to what looks like a flat section up to the hinge line. Frank's attachment shows an airfoil that although it has been reversed, it has an airfoil shape and not flat. For the aerodynamics engineers, is this difference of any significance?
I don't think so, but I'm outvoted two to one so far.
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All airfoils are based on an efficiency coefficient. Not necessarily a glide ratio, but drag vs lift vs speed vs AOA.
Never heard of efficiency coefficient. What's the definition?
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Actually the term "shadow" is used widely in aeronautical engineering.
I've used it myself. I built a 1/10th-scale 777 wing to investigate whether the ground maneuvering camera that Dave Fitzgerald doesn't like could see the landing gear when the sun was reflecting off the wing and the gear was on the other side of the wing from the sun: a phenomenon we called "shadow".
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This is all very good stuff.
I have been using a 1/2" flat stab with a molded round LE and TE for years, basically since I started building built up models, and it works really well. I have thought about trying the sharp LE but at my building rate the super pointy will probably be out of style by the time I finish one. I tend to shy away from it as I worry I wont be able to make the Stab perfectly the same across the LE like I can with a molded LE. Also 1/2" think built up is super strong. I fear a thinner stab at my building skill level will flex and that would be a lot worse than a flat stab that is rigid.
I don't have the issues people say I should have, except inverted flight tends to be a little iffy at times but not too bad. Many others have used this same stab with good results as well. Then again maybe I am having these issues and I just fly around it and don't notice....hmmmmm. Why don't I have the issues?
I have an RC friend who says certain areas of CL stunt is purely "VooDoo aerodynamics" sometimes I have to agree. :)
Is Paul's Stab pointed then flat like Dave's or is pointed at the LE and airfoiled back to the TE? Are Elevators airfoiled back to a point as well? I would really like to see a cross section of the Stab and Elevator on his design. Anyone got one? Paul?
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I've used it myself. I built a 1/10th-scale 777 wing to investigate whether the ground maneuvering camera that Dave Fitzgerald doesn't like could see the landing gear when the sun was reflecting off the wing and the gear was on the other side of the wing from the sun: a phenomenon we called "shadow".
;D ;D ;D ;D ;D ;D ;D
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I distinctly remember this topic coming up at a VSC that got rained out about 18 or so years ago. the rain (only time that has happened at VSC) caused everyone to retreat to the Motel meeting room for a general bull session which then became a series of discussion groups, each headed by a genuine EXPERT to discuss various do's and don'ts within each group. I was in process of building my first Trivial Pursuit and the question of level flight stability came up. The group leader, none other than Mr Ted Fancher discussed some experimentation done by some fairly young guy named David Fitzgerald concerning level flight stability. Using different stabilizers (because of course his airplane was a take apart) with different configurations of leading edges on the stabilizer. Ted said the conclusion of the experiments was that a "sharp" leading with no other changes solved the instability problem. Then there was an article in Stunt News by none other than David Fitzgerald some months later showing and discussing the changes and their findings.
Me! I just took Ted at his word, went home and sharpened the leading edge on my Trivial Pursuit and never looked back. No level flight instability ever reared it's head.
Just a great flying and extremely well flying and easy to trim airplane.
The assumption at the time (I think) was that a nice rounded leading edge on the wing was a very good thing because it stalled at a higher angle of attack, but on the stab was less stable because it stalled at a lower angle of attack. Yes I know that isn't really true in aerodynamic terms but in layman terms the stabilizer has very different requirements (like stabilizing a big massive blunt air foiled wing that is inherently willing to turn quickly in response to forces on a long lever) like the fuselage.
AT any rate I just wanted to mention that the idea of sharp leading edges on a stabilizer is not a new trick and the first place I heard about it was from Ted Fancher, and David Fitzgerald Many years ago (I think around 1997 or 1998).
It definitely works...Just ask Ted!
Randy Cuberly
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... I have thought about trying the sharp LE but at my building rate ...
My Atlantis has some really ugly balsa leading edge shoes and scotch tape in its (near) future. I figure I can scotch-tape a pointy LE onto the thing, try, then rip it off and try again. I fear that I'm not good enough of a pilot to tell the difference, but -- I'll find out! All of a sudden, a hand-me-down aircraft with NO attached appearance points seems like an advantage!
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My Atlantis has some really ugly balsa leading edge shoes and scotch tape in its (near) future. I figure I can scotch-tape a pointy LE onto the thing, try, then rip it off and try again. I fear that I'm not good enough of a pilot to tell the difference, but -- I'll find out! All of a sudden, a hand-me-down aircraft with NO attached appearance points seems like an advantage!
Back in 2000 or 2001 I taped 1/4" triangle stock onto the LE of my Saturn stab. It was thick flat and round LE. The plane locked in on rails in level flight. But when I turned it for a square corner it almost crashed. At about 70 degrees into the turn it would stall violently and shudder. I nearly smashed it bad on the first turn. Scared me real bad. I tried it a few more times that day with the same result. Take off the 1/4 triangle piece and it went back to its normal self. Don't know what was happening....
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Thats in level flight. Once AOA changes airflow loses laminar effect on the surface earlier than that of a rounder LE.
That's the point of the pointy LE. Transition stays put.
When deciding an airfoil, the designer is going to choose a shape that follows the air shape as much as possible...
Not me. I choose a shape that determines the air shape (as much as possible).
When you're doing the dishes, fill the sink. Then take a knife and move it through the water. You'll notice the knife wants to take its own direction if you twist it slightly. The faster you go the more force the knife has. The knife is actually stalling through the water.
If your out driving your car, roll down the window and fly your hand like a wing. angle your hand down. It moves abruptly. Same thing as the knife. Typically we don't want that. We want a subtle transition and the flying as much as possible.
Your examples suggest the flow is not stalled. The rate of change of lift with angle of attack is higher with attached flow than it is with the wing or knife or hand stalled.
This isn't always so with all applications. For instance a fighter jet is very thin, and has a very sharp LE. The designers don't really care much about lift as much as drag, so they choose this style.
They care about both, but fighter jets go fast, and they need to worry about squishing the air. The F-104 has a really sharp LE, but not as sharp as Walker's new stab.
But that doesn't matter since it has enough thrust to pull it where it needs to go.
Those were almost the exact words my boss said when we added some draggy external armor to the F-4. It brought a tear to the aero guy's eye, though.
Stalling the stab will result in loss of control of the elevator, since stalling is a loss of airflow.
Curiously, it doesn't seem to, maybe because of what Target mentions above.
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Never heard of efficiency coefficient. What's the definition?
Finding the best compromise of given requirements. Like the lift, drag, etc.. to conclude the best overall decision. That would be efficient.
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I have been using a 1/2" flat stab with a molded round LE and TE for years, basically since I started building built up models, and it works really well. I have thought about trying the sharp LE but at my building rate the super pointy will probably be out of style by the time I finish one. I tend to shy away from it as I worry I wont be able to make the Stab perfectly the same across the LE like I can with a molded LE. Also 1/2" think built up is super strong. I fear a thinner stab at my building skill level will flex and that would be a lot worse than a flat stab that is rigid.
Mine's the same, but with a sanded solid LE, which accounts for some of the ballast in the nose. Between the trip strips on the LE and the elevator downrig that maybe gives some stab angle of attack in level flight, I get away with it. The next one may be pointy. I know what you mean about slow building and fashion. The world went electric before I could make a plane for my four-stroke engines.
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I typed a lengthy response, but has disappeared into SH ether. Will have to re-do.
Ugh....
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hmm. Did you read the article I posted earlier with a link.
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Finding the best compromise of given requirements. Like the lift, drag, etc.. to conclude the best overall decision. That would be efficient.
You've made a personal definition for a standard technical term. You'll fit right in with the stunt community.
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I typed a lengthy response, but has disappeared into SH ether.
Speaking of not being up-to-date, this guy hasn't heard the Michaelson-Morley news.
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In my working days, there used to be a saying we used, and the Air Force used on us: Emperical data trumps analytical data. This is why there are so many full scale aircraft tests for structure, aero, etc. Even at these levels, analytical data has to be proven. Many times it was found that the analytical data/analysis was not "correct". Tests verify! I try to structure tests that minimize variables as much as possible to get a "good" test, in real aircraft and models as well.
I became interested in the sharp HT LE's in the mid '90's. I built a new stab with a sharp LE on a take apart Impact in '97. This test replaced a removable stab with one the same size and weight. I tested it and found no significant improvement. It was dropped at that point.
Years later after I had switched to electric power, I became unhappy with the variation I found in HT effectivness in colder to warmer conditions. Either could be adjusted to quality, but the change was unacceptable. Once again I pulled out that same stab from '97, and installed it on 2 Impacts, and the second Predator. The change in each plane was remarkable. Tracking improved, turn and stop improved, the control feel was more linear. The second Predator was interesting as it had non linear flap mechanisms in it, and still felt more linear.
Since that test, I have built nothing but sharp LE HT's. I have flown a lot of other quality pilots planes, Nat's winners, and World Champ's planes, and as far as I am concerned, the 3rd Predator is the best plane I have ever flown. The control response is very linear, and corners can be executed at what ever radius you are wanting and willing to do. It still does nice rounds, and the turn and stop is perfect. Yeah, it took me years to get it there, but it is there now. There are several factors that have made it this way, but the sharp HT LE is a significant part of it.
As far as the wing goes, the wing LE is fairly blunt, but not as blunt as Brett's Infinity. My Impacts and Predators have a blunt enough radius as they are, and work just fine. Sharp wing LE's are clearly a bad thing. No argument there.
Bottom line for me is the sharp HT LE clearly works, and are on all my planes now.
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Just curious...
