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Author Topic: Stab Aerodynamics question??  (Read 3231 times)

Offline Doug Moon

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Stab Aerodynamics question??
« on: February 18, 2021, 12:28:02 PM »
Hello,

I am starting a new plane. There are a few things I want to change.  The stab may be one of them.
As long as I can remember I have used 1/2" thick flat stab with molded round LE and TE. So far it has served me well.

I have seen over the past several years the move towards a stab with a very sharp LE back to a flat stab.  Not sure on the TE.

Way back in 2000 I had a Saturn with a slight hunt that was giving me fits from time to time.  It was a good model but that hunt was bugging me.  I read David's article again and again and tried a few ideas from there, trip strips, wire on the LE and nothing worked.  Then I taped 1/4" triangle stock to the LE.  This was also a 1/2" flat stab with round LE and TE.  Nothing else was changed on the plane during the tests so I could really see the differences. The triangle stock locked that plane in like a laser.  It was "dead nuts" level.  No hunting.  At whatever altitude I flew it at it was dead on. In the rounds it was locked in. I tried the squares it took alot of stick pressure to get it out of the level flight and at about 60% of the turn it shuddered violently and flew off track. I almost lost the plane. It was scary.  Luckily the first square turn was an upward turn or it would have been in the ground. I flew to a much higher altitude and tried several more square maneuvers and it did the same thing at the same places every time.

It appeared to me that sharp LE on the stab was creating a separation point that would cause the air on the back of it to snap off at higher AoA and it would stall.  That was my conclusion but I am sure it more complicated than that.

I did a couple of flights with this to make sure it wasn't just a fluke.  It acted the same every time. Locked in level and rounds. Lots of stick pressure needed to get out of the level flight.  Square corners caused what appeared to be a violent stall.

I removed the 1/4" triangle stock from the front of the LE and it went back to it's old self. After many adjustments on the pushrod and the handle I was able to get that plane to fly really well until it's untimely demise.

I want to build a new stab with a sharp LE but this experiment all those years ago has scared me off of doing it.

Why did this happen?
Why did it not work like the stabs you see today?
Does the round TE have anything to do with it?

Thank you
Doug Moon
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Online Dave_Trible

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Re: Stab Aerodynamics question??
« Reply #1 on: February 18, 2021, 01:03:40 PM »
Doug I know the pointy stab is supposed to be the 'bees knees' but I didn't like it either-about the same experience as yours on three different airplanes.  I struggled with it through the Nats two years ago and finally built new round leading edge stabs to replace them and they once again became manageable .  These airplanes weren't real light and likely required a little more deflection than a light one would.  Frank Williams thought the sharp stab was driving the airplane at a higher AoA and maybe causing stall of the wing.  I had been thinking the stab was stalling.  In any case I'm done with them...

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Online Ken Culbertson

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Re: Stab Aerodynamics question??
« Reply #2 on: February 18, 2021, 02:19:37 PM »
My experience with pointey stabs been really good with airfoil stabs and not so good with flat stabs.   
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Offline Dennis Toth

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Re: Stab Aerodynamics question??
« Reply #3 on: February 18, 2021, 02:25:21 PM »
Maybe what is needed is a small radius that flares into the flat stab rather than the sharp triangle shape. Also did you have the hinge line sealed?

Best,    DennisT

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Re: Stab Aerodynamics question??
« Reply #4 on: February 18, 2021, 02:38:23 PM »
Maybe what is needed is a small radius that flares into the flat stab rather than the sharp triangle shape. Also did you have the hinge line sealed?

Best,    DennisT

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Offline Scott Richlen

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Re: Stab Aerodynamics question??
« Reply #5 on: February 18, 2021, 02:45:47 PM »
I rebuilt one of my older stunt ships last year.  The foam wing had held up very well over the years but the cowl had gotten pretty fuel soaked and beat up and the fuselage needed attention.  I figured if I am doing all of that I might as well try a new stab.  I built a StarGeezer stab and elevator per plans and used it to replace the old stab.  I've only gotten to fly it a few times thanks to our Chinese Commie friends and their pet running-dog-lackey Covid.  But in the few flights I got I was impressed.  It turns scary sharp in the squares and seems to lock in on the levels.  I'm happy with it.

Offline Trostle

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Re: Stab Aerodynamics question??
« Reply #6 on: February 18, 2021, 03:26:19 PM »
  I built a StarGeezer stab and elevator per plans and used it to replace the old stab.

Maybe I am the only one who does not know, but there could be others --  what do those StarGeezer stab and elevator plans show?

Keith


Offline bob whitney

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Re: Stab Aerodynamics question??
« Reply #7 on: February 18, 2021, 03:46:50 PM »
i had a Scorpio (still being used by my flying Buddy ) it had a 3/8 flat slab stab and elevator rounded front and back .it flew the pattern better than i could but would wonder on level flight  upright and inverted .after a year of playing with it i tapered the elevators to about 1/16 and it took care of all the wondering .
i figured that being tapered the elevator had to move some before it took affect .is my thinking correct??
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Re: Stab Aerodynamics question??
« Reply #8 on: February 18, 2021, 09:06:36 PM »
Maybe I am the only one who does not know, but there could be others --  what do those StarGeezer stab and elevator plans show?

Keith

Yeah, what Keith said!
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Online Crist Rigotti

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Re: Stab Aerodynamics question??
« Reply #9 on: February 18, 2021, 09:09:57 PM »
Doug,

At the Nats, the sharp stab LE I've seen on Paul's airplanes, they were sharp but flowed nicely into the stab.  A nice some what gentle taper.  Not a triangle shape at all.  Maybe that was your problem?
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Offline Air Ministry .

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Re: Stab Aerodynamics question??
« Reply #10 on: February 18, 2021, 09:26:55 PM »
Was the hinge taped or untaped. And was there a bit of a gap, like 1 mm or more, there .

