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Author Topic: Flat vs Airfoil Stab  (Read 2488 times)

Offline Ken Culbertson

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Flat vs Airfoil Stab
« on: August 20, 2018, 07:35:14 AM »
I have been researching this issue for about an hour and I can't find a thread that goes into the pros and cons of a flat stab/elevator vs an airfoil.  I have always used an airfoiled set but most every thing I see at the field recently has flat.  Can anyone point me to a discussion of the differences in them.  I am sure this subject has been beat to death at least once.

Thanks - Ken
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Offline Tim Wescott

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Re: Flat vs Airfoil Stab
« Reply #1 on: August 20, 2018, 08:36:29 AM »
Synopsis of the discussion, from Fancher/Buck/Walker, as best as I can remember.  Note that all of this is hearsay:

It's more important that the LE of the stab have a low radius than the stab be airfoiled or not.  No, I don't know why, but the top guys report hunting with a rounded LE stab, and improved flight characteristics with a small LE radius.  Brett Buck, somewhere in the reams of posts on this site, talks about taking a stab with a round LE and taping a piece of 1/16" music wire centered on it's LE, and seeing improved performance.

I think the driver for the flat stab is (A) the realization that it's going to work about as well as an airfoiled stab if you get the LE right, and (B) it's a heck of a lot easier to align the airplane on the bench.
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Offline Ken Culbertson

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Re: Flat vs Airfoil Stab
« Reply #2 on: August 20, 2018, 10:46:38 AM »
Synopsis of the discussion, from Fancher/Buck/Walker, as best as I can remember.  Note that all of this is hearsay:

It's more important that the LE of the stab have a low radius than the stab be airfoiled or not.  No, I don't know why, but the top guys report hunting with a rounded LE stab, and improved flight characteristics with a small LE radius.  Brett Buck, somewhere in the reams of posts on this site, talks about taking a stab with a round LE and taping a piece of 1/16" music wire centered on it's LE, and seeing improved performance.

I think the driver for the flat stab is (A) the realization that it's going to work about as well as an airfoiled stab if you get the LE right, and (B) it's a heck of a lot easier to align the airplane on the
I am pretty much trying to talk myself into just what you outlined. y1  The diameter on my stab LE is normally about 1/8" on a 3/8" thick stab.  If I keep that and do a nice parabolic curve back to about 1" I have just saved a day of grief trying to get the planking on a double taper without warping it.   The elevator is not a problem.  I use a straight taper from 3/8" to 1/8".  This, in a way produces an airfoil with a 2-3" long high point.  Interestingly the only planes I have now or remember from my "Active" days in the 70/80's that had any plane induced hunting ( I am capable of doing it without the help of the plane) had thick rounded stab LE's.  Maybe Ted/Brett/Paul were on to something - or not -  this is Bear country and the Bear has a thick rounded LE on a flat stab, they don't hunt, but, the folks flying them are pretty good and maybe in the hands of one of us with less than Walker Cup reflexes, they might be all over the place.

I really don't like the look of the flat stab with untapered LE/TE but from 160' I doubt that it matters much what they look like, it is what they fly like.  I really would like to find that thread. 
Ken
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Offline Brett Buck

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Re: Flat vs Airfoil Stab
« Reply #3 on: August 20, 2018, 11:15:23 AM »
I am pretty much trying to talk myself into just what you outlined. y1  The diameter on my stab LE is normally about 1/8" on a 3/8" thick stab.  If I keep that and do a nice parabolic curve back to about 1" I have just saved a day of grief trying to get the planking on a double taper without warping it.   The elevator is not a problem.  I use a straight taper from 3/8" to 1/8".  This, in a way produces an airfoil with a 2-3" long high point.  Interestingly the only planes I have now or remember from my "Active" days in the 70/80's that had any plane induced hunting ( I am capable of doing it without the help of the plane) had thick rounded stab LE's.  Maybe Ted/Brett/Paul were on to something - or not -  this is Bear country and the Bear has a thick rounded LE on a flat stab, they don't hunt, but, the folks flying them are pretty good and maybe in the hands of one of us with less than Walker Cup reflexes, they might be all over the place.

    Airfoiled is the better way to go in general. David wrote an article in SN called "De-Tails" that I had some input to. The alignment issue seems to be less critical than we once thought, because we appear to have discovered that there is a significant tolerance in the alignment, and there's much more tolerance with an airfoiled stabilizer.

