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General control line discussion => Open Forum => Topic started by: RC Storick on January 09, 2014, 12:22:04 PM
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This will be my only post because I don't want to get into a argument.
It was explained to me that the Netzeband wall was the plane not having enough weight to fly smoothly through maneuvers due to loss of line tension.
So my question is (and my fix) Is it possible to hurtle this impossible wall with motor offset and line rake , rudder offset and tip weight. Of course making sure everything weight wise is in alignment. The next thing involved is super free (low friction) controls.
Engineer types answer here .
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So my question is (and my fix) Is it possible to hurtle this impossible wall with motor offset and line rake , rudder offset and tip weight.
Yes, but your airplane may not be in the highest scoring trim. The Netzeband wall, as I understand it, is a condition where having all the tension on one control line is still inadequate to overcome control surface hinge moment. The wall is squishy, though. Before you get to the point of running out of control oomph, the airplane will merely be difficult to fly accurately. I did some calculation and was working on a short monograph on the subject, but got sidetracked, where I'll remain until late summer. There are other ways to get around the problem that don't make the airplane fly poorly.
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My guess is that if you get enough different actions working against each other, like the offset, rudder, etc., you will detract from the overall performance of the airplane. As far as the "wall" theory, I think that it is true, and related to the wing loading of each individual airfoil. Each airfoil, I believe, has a wing loading that it will perform and penetrate best at. I saw this first hand in my R/C sailplane and free flight days. It was very visible, to see a model that was not quite on step and penetrating well, to see it's L/D increase and rate of sink decrease in dead air by just adding a little ballast. Trim changes to the elevator had no effect on the glide ratio, and sometimes made it worse. But when you hit the magic weight, you could really see a ship respond, and the only way I could come to that point was trial and error. I have to believe that a stunt model is no different. Look at different airplanes and how well they glide at engine cut off, some do much better than others. The ones that do better, I feel, have to do better in the maneuvers due to the better penetration. There are of course several factors involved, but I think that an optimum weight for a particular size airplane is a real entity and part of the equation. I have said before that a stunt model is a true sum of all it's parts, no one single feature can make or break a design, they will fly better if all the aspects of a design work with each other, and weight is a part of that. And I stress, a proper weight, not heavy. I am not saying that a heavy airplane has any advantage. I've built too many that have proven the opposite! But a proper weight for a specific wing area and wing loading will let you take most advantage of your power plant and control set up.
Type at you later and off work,
Dan McEntee
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The Netzeband wall, as I understand it, is a condition where having all the tension on one control line is still inadequate to overcome control surface hinge moment.
Aha! That's it. Robert kind of stated it differently than I remembered, because where I saw it come up was in the context of making your control surfaces too big, thereby increasing the control force necessary.
The easy way to make things way better would be to do the fly-by-wire thing: just read the position of the bellcrank with something 'lectronic, and drive servos.
Thinking about it that way, there's only so much advantage you'll get from reducing friction in the controls: at some point the friction forces will be dwarfed by the aerodynamic forces on the control surfaces, and less friction will not lead to discernibly better control.
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The easy way to make things way better would be to do the fly-by-wire thing: just read the position of the bellcrank with something 'lectronic, and drive servos.
Or you could fly RC, or just watch airplanes on television. It kinda removes the essence of control line.
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As far as the "wall" theory, I think that it is true, and related to the wing loading of each individual airfoil. Each airfoil, I believe, has a wing loading that it will perform and penetrate best at. I saw this first hand in my R/C sailplane and free flight days. It was very visible, to see a model that was not quite on step and penetrating well, to see it's L/D increase and rate of sink decrease in dead air by just adding a little ballast. Trim changes to the elevator had no effect on the glide ratio, and sometimes made it worse. But when you hit the magic weight, you could really see a ship respond, and the only way I could come to that point was trial and error. I have to believe that a stunt model is no different. Look at different airplanes and how well they glide at engine cut off, some do much better than others. The ones that do better, I feel, have to do better in the maneuvers due to the better penetration. There are of course several factors involved, but I think that an optimum weight for a particular size airplane is a real entity and part of the equation. I have said before that a stunt model is a true sum of all it's parts, no one single feature can make or break a design, they will fly better if all the aspects of a design work with each other, and weight is a part of that. And I stress, a proper weight, not heavy. I am not saying that a heavy airplane has any advantage. I've built too many that have proven the opposite! But a proper weight for a specific wing area and wing loading will let you take most advantage of your power plant and control set up.
Adding weight will increase both wing loading and line tension, but I don't think it's a useful way to look at the control problem, nor will it give Robert what he wants. I think Robert wants to have a really light airplane without getting near the Wall. That is possible.
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Or you could fly RC, or just watch airplanes on television. It kinda removes the essence of control line.
I know, but it's allowed and it's being done. From a political perspective I wish there was a rule against it, but from a competitive perspective -- you gotta do what wins.
(And please, if someone wants to turn this into a rules debate go spawn a thread over in the rules section -- or pick up the one that's already there, and lying dormant).
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Yes, but your airplane may not be in the highest scoring trim.
BINGO!!!!!
I find that I fly better when the plane pulls harder. More resistance gives me more "feel" which is good in my book.
I have said many times that "I" believe that there is a perfect weight for any given design, not too light and not too heavy. Something like a 63 oz 690 sq in Cutlass XL would do the trick.
Derek
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Eric's Rules for stunt (right or wrong, I still try to go by them)
1) No matter how good or neutral your plane is, trim is still speed sensitive to some extent - So always try to fly the exact same speed so your trim remains optimal.
2) We tend to need to fly faster in the wind in order to penetrate, so get used to flying whatever that "faster" speed is, even on calm days. See rule #1.
3) Trim for as clean of a corner as possible and the most even line tension, then see what it takes to make it comfortable to fly. You have to come to the plane, not make the plane come to you, at least I think we have to in order to score well.
4) Shorter lines will give more line tension on a light plane, given an equal ground speed. Find the planes sweet spot for line length and learn to like it. Oh, and see Rule #1.
5) Line tension has to be even & symmetrical, or you won't have good shapes. Flying slow looks kewel, but when you crawl up hill, pause at the top, and zoom down, your shapes are shot. Turn up the wick until your speed & line tension is constant and you will have pretty rounds. (well, you should). Ya, and you guessed it, See rule #1.
6) Trim that induces yaw (rudder, engine offset, tipweight) is best kept to minimum, because it will make the trim even more speed sensitive and...ya...see rule #1
The Netzeband wall is something the above steps should help avoid, but can still happen. I don't build light enough, or with flimsy enough controls or big enough surfaces to worry about it, or at least I haven't yet. I suppose if it got windy enough, the opposite of too light, perhaps too heavy with too much load on the controls could negate my ability to move them or flex would flatten them out, so it would seem under those conditions a lighter plane with a more controlled run (no runaway 100mph loops) would have the advantage.
Everything is a trade off.
You make your best guess, and live with it until the next build. At some point you have to choose a dancing partner and just learn to dance with them (unless she's got two left feet). Sooner or later you find that changing partners all the time just leaves you with no rhythm, and you end up going home alone...
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I tried to correct that once on a Ringmaster by using gurney flaps....it flew a little better in the upright and inverted, but trying to do an outside loop was murder. I finally added a couple flat washers to the front mm bolts, and it was bearable, but I gave it[plane] away. H^^
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Wsheeewwww...I didn't get to finish my earlier post, had un-expected company, and had to cut it off. Anyway, when all the guys started showing up with flapped planes, it made my ringmaster look kinda un-interesting! Thus the attempt to modify the S-1. That turned out to be a bust! So I bought a P-51 Mustang that was an R/C model and tried to convert it to C/L and that was a disaster[came inside and turned into a lawn dart]. And I destroyed the next attempt which was an Imperial that and hung on the wall too long. By the time i got my first Nobler, all our guys could fly the whole pattern. About then's when I discovered the phenom of "the loss of control on wingovers and other moves that percipitated a loss of line tension. That's when I decided I'd never be competitive, and relegated myself back to the planes without flaps. Except for the Super Clown I built after I came here to SH. That's the first time I heard anyone say that flap to elev. ratios were important. I still have that one, I just haven't had the guts to go past lazy 8s on it. I said all that to say....there's something simple that evades me....that could solve it. I just haven't discovered it yet. H^^
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Eric's Rules for stunt (right or wrong, I still try to go by them)
5) Line tension has to be even & symmetrical, or you won't have good shapes. plane with a more controlled run (no runaway 100mph loops) would have the advantage.
Hey Eric, great post.
"Line tension has to be even" OK got that, but "Line tension symmetrical" ??? Can you explain ?
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The Netzeband wall is something the above steps should help avoid...
Just like those copper bracelets help avoid arthritis.
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Well, having uniform line tension helps get around the situation of having control deflection be a strong function of line tension (which it is near the Wall), but I'd also try to make control deflection less influenced by line tension.
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This will be my only post because I don't want to get into a argument.
It was explained to me that the Netzeband wall was the plane not having enough weight to fly smoothly through maneuvers due to loss of line tension.
So my question is (and my fix) Is it possible to hurtle this impossible wall with motor offset and line rake , rudder offset and tip weight. Of course making sure everything weight wise is in alignment. The next thing involved is super free (low friction) controls.
Engineer types answer here .
The "Netzeband Wall" is as Howard describes - when the line tension is insufficient to move the control enough, or as much as you want. Its about running out of control torque. It takes substantial torque to deflect the controls into the airstream, and the control system has to supply it. The torque is applied by differential line tension.
If you move the controls, the line you are pulling towards you has more tension than the line you are allowing to be pulled away from you. The maximum amount of difference you can get is the full line tension all on one line (one line taking all the load, or "hung on one line"). That amount of force times whatever bellcrank lever arm you might have, is the maximum amount of torque you can apply to the bellcrank. This torque, divided by the lever arm from the bellcrank pivot to the pushrod is the maximum amount of force you can apply to the pushrod. This force, times the lever arm of the control horn, is the maximum amount of torque that can be applied to the flap horn.
In any case, the controls will deflect until the differential line tension x all the mechanical advantage equals the torque required to deflect the controls. Since the line tension is limited, the controls can only move so far. If that is less than you need them to move, you hit the Netzeband wall.
With a conventional straight bellcrank and a normal control setup, the torque available to move the controls goes down as you deflect it. A neutral, on a 4" bellcrank, with 10 lbs of tension, you can get 20 inch-lbs of torque at the bellcrank (2" x 10 lbs). With a 3" bellcrank it's 15 in-lb - that's why you want a big bellcrank.
As you deflect it the lever arm at the bellcrank is reduced. If a 4" bellcrank is deflected 45 degrees, the lever arm goes from 2" at neutral, to 1.41", so you now only have 14 in-lb available. You can see it by figuring what might happen if you let the bellcrank go to 90 degrees - in that case, the leadout and the pivot are in line, so you get *no* torque at all.
