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Author Topic: Transitional Airfoils  (Read 1327 times)

Offline Ken Culbertson

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Transitional Airfoils
« on: February 08, 2020, 05:25:29 AM »
My first ship to rebuild after the fire that claimed my fleet is going to be the PA ship that I had just finished.  It had a rather unique airfoil, designed by Ton Niehbur  that I would like to understand better before I duplicate it.  It was the typical thick blunt fully curved shape common in most of today's top designs at the root but it gradually tapered to a flat back starting just behind the high point at the tips.  The taper started at about mid span.

Any thoughts - Ken
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Offline Tim Wescott

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Re: Transitional Airfoils
« Reply #1 on: February 08, 2020, 08:49:25 AM »
I think you need to document the airfoil for us.  Usually I do this with scrap cardboard, trimmed by whatever method is convenient until it lays nicely along the whole length of the airfoil.  Then I trace it out on paper with a pencil.  Get one at the root, one at the tip, and one in the middle.

And yes -- time consuming PITA.  But sometimes you just need to do the tedious stuff.
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Offline Ken Culbertson

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Re: Transitional Airfoils
« Reply #2 on: February 09, 2020, 06:47:32 AM »
I think you need to document the airfoil for us.  Usually I do this with scrap cardboard, trimmed by whatever method is convenient until it lays nicely along the whole length of the airfoil.  Then I trace it out on paper with a pencil.  Get one at the root, one at the tip, and one in the middle.

And yes -- time consuming PITA.  But sometimes you just need to do the tedious stuff.
Tim:

This is the best I could do without any of my drafting stuff.  It is pretty much freehand.  Actual is a bit more blunt, a bit thicker and the "mid" is a bit thinner at the back.  The point is the transition from rounded to flat.

Ken
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Offline Tim Wescott

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Re: Transitional Airfoils
« Reply #3 on: February 09, 2020, 01:00:04 PM »
Thanks Ken.  I know that Brett Buck, who's about my age but spent his whole life flying instead of taking a 30-year break in the middle for college and kids, doesn't like the ice-cream-cone airfoil -- I can't remember exactly what he doesn't like, but I think it's that its jumpy around neutral.

Having it only flat at the tips may make that better overall, but if the tips have a "jumpy" airfoil and the rest of the wing doesn't, it would make the plane jumpy in roll, which can't be a good thing.
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Offline Ken Culbertson

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Re: Transitional Airfoils
« Reply #4 on: February 09, 2020, 01:32:48 PM »
Thanks Ken.  I know that Brett Buck, who's about my age but spent his whole life flying instead of taking a 30-year break in the middle for college and kids, doesn't like the ice-cream-cone airfoil -- I can't remember exactly what he doesn't like, but I think it's that its jumpy around neutral.

Having it only flat at the tips may make that better overall, but if the tips have a "jumpy" airfoil and the rest of the wing doesn't, it would make the plane jumpy in roll, which can't be a good thing.
I am not promoting this airfoil, I am trying to understand it and why it flew so well.  I didn't have enough flights on it when it burned to tell if it had any minor issues.  It did want to float up in level flight a bit which I traced to flaps not neutral.  That was corrected but never tested.  I agree that jumpy at the tips is a bad thing.  Maybe it has to do with stall characteristics.  It is not a true "Ice-cream-cone" since the flat starts well aft of the high point.  It also could have been a bit of block sanding too late in the evening.  I am also not convinced that the exceptional cornering and smoothness in rounds that the plane had was due entirely to the airfoil.

Ken
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Online Howard Rush

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Re: Transitional Airfoils
« Reply #5 on: February 09, 2020, 05:20:37 PM »
I don't think it would be jumpy around neutral.  All reasonable airfoils have the same lift vs. angle of attack around zero.  Brett's observation (although it's best coming from Brett) is that airfoils like that have high flap hinge moments.  I don't like them flat in the back because they are hard to sand.  I reckon you could use Profili or Javafoil to get a clue as to whether there's a benefit to that airfoil, but I doubt if either would give useful hinge moment information.

On the other hand, Gordan Delaney's wonderful twin Pathfinder has an airfoil like that.

