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What is the effect on the best rate of climb speed with increasing altitude?

• A

Flying at a constant IAS or TAS has no effect on best rate of climb speed.

• B

If flying a constant IAS best rate of climb speed decreases, If flying a constant TAS best rate of climb speed increases.

• C

If flying a constant IAS best rate of climb speed increases, If flying a constant TAS best rate of climb speed decreases.

• D

Best rate of climb speed will remain constant.

Refer to figure.

The Best Rate of Climb speed, VY, is found at the lowest point of the power required curve, where the greatest surplus of power available over the power required occurs, which is also the minimum power required speed, VMP and maximum endurance speed, VMAX END.

As altitude increases, the TAS for a given IAS increases. This fact would cause the power required curve just to be stretched out to the right.

But, power required is: Drag x TAS, so the increase in TAS also causes the power required curve to move up.

Thus, the combined effect is that the power required stretches up along its tangent from the origin.

As a result, the lowest point of the power required curve, which corresponds to VY, moves to the right and increases.

But, VY is normally referenced to IAS in the Helicopter Flight Manual and the increasing disparity between IAS and TAS in the climb is greater than the increase in speed as a TAS. In other words, as air density decreases, IAS reduces against TAS quicker than TAS increases.

The overall result is that VY as IAS slightly decreases in the climb as altitude increases.

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