Which law is involved in the statement that more power is needed to obtain a desired RPM than is needed to maintain the RPM?

Which law is involved in the statement that more power is needed to obtain a desired RPM than is needed to maintain the RPM?

• A. Torque Effect
• B. Action/Reaction
• C. Acceleration
• D. Inertia

Answer: (D)

Rotational inertia is what explains why you need more power to raise the rotor speed than to hold it steady. A rotor resists changes in its angular velocity just like a spinning object resists slowing down or speeding up. To increase RPM, you must overcome that inertia and deliver energy to the rotating mass, doing work equal to the increase in rotational kinetic energy. Since kinetic energy grows with the square of angular speed, the required energy (and thus power, especially while you’re accelerating) is larger as you push toward a higher RPM. Once you’ve reached the target speed, you only need to supply enough power to balance ongoing losses (bearings, drag, airframe drag, etc.), which is much smaller than the peak power needed during acceleration. So the driver of the higher power requirement during speeding up is the rotor’s inertia.

Rotational inertia is what explains why you need more power to raise the rotor speed than to hold it steady. A rotor resists changes in its angular velocity just like a spinning object resists slowing down or speeding up. To increase RPM, you must overcome that inertia and deliver energy to the rotating mass, doing work equal to the increase in rotational kinetic energy. Since kinetic energy grows with the square of angular speed, the required energy (and thus power, especially while you’re accelerating) is larger as you push toward a higher RPM. Once you’ve reached the target speed, you only need to supply enough power to balance ongoing losses (bearings, drag, airframe drag, etc.), which is much smaller than the peak power needed during acceleration. So the driver of the higher power requirement during speeding up is the rotor’s inertia.