PROPELLER EFFICIENCY:
The propeller is 80 - 87% efficient up to approximately 400 mph. Generally, beyond this performance will fall off, although new materials and improved blade technology are tending to increase efficiency.
This efficiency can be expressed as followed:
Propeller efficiency = Work done by propeller/work done by engine * 100
The drag on an aircraft travelling at 300ft/sec is 1100 lbs and if the engine produces 750 Shaft Horsepower the propeller efficiency is as follows:
Work = Force * Distance
Drag = Thrust (in level flight)
Work Done by Propeller / Sec = Thrust * Speed
= 1100 * 300 ft lbs
Work Done by Engine = HP / Sec
= 750 * 550 ft lbs (1 HP)
Propeller Efficiency = (1100*300)/(750*350) * 100 = 80%
Note that if the aircraft is stationary with engine running, thrust is produced, but as there is no forward movement, propeller efficiency is zero. At high forward speeds the slip could be zero, i.e. no angle of attack, therefore no thrust. With no thrust the propeller efficiency is zero.
When power is changed into thrust, the drag (or torque) created by the propeller limits engine speed. To be efficient, obviously the propeller should absorb all the power available. This is achieved by making a compromised design as power absorption creates limitations.
Propeller design with regard to diameter, number of blades and blade shape is governed by the power to be absorbed. The tip speed must not approach the speed of sound or efficiency will be lost; this limits diameter. Aircraft design also limits propeller design. Low slung engines mounted close to the fuselage require small diameter propellers; larger propellers require a longer undercarriage.
- Diameter
- Blade Angle
- Chord
- Change of Angle of Attack
- Camber of Aerofoil
In higher powered engines, a reduction gear is usually fitted. This allows the engine to run at its most efficient speed while allowing the propeller to turn at its most efficient speed.