Pat Johnston likes airfoils that are ellipses in the front and straight in the back.  Pat draws a lot of airfoils.  Does anybody remember National Lampoon's Famous Comic Artists' School?
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Online Brent Williams

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Re: Transitional Airfoils
« Reply #6 on: February 10, 2020, 02:09:00 PM »
Just an observation.  Perhaps the difference in perceived curvature aft of the high point can be attributed to trying to fair the dimensionally smaller airfoil into a fixed thickness trailing edge. ie, 1/2", and the usually flat-ish trailing edge sheeting. 
Percentage wise, it requires less curvature to fair the shorter height and shorter length tip ribs into that fixed height trailing edge dimension than the taller and longer root ribs.  There is less distance to make a curve move smoothly into the ~1.5"(ish) of flat 1/16" trailing edge sheeting stock.  So, it can appear to flatten slightly aft of the high point as you move out to the tips ribs.   

I have attached an overlay of the Impact and Trivial Pursuit root and tip airfoils showing this effect.  Interestingly, when comparing the SV airfoils (in this case, taken from a Tempest,) there appears to be more aft curvature as you move from root to tip.  SO...something to ponder upon as you think about your next plane. 
To the extent that it matters, at all, is another conversation.  Pick any of the known, good airfoil shapes, build it straight, and it will probably work just fine.
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Offline Geoff Goodworth

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Re: Transitional Airfoils
« Reply #7 on: February 11, 2020, 09:17:37 AM »
If the section is as suggested in some of the comments, you may find this interesting.

On the comparison drawing, the red line is NACA 00xx, the blue line is NACA 63A with the max thickness moved forward tp 30% to match NACA 00xx and the white line is an ellipse with straight lines from the TE to be tangent to an ellipse.

The One Design Airfoil Analysis is interesting but only partly relevant to our models because is is the airfoil for full size aerobatic aircraft.
« Last Edit: February 26, 2020, 07:26:55 PM by Geoff Goodworth »

Offline Mark wood

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Re: Transitional Airfoils
« Reply #8 on: March 14, 2020, 08:35:45 AM »
The "flat back", "ice cream cone" airfoils are design for the 4/4 scale airplanes so they can snap roll very easily on command. As a model airfoil they wouldn't be very desirable, especially on the wing tip. The airfoils presented would be done backwards to how I would have utilized them in a design. Since this airfoil has a rapid departure in the Cl/Alpha it is very suitable of creating the differential lift required for the snap roll. To use this type of airfoil to make a more well behaved aircraft using it at the root transitioning to a 00xx at the tip would tame the "tip stall" by causing the stall progression to begin at the root.

The straight transition from the high point to the TE causes a rapid pressure rise in the pressure recovery region which, in turn,  causes rapid separation. Opt instead for a more classic airfoil. If a person claims the airplane is drifting around neutral, the airfoil very well could be the trouble. High pressure air will "leak" up the TE and push the moment around which would result in hunting in level flight. While this airfoil is easy to create, I wouldn't use it in a model opting instead for a size 13 curve in the pressure recovery region.

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Offline Ken Culbertson

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Re: Transitional Airfoils
« Reply #9 on: March 14, 2020, 09:32:27 AM »
The "flat back", "ice cream cone" airfoils are design for the 4/4 scale airplanes so they can snap roll very easily on command. As a model airfoil they wouldn't be very desirable, especially on the wing tip. The airfoils presented would be done backwards to how I would have utilized them in a design. Since this airfoil has a rapid departure in the Cl/Alpha it is very suitable of creating the differential lift required for the snap roll. To use this type of airfoil to make a more well behaved aircraft using it at the root transitioning to a 00xx at the tip would tame the "tip stall" by causing the stall progression to begin at the root.

The straight transition from the high point to the TE causes a rapid pressure rise in the pressure recovery region which, in turn,  causes rapid separation. Opt instead for a more classic airfoil. If a person claims the airplane is drifting around neutral, the airfoil very well could be the trouble. High pressure air will "leak" up the TE and push the moment around which would result in hunting in level flight. While this airfoil is easy to create, I wouldn't use it in a model opting instead for a size 13 curve in the pressure recovery region.
It is apparent to me that you know far more about this than I do.  I did not have the opportunity to fully trim the plane this airfoil came from before it was destroyed by fire.  I intentionally build my planes with at least 45 degrees elevator movement.  When I fly I set my handle to give me 30 degrees with normal maximum handle movement.  I too was confused why it flew so well.  I was able to turn consecutive corners of about 10' (maybe less I never got to plot them) without any stall, even at the tips.  The plane didn't hunt.  There was a slight flap/elevator alignment
that caused it to want to float or pull down a very slight amount but it still grooved.   Tom Niebuhr designed the airfoil.  Unfortunately he is not around any more to explain it.
The more I think about it the superior flight characteristics may have been due to other factors and not the airfoil.

