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Washout... teach me!

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Posted

Planning on doing the  factory wing extensions on my short wing KF5.

Think the washout will need to be reset?  (The KF instructions do not specify to do so)

And bigger picture: what is the effect of having more (or less) washout than spec?

 

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Posted

The washout makes the root of the wing stall first, thus preserving roll control as the stall announces itself, since the tips will have nice regular attached flow and the ailerons/flaperons will act normally in that area. My gut feel is that the flaperons will have the same control with or without tip extensions, so leave the washout as it is. 

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Posted

It depends on what you have in the wing now and what "spec" your bird was built to.  Over the years the amount of washout has gone from almost 2" to .5".  If it was me, I would be building with no more than .5" of washout if I could do it.  You are limited to how much you MAY be able to take out by the way the ribs are glued in, the tank fastened in and if you have adjustment on each side.  In the days of model airplanes we would twist the wing to put more or less in and hold it while we hit it with the heat gun to shrink the wrinkles in the covering.  The covering would then hold it in place.  

I am going to attempt to take some of the washout from my Avid wing by making new struts from the rear lift struts from a PA-18 (streamlined tubing) and putting adjustment on both rear legs so I can crank out a bit of the twist.  I can do it by hand and pull some out so I think I can get away with treating it pretty much like a model airplane and hitting the fabric wrinkles with the heat gun.  If not, I will rebuild the wings and extend them and take out the 1.750" washout that is in it now.

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Posted

Many aspect-ratio 6 wings have no washout at all, since the induced flowfield unloads the tips anyway, so the root stalls first.  Avids are a bit over 7, so one might think we don't need the 4.5 degs of washout spec'd for them.  That may well hold if you have VGs, and can't stall when landing anyway, since your nose would be pointed so ridiculously skyward.  Without VGs, the stall, at least with the Avid STOL airfoil with cylindrical leading edge, is abrupt, so the washout may be there to soften the stall.  Another argument for the washout is that it allows tighter-radius turns at low speed without the inboard tip letting go first.  It could save your bacon if you inadvertently got too deep into a box canyon.

If your short wings are already built, you may be stuck with the washout you get by extending them.  Otherwise, you'd be adding stress to the wing structure, unless there's a way to relieve it.

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Posted

Thanks guys.  Yeah I'm running the stolspeed VGs.  I'll ref the builder's manual and check to see where my washout is set.

I know it can be adjusted up on the top of the lift strut attach points.

If you have a lot of washout set what is the effect?  More draggy?

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Posted (edited)

If I remember right , when building the short wing Kitfox model 4, the manual instructs to build it as a long wing with 1/2 inch washout, then cut 18 inches off the root end.  So. if you extend the wing 18 inches you should be back to 1/2 inch washout.

If you have a Model 5 builder's manual I suspect you will find it instructs to build the model 5 short wing the same way with 1/2 inch of washout.

For what it's worth, there was an option to build the model 5 with removable wing extensions.  You might check with the factory about retro fitting yours with that option.

 

Edited by tcj
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Posted

If I remember right , when building the short wing Kitfox model 4, the manual instructs to build it as a long wing with 1/2 inch washout, then cut 18 inches off the root end.  So. if you extend the wing 18 inches you should be back to 1/2 inch washout.

If you have a Model 5 builder's manual I suspect you will find it instructs to build the model 5 short wing the same way with 1/2 inch of washout.

For what it's worth, there was an option to build the model 5 with removable wing extensions.  You might check with the factory about retro fitting yours with that option.

 

Wouldn't cutting the spars down at the root end move the strut attach points inboard?  This would add unnecessary complexity while limiting ability to extend the span later.

And yeah, that washout is a tradeoff of increased drag for increased safety.

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Posted

If I remember right , when building the short wing Kitfox model 4, the manual instructs to build it as a long wing with 1/2 inch washout, then cut 18 inches off the root end.  So. if you extend the wing 18 inches you should be back to 1/2 inch washout.

If you have a Model 5 builder's manual I suspect you will find it instructs to build the model 5 short wing the same way with 1/2 inch of washout.

For what it's worth, there was an option to build the model 5 with removable wing extensions.  You might check with the factory about retro fitting yours with that option.

 

Wouldn't cutting the spars down at the root end move the strut attach points inboard?  This would add unnecessary complexity while limiting ability to extend the span later.

And yeah, that washout is a tradeoff of increased drag for increased safety.

You locate the strut attach brackets during the rigging process, after the wings are built. I don't follow your thinking on limiting ability to extend them later.

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Posted

O.k., it's in the sequence of operations.  My assumption was that the strut attachment location had been established first.  I would think that if you were building the short-wing version, the first thing you'd do is cut the spars down.

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Posted

Many aspect-ratio 6 wings have no washout at all, since the induced flowfield unloads the tips anyway, so the root stalls first.  Avids are a bit over 7, so one might think we don't need the 4.5 degs of washout spec'd for them.  That may well hold if you have VGs, and can't stall when landing anyway, since your nose would be pointed so ridiculously skyward.  Without VGs, the stall, at least with the Avid STOL airfoil with cylindrical leading edge, is abrupt, so the washout may be there to soften the stall.  Another argument for the washout is that it allows tighter-radius turns at low speed without the inboard tip letting go first.  It could save your bacon if you inadvertently got too deep into a box canyon.