Youve effectively got a sharp split 90 / 45 degree ' seperation point , with the tri stock. On a L E itll break off the top flow ( on hard inside turns ) but still go round sitting on the air under .

Ifits doing the same , might be a bit wierd & varyable, aft . A touch of sandpaper , so theres a 1/8 or 5/32 diameter dowel, in effect , at the front . with a radius from that into the flat ,
is about a standard sharpish L E . or at least it has some radius rather than a knife edge. For Aerobatics . A 45/90 wouldnt be very good on a knife , but its still a ' Hard Edge ' unless you softened it .

Offline Chuck_Smith

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Re: Stab Aerodynamics question??
« Reply #11 on: February 19, 2021, 05:11:58 AM »
I suspect the triangle caused a separation bubble and the previously unstable flow over the stab now moved smoothly over the bubble. In effect, you use the entrained vorticity to create laminar flow above it.

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Offline Joe Ed Pederson

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Re: Stab Aerodynamics question??
« Reply #12 on: February 19, 2021, 12:43:12 PM »
The Dave Fitzgerald article on leading edge shapes (sharp/blunt/in between)for stabilizers is in the March/April 1998 issue of Stunt News.

https://stunthanger.com/smf/open-forum/copy-of-the-david-fitzgerald-article-'de-tails'-from-the-marapr-98-stunt-news/msg511851/#msg511851

Well worth reading.

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Offline Alan Resinger

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Re: Stab Aerodynamics question??
« Reply #13 on: February 19, 2021, 01:49:33 PM »
I use the very sharp leading edge on my stabs as does Paul , Chris Cox and others.  That being said, we fly electric.  In the past when l flew IC, l  had one ship l built with a sharp LE and it hunted until l blunted the egde.  The sharp LE seems to be favored by electric flyers along with forward CG locations  and rearward leadout locations.  Don't ask me why, it just works better.

Online Ken Culbertson

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Re: Stab Aerodynamics question??
« Reply #14 on: February 19, 2021, 02:05:42 PM »
The Dave Fitzgerald article on leading edge shapes (sharp/blunt/in between)for stabilizers is in the March/April 1998 issue of Stunt News.

https://stunthanger.com/smf/open-forum/copy-of-the-david-fitzgerald-article-'de-tails'-from-the-marapr-98-stunt-news/msg511851/#msg511851

Well worth reading.

Joe Ed Pederson
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Thanks for posting that.  I have been looking for support for my airfoiled stabs since I live in the "flat stab lands"  I use David's "C1".  It had a 1/16" radius when it was IC and since going to electric I keep it really sharp.  On both on the ships that I sharpened the "LE" after flying them tracking improved.  One was electric, the other IC but the improvement on the electric was greater.

Ken

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Online Dave_Trible

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Re: Stab Aerodynamics question??
« Reply #15 on: February 19, 2021, 02:39:33 PM »
I use the very sharp leading edge on my stabs as does Paul , Chris Cox and others.  That being said, we fly electric.  In the past when l flew IC, l  had one ship l built with a sharp LE and it hunted until l blunted the egde.  The sharp LE seems to be favored by electric flyers along with forward CG locations  and rearward leadout locations.  Don't ask me why, it just works better.
This starts to make sense to me-especially the more forward CG with electric.  That would be less likely to stall.  Alan you may have provided us a clue....

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Online Matt Colan

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Re: Stab Aerodynamics question??
« Reply #16 on: February 19, 2021, 03:18:12 PM »
I use the very sharp leading edge on my stabs as does Paul , Chris Cox and others.  That being said, we fly electric.  In the past when l flew IC, l  had one ship l built with a sharp LE and it hunted until l blunted the egde.  The sharp LE seems to be favored by electric flyers along with forward CG locations  and rearward leadout locations.  Don't ask me why, it just works better.

Here’s something to throw a wrench into the fire. I have a Staris, with a tapered stab to a pretty small radius leading edge. That plane has a tendency to hunt a lot more in level flight than any of my other planes. The next plane was my SV-11 which has a flat stab and semi sharp leading edge. The plane was very well locked in everywhere in flight and you could fall asleep in inverted and level flight and it would stick. Dracula has the sharp leading edge and is the best tracking airplane I have by far. Each time I’ve gone to progressively sharper leading edges on the stab, the airplanes have tracked better. My new one is just as sharp as Dracula (sharper if anything) so I don’t foresee any reason I should see different results
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Online Ken Culbertson

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Re: Stab Aerodynamics question??
« Reply #17 on: February 19, 2021, 03:38:36 PM »
Here’s something to throw a wrench into the fire.
Please, no fire.  ~^ The wrench will melt. :'(

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Online Crist Rigotti

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Re: Stab Aerodynamics question??
« Reply #18 on: February 19, 2021, 06:31:24 PM »
Please, no fire.  ~^ The wrench will melt. :'(

Ken

LOL!
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Offline Dennis Toth

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Re: Stab Aerodynamics question??
« Reply #19 on: February 20, 2021, 06:25:05 AM »
Great discussion. Since there seems to be no one hard and fast rule that works for all designs or even ships of the same design, it makes a good case for building with a take-a-part stab/elevator. This would allow trying several arrangements (ala Dave F's testing) to find which one works for a particular ship. Once found one could make it permanent. On electric one could do the testing before final finishing since no oil on the surface to mess up the finish.

Best,    DennisT

Online Ken Culbertson

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Re: Stab Aerodynamics question??
« Reply #20 on: February 20, 2021, 10:18:03 AM »
Great discussion. Since there seems to be no one hard and fast rule that works for all designs or even ships of the same design, it makes a good case for building with a take-a-part stab/elevator. This would allow trying several arrangements (ala Dave F's testing) to find which one works for a particular ship. Once found one could make it permanent. On electric one could do the testing before final finishing since no oil on the surface to mess up the finish.