While you are moving the bellcrank and reducing the torque available, the flap and elevator deflect, too, increasing the torque required. At neutral it takes nearly nothing to move the controls. As you go further, the required torque goes up very quickly. This torque is called the "hinge moment". So as you move the controls as a system the torque available goes down as the controls require more torque, and it will stop and deflect no further at some point.
Note that you might be tempted to slow down the control ratio from the bellcrank to the flap to make better use of the available bellcrank torque. That works to a limited extent (as Paul Walker has done it) but at some point, you have to move the bellcrank through such a large angle that you start losing torque because of the bellcrank angle.
Note also that making the handle larger doesn't make effect the available bellcrank torque at all. It's not like using a "cheater bar" on a wrench as some have suggested. You still only have so much line tension, once you hang it on one line, you are done, and with the other line hanging slack, it doesn't matter how far that side of the handle moves.
You can only do one of two things to improve this - either get more bellcrank torque, or reduce the control torque required for a given deflection. To get more bellcrank torque, you can make the bellcrank larger, making the lever arm larger, or get more line tension. The bellcrank can only be so big and still fit in the airplane, to you are necessarily limited there. Getting more line tension is certainly possible, and your first thought might be to speed the airplane up so you get more tension. Unfortunately, speed it up also increases the hinge moment, since it is a function of aerodynamics, it goes up quickly as the speed increase. As it turns out, many times speeding it up doesn't help at all, its faster but you haven't been able to increase the control deflection.
If you try to fly it slower, it reduces the hinge moment, but also reduces the line tension and thus the bellcrank torque.
You can try to get more line tension by other trim adjustments, without increasing the speed. This usually involves putting in extra tip weight, extra rudder offset, etc to "manufacture" line tension. The Nobler was set up like this, with an idea to fly it very slowly (compared to others at the time) by putting in large amounts of rudder offset and a far aft leadouts to yaw the airplane outboard. You can still do it that way, but it definitely compromises the trim in other ways.
You can reduce the hinge moment by changing the aerodynamics of the airplane. For example, reduce the flap or elevator chord, and make the span larger to compensate, or use aerodynamic counterbalances, or a servo tab like Howard did, or a number of things like that. That doesn't help the available torque, but it does reduce the torque required.
The other half of Paul Walkers "Impact" approach is to reduce the control deflection required by running the CG aft. If the CG is further aft, it takes less deflection, so less hinge moment for a given amount of turn, so you have more margin over running out of torque, or get more corner for a given hinge momement.
Finally, you can just make the airplane heavier. The weight is more-or-less a linear relationship to the line tension. Add 20% to the mass, and you get 20% more line tension, all other things being equal. That way, you can definitely deflect the controls further at a given speed. Or, you can get the same deflection at a lower speed.
Ted's Tucker Special experiment was the latter. The airplane was already built, so most of the other options didn't exist. The aerodynamics were set by the design, and since it was a classic airplane, couldn't be changed. The controls were installed and couldn't be adjusted in any useful way. It was pretty light to start with, with light line tension, and otherwise trimmed conventionally without compromises to increase the line tension (and create other problems). Plenty of power. It didn't turn very well, and was very floaty. The first thing he tried was to fly it faster. Eventually it was down to maybe 4.6 seconds a lap, but it wasn't any better because although it had more line tension, it also required more hinge moment, so it was no better off. We then added weight in 2 ounce increments, right on the CG. The first 2 ounces greatly improved the turn, and permitted significantly slower lap times. We then went to 4, then 6, then 8 ounces. Each time, the turn got better, the airplane could fly more slowly, and generally looked far more solid overall.
It was not a subtle effect, or a figment of imagination, it was a very large and blatantly obvious improvement.
In traditional lore, the turn should have degenerated because the wing loading went up dramatically, and the vertical performance would have suffered because dragging 46 ounces to the top of the circle is a lot harder than 38. In fact, the turn got *much* better, because the lift was not a limiting factor in the first place, and the controls could be deflected much further than before, he could get more lift (more flap deflection) and a higher pitch rate (more elevator deflection). By the same token, the vertical performance was hardly affected at all since it had much more power than the original.
Point being, of course, that if the weight is not the limiting factor in the first place, adding weight doesn't hurt, and may help. Of course, eventually, adding more and more weight, it would become a limiting factor and starts hurting. Even on a vintage design like the Tucker, with modern power, this is much higher than most people seem to think.
My other example was actually a real one - two 585 square inch airplanes, both with PA65s, one at 54 ounces, the other at 66. The 66 ounce airplane flies *much* better because it can be trimmed conventionally and still have enough margin over the Netzeband wall, and the light one has severe issues with trim and turns poorly because it runs out of control effort (among other issues).
So, the Netzeband wall is where the control torque required equals the control torque available. There are number of ways to improve the situation, but simply adding weight (either otherwise useless dead weight like this, or in beneficial added structure like the Infinity) can improve it, up to the point weight does start hurting in other ways.
Brett
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Hey Eric, great post.
"Line tension has to be even" OK got that, but "Line tension symmetrical" ??? Can you explain ?
Sure... Even meaning same tension at all altitudes in the hemisphere (said no or minimal speed loss up top) symmetrical meaning same tension and input in consecutive pos and neg g maneuvers like inside loops and squares as the outsides, or the right loop of an eight as the left loop or the top loop of a vert eight as the bottom.
Thanks Howard, but copper bracelets are so yesterday. You need to get some rare earth magnets and healing crystals, that's the ticket now! Oh, and don't forget to pee on your PH strip while you are at it. ;)
Edit: I guess I should actually explain why I care about consistent line tension and symmetry in trim in relation to the original question about hitting the Netzeband wall. Sorry, I sometimes assume people can just read my mind like my wife does. Heh. Well, one thing I have noted, with the trend to droop elevators for instance, opposed to say stab incidence. With drooping the elevators, your flaps are deploying early in one direction and load up the controls more in that direction. So, I prefer minimal or no asymmetry for a more consistant feel at the handle.
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Brett
as a point of interest,,for me anyway ( by the way, thanks for taking the time to write this all out so clearly,)
as you reach that point where you are carrying all the tension on one line,, would not the arc of the line ( I canno recall the correct name,, parabola is not correct) would not by definition the arc of the line be reduced as well? I would think that would lend some springiness" to the control response at higher input levels.
My understanding, and basis for this is that the arc in the line is a function of tension on the lines, or line, and the airspeed creating drag as well as gravity playing on the weight of the lines..
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Thanks, Brett. Good explanation. One might add that "centrifugal" force goes up as the square of inertial speed and hinge moment goes up as the square of airspeed.
I don't know about yours, but mechanical advantage in my dog goes up (if I read the graph correctly) with increased control deflection. You gotta do a 3D analysis, looking at d flap / d leadout.
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as you reach that point where you are carrying all the tension on one line,, would not the arc of the line ( I canno recall the correct name,, parabola is not correct) would not by definition the arc of the line be reduced as well? I would think that would lend some springiness" to the control response at higher input levels.
My understanding, and basis for this is that the arc in the line is a function of tension on the lines, or line, and the airspeed creating drag as well as gravity playing on the weight of the lines.
I'm not Brett, but yes, you got it. That springiness (and the slack line goes way bowed) is nonlinear and gets worse as you approach the Wall. Add to that the line elasticity and it's hard to fly accurate stunt.
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Thanks,, I suspected it was part of the reason,,
I have not really had to worry about it,, none of my airplanes have been light enough to be concerned about the wall
YET,,
but the new one may be
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Brett
as a point of interest,,for me anyway ( by the way, thanks for taking the time to write this all out so clearly,)
as you reach that point where you are carrying all the tension on one line,, would not the arc of the line ( I canno recall the correct name,, parabola is not correct) would not by definition the arc of the line be reduced as well? I would think that would lend some springiness" to the control response at higher input levels.
My understanding, and basis for this is that the arc in the line is a function of tension on the lines, or line, and the airspeed creating drag as well as gravity playing on the weight of the lines..
Of course, and it's not just when hanging on one line, it's any time the controls are deflected - the line you are pulling on straightens a bit, and the line being pulled gets more curved. That's the idea behind the "front up leadout" - it takes advantage of this fact to compensate for the (purported) nose-out yaw on insides, and nose-in yaw on outsides.
I think you can even compute the sag angle of the tighter line with the nomographs and LINEII or LINEIII, by doubling the weight of the airplane and leaving everything else alone. It won't be exactly right but it will be in the ballpark, at least as close as the rest of the calculations.
Brett
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snip
Ted's Tucker Special experiment was the latter. The airplane was already built, so most of the other options didn't exist. The aerodynamics were set by the design, and since it was a classic airplane, couldn't be changed. The controls were installed and couldn't be adjusted in any useful way. It was pretty light to start with, with light line tension, and otherwise trimmed conventionally without compromises to increase the line tension (and create other problems). Plenty of power. It didn't turn very well, and was very floaty. The first thing he tried was to fly it faster. Eventually it was down to maybe 4.6 seconds a lap, but it wasn't any better because although it had more line tension, it also required more hinge moment, so it was no better off. We then added weight in 2 ounce increments, right on the CG. The first 2 ounces greatly improved the turn, and permitted significantly slower lap times. We then went to 4, then 6, then 8 ounces. Each time, the turn got better, the airplane could fly more slowly, and generally looked far more solid overall.
It was not a subtle effect, or a figment of imagination, it was a very large and blatantly obvious improvement.
In traditional lore, the turn should have degenerated because the wing loading went up dramatically, and the vertical performance would have suffered because dragging 46 ounces to the top of the circle is a lot harder than 38. In fact, the turn got *much* better, because the lift was not a limiting factor in the first place, and the controls could be deflected much further than before, he could get more lift (more flap deflection) and a higher pitch rate (more elevator deflection). By the same token, the vertical performance was hardly affected at all since it had much more power than the original.
Point being, of course, that if the weight is not the limiting factor in the first place, adding weight doesn't hurt, and may help. Of course, eventually, adding more and more weight, it would become a limiting factor and starts hurting. Even on a vintage design like the Tucker, with modern power, this is much higher than most people seem to think.
My other example was actually a real one - two 585 square inch airplanes, both with PA65s, one at 54 ounces, the other at 66. The 66 ounce airplane flies *much* better because it can be trimmed conventionally and still have enough margin over the Netzeband wall, and the light one has severe issues with trim and turns poorly because it runs out of control effort (among other issues).
So, the Netzeband wall is where the control torque required equals the control torque available. There are number of ways to improve the situation, but simply adding weight (either otherwise useless dead weight like this, or in beneficial added structure like the Infinity) can improve it, up to the point weight does start hurting in other ways.
Brett
Thanks, Brett. You saved me an hour of typing!
Only additional comment I'd make (and you alluded to it) has to do with the size of our movable surfaces and whether bigger is always better.
