35 What effect does high density altitude have on the efficiency of a UA propeller?
B. Propeller efficiency is decreased.
This is correct.
FAA-H-8083-25B, Pilot’s Handbook of Aeronautical Knowledge, Page 4-4
• Power because the engine takes in less air
• Thrust because a propeller is less efficient in thin air
• Lift because the thin air exerts less force on the airfoils
"Density altitude is the vertical distance above sea level in the standard atmosphere at which a given density is to be found. The density of air has significant effects on the aircraft’s performance because as air becomes less dense, it reduces:
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Why is this important to  Part 107 pilots?
Most of the Part 107 pilots are unlikely to fly high enough to be concerned with high density altitude.  But if you are flying from a mountan and start at 5,000 ft MSL, then you should be aware that the thin air compromises your aircraft performance.  At the least your flying time will be lower since the propellers have to spin faster to account for the lower efficiency of the props.  At the worst, you won't even get off the ground.
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