A quick google search usually produces a number of documents providing conflicting definitions of the purpose of the propeller and the formula for propeller efficiency. Many of these documents include diagrams which show propeller efficiency of zero when TAS is zero.
In trying to resolve the problem we should probably start by asking the question "what is the purpose of the propeller".
Some people (and texts) would say "to produce thrust", while others would say "to propel the aircraft forward".
If the purpose is to produce thrust then:
1. We would probably call it a thruster.
2. Its output is a force (measured in lbf) and its input is power (measured in ft lbf / min)
If we use the standard convention that efficiency = 100% x (output / input)
we get the formula
Propeller efficiency = 100% x (lbf) / (ft lbf / min)
But this formula yields units of ( % min/ft)
We might well ask what exactly a (% min/ft) looks like (I do not know)
If the purpose is to propel the aircraft forward, then
1. We can continue to call it a propeller.
2. Its output is propulsive power (measured in ft lbf / min) and its input is power (measured in ft lbf / min)
If we use the standard convention that efficiency = 100% x (output / input)
we get the formula
Propeller efficiency = 100% x (ft lbf / min) / (ft lbf / min)
This formula yields units of %
Last edited by keith williams; 21st Mar 2013 at 08:18.