If your short wings are already built, you may be stuck with the washout you get by extending them.  Otherwise, you'd be adding stress to the wing structure, unless there's a way to relieve it.

As I have stated before, I am pretty sure that Dean did his best to build an idiot proof plane so we could build them in the garage then learn to fly in our creation.  While all that washout might help a new pilot from becoming a smoking hole in the ground, I truly believe that it also makes us leave a lot of performance on the table.  For someone that is a competent pilot and pays attention to flying the airplane first and foremost, I think taking A LOT of the washout out of the wing will make it a better performing bird.  If your one that likes to haphazardly fly and just kind of meander around the sky, leave it as is.

:BC:

 

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Posted

Many aspect-ratio 6 wings have no washout at all, since the induced flowfield unloads the tips anyway, so the root stalls first.  Avids are a bit over 7, so one might think we don't need the 4.5 degs of washout spec'd for them.  That may well hold if you have VGs, and can't stall when landing anyway, since your nose would be pointed so ridiculously skyward.  Without VGs, the stall, at least with the Avid STOL airfoil with cylindrical leading edge, is abrupt, so the washout may be there to soften the stall.  Another argument for the washout is that it allows tighter-radius turns at low speed without the inboard tip letting go first.  It could save your bacon if you inadvertently got too deep into a box canyon.

If your short wings are already built, you may be stuck with the washout you get by extending them.  Otherwise, you'd be adding stress to the wing structure, unless there's a way to relieve it.

As I have stated before, I am pretty sure that Dean did his best to build an idiot proof plane so we could build them in the garage then learn to fly in our creation.  While all that washout might help a new pilot from becoming a smoking hole in the ground, I truly believe that it also makes us leave a lot of performance on the table.  For someone that is a competent pilot and pays attention to flying the airplane first and foremost, I think taking A LOT of the washout out of the wing will make it a better performing bird.  If your one that likes to haphazardly fly and just kind of meander around the sky, leave it as is.

:BC:

 

Can't disagree there.  4.5 degs washout on an AR 7ish  wing is a lot!  Interesting that Denny pulled a lot of that back out in the KF.  I'm not going to change mine, as it looks like a lot of work, and I'd rather fly anyway.  Best of luck with your wing rebuild.  I'm sure you'll end up with a better performing bird.

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Posted (edited)

O.k., it's in the sequence of operations.  My assumption was that the strut attachment location had been established first.  I would think that if you were building the short-wing version, the first thing you'd do is cut the spars down.

 

O.k., it's in the sequence of operations.  My assumption was that the strut attachment location had been established first.  I would think that if you were building the short-wing version, the first thing you'd do is cut the spars down.

The purpose of leaving the spars full length is to get the proper amount of twist in the wings.  A jig is made of a level saw horse under each end of the full length spars with a 1/2 inch block under the tip end of the rear spar.  If you are building a short wing...AKA Speedster...you do not install ribs numbers 1 and 2.  Rib bay #1 (18 inches) gets cut off.  Rib number two is left out along with the drag/antidrag tubes to make room for the fuel tank which also serves the purpose of the drag/anitdrag tubes 

Photos of the tip end of a left wing.  Note the 1/2 block under the rear spar.

IOyq7kz.jpg

 

SSrZ54s.jpg

Edited by tcj
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Posted

In comparing the stock Avid HH wing to the later versions of mods and extended wing I have progressed to, I have found all the changes to just be gradual improvements but none have resulted in a real large single improvement.  I considered the stock Avid HH wing to be very mild in the stall and not abrupt at all; mine would just stall, drop the nose lightly and start flying again.  I added VG's and they made a slight difference but the biggest improvement I found from them was improved aileron control.  I added the Kitfox leading edge cuff and that improved the climb and stall a little bit. I increased the flap travel to 26 degrees and that made a fairly significant improvement In shortening the takeoff and landing but in some cases it blocks out the rudder when you are not in a calm stable approach so you cannot use it fully all the time.

On my current wings I extended the wings by 16.5" and the flaperons by 18", I reduced the washout to 7/8" and would have gone to 1/2"  but the built in tank twist would have required a lot of anti-twist by the lift struts, so I compromised at 7/8" because I didn't want to induce that much stress into the wings to get them to 1/2".  I also added aluminum  leading edge to 18" back and added the KF cuff and installed the VG's at 1.5" from the tip of the leading edge and significantly reduced the spacing of the VG's on the outboard 1/3 of the wing.  Overall it is an improvement but not as dramatic as you might think, but the improvement is especially dramatic when the plane is loaded heavy.  It gets off much better and lands more controlled and slower when I have it loaded very heavy than it used to with the stock wings.

I flew Jack's Avid+ last weekend and I had not flown it for about a year.  It becomes very dramatic when you do that because even though the stall speeds are not much different by the numbers, I can easily control mine at landing speeds approaching just above the stall and easily arrest the sink rate with a slight amount of power, and I am unable to do that with Jack's plane.  The predictability and level of control at slow speeds is pretty dramatic.  With fairly light loads mine will land in 200' or less if I get on the brakes fairly hard; and with a little headwind, 100' is not too hard to do; where I have a difficult time landing Jacks in 400', in fact I don't think I could get it under 600' without having time to really get tuned into it.

So all in all, I am very happy with the  new extended wings and all the mods but each thing is just an incremental improvement.

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