Building wings without washout

11 posts in this topic

Posted

I have read many posts about wing building and almost universally it has been suggested to remove all or most twist. I have aerobat speedwing components but am going to make new ribs to the Riblett GA30-613.5 section and extend the wings for better low speed performance. Hopefully someone here on the forum has built wings without twist and can share their experience. I have a stock 18ga tank and another tank (same length) with a built-in storage area. I can't find where at the moment but read in one of the build manuals that the twist can be reduced from 2" to 1 1/2" but cannot be reduced further due to the built-in twist of the fuel tanks (had not considered that before.) That would be about 11/16" of twist in the length of the tanks. For the lift struts, the front spar bracket would need to move inward about 1/2" which shouldn't be a problem but that would rotate the strut slightly which would put the fuselage attachment point in a twist.

Also as far as extending the wing spars, I haven't found any information on how the connection is made between the spar extensions and the original spar. The extensions would have to be .065" wall (Can't get the .083") so the ID would be slightly off. I assume an external sleeve would have to be used. Does anyone know how long the sleeve was that was provided in the factory extension kit? Another question is how much you can safely extend the wing without needing to go to new full length spar tubes. Not a big issue but still another $500 add to the build.

Here is a drawing to go with my questions. Hopefully it is legible. Any guidance or suggestions would be appreciated.

 

 

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Posted

I don't think the lift strut being attached 1/2" in will cause any concerns.  Also, I believe when the speed wings are extended people go 3' wider on each side.  Here is a drawing from Avid on how they sat to do the extensions.  JImChuk 

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Posted

wing spar extention.jpg

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Posted

Thank you Jim. Do you know of anyone who has built there wings without washout and what they thought of the change. Is the drawing for the extensions posted on the forum? I haven't seen it. Thank you. 

Dean

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Posted

Thanks again - the drawing helps a lot! I guess I should have thought of using shim material to make up the difference.

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Posted (edited)

Another thing, if you build the wings with the new thicker profile, the tanks should be no problem.  as long as they don't stick up and make a lump it shouldn't change the airfoil shape like it would with a regular wing.  The tanks are fiberglassed to the spars, so you could block them to stay secure while you fiberglassed them, I would think.  Don't know of anyone who used this airfoil.  Maybe they will pipe up.  JImChuk

Edited by 1avidflyer

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Posted

The top of the new profile looks like it is OK with the tank. The bottom is very close toward the back half. I originally looked at the GA30-612 like the Kitfox but it was nowhere close to working with the original tanks. The 613.5 profile also allows the rib to be rotated for a little more positive incidence.

 

612 speed 613.JPG

Bottom.JPG

GA30-612 - 613.JPG

Speed 612 613.JPG

Speed 613.JPG

Top.JPG

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Posted

Washout is primarily a safety concern. Making the root at a higher angle of attack means it stalls first, giving you stall warning ( buffet and pitch nods), but keeping the tips unstalled and roll control still available. If the whole wing stalls at once, roll control can actually reverse, since the flaperons call for more angle of attack, so the down flap wing might stall first.

 

I'm not sure what advantage you think you'll get with zero washout.

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Posted

Sorry, I should have mentioned in my original post. I would not remove all washout but would reduce it considerably - More like 3/8"-1/2". I believe that the J3 cub has a specified washout of 3/8" in 30". That should prevent wingtip stall at the same time as the root, but still provide more uniform lift along the length of the wing. The advantage would (hopefully) be a lower stall speed and lower drag in cruise flight. From everything I've read on the subject, 2" seems excessive. I plan to incorporate the F7-A control linkage geometry to help prevent any control reversal issues. I have never flown an aircraft with flaperons and don't know how that would play into the mix. I definitely appreciate your input. It would be great to hear from anyone has done this and what the outcome was. Thanks, Dean

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Posted

There is a letter from Harry Riblett concerning modifying the Avid airfoil. At the end of the letter (second page) he states that because the original Avid stol airfoil and the modified airfoil have such mild stall characteristics, the wing twist could be reduces or eliminated completely. There is a copy of this letter in the files and forms section but it is hard to read. 

1st pg of Riblett letter.JPG

2nd pg of Riblett letter.jpg

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