Page 235 - Introduction to Naval Architecture

220 PROPULSION

aerofoil will be finite in span and there will be a tendency for the
pressures on the face and back to try to equalize at the tips by a flow
around the ends of the span reducing the lift in these areas. Some
lifting surfaces have plates fitted at the ends to prevent this 'bleeding*
of the pressure. The effect is relatively greater the less the span in
relation to the chord. This ratio of span to chord is termed the aspect
ratio. As aspect ratio increases the lift characteristics approach more
closely those of two-dimensional flow.

Pressure distribution around an aerofoil
The effect of the flow past, and circulation round, the aerofoil is to
increase the velocity over the back and reduce it over the face. By
Bernouilli's principle there will be corresponding decreases in pressure
over the back and increases over the face. Both pressure distributions
contribute to the total lift, the reduced pressure over the back making
the greater contribution as shown in Figure 9.11.

Figure 9,11 Pressure distribution on aerofoil

The maximum reduction in pressure occurs at a point between the
mid-chord and the leading edge. If the reduction is too great in
relation to the ambient pressure in a fluid like water, bubbles form
filled with air and water vapour. The bubbles are swept towards the
trailing edge and they collapse as they enter an area of higher pressure.
This is known as cavitation and is bad from the point of view of noise
and efficiency. The large forces generated when the bubbles collapse
can cause physical damage to the propeller.

PROPELLER THRUST AND TORQUE
Having discussed the basic action of an aerofoil in producing lift, the
action of a screw propeller in generating thrust and torque can be
considered. The momentum theory has already been covered. The