As I look at the Tucker on the wall beside me one of the big things that jumps out is the large chord of the full span flaps. In the "early stunt designers' minds" we needed big flaps to get the necessary lift to do our tricks. To avoid going off on a tangent from the subject matter, let me just say that they were wrong for probably 99% of the flapped classic era designs and nearly as many of the modern ones.
What we need in a stunt ship is enough combination of wing area or wing and integrated flap area (at an appropriate aspect ratio) to provide the lift "necessary to do our tricks" while producing minimal induced drag plus a small amount to prevent ugly surprises. More than enough flap is seldom an advantage.
With respect to the "wall" large chord movable control surfaces all contribute to increased hinge moments and should be minimized unless there are valid reasons in the designers' minds to do other wise. I believe this is almost never the case with flaps and will make only a modest argument in favor of wide chord elevators (believing instead that a low aspect ratio tail should utilize a much smaller than 50/50 split between the stab and elevator.
Minimizing the chord (increasing the aspect ratio) of movable surfaces directly reduces hinge moments and reduces the need for "methods" to increase line tension because it will be unnecessary to do so to obtain the necessary deflection with the available line tension. Rather than using the "classic" flap layout (three inch chord at the root because that's the width of the sheet of balsa) tapering to one inch at the tip (because it looks nice) I've postulated that a three percent more intelligent approach is to utilize a nearly full span, much smaller chord flap that is the same percentage of the wing chord at every station. My designs have utilized chord ratios between 15% (the Imitation and Excitation types prior to the heavier power trains) up to 20% on the Trivial Pursuit in recognition of the higher wing loading...I didn't want a huge wing area and wanted to utilize the larger % high lift device to produce the necessary lift on the modest 650 or so square inches.
Sorry, getting off the track. Bottom line, avoid large chord flaps because: 1. They move you closer to the Netzeband wall and 2. You don't need them. 3. If you're going the low aspect ratio tail route reduce hinge moment by using at least a 55/45 split and don't worry at all going to 60/40. The Infinite is that way and does pretty darn well. Remember, it isn't the "flipper" that's producing the pitch change, it's the lift produced by the entire stab/elevator.
Ted
Oh, a quick edited p.s.
I didn't want to cut it up but an additional experiment could have been performed on the Tucker by slicing off a big chunk of the flaps to reduce the hinge moment. I've zero doubt that doing so would have prevented the wing from developing the lift necessary to do our tricks. There have been plenty of unflapped stunt designs of the same area as the Tucker that would perform very presentable patterns.
Oh, and second p.s. Howard can probably tell us the answer to this. How much would the negative pitching moment developed by bigger than necessary flaps/chord retard the rate of pitch change...simultaneously requiring more deflection for the desired turn, which gets you closer to the wall, etc. etc. etc.
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As you deflect it the lever arm at the bellcrank is reduced. If a 4" bellcrank is deflected 45 degrees, the lever arm goes from 2" at neutral, to 1.41", so you now only have 14 in-lb available. You can see it by figuring what might happen if you let the bellcrank go to 90 degrees - in that case, the leadout and the pivot are in line, so you get *no* torque at all.
Brett
Wonderfully worded as usual, thanks Brett.
But would the above make a case for a circular bell-crank that does not suffer from an effectively diminishing arm length during rotation? (Perhaps the complications of having a rolling contact for the lead outs would negate any perceived gains.)
Cheers.
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My other example was actually a real one - two 585 square inch airplanes, both with PA65s, one at 54 ounces, the other at 66. The 66 ounce airplane flies *much* better because it can be trimmed conventionally and still have enough margin over the Netzeband wall, and the light one has severe issues with trim and turns poorly because it runs out of control effort (among other issues).
So do you think you could thunk a chunk of lead into the light one and make it fly better? I would be intensely interested in the results of that experiment.
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So do you think you could thunk a chunk of lead into the light one and make it fly better? I would be intensely interested in the results of that experiment.
Yes, that, and getting rid of the 13.5" 3-blade.
Brett
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Sorry, getting off the track. Bottom line, avoid large chord flaps because: 1. They move you closer to the Netzeband wall and 2. You don't need them. 3. If you're going the low aspect ratio tail route reduce hinge moment by using at least a 55/45 split and don't worry at all going to 60/40. The Infinite is that way and does pretty darn well. Remember, it isn't the "flipper" that's producing the pitch change, it's the lift produced by the entire stab/elevator.
But do I have the courage to try?
I'm recalling a recent flying session where the wind and other circumstances placed me at the perfect spot to watch my flying buddy do his stuff with a Score ARF. Even on the square corners I never saw the elevator deflect more than 15 degrees or so; on the round maneuvers I could barely see deflection at all. That would certainly suggest that there's room for less elevator with higher deflection.
Has anyone equipped one of our dogs with telemetry to monitor the control surface deflections during tricks?
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Sorry, getting off the track.
No, that's right on track. It's one of the main things one can do about hinge moment. I had an airplane once that suffered from excessive hinge moment. The more elevator chord I added, the less it would turn. I wish I'd read your famous article before I did that.
Howard can probably tell us the answer to this. How much would the negative pitching moment developed by bigger than necessary flaps/chord retard the rate of pitch change...simultaneously requiring more deflection for the desired turn, which gets you closer to the wall, etc. etc. etc.
It's not obvious to me. The bigger the flaps, the less they'd have to deflect, which thickens the plot. I reckon I could use XFoil to figure pitching moment vs. flap chord for a given amount of lift. That part might be easy. Then there's trying to figure hinge moment. As Gary Letsinger says, hinge moment calculations are easy to do, but hard to believe. Wait until after August.
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Has anyone equipped one of our dogs with telemetry to monitor the control surface deflections during tricks?
Stay tuned.
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Hey Howard....Did you move your computer into the model shop?????
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Here's some ciphering from that short monograph in progress. The first picture shows the weird behavior of lines with differential tension from reacting hinge moments. The blue line is the shape of the lines with no differential tension. The purple and green lines are the shapes of the tighter and slacker line, respectfully, with equal amounts of line tension added and subtracted. I forget the flight condition, but the X axis is fraction of distance from handle to airplane, and the Y axis is line sag in feet.
The second picture may actually answer Robert's original question (high time). This is for a 64 oz. airplane on approx 68-ft. lines flying level at a 5.3-second lap time. The X axis is differential line tension needed to react from 0 to 4 lb. of hinge moment. The Y axis is the amount of differential line position in feet at the handle: how much difference you have to put in between the up and down line length at the handle. The vertical asymptotes are the Netzeband wall. The different lines represent different amounts of force on the airplane in addition to centrifugal force: gravity or aerodynamic forces from stuff like side force on the fuselage, engine offset, or lift vector component toward or away from you. Lines on the graph go from -6 (6 pounds of extra force aimed at the pilot) to +3 (3 pounds of extra force aimed away from the pilot). I hope I remembered the units right. Making the airplane lighter would move you toward the upper lines that tickle the Wall. Remedies would be adding side force, which Robert asked about, or requiring less differential force to react hinge moment: moving to the left on the X axis.
OK, Paul, back to the laboratory.
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I believe somebody did a simple experiment to determine maximum control deflection (Frank Williams or Al Rabe?) with a mini-marker mounted on a flap, scratching on a mini-chalkboard attached to the fuselage side. Not that hard to try, if you can find the right sort of marker doodad. It may have been in SN or MA many years ago? D>K Steve
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when the line tension is insufficient to move the control enough, or as much as you want. Its about running out of control torque. It takes substantial torque to deflect the controls into the airstream, and the control system has to supply it. The torque is applied by differential line tension.
If you move the controls, the line you are pulling towards you has more tension than the line you are allowing to be pulled away from you. The maximum amount of difference you can get is the full line tension all on one line (one line taking all the load, or "hung on one line"). That amount of force times whatever bellcrank lever arm you might have, is the maximum amount of torque you can apply to the bellcrank. This torque, divided by the lever arm from the bellcrank pivot to the pushrod is the maximum amount of force you can apply to the pushrod. This force, times the lever arm of the control horn, is the maximum amount of torque that can be applied to the flap horn.
In any case, the controls will deflect until the differential line tension x all the mechanical advantage equals the torque required to deflect the controls. Since the line tension is limited, the controls can only move so far. If that is less than you need them to move, you hit the Netzeband wall.
Brett
Outstanding, I think I can wrap my head around that. Time to drag out the Super Clown and Nobler once again. The worst I could possibly do is crash them. I'll let the clown be my test platform! H^^
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Hey Howard....Did you move your computer into the model shop?????
You heard the man, get to work. mw~
Derek
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I believe somebody did a simple experiment to determine maximum control deflection (Frank Williams or Al Rabe?) with a mini-marker mounted on a flap, scratching on a mini-chalkboard attached to the fuselage side. Not that hard to try, if you can find the right sort of marker doodad. It may have been in SN or MA many years ago? D>K Steve
Big Art did such back when. Take off, one square corner, land, 12 degrees. Obviously many thing affect that from weight to power to trim but I think it's still a lot less then many think. I would guess 20 degrees max for most 'normal' airplanes. Anything much more is airbrakes. (Flapped airplanes). I also think if you needed to go much further the handle travel/ response time would be so slow to return to neutral you couldn't avoid jumping on exit of a square corner. You can see that simply by watching a too-nose-heavy machine fly.
Dave
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When I Google "Netzeband Wall" (I had no clue what it was) I get returns of several anecdotes regarding hitting this wall in windy conditions, when the plane is otherwise in proper trim.
I have plainly experienced this. With 1/2A it doesn't take much wind to reduce control ability.
With changing weather conditions and the desire to fly in less than ideal situations how can one limit or prevent hitting this "wall" of insufficient line tension? Is it just trim and ballast?
Phil
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Is it just trim and ballast?
As seen above, get more leverage over the control surface and don't have more control surface chord than you need. The easy solution (if you haven't yet built the airplane) is to use a big bellcrank.
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As seen above, get more leverage over the control surface and don't have more control surface chord than you need. The easy solution (if you haven't yet built the airplane) is to use a big bellcrank.
So big bell crank, proper size control surfaces (long and narrow), trim, and ballast if the airplane isn't built, trim and ballast if it is. Adding ballast isn't what I think about when trying to trim an airplane, especially these little ones.
On a related note, I trimmed a coroplast wing for a cox reedy by trimming the elevator significantly (cutting away the trailing edge). It not only balances better, it turns better. The elevator ended up about 1/4 inch more narrow than plan.
I also learned immensely about too-stiff hinges, and why pin hinges are desirable. For winter time flying a cut flute provided little up and no down. (I suppose that would be the Netzenband wall in practice due to stiff controls.)
Phil
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BINGO!!!!!
I find that I fly better when the plane pulls harder. More resistance gives me more "feel" which is good in my book.
I have said many times that "I" believe that there is a perfect weight for any given design, not too light and not too heavy. Something like a 63 oz 690 sq in Cutlass XL would do the trick.
Derek
WoW!!!!! There is tons of good info in here. Thank you to all of those who took the time to post it as well!!
