Hallo Gerrit
Fehlerteufel verjagt
Gruß Peter
Kim-
Mike e-mailed me that he had moved his c.g. back some from the 2" specified and liked it better on his prototype. This was several years ago, but I think he said something like 2 1/4" behind the root l.e.
Anyway, I felt that mine too, at 40+ oz, was not as responsive as other P-Forces and wondered whether I'd overloaded the wing or tail. Mine had quite a bit of sag between ribs for the 17% (measured) or 18% (specified by Mike) NACA sections. I had not turbulated the tail, something that seemed to improve another model's performance. You might try that.
I have used the LA .40 mostly, after starting with an FP .35 (pictured on plans). I had decided to redrill for an LA .25, since the heavier engines required 3/4 oz or maybe even more tail weight. However, the fuselage is now in 3 pieces, and rather than CyA it back together, I'll strip all finish and silkspan, add stub ribs, and make another lighter fuselage (and finish!) with a stiffer nose for the LA .25. At this point, I may just use the wing; I'm not sure that my round stab leading edge is the best way to go, even though I'm sure it works fine for the many P-Forces out there. Igor posted some thoughts on airfoil shapes and thicknesses at such low Reynorlds numbers, and I also noticed some NACA material that led me to sharpen the leading edges slightly on my other flapless LA-.25 stunter. The resulting stab on that one, whose plans you have, seemed to work well.
One thing I did notice was that using an APC 11" prop cut down to 10.5" worked pretty well, but the full 11" prop really damaged the model's turning performance. I think a less efficient, but much lighter, Zinger prop worked better, and I mostly used a 10" x 4" APC. If you're using a heavy 11" prop, you might try cutting back of the prop diameter and/or weight.
As usual, "for what it's worth."
Hi Kim,
The C.G. for the P-Force should be close to the 2” back from the leading edge of the wing, measured at the fuselage. Going any further back can make the P-Force a little jumpy at times. At forty ounces, yours shouldn’t have much on an effect on its turn rate at all. I added 6 ounces of weight to the C.G. of the prototype P-Force without greatly affecting the turn rate of the model.
In your post, you mentioned that you have a full 45° of elevator deflection, if indeed you are using all of that deflection, then your handle spacing is way too much for the model. The handle spacing should closer to 3-3/4” for the best all around performance.
The Brodak .40 is a good choice for the P-Force. It’s lightweight and power should make good combo for the P-Force in most situations. My thinking is that you have too much pitch on the prop and causing speed to go up when the engine breaks into a 2-cycle. Switch to a 4 pitch to control the speed and the lap times of the model. One of the best flying P-Force I ever flew was built by Jim Hoffman. It was powered by a Randy Smith AeroTiger 36 and he was almost unbeatable with it (just ask the guy’s in the southwest).
Later,
Mikey