   The issue with the LE radius is, I think, roundly misunderstood. I think the LE radius itself is probably irrelevant, but, to get a small LE radius with an otherwise 1/2" flat stab, you have to reshape the front of the stab to also create an extended curvature, which moves the likely separation point further aft. The problem with the flat stab is, I think, the separation right near the leading edge, as the radius ends and the flat part begins. All the flat stab experiments ended up with turbulators in the right places to trip the flow, usually one right at the LE, and one top and one bottom, right at the end of the radius and beginning of the flat. I have gone back to airfoiled stabilizers, with a very long "arc" airfoil all the way back to the elevator hinge line, and have never had any improvement from VGs, tripper strips, zigzag strips, etc, probably because my flow stays attached anyway without them, at least to near the hinge line. My LE radius is about 1/16" - 3/32", but that is incidental to the fact that the airfoil keeps getting thicker until about 65% of chord.

     Plenty of people have done OK with flat stabs, but I think the alignment is super-critical, and also, it is prone to uncertainty around neutral, as tiny changes in the AoA causes it to separate early on either the top or the bottom, from variation of the surface normal pressure.

   By the way, the tolerance on the alignment seems to be in the direction of "positive" incidence, that is, the LE higher than the TE, with an otherwise normal layout. Not that I would ever use a large angle like 2 degrees, but when aligning it, for sure, err on the side of the LE too high rather than too low. Most of the flat-stab models end up with a large amount of down elevator, like 1/4" on a half-inch stabilizer. Get it right, and not too much, and the inside/outside turn is unaffected, but the tracking in either direction (upright or inverted) can be remarkably improved. Based on experience, I would not build in any incidence on a flat stab airplane, but start with the elevator drooped at neutral flap.

   I build in 1/4 degree of positive incidence with my airfoiled stab. That's about 1/64" over the chord of the stabilizer.
    Brett

Offline Brent Williams

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Offline Ken Culbertson

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Re: Flat vs Airfoil Stab
« Reply #5 on: August 20, 2018, 12:08:17 PM »
    Airfoiled is the better way to go in general. David wrote an article in SN called "De-Tails" that I had some input to. The alignment issue seems to be less critical than we once thought, because we appear to have discovered that there is a significant tolerance in the alignment, and there's much more tolerance with an airfoiled stabilizer.

   The issue with the LE radius is, I think, roundly misunderstood. I think the LE radius itself is probably irrelevant, but, to get a small LE radius with an otherwise 1/2" flat stab, you have to reshape the front of the stab to also create an extended curvature, which moves the likely separation point further aft. The problem with the flat stab is, I think, the separation right near the leading edge, as the radius ends and the flat part begins. All the flat stab experiments ended up with turbulators in the right places to trip the flow, usually one right at the LE, and one top and one bottom, right at the end of the radius and beginning of the flat. I have gone back to airfoiled stabilizers, with a very long "arc" airfoil all the way back to the elevator hinge line, and have never had any improvement from VGs, tripper strips, zigzag strips, etc, probably because my flow stays attached anyway without them, at least to near the hinge line. My LE radius is about 1/16" - 3/32", but that is incidental to the fact that the airfoil keeps getting thicker until about 65% of chord.

     Plenty of people have done OK with flat stabs, but I think the alignment is super-critical, and also, it is prone to uncertainty around neutral, as tiny changes in the AoA causes it to separate early on either the top or the bottom, from variation of the surface normal pressure.

   By the way, the tolerance on the alignment seems to be in the direction of "positive" incidence, that is, the LE higher than the TE, with an otherwise normal layout. Not that I would ever use a large angle like 2 degrees, but when aligning it, for sure, err on the side of the LE too high rather than too low. Most of the flat-stab models end up with a large amount of down elevator, like 1/4" on a half-inch stabilizer. Get it right, and not too much, and the inside/outside turn is unaffected, but the tracking in either direction (upright or inverted) can be remarkably improved. Based on experience, I would not build in any incidence on a flat stab airplane, but start with the elevator drooped at neutral flap.

