A propeller, or ‘airscrew’, converts the torque of an engine (piston engine or
turboprop) into thrust. Propeller blades have an airfoil section that 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.
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 Fig. 10.1 the following equations can be expressed
Pitch angle φ = tan–1 (Vo/πnd)
u = velocity of blade element = 2πnr
The propulsion efficiency (ηb) of the blade element, i.e. the ‘blading
efficiency’, is defined by
Engineers’ Guide to Rotating Equipment
D = drag
L = lift
dF = thrust force acting on blade element
dQ = corresponding torque force
r = radius
The value of φ that makes ηb a maximum is termed the ‘optimum advance
Fig. 10.1 Aeropropeller design
Maximum blade efficiency is given by
The pitch and angle φ have different values at different radii along a
propeller blade. It is common to refer to all parameters determining the
overall characteristics of a propeller to their values at either 0.7r or 0.75r.
Lift coefficient CL is a linear function of the angle of attack α up to the
point where the blade stalls, while drag coefficient CD is a quadratic function
of α. Figure 10.2 shows broad relationships between blading efficiency,
pitch angle, and L/D ratio.
Fig. 10.2 A square key end shape
It can be shown, neglecting the compressibili