Wing incidence degrees for a low wing monoplane?
 

I want to enlarge some plans I have for a low wing monoplane that was originally draw for free flight. It has a flat bottom airfoil. Whan power is added should the incidence of the wings be changed? I believe it is a Clark Y airfoil. It is flat on the bottom and at the trailing edge but is rounded up at the leading edge. If I understand correctly, in this plan, the airfoil has a 2 degree incidence if the flat portion of the wing is level with the centerline of the airplane. Would this be ok on a powered model?


Thanks
Jim

RE: Wing incidence degrees for a low wing monoplane?
 

I'm gonna say no, 2 deg is about right for a cambered airfoil for best lift/drag. If it's going to be R/C, you'll have elevator and trim control. Just don't over-power it. What kind of plane?

RE: Wing incidence degrees for a low wing monoplane?
 

If it flew well on the original incidence setup, no need to change. The forces should be the same even if you blow it up. The main thing to watch is the engine thrust line. You will probably want a bit of right thrust and possibly 2 to 3 degrees of down thrust.

RE: Wing incidence degrees for a low wing monoplane?
 

The airplane is an Earl Stahl designed Miles Magister redraw by Jim Wilkerson. The plan is in cad at 30" wingspan and I want to enlarge it 3x to 90" ws. I have converted several of these rubber plans to electric in the past and most have been sensitive to power changes. The planes wanted to climb when power were added. What I want out of this airplane is a slightly airobatic kick around plane for relaxed flying. This one will be gas powered most likely with a 25cc weedie conversion.

Thanks
Jim

RE: Wing incidence degrees for a low wing monoplane?
 

With the increased cruise speed of powered flight I am wondering if you shouldn't take the incidence out. It seems that less angle of attack will be needed because of speed and the fuse will be pointed down in level flight. Also the change in horizontal stab angle in relationship to the wing may help keep it from climbing so much when power is added.

I am a pilot and not an engineer just throwing out some thoughts.

RE: Wing incidence degrees for a low wing monoplane?
 

The wing sounds fine but you need to realize that it's the angle of the horizontal stabilizer to the wing that is important. The fuselage just goes along for the ride. If the wing is at roughly +2 as you say and the tail is at 0 with reference to whatever line you're using (it sounds like the lower wing surface in this case and that's fine but keep it consistent). A 2 degree difference is a good starting place for normal flying with a normal CG setup. Add elevator trim as needed from there.

The engine downthrust suggested above isn't a bad idea either and again that would be relative to the lower surface since that's what you want to use. Except I'd probably go with 1 or 2 degrees. This is because compared to the true wing section center line the actual downthrust will be more like -3 to -4 degrees then.

RE: Wing incidence degrees for a low wing monoplane?
 

The original Stahl plan has an airfoil 14% thick (Clark YM?) set at +3deg. per stab. at 0. This airfoil is 50% thicker than the normal Clark Y which would sit at 2deg. in the same position. Either is the correct relationship for pitch stability. The full scale plane had a high-mounted inverted engine like the PT-19. The thrust-line is above both the C.G. and the center of drag so should need no down-thrust, and with the high-drag air-foil shown would likely pitch down with added power. As is usual with these, the vertical stab. is high enough above the thrust-line to require some right-thrust to balance out.

RE: Wing incidence degrees for a low wing monoplane?
 

So I should go with it as is with no down thrust and add right thrust?

Thanks
Jim

RE: Wing incidence degrees for a low wing monoplane?
 

I can guarantee you that the rubber model will have ended up with some down and right thrust and so should your engine powered one. It's typical on rubber models to not show thrustline offsets since it's understood that adjustments WILL be needed during the trimming phase. Smart rubber modelers will drill the hole for the thrust bearing with some down and right to get a head start on things and avoid large unsightly amounts of packing shims.

Adding on a couple of degrees of down and a couple of degrees of right thrust would be a wise thing to do.

RE: Wing incidence degrees for a low wing monoplane?
 

So I should add downthrust. Will this offset the tendency to climb when power is added?
The planes that I have converted from Stahl plans in the past have all been excellent flyers. Just had to add a good bit of downtrim. They were all warbirds. P-40 and Bf 109.


Thanks
Jim

RE: Wing incidence degrees for a low wing monoplane?
 

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