Best,    DennisT
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Online Brett Buck

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Re: Stab Aerodynamics question??
« Reply #21 on: February 20, 2021, 01:29:05 PM »
This issue is the best illustration ever of the problem of "cut-and-try" to test design features. You/we are trying to pick out a tiny signal out of a giant mass of noise - along with textbook examples  of "confirmation bias". I am not trying to pick on anyone, because I am doing the same thing.

     The problem is that we are comparing different airplanes with different stab airfoils, and drawing conclusions about how they fly - they hunt, indistinct around corner exit, etc.  - and just assuming that the stab airfoil is difference because that's what we are looking at - and not the thousands of more important things what we aren't looking at or controlling. That's why it is so hard to make good conclusions simply by observation.

     Brett

Offline Chuck_Smith

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Re: Stab Aerodynamics question??
« Reply #22 on: February 20, 2021, 02:16:17 PM »
This starts to make sense to me-especially the more forward CG with electric.  That would be less likely to stall.  Alan you may have provided us a clue....

Dave

Hmmmm. Not sure about that one. The further forward the CG the more the tail needs to "push" so seems like it would stall earlier with a forward CG.

And in real life Ted and the other pilots will tell you that a a plane with the CG forward does in fact, stall at a higher airspeed than the same one with the CG shifted aft. Pretty easy to work out why.

I suspect that most "hunting" on stunt planes is not because of the stab leading edge, but the fuselage, and the wake of the wing. So all the "fixes" that we do address the symptoms and not the problem.
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Offline Ted Fancher

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Re: Stab Aerodynamics question??
« Reply #23 on: February 20, 2021, 03:44:19 PM »
While I'm well known for pontificating about aero "stuff" without a degree in aerodynamics, nonetheless into the breech!

If you're not flying an SR71 at supersonic speeds you should probably not be looking at a razor sharp leading edge on either the wing or tail...especially of an aircraft flying in circles at more or less 50Knots or so...and even more especially not if the aerodynamic surface to which it is attached is flat...re no air'foil' involved.

The leading edge of a lifting surface only does a handful of things based on its shape but the primary one is to divide the airflow above and/or below the surface to which it is attached to provide lift (either to "get the ship in the air and keep it there" or...in the case of tails...provide a lifting force in either up or down direction so as to "fly" the tail "about" the CG of the aircraft to which attached and thus alter the angle of attack of that aircraft's primary lifting surface...generally, a wing.

Airfoil on the wings of "real" airplanes are pretty much never "flat".  They have curved surfaces top and bottom so as to allow/utilize the airflow over (or under in wings and tails of aerobatic machines) and their leading edges are generally modestly rounded (unless it's one of the SR-71 sorta ships...which our stunters aren't).  For the lifting surface to do its job the air through which it flies must be divided to pass above and below in an orderly fashion.  By and large we've learned that in order for the lifting surface to do its job it is desirable to have the upper and lower surfaces curved...and then call it an air"foil".  They generally do this to aide/assist/assure that the airflow above and below the the surface remains "attached" to that surface at at least the angles of attack necessary to achieve the design intent of the airplane to which attached.

They've generally come to the conclusion that airplanes intended to perform tricks in the pitch axis at modest airspeeds need to have surfaces configured to keep that airflow "attached" to the surface and delay the inevitable stall to an angle of attack adequate to perform those tricks without stalling.

A "sharp" leading edge isn't conducive to delaying the surface's stall especially at modest airspeed...i.e. 50-60MPH for instance.

A "flat" surface behind either a sharp or rounded leading edge isn't conducive to delaying the surfaces stall, especially at modest...oh, wait...just said that.  Sorry. 

IOW, the sharper-flatter a lifting surface is the poorer it will perform the functions necessary for aerobatic flights at modest speeds (see Brett's oft repeated discussion of the improved performance of the famous design ARF with the sharp leading edge that stalled dramatically when asked to corner briskly but...after rounding the leading edge in less than Concours fashion...performed very respectfully...posted frequently on SH.

What we don't want on a stunt ship are design elements that will predictably reduce the angle of attack at which the lifting surfaces stop doing that..."lifting"...that is!  Both a sharp leading edge and a flat surface are a step in that direction!  As in so many things, the solution is moderation.  A modestly rounded leading edge attached to a modestly airfoiled lifting surface of adequate area; flown at the speeds we fly is a low tech but sensible sure road to a functional machine.


Offline Paul Walker

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Re: Stab Aerodynamics question??
« Reply #24 on: February 20, 2021, 07:09:13 PM »
It took me a few minutes to decide how to answer your post Ted. I have read MANY of your posts in the past and enjoyed most all of them. Yes, there were lots of words there. This one just hit nerve though.

In my days at work, when it came time to certify an airplane, full scale tests were done for verification. As one Air Force representative told me, emperical data from test trumps analytical data. My friend, your post had neither any analytical data or empirical data to base your conclusion on.

Before I committed to this route, I tested them. I built a removable stab/elevator assembly that could be fitted to al last three different planes. They varied in weight, and geometry. The result in each case was the same. A noticable improvement in track, both in level flight and in rounds, and the corners had a better turn and stop. I have had them in every plane since.

I have had other quality fliers try it, and none have gone back to their rounded LE stabs. It apparently works.

So, I have empirical data that say it works. I will leave it at that!

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Re: Stab Aerodynamics question??
« Reply #25 on: February 20, 2021, 07:40:36 PM »
HOW SHARP is this here leading edge ? . Any Pictures , Please.

