Page 143 - Aeronautical Engineer Data Book

118 Aeronautical Engineer’s Data Book
where d = propeller diameter (ft)
n = speed in revs per second
Q = torque (ft lb)
F = thrust (lbf)
P = power (ft lb/s)
4
2
= air density (lb s /ft )
8.1.4 Activity factor
Activity factor (AF) is a measure of the power-
absorbing capabilities of a propeller, and
hence a measure of its ‘solidity’. It is defined
as:
r/R=1 c
r
r
100 000
3
AF =
d
16 r /R d P R R
h
8.1.5 Propeller mechanical design
Propeller blades are subjected to:
• Tensile stress due to centrifugal forces.
• Steady bending stress due to thrust and
torque forces.
• Bending stress caused by vibration.
Vibration-induced stresses are the most serious
hence propellers are designed so that their first
order natural reasonant frequency lies above
expected operating speeds. To minimize the
chance of failures, blades are designed using
fatigue strength criteria. Steel blades are often
hollow whereas aluminium alloy ones are
normally solid.

8.2 The gas turbine engine: general
principles
Although there are many variants of gas
turbine-based aero engines, they operate using
similar principles. Air is compressed by an
axial flow or centrifugal compressor. The
highly compressed air then passes to a combus­
tion chamber where it is mixed with fuel and
ignited. The mixture of air and combustion
products expands into the turbine stage which
in turn provides the power through a coupling
shaft to drive the compressor. The expanding