(hrwc10p80) A uniform helicopter rotor blade (see the figure) is 7.80 m long and
ID: 1366762 • Letter: #
Question
(hrwc10p80) A uniform helicopter rotor blade (see the figure) is 7.80 m long and has a mass of 110 kg. What force is exerted on the bolt attaching the blade to the rotor axle wh en the rotor is turning at 350 rpm? (Hint: for this calculation the blade can be considered to be a point mass at the center of mass. Why?)
Calculate the torque that must be applied to the rotor to bring it to full speed from rest in 6.5 s. Ignore air resistance. (The blade cannot be considered to be a point mass for this calculation. Why not? Assume the mass distribution of a uniform, thin rod.)
How much work did the torque do on the blade in order for the blade to reach a speed of 350 rpm?
Explanation / Answer
A uniform rod of length L and mass m, rotating about its end, has moment of inertia
I = 1/3 m L^2
I = (110 * (7.80)^2) / 3 = 2230.8 kgm2
350 rpm = 36.63 rev/s
T = 2230.8 * 36.63 / 6.5 = 12571.42 Nm
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