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Re: Stab Aerodynamics question??
« Reply #26 on: February 20, 2021, 09:33:09 PM »
Like this. One of Matt's.
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Re: Stab Aerodynamics question??
« Reply #27 on: February 21, 2021, 05:30:51 AM »
So, I've tried to avoid talking about circulation because it causes the folks like High School or undergraduate science teachers that still believe Bernoulli's Principle causes an airplane to fly's heads to explode - but here goes:

What happens at the leading edge is a direct result of what happens as the trailing edge.  The incoming direction of the air is not straight into the surface. In aerodynamics we talk about the "stagnation streamline1", the streamline that represents the path of an air molecule that hits the incoming surface  so dead nuts square to the surface that it would stop, rather than deflect over the the top or under the bottom of the body.

When the surface changes velocity, AoA, or defects it sheds a vortex off the trailing edge2. Since our little plane is flying significantly slower than the speed of sound the flow field around the surface - in this case the stab - gets warped and the incoming stagnation streamline now moves up or down on the leading edge. And it is for this reason we want a round leading edge. Were it sharp the streamline would flip to one side and possibly cause a stall or at the very least a non-linear control response. When the leading edge is round, the stagnation streamline can move smoothly up or down on the leading edge allowing smooth control.
 

Nobody here would ever propose a sharp LE on the wing. The horizontal tail is not imbued with any magical quantities that make the air it's flying through react any differently than the wing.

To add to Ted's post, I'm sure he and others have noticed "stall strips" on wings of full scale aircraft near the wing root . These are basically triangular protrusions added onto the leading edge that add pointy bits to said LE, their purpose being to initiate the stall sooner. Their purpose is to have the inboard portion of the wing stall first allowing the pilot to maintain roll control and avoid a stall-spin accident. That right there should be enough to convince you rounder is better.


1.And again, I'm using a 2D model to explain so remember that in real life things may be more complicated, but in this case it's a good way to visualize the effects of circulation. BTW, if you're a real geek you should know that his circulation is usually represented mathematically as capital Gamma. Unless there's a non-trivial representation of the greek alphabet it's not real aerodynamics, lol.

2.If by now I've piqued your interest you might want to do a search on the "Kutta condition".



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Offline Paul Walker

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Re: Stab Aerodynamics question??
« Reply #28 on: February 21, 2021, 09:29:36 AM »
So, I've tried to avoid talking about circulation because it causes the folks like High School or undergraduate science teachers that still believe Bernoulli's Principle causes an airplane to fly's heads to explode - but here goes:

What happens at the leading edge is a direct result of what happens as the trailing edge.  The incoming direction of the air is not straight into the surface. In aerodynamics we talk about the "stagnation streamline1", the streamline that represents the path of an air molecule that hits the incoming surface  so dead nuts square to the surface that it would stop, rather than deflect over the the top or under the bottom of the body.

When the surface changes velocity, AoA, or defects it sheds a vortex off the trailing edge2. Since our little plane is flying significantly slower than the speed of sound the flow field around the surface - in this case the stab - gets warped and the incoming stagnation streamline now moves up or down on the leading edge. And it is for this reason we want a round leading edge. Were it sharp the streamline would flip to one side and possibly cause a stall or at the very least a non-linear control response. When the leading edge is round, the stagnation streamline can move smoothly up or down on the leading edge allowing smooth control.
 

Nobody here would ever propose a sharp LE on the wing. The horizontal tail is not imbued with any magical quantities that make the air it's flying through react any differently than the wing.

To add to Ted's post, I'm sure he and others have noticed "stall strips" on wings of full scale aircraft near the wing root . These are basically triangular protrusions added onto the leading edge that add pointy bits to said LE, their purpose being to initiate the stall sooner. Their purpose is to have the inboard portion of the wing stall first allowing the pilot to maintain roll control and avoid a stall-spin accident. That right there should be enough to convince you rounder is better.


1.And again, I'm using a 2D model to explain so remember that in real life things may be more complicated, but in this case it's a good way to visualize the effects of circulation. BTW, if you're a real geek you should know that his circulation is usually represented mathematically as capital Gamma. Unless there's a non-trivial representation of the greek alphabet it's not real aerodynamics, lol.

2.If by now I've piqued your interest you might want to do a search on the "Kutta condition".

I am quite aware that the main lifting surface need to have  nice ROUND LE.  Mine clearly do.  You have taken this further than Ted, by adding some analytical information.

What is missing is your empirical verification of this. Have you ever put a stab of the pointy LE variety in one of your stunt planes?  How did it feel? Did this verify the theory you explained?  You should also ask Igor about his stab. It is not as sharp as mine, but has a small radius at the LE.

My inputs are based on real world data. This sharp LE has done more for making a linear feel than many other things I have tied. The real world tests on the original 3 test vehicles was very conclusive. It worked on a very light plane, on a medium weight plan, and a heavier plane. The difference on each was easy to feel. The turn and stop on the light plane was simply amazing.

Once again, my empirical data shows that it does work.


Someone asked how sharp. I embed a carbon strip at the CL of the LE  and shape it to that. Before paint, it is likely sharp enough to shave with. Then after paint, it has a slightly larger radius due to the thickness of the paint.

Online frank williams

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Re: Stab Aerodynamics question??
« Reply #29 on: February 21, 2021, 10:55:33 AM »
There is also analytical data that shows that it works.

The triangles and circles in the charts are measured data .... the red dashed line is theoretical from a math model.

The thing to note is the fact that the reversed airfoil, with extremely "sharp" end pointed into the wind, has a very linear passage through zero angle of attack.  The rounded end forward has some wanderings around zero before it settles on a slope.  Yes the Re number for this data is less than our applications, but I suggest that its still applicable data.

Now you are going to say that, "I know at 10 or 15 degrees a sharp stab is gonna stall like an sob.  Who says that the stab ever "sees" an angle of attack that high?  Remember in a sharp turn the tail is being rotated very rapidly in a direction that reduces the aoa.   Save that for another time.

Empirically, yes I have tried it, .... it works.  I went for a year of feeling Paul's stabs at the Nats before I tried it.  It doesn't stall.  I was somewhat surprised.   The feeling I got was just what the charts show, a more rapid initiation of the turn with no degradation in tracking.