I tend to be right in line with Derek. I believe there is an optimal weight for each design. Since I can't really figure out what it will be prior to flying I tend to try to build as light as I can without sacrificing the integrity of the model. I was always told, "It's alot easier to add weight to the plane then it is to take weight off of the plane." That still holds true today. BUT...last years plane gained some weight due to a repair and it came in at a hefty 68.5 oz. "YIKES" I thought!! Performance didn't seem to suffer much. Plane still penetrated everywhere and didn't sink anywhere at the same time. I do think I was out on it's heavy end for sure. But (there's that word again) I did notice it had a smaller window of speed in which it would perform. I have found over the years on the lighter side of the optimal will give you a larger speed window you can operate in and still have the needed tension to be precise everywhere you need to go. On the heavier side and you will be cornered into a smaller range of speed for optimal performace. In both situations line tension is steady and strong.
I want to stay as far away as possible from any of the netzeband wall characteristics. I try my best to trim the model to fly on both lines at all times everywhere in the hemisphere. When it feels the tension is equal across both lines throughout the corner I actually feel as though I can fly the model through the corner and not just snap it and watch it flip through the corner hoping for the best. It almost becomes slow motion as it happens. The quality and repeatability of the corner and rounds is greatly improved when trimmed for equal line tension on both lines. It has been described by Brad Walker as a "nonevent" corner. Plane flys to a point then it turns and flys away from that point with no oscillation or hinging of any kind. Once that trim is achieved the mistakes are all me and not the plane. Eric, said we have to come to the plane and not the other way around. This is partially true in the fact that we will always be way more capable than our planes but through trimming we get the plane to come to us as much as possible. Then the rest is up to us.
This may seem to be a bit off track here but the point of all of this writing is to say RUN AS FAR AWAY FROM THE NETZABAND WALL AS POSSIBLE!
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Back in the day when Wild Bill wrote this stuff for the magazines, I tried to read it. It was way over my head at the time. All those algebraic formulas and such. I figured when I built from a kit or plans, stay as close as possible to what the designer did. Never had much of any problems. As someone stated, horse power, that is what my RSM Doodle Bug lacked in reality. Will get around to building another one of these years if I live long enough. Now I need to pull the King Sweep off the hook and fire up the tea kettle. Also change power plants.
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The philosophy in building race cars is make it as light a possible and then add lead to make the class weight break. The idea is put it where it will it provide the best benefit or minimize the handicap.
Joe
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So many times I go back and read stuff from BB, Howard and others and I realize how much I have forgotten over the years.... I read BB's link the trimming thread, wow so much good stuff.
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I have built and flown two Big Frys in OTS. It is a high wing British design with a huge elevator. I was flying with the sun just right where I could see the lines while flying overhead 8s. I was amazed at how slack the non-pull line became. I came to the conclusion that making the transition from inside to outside in figure 8s requires a quick movement of the handle because you have to take up the slack in the non-pulling before the airplane would change direction.
My never applied idea for a linear bellcrank is rack and pinion, a circular bellcrank with gear teeth, the pinion for a line of teeth on the pushrod, the rack.
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Off topic, but needs to be said: This very quickly emerging thread is possibly the best ever. Lucid posts, notably Brett's, take time and effort, and as a (former) teacher, I really appreciate the ability and generosity that went into some of these submissions. I've certainly enjoyed the finely crafted responses.
SK
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So many times I go back and read stuff from BB, Howard and others and I realize how much I have forgotten over the years.... I read BB's link the trimming thread, wow so much good stuff.
Tru dat! Amazing what I lose and forget if I don't use it... fortunately we have this great resource to search! (As long as it doesn't get deleted. :X )
I was just ruminating about a friend who passed away. Whenever I flew one of his planes, it was almost always trimmed the same way, some of the smaller ones weren't, but his big stunters always were where the pull was all on the down line and almost no pull on the up line, darn near slack, yet up was so sensitive, you just had to think up and it would turn like crazy. That doesn't make sense to me, especially with the N-Wall in mind as described in this thread? Even just level flight was painful to hold, the downline pulled so hard. When I'd land, I'd rub my pinkie finger for 5 minutes trying to get the feeling back.
Somehow, he flew OK, and was reasonably competitive at the local level. If you watched him fly and didn't watch the plane, he moved his hands, arms, feet, hips, shoulders, well you get the idea, moved a whole lot during the square8, especially for down turns. He flew a pistol grip biased, front bar handle with a lot of overhang too. I never could understand how he just didn't simply crash on every flight, especially when the wind came up. He must have been an outstanding pilot, with just terrible taste in trim... super nice guy, still miss him at our field.
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When I Google "Netzeband Wall" (I had no clue what it was) I get returns of several anecdotes regarding hitting this wall in windy conditions, when the plane is otherwise in proper trim.
I have plainly experienced this. With 1/2A it doesn't take much wind to reduce control ability.
With changing weather conditions and the desire to fly in less than ideal situations how can one limit or prevent hitting this "wall" of insufficient line tension? Is it just trim and ballast?
Phil
Phil,
That's an excellent point and one that needs to be added to the discussion. Well designed airplanes that hit the Netzeband Wall when flown in strong winds are almost certainly nose heavy. As we all know, stunt ships can accelerate when performing consecutive manevuers in the wind. As they accelerated the G forces on the airplane increase proportionately (Howard can tell you how big the proportion is...I can't). If the Center of gravity is well ahead of the center of lift (25% of the average chord is good ballpark figure) the moment between the CG and CL will have to be corrected by downforce on the tail. The greater the G force the greater the demand on the tail to overcome that "pitching moment."
For instance. Let's say a four pound super stunter has the CG (improperly) one full inch forward of the CL. In level flight the tail will have to produce four inch pounds of downforce to maintain level flight...not very much. Now, let's say round loops produce an increase in load of 10 Gs. Ten times four pounds is 40 oz acting at the CG and the tail must now produce 40 inch pounds just to maintain the desired constant radius loop. small radius maneuvers in squares and triangles multiply those factors significant as the radius of the pitch change reduces.
Again, we've all seen how some stunters want to accelerate and open up maneuvers when flying in substantial winds. This is generally where the model described above runs out ideas and altitude at the same time. The faster the airplane goes while flying a given sized radius the greater will be the G forces. The faster the airplane goes the more control deflection will be required to maintain the radius as more of the tail down force is busy simply overcoming the moment between the CL and the increasing mass at the CG. It is "entirely" possible to have insufficient tail authority to do so.
If an airplane is designed so as to allow a further aft CG (let's just pick @25% of the average chord for a ball park) the moment arm between lift and the Center of Gravity now is more or less zero and the adverse moment between the two that must be supported by downforce from the tail essentially disappears. Airplanes designed for and trimmed to that approximate CG/CL relationship have greatly reduced tendency to open up maneuvers and/or accelerate in high winds. The primary source of increased hinge moment now becomes the need to deflect the surface roughly the same amount as in good conditions but doing while the airplane is flying faster...thus modestly increasing necessary input forces.
Other advantages are the fact that turn rates are very little affected in high winds and it is possible to fly the same maneuvers in that windy air as in perfect conditions. More demanding, for sure...but your airplane will remain your friend rather than fighting you.
Ted
p.s. Over the years of stunt history way too little attention has been directed at the importance of a properly located CG and the design factors necessary to support it (primarily a large enough stab and elevator). I believe more attention has been directed at the far less important vertical location of the CG due to heavy wheels, dihedral in the wing, overweight and unnecessary batteries splattered all over the front end, etc.
In concert with the subject matter, larger (or further deflected than necessary) flaps are also a contributor to attacks on the Wild Bill wall. Deflecting flaps is another source for adverse pitching moment that must be countered by downforce at the tail end. The existence of flaps on airplanes that don't really require them to fly the pattern (38 oz Tucker Specials with 550 or square inches of perfectly capable wing, for instance) display proof positive that their CGs ought to be further aft to overcome both the CG/CL moment plus the pitching moment of the "cambered by virtue of the deflected flaps" airfoil. Take a good flying, well trimmed flapless profile like a Ringmaster, Coyote, Doctor ,etc. and, without changing the CG add full span flaps to it and then fly it. I guarantee that, despite all the additional lift of which the wing is now capable, the pitch response of the airplane for the same control input will suffer dramatically.
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Tru dat! Amazing what I lose and forget if I don't use it... fortunately we have this great resource to search! (As long as it doesn't get deleted. :X )
I was just ruminating about a friend who passed away. Whenever I flew one of his planes, it was almost always trimmed the same way, some of the smaller ones weren't, but his big stunters always were where the pull was all on the down line and almost no pull on the up line, darn near slack, yet up was so sensitive, you just had to think up and it would turn like crazy. That doesn't make sense to me, especially with the N-Wall in mind as described in this thread? Even just level flight was painful to hold, the downline pulled so hard. When I'd land, I'd rub my pinkie finger for 5 minutes trying to get the feeling back.
Somehow, he flew OK, and was reasonably competitive at the local level. If you watched him fly and didn't watch the plane, he moved his hands, arms, feet, hips, shoulders, well you get the idea, moved a whole lot during the square8, especially for down turns. He flew a pistol grip biased, front bar handle with a lot of overhang too. I never could understand how he just didn't simply crash on every flight, especially when the wind came up. He must have been an outstanding pilot, with just terrible taste in trim... super nice guy, still miss him at our field.
Wow, Eric. A ton of good stuff in a couple of paragraphs which are good for several independent threads! Especially the part about the Latin salsa dance your friend did to fly the pattern. I've seen that dance more than a few time and generally, the sort of feel you describe when flying his airplanes were the accompaniment for the dance!
Ted
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If the CG is too far forward, the result would be the maneuver will open AND/OR added input would be required to maintain the shape of the maneuver.
The CG being too far forward will exaggerate this tendency if there is added velocity to the maneuvers due to being propped too fast AND/OR increased velocity entering the maneuver due to wind...
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I need to digest this for a while.
Phil
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I need to digest this for a while.
Phil
Me, too. Everything goes up as speed squared, but in the wind airspeed is different from ground speed, and the two act on different things.
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That's an excellent point and one that needs to be added to the discussion. Well designed airplanes that hit the Netzeband Wall when flown in strong winds are almost certainly nose heavy.
I touched upon it above, but I was already droning on a bit. Running the CG aft has the effect of requiring less elevator deflection for a given rate of turn, and thus also deflecting the flap less. It doesn't help you move the controls further, just makes it less important, so you are still better off. The part I barely mentioned above about Paul's slow control ratios - which are enabled by the aft CGs, and the slow control ratios improve the margins because you get better translation of the bellcrank torque to control horn torque.
The downside to the aft CGs is exactly the same as the advantage. By requiring less control deflection, you get less flap deflection, and less camber in the wing from the flap deflection (for a given pitch rate). If you were going to run out of lift, moving the CG aft and doing nothing else would hurt you and make the stall happen sooner. Al Rabe used the inverse of this to good effect, theorizing that running out of lift was his problem, he moved the CG forward (in the days of unadjustable controls) so he would would have more flap deflection at a given pitch rate, so he didn't run out of lift. The same side effects happened, of course, but it's better than stalling. So he ended up improving his corners by running the CG forward.