Thanks - I am attaching the airfoil from the plans for the rebuild.  Any comments will be appreciated.

Ken
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Re: Transitional Airfoils
« Reply #10 on: March 14, 2020, 11:39:54 AM »
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Offline Ted Fancher

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Re: Transitional Airfoils
« Reply #11 on: March 14, 2020, 07:58:50 PM »
Ken,

I think it looks as good as 99% of the usual suspects and I'm unsure which of the remaining 1% would be considered #1.

Build it straight and reasonably light, seal the flap (and elevator) hingelines and trim the resulting model and the control system to to allow you to do all the tricks under ideal conditions with finger, wrist (and elbow input only under  difficult air conditions) and go win contests.

Ted

That should get some comments!!!

Offline Mark wood

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Re: Transitional Airfoils
« Reply #12 on: March 17, 2020, 10:22:13 AM »
It is apparent to me that you know far more about this than I do. 

The more I think about it the superior flight characteristics may have been due to other factors and not the airfoil.

Thanks - I am attaching the airfoil from the plans for the rebuild.  Any comments will be appreciated.

Ken

Well, I donít know about the knowing more part. I am one of those people who have been around a while and airfoils are an intellectual hobby of mine. Several full scale aerobatic designers have endeavored to I,prove the snap characteristics of their airplanes. I donít think Nieghbower was the originator as it seems to have shown up first in a Sukoy. What they are striving for isnít necessarily suitable for models.

Your conclusion is probably more correct in that planform makes a huge impact on the model.  The airfoil can make big impacts as well but not on the same scale.

It is fairly easy to eyeball a basic airfoil. The size 13 section generally works and improvement are subtle.  Adding thickness behind 5e high point aids in the transition and helps reduce the sudden stall which is undesirable in a model. My eye says the new airfoil will be better the than the ice cream cone.

My $0.02 FWIW
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Online Howard Rush

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Re: Transitional Airfoils
« Reply #13 on: March 17, 2020, 04:29:00 PM »
I suspect that airfoil behavior at full-scale Reynolds numbers is a lot different than at ours.  Gary James or Frank Williams or Igor could elaborate as to why.  My airfoil notions are based mostly on experience with models at Reynolds numbers between about 400,000 and 700,000, particularly single-element (no flap) airfoils used on combat planes.  It seems to me for this application that airfoils with the highest max lift have the most abrupt stall.  For several years I used an NACA 0016.5 with the max thickness squished forward to 25% chord.  It worked great, but I had to use bellcrank stops, and the slightest warp made an airplane useless.  Any anomaly around 25% chord significantly killed off lift capability, but I don't remember it causing an abrupt stall.  When I used a foam wing with a spar at 25%, I carefully planed the spar, then filled any remaining little dips with spackle.  Likewise, a pointy leading edge caused reduced max lift, but didn't cause an abrupt stall.  If anything, a pointy LE made the stall gentler. 

I fly an Impact for stunt.  I plotted its airfoil using the NACA four-digit formula, fiddling with the parameters until I matched the root and tip airfoils.  As Brent observed, the trailing edge thickness and slope are the same from root to tip.  The Impact airfoil is very nearly a straight line for the last 20% or so before the flap, causing it to be hard to sand. 

My last two Impacts have a wing structure with slanty ribs that make for pretty bad bumps in the wing cross section aft of the fattest part of the wing.  Although the airplane has significant, audible separation, it doesn't do anything perverted.  It's a pretty good stunt plane.  Vortex generators may have ameliorated the effect of the bumps. 

The aft part of Igor's airfoil has a lot of curvature and fairs nicely into the flap. It wins a lot of stunt contests.  The only comparison I've done between it and the Impact is with XFOIL via Profili, which favored the Impact airfoil.

Aerodynamically I would reckon--based on no theory--that a ice-cream-cone-looking airfoil at our Reynolds numbers could be OK.  I'd use a blunt leading edge and keep the curvature low at the transition to the straight part. Looking at the Endgame II  tip template above causes me to reach for a sanding block--just a wee swipe in front of the straight part.  Structurally, I'd think that a curve in the wing surface would be better than a straight line.

(edited to remove some stupidity)

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Offline Ken Culbertson

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Re: Transitional Airfoils
« Reply #14 on: March 17, 2020, 07:59:51 PM »
Howard, I think I am going to keep the curve the same to the tip.  I think your description  The more I think about it the superior flight characteristics I had with Endgame I was due to the overall design.  It had a 19.5" tail movement with a 26% sharp LE stab.  710 squares at 65oz. and Keith's CAM rudder (which I love).  In other words it was right.  I am noticing all of that as I redraw the plans for II.  I am agreeing with Ted that the airfoil id good enough to not be a problem.