Offline Chuck_Smith

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Re: Stab Aerodynamics question??
« Reply #30 on: February 21, 2021, 11:07:45 AM »
Paul,

That's pretty neat. Without a lot of pictures and math, when you sharpen the LE enough it starts behaving like a thinner airfoil than it really is (in some respects). Again, from an aero point of view it matters not whether it's a stab or main surface or a rudder.

I don't doubt your evidence. It's sounds intriguing!  It implies that there's something else going on with your ships we haven't  identified and addressed. You are almost certainly creating a separation bubble aft of the LE and smoothing out the streamlines flowing over it. I also suspect that the extra drag that creates is adding a stabilizing moment and that could be helping.

An airplane is stable in pitch if the slope of moment vs AoA is negative. That's all there is too it. If the plane is hunting that means it's either a) not properly damped in pitch and you're really fighting the short period phugoid oscillation due to gusts or other perturbations, or b) something else is wrong somewhere.

A stable aircraft wants to return to it's trim condition - it's built in by the math. This is pretty basic aero stuff that's been understood for over 100 years.
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Re: Stab Aerodynamics question??
« Reply #31 on: February 21, 2021, 02:15:08 PM »
Without a lot of pictures and math, when you sharpen the LE enough it starts behaving like a thinner airfoil than it really is (in some respects).

That respect being higher Cl.  The pertinent phenomenon here is the behavior around zero Cl.

It implies that there's something else going on with your ships we haven't  identified and addressed. You are almost certainly creating a separation bubble aft of the LE...

Stop there.  Yep.  Igor posted a piece on the Preacher's board in 2009 explaining it.  Frank elaborated at the time. The sharp LE prevents the laminar-turbulent transition point from moving abruptly as elevator is moved a little, changing the stab-elevator Cl abruptly.

I also suspect that the extra drag that creates is adding a stabilizing moment and that could be helping.

Ted suspected that, too.  Drag effect is infinitesimal.  I posted some calculations showing that awhile back, but you can do them, too.

If the plane is hunting that means it's either a) not properly damped in pitch and you're really fighting the short period phugoid oscillation due to gusts or other perturbations, or b) something else is wrong somewhere.

b is the correct answer. 

I presume you forgot a comma before phugoid.  Do these airplanes have two pairs of complex roots?  Beats me.  Maybe something like the phugoid is involved, but I think Igor and Frank explain it. 
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Re: Stab Aerodynamics question??
« Reply #32 on: February 21, 2021, 02:24:04 PM »
It implies that there's something else going on with your ships we haven't  identified and addressed.
And mine and just about everybody else who has tried it.  Sometimes we get really bogged down in the science and forget to simply observe.  It is entirely possible that the reason many of us feel that the sharp LE works has nothing to do with why it does.  I use Paul's method of making the LE then allow the covering to wrap around it giving me something in-between Paul and Igor.  Personally I didn't see much change in hunting.  When you have a plane trimmed to fly straight and level with no control input it is not going to hunt on it's own but it will if you tell it to.  I think much of what we call hunting is simply poor level flight technique.  What I was thrilled at was how well it tracked in the round maneuvers and how much better it transitioned back to level in corners.

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Offline Ted Fancher

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Re: Stab Aerodynamics question??
« Reply #33 on: February 21, 2021, 06:15:50 PM »
It took me a few minutes to decide how to answer your post Ted. I have read MANY of your posts in the past and enjoyed most all of them. Yes, there were lots of words there. This one just hit nerve though.

In my days at work, when it came time to certify an airplane, full scale tests were done for verification. As one Air Force representative told me, emperical data from test trumps analytical data. My friend, your post had neither any analytical data or empirical data to base your conclusion on.

Before I committed to this route, I tested them. I built a removable stab/elevator assembly that could be fitted to al last three different planes. They varied in weight, and geometry. The result in each case was the same. A noticable improvement in track, both in level flight and in rounds, and the corners had a better turn and stop. I have had them in every plane since.

I have had other quality fliers try it, and none have gone back to their rounded LE stabs. It apparently works.

So, I have empirical data that say it works. I will leave it at that!

My bad, Paul.  My apologies.  (No worries.)

Just for the record, were the optimum stabs "flat" aft of the sharp leading edge in all cases or were some "airfoiled" (is that even a word???) and some flat?  I'm guessing the elevators were/are tapered aft of the hinges to a  more or less "sharp" trailing edge?

Ted

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Re: Stab Aerodynamics question??
« Reply #34 on: February 21, 2021, 06:50:35 PM »
My bad, Paul.  My apologies.  (No worries.)

Just for the record, were the optimum stabs "flat" aft of the sharp leading edge in all cases or were some "airfoiled" (is that even a word???) and some flat?  I

They were all flat aft of the transition from the point LE. That is around 1" aft of the LE.


'm guessing the elevators were/are tapered aft of the hinges to a  more or less "sharp" trailing edge?

Yes, they are tapered to around 1/8" at the TE.

Ted

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Re: Stab Aerodynamics question??
« Reply #35 on: February 21, 2021, 10:16:27 PM »
There is also analytical data that shows that it works.

The triangles and circles in the charts are measured data .... the red dashed line is theoretical from a math model.

The thing to note is the fact that the reversed airfoil, with extremely "sharp" end pointed into the wind, has a very linear passage through zero angle of attack.  The rounded end forward has some wanderings around zero before it settles on a slope.  Yes the Re number for this data is less than our applications, but I suggest that its still applicable data.

Now you are going to say that, "I know at 10 or 15 degrees a sharp stab is gonna stall like an sob.  Who says that the stab ever "sees" an angle of attack that high?  Remember in a sharp turn the tail is being rotated very rapidly in a direction that reduces the aoa.   Save that for another time.