Has anyone considered the ACTUAL PATH the model flies in executing a 'square corner' (at any elevation)?
Seem to recall a photo of a stunter oriented vertically (reference to the ground below the model) at LESS THAN the model's fuselage length above the last 'level flight' posture before the turn. (Brett's model, 4, 5 or more years ago?)
Thoughts of standard aerodynamics, presuming smooth, steady flow conditions, cannot apply! The model had to be 'dirt-tracking' like an "outlaw" stocker! I could only conclude that the model rotated - in pitch - to vertical, but did not FLY there. If you've ever stood near the edge of the flight circle where good squares are being done, you've felt a strong smack of wind - like someone dropping a door flat next to you, right? That ain't smooth, streamlined flying...
The momentum of level flight at usual speed apparently is absorbed by this non-aerodynamic event. We do have sufficient power to reduce any apparent velocity loss, but remember, this is a time interval so brief that I suspect we humans cannot actually control it. Instead we develop models that can dirt track around to the desired rotation, then are positioned and powered to resume (apparently) what we wish.
IM(amateur)(not so)HO...
Just another candy bar in Caddyshack's swimming pool?
\LOU
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Has anyone considered the ACTUAL PATH the model flies in executing a 'square corner' (at any elevation)?
Seem to recall a photo of a stunter oriented vertically (reference to the ground below the model) at LESS THAN the model's fuselage length above the last 'level flight' posture before the turn. (Brett's model, 4, 5 or more years ago?)
Thoughts of standard aerodynamics, presuming smooth, steady flow conditions, cannot apply! The model had to be 'dirt-tracking' like an "outlaw" stocker! I could only conclude that the model rotated - in pitch - to vertical, but did not FLY there. If you've ever stood near the edge of the flight circle where good squares are being done, you've felt a strong smack of wind - like someone dropping a door flat next to you, right? That ain't smooth, streamlined flying...
The momentum of level flight at usual speed apparently is absorbed by this non-aerodynamic event. We do have sufficient power to reduce any apparent velocity loss, but remember, this is a time interval so brief that I suspect we humans cannot actually control it. Instead we develop models that can dirt track around to the desired rotation, then are positioned and powered to resume (apparently) what we wish.
Mine's so bad you can hear it.
I'd like to know the pitch rate in these corners so I can get more of a clue as to what's happening at the tail. That's why I'm excited to get a data TUT, but you can hardly get a regular TUT anymore.
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Emperical data trumps analytical data.
It certainly trumps misapplied analytical data. In the case of the stab LE, Igor and Frank have given us some plausible analyses. See post #17 here: http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=133980&mesg_id=133980&listing_type=search#133990 .
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Hello Paul,
Thank you for the reply. HT's?
Also is it pointed and then flat or is arifoiled the whole back?
Elevators airfoiled too?
Thank you
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Hello Paul,
Thank you for the reply. HT's?
Also is it pointed and then flat or is arifoiled the whole back?
Elevators airfoiled too?
Thank you
HT = Horizontal tail
My tail is tapered from the LE to about 0.80" aft of there. It is then flat. The elevators are straight tapered from the hinge line to the TE.
I am not suggesting that Howard's or Frank's or Igor's analytical data is wrong. That data gas some emperical data that helps validate it. Data without any emperical data to validate it is simply analytical data.
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I became interested in the sharp HT LE's in the mid '90's. I built a new stab with a sharp LE on a take apart Impact in '97. This test replaced a removable stab with one the same size and weight. I tested it and found no significant improvement. It was dropped at that point.
Years later after I had switched to electric power, I became unhappy with the variation I found in HT effectivness in colder to warmer conditions. Either could be adjusted to quality, but the change was unacceptable. Once again I pulled out that same stab from '97, and installed it on 2 Impacts, and the second Predator. The change in each plane was remarkable. Tracking improved, turn and stop improved, the control feel was more linear. The second Predator was interesting as it had non linear flap mechanisms in it, and still felt more linear.
Do you think the reason there wasn't an apparent difference the first time you tested is one of those slime vs. 'lectric things? Or do you think there may be some other reason that you didn't notice much difference on your '97 Impact?
Someone needs to run a deliberately unbalanced electric to see -- and they can solve their control stickiness problems at the same time!
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Once again let me point out that this data was for a Re = 20,000 and our typical stab is at ~ Re = 200,000. But none the less, I found it interesting anyway, in that the reversed airfoil had a linear lift response through zero alpha. The conventional direction for the airfoil at this Re was interesting in its “deadness” around zero alpha +/- 1.5 degrees, but once again, not at our Reynolds number. Hoerner (Fluid Dynamic Lift) has data for a reversed 0012 at a Re=2,500,000, once again not at our Re. It shows the same very linear effect through zero as does the Re = 20,000. I make a leap to assume that for our speeds the effect is the same. So the force from the stab around zero alpha is responsive and predictable.
The “dead effect” around zero is very pronounced for the Re 20,000 case. Data for a 0012 at high Re generally doesn’t show it. At our Re, it’s probably there, dependent on the leading edge shape.
At any rate, my gut feel, like David’s, was that the sharp leading edge would be disaster with any incidence at all. But, as we see, it isn’t too shabby at all, very one-to-one for angles of +/- 8-9 degrees.
Which brings up another question. What incidence does the stab see during a square corner? As Howard’s diagram above depicts, as the pitch rate starts, the driving angle-of-attack on stab is decreased, due to the tail length and the rotation rate. So, what is the range of alpha of the stab? It may be that it never gets above +/- 5 degrees or maybe 10 degrees. Is there any way to capture that in flight?
A guy walks into the wind tunnel area and asks, “which one of those guys is Richard Whitcomb?” The response was, “the one holding the metal file”. Whitcomb apparently was always willing to do a little filing to the model to seek a better result.
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This theory-vs.-practice thing with Paul goes way back. When Paul and I were out flying he'd suggest a counterintuitive trim change. I'd do it, and it would work. I'd ask, "Why did that work?" He'd just roll his eyes. That went on for years.
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This theory-vs.-practice thing with Paul goes way back. When Paul and I were out flying he'd suggest a counterintuitive trim change. I'd do it, and it would work. I'd ask, "Why did that work?" He'd just roll his eyes. That went on for years.
When I was a newbie engineer fresh out of college it was theory all the way (in spite of a sensible practical blue-collar upbringing). Then it was "OK, we'll go with counter-intuitive practice after we've expanded the theory to account for it". Now it's "OK, fine, whatever works; I'll reconcile theory and practice in my spare time".
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Do you think the reason there wasn't an apparent difference the first time you tested is one of those slime vs. 'lectric things? Or do you think there may be some other reason that you didn't notice much difference on your '97 Impact?
Someone needs to run a deliberately unbalanced electric to see -- and they can solve their control stickiness problems at the same time!
1) I do not know why the IC version didn't work. The difference between IC and electric is the CG and leadout position. The IC CG was much further aft (even considering the fuel), and the leadouts much farther forward.
2) what stickiness issue?
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This theory-vs.-practice thing with Paul goes way back. When Paul and I were out flying he'd suggest a counterintuitive trim change. I'd do it, and it would work. I'd ask, "Why did that work?" He'd just roll his eyes. That went on for years.
What's missing here is the fact that the change is counterintuitive in my eyes as well, but years of testing showed me certain things work....thus the rolling eyes.. as there was no explaining why it worked.
As in our current LO positions on NW electric planes......
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As in out current LO positions on NW electric planes......
Or having to move the vertical CG up on electric planes.
As in out current LO positions on NW electric planes......
Got mine way back.
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2) what stickiness issue?
People who don't build control systems as well as you have complained about their 'lectric stunters hunting, and the cure has been to git rid of friction in their control systems. (Well, from my perspective the cure has been to tell them to get rid of friction in their control systems). Doing so is reported to work. It has been hypothesized that flying an airplane that doesn't have a giant powerful vibrator in the front of it makes the control more sensitive to stiction. It has been further hypothesized, by those of questionable senses of humor, that intentionally unbalancing the motor would clear those stiction problems right up.
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People who don't build control systems as well as you have complained about their 'lectric stunters hunting, and the cure has been to git rid of friction in their control systems. (Well, from my perspective the cure has been to tell them to get rid of friction in their control systems). Doing so is reported to work. It has been hypothesized that flying an airplane that doesn't have a giant powerful vibrator in the front of it makes the control more sensitive to stiction. It has been further hypothesized, by those of questionable senses of humor, that intentionally unbalancing the motor would clear those stiction problems right up.
As long as we are "topic bending" I think the vibration coming from an IC vs an electric is more acoustic than harmonic. Both will vibrate if they are out of balance and it is almost impossible to get either in perfect balance. I have seen how a ball joint can stick when it is just sitting there but I don't know how it could do it in the air once the plane has line tension and with no slop, those flaps are not going to move on their own, (especially if it is the plane that is pulling Ted out of the circle on another thread). I could be all wet on this one and I am not advocating for stickey controls, it just seems that eliminating hunting is more of a trim problem and will depend a lot on the pilot's style.
Just to be clear, I don't consider "Binding" to be the same as sticking. Controls should never bind. And, I don't consider hunting to be bouncing around in crappy air.
Ken
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NASA proposed piezoelectric devices that would hum on a wing to keep the flow attached. I think they tried it in a wind tunnel. We investigated and decided it wouldn't scale up well. As I recall, it would have used a lot of power and would have been loud. Maybe it would work on a stunter.
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NASA proposed piezoelectric devices that would hum on a wing to keep the flow attached. I think they tried it in a wind tunnel. We investigated and decided it wouldn't scale up well. As I recall, it would have used a lot of power and would have been loud. Maybe it would work on a stunter.