   I build in 1/4 degree of positive incidence with my airfoiled stab. That's about 1/64" over the chord of the stabilizer.
    Brett
Thank you - The plans I have drawn up have this exact setup so I am not going to change it.  I originally had an embedded elevator using roberts hinges but I have concluded that the performance increase I used to get from the 70's using this method was due to not sealing the hinges.  With them sealed it is just more work for virtually no performance increase.  Since we seem to agree on the airfoil vs flat I would like your opinion on the shape of the elevator.  Left alone it will be tapered from 1/16" the height of the stab to 1/8" at the TE with bevels (not round) on the LE.  The TE of the stab will be flat, not rounded.

You have explained something that I am experiencing with an ARF Nobler I use for practice.  Unlike a real Nobler, the ARF has a yucky flat stab/elevator.  In calm conditions or steady wind she flies great but in an unsteady wind she hunts like Davy Crockett getting ready for winter.  Your 2nd paragraph explains why - thanks.

Follow Up - Brett, if you haven't already, no response is necessary.  y1  Brent Williams dug up the article you contributed to on this subject and it explains everything I wanted to know....twice! <=
« Last Edit: August 20, 2018, 12:44:25 PM by Ken Culbertson »
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Offline Ken Culbertson

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Re: Flat vs Airfoil Stab
« Reply #6 on: August 20, 2018, 12:52:08 PM »
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Offline Brett Buck

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Re: Flat vs Airfoil Stab
« Reply #7 on: August 20, 2018, 01:09:04 PM »

You have explained something that I am experiencing with an ARF Nobler I use for practice.  Unlike a real Nobler, the ARF has a yucky flat stab/elevator.  In calm conditions or steady wind she flies great but in an unsteady wind she hunts like Davy Crockett getting ready for winter.  Your 2nd paragraph explains why - thanks.

 Possibly...   There could be a lot of other things causing it, too. I wouldn't overlook the possibility that the alignment is just off, in particular, that you have some positive incidence in the stabilizer, or not enough elevator droop. If it's unstable, you might be able to handle it in perfect conditions, but not when you have something disturbing it.

    If I had a suggestion, it would be to tape 1/32 wires or something like it to the very apex of the LE, and the point at which the curvature ends and the flat begins on the top and bottom, from root to tip, both sides. This is where they generally ended up on David's flat-stab airplanes. Then see if it gets better. He used either 1/32 music wire or old flying lines, but you are taping it down from root to tip, so all it has to be is a protuberance, could be plastic or anything, because there's no load to speak of. This is supposedly tripping the boundary layer to keep it attached longer, or possibly, dragging the air around the corner with viscous effects.

    I am sure that Howard and others are rushing to point out that it couldn't work that way, but that's what they said about vortex generators, too, and last I saw, Howard's airplane was literally festooned with them.

   Brett

Offline Ken Culbertson

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Re: Flat vs Airfoil Stab
« Reply #8 on: August 20, 2018, 02:14:51 PM »
Possibly...   There could be a lot of other things causing it, too. I wouldn't overlook the possibility that the alignment is just off, in particular, that you have some positive incidence in the stabilizer, or not enough elevator droop. If it's unstable, you might be able to handle it in perfect conditions, but not when you have something disturbing it.

    If I had a suggestion, it would be to tape 1/32 wires or something like it to the very apex of the LE, and the point at which the curvature ends and the flat begins on the top and bottom, from root to tip, both sides. This is where they generally ended up on David's flat-stab airplanes. Then see if it gets better. He used either 1/32 music wire or old flying lines, but you are taping it down from root to tip, so all it has to be is a protuberance, could be plastic or anything, because there's no load to speak of. This is supposedly tripping the boundary layer to keep it attached longer, or possibly, dragging the air around the corner with viscous effects.

    I am sure that Howard and others are rushing to point out that it couldn't work that way, but that's what they said about vortex generators, too, and last I saw, Howard's airplane was literally festooned with them.

   Brett
I will give it a shot.  Can't hurt anything and that plane is so much fun to fly.  Can't believe I am saying that about an ARF but it truly is fun.  I need to get out the tape anyway.  Seals are getting a bit gooey.
Cool part is that with MonoKote you can just MonoKote it permanently if it works.

Quote
If it's unstable, you might be able to handle it in perfect conditions, but not when you have something disturbing it.
[\quote]

All other aspects of flight are fine.  It just wants to hunt in level flight when it gets upset in bumpy air.  I gave my Cat Figure in the cockpit an ejection seat and that helped him get through the trimming process, now maybe he needs some no doze or a barf bag.

Ken

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