Turbo

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Everything posted by Turbo

  1. 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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  2. 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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  3. 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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  4. 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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  5. 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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  6. 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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  7. 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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  8. 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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  9. 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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  10. 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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  11. 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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  12. Turbo added a post in a topic Bought an Avid Speedwing, need advice   

    Being that it's a taildragger speedwing, it could prove exciting to land.  Mine's the STOL and it's exciting enough, even with its lower stall speed.  But I don't carry flap for landings.  With less span and the lower-cambered, essentially flat-bottomed airfoil, you may be able to carry more flap on final and into the landing flare wthout running short on elevator authority, which could help.  My advice is to just replace the LG bungees; the current ones are most likely more than 5 years old.  They get soft with age.  And validate that the safety cables allow more than 3" bungee stretch (5" if you want it to survive a 3-G plop-down!).  On mine, first flight, I misjudged height after the flare, and banged her in.  Too-short safety cable came up tight on the seat truss tube, crimping and bending it!  Next stop was the weld shop!  And hardware-store 3/8" bungees don't cut it; go with Aircraft Spruce's "type-1" bungee.  It's twice as stiff as the hardware-store stuff!  There are lots of posts on here on how to wrap the ends, do the installation, etc.  Good luck, and welcome to this forum!  I think we've got a great bunch of guys here, with a lot of valuable experience to share.
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  13. Turbo added a post in a topic latest air vehicle (not really)   

    Any problem and you slam down the collective and go into autorotation!  Oops - no collective!  Great for clearing leaves off the lawn, though!  Maybe I've grown stodgy in my old age, but I can't consider these things as viable aircraft - ever!  So original.  Scaling up a drone.  Had to happen eventually.  I'm way more impressed with Brian Austein's Woodpecker.
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  14. Turbo added a post in a topic Fuel gage in Wingtank   

    FWIW, another approach is a fuel flow meter & totalizer.  Fuel flow data are valuable too.  This is the approach I have taken.  I'm thinking that once I have gathered enough flight data I will be able to make good estimates of speed to fly in various headwinds/tailwinds to maximize range.  I know I'm waxing a bit nerdy here, but hey, why not?
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  15. Turbo added a post in a topic covering process   

    Doesn't isopropyl alcohol dissolve latex paints?  Seems to me it does so quite slowly. FWIW.
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  16. Turbo added a post in a topic Need help identifying this pully   

    Wow, that must be a Magnum thing!  My old model C doesn't have these pulleys.  Just a couple of cable guides.
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  17. Turbo added a post in a topic Avid Flyer Model C Seat   

    Epiphany!  Buy matching aluminum-framed lawn chairs, cut off the unneeded parts, keeping the seat parts, then string them in like the original sling seat!  Still not too good for lumbar support, though, but lightweight.  Less padding means more leg-and-headroom.  Got to have been tried, wouldn't you think?
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  18. Turbo added a topic in Avidfoxflyers General Hangar   

    How best to descend
    For us who fly two-strokes, descending from cruise altitude is not so simple.   C5 , Leni, and others with lots of experience know what not to do when descending, in order to be kind to our little engines, and have made mention of same.  It seems like it's either descend at high speed under power if the air is smooth and it's not too cold out, go back to idle and descend rapidly if it's bumpy, and also not too cold out.  I was thinking yet another way is to pull the stick back and carry some power, riding down on the back side of the power curve, using high induced drag as a way to keep the power up.  This would seem to be a viable approach, particularly if the air is rough, and especially if it's bloody cold out.  What say the voices of experience here?  You guys with the draggy monster off-airport tires are cheating, and have it too easy, though!  Some of us still have golf-cart Carlisles; I even have wheel pants!  JimChuk says his skis are relatively clean.
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  19. Turbo added a post in a topic Avid Flyer Model C Seat   

    Uuh - doesn't it have to -like- tie to the frame tubes?  What other reasonably simple and lightweight mounting options are there, realistically?  We don't have a for-real floor here.  I'm now thinking of a recurving stiff back with a butt sling as perhaps a reasonable approach.  The sling seat's padding for the backs of one's thighs over the seat truss is nice, though.  Maybe that's overkill, though, as I typically find my knees bent at almost 90-degrees while flying anyway, which should lift my thigh-backs up away from the seat truss.  Kinda like a circus clown car!
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  20. Turbo added a post in a topic What model is it?   

    A mark IV with an offset fin!  Live & learn!
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  21. Turbo added a post in a topic First Flight   

    Congratulations!  Nice looking bird!  
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  22. Turbo added a post in a topic 582 unexplainable stoppage   

    Vance, is it possible that a piece of ash flaked off the head or piston top and wedged its way sideways between the two?  Maybe that would just crunch!  Maybe a spin with plugs out would cause it to show itself, if you don't have a borescope.
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  23. Turbo added a post in a topic Avid Flyer Model C Seat   

    Gotta admit I have the same concerns.  The sling seat seemingly offers absolutely no chance of lumbar support.  I am looking into an alternative based on a couple of slabs of thin, maybe reinforced plywood with thin foam covering for comfort, with a piano hinge between the two and laced to the tubes like the sling seat.  Has anyone tried this?  With some stiffness, there's at least a chance of some lumbar support.  Of course, you wouldn't want it to fail if you hit an updraft too hard or otherwise inadvertently pulled a few G's, so some engineering needs to go into such a design.  Maybe a hi-tec version with a foam and carbon sandwich for the lower piece? Carve in some bun dents?
    A lighter, simpler alternative is a "back-board" with thin padding to place over the flexible seatback.
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  24. Turbo added a post in a topic New toy coming for the shop   

    Greater chord can create higher loads, so you will have a lower maneuvering speed, even with the same span.  V-squared will come down roughly as wing area goes up (ignoring aspect ratio reduction)  in other words, the product of S x V- squared is constant.  You will want to keep the quarter-chord point at the same lengthwise location, I should think.  This will help to preserve the relative loadings on the two spars, and will minimize changes in the aerodynamic center location although the larger H-tail will move it aft somewhat, increasing pitch stability.   Max lift coefficient of the new airfoil, if higher than that of the Avid, will also drive maneuvering speed down, just like VGs do.  The difficult part here is assessing that difference, since with the flaperons we have to consider this a two-element lifting system.  The wing airfoil never flies alone.  It's like having camber-changing flaps, like on a sailplane.  Comparing at cruise flaperon setting makes the most sense, though the optimum may be different for the two section shapes.  Fortunately, this can be assessed after-the-fact, in flight test, by measuring your new stall speed. 
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  25. Turbo added a post in a topic Hi pitched IVO prop blades on a 582???   

    I have the 72" 3-blade IVO, with pitch currently at 52", so I guess I have that prop.  Seems to work fine on my 582GH with 3:1 box.
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