Avid STOL airfoil -some calculations

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Posted

I was curious about the Avid STOL airfoil, so made some calculations using an inviscid panel method code.  The results are interesting.

We aerodynamicists tend to do everything we can to remove scale, or physical size from consideration.  For example, we divide the wing’s lift by everything it’s proportional to: wing area and dynamic pressure, to get a lift coefficient.  The dynamic pressure is just the difference between total pressure, that measured with a forward-facing Pitot tube, and freestream static pressure Pfs, that of the air at that altitude away from the influence of the airplane.  It turns out that at our speeds this difference is ½rVfs2, where r (Greek r) is the air density.  What’s great about total pressure is that it’s constant almost everywhere, except in regions of flow separation or where viscous effects are important, like right next to the surface, in the boundary layer.  Bernoulli would say that this means that in locales where local velocity is high, pressure will be low, to preserve the total pressure: Pt=Ps +½rV2.

 We talk about the pressure distribution around a body in terms of a pressure coefficient, Cp, which is just the difference between local pressure on the body’s surface and freestream pressure divided by the dynamic pressure

(Cp = (p-pfs)/½rVfs2).  When local airspeed, say, over the wing, goes up, pressure coefficient goes down.  That partial vacuum on the upper surface is a lot of what keeps us afloat so-to-speak.  But y’all already knew that!

So just for fun, I looked at not only the STOL airfoil shape, but a version for which a straight line defines the lower surface.  This is what Manu (Efil01) has on his Avid.  It has more camber than the speedwing airfoil, since the upper surface is more curved.  Here’s a plot of the airfoil shapes.

 

Avid_STOL_Airfoil.thumb.JPG.56723783f601

The first thing I looked at was the pressure distribution at takeoff, assuming the landing gear arrangement allows a maximum angle of attack of no more than 8 degrees, relative to the max length line (here the x-axis).  Taller main gear or a smaller tailwheel would help here.  By the way, the plot seems to chop off the trailing edge of the airfoil.  That’s just an artifact of what was plotted; the inviscid panel code used for the computation plotted the “control points” which are midway between the defining points.  I’m just being lazy here, so I beg your forebearance. 

So here’s what the Cp distribution looks like at takeoff:

Avid_STOL_takeoff.thumb.JPG.647933df1732

Per aerodynamics custom the Cp is plotted upside down, with negative values going up the ordinate, not down.  The blue is the original airfoil, the red is the flat-bottomed version.   At 8 deg AoA, the flat-bottomed airfoil delivers 7% less lift than the original version.  As you can see, the lower surface has positive pressure on it in both cases.  The upper surface features an overspeed right at the leading edge, perhaps the consequence of having too large an angular range for the round shape of the front spar tube, with a rather abrupt change in the surface curvature.  Note also that the upper surface, with its overall overspeed, contributes about 2.5  to 2.8 times as much lift (area under the curve) as the lower surface.

Still, the slowing down of the airflow on the upper surface is relatively gradual, which is a good thing (Thank you Dean!).  Note also that the lower surface flow is accelerating as it goes back towards the trailing edge.  However, the effect of the under-surface camber is about a 40% increase in nose-down pitching moment for the STOL section vs the modified shape.  All that camber is like flying around with the flaps down.  Yes, it gets us off of the runway faster, since with taildragger gear we’re angle-of-attack limited on the ground, and yes, it delivers glorious in-flight visibility over-the-nose.  But this is one of the great things about the Junkers-style flaperons:  The camber can effectively be increased for takeoff & landing, but that dang nose-down pitching moment can be banished by pulling flaps back up for cruise.  Having the wing and tail not fighting each other lowers the induced drags of both and allows us to cruise more efficiently, and faster

O.K. now, so what do the Cp plots look like in cruise?  I ran both airfoils at CL=0.5, a typical cruise value.  Here’s what showed up. 

Avid_STOL_cruise.thumb.JPG.77f181a4e3d73

A bit different looking, eh?  That giant -Cp spike at the leading edge is on the lower surface!  Does the flow stay attached after that steep recovery?  Do we need to VG the lower surface?  Tufts would tell the tale.  I’ll bet some of you have already done this.  Note how much milder it is for the flat-bottomed modification.  By contrast the upper surface has it easy.  So it looks like our friend Manu will have a sweet-flying airplane!

