Tail Rotor Pitch - Is this Normal

I’m very familiar with ArduPlane and ArduCopter on MultiRotors

This is on a 800 size Heli.

  1. I’m ready to do my first test flight and my question is the tail rotor pitch seems to be in slow motion, I come from 3d helicopters and on 3d Heli’s the tail pitch is snappy. If I hook the tail servo direct to the Rx it’s snappy and if I hook the tail rotor servo to one of collective/pitch outputs it’s snappy, but when hooked to Ch4 out it is pretty slow. It seems to slow to properly control the tail. Is this normal?

  2. I’m going to be running a low head speed and want to go to a DDVP tail rotor. My question there is what motor can someone recommend to use?


  1. Yes, it’s normal on the ground/bench. (Or at least, mine behaves the same).

There is a parameter, H_SV_TEST, that you can set to cycle the servos when you boot your Pixhawk. It is a nice feature to make sure the servos are responding normally and not binding. That test will show proper tail servo function. On the bench, due to the way the control loop works, it will only respond slowly to the stick input. In flight it is very fast.

Thanks Jakob and Chris, I’ll try the H_SV_TEST.

Also, do you have a suggestion for the DDVP motor? Kv in particular. What kind of tail rotor RPM should I be looking for ?

I’m just trying to keep from having to buy several motors and experiment on which one will work.

Thanks DougB

The tail rotor speed will typically be 4 - 4.5x headspeed. Can’t say for sure what kV motor to use, as that depends on voltage and size of the tail rotor blades too.

Thanks Chris, Ya, Kv is the wrong question, RPM would be better, then I can figure the rest out. From what I’ve read if I’m running <1000 RPM for the head-speed the tail rotor at 4 to 4.5 times the head-speed would probably not be fast enough to counter act the torque. I’m running 115mm tail blades. Here’s another question, assuming the same heli in a hover, would the torque that the tail-rotor has to counter be higher with a higher pitch lower head-speed say around 800 vs a lower pitch higher head-speed of 1800

The amount of torque produced on the frame is directly proportional to shaft power at the mainshaft. In theory at lower headspeed it takes less shaft hp to turn the main rotor, which means less torque on the frame.

<1,000 rpm would probably require a higher tail drive ratio from 5-6:1. Or simply swing bigger tail rotor blades. You have to be the judge of that because it depends on takeoff weight too and the ability of the tail to hold in the wind without blowouts. 115’s aren’t very big for a 800 with a heavy load. For UAV heli I run 116’s on my Synergy 696 @ 4:1 tail drive ratio, 1,350 rpm and 17.7 lbs takeoff weight. I run 106’s on my Synergy 626 @ 1,610 rpm, same 4:1 tail ratio, 14.5 lb takeoff weight. Just to give an idea.

If you don’t have main rotor blade tip clearance to swing bigger tail blades for a UAV, you’ll have to run it faster.