Asymmetrical Washout Concern

[MikeEasley]

I have a composite, kit built aircraft that I'm dealing with that was assembled with one wing in an incorrectly aligned jig. The result: the right wing is right on spec, with 2 degrees of washout. The left wing, however, only has .3 degrees of washout. The outer two cradles were out of alignment when the builder closed the wing (locking in the geometry). The problem wasn't discovered until after the first flight when a significant right banking tendency was discovered (no surprise there). The bulider took careful measurements along both wings and compared his measurements to the factory specs and blueprints.

The kit company suggested using eccentrics to get rid of the banking problem, tip the right wing up and the left wing down. I know the certified companies routinely use eccentrics to get planes flying straight, but that's with symmetrical (or nearly symmetrical) wings.

The builder installed eccentrics to adjust the wing incidence at the roots. Now the plane flies straight in cruise but the stall characteristics are not ideal, a right wing drop at the stall (no surprise there either). At least theoretically, the right wing now has more incidence at the root, so it stalls first, followed by the entire left wing, then the right tip stalls last.

Now to the question, is the eccentrics solution acceptable, or should the builder insist of re-closing the wing in properly aligned wing cradles?

I don't consider a "straight flying airplane" to be the same as a "straight built airplane". Tipping one thing one way to compensate for something else that's crooked isn't a solution in my opinion.

The original wing designer, Rick McWilliams, put those two degrees of washout in there for a reason. Some washout is important for lift distribution on a tapered wing, but it really makes the plane controllable at the onset of the stall, the ailerons still work. In addition, the lift distribution along that left wing is way off from the original design, structural issues???

I'm curious to hear what some experts have to say on the issue.

Thanks in advance,

Mike Easley
Colorado Springs

 

[rb1957]

we (a reasonably reputitable airplane manufacturer) had a plane once that became, 1/2 way through its manufacture, a hangar queen. it was hanging around the plant for years. eventually someone bought it and discovered that one wing was twisted (washed out) 7 degrees. this was fixed by extending the aileron control rods, and rigging to counter the wash out, a "straight flying" plane. extensive engineering showed that this had negligible impact on the plane.

maybe ... MAYBE ... the same appraoch can be used on a kit plane, i'm only posting as an example solution ... "straight flying" is (atleast can be) almost as good as "straight"

 

[MikeHalloran]

There's no substitute for making it like the damn print.



Mike Halloran
Pembroke Pines, FL, USA

 

[MikeEasley]

rb1957, I agree, somewhat, with what you say. I'm not sure what manufacturer you're with, but most certified aircraft have very conservative airfoils compared to this Lancair. An airfoil that had a very slow and predictable stall profile would be less succeptable to asymmetrical washout, and could be "fixed" by simply adjusting a few things.

I talked to a Beechcraft "old timer" today. He said that they do some tweaking at the factory to get the Bonanzas to fly straight. The small stall strips on the leading edge are taped on to start with, tested until the plane drops straight ahead in a deep stall, then they permanently attach the stall strips before the bird is delivered.

Interesting....

My assumption is that a Lancair is built with a very high performance airfoil, not very tolerant of any build errors.

I walked around my home airport today with my digital level, checking the various certified aircraft sitting out where I could get at them. I had Pipers from the 70s, Bonanzas from the 50s. And everything in between. I took measurements from both wings, just to compare left to right at the same places, checking for wing incidence symmetry. I was pretty impressed. Virtually every aircraft was within a few tenths of a degree anywhere along the wings I checked. One Piper Archer from the 70s was dead on at three different places.

 

[berkshire]

As someone who has messed around with advanced composite aircraft since the 1970s. The practice of using eccentric wing mountings to trim an aircraft is normally considered a last resort. Except on those machines that have an active trailing edge, i.e. differential flaps and ailerons.
It appears you have 3 choices: Leave it like it is. Split the wing and reglue it. Buy a new wing from from the factory.
I did not think that the airfoil used on that aircraft, which I believe is the NLF (1)-0215F, has a particularly sharp stalling characteristic, so even with the lack of washout apart from the fact that one wing drops first the stall should not be violent.
Since the aircraft is flying, have you done a test flight series? Checking the stall characteristics in the turns. Checking for a tendency for an over the top spin entry into the wing without enough washout. If the machine behaves itself leave it alone. If it does not, then fix it.
B.E.

 

[thruthefence]

Regarding the Bonanzas, ( and Barons ) the wings are attached to the center section with loosly fitted "bathtub" fittings & tension bolts. The aircraft is normally washed in or out by loosening all four bolts, & shifting the incedence of the wing panels. It is a trial & error process, and a hair puller. When the wing bolts are retired, you normally change one at a time, to avoid affecting the flight characteristics of the aircraft. I have found, as a mechanic, that most ill-flying aircraft are miss-rigged. In my mind, when you get a squawk like 'half-a-ball-out' at such & such airspeed, it's time ahead to check ALL the surfaces & set them to spec. Often folks will pick up a heavy wing by drooping a flap slightly,( see the above post regarding ailerons, altho I can't see how that would be legal ) or tab the rudder to similiarly pick up a wing. This make the aircraft fly 'straight' but often you end up with controls fighting each other. This is induced drag, & slows the airplane down