Apr 21, 2005, 08:50 PM Registered User Joined Jun 2003 676 Posts servo torque for a 6 to 8 foot rotor? How do you determine how much servo torque is needed for a given rotor size for roll tilt?
Apr 22, 2005, 06:41 AM
Mickey from Orlando. Really.
Joined Nov 2004
3,844 Posts
Quote:
 Originally Posted by grant31781 How do you determine how much servo torque is needed for a given rotor size for roll tilt?
The variables are 1) gyro weight 2) coning angle 3) flapping offset
Here is a diagram showing the forces involved.
Here is also a table showing the torque required for a 1 lb gyro, with 5 degrees of rotor tilt and a 3 lb gyro with 8 degrees of tilt. The chart is for various coning angles and flapping offsets.

Clearly the least load on the servo is small flapping offsets Small flapping offsets create unstable rotors with very little control authority. The design optimization is toward the largest flapping offset that the servo can take.
Hope this helps.
mickey

reason for edit : stupidity

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Last edited by mnowell129; Apr 22, 2005 at 07:27 AM.
Apr 22, 2005, 06:47 AM
Mickey from Orlando. Really.
Joined Nov 2004
3,844 Posts
Quote:
 Originally Posted by grant31781 How do you determine how much servo torque is needed for a given rotor size for roll tilt?
Clarification:
Previous post was assuming a DC head.
If you are using cyclic pitch and the blades pivot at
the aerodynamic center the similar calculation is
always 0 torque.
 Apr 25, 2005, 02:57 PM Registered User Joined Jun 2003 676 Posts Wow, thats pretty complicated to figure! Can you do an example of my gyro? It weights 6.6 lbs, coning angle unkown? I have 15o upstops but it doesnt cone up much. it just has a slight upward coning angle. flapping offsets are 1.5 inches from the rotor shaft. Would this be a simple way to determine servo tourqe needed? Simply hang the model by the rotor shaft and see if the servo is strong enough to tilt the body under the rotor? If it can swing the body to the side it should be strong enough right?
Apr 25, 2005, 03:57 PM
Mickey from Orlando. Really.
Joined Nov 2004
3,844 Posts
Quote:
 Originally Posted by grant31781 Wow, thats pretty complicated to figure! Can you do an example of my gyro? It weights 6.6 lbs, coning angle unkown? I have 15o upstops but it doesnt cone up much. it just has a slight upward coning angle. flapping offsets are 1.5 inches from the rotor shaft. Would this be a simple way to determine servo tourqe needed? Simply hang the model by the rotor shaft and see if the servo is strong enough to tilt the body under the rotor? If it can swing the body to the side it should be strong enough right?
Body swing is probably not good enough.

Lets say each blade is 3 feet by 3 inches.
The rotor will have to turn around 400 rpm to fly level.
If each blade were to weigh 3 ounces, the centrifigal force
on each blade is 18 pounds.
This gives a coning angle of 7 degrees.
Your flapping offsets are 1.5 inches.
When you run the formula you get

121 ounce inches.

Suppose you really start flying fast and the RPM gets up around 600.
Then the servo torque required is:

212 ounce inches.

If the blades weigh 4 ounces each these numbers change to
162 and 327 ounce inches respectively.

If the blades weight 2 ounces each:
80 and 121 ounce inches repectively.

I'd plan on something > 100 ounce inches no matter what.
Does this help?