Avid STOL airfoil -some calculations

94 posts in this topic

Posted

Every take off I do is with full flaps.  Start the roll with no flaps, as soon as the tail comes up yank in full flaps and pull back on the stick.  Once in ground effect I will gain more speed and bleed flaps if I have room in front of me before really starting the climb.  If I have trees to clear I keep the nose pointed skyward as I slowly bleed flaps.

Every landing is also with full flaps unless nasty cross wind dictates otherwise.  The second the wheels touch I dump the flaps and she is planted on the ground and done flying.

:BC:

 

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Posted

Wow!  Gotta try this!  Full flaps appears to only be a 10 deg or so deflection, so I reckon there's little concern for running out of roll authority.

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Posted

I have mine dialed in for a bit more deflection.  My turtle deck is my control stop :lol:

:bc:

 

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Posted (edited)

guys;

I have a very short airstrip, so short you can´t call it airstrip, just a patch of grass, my take off technic works pretty fine, standing still, WOT, no flaps, start ground roll until I reach 35Kt full flaps and it will jump in the air shorter than starting the ground roll with flaps; conversely when I coming in for landing as slow as I can full flaps; touch down at 37kt (I´m heavy @$$pilot) kill the flaps and it will stay on the ground; if I leave the flaps on, I will be hoping along until I run out of space.-

Playing with the flaps does have a good effect on short take off and landings.-

Though I would love to try the suggested modifications of Riblett on the undercamber to improve cruise speed without sacrificing the take off roll.-

Regards

Dimi3

 

Edited by wolves200
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Posted

I totally agree.  The induced drag due to that pitch moment is inescapable via speed, as tail download simply increases!  You feel like you've hit a wall or something.  I am currently having trouble learning how to land gracefully, but I think it's a different aspect of this problem, more related to the cylindrical leading edge, as it puts too much surface curvature in a region too far from the stagnation point, so it adds to the overspeed!  Elliptical shapes do better in the leading-edge region.  The airfoil seems to be a leading-edge staller.  You just fall out of the sky, except the wing's twist and lack of taper mean that the wing quits abruptly inboard, with the outboard still flying, so it doesn't roll sharply upon stall.  Guess I've got to learn to fly it on - or do carrier landings!  I've banged it in a time or two!  Still, the Avid's a hoot to fly!  But I'm not ready to paste on VGs - yet!  Those aren't all goodness either.  They reduce maneuvering speed,  slow you down, and make the airplane hard to wash!  The airplane does almost flare itself, but it's like playing chicken with the ground!

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Posted

I did not see a noticeable speed drop with the VG's my MK IV cruises at 110-115 mph @ 5500 rpm with the STOL wing.  Not to sure where you get the idea that the wing is hard to wash with them on, using a soft 3" bristle brush just simply brush front to rear right over them and presto, nice and easy.  I have VG'S on both of my planes.  This is of course just my experience so take it for what it is, one data point, might not suit you as well. 

I do have the KF plastic leading edge on my wing and I think you are right about the leading edge of the standard wing.

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Posted

Great feedback.  My remaining objection to VGs is that, by reducing stall speed, they also reduce maneuvering speed.

Maybe they are good for landing: at least you don't stall & plop down so easily, as I have done many times.

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Posted

I have had the best luck landing with power, on gravel, 3 point. I supposed a guy better know what it will behave like power off like during an engine failure... light airframe = low inertia= high drag=higher drag as the AOA increase and that last 10 or 20 mph bleed of in an accelerating fashion very quik.

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Posted

Here was the plane before I bought it. The seller kept the grove gear and put kitfox bush gear with coil springs (hatz type gear) 

He used a technique where he got rolling on takeoff and popped the flaps down to get in the air.. .It was pretty quik with a 582 no matter how you did it when empty.

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Posted

24K for a fox w/new engine might seem a tad on the high side right now. Wait a couple years and that will be the going price for one in good shape. Experimental kits are now going out of sight, pricewise. Even a single seat ultralight kit completed will get close to that.  GA aircraft on the other hand, are dropping in price. Yes, there are more of them, more for sale, and maintaining one is getting expensive. I think that the pilot population is getting older and they are moving to experimentals in the LSA category. And that will drive the prices up and the supply down. A new Ford pick-up is pushing 50K so a good condition older Avid/Fox for half or less is a good deal. I'll probably have 15K in mine when its finished with all new parts and hardware, but a new Fox for less than 18K. We all know people that will spend 20-30K for a bass boat or camper to use a half dozen times a year and they drop in value as time rolls on. If you pay attention, the older Avid/Fox available market is drying up. A common man's bush plane (with limitations) will always be in demand. You guys that already have one, better hang on to it, at least for a while. Might pay better interest than a bank!

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Posted

After 15+ hours I'm still trying to do graceful landings consistently, so I feel your pain.  The old "keep it off, keep it off, keep it off"  mantra doesn't seem to work too well, although Avid advises us to land 3-point.  "Fly it on" seems better advice.  Still, that dance with the feet still feels a bit frenetic to me, so I'm a bit loathe to land with higher speed than is absolutely necessary.  So maybe with full flaps.  But lots of planes have airfoils that are nasty leading-edge stallers.  Like NACA23012, and virtually anything thinner than, say 12%.  Usually it's tamed with a little washout, and holy moly! the Avid's got 4.5 degrees of twist!  That's a lot!  

