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How much incidence

K

koda2

Well Known Member
My 6AQB came with, free of charge, a twist in the midportion of the fuselage which lowers the rear spar stub on the left side enough that setting the incidence per Vans, is impossible; it violates the 5/8" rule.

After exhaustive measurements by multiple means and having several builders look at it, the best we can do is to compromise and increase(lower the rear spar) on both sides until the incidence is an additional .2 degrees(total of 1.2 positive?). There will be no room for error whatsoever on drilling the holes at this setting.

If my ciphering is correct this is over a quarter inch on the wing and will raise(?) the tail in flight four or five times that. I am under the impression that the front of the horiz. stab will have to be raised .2 degrees as well to mitigate the change.

Is this much change a big deal?

What happens to the flight characteristics in this kind of setup? Speed, handling, propensity to and recovery from spins, etc. a problem? Everything else appears to be dead on: sweep, squareness, dihedral etc.

Dave A.
6AQB
 
Dave, Adding incidence to the wing changes the dynamic response of the horiz tail toward making the airplane more stable. Adding incidence between the wing and h-tail would drive dynamic stability in a more positive direction, reduce the period of phugoid type maneuvers, and require more trimming with airspeed change. Too much of this can be a bad thing, but IMHO .2 degrees isn't too much.

You are correct in understanding that you could just raise the tail .2 degrees to mitigate the change. That would only affect the incidence of the fuselage in flight by the same amount - in this case, the fuse would fly .2 degrees flatter than before. I can't think of how such a small change would affect stability or performance at all.

As a reference, I built my RV4 with relaxed dynamic stability by taking .5 degrees away from the wing/tail system (at the tail). It flew like a dream; a better setup for the 200HP installed up front.

One last thought - if its a QB defect you're addressing here, wouldn't Vans be responsible for that?
 
QB

Bill,

Thanks for the info. I don't have any training in aeronautical engineeering but I was under the impression that the tail always has a little reverse lift to counteract the weight and thrust from the engine. From what I can gather, Vans has designed his planes to have the same incidence?
I am the second builder. I bought the kit from the original purchaser. It has also exceeded the warranty time limits.

Dave A.
 
Have you considered replacing the spar carry-thru? I did this on my -4 and it was really not that much trouble. Perhaps a -6 is a bigger operation.
 
This is repairable and easier than you think. A friend of mine had a quickbuild fuse that had a twist in the fuse as well. What we did was to drill out the rivets in the longeron from the bagage bulkhead rearward. We then applied a opposite twisting force on the rear bulkhead till the aft deck was square with the forward longeron in the spar area(acually with the spar as well). Next we would rivet a section on both sides and recheck/readjust to make sure everything lined up. think it took all of about 3 hours from start to finish. was really quite easy ad took very little twisting force to bring it back to square.
 
Done

Talked to Van's when I first discovered it a couple of years ago. They said it was a non-issue. Too late now. Wings are pinned. Roll bar is on and canopy done.
It will have to fly twisted.
Dave A.
 
I had the same problem

I did not know about any 5/8" rule. I raised the trailing edge of the left wing until the incidence was correct relative to the leveled canopy deck reference plane. As you did, I rechecked it many times and I agonized over it a lot before I clamped the spar to the stub and drilled the hole. Every flying surface on my RV-6A (purchased direct from Van's in 1996, S/N 2499 was on the firewall) is rigged with respect to that plane and the mutually perpendicular axis resulting from leveling that plane. My plane flew right at book speed in the stock configuration. I have modified it quite a bit since then and gained approximately 14 kts without changing the rigging.

After six years of flying and several years of racing I am planning to reduce the difference in angle between the wing and stabilizer by raising the leading edge of the stabilizer in search of a little more speed.

Bob Axsom
 
Twist

Gary,
In regards to trying to fix the twist, I did remove the afterdeck to repair some damage. The first builder started to change the plane to a tail dragger and badly drilled out some rivets but not enough loosen the fuselage. I checked the fuselage before removing the afterdeck and discovered the problem.

I fabricated a new aft deck and riveted it in place with the back level. It didn't occur to me that taking out that many rivets was possible. I might have been able to reduce the twist, but I am afraid the rivet holes would no longer have matched.

Dave A.
6AQB
 
Suprisingly the rivet holes stayed aligned. There were clecoes in all holes and we removed one at a time as we riveted. I dont recall having to drill to get hole alignment. We were very carefull in drilling out the rivets as to not enlarge the holes.
 
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