Empirically, yes I have tried it, .... it works.  I went for a year of feeling Paul's stabs at the Nats before I tried it.  It doesn't stall.  I was somewhat surprised.   The feeling I got was just what the charts show, a more rapid initiation of the turn with no degradation in tracking.

    Mine looks like this, except it has the front rounded off to about a 1/16" radius and much thinner. The part that I think matters (after having done some experiments myself, and witnessing and analyzing the results from numerous other's tests, like David's documented tests, Ted and David's largely undocumented tests with flat stabs and turbulators, and Paul's various solutions) is that you maintain some amount of normal component of q as far back as you can.

    This is my analysis of the various experiences (back to Gid Adkisson's "Laser", which is where a lot of this started), I emphasize that this is merely my analysis of others experiences and a few of my own experiments (with various “slippers” over the stab/elevator of several airplanes). I don’t claim it will meet anyone else’s standards for “validity”, however it does seem to cover most examples for which the stab/elevator was the relevant factor and for which the experimenters were known to be sound and objective observers. It does meet my standards (low or otherwise):



   Thin flat stabs of low aspect ratio and radiused LE- thin means the pressure change at the corner is so abrupt that the separation bubble is always present and more-or-less in the same place, and, the vortex from the tips curls around and affects a significant fraction of the stab all the time, and adds normal force, or at least, turbulates the surface

   Moderate flat stabs - These have the issue with the separation bubble forming or not forming, because the radius is such that it doesn't always guarantee that it forms on both sides all the time, and either forms or goes away with tiny changes in AoA. If you make it low aspect ratio, the interference from the tip interferes with it some, and maybe helps. Even more so with low aspect ratio and a large LE sweep angle. High aspect ratio allows the "bubble/no bubble" effect because most of the stab is free of any influence from the tip. Even worse with tip plates, that interrupt the main vortex and replace it with a much smaller one that has nearly no effect at the LE

  Moderate flat stabs with turbulators - experiements show most of the problems if you have have the "bubble/no bubble" problem with the medium flat stab are eliminated if you put turbulators on it, top and bottom, right near where the radius ends and the flat begins. It is better than nothing to put them further back, but the further forward you put them, the better. My theory on why that is is that it forces those bubbles to form and be stable at the same place most of the time at most AoAs, in addition to deflecting a bit of normal force back due to viscous shear effects.

          The “moderate flat stab” comments and analysis are based on very extensive experiments by David, Ted, with me observing/commenting, and with me finding that my similar stabs saw no real problem and no improvement from turbulators, and Paul's observation of the same - not a problem. The big difference was that my stab and Paul's stabs were lower aspect ratio than those Ted and David were using, and also Doug's bear, who with tip plates, came a lot closer to infinite aspect ratio and gave a negative example (gleaned through comments on SSW). This was what lead up to the "De-Tails aticle, after David decided that, based on the analysis above, that he wanted to experiment with further variants and airfoils. It was already posted so I won’t belabor it.

As far as I can tell, David’s results with flat stabs and various LE’s were consistent with the “separation bubble” theory and I was able to pretty reliably predict the results using that theory, particularly the thin and triangle-shaped LE. The “smaller elevator” was no surprise to anyone, but that is a different topic.

So, to the extent I understand it, with Ted’s “pointy” rework of the second Trivial Pursuit stab, which is very close to if not identical to what Paul is currently using, and looks very similar to the one Craig Beswick posted earlier. Same theory works there – it’s *better*, because the abrupt change in shape is less abrupt – a slope leading into the discontinuity at the radius/flat transistion point and less likely for the bubbles to form/disappear with small changes in the AoA, although apparently, still not immune because on both Ted’s model (wire turbulators) and Pauls (zigzag strips) it still helps, but perhaps not as much as turbulators on the flat stab with a blunt radius. The gadgets help by stabilizing the existence and position of the separation bubble.

Also from David’s article – the airfoiled C1 stab. This was at my suggestion and more-or-less straight off the second Infinity plan, with the exception the squared-off corners at the hinge line (another thing I shamelessly ripped off from Paul). This was designed using the theory of separation bubbles from the various flat-stab experiments, trying to keep at least some positive slope until I couldn’t avoid it any more, and eliminating the bubbles at least at low AoA where it matters most to tracking and corner exit. Effectively, this extends the point section that is more-or-less “tacked on” to the front of the otherwise flat stab all the way to the hinge line, so, similar idea just taken to the obvious conclusion. Note it is also very similar to the Green Box and ’52 Nobler plans, particularly once you round off the hinge lines.

   I note that this also explains why, so far, I haven't had *any* significant positive effect with tripper strips, VGs, or turbulator wires, turbulator wires being a moderate *negative* and the others a wash. David had very positive effect with tripper/zigzag strips, but he also didn't round of his corners at the hinge line, so that may be a different (also very interesting) issue.

Again, C2 the same thing with the thinner stab, which predictably had poor tracking by running the elevator in turbulence.

The advantage to the flat stab, or variations, is that it is easy to align. The second half the theory is that, after a lot of examples slapping us in the face, there is a fair bit of tolerance in the alignment in one direction (positive) and not the other (negative). However, if you have a flat stab, it is far, far more critical, because you can’t have it running at a tiny positive or negative angle of attack due to the same separation bubble theory – that’s where it is the most critical. So, with an airfoiled stab as shown, you can’t align it as well, but you also don’t have to.

Of course, if make it removable in the most convenient way, you can also experiment with the alignment instead of having to glue it in place at construction and then hope it never changes - in my case by building the fuselage like a bridge girder, and in other cases just crossing your fingers. The alignment (positive/negative) was barely mentioned in David’s article but was a much bigger effect than the stab airfoil, with the flat stabs super-critical and the airfoiled stabs much less so – as long as you never put in even the slightest breath of “negative”. David actually ended up with FAR MORE positive than I ever would have had the guts to build in, inadvertently leading some others (particularly with flat stabs) to grief – the sort of grief that leads to the use of Zona saws.

