A helicopter has two blades (see figure), each of which has a mass of 270 kg and

Question

A helicopter has two blades (see figure), each of which has a mass of 270 kg and can be approximated as a thin rod of length 6.7 m. The blades are rotating at an angular speed of 48 rad/s. (a) What is the total moment of inertia of the two blades about the axis of rotation? (a) Determine the rotational kinetic energy of the spinning blades.

Explanation / Answer

(a) M.I. of a thin rod of mass M about an axis passing through its centre and perpendicular to its length L is given by ML^2/12. => M.I. of the blades = (2*270) * (2*6.7)^2 / 12 kg-m^2 = 8080.2 kg-m^2. (b) Rotational KE of the blades = (1/2) * M.I. * ?^2 = (1/2) * 8080.2 * (55)^2 J = 12,221,302.5 J = 12,221.3 kJ.

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