Now, to accomplish the same thing, you might be tempted to use your adjustable control ratios to decrease the elevator travel in relation to the flap to solve the same problem without moving the CG at all. Or make the flap bigger, since many people are making the flaps removable.
This illustrates several interesting points, but mostly, that there isn't one dogmatic solution for everything. You design the airplane the best you think you can, build it the way you think it should be, and then solve the issues you have by diagnosing them correctly and devising a solution. When you reach some compromise solution that you would rather remove, you design the next one with changes necessary to resolve that compromise. That's why most successful designs are evolved over a long period of time - people usually start with a sort of dogmatic approach, or some killer feature or approach that they take to an extreme, trying to optimize one aspect. They build it, find some problems, and either realize where the shortcomings are and address them, or, they figure the problems are because they didn't go extreme enough, and double down on the same ideas.
The former results in what is biologists call "convergent evolution" where you might have people start with wildly different ideas and have everyone end up and just about the same spot. Usually, the latter is a dead end, mostly because the person is more wedded to the idea that their killer feature is really killer and is unwilling to compromise on that. I have mentioned it before but in many cases people care more about maintaining their one solution regardless of the consequences, and will also observe others having success, and intentionally do just the opposite, just to show they aren't part of the herd. Howard's "radical independents".
It doesn't really matter, ultimately, since the entire point of the event is to enjoy yourself. You can do it by winning trophies, or by losing while sticking it to the man, who cares, as long you are having fun doing it. I can assure you that you aren't going to get money or girls doing stunt, so you had darn well better be having fun.
Brett
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Say there was a guy that built, oh, say, four new stunt planes every year for 40 years. That would be a lot of planes! But I don't see how that one guy would be able to fly enough to get 4 new planes really well trimmed each season. Without getting them well trimmed, will he be able to correctly evolve his design, even if they are all basically the same? I don't see that being possible, plus, they are not all basically the same. Hypothetically speaking, of course... H^^ Steve
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Steve,
??????? care to explain that?
Joe
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Say there was a guy that built, oh, say, four new stunt planes every year for 40 years. That would be a lot of planes! But I don't see how that one guy would be able to fly enough to get 4 new planes really well trimmed each season. Without getting them well trimmed, will he be able to correctly evolve his design, even if they are all basically the same? I don't see that being possible, plus, they are not all basically the same. Hypothetically speaking, of course... H^^ Steve
Easy, you are a millionaire with unlimited resources and tons of time on your hands. When it is day time you go fly. When it is night or weather really unbearable, you work on planes. But, it is fun watching people through the years. Someone win the NATS with a big plane. The next year there are more big planes. Then the pipes came out and now it is electric. I am waiting for someone to win the NATS with a 40 size plane with electric propulsion.
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I am waiting for someone to win the NATS with a 40 size plane with electric propulsion.
Really? I thought that already happened last year, no?
If you are going to dream, you need to dream bigger! How about winning the Nat's with the new YS63 C/L 4 stroke in a Bipe! :! :o <=
Then everyone will run out and build one...right? The pits full of 4 stroke Bipe's, now how cool would that be?
(I think I just heard Milton fall out of his chair, sorry buddy, that was low, wasn't it?)
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Finally, you can just make the airplane heavier. The weight is more-or-less a linear relationship to the line tension. Add 20% to the mass, and you get 20% more line tension, all other things being equal. That way, you can definitely deflect the controls further at a given speed. Or, you can get the same deflection at a lower speed.
Brett, are you sure wit that? Because I am not so sure :- )))
Yes you can get 20% more line tension, but 20% heavier model will need also aproximately 20% stronger hinge moment as it comes from pressuse on wing (that pressure which must overcome 20% centrifugal force necessary to carry 20% heavier model on the same flight path). So I would say just oposite, heavier model will not make situation with Netzeband wall any better :- )) ... well it can, if you push the weight to partial (flap) stall, then yes, it could help because hingemoment on flap can drop to 1/2 of normal but airflow separated on hinheline is last think I would like to have on my model :- )))
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Brett, are you sure wit that? Because I am not so sure :- )))
Yes you can get 20% more line tension, but 20% heavier model will need also aproximately 20% stronger hinge moment as it comes from pressuse on wing (that pressure which must overcome 20% centrifugal force necessary to carry 20% heavier model on the same flight path). So I would say just oposite, heavier model will not make situation with Netzeband wall any better :- )) ... well it can, if you push the weight to partial (flap) stall, then yes, it could help because hingemoment on flap can drop to 1/2 of normal but airflow separated on hinheline is last think I would like to have on my model :- )))
Igor,
A hypothetical question. If you had a zero ounce 700 square inch flapped stunter flying on 70" lines at five second a lap how much line tension would there be? How much control deflection would be achievable? >:D >:D >:D
How about 10 oz? 20 oz? thirty ????
Ted
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Brett, are you sure wit that? Because I am not so sure :- )))
Yes you can get 20% more line tension, but 20% heavier model will need also aproximately 20% stronger hinge moment as it comes from pressuse on wing (that pressure which must overcome 20% centrifugal force necessary to carry 20% heavier model on the same flight path). So I would say just oposite, heavier model will not make situation with Netzeband wall any better :- )) ... well it can, if you push the weight to partial (flap) stall, then yes, it could help because hingemoment on flap can drop to 1/2 of normal but airflow separated on hinheline is last think I would like to have on my model :- )))
If you were running out of lift before, no. If the problem is that you can't deflect the elevator enough (which was the case here), you don't need more just because it is heavy.
Brett
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If you were running out of lift before, no. If the problem is that you can't deflect the elevator enough (which was the case here), you don't need more just because it is heavy.
Brett
I thought we are speaking about flapped stunter, in that case most of feedback comes from flaps and if weight is 20% higher, also feedack is aproximatey 20% higher ... more in next post ...
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I thought we are speaking about flapped stunter, in that case most of feedback comes from flaps and if weight is 20% higher, also feedack is aproximatey 20% higher ... more in next post ...
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Bear in mind that the lap times also go up, from 4.6 to 5.2 secs/lap or so, so you get a 27% reduction in the control torque at a given deflection.
Brett
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Igor,
A hypothetical question. If you had a zero ounce 700 square inch flapped stunter flying on 70" lines at five second a lap how much line tension would there be? How much control deflection would be achievable? >:D >:D >:D
How about 10 oz? 20 oz? thirty ????
Ted
I do not think it makes too much sense to explore singularities like 0 weight needed 0 lift to make a corner, but if you take 10 oz stunter which needs some lift L for making some given radius which is on edge of Netzeband wall (means line tension equals force necessary to make that radius) and having some C centrifugal force, then if you go to 20oz stunter, you will have 2*C centrifugal force and thus twice stronger input to controls. That is where we agree.
That heavier stunter will certainly need also twice the lift of that lighter 2*L at the same speed and the same radius. It is because of the same equation for centrifugal force which gives higher line tension 2*C. Just the radius is not line length, but radius of corner instead. (note: I ignored gravity to make it simpler)
The pressure distribution chordwise (the quality) on wing will be very similar on both models because it does no change too much with changed lift, just that heavier, which needs 2*L lift will have twice higher pressure (its quantity, but distribution will be very similar). It means the pressure of flaps will be twice higher what will make twice higher hinge moment which will be transformed to twice higher force necessary to make the same corner, so we re back on the same minimal radius.
Most of the feedback comes from flaps, I am now not able to say hom much, because i am on trim in CA now and all my calculators are home, but elevator hinge motment is really small. But ok Brett is right, we have also elevator. Elevator must counterbalance moment of CG to AC. That moment will need some lift and thus pressure distribution on elevator. But is very similar to flaps. Twice heavier model will have twice higher CG moment and thus it will need twice higher elevator lift and thus also absolute pressure. However we have still the same elevator and stab, so its distribution is still similar, it means also elevator hinge moment will be twice stronger on heavier model then on lighter.
So from this point of view weight does not change the situation. However if you count that heavier model will probably need little more deflected controls (that twice higher lift must be gained somewhere somehow – probably by more deflected flaps) means lift will be concentrated more on back side of airfoil, making worse pressure distribution and also higher airfoil moment which must be counterbalanced by elevator, so it can easily happen that lighter model could be even little better in meaning minimal flyable radius.
So this is probably what you did not want to hear : )))) ... however it is still only static situation. We have also some dynamic effects here allowing little tighter corner then limited by Netzeband wall - you can pull handle during comer and heavier model will give you better mass inertia to do it, but it has nothing to do with Netzeband wall, which is defined statically.
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Bear in mind that the lap times also go up, from 4.6 to 5.2 secs/lap or so, so you get a 27% reduction in the control torque at a given deflection.
Brett
Could be, but as you aleady wrote and I fully agree, the lap times also do not change it. Nececcary line force goes up with speed at the same magnitude like line tension - it is still the same equation for centrifugal force.
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Could be, but as you aleady wrote and I fully agree, the lap times also do not change it. Nececcary line force goes up with speed at the same magnitude like line tension - it is still the same equation for centrifugal force.
Right, if you merely speed it up or down, it doesn't get you anywhere. Adding mass and reducing the speed alters the required control torque for a given deflection more than it alters the line tension. That's the whole point.
Take the example above, the line tension goes down to 94% of the original but the aerodynamic loads go down to 79% of the original, so you are ahead of the game substantially. And, in fact, that's pretty much exactly what happened in the experiment (and the many similar examples over the years).
Since Howard always wants me to show my work,
Original = 36 oz at 4.6 sec laps, 89 fps, .069 slugs, 65 feet; line tension = 8.52 lb, q = 9.3 psf
final = 44 oz at 5.2 sec laps, 78 fps, .086 slugs, 65 feet; line tension = 8.04 lb, q= 7.3 psf
ratio of line tensions = .94, ratio of q = .79
As an aside, note also that all the aerodynamic forcing functions for out-of-trim conditions also go down substantially, while the line tension that acts to stabilize it (essentially all the roll restoring torque and maybe 50% of the yaw) changes only a little bit. This would have drastically assisted the 585 square inch/54 ounce airplane since it also had some serious aerodynamic trim issues that were not that easy to fix, without taking a saw to the fin.
Brett
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One curious thing is that Brett, Igor, and Ted are within a kilometer or so of each other now.
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well thankfully they are not sitting at a table sipping a beer talking about this, or we would all miss out on the exchange!!
