Expert: Paul Soderman - 3/1/2010

Question
The propeller design diagram at
http://www.woodenpropeller.com/Basic_Propeller_Design.html

shows a single propeller blade with its centre line as the horizontal axis, with blade pitch shown at different places along its length.  The datum lines for the blade cross sections from each station all intersect at the same place on the vertical through the propeller centre.

The distance from the propeller centre to the point of intersection thus represents forward speed of the propeller.  This forward speed can be calculated by taking the distance from the prop centre to a particular cross-section, multiplied by the tangent of the angle between the blade axis and that datum line, the angle of attack.  And feeding in the rpm of course.

If I then simulate pitch change by adding say 5 degrees to each angle of attack, the datum lines then cross the vertical at different places.  This suggests to me that the twist on a propeller blade must be optimal for just one speed, and changing pitch away from this optimum will impair efficiency.

How significant is this impairment?

Answer
Yes Bill - one definition of propeller pitch is the distance it would travel forward in one revolution.  This is common for boat propellers and old references on airplane propellers or "airscrews".  A more modern definition is the angle of the blade chord at 3/4 radius relative to the propeller disc plane.  And as you say, the twist or pitch distribution is optimum for one airspeed.  As the aircraft speed changes the propeller efficiency and thrust changes because parts of the blade will become unloaded or even stalled.  The propeller chart I am looking at shows propeller efficiency (TV/Pshaft) versus advance ratio (V/nD) that, for example has a maximum of 0.82 at 60° pitch and V/nD = 3.6.  If the airspeed is cut in half, the efficiency drops to 0.45.  That is why most propellers are variable pitch so performance can be optimized at every flight speed.  You can see propeller efficiency curves by googling 'propeller efficiency' (for example:  http://www.epi-eng.com/propeller_technology/selecting_a_propeller.htm).

In the above discussion:

T = thrust
V = flight speed
Pshaft = engine shaft power
n = rotational speed
D = diameter

Paul