One caveat here: these calculations did not include the boundary layer or any of its effects on the outer flow, and were incapable of modelling any flow separation.  In subsonics, everything affects everything, so these results, while indicative of what’s going on, are not accurate to the nth degree.  I also did not include any effects of the flaperon, assuming it was neutral, not lifting upward or downward.  Retired, I don't have a tool available to model the flaperon too.

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Posted

Thanks for posting this I don't understand all it but still like it. 

You Mr. must eat sleep and breath airplane.

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Posted

It would be interesting to plot the KF 4 airfoil in comparison.  I know VG's have been tried on the lower surface but I have often wondered if they were placed far enough forward.  What are your thoughts on placement (% of chord) for them on both the upper and lower surfaces?  I have plenty of them around but I guess I have been to lazy to date to give it a try.  I do know that the KF 4 is faster than the Avid MK IV even with the added area.  It will be interesting to see if Manu's wing is faster also.  I install the KF leading edge on my wing and I believe it is a little faster. 

PS I do have VG's on the top of the wing.

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Posted (edited)

O.K. I've got to admit I had a couple of extra ribs, so I traced and measured one, so that's how I got the coordinates.  I also admit to not being a big VG fan, as they make the wing harder to even dust, much less wash, and reduce maneuvering speed along with stall speed.  To figure out where to put them without a trial-and-error flight test series requires modelling of the boundary layer.  I have 2 codes to do this, and both have passed muster in the compiler,  but that doesn't guarantee they will run without further modification.  So that's for later, like after summer, in the rainy season!  If someone has the KF airfoil coordinates, it might be fun to take a peek, though.  Flying season is upon us (well, we fair-weather flyers anyway!).  I'm excited at the prospect of testing out all of last winter's mods!  You Alaskans and Midwesterners are admirably tough for sure!

Edited by Turbo

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Posted

With a flat bottom the rib profile looks like an Aeronca Champ wing rib. NACA 4412 was used for those and it's been a popular airplane for decades. While you may be able to speed it up, I wonder if you can do so without raising the stall as well?

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Posted (edited)

Hey, CD,  4412 does indeed look a lot like the flat-bottomed STOL airfoil.  4415 does too.  Both are better in that they have more elliptically rounded leading edge shapes, avoiding that leading-edge -Cp spike.  With TD gear we are AoA limited on the ground, and if the AoA on ground is much lower than that producing max lift, we have to run along on the runway for a greater distance, so. we're leaving some TO performance on the table.  This is the "killer app" for these full-span Junkers-style flaperons: we can both increase CL max and reduce the AoA at which it's achieved by just dropping a little flap!  They're perhaps not so good for landing, since we can't go to high deflections without risking loss of roll authority, but full-span, they've got to be great for minimizing takeoff roll!  I now wonder if that's what Dean was thinking.  If so, great, but I think the STOL airfoil is overendowed in the camber department.  A minor miscalculation by a  smart non-engineer, understandable and certainly forgiveable!  So yeah, drop 5-10 degrees of flaperon!

Edited by Turbo

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Posted

So yea, totally understood that...but maybe for the lay person....what does your testing show? 

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Posted (edited)

My conclusion here is that the STOL airfoil is overcambered, and that a little flap on takeoff would have sufficed for minimizing takeoff roll.  With less camber, the airplane could be more efficient and faster in cruise, with the flaperons pulled back up.  I am told that some folks cruise with flaperons up 3 degs or so, but it looks like they are on the edge of (negative flaperon) stall, while providing downward lift to counter the wing's nose-down moment.  This is not efficient.

Edited by Turbo
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Posted (edited)

So where do stol length speedwings fit In That equation

Edited by Matthewtanner

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Posted

As it turns out, I have a speedwing rib too, but have not analyzed the shape - however:  that shape has less camber overall, and even has a slightly convex lower surface.  My intuition says that you may need a little flaperon deflection in order to minimize takeoff roll, but even with the STOL wing's span, you should have a significantly faster airplane in cruise.  Minimizing ground roll, and finding the best flaperon deflection to do that is a flight test experiment.  If STOL operation is not that important to you, I think you would have a more versatile and efficient airplane with the speedwing airfoil shape.

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Posted

Hello Turbo;

Some time ago there was a discussion about a better airfoil and the usage of the VG on the bottom of the wing, one member tried it without luck, however there is a letter from Riblett  saying that Avid airfoil will not sacrifice STOL performance taking the bottom section of Riblett GA35-614; where the top of the wing remain untouched (Avid original airfoil) and the bottom will have same as GA35-614.