Thanks for sharing this cautionary account.

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Posted

I did not see a noticeable speed drop with the VG's my MK IV cruises at 110-115 mph @ 5500 rpm with the STOL wing.  Not to sure where you get the idea that the wing is hard to wash with them on, using a soft 3" bristle brush just simply brush front to rear right over them and presto, nice and easy.  I have VG'S on both of my planes.  This is of course just my experience so take it for what it is, one data point, might not suit you as well. 

I do have the KF plastic leading edge on my wing and I think you are right about the leading edge of the standard wing.

Hello Paul;

110-115mph on your avid, I guess your supercharger 140hp engine doesn´t care about under camber and plow right through the DAM drag :-) I always thought that buying such expensive engine it´s not the solution because 80 or 90hp still won´t give you more than 75kt on cruise.-

Regards

Dimi3

 

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Posted

Hey Dimitri nice to hear from you.  I'm still running the 912ULS but did do some fairing on the jury struts,gear legs and horizontal stabilizer.

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Posted

Well Trent's been looking at leaving edge cuffs as well. His execution didn't seem to do anything for his performance. It'd be interesting to see how they would do on the original airfoil

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Posted

Ian's attempt at a modified airfoil is a little more permanent.

 

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Posted (edited)

Ian's attempt at a modified airfoil is a little more permanent.

 

Well Trent's method has the advantage that it's not permanent. It didn't work so 10 minutes work returned the plane to the previous configuration. Hopefully Ian likes what he's doing when it's done because it's months more work if he doesn't or sell it off and let someone else deal with it. 

Edited by Willja67
Corrected incorrect fact

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Posted

Turbo, did you ever plot the Avid speed rib together with the pvc leading edge extrusion used on the Riblett laminar flow rib profile from Kitfox model 4-7? I'd love to see that combo compared against the KF 4-7 rib profile to see just how much the slight lower under camber on the KF profile adds to lower stall of shorter T/O.

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Posted (edited)

Doug,

I have a speedwing rib, which I could trace and measure, but no coordinate data for any leading-edge extension.  Once the rainy season sets in this fall, I plan to look at both the speedwing section and some trial geometries for a minimum-extension leading-edge glove to try to tame those nasty Cp spikes. 

Laminar boundary layer flow over fabric-covered wings, sad-to-say, is largely hype.  All it takes is the roughness equivalent of a fruit fly's post-impact guts to cause the boundary layer to transition to its turbulent form.  Considering the surface roughness due to the doped-over fabric weave, coupled with stringent requirements on the pressure distribution to achieve laminar flow, and my read is most likely we all have turbulent boundary layers on our wings.  This is actually a good thing, as turbulent boundary layers are much more tolerant of abuse.   I worked R&D on these issues for most of my professional career, mostly for large, swept, transonic wings, with some supersonic stuff tossed in there for fun, but ultimately culminating in the first production controlled laminar flow system, on the tailfeathers on the 787-9 and -10 airplanes.  Ironically, I am more concerned now with assuring that those 8"-chord flaperons have a turbulent boundary layer.  Don't polish the damn things!

Anyway, it wasn't until metal-skinned wings became practical in the 30's that NACA developed the laminar-flow 6-series airfoils.  I don't think the possibility was even envisioned for fabric wings.

Edited by Turbo

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Posted

Was just watching Trent's video comparing his kitfox to the highlander and about 2:35 they start talking about using the kitfox leading edge on the highlander (same airfoil as the early foxes and avids). He said he got 3-5 mph speed increase on the top end and about the same in stall speed reduction. In conjunction with vgs it made a huge difference. 

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Posted

Just to clarify, the airfoils on KF 1-3 vs. Avid HH/STOL are quite similar but are, in fact, different.

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Posted (edited)

Will, that is an older video. After he filmed that one, Trent glued a whole bunch of foam on his leading edge, as he promised, and found it did squat. He ripped it off because it cost him gobs of performance.

Here is the story of his leading edge experiment:

Edited by nlappos

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Posted

The one thing Trent did not do is reposition the VGs , so they probably ended up too far aft with the LE extension on.  Can't say I blame him though; moving them is a lot of work!  Then there's the question of where they should be placed with the LEX on.

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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.

 

thank you very much turbo for your analysis, this help me a lot to get some numbers about my mod !

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Posted

So, Manu, how is the new trim tab working out?

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Posted (edited)

Will, that is an older video. After he filmed that one, Trent glued a whole bunch of foam on his leading edge, as he promised, and found it did squat. He ripped it off because it cost him gobs of performance.

Here is the story of his leading edge experiment:

You need to be a little more careful reading and listening. They were talking in the video i posted about the Highlanders wing which according to them is the same as the early foxes. The STI wing which is on Trent's plane is an entirely different airfoil. It already has a leading edge piece of plastic or fiberglass glued to the aluminum spar. The video you linked to stated this was a new design from kitfox that hadn't been tested yet. There was some hope that there was more performance to be had. As you pointed out there's not at least with that particular design. It may also be pointed out that Trent didn't smooth the transition between the new leading edge and the rest of the wing. That could have had a significant impact. 

SuberAvid has also added the KF leading edge to his plane and I don't remember specifics but I do remember he reported improved performance. 

Edited by Willja67

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