Pulsing in a spin
 

An ultimte 40 with a super tigre 61 swinging a 12 1/4 x 3 3/4 apc prop. While spining (inverted or upright) this airplane pulses faster in half of the spin! Why? Ollie, see if you can fix this. I like your answers. C.g. is pushing the aft limit real hard. Knife edge the nose tucks down real hard. Had to brace the tail to save the servos. (after three ) Have not addressed the tuck yet. Looking for ideas. This plane is a blast. Matt

Pulsing in a spin
 

Do you mean the plane spins faster for half a turn and slower for the next half a turn, then repeats the cycle? How many 360 degree turns per minute does it spin? Is the spin flat or nose down? Is the engine surging during the spin? What position are the control sticks in during the spin? Does it behave the same with a near full tank as it does with a nearly empty tank? Have you tried varying the rate of spin with the controls? Have you tried varying the throtle during the spin?

Please try to give us a more complete and detailed report of what is happening.

Pulsing in a spin
 

No throttle change. no stick movement. Yes half of the spin is faster than the other. Have not tried altering the control movement. Attitude is real flat. This is with about 10 revolutions. A couple of times the spin would look to be offset like a tilt-a-whirl a the fair. Wind was lite today about 8 to 10 mph. Appeared to pulse faster with the wind. Matt

Pulsing in a spin
 

I think what you are seeing is the ground speed of the model in a spin rather than the airspeed. The ground speed is the airspeed plus the wind speed over half the turn and the airspeed minus the wind speed for the other half of the turn. if you subtract the wind speed you are left with a constant airspeed and no pulsing.

To help you understand the two points of view consider the following thought experiment. You are in the gondola of a balloon drifting with the wind. You hand launch your model and fly it in perfect circles around the balloon at a constant angle of bank and a constant speed at a constant altitude. An observer on the ground won't see the perfect circles at a constant speed that you are flying. The observer on the ground will see a plane following an epicyclic path with high speed downwind and low speed up wind. To the ground observer the speed of the plane will seem to pulse slowly. Now you, in the balloon, steepen the angle of bank and fly smaller perfect circles around the balloon. The observer on the ground will see the airspeed seem to pulse more quickly. The flat spin you describe is an unbanked very, very tight circle and if you viewed it from the vantage point of a balloon drifting with the wind, I don't think you would observe any pulsing.

Always remember that models fly relative to the air and land or crash relative to the ground.

Pulsing in a spin
 

The doppler shift just makes it sounds as if it is pulsating. I'm guessing.

me too
 

I've had this happen on a plane or two. Probably just some slightly unstable airflow over the wing or rudder while spinning.

Pulsing in a spin
 

I take that during these pulses, the pitch of the plane does not change. So if in a flat spin, the nose neither tilts up or down?