    Frank's data tends to confirm the situation I envisioned, he posted a picture earlier that was nearly identical to what I drew out in the pits at the old Napa field when we were doing the flat stab turbulator experiments.



   So, anyone can throw darts if they will, but I was able to predict what was going to happen and what seems to happen in most of the variations.

     Brett

Offline Scott Richlen

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Re: Stab Aerodynamics question??
« Reply #36 on: February 22, 2021, 06:53:16 AM »
I have a really great suggestion!

Oh-Oh      ;D

I volunteer Brett to become a famous author!

Easy - see all he has to do is assemble all of his various posts on aerodynamics (if he wants he could also assemble all of his political posts - but that would be a different book) and then edit them to create the soon to be famous tome: "All You Ever Needed to Know About Model Aerodynamics - But Were Afraid to Ask".

Here's the process:
1) Identify all relevant posts in Stunt Hanger (and even Stuka Stunt if assessable)
2) Set them in relative order (this would be by various topics and sequence of explanation)
3) Edit out the repetitions
4) Get it reviewed by a non-engineer (or by an engineer who has mostly forgotten this stuff because he went into management)
5) Revise as necessary based on reviewers comments
6) Publish it (or maybe serialize it in Stunt  News....oh, wait...)

See, I've got the top-level part done for you and haven't even broken a sweat.  The rest should be straightforward.

Plus, you can cut into it all of the nice graphics by Frank Williams and others so you wouldn't have to mess with that part of it.

Great idea, right?  When can we...I mean, you...start?

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Re: Stab Aerodynamics question??
« Reply #37 on: February 22, 2021, 08:12:27 AM »
Easy - see all he has to do is assemble all of his various posts on aerodynamics (if he wants he could also assemble all of his political posts - but that would be a different book) and then edit them to create the soon to be famous tome: "All You Ever Needed to Know About Model Aerodynamics - But Were Afraid to Ask".

   How about All You Ever Needed to Know About Model Aerodynamics - But Were too classy to steal!".  Stealing, er, "being inspired by" ideas is certainly not beneath my dignity, so your can benefit from it with a clear conscience. I have a lot of issues but knowing how to rip off good idea is not one of them.

    Brett
« Last Edit: February 22, 2021, 02:34:48 PM by Brett Buck »

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Re: Stab Aerodynamics question??
« Reply #38 on: February 22, 2021, 08:58:19 AM »

   Moderate flat stabs - These have the issue with the separation bubble forming or not forming, because the radius is such that it doesn't always guarantee that it forms on both sides all the time, and either forms or goes away with tiny changes in AoA. If you make it low aspect ratio, the interference from the tip interferes with it some, and maybe helps. Even more so with low aspect ratio and a large LE sweep angle. High aspect ratio allows the "bubble/no bubble" effect because most of the stab is free of any influence from the tip. Even worse with tip plates, that interrupt the main vortex and replace it with a much smaller one that has nearly no effect at the LE

     Brett

Can you expand on the stab leading edge sweep comment a bit?  Is more sweep or less sweep preferable on a low aspect ratio stab?
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Offline Chuck_Smith

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Re: Stab Aerodynamics question??
« Reply #39 on: February 22, 2021, 09:26:44 AM »
frank, the data you show are making me curious. Generally on the 00xx series we see the exact opposite behavior around 0° AoA, more commonly other data are often showing the slope of Cl vs. AoA actually steepens slightly with lower Re. numbers.

Another thing to consider - the Cl vs. AoA for the section isn't applicable when discussing a stunt stabilizer/elevator combination. it would be for an all-flying tail but an elevator works differently.  What happens when we deflect the elevator is we alter the camber line, and that shifts the Cl vs AoA in either direction as well as changing the AoA for the "new" airfoil created. That creates the tail force in the direction required. It also creates a pitching moment, but lucky for us, wing's moment is much much larger than the tail's so the system remains stable.

The fun part is we get to opine and have peer review. Too cold in the basement to work on the new ship this time of year.

Well, the main thing is always use what works. I'm a proponent of round LE's and razor sharp TE's. Works for me. I say make the tail as thin as you can and still have it hold together.  Others have a system that works for them that may be vastly different.  For decades we've been beholden to the geometry of our control systems when it comes to horizontal tails. :)

Chuck




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Online Brett Buck

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Re: Stab Aerodynamics question??
« Reply #40 on: February 22, 2021, 10:49:07 AM »
Can you expand on the stab leading edge sweep comment a bit?  Is more sweep or less sweep preferable on a low aspect ratio stab?

  Preferable?  I think it tends to create even more interaction between the planform (particularly air swirling around and interfering with the other flow characterstics) than just having low aspect ratio. Whether that is preferable or not depends on what you are trying to do.

    I offer this as an explanation of the results with one of Paul Walker's experiments which we briefly discussed, specifically, the usual Impact "flat stab" except with a much higher LE sweep angle, and it's apparent better characteristics related to what we are talking about.

   My gut feeling is it was better in that case because it made it less likely to form/destroy the separation bubble by increasing the finite-span effects, and also probably by shortening the distance from the LE (where the problem starts) to the elevator (where you are stuck dealing with it in any case, since something far more dramatic happens at the hinge line). If nothing else, it makes absolutely sure that the same condition is less likely to occur at all points along the span at the same time, breaking up any sort of tendency to have dramatic changes with tiny variations in the AoA or elevator movement.

    One observation is that it seemed like the worst problem with the flat stabs were when they were also of reasonably high aspect ratio, even the "normal" range (like 5.5:1) . This makes at least some sense, because while just looking at section characteristics, all airfoils have about the same Cl vs AoA curve (~.1 Cl for 1 degree AoA), this starts to flatten out with finite span effects, and seems to start showing dramatic deviations in the range we currently use. Same as the "leadouts on wrong side" thread, the *drag* seems to start falling off a cliff at <5:1, presumably because you have to start cranking in more AoA to get the same Cl. This takes us all the way back to the mid-80s and Gid Adkisson's "Bud Light Laser" that started this whole line of reasoning.   