A word to all of you out there that have spent years collecting plans, airfoil templates, drawings, etc.  STORE IT SOMEWHERE OTHER THAN IN YOUR SHOP!  When mine went up all of my plans went with it.  I almost never build kits and I am having to design totally from pictures and memory.  Camera's distort and my memory at 73 may be a steel trap....a rusty one!
When I get it redrawn I will shoot a picture and give you a good laugh!

Ken
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Offline Geoff Goodworth

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Re: Transitional Airfoils
« Reply #15 on: March 19, 2020, 09:47:57 PM »
Ken, have you looked at Hip pocket Aeronautical and Outer Zone for PDF files of the plans you lost?

There are a great many plans on both those lists. You should find something interesting.

Offline Ken Culbertson

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Re: Transitional Airfoils
« Reply #16 on: March 19, 2020, 11:29:05 PM »
Ken, have you looked at Hip pocket Aeronautical and Outer Zone for PDF files of the plans you lost?

There are a great many plans on both those lists. You should find something interesting.
I have looked at a lot of plans there.  I really enjoy seeing what others come up with.  But, except for a Twister I built to get back in shape that I totally Fancherized and an ARF Nobler that I bought just to have something to fly a few years back, I have never flown a PA plane I didn't design going all the way back to the early 60's.  None were ever published mainly because I never submitted any.  I love all three phases of our hobby;  design, building and flying pretty much equally so, until I get my shop rebuilt I will content myself with wasting rolls of butcher paper!  The great part of a "II" design is that you get to make all of the things you screwed up and had to do over on the "I" part of the design! y1

Ken
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Offline Ken Culbertson

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Re: Transitional Airfoils
« Reply #17 on: September 17, 2020, 12:41:29 PM »
I was going to start another thread for my latest question but so much of this one is relevant so I decided to revive it.  I was just about to place an order with Bob for a wing with the airfoil we discussed here which is similar to a Thundergazer airfoil with a 1/2" TE to allow for recessed flaps. (Not that they are better, they just look cool).  Then the worst thing that could happen, happened.  I got to watch Mike Scott's new MaxBee fly.  It flew nearly identical to my Endgame, only it seemed to corner faster without losing that "T-Square on a string" tangency to the circle in the corner.  So I decided to see why.  MaxBee nose 1/2" longer, Tail 1/2"shorter, area/span about the same Stab% about the same, stab airfoil close, thrust lines same.  Not enough there to be the difference.  It had to be the airfoil. 

Mike gave me a copy of the airfoil and I overlaid it onto the one I was about to order.  To the high point they are nearly identical.   Length is the same, LE radius is the same, thickness at the high point is the same.  Where they differ is curvature.  MaxBee has a little bit less forward curvature but, the aft curvature of the MaxBee would make Al Rabe blush.  I had always operated under the assumption that such an airfoil was effective when you had high wing loading and large flaps. 

Have any of you out there flown both enough to make a comparison?

Ken

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Re: Transitional Airfoils
« Reply #18 on: September 17, 2020, 05:29:56 PM »
Ken, here are Igor Burger's links to his Stunt News articles.  They are a good place to start to understand his philosophies on his airfoil and stunt design concepts.  As I interpret his methodology, it appears that he focuses on the effectiveness and lift of the wing with the flaps deployed in the corner, and he's much less concerned with level flight airfoil efficiency.  The idea is that his design doesn't scrub off as much speed in the corner at higher angles of attack which helps it fly cleanly through the corner. 

Perhaps some more technical observations will develop from the engineers on here.

Quote from: Igor Burger:

http://www.maxbee.net/download/MaxBee_1.pdf

http://www.maxbee.net/download/MaxBee_2.pdf
« Last Edit: September 18, 2020, 05:00:54 PM by Brent Williams »
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Re: Transitional Airfoils
« Reply #19 on: September 17, 2020, 07:40:55 PM »
Quote
Just an observation.  Perhaps the difference in perceived curvature aft of the high point can be attributed to trying to fair the dimensionally smaller airfoil into a fixed thickness trailing edge. ie, 1/2", and the usually flat-ish trailing edge sheeting.
Percentage wise, it requires less curvature to fair the shorter height and shorter length tip ribs into that fixed height trailing edge dimension than the taller and longer root ribs.  There is less distance to make a curve move smoothly into the ~1.5"(ish) of flat 1/16" trailing edge sheeting stock.  So, it can appear to flatten slightly aft of the high point as you move out to the tips ribs.   

