Game Development Reference
In-Depth Information
The engine power is sometimes referred to as the
engine brake power
because engine
power is tested by applying a brake to the engine shaft. The thrust power is equal to the thrust
produced by the propeller multiplied by the airplane velocity,
v
.
PFv
=
(10.6)
T
T
The propeller efficiency, which has a value between 0 and 1, is not constant but instead
is a function of propeller forward velocity and turnover rate. Propeller efficiency is usually
plotted as a function of a quantity called the
propeller advance ratio
,
J
, which is the ratio of the
velocity divided by the propeller turnover rate,
n
, and the propeller diameter,
d
.
v
J
=
(10.7)
nd
In Equation (10.7), the engine turnover rate is in units of revolutions per second. The
propeller efficiency curve for a typical propeller,
2
the McCauley 7557, is shown in Figure 10-13.
It is clear from the figure that the propeller efficiency coefficient is not constant. The efficiency
reaches a peak value of about 0.83 at an advance ratio value of 0.65 and then decreases quite
sharply after that.
Figure 10-13.
Propeller efficiency coefficient, McCauley 7557 propeller
In order to use the propeller efficiency data shown in Figure 10-13, a mathematical equation
must be found that simulates the propeller efficiency curve. One way to model the curve is with
a simple cubic equation that is a function of the advance ratio,
J
.
3
h
=+
aJ
bJ
(10.8)
p
The expression shown in Equation (10.8) satisfies the requirement that the propeller effi-
ciency must be zero when the advance ratio is zero. The constants
a
and
b
can be chosen to
