Helicopter tail rotor thrust requirement

PaulHP

How can you calculate the amount of thrust needed by the tail rotor of a helicopter to counter the torque of the main rotor?

russ_watters

Welcome to PF.

The answer to the question depends on what you know. If you know the horsepower and rpm of the main rotor, you can calculate the torque easily enough. More information is needed before we can help you...

PaulHP

lets say that this is for a model helicopter with .05 hp (37 watts), has blades that rotate at 300 rpm and has a 3.5 foot long tail boom. Wouldn't the required thrust of the tail rotor be cut in half if the tail boom length is doubled?

Cyrus

Making the tail boom longer would decrease the power requirements of the tail rotor.

PaulHP

Thanks, I thought it would, but how would you determine exactly how much thrust would be needed for the tail rotor at various tail boom lengths?

Cyrus

Just do a moment balance.

russ_watters

Well, you have the horsepower and rpm. Torque is horsepower/rpm....

PaulHP

ah. So, 0.88 ft-lb would be required to counter balance to main rotor and the thrust required for the tail rotor at 3.5 feet would be 0.25 lbs, right?

Cyrus

I can tell you yes or no if you show some steps as to how you got to your final answer.

PaulHP

0.05 hp X 5252= 262.6/300 rpm = 0.88 ft-lb
0.88/3.5 ft= 0.25 lbs of thrust

Cyrus

T = 63025*hp/rpm

this "0.05 hp X 5252= 262.6/300 rpm = 0.88 ft-lb" Makes no sense to me.

I get 3lbs of thrust.

PaulHP

T = 63025*hp/rpm is to find torque in in-lbs, and T-5252*hp/rpm is to find it in ft-lbs..

Cyrus

Yep you're right