So, what would you play? Inna Gadda da Vida? Stairway to Heaven (just the loud parts)? Smoke on the Water?
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So, what would you play? Inna Gadda da Vida? Stairway to Heaven (just the loud parts)? Smoke on the Water?
I was thinking about a small speaker near the flap horn. Led Zeppelin or some Buddy Rich. We could trigger it with the LG option on the timer.
Ken
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I have seen how a ball joint can stick when it is just sitting there but I don't know how it could do it in the air once the plane has line tension and with no slop, those flaps are not going to move on their own, (especially if it is the plane that is pulling Ted out of the circle on another thread).
Ken
[/quote]
This IS one of the problems. They can get sticky after some flight time, and cause real tracking issues. Unfortunately, I have a plane like that and have resisted cutting into it to replace them.
I now have only one ball link in a plane that could possibly bind, and it is at the tail and could be replaced if necessary.
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I have seen how a ball joint can stick when it is just sitting there but I don't know how it could do it in the air once the plane has line tension and with no slop, those flaps are not going to move on their own, (especially if it is the plane that is pulling Ted out of the circle on another thread).
Ken
This IS one of the problems. They can get sticky after some flight time, and cause real tracking issues. Unfortunately, I have a plane like that and have resisted cutting into it to replace them.
I now have only one ball link in a plane that could possibly bind, and it is at the tail and could be replaced if necessary.
I hadn't seen that until last year, but I had also been using the original batch of links I got back in ~1988. Last year as a stunt clinic, we did the "triage" session on Dennis Nunes' airplane, and found a bunch of slop in the controls. Bad, right? So we looked down the leadout slot and saw the whole link tilting. OK, swell, we'll cut holes and tighten them up. Duly accomplished, but then the controls were so bound up we could barely move it, completely hopeless.
This was (I think) the DuBro copy of the Rocket City #87 link. It was absolutely hopeless, unusable. I got uncomfortable and checked my other airplanes, including the original from 1988. Still loose as a goose.
The issue is the plastic binding up for some reason over time. I have started looking for an acceptable metal link to use instead. They are generally plenty free enough and very unlikely to shrink and bind up, however, the construction makes me very nervous, and most of them have visible ridges on the ball part, requiring excessive clearance to be able to move, and very likely wearing out the aluminum link body. I haven't found anything acceptable yet.
Brett
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I now have only one ball link in a plane that could possibly bind, and it is at the tail and could be replaced if necessary.
What type of linkage are you using now, specifically with the adjustable flaps??
Thanks Paul.
Vr,
Target
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What type of linkage are you using now, specifically with the adjustable flaps??
Thanks Paul.
Vr,
Target
Standard "old tech" 3/32 piano wire in brass bushings. Works great.
The one ball link in the adjuster mechanism does not have the bolt fully tightened down on the ball. The ball can spin in the plastic clevis AND the ball can spin on the bolt if the clevis tightens. This has been in several planes now and works fine.
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Thanks Paul.
Makes sense to me.
Vr,
Target
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This IS one of the problems. They can get sticky after some flight time, and cause real tracking issues. Unfortunately, I have a plane like that and have resisted cutting into it to replace them.
I am going to defer to those with more experience. H^^ I have only been back at it for a little over a year and have never had an issue. Evidently it takes more time than that to have the problem. I also tap and thread my horns and use lock nuts so that I do not tighten so much that the ball cannot move and I do not distort it. Maybe that helps.
Given our fondness for hatches and panel lines, what would be wrong with having a hatch or removable canopy that would give us access to a fully adjustable flap horn(s) and going back to full metal linkages? Has anybody tried ball bearings? I have seen them as small as 2mm ID? I love IC and do miss all of the challenge associated with getting them to run the same regardless of the weather but I am also convinced that electric is the future, that is why I am going that way. If ball links are a problem with electric then let's find a way to eliminate them or acceptance of ways to replace them periodically.
Ken
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Howard,
Thanks for your comment!
Prandtl and Tietgens (sp) long ago considered ways to consider flow conditions in fluids. (I read it, but can't do the math; I could follow it, sorta. Air is a fluid, but not a liquid fluid - a gaseous fluid fills a container, a liquid in an oversize container will form a surface preserving its actual volume.)
Situation in our "squares" seems to me to be like rapidly plunging a flat rectangle of, say, wood perpendicularly into a liquid surface. Flow around the object will be very disturbed zone... That may affect what happens to a faired surface following the disturbance... Can aerodynamics of reasonably smooth flow apply?
Instead of "path the model flies through, perhaps I should have said "path the model moves through". It is actually more like a 4-wheel drift than a loose rear pair oversteering outlaw cornering posture, no?
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I am going to defer to those with more experience. H^^ I have only been back at it for a little over a year and have never had an issue. Evidently it takes more time than that to have the problem.
It also varies from part to part. I have a Twister that's about six years old (I'd need to check). It has three ball links in it and has probably 500 flights on it, and the control system is as free as when I built it. But I know that other folks have problems.
The issue is that if you make a nice slop-free ball joint, it'll be subject to binding if the outside part shrinks or the inside part grows. With the kind of precision that you get with plastic-on-metal, this means you can't anticipate which ones will last forever and which ones will seize up.
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Well, as "fate" would have it, today, I was collecting all the pieces needed to assemble my new plane. Before construction I had "ONE" free ball link, and I put it aside. Pulled it out to put on the pushrod, and you know I don't have to tell you what happened.
Yup..it had tightened up. Total garbage now.
No problem, I will just raid one off an old plane that I dob't use anymore. No dice, it was tight also. It's not as if my shop was freezing, because it is a toasty 63F inside.
So, necessity is the mother of invention. I came up with a way to make the "ball link" work perfectly. I will take a few pictures and show them here when complete.
Why didn't I think of this before???
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Given the variation in the ball links, I have taken to pressing out the ball, run a 4-40 bolt through it, and lock down with a nut. Mount the bolt in a drill press , and using a fine file turn it and just kiss the sphere a bit. Check for fit by pressing back in place, if not happy, refile until it's good. It's actually quick and easy, but I hope Paul's remedy is easier.
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HT = Horizontal tail
My tail is tapered from the LE to about 0.80" aft of there. It is then flat. The elevators are straight tapered from the hinge line to the TE.
I am not suggesting that Howard's or Frank's or Igor's analytical data is wrong. That data gas some emperical data that helps validate it. Data without any emperical data to validate it is simply analytical data.
Hi Paul;
In a previous post, Doug Moon has mentioned adapting a triangle balsa stick to the leading edge on a model to test the theory and had issues. removing the stick brought the model back to it's previous stable trim. That's a pretty steep angle but still having a sharp leading edge. The dimensions above have been the only mention of how long to make the "slope" of the leading edge and I have been wondering about that. I had been reading this thread as it went along hoping that the length of the leading edge would be discussed more. Even though Doug had a " sharp" leading edge with the triangle stock, I think this is still relatively blunt and the air going over the top and bottom can still get turbulated as it goes across the edges. I'm thinking that having a longer, shallow angle is the key here. Can you elaborate on this aspect of it?
Type at you later,
Dan McEntee
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Since all my ball joints are stiff, I decided to remedy the situation. First I pressed the ball out of the clevis. Then measured the hole and found the nearest size of brass tubing. It was a touch loose, but that was perfect. I cleaned the inside of the ball race, and applied JB Weld inside to fill the contour of the race. Next, a length of the brass tubing was cut to length, cleaned, deburred, and inserted in the hole. Another length of tubing was necessary, and it was cut a tad longer than the first, and also JB Welded into place. Liberal application of the glue was applied around the protruding tube, and the face of the clevis. The first picture shows this. When cured, they will be faced off.
The second picture shows the final piece. A 4-40 bolt, a washer, another length of smaller tubing, and this will fit inside the clevis assembly.
The final picture shows how it will fit up.
There is enough "gap" between the last two tubes to allow a small amount of rock, and a very free rotation that will not eventually bind. It is in the tail so any significant rock is not necessary.
Tomorrow after the glue cures, it can be assembled.
Hope this explains it sufficiently.
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Dan,
Here is a photo of the geometry of the HT LE.
It reaches the full stab thickness in 0.80"
Help?
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Dan,
Here is a photo of the geometry of the HT LE.
It reaches the full stab thickness in 0.80"
Help?
Oh, I understood what the dimension represented, I'm just curious with how many different "lengths" of the leading edge did you try? Is this a minimum? A certain ratio depending on stab thickness? Given what I have read in this thread, I think you can be too short, but is there a maximum length? I have a model that I am going to do a face lift on, and I might redo the stab in this fashion, but just want to do it once! Is this treatment good for any stab location in relation to the wing/thrust line? Or mainly something for 0-0-0 set ups? I have two airplanes that I tried to build identical, but they were ten years apart! I used same engine/tank combination and tried to make the second airplane as much of a clone as the first in order to minimize trimming issues. The second model came out just a bit lighter and my plan worked as it flew exactly the same as the first model. So much so that I was able to fly a pretty decent pattern on the first flight after an upright and inverted wings level eyeball check and lap time check. But both models turn better outside than they do inside and I haven't been able to figure out why. They do fly well enough for me to compete with and have done well with them over the years, but the turn thing still bothers me a bit. The stab on this design might be a bit higher than the norm I think but I really haven't had the time with work and family stuff through the years to really dig into it, just put up with it and dealt with it. This sharper leading edge has me wondering if that may help in this circumstance.
