What effect does high density altitude have on the efficiency of a UA propeller?
ADecreased efficiency because the propeller is less effective in thin air.✓
BIncreased efficiency because the propeller moves through less resistance.
CNo effect: propeller efficiency depends only on RPM, not air density.
Why →At high density altitude, air is less dense. A propeller blade moving through thin air encounters fewer air molecules per rotation, generating less thrust for a given RPM. To maintain the same thrust, the motor must spin faster, drawing more current and reducing battery endurance.
The trap →"Less resistance" sounds like it would improve efficiency, but propellers need air molecules to push against to generate thrust. Less density means less thrust per rotation, not better performance.
Field note →High density altitude is the most common surprise for drone pilots operating in mountain environments. Drones rated for 25 minutes at sea level may deliver only 18 minutes at 6,000 feet MSL on a warm day.
SOURCE → PHAK Chapter 11, FAA UAG Sample Question 38CHECKED JUL 16ACS III.B.K2MED