I have not heard anyone that has applied this modification to the bottom of the wing, but as it looks like will give the best out of the 2 worlds with little modification to the wings.-

Regards

Dimi3

p.s. here is the link to the discussion including the letters (pictures) in question.-

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Posted

I agree.  Even a straight-line, flat underside will work fine.  I sure wish there was an easy way to put detents on the flaperon handle.  I've got to at least upgrade my friction discs.

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Posted (edited)

There was a pretty simple setup for that posted here or on the Yahoo sight I believe. Check the files section on Yahoo first. 

https://xa.yimg.com/df/avid_flyer/Avid+flap+detent+template.pdf?token=D9hqLSzEzFW3jqOZI3i0JyFY0fnPdL37EuTGFEAWnJDGV4ctfm-CEL6P4iw14XbUwDdhEhwO7vUqtzUT7y1tJXL1RhdZjsQPH4c49LpC22Vq4U0SSkdnBfIE1whBElBTe9Q&type=download

Edited by wypaul

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Posted

The link goes to a September 1996 one, not June 1996

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Posted

The link goes to a September 1996 one, not June 1996

should work now

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Posted

Read it. Thanks!  Seems to be in line with what I'm saying, but it appears that Dean didn't like the idea of using flaps for takeoff and climbout as a way to increase the effective camber, for steeper climbout.  This is probably better for most of us fudpuckers, especially green TW pilots like me!  Flew the other day, and noted how happy the bird is at 60mph!

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Posted

Still, getting off the ground in minimal ground run requires the wing to be close to max-lift AoA when you're still on your wheels.  The camber helps with that, no question. I expect other Avids with the 582 accelerate strongly; mine sure does at WOT!  Àfter you're up, and have accelerated through flaps-up stall speed, your attitude is irrelevant as long as you keep it flying. From then on it's all a matter of wing loading and power loading; the wing's camber level has nothing to do with achievable climb angle.

So for the very brief time period in which you're trying to break free of the ground, the wing's camber is useful.  My argument is that this could easily have been achieved with a little flaperon deflection.  A wing with lower camber level would make for a faster, cleaner airplane.  It's like, with all that camber of the STOL airfoil, we're essentially flying around with the flaps down all the time unnecessarily.  So yeah, Dean got a lot right, but here he either goofed, or was trying to protect us from ourselves.

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Posted

Turbo,

I think you missed the point of the climb out in the article.  The point was that the Avid achieves maximum climb rate at a low speed (60mph in the examples).  If you are climbing 1000fpm at 60mph, then you have a steeper angle of climb than an airplane that climbs 1000fpm at 100mph.   This is a big advantage when  you are in a confined takeoff area.

My Cherokee 235 will climb at 1000fpm, but it is going 110mph to do so.  At 60mph, it would be mushing along with little to no climb - with full flaps.  So the Avid advantage in a confined area is much greater than just the 600-700 feet shorter takeoff run.

Always remember these kinds of articles are as much about marketing as they are education- they will stress the strengths of their product, while glossing over the disadvantages.  In this case, I'm sure it was inspired by a competitor talking about how much faster they were, with a slightly longer takeoff run.

Of course, the Cherokee would be doing that with 4 people, full fuel, and 100lbs of baggage, (or 2 people, full fuel, a couple of dressed moose, well maybe alligators in my case :P ), which  is a different kind of advantage depending on what your mission is.  It would also be burning about 20gph in the climb, then 12.5 in cruise, but cruising at 150mph.  Again, different mission, different plane.

Mark

 

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Posted

Hey Turbo,

 

I'd like to ad that Dean designed the avids to be fitted with floats...and for a floatplane config (avids on floats/amphibian & Catalina) there is absolutely nothing better than a very high lift wing to get you off the water ..in a short distance ......avid has the water take off distance record...https://www.youtube.com/watch?v=qjDSatUSoCY

On the other extreme...the worst floatplane ever....the cessna  caravan on floats (not even amphibs) at 30 seconds still on the water making wind. I have a caravan driver friend who has experience and he said some water take offs can take forever/never happen . The Caravan was not destined to be on floats, the wing is optimized for good cruise and payload...not at all a low stall high lift wing...flaps and turbine don't help. Many fitted with floats have crashed https://www.youtube.com/watch?v=6dDmUI2vEdM

There are so many tiny mountain lakes I can land on no other floatplane can (except the light Avids and similar of course)....

