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HELP re: Aerodynamics / Flight Safety

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Victor Bravo

Well-Known Member
Supporting Member
Joined
Jul 30, 2014
Messages
13,447
Location
KWHP, Los Angeles CA, USA
This is more than an old washed up model airplane guy like me can safely determine. I need the opinion of trained aerodynamicists or aero engineers who understand this more than me.

I have a Kolb Firestar, with a flat bottom airfoil and a rectangular wing planform. Therei s NO geometric twist built into this wing, it is flat from root to tip.

On the outboard half of each wing panel is a large chord aileron, approximately half of the span of the main wing panel. This aileron extends 11 or 12 inches past (aft) of the "main" wing panel trailing edge. The "trailing edge" of the inboard half of the wing is the aileron torque tube.

I am having a pitch trim problem, I need to hold significant forward stick pressure to keep the aircraft in level flight. So much so that I have not been able to let go of the stick at any speed.

The CG is towards the rear of the factory's acceptable CG range, but it is clearly within that published range. If it is "tail-heavy" then the factory CG range somehow specifically allows it to be tail-heavy.

One of the Kolb owners reported that he had re-rigged the leading edge of the stabilizer upward by 7/8 of an inch, which solved his problem. So I started by raising my stabilizer 3/4 of an inch, which took about half or 2/3 of the problem out. So now I have increased the stabilizer incidencee to 1.25 inches higher. This is similar to the way the picth trim works on the J-3 Cub and Cessnaa 180/182.

I posted photos of this on the Kolb e-mail list, and some people commented that the angle looked excessive, perhaps unsafe, and they've never seen a Kolb with the stabilizer jacked up that high.

At the same time, several of the experienced Kolb owners have advised me to re-rig the ailerons to "droop" them downward a few degrees, which would have the effect of pitching the aircraft a little nose-down. Several of the experienced guys said this would probably solve the problem. I get this, and I agree it would likely reduce or mitigate my current nose-up tendency.

BUT, if you lower the ailerons on the outboard half of a wing, and leave the inboard half alone, you are creating "Wash-In" instead of wash-out, and according to everything I have learned this will create a tendency for the wing tips to stall before the root. Exactly the opposite or "normal" wing twist.

I have no need or qualifications to become a spin-test pilot on this aircraft.

So my aerodynamics question is this: If you have ailerons that are half-span, and you droop them in order to increase the pitching moment of the wing, is this going to create a potential tip-stall and "wrong way twist" situation... or is half of the wing span enough to aerodynamically consider it as not creating aerodynamic twist for the whole wing????

Lowering the ailerons is an easy and likely effective fix for the trim problem, but I absolutely want to know whether I am poking at a different bear by doing this.
 
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