Rob, got back to my shop.
Here is an example plot of yaw vs tail rotor ESC signal on the DDFP. Since the fixed pitch tail throttle signal is related to thrust, it’s kind of interesting. It took quite a bit of fiddling with it to get it to throttle the tail motor at ~50% throttle for hover. This shows the plot of the throttle signal to the tail ESC in a static hover in Loiter, not moving, no yaw changes being made.
Now we’re in dynamic flight with constant heading between some waypoints, and making left and right turns between others. The throttle signal to the tail actually changes very little if you plot the mean of the signal to the ESC. From waypoints 6 to 8 where we were on a southerly heading, then make a 180 degree left hand turn, requiring reduced thrust from the tail rotor, to let the main rotor torque turn the airframe, the change is barely perceptible. And there may have been the effect of a crosswind on this too, I don’t remember on this flight.
Anyway, what I learned from experimenting with the DDFP is that the yaw of the tail is changed with the blade pitch in the anti-torque direction with a normal tail in all normal flight conditions. To use reverse tail thrust, it would be the equivalent of throttling the DDFP tail ESC to 1000 and completely shutting down the tail rotor motor because it wanted reverse tail authority. So it never does actually crosses the zero pitch position on the tail rotor on a conventional tail. In all the testing I’ve done with the DDFP I have not seen that yet, and I’ve flown it in 30mph crosswinds without ever having a blowout. The main rotor torque has always been sufficient to yaw left under any conditions I’ve tested it in, by simply reducing the thrust on the tail rotor.
So I think I arrived at the fact that the change in pitch on the tail rotor blades with a normal tail is so minute in UAV type flight that it there’s any binding in the linkage, even backlash in the servo gears, the tail will not be steady.
The power I measured when I figured out the DDFP was 23.5 watts developing 212.3 grams of thrust with a 970mm main rotor turning at 2,100 rpm at 7 degrees of collective pitch (hover). And the length of the arm (tail motor shaft centerline to main rotor shaft centerline) was 585mm. That’s from all my notes when I engineered it to get it to work.
Is that the kind of data you fellows were hoping to measure?
Here’s some additional info I measured on the bench. The power was measured with my Doc Wattson RC power meter to the ESC. Prop rpm was 5700 @ 1510pwm to the ESC @ 212 grams thrust, 222mm diameter, 4" pitch. My rough calculations on static thrust efficiency of the tail prop was that it was producing actual mechanical power of ~18 watts, or about 78% static thrust efficiency. Frame torque 1.216Nm.