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Those 70" lines have me concerned, tho! VD~ Steve
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Original = 36 oz at 4.6 sec laps, 89 fps, .069 slugs, 65 feet; line tension = 8.52 lb, q = 9.3 psf
final = 44 oz at 5.2 sec laps, 78 fps, .086 slugs, 65 feet; line tension = 8.04 lb, q= 7.3 psf
ratio of line tensions = .94, ratio of q = .79
Hmmm ... I am lost in numbers and especially units and where they come from, as I am here only with handy ... so I affraid I can continue when I come back home, however it is no queston that proper weight is propper weight, as much as wing can carry and motor can pull, but now I do not see directly how it can help to push Netzeband wall, especially if heavier and slower model will need larger controlls deflection, I will try it home on my stnt calculator, I am already curiouse : -))
BTW I am now some 50 miles from SF .. exactly 53 from Alcatraz .. at least that is waht google maps told me when I started my way yesterday :- ))) ... I am in Mountain Vew visiting my son :- )))
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GREAT thread!
Only two thoughts:-
1.) The term 'hinge moment' may confuse some of us.
Try it this way: A moment is a force at an arm distance, tending to cause rotation about a point. When we move our control surfaces off neutral, they meet airloads trying to return them to neutral. A point where, in effect, all the airload acts on a surface can be SWAG'ed or estimated. The distance from that point to the hingeline is the moment arm, and the total force x that distance is the hinge moment.
To hold the control surfaces to angles needed to perform figures, we need to match the airload hinge moments, by shifting pull between the lines, sharing the full value of centrifugal force - split equally at neutral - to more on the 'loaded' line and less on the 'unloaded' line. The pull difference, led through the series of levers (bellcrank line radius, bellcrank pushrod radius, horn radii at each control surface) applies the matching counter moment.
The Netzeband Wall is hit when the pull (basically CF) cannot reach the values needed to hold or increase control surface deflection for conditions. The model does not turn tighter, however much we want it to. If too low, and it opens up... Next model please...
Note: It IS possible to add briefly to pull's CF value by yanking the handle away from the model. (Inertia permits a very brief boost to CF. Paul Walker, in a video I've seen, and in seeing him fly, uses this, quite violently, for corners. It does work!)
2.) Line curvature due to drag: (weight IS uniform, but very small and not sigificant for the following remarks.)
I've seen the curve our lines take in flight called an "accelerated catenary." A simple catenary is the curve we see in a cable suspension bridge, supported at both ends, for a cable of uniform weight load along its length. Our lines, in flight, do not meet a uniform load. Air drag varies with the square of velocity - which, itself, varies from (in effect) zero at the handle, to whatever value exists at the leadout guides.
Incidental to this: If the line pull direction, when it reaches the leadout guides, can be aimed at the CG, we gain. "Moments," again... LINE I and LINE II help for these concepts...
If the line pull aims ahead of the CG, it tends to pull the model's nose "in" toward the flier. And vice versa...
These are 'yaw' disturbances, and at extreme, can cause bad hinging. It is ideal when yaw and roll tendencies are at minimum.
Gyro effects from the propellor and crankshaft, for conventional (CCW rotation props, seen from in front,) ""nitro"" models (flown CCW), cause a nose-out tendency on inside turns, and, again, vice versa.
(It may be possible, if good numbers exist, to spread the leadout guides chordwise enough to reduce, or even counter, the yaw tendencies! For years, I've tried to cancel these for the worst case I could estimate, hoping that that would deal with "lesser included" conditions. I'm no rocket scientist -Howard and Brett are - but my approach seemed to work for my level of seriousness and potential...)
Over to you,Chet...
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Hmmm ... I am lost in numbers and especially units and where they come from, as I am here only with handy ... so I affraid I can continue when I come back home, however it is no queston that proper weight is propper weight, as much as wing can carry and motor can pull, but now I do not see directly how it can help to push Netzeband wall, especially if heavier and slower model will need larger controlls deflection, I will try it home on my stnt calculator, I am already curiouse : -))
BTW I am now some 50 miles from SF .. exactly 53 from Alcatraz .. at least that is waht google maps told me when I started my way yesterday :- ))) ... I am in Mountain Vew visiting my son :- )))
That would be about 5 miles from here...
I can convert it to old-world units when I get a chance, but the ratios will be the same.
Brett
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[I will try it home on my stunt calculator, ... [/quote]
A truly interesting thread, indeed! How about all of this when I fly corners above level, such as upper square, triangle and hr. glass corners? Could it be the Netzeband wall is moving with line elevation angle?
Peter Germann
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[I will try it home on my stunt calculator, ...
A truly interesting thread, indeed! How about all of this when I fly corners above level, such as upper square, triangle and hr. glass corners? Could it be the Netzeband wall is moving with line elevation angle?
Peter Germann
While the line weights will balance each other out due to having the same drag and weight to them, at elevation (directly overhead) the weight of the model is removed from the line tension, and at a steep angle a large fraction of the weight of the model is removed from line tension (apply some vector math, trig and all), so yes, the upper portion may have less tension compared to the level portion at shoulder height.
Phil
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[I will try it home on my stunt calculator, ...
A truly interesting thread, indeed! How about all of this when I fly corners above level, such as upper square, triangle and hr. glass corners? Could it be the Netzeband wall is moving with line elevation angle?
Peter Germann
Yes it does, All we wrote until now is in level flught. Overhead we have liine tension less gravty and it makes situation completaly diferent. Minimal radius is narualy larger due to lower line tension, but it chages also effect of different weight and different speed. While the weight does not change situation + that similar like in level, the the higher speed wit improve it, because the higher speed we fly, the higher centrifugal acceleraion we have and the lower proportional effect gravity has. ... at least that it how I see it now, just without couning numbers. So I think that quicker lighter model from Bretts example will have advantage overhad.
However there is still effect of gravity for necessary lift, which is present in level, but not overhead, so it is little more complicated, and the difference is not so large ... it needs really count numbers then just guess.
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Note: It IS possible to add briefly to pull's CF value by yanking the handle away from the model. (Inertia permits a very brief boost to CF. Paul Walker, in a video I've seen, and in seeing him fly, uses this, quite violently, for corners. It does work!)
Yes but it has also its limits, if you fly square loop, you can use little enegy from model mass inertia, but you must allow model to gain it back, it means you need some time and some distance between corners to release handle back. There are many pilots flying like that. And not only because pure design of their models, some people simply like such feeling, they can easier feel impulse given to handle then deflection and time. Once I even saw friend of mine from Ukraine (they typically fly like that) trying to fly my 200g indoor like that ... well without too much success n first corners :- )))
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That would be about 5 miles from here...
I can convert it to old-world units when I get a chance, but the ratios will be the same.
Brett
5 miles? it must be somewhere close, I see you have perfect weather for flying whole year, you not need need winter sleeping like we do :- ))))))))))))
yes, may be it is not question of units, may be if you can tell how you got those numbers ... the thing which I cannot get is how you got that difference if you combined speed and weight difference together, if speed and also weight difference alone does not make any difference ... hmm ... I am missing my notebook and my spread sheets ... we are going o visit some aquarium, may be I will get it looking to blue water :- )))))
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[I will try it home on my stunt calculator, ...
A truly interesting thread, indeed! How about all of this when I fly corners above level, such as upper square, triangle and hr. glass corners? Could it be the Netzeband wall is moving with line elevation angle?
Peter Germann
Absolutely. The worst case is probably the hourglass. But don't overlook the effects of varying speed and the airplane getting "out of shape" due to trim issues during other maneuvers, particularly the square 8. If everything is not perfect, the available corner can vary dramatically from corner to corner. That's the reason nearly no one flies the maneuvers the right size - these sort of transient effects have to mostly go away before you can successfully do a clean corner. They aren't too big because the corners are too big, they are too big (for the most part*) because you have to let it recover after the corner. Once you get to a certain skill level, this is what separates the competitors in many cases.
That's why trim matters so much and why getting it perfect is so important - and takes up the vast majority of the practice time.
We are only talking about quasi-steady-state, for the most part, above. That's why I mention some improvement in the restoring forces above, more is going on that just running out of control torque.
Brett
*check some videos of Igor's WC flights, and compare the sizes to what you see at local contests! When Ted/David/I practice, we spend time from flight to flight on correcting shape errors, but we talk about sizes over periods of decades.
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5 miles? it must be somewhere close, I see you have perfect weather for flying whole year, you not need need winter sleeping like we do :- ))))))))))))
What do you mean, it rained for about 5 minutes several times this week, and the highs are only in the 60s? They call that a major winter storm around here. I laugh at it, too, I grew up in the Midwest, where brass monkeys are an endangered species.
yes, may be it is not question of units, may be if you can tell how you got those numbers ... the thing which I cannot get is how you got that difference if you combined speed and weight difference together, if speed and also weight difference alone does not make any difference ... hmm ... I am missing my notebook and my spread sheets ... we are going o visit some aquarium, may be I will get it looking to blue water :- )))))
m(V)**2/r (line tension) is the same no matter where you are! same with 1/2*rho*v**2 (q, AKA dynamic pressure). I am using the values shown with because those are roughly the parameters of the Tucker Special tests mentioned above.
4.6 seconds a lap and 36 ounces ("before")
5.2 seconds a lap and 44 ounces ("after", same airplane with 8 ounces added)
r= 65 feet in either case (~60ft lines+length of Ted's arm+length of inboard wing), circumference = 408 feet
m = mass in slugs, mass= weight in lbf/32.174, weight in lbf = weight in ounces/16
v= velocity in feet per second = 408 feet/(lap time)
rho = density of air = 0.00237 slugs/cubic foot (I assumed STP, which is close enough since we were in the Mission College parking lot about 8 miles from your current position, and you will pass right by if you are headed to the Monterey Bay Aquarium, elevation is about 8 feet - it's by Great America amusement park {which is closed for the season, sorry}).
psf = pounds per square foot
And before I get another lecture from the "USA Metric in '78!" crowd... Anyone reporting their weight in Kg is committing all the same offenses, because the SI unit of force is Newtons, not Kgf. and if you try to get it all right, you have to put in a fudge factor of 9.8 somewhere - just like the 32.174
Brett
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I see we have different approach here, I think rho does not play any effect here, as the wing pressure is given from centrifugal force and wing area, so the line tension from centrifugal force out of the circle and centrifugal force out of the flight radius goes up at the same ratio, so that is why I think change of mass does not change situaton ... ok, at least i have enough for think about :- )))
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What do you mean, it rained for about 5 minutes several times this week, and the highs are only in the 60s? They call that a major winter storm around here. I laugh at it, too, I grew up in the Midwest, where brass monkeys are an endangered species.
be happy, we have only one rain during winted ... 3 monthes long :- ))))))))))))))))
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It may be possible, if good numbers exist, to spread the leadout guides chordwise enough to reduce, or even counter, the yaw tendencies!
I had an interesting discussion on this yesterday. It was about forwards props and backwards props and making a new plane with up on the aft line (or maybe up on the forward line) instead of the other way around, all to fix a funny on the third lobe of the clover. Anyhow, I lost track of all the sign changes.
Good explanation of hinge moment. Hinge moments are kinda hard to predict, which is another bone I'd pick with Bill Netzeband's article about them. He overgeneralized the calculation.
