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conversion of undercamber airfoil to Ribblett


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Posted

THe sky raider and ridge runner use the under camber airfoil. SO did my Kitfox model 2. My kitfox model 4 classic had what I think was the Harry Ribblett airfoil and it was more predictable than the undercamber wing. My question is, how much trouble is it to span across the bottom of the wing ribs to change the undercamber airfoil to the RIbblett airfoil??

Have any of you fine feathered friends tried this??? IF so was it worth the trouble??? Did ti fly better???

Thanks Mark Smith

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Posted (edited)

Mark,

See my post on the undercambered STOL airfoil under Avid Model C.  There's no special magic in Harry Riblett's airfoil.  The USA35B airfoil, and NACA 4415 are similar.  One could help cruise performance by simply filling in the lower-surface concavity with a straight line!  The reduction in nose-down pitching moment is about 40%, which reduces the H-tail download, and induced drag.  One might acccomplish this by gluing on lower-surface ribs shaped out of foam, then skinning on a new second lower surface.  The subtleties of the lower surface shape are relatively unimportant to lift.   Your airplane might lose a little of its STOL magic, though, as the filled-in lower surface in effect shows up as a slightly reduced incidence relative to the fuselage, and effectively lower angle-of-attack with wheels on the tarmac.  The flaperons' killer app is popping you off the tarmac!  From there on, it's all gravy!

Edited by Turbo
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Posted

I think the biggest gain would be to take all the washout out of the wing.  2" is way too much and if you look at the wing in flight you will see that the outboard 2/3rds of the leading edge is pointed at the ground.  That has to be inducing a ton of drag and its pulling you out of the sky.  I seriously wish I would have taken most of it out and only left 1/2" of the washout in the wing when I rebuilt mine.

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Posted (edited)

Mark - That last pic is a heartbreaker!  

Leni - all that twist might have been to soften up the stall, in addition to keeping it rightside up, by confining onset to the root region.  The cylindrical leading-edge shape is decidedly non-optimal, and strikes me as a significant part of the problem.  

Yeah, that twist seems excessive.  On the leading edge shape: I wonder how much LE extension it would take to cure, of course with a major redistribution of the streamwise surface curvature locally.  Perhaps a set of glue-on foam sections, wire cut, would do the trick.  This wnter when it's raining...?

Edited by Turbo

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Posted

mine has aluminum leading edge sheet that is wrapped over the front spar and looks a lot like the kitfox plastic leading edge profile.  I am not at all convinced that it was really done right or has the optimum shape.  The kitfox STOL wing is actually faster than mine is and climbs better as well.  I am still kicking around what I want to do with my wing when I am ready to rebuild it, but I am pretty sure I will be going with a KF Mod IV rib and extending the wings.  I am also going to pick up some cub lift struts and make new struts for mine using the cub rear struts.  

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Posted

Mark - That last pic is a heartbreaker!  

Leni - all that twist might have been to soften up the stall, in addition to keeping it rightside up, by confining onset to the root region.  The cylindrical leading-edge shape is decidedly non-optimal, and strikes me as a significant part of the problem.  

Yeah, that twist seems excessive.  On the leading edge shape: I wonder how much LE extension it would take to cure, of course with a major redistribution of the streamwise surface curvature locally.  Perhaps a set of glue-on foam sections, wire cut, would do the trick.  This wnter when it's raining...?

It was to help the stall and trying to make the plane idiot proof.  Later models of the KF went from 2" to .5" I think.  I know for sure in the hands of a decent pilot it does not need all that washout. 

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Posted

Just measured mine.  That 2" translates into a 4.5-degree washout!  That's a lot, especially for an AR 7ish constant-chord wing!  Gotta be to protect the most ham-handed of us from ourselves, as you say.

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Posted

A sensible man once said in not so many words, "if you want to go  faster than 85, buy another aircraft". A decked out Super Cub may do 95 full bore, but look at the other qualities it has. Everybody relates high performance with speed but it doesn't always mean speed. Short take off and short landing is classed as high performance, also. If it does something quicker than you're familiar with, then its high performance to you. Fly a C-150 for a while and then go jump in a light Avid/Fox and becomes clear. Oh well. just my monthly rant!

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Posted (edited)

Point well taken.  With max AoA set by the landing gear, the takeoff run is shortest with the high camber, and of course the flaperons deployed at the proper time.  Being on floats makes this even more critical.  However, once you're in the air, you lose that AoA constraint, and the steepest angle of climb is set only by wing loading and power loading.  The wing camber level becomes unimportant.  As a TD landplane the penalty for not having so much camber is only in a longer ground run.  The benefit is a faster, more efficient cruise.  

Edited by Turbo

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Posted

Leni,

think we talked about this, on my new wings I set the washout to 7/8" in the 160.5" extended wing.  I was planning to go to 3/4"  or less but the factory fiberglass tanks are set for the 2" washout and it is very difficult to take it out more than what I did; and I had to do that by setting the twist back using the extended lift struts that you welded.  It still stalls very docile with 7/8" washout but I also increased the vortex generators on the outside 1/3 of the wings.

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Posted

Point well taken.  With max AoA set by the landing gear, the takeoff run is shortest with the high camber, and of course the flaperons deployed at the proper time.  Being on floats makes this even more critical.  However, once you're in the air, you lose that AoA constraint, and the steepest angle of climb is set only by wing loading and power loading.  The wing camber level becomes unimportant.  As a TD landplane the penalty for not having so much camber is only in a longer ground run.  The benefit is a faster, more efficient cruise.  

Every designer or engineer out there has been hunting for the holy grail of wing design. Low stall speed and fast cruise speed. a few have come close but no cigar just yet. Two different problems working against each other. It all comes down to the desired mission one wants in an aircraft  Always a trade-off somewhere in the process.

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