81 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:

The propulsion efficiency of the blade element, i.e. the blading efficiency, is defined by:

V0dF udQ

u = velocity of blade element = 2nnr where D = drag

L = lift dF = thrust force acting on blade element dQ = corresponding torque force r = radius

Vector diagram for a blade element of a propeller O'

Vector diagram for a blade element of a propeller O'

Aerodynamic forces acting on a blade element

Projection of axis of rotation

Chord line

Aerodynamic forces acting on a blade element

Projection of axis of rotation

Chord line

Fig. 8.1 Propeller blade elements

Fig. 8.1 Propeller blade elements

The value of $ which makes ^b a maximum is termed the optimum advance angle $opt. Maximum blade efficiency is given by:

8.1.2 Performance characteristics

The pitch and angle $ 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 (a) up to the point where the

Pitch angle, $ Fig. 8.2 Propeller parameter relationship

blade stalls whilst drag coefficient CD is quadratic function of a. Figure 8.2 shows broad relationships between blading efficiency, pitch angle and L/D ratio.

8.1.3 Propeller coefficients

It can be shown, neglecting the compressibility of the air, that:

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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