Page 140 - Aeronautical Engineer Data Book

Section 8

Principles of propulsion

8.1 Propellers
A propeller or airscrew converts the torque of
an engine (piston engine or turboprop) into
thrust. Propeller blades have an airfoil section
which becomes more ‘circular’ towards the hub.
The torque of a rotating propeller imparts a
rotational motion to the air flowing through it.
Pressure is reduced in front of the blades and
increased behind them, creating a rotating
slipstream. Large masses of air pass through the
propeller, but the velocity rise is small compared
to that in turbojet and turbofan engines.

8.1.1 Blade element design theory
Basic design theory considers each section of
the propeller as a rotating airfoil. The flow over
the blade is assumed to be two dimensional (i.e.
no radial component). From Figure 8.1 the
following equations can be expressed:
–1
Pitch angle = tan (V /πnd)
0
The propulsion efficiency of the blade element,
i.e. the blading efficiency, is defined by:
dF tan L/D – tan
V 0
= = =
b
udQ tan( + ) L/D + cot
u = velocity of blade element = 2πnr
where D = drag
L = lift
dF = thrust force acting on blade
element
dQ = corresponding torque force
r = radius