Thanks a lot,
Dan McEntee
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Oh, I understood what the dimension represented, I'm just curious with how many different "lengths" of the leading edge did you try? Is this a minimum? A certain ratio depending on stab thickness? Given what I have read in this thread, I think you can be too short, but is there a maximum length? I have a model that I am going to do a face lift on, and I might redo the stab in this fashion, but just want to do it once! Is this treatment good for any stab location in relation to the wing/thrust line? Or mainly something for 0-0-0 set ups? I have two airplanes that I tried to build identical, but they were ten years apart! I used same engine/tank combination and tried to make the second airplane as much of a clone as the first in order to minimize trimming issues. The second model came out just a bit lighter and my plan worked as it flew exactly the same as the first model. So much so that I was able to fly a pretty decent pattern on the first flight after an upright and inverted wings level eyeball check and lap time check. But both models turn better outside than they do inside and I haven't been able to figure out why. They do fly well enough for me to compete with and have done well with them over the years, but the turn thing still bothers me a bit. The stab on this design might be a bit higher than the norm I think but I really haven't had the time with work and family stuff through the years to really dig into it, just put up with it and dealt with it. This sharper leading edge has me wondering if that may help in this circumstance.
Thanks a lot,
Dan McEntee
I have several both ways. The position of the stab does not appear to make any differencs.
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A certain ratio depending on stab thickness?
Thanks a lot,
Dan McEntee
Seemingly so, 5T : 8L.
I'm also interested in if the ratio matters. Sounds like it needs to be lesser than 1 : 1 at least.
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Seemingly so, 5T : 8L.
I'm also interested in if the ratio matters. Sounds like it needs to be lesser than 1 : 1 at least.
Ratio: I assume you mean elevator deflection to flap deflection.
Mine run fairly close to 1:1. Each is adjusted to its own preference.
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Ratio: I assume you mean elevator deflection to flap deflection.
Mine run fairly close to 1:1. Each is adjusted to its own preference.
Nope, not elevator/flap, but length of the 'slope" of the leading edge, is what I'm referring to. This is all about airflow over the stab to the elevators, correct? In the model I am talking about, I have enough deflection. In fact, I can stall the airplane going into an inside square or coming out of it if I'm not careful. Having two airplanes that fly almost exactly the same way built ten years apart made me wonder if it was something in the design? I had read a long time ago something in which Ted Fancher tried taping a piece of wire to the leading edge of a stabilizer on some one's airplane and helping with an issue. I even tried that but did not see any difference. This is much like Doug taping some triangle stock to his leading edges and things went south for him. What Paul has drawn up can be incorporated into my airplane when I refinish it, but like I said in the beginning, I want to try and get it right the first time.
Type at you later,
Dan McEntee
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Ratio: I assume you mean elevator deflection to flap deflection.
Mine run fairly close to 1:1. Each is adjusted to its own preference.
Nope, sorry to not be more clear.
Dan and i are intrigued by your HT LE slope/shaping. Did you try a steeper slope initially or is that .8" arc per (i assume by your plan) .5" thickness random?
From reading your other posts, I'm guessing it's not random.
Thanks in advance.
Vr,
Target
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I'm curious, is Howard currently flying with the pointy L/E?
No, not yet. I have a flat stab with a semicircular LE, as on Impacts of old. There are clear vinyl lattice trip strips, made by Mike Haverly, on the LE intended to keep the laminar-turbulent transition fixed. They are almost invisible, but look cool in sunlight.
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No, not yet. I have a flat stab with a semicircular LE, as on Impacts of old. There are clear vinyl lattice trip strips, made by Mike Haverly, on the LE intended to keep the laminar-turbulent transition fixed. They are almost invisible, but look cool in sunlight.
On the similar "flat stab" Trivial Pursuits, they ended up with wires taped to the LE at various points for the same reason. Right at the nose (effectively creating a point) and top and bottom, right were the LE went from curved to flat.
That was the motivation behind David's DeTails article, to experiment with different stab airfoils (and at my behest, incidence, which not surprisingly, made more difference in the results).
Brett
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The geometry in the picture was from the stab test I did in '97.
It worked so well in two Impacts, two Predators, that I continued to use it. It is also in two P-47's, two Hellcats, and now Bearcats.
I just haven't felt like building a fleet of different stabs to test them out, as the one I have works so well.
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So, Paul, you're saying it was just a "lucky stab" that you arrived at a ratio LE design that works for you.
Pun intended.
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So, Paul, you're saying it was just a "lucky stab" that you arrived at a ratio LE design that works for you.
Pun intended.
::) ;D
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The geometry in the picture was from the stab test I did in '97.
It worked so well in two Impacts, two Predators, that I continued to use it. It is also in two P-47's, two Hellcats, and now Bearcats.
I just haven't felt like building a fleet of different stabs to test them out, as the one I have works so well.
Paul:
I have a new ship that I test flew yesterday (without final finish - gotta love electric) that has a "sharp" LE (1/16 Diameter). Would it be worth my effort to cap it with a more pointed one before final finish? I am not sure what characteristic is improved. It doesn't hunt and it locks out of corners really well now.
Ken
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Paul:
I have a new ship that I test flew yesterday (without final finish - gotta love electric) that has a "sharp" LE (1/16 Diameter). Would it be worth my effort to cap it with a more pointed one before final finish? I am not sure what characteristic is improved. It doesn't hunt and it locks out of corners really well now.
Ken
If it flies up to your expectations, then I wouldn't touch it.
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If it flies up to your expectations, then I wouldn't touch it.
Thanks, I will leave it as is. I thought mine were "sharp" till I saw yours!
Ken
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Paul:
I have a new ship that I test flew yesterday (without final finish - gotta love electric) that has a "sharp" LE (1/16 Diameter). Would it be worth my effort to cap it with a more pointed one before final finish? I am not sure what characteristic is improved. It doesn't hunt and it locks out of corners really well now.
It doesn't have to be super-sharp like a knife-edge. Mine have had a LE radius of about 3/32" - but the rest of the stab is an airfoil, and gets thicker all the way to the hinge line. It flies very well, never has these sorts of issues that David had with the flat-stab Trivial Pursuits, and using either tripper strips, zigzag strips, or vortex generators has never made any consequential difference in the performance. I could hear the difference with VGs, it made a faint whooshing sound, but that's about it.
The original Trivial Pursuit, and David's first model, had an airfoiled stab, but the high point was far forward like the wing and the nose was pretty blunt. I think that created a similar issue at times, with the separation bubbles forming further back. But they were relatively stable, so they didn't have the sorts of issues the flat stab/round LE seemed to cause on the later examples (and subsequently required gadgets like turbulators to optimize).
My elevators have something else I ripped off, unapologetically, from Paul - there are no sharp edges along the hinge lines. The elevator comes to a point at the hinge line, but everywhere else, the corners are rounded off with a about a 1/4" radius.
Brett
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It doesn't have to be super-sharp like a knife-edge. Mine have had a LE radius of about 3/32" - but the rest of the stab is an airfoil, and gets thicker all the way to the hinge line. It flies very well, never has these sorts of issues that David had with the flat-stab Trivial Pursuits, and using either tripper strips, zigzag strips, or vortex generators has never made any consequential difference in the performance. I could hear the difference with VGs, it made a faint whooshing sound, but that's about it.
The original Trivial Pursuit, and David's first model, had an airfoiled stab, but the high point was far forward like the wing and the nose was pretty blunt. I think that created a similar issue at times, with the separation bubbles forming further back. But they were relatively stable, so they didn't have the sorts of issues the flat stab/round LE seemed to cause on the later examples (and subsequently required gadgets like turbulators to optimize).
My elevators have something else I ripped off, unapologetically, from Paul - there are no sharp edges along the hinge lines. The elevator comes to a point at the hinge line, but everywhere else, the corners are rounded off with a about a 1/4" radius.
Brett
You are probably right. I have never had a flat airfoil on a competition ship. This is the airfoil from the one pictured in my question to Paul. It is 1/2" thick at the highpoint which is just ahead of the hinge line. The elevators are recessed and 7/16 thick at the hinge. If that looks good I am moving on. Probably the last time I will do recessed anything. Looks good but doesn't fly any better.
Ken
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In response to a question about how he connects the bell crank to the flaps, Paul Walker responded:
Standard "old tech" 3/32 piano wire in brass bushings. Works great.]
[I can't figure out how to keep my remarks below from looking like they are a quote of something Paul wrote]
In that vein, I'm going to follow Tom Dixon's control pushrod system of 3/32" music wire with carbon fiber tubing (.100" ID) slid over the wire and super glued to the piano wire.
I ordered .100" ID CF tubing from CST products and got an email saying they were out of stock and their supplier wasn't going to make CF tubing to .100" ID any more.
I found .098" ID CF tubing at Midwestproducts.com. Forty inches was $7.46. I ordered two tubes and the shipping was only $5.00.
Joe Ed Pederson
Cuba, MO
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In response to a question about how he connects the bell crank to the flaps, Paul Walker responded:
Standard "old tech" 3/32 piano wire in brass bushings. Works great.]
[I can't figure out how to keep my remarks below from looking like they are a quote of something Paul wrote]
You have to close the quote tag before you type your response, that is:
{quote} text being quoted {/quote} with the curly brackets replaced with square brackets.
Brett
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.....Danged if I know how the stab could get much lift in this condition:
The picture you show, Howard, looks a lot like the shape used on many free-flighters- a seriously cambered airfoil.. The air going over the bottom side of the stab gets all its lift from the large flaps. The bottom of the airfoil(pointing up) generates very little of the lift. In that condition the size of the leading edge doesn't matter as much, but a sharp leading edge would make sure that whatever the stab was doing happened with very little control.
