Question

A radio-controlled helicopter with rotor length, L = 286 mm and rotor mass (of both blades together) of 51 grams, must spin it's blades up to an angular velocity of about 300 radians per second in order to take off. Assume that for the purpose of determining rotational inertia, that the pair of helicopter blades approximate a long uniform rod rotating through its center. Calculate the net torque needed to bring the rotor up to take-off speed in 0.73 seconds. Answer in units of Nxm [Hint: You may need to review the tables in section 8-5 of the course text.]

Answer 1

Sol:
Here length of rotor L = 286 mm = 0.286m and mass M = 51 grams = 0.051 kg

Hence moment of inertia about its center is given as-

Here initial angular velocity   and final angular velocity

time t= 0.73 s

Now angular acceleration is given as-

Now required torque is given as-