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Old 01-04-2009, 09:02 PM
  #11  
otrcman
 
Join Date: Oct 2004
Location: Arroyo Grande, CA
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Default RE: Sterling Spitfire

Hi Chris,

I'm presuming from your question that you're talking about your P-63, so that's how I'll reply. First, I should plead insanity as far as the effect of wing/stab incidence upon aerobatic maneuvers. That's a really complicated subject and I don't know enough to even make educated guesses. So now that you know my limits, I'll comment on your questions.

First the airfoil. It's about 14% thick at the root, and the shape looks like it would work out to be a NACA 2414. Note that the four digit 24xx series is a completely different animal than the five digit 23xxx series. The four digit series is semi-symmetrical and has pretty good stall characteristics but not particularly low drag. The five digit 23xxx series, also semi-symmetrical (like on a Taylorcraft), has lower drag but comes at the expense of a sharper stall.

My suggestion for relative incidence angles was based primarily on being able to trim the airplane for level, upright flight with little or no elevator deflection. Obviously if your stab is incorrectly angled relative to the wing you can trim for level flight with some elevator deflection, but we usually shoot for no elevator. So what I was suggesting was that the angular difference between wing zero lift angle and stab zero lift angle should be about 1°. That is, the stab should be about 1° negative relative to the wing. If you are going to run a really powerful engine you might want to go down to 1/2° and if you have a low powered engine you might go up to 2°. Your CG at 28 to 30% sounds like a good starting point.

Now, to really get into the guesswork involving aerobatics. The only comment I can venture here is that you probably want to rotate the airplane around the fuselage principal axis for cleanest rolls. If the fuselage is well aligned with the velocity vector then the tail will not describe an arc as you roll, and fewer yaw and pitch excursions will be mixed in with the rolling motion. For this, it seems like the lower wing incidence the better. I'd expect that with your 90FS you will be moving right along in level flight and your angle of attack will be very low, maybe 1 to 2° above zero lift. So to have your fuselage aligned with the velocity vector you would need the wing to be at +1 to +2° above the zero lift angle.

Most of my career as an engineer was in flight test, so I have a healthy appreciation for just how wrong the theoretical guys can be. In the end, it's called cut and try. That's what is so neat about RC models: Doesn't cost all that much to cut, and you don't get hurt too badly when you try.

Dick