Many years ago I did something similar for profile carrier. The flaps were something like 1/3 the chord made from sheet balsa. It also had a very large stab, maybe 40% 'cause I knew I needed something to control that wing.
It flew perfectly fine but its carrier maners left somthing to be desired. During low speed flight the nose pointed about 20° down. A 28 oz plane at 13mph slow didn't need but may 10% of the lift available. 5 deg° flap, maybe zero flaps would have worked well. Carrier wasn't my bag and I quit when they all started prop hanging for low speed flight.
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I've used it myself. I built a 1/10th-scale 777 wing to investigate whether the ground maneuvering camera that Dave Fitzgerald doesn't like could see the landing gear when the sun was reflecting off the wing and the gear was on the other side of the wing from the sun: a phenomenon we called "shadow".
Ok Howard, If it works that seems to be good enough for you.
But not me. I want to know Why it works.
I actually have a degree in aerospace engineering, with a study of high lifting low velocity airfoils, that I got back in 1991. Useless because I chose a different path in my career.
But yes indeed when wind is deflected, in any means, there is a "shadow".
Now all I've been mentioning is "what happens to the air when you choose a sharp airfoil. If it works in Stunt, then I'd like to know why it works.
Also, I'm not sure you understand what the word "efficiency" means. As you said it's not what stunt pilots are after.
Efficient and Coefficient are to separate words. They are not a term. I later just rephrased my sentence to mean the same thing.
Others:
I still wonder of the practices in stunt. It's a very unorthodox method and very intriguing. The only way I'll learn is by asking. And the only way I'll understand is by questioning, Why..
Earlier I mentioned an idea of why a sharp LE would be better for these stunt pilots. I made this guess in relation to what I know happens physically to the surface of the stab.
I'm not trying to act as if I know everything, on the contrary. I don't know. But I do want to know. That's why I ask.
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Ok Howard, If it works that seems to be good enough for you.
But not me. I want to know Why it works.
Brett has offered a cognizant explanations of why it is believed to work. If you keep in mind that the purpose of the part is a control surface and not surface to provide lots of lift, then the notion that we're trading off some drag for a lot of consistency works for me.
Beyond that, I can only see two ways to resolve your question, because none of the freaking experts we have on the group (i.e., Brett, Paul, possibly Howard, and possibly Lou) know the answer, which is -- go do a lot of work, and find out for yourself. Either find a computational fluid dynamics program that you trust (I suggest x-foil and Martin Hepperle's Java app, but the last time I was really serious about this was about ten years ago -- there may be better open-source CFD programs out there), do the simulations (at various elevator deflections) and check, or build a wind tunnel and a stab and check. Or if you know some university types, build a stab and get some time on a wind tunnel somewhere.
Because what we have here is a water-cooler discussion that involves some pretty heavyweight technical types; any junior engineer that's not satisfied with the answer needs to go off quietly and do some careful work to either confirm or contradict what's been said.
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Earlier I mentioned an idea of why a sharp LE would be better for these stunt pilots. I made this guess in relation to what I know happens physically to the surface of the stab.
I'm not trying to act as if I know everything, on the contrary. I don't know. But I do want to know. That's why I ask.
I don't think anyone has a problem with that, however, you are also not listening to the answers, or rather, dismissing them out of hand for differing with your preconceived notions. That's not a crime, at most it is mildly irritating. But it's also common bordering on ubiquitous for people to have a theory that sort of works for them, then hold on to it like grim death even in the presence of abundant contrary information. No one particularly begrudges that - you are free to use whatever theory you want.
Fortunately, we have a method for determining who is right, it's in Muncie in July. And it's not like we are curing cancer, no one is harmed if any of us guess wrong.
This was a well-understood issue 20 years ago, there have been multiple internet posts and SN articles about the phenomenon, particularly with blunt flat stabs. Paul has since adopted the same idea and there is no one who is a more careful experimenter or more capable of evaluating the results. I think making it pointy but otherwise flat is a transitional state to making it airfoiled, but there are tradeoffs in everything. The underlying concept is proven (by dint of endless hours in the 100+ degree sun experimenting with it over the years) to the extent I think it needs to be proven. If that's not good enough, well, you know what to do.
I am a little concerned when I see talk of "helical flow pictures" or "stunt planes don't roll so what does dihedral do", from the other threads, which are pretty naive comments for an aeronautical engineer, but again, naiveté is not a crime, either. There are also plenty of things we definitely *don't* understand very well or at all, so the rest of us are on the same path - just much further along it.
Brett
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I don't have anything to offer in the way of explanations or ideas, but I think that the "sharp leading edge" statement by itself doesn't give the whole picture now that we have gotten this far. "Sharp leading Edge" tends to infer just the actual point on the leading edge, but I think it needs to be kept in mind that the transition to the surface is part of any success or improvement. Just a blunt, shallow "sharp, pointy leading edge" isn't the same thing. I just was looking at the plans for Charles Parrot's P-47 Thunderbolt classic legal model and the profile of the stab airfoil on that airplane. That was about 1964 or so but it's right on par with the discussion here, I think.
Type at you later,
Dan McEntee
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I don't have anything to offer in the way of explanations or ideas, but I think that the "sharp leading edge" statement by itself doesn't give the whole picture now that we have gotten this far. "Sharp leading Edge" tends to infer just the actual point on the leading edge, but I think it needs to be kept in mind that the transition to the surface is part of any success or improvement. Just a blunt, shallow "sharp, pointy leading edge" isn't the same thing. I just was looking at the plans for Charles Parrot's P-47 Thunderbolt classic legal model and the profile of the stab airfoil on that airplane. That was about 1964 or so but it's right on par with the discussion here, I think.
Type at you later,
Dan McEntee
So is the one on the Green Box Nobler. But you don't know whether something is better or not, without trying to make it better and failing.
Brett
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I don't think anyone has a problem with that, however, you are also not listening to the answers, or rather, dismissing them out of hand for differing with your preconceived notions. That's not a crime, at most it is mildly irritating. But it's also common bordering on ubiquitous for people to have a theory that sort of works for them, then hold on to it like grim death even in the presence of abundant contrary information. No one particularly begrudges that - you are free to use whatever theory you want.
Fortunately, we have a method for determining who is right, it's in Muncie in July. And it's not like we are curing cancer, no one is harmed if any of us guess wrong.
This was a well-understood issue 20 years ago, there have been multiple internet posts and SN articles about the phenomenon, particularly with blunt flat stabs. Paul has since adopted the same idea and there is no one who is a more careful experimenter or more capable of evaluating the results. I think making it pointy but otherwise flat is a transitional state to making it airfoiled, but there are tradeoffs in everything. The underlying concept is proven (by dint of endless hours in the 100+ degree sun experimenting with it over the years) to the extent I think it needs to be proven. If that's not good enough, well, you know what to do.
I am a little concerned when I see talk of "helical flow pictures" or "stunt planes don't roll so what does dihedral do", from the other threads, which are pretty naive comments for an aeronautical engineer, but again, naiveté is not a crime, either. There are also plenty of things we definitely *don't* understand very well or at all, so the rest of us are on the same path - just much further along it.
Brett
I'm not nor have I questioned whether or not it is successful. I simply questioned why it works for these guys.
I don't think too many have an understanding of this. Why it works. But just that it does. Having thin airfoils with sharp leading edges are not creating more drag, as Dan just stated, but less drag. You guys should know that. Just look at CL speed rigs.
And then Howard stating that Efficiency and Stunt do not go together (I'm probably paraphrasing) makes me believe he doesn't understand the word efficiency. Because that doesn't make sense.
Or that sometimes the laminar airflow is never separated. It always has some point of separation.
The effects of the elevator is what I have been questioning. During transition the sharp LE will cause more loss of airflow, or a shadow, over the surface. Which is a stall. And the elevator will be less sensitive. That Is what happens. Why it would be preferred among these pilots is questionable at this time. My theory would be that since this creates a exponential loss of sensitiveness on the elevator, It makes the airplane behavior more manageable. And finding that spot to ease off when ending a transition, like doing a hard 90* pull up and having to stop precisely at 90* makes it easier. This is what I said earlier.
I know What happens, I just want to know why it's preferred. From knowing what happens, this seems to make the most sense to me.
LOL
I asked about dihedral in CL because in aviation dihedral effects the roll stability. I thought everyone interested in planes knew this?
I believe that's a bit difference as well.
I'm interested in CL Stunt because I have a passion for airplanes. I'm not involved in Airplanes because I have a passion for Stunt.
I seriously question that you actually read the link about propeller theory. But there's nothing wrong about that. It's just that if you did, you'd realize that once again, Helical is not my interpretation. It's what is understood in that industry. Just like "shadow".
I believe the I'm having a problem understanding what some are saying here and the explanations are being lost in translation.
Probably better to sit down with a glass of ale and share ideas with some impartial individuals. Hopefully that day will come.
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And then Howard stating that Efficiency and Stunt do not go together (I'm probably paraphrasing) makes me believe he doesn't understand the word efficiency. Because that doesn't make sense.
It may well have been me. And having been a successful circuit & systems designer for the last 30 years, I certainly feel that I know what it means.
So how do you define it? I would say that a 64-ounce stunt plane that can fly the entire pattern on two ounces of fuel is way more "efficient" than one that can do the same thing on seven ounces of fuel.
But there are folks out there who fly .77 cubic inch engines that need seven ounces of fuel to slobber their way through the pattern, and who place in the top 5 at the Nationals, and who would be (and sometimes are) competitive at the Worlds. On the other hand, if you put a 40 FP on a 64-ounce plane, prop it right, and lean it out, you can do the pattern with just two ounces of fuel. Woo hoo! 2 1/2 times as efficient! And your plane flys so crappy that if you're a freaking expert you may score in the low 500's.
