Page 138 - Understanding Flight

CH05_Anderson 7/25/01 8:58 AM Page 125

Airplane Propulsion 125

importance is matching the propeller rotation speed to the type of
engine provided. Couple these requirements with the need to keep
the propeller-tip speed below the speed of sound (for reasons of
noise and additional power loss) and you have propellers that we see
on airplanes today. Airplane piston engines are usually designed to
operate at between 2200 and 2600 rpm (revolutions per minute).
Limiting the tip speed to roughly the speed of sound in normal flight
situations gives a typical propeller diameter of 72 to 76 inches (182
to 193 cm).
The power added to the air is proportional to the propeller’s
rotational speed cubed. This can be easily understood by considering
a wing with a fixed angle of attack. We know, from Chapter 2, the
power imparted to the air by the wing is proportional to the amount
of air diverted times the vertical velocity of that air squared. If the
speed of the wing is doubled, while keeping the angle of attack
constant, both the amount of air diverted and the vertical velocity of
that air will also be doubled. Thus the power will have gone up by a
factor of 8. Likewise with a propeller, with a fixed pitch and constant
forward speed, the power transferred to the air is proportional to its
rotational speed cubed. What this means is that the power needed to
turn a propeller increases very rapidly with its rotation speed. It is
therefore very important to get the area of the propeller correct for the
engine size. If the blade is too small, the load on the engine
will be low and the engine will “over-rev” by going to a high
The radial engines in WWI
rpm. This can damage an engine. If the blade is too large, the
fighter airplanes actually rotated
engine will not be able to reach its optimum operating speed
with the propeller.
and thus will not be able to deliver full power to the propeller.
Multibladed Propellers

The total area of the blades on a propeller determines the ability of the
propeller to convert the engine’s power into thrust. The more area, the
more power the propeller can convert. For most small planes the
appropriate blade area is achieved with two blades, with the area of
the blades becoming greater with increasing engine size. Some pro-
pellers increase the total area by using three-bladed, four-bladed, and
even six-bladed propellers. The reality is that if the total area is the