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A truly interesting thread, indeed! How about all of this when I fly corners above level, such as upper square, triangle and hr. glass corners? Could it be the Netzeband wall is moving with line elevation angle?
Yes, see the second graph in reply 29 above (sorry about the proprietary measurement units). There are also the destabilizing effects on line tension of thrust and Clβ. Less line tension or more wind cause the airplane to point toward the pilot. The effect of the prop disk being at an angle to the flow is a lot more than thrust * sin β. These effects are exacerbated close to the Wall, as seen in that graph.
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be happy, we have only one rain during winted ... 3 monthes long :- ))))))))))))))))
Be happy is is only rain !!!
We get snow here!
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We get snow here!
[/quote]
"that's not Snow, This is snow...." says Polarbear Dundee.
hahaha
B
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We get snow here!
"that's not Snow, This is snow...." says Polarbear Dundee.
hahaha
B
Bruce, get back to charging your Li-Po's.....
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Brett, are you sure wit that? Because I am not so sure :- )))
Yes you can get 20% more line tension, but 20% heavier model will need also aproximately 20% stronger hinge moment as it comes from pressuse on wing (that pressure which must overcome 20% centrifugal force necessary to carry 20% heavier model on the same flight path). So I would say just oposite, heavier model will not make situation with Netzeband wall any better :- )) ... well it can, if you push the weight to partial (flap) stall, then yes, it could help because hingemoment on flap can drop to 1/2 of normal but airflow separated on hinheline is last think I would like to have on my model :- )))
Igor,
Where does the extra 20% lift come from?
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Howard, to your #70, thanks for the kind comment.
As to CW shaft v. CCW shaft (as seen from out front), my only practical application was in the early 1980's on an OT AA, Sr., Fox 35Stunt, factory shaft.
Main difference noted at my cuneiform in clay skill level computer was that the basic engine torque reaction on the fuselage, being reversed, held the inboard wing up better. No tip weight needed. Takeoffs were easier, and the bellcrank connections could be Jim Walker Basic (down line forward.) No tabs or warts needed.
All other practical handling actions the same both ways, except that flipping that CW prop with the wheels on the ground consisted of punching the pavement, Yes, ouch, and with the flying hand...
To Igor - earlier post:
Of course the 'yank' to increase line pull was - I thought - understood to be only for that brief moment when it was needed to move Netzeband's wall back a small distance. Unless we are whipping the model continuously, we should have enough CF pull to be comfortable almost everywhere.
Howard, estimating the force and effective chord distance from hingeline where it can be considered to act is something I'd gladly leave to you, Brett and Igor... For my rather crude uses, I applied what the NACA wind tunnel pressure profiles in approx WW2 basic texts seemed to imply for deflected flap surfaces. SWAG, not rigorous, but a more credible start point than none at all. At my highest level of performance, that was about a match...
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Less line tension or more wind cause the airplane to point toward the pilot.
Depends what is the model and who is the pilot. ... certainly not mine to me :- )))
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Be happy is is only rain !!!
We get snow here!
we get snow afterwards, March and April :- )))
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Igor,
Where does the extra 20% lift come from?
If it should be the same model, then the only way is elevator and flap deflecttion, but it is hard to predict without more data, so I sinply ignored it. However as I wrote before, especially flaps which will be more deflected will push center of pressure more back (if it does not stall), it means the pressure distribution will be moved more to flaps and so neavier model will fly even more opened corners, instead of tighter. But I think that modified pressure distribution will make very small difference.
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Right, if you merely speed it up or down, it doesn't get you anywhere. Adding mass and reducing the speed alters the required control torque for a given deflection more than it alters the line tension. That's the whole point.
I don't think so, nor do I think that's the point. I shall attempt to explain.
Take the example above, the line tension goes down to 94% of the original but the aerodynamic loads go down to 79% of the original, so you are ahead of the game substantially. And, in fact, that's pretty much exactly what happened in the experiment (and the many similar examples over the years).
Since Howard always wants me to show my work,
Original = 36 oz at 4.6 sec laps, 89 fps, .069 slugs, 65 feet; line tension = 8.52 lb, q = 9.3 psf
final = 44 oz at 5.2 sec laps, 78 fps, .086 slugs, 65 feet; line tension = 8.04 lb, q= 7.3 psf
ratio of line tensions = .94, ratio of q = .79
These numbers are a tad off--Brett must have been using his 5" slide rule, rather than the big one--but pretty close. I get line tension 2 / line tension 1 = .96 and q2/q1 = .78 . But as Igor was saying, the heavier model requires more lift for a given loop radius. His simplifying assumption was that the pressure distribution on the wing and flaps had the same shape, but was multiplied by the ratio of lift required for the heavy airplane to lift required for the light airplane. Thus the hinge moment as well as the line tension is proportional to mV2. The ratio of hinge moments is therefore .96 also, given Igor's simplifying assumption. So at best, the ratio of line tension to hinge moment is 1 ratio (line tension 2/hinge moment 2) to (line tension 1/hinge moment 1) is 1. Actually, for the same elevator/flap ratio, you'd need a bit more flap deflection to get the increased Cl you'd need for the same loop radius with the heavier model, so hinge moment would go up. Thus adding weight decreases the ratio of line tension to hinge moment. But it worked; it made the Tucker fly better.
Here's where it gets weird. I took the program that generated the plots in reply 29 and did runs for the Tucker Special at the two different weights, but at the same speed: 5-second laps. I don't know how much differential line tension it takes for a given maneuver. I'll tell you when I determine hinge moment. However, for Igor's simplifying assumption of the same pressure distribution shape, the same maneuver would require the same difference in line tension for both airplane weights. On the attached plot, you can see that for a given difference in tension between the lines, the 44 oz. airplane has a less perverted response to control inputs than the 36-oz. airplane. Even if hinge moment coefficient goes up some for the heavier airplane, as it will, there could still be a big advantage to adding the weight. I picked a couple of points as samples. The square on the 44-oz. line requires more differential line tension to react hinge moment than the square on the 36-oz. line, but still gives better control response and is further from the Wall.
To include the speed difference in the comparison, I'd need to change the plot to normalize the X axis by V2, but I have parts to sand and glue, so it can wait until after August when I finish the short monograph on hinge moment.
My conclusion from all this is that an airplane probably does have an optimal weight, as folks have said above, but that weight should be determined by turbulence response, rather than hinge moment issues. If you have a good control system, you won't have to worry about getting the plane too light for the Netzeband wall. Hint: you won't get there looking at the leverage of control surface to bellcrank. A little added side force can help, too. Sparky is on the right track.
Edited to make a little more sense.
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I don't think so, nor do I think that's the point. I shall attempt to explain.
These numbers are a tad off--Brett must have been using his 5" slide rule, rather than the big one--but pretty close. I get line tension 2 / line tension 1 = .96 and q2/q1 = .78 . But as Igor was saying, the heavier model requires more lift for a given loop radius. His simplifying assumption was that the pressure distribution on the wing and flaps had the same shape, but was multiplied by the ratio of lift required for the heavy airplane to lift required for the light airplane. Thus the hinge moment as well as the line tension is proportional to mV2. The ratio of hinge moments is therefore .96 also, given Igor's simplifying assumption. So at best, the ratio of line tension to hinge moment is 1. Actually, for the same elevator/flap ratio, you'd need a bit more flap deflection to get the increased Cl you'd need for the same loop radius with the heavier model, so hinge moment would go up. Thus adding weight decreases the ratio of line tension to hinge moment. But it worked; it made the Tucker fly better.
You appear to be assuming that all the additional lift comes from deflecting the flap more, in which case you are correct. But I don't think that's what happens when you have coupled flaps. I think the flaps are deflected less than you think on the heavy airplane, but more than on the light airplane.
Do the same thought experiment with no flaps at all, in which case the hinge moment per turn rate is almost unaffected by the weight of the airplane. Then do the same experiment with the flaps set to move only as far as they do in the "light case" and then stop (while the elevator can continue to move). This is pretty much the same as doing the same experiment with the flaps glued in place as some fixed angle.
The heavy airplane ends up flying at a higher angle of attack for a given turn radius, but I think my ratio holds. The required pitch rate at "equilibrium" is actually lower on the heavier but slower airplane, you end up needing less deflection to achieve it. In fact the same theory can hold until your AoA in the corner exceeds the stall angle.
This is now verging on the Imitation experiments with variable-sized flaps, and why heavy airplanes "hop" when the flaps are too small or have the wrong flap/elevator ratio.
Perhaps we aren't on the same page with what happens when you are in the middle of the turn in quasi-steady state conditions and/or how it gets to a particular angle of attack.
Brett
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I touched upon it above, but I was already droning on a bit. Running the CG aft has the effect of requiring less elevator deflection for a given rate of turn, and thus also deflecting the flap less. It doesn't help you move the controls further, just makes it less important, so you are still better off. The part I barely mentioned above about Paul's slow control ratios - which are enabled by the aft CGs, and the slow control ratios improve the margins because you get better translation of the bellcrank torque to control horn torque.
The downside to the aft CGs is exactly the same as the advantage. By requiring less control deflection, you get less flap deflection, and less camber in the wing from the flap deflection (for a given pitch rate). If you were going to run out of lift, moving the CG aft and doing nothing else would hurt you and make the stall happen sooner. Al Rabe used the inverse of this to good effect, theorizing that running out of lift was his problem, he moved the CG forward (in the days of unadjustable controls) so he would would have more flap deflection at a given pitch rate, so he didn't run out of lift. The same side effects happened, of course, but it's better than stalling. So he ended up improving his corners by running the CG forward.
Brett
Delete this if I muddle up this excellent thread.
Generally, the CG will be somewhere forward of the CL (center of lift). Moving the CG closer to the CL makes the airplane more maneuverable, needing less force to rotate the aircraft around the lateral axis, but also makes it less stable.
I would think that at some point, the increased maneuverability would be offset by the instability, leading to over-controlling attempting to fly smooth maneuvers.
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Delete this if I muddle up this excellent thread.
Generally, the CG will be somewhere forward of the CL (center of lift). Moving the CG closer to the CL makes the airplane more maneuverable, needing less force to rotate the aircraft around the lateral axis, but also makes it less stable.
I would think that at some point, the increased maneuverability would be offset by the instability, leading to over-controlling attempting to fly smooth maneuvers.
Oh, certainly, if you move the CG back enough, it will be unstable and/or unflyable. The theory in this thread is that weight was added, but the CG didn't change.
Brett
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Yes I did not include different controls deflction for both weights, also for simplicity, and also because it can change situation depending on arms geometry, it can easily happen that line difference / hinge moment ratio changes with deflection (unequal bellcrank and flap horn arms) and there are also another effects which are not included to equaton like mentioned mass inertia but aslo transient effects of moment of inertia - heavier fuselage can be accelerated before it enters circular path and that will be different on lighter and heavier models. And also my logarithmic device (making feedback smaller at higher deflection necessary for heavier model) will make heavier plane controll easier then lighter.
