S

szicree

Well Known Member
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.
 
Fix it when flying

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.
 
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.
 
Smart Level vs Dumb Builder

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" :p
 
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.
 
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Kevin Horton said:
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.
Click to expand...
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.
 
The real world.

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.
 
fodrv7 said:
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.
Click to expand...

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".
 
greater resolution for your level

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
 
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
 
Steve,

Just thinking out loud here... One thought is "fly it the way it is", and the other is "if it can be fixed easily, go ahead"

If you choose to fly it the way it is, you are in the fortunate (?) position of having more incidence on your left wing than the right wing. That will help offset propeller torque. If it was the other way around (more incidence on the right wing), you'd have torque compounding the incidence issue.

If you're thinking about correcting the problem, are all of the inboard wing skins attached? If not, it wouldn't be a huge deal to replace the aft spar attach fitting on one wing.
 
szicree said:
Pete,
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".
Click to expand...



You can squeeze the TE's on one of your Ailerons to fix the out of trim condition after you start test flying. The diamond I rent has a trim tab screwed on to one ailerons which I'm sure is to compensate for "out of trim".
My point is factory aircraft are not perfect either. Would your performance be better if the plane was perfect? Most likely yes. How much? I don't know.


Van's will say build on and IMO they are right.
 
One thing I haven't seen talked about yet is the fact that the higher incidence wing will always stall first. You may or may not be able to correct that by tweaks to your flaps. Remember each adjustment you make to compensate will also add extra drag. :(
Tom
RV3 2000+
 
This Might Help You

I called Van's last week agonizing over a minor detail in the tailwheel attach weldment on the tail of my RV-9. After patiently listening for about 3 minutes to my concerns, a senior Van's official, who will remain nameless said, and I quote "Bolt it together, it's a farm tractor"

All too frequently, I find myself trying to build in a level of precision not necessary or even attainable in this little airplane. I think it drives the guy's at Van's absolutely nuts!
 
You know what would make me feel a whole lot better? If I could take my Smart Level down to the local airport and check the incidence on a half dozen or so spam cans. Since the thing works at any angle there'd be no need to level the planes or anything. I'd be real curious to see what exists out there. Unfortunately I don't have access to other folks' bug smashers...but if any of you do, maybe you do a little research to end or further this discussion?
 
szicree said:
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 (and best case to 0.07). 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.
Click to expand...

I've been outside checking all dimensions and here's what I got.

1. Both parts are 1.5 inches wide.

2. The centers of the current holes are 1/32 above center on the fuse clevis, and 1/32 below center on the spar stub.

3. The current holes are about 7/8 inch in from the ends on both pieces.

So if each hole is moved 1/32 it will place them smack in the middle of the material and give me the 3/4 inch edge distance. I will consult with Vans on this and I'm sure I'll get "build on, it's a tractor, etc.", but I am puzzled by the edge distance rules. Specifically, using the 2 x diameter rule, you end up with 1.5 x diameter distance between the edge of the hole and the edge of the material, thus the bigger the bolt, the more material you actually end up with between bolt and edge. I assume this is because a bigger bolt implies they'll be a greater load there, and so the material is sized accordingly. But in my case, the load is not increased. :confused:

I know we've got a lot of engineers on here and I'd love to get a better grasp of this stuff, so feel free to expound. Thanks to all who've advised up til now.
 
.2 deg wing split requires how much aileron?

Steve, I took Kevin's numbers and did a back of the envelope to see how much aileron deflection would be required. Since the aileron is further out on the wing, it needs to lift less than the lift generated by wing incidence. I took measurements from my -9A. The aileron trailing edges would have to move about .15 inches, one up and one down. Remember that the ailerons would try to center. That would be the same distance for any speed as we are balancing coefficients. If the coefficients are balanced, the forces will remain balanced. If both the flaps and ailerons were offset, the TE would move about .1 inches, one set up, one set down. For Kevin, I used a lift curve slope of .1/deg for both the wing and aileron and their respective areas, 124 sqft and 4 sqft. I'll see if I can find a better aileron lift curve slope value.

Steve, I also like the point you raise about edge distance. It would seem that the bigger bolt would spread the original load over more surface area, thus the edge distance for the origanal size bolt may be ok, but get somethin in writing so that your inspection will go ok. Maybe a structures guy can jump in. I will ask our structures guys there thoughts on Tuesday.

Regards, John.
 
John,

Nice info. I guess I'm just trying to build it as straight as possible to avoid having to tweak things later. I feel that up till now I've got a very straight plane with no wing twist, aileron twist/warp, etc. My dream is to have this thing fly hands off right out of the box, but I know that doesn't always happen. I'm just trying to have less to fiddle with later.
 
szicree said:
Specifically, using the 2 x diameter rule, you end up with 1.5 x diameter distance between the edge of the hole and the edge of the material, thus the bigger the bolt, the more material you actually end up with between bolt and edge. I assume this is because a bigger bolt implies they'll be a greater load there, and so the material is sized accordingly. But in my case, the load is not increased. :confused:

I know we've got a lot of engineers on here and I'd love to get a better grasp of this stuff, so feel free to expound. Thanks to all who've advised up til now.[/QUOTE


Not an engineer but..... As I understand it, the edge distance rule is because stress concentrations have a significant affect on aluminum and holes (not bolts or rivets) cause the high stress areas, larger hole = higher stress.


