SU5: Airplane Performance and Weight & Balance

Question 1

What effect does high density altitude, as compared to low density altitude, have on propeller efficiency and why?

Answer 1

Efficiency is reduced because the propeller exerts less force at high density altitudes than at low density altitudes.

(FAA-H-8083-25B Chap 11) ( ? )
The propeller produces thrust in proportion to the mass of air being accelerated through the rotating propeller. If the air is less dense, the propeller efficiency is decreased. Remember, higher density altitude refers to less dense air.

Question 2

If the outside air temperature (OAT) at a given altitude is warmer than standard, the density altitude is ______in relation to pressure altitude?

Answer 2

higher than pressure altitude.

(FAA-H-8083-25B Chap 11) ( ? )
When temperature increases, the air expands and therefore becomes less dense. This decrease in density means a higher density altitude. Pressure altitude is based on standard temperature. Thus, density altitude exceeds pressure altitude when the temperature is warmer than standard.