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About Turbo

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    Advanced Member
  • Birthday 05/07/1945

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  • ICQ rthrpwll@gmail.com

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  • Gender Male
  • Location Columbia Gorge, WA
  • Interests Avid model C, windsurfing, hunting

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Turbo's Activity

  1. Turbo added a post in a topic Tail Wheel Swap to a Matco Pneumatic 8"   

    Heat the spring to red-hot in the vicinity of where you will be drilling, then let it cool in still air.  After drilling the hole, heat the spring again to red-hot, then dip it into water. This will undo the anneal and re-establish the martensitic, body-centered cubic lattice structure, returning it to spring steel.  Materials 101.
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  2. Turbo added a post in a topic Avid STOL airfoil -some calculations   

    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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  3. Turbo added a post in a topic Avid STOL airfoil -some calculations   

    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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  4. Turbo added a post in a topic Avid STOL airfoil -some calculations   

    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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  5. Turbo added a post in a topic Avid STOL airfoil -some calculations   

    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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  6. Turbo added a post in a topic Avid STOL airfoil -some calculations   

    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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  7. Turbo added a post in a topic Avid STOL airfoil -some calculations   

    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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  8. Turbo added a post in a topic Avid STOL airfoil -some calculations   

    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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  9. Turbo added a post in a topic Avid STOL airfoil -some calculations   

    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.
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  10. Turbo added a post in a topic Avid STOL airfoil -some calculations   

    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!
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  11. Turbo added a post in a topic Avid STOL airfoil -some calculations   

    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!
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  12. Turbo added a post in a topic Avid Trailer   

    Man, an enclosed trailer would be nice!  I hate towing in the rain, even drizzle!  My builder put together an ingenious trailer that tows the airplane level and backwards very securely, but semis send you rocking and careening!  In my case the trailer was a necessary part of the deal, as my airport's 35 miles away.
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  13. Turbo added a topic in Avid Model C   

    Avid STOL airfoil -some calculations
    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.


    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:

    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. 

    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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  14. Turbo added a post in a topic Carbon fiber spars   

    There are alloys of aluminum that are much stronger than the 6061-t6 spec'd for these birds.  2024-t3 for one, although it's not as resistant to corrosion, and 7075-t6 which is about 75% stronger than 6061-t6, and has good corrosion resistance.  Heck, there's even a 5000 series alloy, like they use on boats, that's stronger, with outstanding corrosion resistance!  All this points to the possibility of extending the span by lengthening the cantilevered part outboard of the lift strut attachment.  Doing that would also mean getting rid of the underside camber to lower nose-down pitching moment. Just going to a flat undersurface cuts the pitching moment down by almost 40%!  The alternative, of course, is enlarging the H-tail.    Small flap deflections for takeoff could easily buy back any lengthening of takeoff run for the airfoil mod.  Indeed, that's the killer app for these Junkers-style flaperons!  Dean got a lot right on these birds, hats off to him, but the STOL airfoil is over-endowed in the camber department.  This hurts top-end speed by increasing induced drags of both wing and tail, while, like I say,  small flap deflection would have done the trick on takeoff roll.
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  15. Turbo added a post in a topic Bought an Avid Speedwing, need advice   

    Oh, and on noise:  don't expect it to be even tolerable without a headset.  Most of it is engine noise anyway.  But it's that way on most light planes.  What engine do you have?  I have  the Rotax 582 with a 3- blade IVO prop and yeah, they're ground adjustable.  Not having to fly formation with strangers all the time really takes the stress out of flying vs. driving.  It's way more fun!
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