    Brett

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Re: Stab Aerodynamics question??
« Reply #41 on: February 22, 2021, 11:07:07 AM »
See, I've got the top-level part done for you and haven't even broken a sweat.  The rest should be straightforward.

Plus, you can cut into it all of the nice graphics by Frank Williams and others so you wouldn't have to mess with that part of it.

Great idea, right?  When can we...I mean, you...start?

   When I see my advance check.

     Seriously, the problem with any such book is that it is a moving target - even in the last few months or a year we have had fascinating new results, most of which are consistent with my understanding and reinforce my confidence, and a few that no one seems to have a good explanation for.

    But any of it is heavily compromised/complicated by the factor I noted above. If you work entirely by experiment, unless you are extremely careful, you are changing multiple things at once and have no idea what might work next. That's why I am very careful about whose observations I take, unless I am directly involved.

     Brett

Online Dave_Trible

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Re: Stab Aerodynamics question??
« Reply #42 on: February 22, 2021, 12:54:17 PM »
Brett you said something before that sort of gave me a DUH! moment.  Flat stabs being much more critical to alignment and also the floating aspect of the bubble.  I had forgotten about the positive things putting zig zag trip strips on a couple of my airplanes just past the curve on my rounded stab leading edges did.  Think I will find my pinking shears for my new planes.  My original 'Music" airplanes had an airfoiled stab and flew quite well.  My gripe at the time was getting them aligned as close to zero as I could guess.  This was before I had the incidence meter.  Simply to help make construction fool-proof for others I went to the flat stab.  Never was sure I liked it flying as well but stuck with the practice for simplicity.  I am starring at the drawing board now at a new design and thinking this may be a good time to return to a more shapely stab.  Might be better than trying new shades of lipstick for a flat stab.

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Offline dale gleason

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Re: Stab Aerodynamics question??
« Reply #43 on: February 22, 2021, 01:03:05 PM »
I've enjoyed flying my Impcats with Round Stab LEs and squared off elevator TEs. One day at the NATs I saw a worthy fellow with a swept-back stab LE on his Impact (not to be confused with an Impcat) which removed half of the stab area. I inquired why he did that...in layman's terms, please. His reply? "It flies better."

I had to have one! I built one. I took it to Muncie, where I noticed the Master of the Manor had returned to the straight stab. My crest fallen...I harkened back to a time long ago when my mentor had so eloquently spoken, in a soft voice..."These guys are good!"

Yes, they are, and I'm enjoying trying to keep up with them.

dg

 

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Re: Stab Aerodynamics question??
« Reply #44 on: February 22, 2021, 01:28:08 PM »
frank, the data you show are making me curious. Generally on the 00xx series we see the exact opposite behavior around 0° AoA, more commonly other data are often showing the slope of Cl vs. AoA actually steepens slightly with lower Re. numbers.

This is something weird that happens with the 0012 at low Reynolds number, 15,000 to 20,000 as I recall.  I saw it on XFOIL or Javafoil or maybe both, I forget.  The Stuttgart wind tunnel data shows it.
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Offline Ted Fancher

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Re: Stab Aerodynamics question??
« Reply #45 on: February 22, 2021, 04:38:35 PM »

My bad, Paul.  My apologies.  (No worries.)

Just for the record, were the optimum stabs "flat" aft of the sharp leading edge in all cases or were some "airfoiled" (is that even a word???) and some flat?  I

They were all flat aft of the transition from the point LE. That is around 1" aft of the LE.


I'm guessing the elevators were/are tapered aft of the hinges to a  more or less "sharp" trailing edge?



Yes, they are tapered to around 1/8" at the TE.

Thanks, Paul.

Last queries: 1l Are the transition sections from the sharp leading edge to the flat stab surface "airfoiled" or "flat"?  2. What is the overall thickness of the flat sheet stab prior to final "shaping"?

Ted




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Re: Stab Aerodynamics question??
« Reply #46 on: February 22, 2021, 06:29:27 PM »
Ted,

The flat stab is 1/2 inch thick. The pointy LE  tapers to the 1/2 inch constant section with a smooth round transition about 0.80 inch aft of the LE.

Hope this describes it well enough.

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Re: Stab Aerodynamics question??
« Reply #47 on: February 22, 2021, 06:39:20 PM »

Here is a photo of the geometry of the HT LE.

It reaches the full stab thickness in 0.80"


Here is a link to a previous thread on the this topic.  Paul attached this picture showing the leading edge curvature:
- https://stunthanger.com/smf/open-forum/walker's-stab-leading-edge/msg547251/#msg547251

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Re: Stab Aerodynamics question??
« Reply #48 on: February 22, 2021, 07:00:09 PM »
Here is a link to a previous thread on the this topic.  Paul attached this picture showing the leading edge curvature:
- https://stunthanger.com/smf/open-forum/walker's-stab-leading-edge/msg547251/#msg547251



So much great info here!!!!  This is the kind of discussion I was hoping would happen.  THANK YOU!!

After looking at Matts in person and the pics of the drawing.  I think that slight "foil" slope back to the flat area is a huge key to the equation.  When I taped on triangle stock there was no transition and also there was a bump where the 1/4" triangle butted up against 1/2" LE.  This conversation tells me that there was no way the air could stay "attached" with that setup.  With the ever so slightly "foil" you get what Frank posted...

Paul,

What does the trailing edge look like?
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Offline Paul Walker

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Re: Stab Aerodynamics question??
« Reply #49 on: February 22, 2021, 07:45:49 PM »
TE of stab, 1/4 inch radius.


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