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You appear to be assuming that all the additional lift comes from deflecting the flap more, in which case you are correct. But I don't think that's what happens when you have coupled flaps. I think the flaps are deflected less than you think on the heavy airplane, but more than on the light airplane.
Do the same thought experiment with no flaps at all, in which case the hinge moment per turn rate is almost unaffected by the weight of the airplane. Then do the same experiment with the flaps set to move only as far as they do in the "light case" and then stop (while the elevator can continue to move). This is pretty much the same as doing the same experiment with the flaps glued in place as some fixed angle.
The heavy airplane ends up flying at a higher angle of attack for a given turn radius, but I think my ratio holds. The required pitch rate at "equilibrium" is actually lower on the heavier but slower airplane, you end up needing less deflection to achieve it. In fact the same theory can hold until your AoA in the corner exceeds the stall angle.
This is now verging on the Imitation experiments with variable-sized flaps, and why heavy airplanes "hop" when the flaps are too small or have the wrong flap/elevator ratio.
Perhaps we aren't on the same page with what happens when you are in the middle of the turn in quasi-steady state conditions and/or how it gets to a particular angle of attack.
You can get the extra lift from angle of attack. If you want to do so without changing hinge moment between the light and heavy airplane, you could use fixed flaps and a balanced elevator. I assume, though, that the experiment we are discussing is a given Tucker Special with the same control system at both weights. That airplane will have more hinge moment as lift increases. You did not improve the flyability of the airplane by reducing hinge moment via added weight. You improved it by overcoming the control nonlinearity caused by line drag.
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From Brett's post: "Original = 36 oz at 4.6 sec laps, 89 fps, .069 slugs, 65 feet; line tension = 8.52 lb, q = 9.3 psf
final = 44 oz at 5.2 sec laps, 78 fps, .086 slugs, 65 feet; line tension = 8.04 lb, q= 7.3 psf
ratio of line tensions = .94, ratio of q = .79"
The subject Tucker Special has been ballasted up 8 oz and slowed from 4.6 sec. laps to 5.2 second laps. It will require a little more control input due to the increased weight, but due to the reduced speed/increased lap time, the hinge moments will be reduced quite a bit. It seemed to work; I was there.
One thing Gary Letsinger once told me was that sometimes the corner can be tightened by shifting the CG forward. Theory is this requires more control input, which brings more wing flap to bear. I suspect this would be only possible on heavy models and of course, modern adjustable controls would probably make this the wrong thing to do. But then, not everybody incorporates adjustable controls. H^^ Steve
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From Brett's post: "Original = 36 oz at 4.6 sec laps, 89 fps, .069 slugs, 65 feet; line tension = 8.52 lb, q = 9.3 psf
final = 44 oz at 5.2 sec laps, 78 fps, .086 slugs, 65 feet; line tension = 8.04 lb, q= 7.3 psf
ratio of line tensions = .94, ratio of q = .79"
The subject Tucker Special has been ballasted up 8 oz and slowed from 4.6 sec. laps to 5.2 second laps. It will require a little more control input due to the increased weight, but due to the reduced speed/increased lap time, the hinge moments will be reduced quite a bit. It seemed to work; I was there.
One thing Gary Letsinger once told me was that sometimes the corner can be tightened by shifting the CG forward. Theory is this requires more control input, which brings more wing flap to bear. I suspect this would be only possible on heavy models and of course, modern adjustable controls would probably make this the wrong thing to do. But then, not everybody incorporates adjustable controls. H^^ Steve
Steve,
I would think that could only be true if the ship was stalling with the aft CG. A better solution to that dilemma is to increase the flap movement relative to the elevator as I did with the porky original Trivial Pursuit which still flies pretty respectably. In hot, rainy air in Muncie at one team trials it started to do that (stall) in triangle bottoms and scared me to death. That's when we first experimented with the .018 control line turbulator randomly taped to the high point of the wing as suggested by--guess who--Mr B. The problem disappeared and the plane flew competitively but it wasn't the year it won the trials...don't ask me which it was mw~ mw~ mw~.
Of course, the taped on wire looked tacky and I couldn't live with that so I moved the elevator pushrod out a 1/16 of an inch or so and there hasn't been a stall since (where is the knock on wood smiley???). There was, by the way, almost no difference in input required that I can recall from doing so and no significant difference in the ability to fly corners as tight as I'm capable of.
Ted
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Delete this if I muddle up this excellent thread.
Generally, the CG will be somewhere forward of the CL (center of lift). Moving the CG closer to the CL makes the airplane more maneuverable, needing less force to rotate the aircraft around the lateral axis, but also makes it less stable.
I would think that at some point, the increased maneuverability would be offset by the instability, leading to over-controlling attempting to fly smooth maneuvers.
Bill,
On a more or less conventional canard the CG "must" be well behind the wing (the front lifting surface...i.e. the canard). As the tail grows larger and larger in comparison with the wing the aft most "stable" CG moves with it. As the aft surface approaches the area of the front surface the useable CG range quickly moves aft with it.
The result for a normal stunt ship design is that, if you want, you could make the tail any size and be able to fly it with stability in level flight. When you start to maneuver in the pitch axis, however, lift generated forward of the CG will accelerate the pitch change (positive pitching moment) and (I think) make stopping turns and flying constant radius corners and/or round maneuvers more demanding, especially in the wind as the control inputs to maintain a constant rate pitch change will vary as well...more or less the opposite of the "enhanced" stability we used to seek using forward CGs. Ever seen a competitive aerobatic canard?
Ted Fancher
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One thing Gary Letsinger once told me was that sometimes the corner can be tightened by shifting the CG forward. Theory is this requires more control input, which brings more wing flap to bear. I suspect this would be only possible on heavy models and of course, modern adjustable controls would probably make this the wrong thing to do. But then, not everybody incorporates adjustable controls.
I mentioned both sides of this earlier in the thread. As far as I can tell, Al Rabe first figured this one out, it was in the Bearcat article. It does require more line tension, particularly before large bellcranks were common. You only need it on heavier models, of course, because the lighter ones don't run out of lift.
Brett
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....
The heavy airplane ends up flying at a higher angle of attack for a given turn radius, but I think my ratio holds. The required pitch rate at "equilibrium" is actually lower on the heavier but slower airplane, you end up needing less deflection to achieve it. In fact the same theory can hold until your AoA in the corner exceeds the stall angle.
Brett
Nice thing about flaps. When the flaps go down the AOA of the wing automatically goes up, even if the AOA of the plane doesn't necessarily change.
Found an applicable quote from Brian Hamton:
"It would seem the smallest possible flap deflection to produce the desired lift would be the best solution.
I quite agree. The model I designed had the flaps as part of the airfoil section but also had fully adjustable (independent) movement from zero to around +/-30 degrees. With flaps set at zero there was a trace of stall in the last corners of both triangle and hourglass even with a very light wing loading of 10.25 ounce/sq foot. Flaps set at +/-5 degrees eliminated the stall and kept added drag to a minimum to minimise loss of airspeed in hard turns. Higher wing loading would have required a bit more flap movement on this model."
Brian has shown an unusual flap horn arrangement that allows the flaps and elevator movement to be adjusted independently with virtually no interaction. Seems we need to be building better adjustable controls for trimming. They would solve most of the problems discussed here.
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I mentioned both sides of this earlier in the thread. As far as I can tell, Al Rabe first figured this one out, it was in the Bearcat article. It does require more line tension, particularly before large bellcranks were common. You only need it on heavier models, of course, because the lighter ones don't run out of lift.
Brett
Remember the pictures of Al's modified Hot Rock handles with up to six inch spacing to "increase leverage"? Given we depend on line tension no matter how much mechanical advantage we think we gain at the handle would such extensions have particular value unless inputs are accompanied by an aggressive shortening of the lines (pulling back while rotating the handle) to achieve an advantage??? Sure seemed to work for Al but it kind of goes counter to this thread. Also, Al used three inch bellcranks for most of his competitive career although I heard a rumor he might have gone to the now preferred four inch ones.
Anybody know??
Ted
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You can't get leverage by handle spacing alone.
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You can't get leverage by handle spacing alone.
Also as mentioned above. Of course, if you move the CG forward, you will need to increase the spacing just to get the sensitivity back. You don't get any more torque at the bellcrank. Once the slack line is really slack, it might as well be disconnected.
Brett
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Remember the pictures of Al's modified Hot Rock handles with up to six inch spacing to "increase leverage"?
I decided to omit that part of the discussion. There are enough people mad at me on the average day that I don't need to seek them out...
Also, Al used three inch bellcranks for most of his competitive career although I heard a rumor he might have gone to the now preferred four inch ones.
Anybody know??
4 or bigger, last time I discussed it with him several years ago. Maybe he invented it.
Brett
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Gimme five.
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Gimme five.
Randy <------ has 4.5 inch ones , I used in 2 planes, hard to get larger, and keep the control feel I want.
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Gimme five.
Problem is how to fit it in there, and still have something solid to connect it to. I make it as big as will fit without extraordinary effort to connect it to the spar. 4 1/4 seems OK, and it works well with the small Ted handle. I am just about in the middle of the adjustment range.
Brett
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I did a study awhile back comparing 4", 5", and 6" bellcranks, holding mechanical advantage over the control surfaces constant, to see which had the leadouts sweep the least swath out of the wing for the full leadout range. The 6" one allowed the spar shear web to extend farthest, but it's a squeeze to fit it into the wing and keep it close to the CG. I am putting a 4" bellcrank in my new dog. It's easy, and it's the devil you know.
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I did a study awhile back comparing 4", 5", and 6" bellcranks, holding mechanical advantage over the control surfaces constant, to see which had the leadouts sweep the least swath out of the wing for the full leadout range. The 6" one allowed the spar shear web to extend farthest, but it's a squeeze to fit it into the wing and keep it close to the CG. I am putting a 4" bellcrank in my new dog. It's easy, and it's the devil you know.
We will have to have Rich Porter discuss this with you.......If you can get a word in edgewise!
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I did a study awhile back comparing 4", 5", and 6" bellcranks, holding mechanical advantage over the control surfaces constant, to see which had the leadouts sweep the least swath out of the wing for the full leadout range. The 6" one allowed the spar shear web to extend farthest, but it's a squeeze to fit it into the wing and keep it close to the CG. I am putting a 4" bellcrank in my new dog. It's easy, and it's the devil you know.
What does the 5" and 6" bellcranks feel like?
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What does the 5" and 6" bellcranks feel like?
Kinda big, rigid & triangular? LL~
OK OK... my guess it feels like going from a 3" to a 4", only more so. My question is this: When running a 4" bc, we tend to think of 4" handle spacing as 1 to 1... or at least I think of it that way. So, does a 5" BC point towards a 5" handle if you only want the additional leverage with the same feel as before without having to waive your arms all about?