My 2 cents

 

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Posted

Still, getting off the ground in minimal ground run requires the wing to be close to max-lift AoA when you're still on your wheels.  The camber helps with that, no question. I expect other Avids with the 582 accelerate strongly; mine sure does at WOT!  Àfter you're up, and have accelerated through flaps-up stall speed, your attitude is irrelevant as long as you keep it flying. From then on it's all a matter of wing loading and power loading; the wing's camber level has nothing to do with achievable climb angle.

So for the very brief time period in which you're trying to break free of the ground, the wing's camber is useful.  My argument is that this could easily have been achieved with a little flaperon deflection.  A wing with lower camber level would make for a faster, cleaner airplane.  It's like, with all that camber of the STOL airfoil, we're essentially flying around with the flaps down all the time unnecessarily.  So yeah, Dean got a lot right, but here he either goofed, or was trying to protect us from ourselves.

Dean was doing his best to protect us from ourselves.  Remember, the dream was pretty much to design a plane that anyone could build, then learn to fly their creation as easily as possible.  

I am digging the work your doing here.  The KF leading edge that goes on seems to help a bunch with what your seeing on your modeling.

:BC:

 

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Posted

I'm sure that a smoother curvature change could banish that -Cp spike up at the leading edge; high streamwise curvatures belong where local velocity is low.  So a simple, and relatively small foam add-on up there could help get even higher CL max.

Good point, Leni.  And it seems plausible that Dean designed the wing that way to save us fudpuckers from ourselves!

The floatplane app does perhaps argue for more camber, but again, that's for getting unstuck from the water with even less angle of attack.  With full-span flaperons we can temporarily increase our effective camber, to get lots of lift at low angle of attack.  Here we don't have the large induced-drag penalty you'd get with part-span flaps.  Like I say, this is the killer app for these Junkers-style flaperons - short takeoff runs while temporarily AoA limited.  Think of power loading as thrust/weight ratio, but upside down.  low enough power loading and we can accelerate straight up like an F-16!  At higher power loadings,  (weight/horsepower) we still need that wing.  So with a wing that allows slow flight (think low wing loading) we can steepen the climb up considerably.  But once you're free of the ground, the angular relationship of the wing and fuselage are, within reason, irrelevant to climb angle.  So after you have left the ground, all that camber is just buying better visibility!  It is not helping to steepen the climb!  So now the 64k$ question:  Has anybody used their flaperons to shorten our already ridiculously short takeoff roll?  By how much? 

Oh, and by-the-way, CLmax of the NACA 4-digit airfoils is almost camber-independent.  0012 gets same as 4412.  It just does so at higher AoA.

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Posted

With Dean's design history(ellipse, bellanca eagle, explorer and others) I don't think he goofed  anything when he designed the stol airfoil. It was initially a very light plane design with not a lot of horse power to start and not carrying a lot of inertia over the fence so to speak so he did some things to compensate for the unexpected and unfamiliar. You see and hear of some pretty scary sustained  angle of climb demonstrations at reduced air speeds by pilots who know the plane and what it can do. I am not one of them. Like leni said the design was simple so as to get would be builders into the air successfully. In many cases improving upon the design involves taking what was once simple and creating complexity that can make success of completion a nonevent. 

I learned sme time ago that anything mechanical designed to do something and work is only ever going to be conceived as an amalgamation of compromises. Many considerations made Dean Wilson make the design the way he did. 

 

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Posted

That link showing the Avid with floats jumping off the water was impressive. The description said it was an early model but it didn't list the engine size. If it still had a 503 in it I'm even more impressed. Another video posted by the same guy shows a Kitfox doing aerobatics but again not too many details. The Kitfox looks like all the dihedral has been dialed out of the wing and that makes sense to keep it from wanting to roll out of the top of a loop all the time. I'm curious if dialing out the dihedral in our Avids would help getting that takeoff distance even shorter? Your not losing airflow diagonal across your airfoil with the dihedral dialed down.

How's the Catalina compare to the regular Avid on floats? Which gets out of the water quicker? Is the dihedral on the Catalina the same as the Avid Flyer?

Turbo, the foam leading edge idea needs to be tried. It could be hot wired out of strips of foam in short time. If it works the foam could be glued directly over the fabric covered leading edge and another cap of fabric glued down on top of it. Just got to make sure it's the right kind of foam. Mek is an awful strong solvent and I suspect it eats foam.

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Posted

The foam would have to be urethane for sure, then.  We know what mek does to styrene!  

So I think Leni's right.  Dean was protecting us from ourselves. The bird wastes no time or runway getting airborn with no flaps.  Still, I insist,  a takeoff detent would have done the trick.

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