This is why nobody who wins chases that efficiency -- they just happily build fuel tanks that are as big as they need to be to fly their particular plane/engine combination.
But, clearly, I'm a dolt. Could you please explain what "efficiency" really means, and why we need it in stunt?
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That is correct. That is efficiency. In terms of your fuel consumption over the distance traveled
These are ratios, comparing one aspect to another.
An efficient light bulb would be lumens vs. wattage use.
Thermal Dynamics. This pan is more efficient that that one. It takes less heat to cook an egg and therefore less time.
Everything has a basis of some kind of efficiency. From the food we consume, to the way our eyes see and send those images to our brain, to the way we mow our lawns, etc.
These are Ratios
They help use find the most efficient coefficient (applied mathematics formula) to reach a desired goal for a particular task.
Why looking for efficiency is the aerodynamics of control line stunt planes would be advisable, it seems self explanatory once you consider what efficiency means.
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That is correct. That is efficiency. In terms of your fuel consumption over the distance traveled
These are ratios, comparing one aspect to another.
An efficient light bulb would be lumens vs. wattage use.
Thermal Dynamics. This pan is more efficient that that one. It takes less heat to cook an egg and therefore less time.
Everything has a basis of some kind of efficiency. From the food we consume, to the way our eyes see and send those images to our brain, to the way we mow our lawns, etc.
These are Ratios
They help use find the most efficient coefficient (applied mathematics formula) to reach a desired goal for a particular task.
Why looking for efficiency is the aerodynamics of control line stunt planes would be advisable, it seems self explanatory once you consider what efficiency means.
OK. So, the reason that efficiency is not important in stunt is that chasing it makes you lose. And I want to win. So I say things like "efficiency and stunt don't go together". I should think that the reason to avoid chasing something that, in and of itself makes you lose would be self-explanatory.
So, explain -- why should I expend energy getting a worse score? It just isn't explaining itself to me. Actually, it is explaining itself, but in just about the opposite direction that you're trying to pull me. C'mon, man -- explain! Two ounces of fuel consumption and a losing score -- how is that better than $1 more worth of fuel and a winning score? Please clarify how purposely losing is "self explanatory"!
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To succeed in Stunt one has to be as inefficient as it gets-burn as much fuel as you possibly can. Most else is small stuff.
Dave
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OK. So, the reason that efficiency is not important in stunt is that chasing it makes you lose. And I want to win. So I say things like "efficiency and stunt don't go together". I should think that the reason to avoid chasing something that, in and of itself makes you lose would be self-explanatory.
Ok. I understand now.
You believe efficiency Only relates to your fuel consumption. It does not.
Again, Everything is based on some sort of efficient ratio. And I mean everything as in Not just in Cl Stunt or airplanes for that matter.
So what you're saying is fuel efficiency in therms of distance (it could be a lot of other factors as well) is not important. Ok. But then that isn't what we're discussing here. We're discussing the stab and elevator. Not fuel consumption.
At least I wasn't
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We're discussing the stab and elevator. Not fuel consumption.
At least I wasn't
OK. So. The notion has been put forth that a sharp stab leading edge not only reduces the "efficiency" (presumably the L/D ratio), but increases how predictable the control is (presumably by making the position at which flow separates more consistent). So a pointy LE stab is always a little bit stalled, and always a little bit "inefficient" in terms of L/D, and very possibly lift vs. angle of attack and lift vs. elevator deflection. So in every conceivable way, it's inefficient. But, the airplane does not jump all over the sky.
So, explain to me how an airplane that has the world's most efficient stab, but jumps all over the sky because of chaotic flow separation, is going to help me win vs. a stab that lifts less and drags more, but does so with absolute repeatability.
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OK. So, the reason that efficiency is not important in stunt is that chasing it makes you lose. And I want to win. So I say things like "efficiency and stunt don't go together". I should think that the reason to avoid chasing something that, in and of itself makes you lose would be self-explanatory.
Ok. I understand now.
You believe efficiency Only relates to your fuel consumption. It does not.
Again, Everything is based on some sort of efficient ratio. And I mean everything as in Not just in Cl Stunt or airplanes for that matter.
So what you're saying is fuel efficiency in therms of distance (it could be a lot of other factors as well) is not important. Ok. But then that isn't what we're discussing here. We're discussing the stab and elevator. Not fuel consumption.
At least I wasn't
In my case I really wasn't talking about fuel consumption. I also know the discussion is largely theoretical anyway. Most of todays competitive airplanes are more powerful, and have some reserve performance designed in so that the motivation or need to squeeze the last drop of aerodynamic efficiency out of each part doesn't gain much. We do like to experiment though and sometimes certain things seem to work better or feel better at the control handle. That is all the efficiency I'm looking for.
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Also, I'm not sure you understand what the word "efficiency" means. As you said it's not what stunt pilots are after.
Efficient and Coefficient are to separate words. They are not a term. I later just rephrased my sentence to mean the same thing.
[/quote]
Let me give you an example.
Props.
I used the APC 13*4.5 electric prop for several years. In a standard Impact, I could use a 5S2700 battery for full competition flights, with adequate reserve in the battery.
I then switched to a 3 blade 11" prop and instantly had to go to a 6S2700 battery for enough capacity to have adequate reserve. The plane flew better, but used more battery. The 3 blade is less efficient in converting battery power to performance.
So in this case, better performance is had with a less efficient prop. So, I think Howard has it right!
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It may well have been me. And having been a successful circuit & systems designer for the last 30 years, I certainly feel that I know what it means.
So how do you define it? I would say that a 64-ounce stunt plane that can fly the entire pattern on two ounces of fuel is way more "efficient" than one that can do the same thing on seven ounces of fuel.
But there are folks out there who fly .77 cubic inch engines that need seven ounces of fuel to slobber their way through the pattern, and who place in the top 5 at the Nationals, and who would be (and sometimes are) competitive at the Worlds. On the other hand, if you put a 40 FP on a 64-ounce plane, prop it right, and lean it out, you can do the pattern with just two ounces of fuel. Woo hoo! 2 1/2 times as efficient! And your plane flys so crappy that if you're a freaking expert you may score in the low 500's.
This is why nobody who wins chases that efficiency -- they just happily build fuel tanks that are as big as they need to be to fly their particular plane/engine combination.
But, clearly, I'm a dolt. Could you please explain what "efficiency" really means, and why we need it in stunt?
To my thinking, in PA or stunt, efficiency is the number of flights you need to take first place. That could be number of practice flights before a contest, number of flights from beginner to the first place trophy in expert, or advanced, or intermediate, or the number of flights it took to say "I think that was a pretty good pattern, first one ever". Talking about fuel efficiency is stunt is like talking about beauty in a beer. They just don't jibe.
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To my thinking, in PA or stunt, efficiency is the number of flights you need to take first place. That could be number of practice flights before a contest, number of flights from beginner to the first place trophy in expert, or advanced, or intermediate, or the number of flights it took to say "I think that was a pretty good pattern, first one ever". Talking about fuel efficiency is stunt is like talking about beauty in a beer. They just don't jibe.
OK. I was with you right up until you implied that beer is not beautiful.
WHAT?!?!?!!!
Oh, the sacrilege.
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I used to think that I was a smarter than average laymen aero guy, having flown RC Sailplanes for a number of years, with some success.
"Control line seems pretty simple", I thought.
WRONG!
R,
Target
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David, I think you may have been aiming too low for this audience. The guy who started this topic, for example, is a NASA aero with his own wind tunnel. I think everybody here has known for decades that the airfoils that have the highest Clmax at our Reynolds numbers are thicker, have blunter LEs, and have the chordwise point of max thickness more forward than airfoils for most purposes. I learned that myself from old NACA reports in 1962, verified it in practice, and started winning combat matches because of it. Rather than vectoring us to a nontechnical Web site, you might have shown us data such as post #46 here: http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=371054&sub_topic_id=371056&mesg_id=&page=#371291 . I started picking on you after I explained the stab issue and you went on to show me that Web site.
The virtue of pointy stab LEs on stunters is pretty well established experimentally and theoretically. Three guys who worked on this are Paul Walker, Dave Fitzgerald, and Igor Burger. Paul, whose testimony you heard above, of whom Brett said, "... there is no one who is a more careful experimenter or more capable of evaluating the results", wrote the book on stunt plane trimming: http://flyinglines.org/pw.trimflow1.html . David made and evaluated a series of stabilizers on one airplane and wrote the results up for Stunt News. Igor did an analysis with JavaFoil to show how the laminar-turbulent transition causes the level-flight limit cycle: http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=133980&mesg_id=133980&listing_type=&page=6#133983 , post #17. And those are just the world champions who researched it! I regret that I don't know why the pointy stab works well at high lift. I'd like to get lots of lift at even more negative angle of attack (positive lift with positive elevator deflection), but I haven't figured out a practical way to do it, let alone a way that would help my score.
I presume that "efficiency coefficient" is nondimensionalized efficiency. Maybe you divide efficiency by 2 pi.
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To succeed in Stunt one has to be as inefficient as it gets-burn as much fuel as you possibly can. Most else is small stuff.
Dave
Why?
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Why?
Burning fuel (or ions) mean very many practice flights............fuel in the jug =idle airplane.
Dave
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Rather than worry about “efficiency”, I think we need to fall back and look at the graph in my original post. The measured and the CFD data show that the lift produced by the sharp leading edge configuration is very very linear between +/- 8-9 degrees of angle of attack. I initially thought for sure that the razor sharp LE would not work at all. I thought that it might have a positive effect on grooving or killing any hunting, but intuition said, like David I think believes, that it would be a disaster once I started to maneuver. That’s not the case.