I have attached an overlay of the Impact and Trivial Pursuit root and tip airfoils showing this effect.  Interestingly, when comparing the SV airfoils (in this case, taken from a Tempest,) there appears to be more aft curvature as you move from root to tip.  SO...something to ponder upon as you think about your next plane.
To the extent that it matters, at all, is another conversation.  Pick any of the known, good airfoil shapes, build it straight, and it will probably work just fine.

People speak of lighter control loads with the aft curvature Vs flatter aft section . And perhaps less ' loading up ' in wind ,  of control loads .

Offline FLOYD CARTER

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Re: Transitional Airfoils
« Reply #20 on: September 27, 2020, 05:19:21 PM »
All my designs use the proven NACA 0018 or thicker airfoil.  No point in re-inventing the wheel
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Re: Transitional Airfoils
« Reply #21 on: September 29, 2020, 11:00:09 PM »
If thinking the flat rear flank , has more unsteady stability. when Stalled . But good recovery . And dosnt 'cept way outside usual airspeed etc .

Good modulation ( correction ) for wind blown round manouvers .

May even rtate ( stall turn ) with wind attacked under but broken off topside ! .


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Re: Transitional Airfoils
« Reply #22 on: September 30, 2020, 04:12:42 PM »
All my designs use the proven NACA 0018 or thicker airfoil.  No point in re-inventing the wheel

  How does it compare to, say, an Imitation airfoil? Or one of Phil Granderson's? What did a NACA 0018 do better than Phil's?

    Brett

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Re: Transitional Airfoils
« Reply #23 on: September 30, 2020, 04:55:13 PM »
All my designs use the proven NACA 0018 or thicker airfoil.  No point in re-inventing the wheel

Thatís what the B-17 used.   Not the model, the real one.
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Online Brett Buck

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Re: Transitional Airfoils
« Reply #24 on: September 30, 2020, 08:15:27 PM »
Thatís what the B-17 used.   Not the model, the real one.

   To be fair, Phil wasn't around when they did it.

    Brett

   

Offline Ted Fancher

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Re: Transitional Airfoils
« Reply #25 on: October 01, 2020, 05:41:04 PM »
For some reason I can never restrain myself when these mile long excursion into the "perfect" CLPA airfoil arise...

A number of years ago Big Art Adamisin went down under  to visit stunt folks and judge a contest or two.  One of his more curious /surprising  revelations upon his return was that there were several "competitive" entries with flat plate airfoils...i.e. nothing more exotic than a radius to the leading edge of conjoined sheets of balsa (don't recall if he ever mentioned the thickness) and (most likely although I don't know that I ever heard it from Big Art) a tapered trailing edge...nothing but flat balsa between the two!

One Adamisin or another forwarded pics of the ships following an earlier iteration of the above but, unfortunately, although they're likely somewhere in my office they would have to be somewhere within the multiple stacks of unlabeled or cataloged stunt stuff piles.

Just a hint to remember we're not talking supersonic helicopter rotor airfoils here!

Ted
« Last Edit: October 04, 2020, 04:58:56 PM by Ted Fancher »

Online Brett Buck

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Re: Transitional Airfoils
« Reply #26 on: October 05, 2020, 09:57:07 PM »
One Adamisin or another forwarded pics of the ships following an earlier iteration of the above but, unfortunately, although they're likely somewhere in my office they would have to be somewhere within the multiple stacks of unlabeled or cataloged stunt stuff piles.

Just a hint to remember we're not talking supersonic helicopter rotor airfoils here!

   As a counterpoint, we have about 100-post email thread about how adding a .004" thick tripper strip has no effect, and a .006" thick tripper strip has a dramatic effect. .002 is smaller than the ridges around my trim colors.

   I would also note the experiences of The Unknown Pilots #1 and #2 at the inaugural ARF-Off. You and I witnesses,  close-up,  to the effects of an otherwise reasonable size/weight ARF Strega that nonetheless was stalling in the third corner of triangle in 75 degree sea level air, and ended up stalling in *the 4th loop of the 4-leaf* by the time we got over 90 degrees in the afternoon. And how it was fixed by 5 minutes of very crude rasp work to lightly blunt the leading edge.

    Brett

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