Effects of Wing Incidence

[szicree]

A while back I realized that the incidence of my wings differ by 0.2 degrees. I've measured and remeasured with several different devices and it definitely is about .2 degrees difference. It appears that the right is probably responsible for most of the problem (spar too high). I emailed Van's and they said not to worrry, that lots of planes were built before digital levels. Well, as it happens, you can definitely see .1 degrees on a bubble level. In any case, I am still worrying about it. Can anybody give me an estimate of the actual effect of .2 degrees difference? I hear about heavy wings, and some REALLY heavy wings. I'm still on the fence about trying to fix it. Thanks folks.

[fodrv7]

Steve,
I had 0? on one wing and between 0 & 0.1? on the other . That is a slight twist on QB wings.
I then found I had a pronounced heavy left wing and traced it to the right aileron being 1/16" lower than the left. Once again QB ailerons.
So I slotted the ailerons bolt holes 1/16" and did a test flight.
The effect was so great that I now had a slightly heavy right wing.
I then took a new pair of aileron hinge brackets and drilled them 1/32" from the original pre-punched holes and she flies straight as an arrow.

So the point is, there is so much wing leveling ability in raising or lowering an aileron I wouldn't worry about it until you fly.

Pete.

[Kevin Horton]

According to Xfoil, the lift curve slope of our airfoil is about 0.12/degree. I.e. the coefficient of lift changes by about 0.12 for every degree change in angle of attack. A 0.2 degree difference in angle of attack between the two wings would create a difference in lift between the two wings of about 22 lb at 80 mph IAS and about 136 lb at 200 mph IAS. As Peter James said, these small differences should be easily corrected by adjustments to the ailerons.

Build on.

[szicree]

Thanks for the info guys. If I assume that the 136 lbs is acting at the center of the wing, that's like 22 extra gallons of fuel midwing. I'm sure I can correct with rigging, but I wish it weren't so.

I've got some new ideas and observations too. I've been out there all day measuring and checking, and here's what I now believe. My left wing, longerons and h-stab all measure the same. This was measured using the recommended blocks and a Smart Level (The Devil's Plaything). Unfortunately, the right wing is nose low by about 0.2 degree. Judging by the back and forth on the Smart Level, I'd say it's closer to 0.25 degree. I still can't figure how this happened, since I spent all day checking and rechecking before I drilled. The only thing I can figure is that the parts moved while the hole was being enlarged to final size. For those that haven't drilled this yet, do yourself a favor and get a precision bubble level for this operation. They have far greater precision than this digital gadget, however the Smart Level can't be beat for setting control surface travel limits.

Now for my thoughts on erasing this screwup. I'm considering relocating the center of the hole on the fuse clevis downward by 1/32 and redrilling to 3/8 with a really secure guide block, and then doing likewise (only upward) to the hole in the spar stub. This should move the whole trailing edge down by 1/16, which will produce an increase in incidence of about 0.13 degree. Worse case scenario this reduces my difference in incidence from 0.25 to 0.12. Obviously I'd use undersize drills and ream up to the final size. I also figure on practicing this entire procedure on mocked up scrap pieces a thousand times first. Regarding edge distances, If understand correctly, the center of a 3/8 hole should be 3/4 from all edges. Based on what I've measured so far, I can meet this requirement.

Any thoughts or advice would be greatly appreciated. I know many think I should just build on, but this one is really bugging me. If it was 0.1, I'd definitely put it behind me, but 0.2? 0.25?? AARGH!

One last bit: I'm out in the yard today puzzling over all this when a soccer-mom walks up and says "Is that a glider?!" After pausing for a beat I reply "I sure hope not"

[captainron]

My only advice (other than to use a precision level) would be to make sure that whatever you are using to check wing incidence angle, is to keep the instrument (level or "smart level") in the same spatial orientation to check each side. In other words, say you have a level taped to a straight edge and you are checking the right wing. After taking your reading, do not turn the straight edge end for end when you walk around to the left wing. Mark your measurement tool "forward" and "aft" so you get accurate readings with the tool orientated the same on both wings.

[captainron]

Originally Posted by Kevin Horton

A 0.2 degree difference in angle of attack between the two wings would create a difference in lift between the two wings of about 22 lb at 80 mph IAS and about 136 lb at 200 mph IAS. As Peter James said, these small differences should be easily corrected by adjustments to the ailerons.

Build on.

136 lb should not be considered a "small" difference. I've been looking for one of my old Hawker manuals, but if I remember correctly, the max imbalance for T/O and landings was 400 lbs. And this is a 27,000 lb airplane rotating at around 127 kt. I'm sure I wouldn't want a built-in difference in lift of 136 lb in a 1500 lb airplane.

[fodrv7]

I have tip tanks that hold 40 litres. Multiply by 0.72 means the hold 28KG of fuel, (Don't you just love the Metric system.) Now, for the Imperial stuff;
28KG x 2.2 = 61lb at the wing tip.
My arithmetic tells me that this is 122lb at mid span. Pretty close to Kevin's figure.
I have no trouble trimming wings level with aileron trim when one tip is empty and the other one full.

Steve, if you want to drill out the rear spar bolts to 3/8" why not wait until you have done a few flights. It may not be necessary and you can do it just as easily then. I also seem to remember edge distance was critical when drilling the rear spar attach.
Pete.

[szicree]

Originally Posted by fodrv7

Steve, if you want to drill out the rear spar bolts to 3/8" why not wait until you have done a few flights. It may not be necessary and you can do it just as easily then. I also seem to remember edge distance was critical when drilling the rear spar attach.
Pete.

Pete,

I definitely appreciate your experience on this and would tend to agree that it could wait. However, to relocate the holes would be much easier with the wing off. I just hate the idea of having such a large out of trim condition built into the plane. I'm gonna run it by Vans next week regarding the edge distance concern, but we all know they're just gonna say "build on".

[tcone1]

Hey there,

You can double the resolution of you existing level by rotating it 45 degrees from the chord line...leave the fixture you are using to hold the level where it is, but rotate the level on your fixture. you should now be able to tell whether it's .2 or .25.

BTW the maximum fuel imbalance on all Hawker models is 500 pounds.

Tim Cone
Hawker dork
Mooney owner
RV-8 builder

[Tom Martin]

Steve
You will not be able to drill up to 3/8" and maintain an edge distance of 3/4". Fly the plane like it is. As many others have pointed out there are ways of correcting an imbalance in these wings. It might even surprise you and fly level the way it is.

Tom Martin
one RV4 and six rockets