All along the problem with the tail end of a stunt ship has been getting a repeatable linear effect from the stab/elevator. Sealing the hinge line helps greatly with repeatability. The shape of the stab appeared to be very important also. Many different sections have been tried through the years, some better than others. The “pointy leading edge” is the latest attempt that has shown good results. I was looking for published measured data that supported the empirical results, and it’s there.
Since flying one of these “razor tails” work and we don’t feel a stall of the tail, I think we can conclude that the tails don’t actually see a very high angle of attack like we might suspect. Surely something less than +/- ten degrees. This is a question that would be fun to look at, “what is the incidence of the flow at the tail during a tight square maneuver?”. The dynamics of the flow at the tail in a tight turn is effected by several factors; the downwash from wing and flaps, the induced up wash of the elevator, the length of the tail swinging through the arc of the turn, and the rate of the turn. We need a simple simulation to look at this.
So. look again at the data and observe the very linear relation between the angle of attack and the lift for the reversed flow condition. Don’t try to look at a big picture of a backwards flying airfoil at large angles of flow.
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What I find interesting is the linearity from the origin with the reversed airfoil. The "standard" direction is flat from the origin for about 2 degrees, then increases. This could be the reason I can feel a difference!
Thanks Frank!
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Frank thanks for the data. It's comforting to think my new stabs shouldn't be hyper-critical to either incidence adjustments or minor control inputs. On one of my take-apart airplanes with a rounded stab leading edge, I noticed a pronounced habit of it to fly tail low in level flight but otherwise act perfectly normal and linear to the handle. I fly next to a river levee and the visual height in level flight is exactly the same as the top of the levee so it stuck out like a sore thumb. I started to shim under the stab leading edge and needed to lift it over 1/32" to get the tail up to level. This didn't seem to change any other way the airplane flies.* I felt the leading edge radius was being tolerant of these incidence changes. From my free flight experience and especially hand launched gliders where we try to be as close to 0-0 incidence as possible it sometimes doesn't take much of an incidence adjustment or slight warp to get the airplane to "go negative" and start an outside loop mid-launch. It's that sensitivity to changes that concerned me in trying this razor leading edge. Now I'm more confident about it.
Dave
* This is now why all my new airplanes have the stab incidence adjustable....
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We need a simple simulation to look at this.
Simple?
Looks like that's a CFD analysis, although the caption mentions experimental data. It's interesting that it shows air density ratio. It's close to one. Is that what really happens in "incompressible" flow, or is this at some higher Mach number?
Of course the scale would have to be in density altitude for Dave Fitzgerald to make sense of it.
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The solid line and dashed line are the CFD results.
The circle and the triangle are the experimental results from tunnels.
The normal facing data shows a dead spot through zero. However, once again, this particular study is for a lower Reynolds number than us and is more susceptible to these problems at zero alpha. There are other studies that show similar results for the reversed flow that are at our speeds.
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Just curious if it is at all possible that the various anomalous behaviors of the many planes being diagnosed in these 120 odd posts might be the result of separate and disparate construction/trim/alignment etc. issues in the other 99% of the airplane not being considered, i.e. the relative bluntness of the stab LE?
Paul and David have won about 100 or so Nats and WCs with stabs varying from flat planks from Home Depot with a few strokes of sandpaper rounding the leading edge to subtle and not so subtle airfoils with and without pre-planned and/or after the fact incidence adjustments. Paul, in particular, has done it both ways (leading edge entry) and advocated for each at different times (although,not to my memory, has he utilized the positive incidence approach...could be totally wrong in that regard).
I'd be interested in a discussion that included the once favored versus the newly favored and why the change in preference from an aerodynamic/performance perspective inasmuch as significant victories would point to either approach as--at least--a valid choice.
Ted
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My first two full body planes were a Nobler and Ares (from plans). I have put a 1/16" diameter LE on everything I have designed ever since. Didn't know any better. Amazing what we rediscover every 50 years or so.
Ken
Where's that "Like" button???????
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Some thickening of the plot. I ran the normal-facing 0012 at different Reynolds numbers with Profili (XFOIL) and saw pretty much the same thing at low Reynolds numbers (see below). Our stabs operate at about 300,000, where the lift curve slope is normal. Then I looked at the Stuttgart wind tunnel data for the 0012. It, too, shows the perversion at Reynolds numbers of 40,000 and 60,000, with a noticeable slope reversal at 40,000 and 60,000. The lift curve slope is well-behaved at 150,000. Here is what I did awhile back for the old Impact stab: https://stunthanger.com/smf/engineering-board/impact-stab-analysis/msg294246/#msg294246 . It needs more work, but the Reynolds number plot shows that it's well-behaved from 200,000 to 400,000. I don't think this lift-curve-sign-reversal effect is what Paul saw with the blunt LE. Rather, I think it's the transition wandering around as described by Igor and Frank.
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Paul and David have won about 100 or so Nats and WCs with stabs varying from flat planks from Home Depot with a few strokes of sandpaper rounding the leading edge to subtle and not so subtle airfoils with and without pre-planned and/or after the fact incidence adjustments. Paul, in particular, has done it both ways (leading edge entry) and advocated for each at different times (although,not to my memory, has he utilized the positive incidence approach...could be totally wrong in that regard).
I'd be interested in a discussion that included the once favored versus the newly favored and why the change in preference from an aerodynamic/performance perspective inasmuch as significant victories would point to either approach as--at least--a valid choice.
David's experiment, as I recall, showed that the flat, round-LE Impact stab worked OK. The difference seems to be whether it's being pulled by a .40VF or a Plettenberg.
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Speaking of not being up-to-date, this guy hasn't heard the Michaelson-Morley news.
1887? Lots of time to get up to date....
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I am not an engineer so I have to wait for the punch line (Yes It's better/No it Sucks) when you guys go off into the weeds. I understand most of it but I still have not been able to get a line on whether the LE simply performs better if it is thinner (sharp being the ultimate thin) OR is it the point that makes it better?
The reason I am asking this is a post from Paul earlier where he recommended I leave it at 1/16 dia rounded since I stated that it flew well. I may have been misleading when I said that. I "fly" my electric's as soon as there is enough dope on the wings to keep the covering on just to make sure I don't have a serious issue that can be fixed before the finish is applied. My criteria for these "flights" are simple: a strenuous pull test, doesn't hit the prop taking off,wings level enough, fuselage level, controls don't bind, will do a 10' corner both ways, an overhead 8 and stays on the ground when it lands. A one and a two minute flight and it is back to the shop.
When I said it flew fine it was using those very simple criteria. When it is finished, flaps and elevator sealed then it will get "trimmed". It would be super simple at this point (I am not into gray yet) to stick a strip of basswood on the LE and make it pointed IF pointed is the goal, not just thinner.
Ken
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Paul, could pleas you post a picture of your stabs entire cross section? Thanks.
Bruce
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2nd page .... reply #77
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It is interesting that Paul, Igor, David F., Brett, ect, have taken different paths to arrive at the same basic conclusion that moderately sharp to very sharp leading edges on the stab are advantageous for them. All have different preferences for the overall stab design, ie, flat(pw),curved(df/bb), airfoiled(Igor).
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I've been experimenting with adding some fixtures to the leading edge of a flat stab. I used 1 inch alluminum blind slats taped together along one edge and taped to the normal flat stab. It gives a pointy leading edge. See photo.
The results are not awful. It flew, was able to loop, didn't fall out of the sky, made impressive corners and generally felt like a quicker response. I still need to verify that I didn't fool myself with just increased tail area and a bit more tail weight. I haven't completed that yet, but initially, I'm impressed.
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2nd page .... reply #77
Thanks Frank, I saw that but it is only the leading edge. I was curious as to the cross section if the entire stab and elevator combination.
Bruce
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I must mention that all of this theory versus reality talk reminds me of when the eight year old boy asked his Dad ;'What's the difference between theory and reality?".
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Thanks Frank, I saw that but it is only the leading edge. I was curious as to the cross section if the entire stab and elevator combination.
Bruce
It is flat to the rounded hinge line.
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I must mention that all of this theory versus reality talk reminds me of when the eight year old boy asked his Dad ;'What's the difference between theory and reality?".
In theory, there is none. In reality, things are a bit different.
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I have been away for a few days and a complete 3rd page shows up. Great stuff! Pretty far above my pay grade but I get through it as best I can. The triangle and circle graph really show the difference! Thank you Frank for showing us this stuff and thank you to others for explaining things as you always do.
I liked Ted's post alluding to the fact there are several ways to skin the stunt cat. ;D
Now let's dive into the next section of this discussion....... How does one build this sharp leading edge and keep it perfectly straight all the way across. I can assume its not just a sanding block.... ahem,,,Paul,,,you got any pics of how you construct the LE on the stab and the stab in general? Do you do anything to it to make resistant to dings or dents. Seems even the slightest bump and you have a dented LE that could negatively effect flight performance.
I know once I went to a molded LE on the stab, even though it was blunt, my flying consistency skyrocketed. Any building details would be greatly appreciated. Or if they are already out there just point me in that direction.
Thank you
Doug
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... Do you do anything to it to make resistant to dings or dents. ...
https://www.mcmaster.com/tool-steel
Weight may be an issue, though.
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Doug,
The LE is a laminate construction.
The stab has a 3/8" "core" if 1/16" ribs, with 1/16" sheeting top and bottom. The TE is a laminate construction with a 1/32" core for hinge slots, a layer on each side of the core to make tge stack up 3/8", and then the1/16"to get it to 1/2".
