Thread: Q500 Racers!
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Old 04-18-2008, 11:35 PM
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HighPlains
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Default RE: Q500 Racers!

Yeah, I've posted that formula before. Tractor conventional airplanes (prop on the front, tail in the back) are destablized by the prop. The bigger the prop diameter, the larger a rudder is required. Racing models have quite small props, so the vertical area need is pretty low. I have flown quickies with the V-tail at 130 degrees, and it was perfectly stable in flight. But with the limited surface area and throw, I had almost no steering on the ground when landing. So I decreased the angle to 120 degrees.

But you say, so many the designs are set at 110 degrees and work fine. True, but this angle was copied from gliders which need massive vertical area and little horizontal due to the long wings high inertia and limited pitching moment due to the narrow chords. The flatter V-tail have less drag, so those are the trade-offs.

If you get real clever with your fuselage design, you can arrange the slope of the top of the fuselage so the tail comes out at the corner. It takes a couple of measurements and a bit of trig.

First, measure the width of the fuselage where the leading edge and trailing edge of the V-tail meets the fuselage. Subtract the TE width from the LE width and divide by two. I try to measure this to a 1/32" accuracy. Take the Tangent of the angle to horizontal (180 – {V-tail angle})/2. For example on a 120 degree V-tail, it’s 30 degrees. So working out the trailing edge position is : [(LE width– TE width)/2] x tan (30). This distance is the amount that the TE of the V-Tail is lower on the side of the fuselage than the LE for zero incidence. Place the fuselage with the wing mounted so that the leading and trailing edges are blocked up an equal amount (zero-zero) on the obligatory flat surface.
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