Please anyone correct me if I'm wrong.
Click to expand...
 
Steve
After having built a number of these planes and helped quite a few other guys I can certainly understand your wish to have this plane fly hands off out of the box. Although you often hear this from builders after a first flight it has not really been my experience. It really takes the average pilot quite a while to get use to his plane before he can really see if, for example, it needs a rudder tab. All of the first flights that I have been on have needed some sort of aileron tweaking. I am a fussy builder and an increasingly fussy pilot. Others pilots are often happy with the trim of their aircraft and I am not.
I believe that some real research is needed into the roll trim of these aircraft. In my opinion is it quite possible that we should be building the planes with more incidence on one wing than the other. You just might have it right at this point in time. Any of the fixes that you are considering might in fact put your plane out of trim!
 
I totally agree with Van's and many others here. FLY IT. I had what I thought was the exact same condition as you. Trust me after I drilled my spars I went back and measured and measured and measured and came up with what you describe. All kind of thoughts and fixes went through my mind.

In the end, I ended up with a slightly heavy wing which was easily fix by the aileron trick. You'd be surprised how little tweaking is really necessary.

Being an engineer used to dealing in .001 and .0001 and even smaller tolerances, it's hard to get over the "farm tractor" mentality. But sometimes you just have to suck it up and trust them.
 
The word from Van's

I sent a detailed, but brief, description of the problem to Van's, along with my proposed fix. I asked in the email for them to please run my plan past an engineer. Well, first thing this morning I got the reply: the engineers will not approve drilling to 3/8. The only fix (if you want to) is to replace the parts. They had previously advised me to not worry about it since plenty of planes had been built with less accuracy.

Well today I decided to bite the bullet and replace the spar carry-thru. It required the drilling out of 21 rivets way down near the floor of the canoe, but I had it out within 2 hours. I ordered replacement parts from Van's for about 40 bucks and we should be good to go. I figure on just using the existing part to duplicate all holes, except the right wing attach hole, which I will lower by 3/32. This will give me back 0.2 degrees, give me edge distance to spare, and stick with the called-out hardware.

I know everybody thinks I should just let it go, but it's my plane and I'd know it was wrong. Thanks to everybody for the advice. Hopefully this'll be the last big screwup. Heck, I've built so many parts twice I'm almost ready to consider myself a repeat offender. :rolleyes:
 
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Hi Steve-

I'm sure you've had a few sleepless nights the past week or two. Although I probably would have just lived with 0.1 off, I too sometimes stress about having things perfect, so I know the feeling. I think Bob Collins just went through the same thing in his -7.

In any case, good luck getting the replacement part riveted in. If the 4 carry-through is anything remotedly like the 9, there will be some pretty tough rivets to set that might require an offset rivet set. If you need to use one, and haven't discovered them already, Avery sells "snapsocs" which work fantastic for keeping an offset rivet set on top of the rivet. I've NEVER shot a bad rivet while using them. It might make what you are going to do a little less stressful anyway.

Good luck again.
 
Update on Incidence Reset

Today I fabbed a new rear spar pass through, using the old one as a drill guide. All holes were duplicated except for the right spar attach hole, which was moved downward 3/32. I clecoed the new part in the plane and attached the wings, and everything worked out perfectly. I jacked up the tail and rechecked h-stab incidence and level (not that these would be affected), wing incidence, sweep, perpendicular to fuse centerline, etc. and it was all spot on. Before, my level was indicating between .2 and .3 difference between wings and now it reads zero. Once I was happy with it, I rivetted it in place and all is terrific. For anybody thinking of doing this, fear not. It was really no big thing. The hardest part by far was drilling out the old rivets. It feels pretty great to have fixed this one. :)
 
briand said:
Not an engineer but..... As I understand it, the edge distance rule is because stress concentrations have a significant affect on aluminum and holes (not bolts or rivets) cause the high stress areas, larger hole = higher stress.

Please anyone correct me if I'm wrong.
Click to expand...

Actually stress concentration in metal defects is somewhat counter-intuitive. A smaller whole will actually have higher stress because it is the same pressure/force in the component acting on a smaller area. Thus smaller hole=higher stress. Cracks are the worst because stress compounds at their ends (being in theory, the smallest possible hole). This is why a good (temporary) fix for a crack in a metal component is to have the ends drilled out to a larger radius.

I'm assuming that my courses in steel design apply to aluminum (aerospace eng. please correct me), but edge distance requirements apply to different kinds of failure (sorry, I don't feel like breaking out the steel notes to propound) :). But I'm sure you can imagine what consequences should arise by drilling too close to an edge.
 
Tension Release

Steve,
Admire your inititive and determination to put it right to your own satisfaction.

I think we have all looked at something during building and said; "Not good enough" and chucked it in the bin.

It always seems a big task until you take the decision and then it is less of a hassle once you get your teeth into it.

Your attutide is going to produce a great flying machine at the end.

Pete.
 
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