The LE is similar. The core is a single layer of unidirectional tape, balsa on each side to get it to the 3/8" and then the 1/16" sheeting. Once the entire stab is assembled, a planer is used to start shaping the LE. Then a sanding block to finish it off. The carbon strip acts as a guide for the shaping and sanding. It is sanded sharp at the front. Once happy, the LE is soaked with thin CA, and then sanded smooth. Two applications of CA are applied.
At this point it is ready for paint. With this sharp LE, one has to be very careful sanding the paint so that the paint is not removed.
Sanding once the clear is on is nerve racking. Real easy to cut through.
Hope this helps.
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I have been away for a few days and a complete 3rd page shows up. Great stuff! Pretty far above my pay grade but I get through it as best I can. The triangle and circle graph really show the difference! Thank you Frank for showing us this stuff and thank you to others for explaining things as you always do.
I liked Ted's post alluding to the fact there are several ways to skin the stunt cat. ;D
Now let's dive into the next section of this discussion....... How does one build this sharp leading edge and keep it perfectly straight all the way across. I can assume its not just a sanding block.... ahem,,,Paul,,,you got any pics of how you construct the LE on the stab and the stab in general? Do you do anything to it to make resistant to dings or dents. Seems even the slightest bump and you have a dented LE that could negatively effect flight performance.
I know once I went to a molded LE on the stab, even though it was blunt, my flying consistency skyrocketed. Any building details would be greatly appreciated. Or if they are already out there just point me in that direction.
Thank you
Doug
Doug:
There may very well be a better way but I had a few thin (1/32" x 1/8") carbon fiber strips we used as spar stiffeners for zoom launches on sailplanes so I decided to see what they would do for keeping flap T/E's from warping if added to them vertically. It worked. I am leaning towards putting "sharpies" on my new ship and I am going to use one of these strips sideways sandwiched between two pieces of 1/16 balsa to form the new LE. That thin black centerpiece will make a perfect sanding line so it will be perfectly straight and ding resistant. I have had enough of the "why" to be convinced - on to the "How"! #^
Well it seems that Paul has beat me to it. Guess maybe he flew sailplanes too!
Ken
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My choise would be a carbon microtube glued in a groove in center of stab. Or even a solid rod, they are nott so heavy. What kind of r are you looking for? L
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My choise would be a carbon microtube glued in a groove in center of stab. Or even a solid rod, they are nott so heavy. What kind of r are you looking for? L
I think you could shave with it before paint!
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My choise would be a carbon microtube glued in a groove in center of stab. Or even a solid rod, they are nott so heavy. What kind of r are you looking for? L
I may be wrong here and since I am probably going to try this I hope it gets clarified. I don't think that there is an "r". I think that the point is that there is a point. ??? You are right about the CF tubing and I would totally use it for any place you need a smooth radius.
Ken
I think it just did - thanks Paul.
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I think you could shave with it before paint!
Oh, *That* sharp! What do you use Paul?
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This is making sense. I have used a small carbon blade spar on its side laminated top and bottom in the TE of my elevators for several models now. Helps keep them straight and they are much stronger in the end. This is very similar to how Paul is doing the LE. I will have to try this out. Sounds like I will have to build a new model around a new stab! ;D
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This making sense. I have used a small carbon blade spar on its side laminated top and bottom in the TE of my elevators for several models now. Helps keep the straight and they are mush stronger in the end. This is very similar to how Paul is doing the LE. I will have to try this out. Sounds like I will have to build a new model around a new stab! ;D
You do realize that this will require actually Flying the plane... LL~ LL~ LL~
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You do realize that this will require actually Flying the plane... LL~ LL~ LL~
Yes I know this, don't worry many flights for me are coming.
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Oh, *That* sharp! What do you use Paul?
It is an inlaid single layer carbon strip.
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Yes I know this, don't worry many flights for me are coming.
I thought you were one of these guys who don't need to fly much!!!
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I thought you were one of these guys who don't need to fly much!!!
Sometimes it is that way and other times not so much.... As of late, last several years, if I wanted to get a lot of flights it wasn't happening with life getting in the way. Last year I burned some fuel towards the end of the season and it felt really good and my last contest flight was one of my better ones in a really long time. I do really well with 4 sessions a week at 4-6 flights per. Some call that a lot and others not so much. I haven't been able to get that much in the past 6-8 years. Our weather has sucked for a long time now.... Before 2012 nats I think I had 28 flights on the new plane, then another 30 or so at Muncie. In 2014 I had 5 patterns on the new plane before Muncie then another 30 or so there. But thanks t0 100s and 100s and 100s of flights in years past when things were more agreeable the pattern was ingrained pretty good...
But....I ALWAYS want to go flying and still check the trees for wind direction almost every day! hahahaha
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Sometimes it is that way and other times not so much.... As of late, last several years, if I wanted to get a lot of flights it wasn't happening with life getting in the way. Last year I burned some fuel towards the end of the season and it felt really good and my last contest flight was one of my better ones in a really long time. I do really well with 4 sessions a week at 4-6 flights per. Some call that a lot and others not so much. I haven't been able to get that much in the past 6-8 years. Our weather has sucked for a long time now.... Before 2012 nats I think I had 28 flights on the new plane, then another 30 or so at Muncie. In 2014 I had 5 patterns on the new plane before Muncie then another 30 or so there. But thanks t0 100s and 100s and 100s of flights in years past when things were more agreeable the pattern was ingrained pretty good...
But....I ALWAYS want to go flying and still check the trees for wind direction almost every day! hahahaha
I tried to tell Paul that planes fly better brand new. No need to wear them out before you get to Muncie.
Derek
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I tried to tell Paul that planes fly better brand new. No need to wear them out before you get to Muncie.
Derek
Totally agree!! :) :)
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I see I missed funny thread, so I will try to collect my thoughts to one message:
1/ reason - I think I saw reasons mentioned, but shortly there are 2:
- one is problem of blunt LE in level flight which can make unstable bubble just after the end of radius on both lower and higher surface. That bubble can appear or disappear at very small elevator deflections used for controlling in level flight. That CAN lead to hunting or make other ill effects. Howard posted link to old thread where we spoke about it. Solution with turbulator proofs that idea (described by DF in SN article)
- the other is separation at LE on high pressure side in corners, when air steam comes at -16 deg AoA to LE - that separation will allow stronger effect of tail. Difference for pilot is feeling that model is nose more heavy with blunt LE in corners. Translated to pilot feeling - sharp LE will make more nose heavy feeling in level and easier corners, while blunt will make sensitive model in level and less sensitive model in corners. I prefer that stable in level and easy corners.
2/ That funny picture posted by Frank only shows known fact, that thick airfield wings with round LE do not work well in low RE number (sub critical). Sharp LE can move critical RE number down, easily to or under 20 000. So no wonder that reversed airfoil will work better. That is reason why we fly FLAT wings on indoor models. Airfoiled wing simply do not work in such conditions. Our tails are over that value, but tails on smaller models (.15 size) need FLAT stabs. Tested and gives repetitive results. That is why I used flat stabs on smaller models for beginners:
http://www.netax.sk/hexoft/stunt/thetiny.htm
http://www.netax.sk/hexoft/stunt/the_middle.htm
3/ Stab construction. Actually I use composite stabs, but when I build first Max Bee I used carbon rod R1 mm . I have fixtures which allow precise positioning of that rod on flat LE. Then I fill the gap and sand it to airfoil surface. It makes strong LE and even after years it does not show any problems. Pictures show it.
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Thanks for your response. I’m going to guess that the translator meant “interesting” rather than “funny”.
I like your jigs for the placement of the carbon rod on the stab leading edge. That’s a very nice idea for accurate building.
To summarize your comments …..
Sharp leading edge = solid level flight …. Easy corners
Blunt leading edge = sensitive level flight …. Softer corners
I must say that with just a couple of days flying with the sharp leading edge, that was my first impression.
“.... the other is separation at LE on high pressure side in corners, when air steam comes at -16 deg AoA to LE - that separation will allow stronger effect of tail. Difference for pilot is feeling that model is nose more heavy with blunt LE in corners”
I don’t understand the “separation … on high pressure side …” Could you explain?
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At least Doug does not build hanger queens. I too would like to get out more but old age and weather interrupts once in a while. D>K
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Stab construction. Actually I use composite stabs, but when I build first Max Bee I used carbon rod R1 mm . I have fixtures which allow precise positioning of that rod on flat LE. Then I fill the gap and sand it to airfoil surface. It makes strong LE and even after years it does not show any problems. Pictures show it.
The Jig is a great idea. I was going to use a thin tube until the thread explanations got more into detail and I was convinced to try truly sharp. The surgery is complete and the patient is hanging in the recovery room awaiting some sanding. Unless I am missing something your LE is very small but round, Paul's is sharp.
Is there a difference?
Ken
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“.... the other is separation at LE on high pressure side in corners, when air steam comes at -16 deg AoA to LE - that separation will allow stronger effect of tail. Difference for pilot is feeling that model is nose more heavy with blunt LE in corners”
I don’t understand the “separation … on high pressure side …” Could you explain?
I mean side where is elevator deflected to (for example UP). That side (upper) has higher pressure than the opposite and thus it is pushed DOWN. The LE of stab is in that regime attaked at angle say 15 degrees (Howard posted picture showing that case earlier in this thread). So that makes small spot with vely low pessure just behind LE on upper - means high pressure side which can cause separation. The pressure then goes rapidly up (because of deflected elevator) and flow can attach back, so that makes very good conditions for separation bubble which can cancel that low pressure which actst agains the eleator effect and thus can allow better net effect of the tail.
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OK..... I understand now ....... you were talking about the "induced" angle-of-attack due to the a/c pitch rate.