An aircraft propeller or airscrew
converts rotary motion from a piston
engine, a turboprop or an electric
motor, to provide propulsive force. Its
pitch may be fixed or variable. Early
aircraft propellers were carved by hand
from solid or laminated wood, while
later propellers were constructed of
metal. Modern designs use
high-technology composite materials.
The propeller attaches to the crankshaft
of a piston engine, either directly or
through a reduction unit. A light
aircraft engine may not require the
complexity of gearing, which is
essential on a larger engine or on a
turboprop aircraft.
History
Italian scientist Leonardo da Vinci made
sketches of an “air screw” human-powered
helicopter with no practical application
between 1487 and 1490.
The twisted airfoil shape of an aircraft
propeller was pioneered by the Wright
Brothers. While some earlier engineers
had attempted to model air propellers on
marine propellers, the Wright Brothers
realized that a propeller is essentially
the same as a wing, and were able to use
data from their earlier wind tunnel
experiments on wings, introducing a
twist along the length of the blades.
This was necessary to maintain a more
uniform angle of attack of the blade
along its length. Their original
propeller blades had an efficiency of
about 82%, compared to the 90% of modern
propellers. Mahogany was the wood
preferred for propellers through World
War I, but wartime shortages encouraged
use of walnut, oak, cherry and ash.
Alberto Santos Dumont was another early
pioneer, having designed propellers
before the Wright Brothers for his
airships. He applied the knowledge he
gained from experiences with airships to
make a propeller with a steel shaft and
aluminium blades for his 14 bis biplane.
Some of his designs used a bent
aluminium sheet for blades, thus
creating an airfoil shape. They were
heavily undercambered, and this plus the
absence of lengthwise twist made them
less efficient than the Wright
propellers. Even so, this was perhaps
the first use of aluminium in the
construction of an airscrew.
Originally, a rotating airfoil behind
the aircraft, which pushes it, was
called a propeller, while one which
pulled from the front was a tractor.
Later the term 'pusher' became adopted
for the rear-mounted device in contrast
to the tractor configuration and both
became referred to as 'propellers' or
'airscrews'.
The understanding of low speed propeller
aerodynamics was fairly complete by the
1920s, but later requirements to handle
more power in a smaller diameter have
made the problem more complex.
Theory and design of aircraft propellers
A well-designed propeller typically has
an efficiency of around 80% when
operating in the best regime. The
efficiency of the propeller is
influenced by the angle of attack. This
is defined as α = Φ - θ, where θ is the
helix angle and Φ is the blade pitch
angle. Very small pitch and helix angles
give a good performance against
resistance but provide little thrust,
while larger angles have the opposite
effect. The best helix angle is when the
blade is acting as a wing producing much
more lift than drag. Angle of attack is
similar to advance ratio, for
propellers.
A propeller's efficiency is determined
by
Propellers are similar in aerofoil
section to a low-drag wing and as such
are poor in operation when at other than
their optimum angle of attack.
Therefore, some propellers use a
variable pitch mechanism to alter the
blades' pitch angle as engine speed and
aircraft velocity are changed.
A further consideration is the number
and the shape of the blades used.
Increasing the aspect ratio of the
blades reduces drag but the amount of
thrust produced depends on blade area,
so using high-aspect blades can result
in an excessive propeller diameter. A
further balance is that using a smaller
number of blades reduces interference
effects between the blades, but to have
sufficient blade area to transmit the
available power within a set diameter
means a compromise is needed. Increasing
the number of blades also decreases the
amount of work each blade is required to
perform, limiting the local Mach number
- a significant performance limit on
propellers.
A propeller's performance suffers as the
blade speed nears the transonic. As the
relative air speed at any section of a
propeller is a vector sum of the
aircraft speed and the tangential speed
due to rotation, a propeller blade tip
will reach transonic speed well before
the aircraft does. When the airflow over
the tip of the blade reaches its
critical speed, drag and torque
resistance increase rapidly and shock
waves form creating a sharp increase in
noise. Aircraft with conventional
propellers, therefore, do not usually
fly faster than Mach 0.6. There have
been propeller aircraft which attained
up to the Mach 0.8 range, but the low
propeller efficiency at this speed makes
such applications rare.
There have been efforts to develop
propellers for aircraft at high subsonic
speeds. The 'fix' is similar to that of
transonic wing design. The maximum
relative velocity is kept as low as
possible by careful control of pitch to
allow the blades to have large helix
angles; thin blade sections are used and
the blades are swept back in a scimitar
shape; a large number of blades are used
to reduce work per blade and so
circulation strength; contra-rotation is
used. The propellers designed are more
efficient than turbo-fans and their
cruising speed is suitable for
airliners, but the noise generated is
tremendous.
= Forces acting on a propeller=
Five forces act on the blades of an
aircraft propeller in motion, they are:
Thrust bending force
Thrust loads on the blades act to bend
them forward.
Centrifugal twisting force
Acts to twist the blades to a low, or
fine pitch angle.
Aerodynamic twisting force
As the centre of pressure of a propeller
blade is forward of its centreline the
blade is twisted towards a coarse pitch
position.
Centrifugal force
The force felt by the blades acting to
pull them away from the hub when
turning.
Torque bending force
Air resistance acting against the
blades, combined with inertial effects
causes propeller blades to bend away
from the direction of rotation.
= Curved propeller blades=
Since the 1940s, propellers and propfans
with swept tips or curved
"scimitar-shaped" blades have been
studied for use in high-speed
applications so as to delay the onset of
shockwaves, in similar manner to wing
sweepback, where the blade tips approach
the speed of sound. The Airbus A400M
turboprop transport aircraft is expected
to provide the first production example:
note that it is not a propfan because
the propellers are not mounted directly
to the engine shaft but are driven
through reduction gearing.
Propeller control
= Variable pitch=
The purpose of varying pitch angle with
a variable-pitch propeller is to
maintain an optimal angle of attack on
the propeller blades as aircraft speed
varies. Early pitch control settings
were pilot operated, either two-position
or manually variable. Following World
War I, automatic propellers were
developed to maintain an optimum angle
of attack. This was done by balancing
the centripetal twisting moment on the
blades and a set of counterweights
against a spring and the aerodynamic
forces on the blade. Automatic props had
the advantage of being simple,
lightweight, and requiring no external
control, but a particular propeller's
performance was difficult to match with
that of the aircraft's powerplant. An
improvement on the automatic type was
the constant-speed propeller.
Constant-speed propellers allow the
pilot to select a rotational speed for
maximum engine power or maximum
efficiency, and a propeller governor
acts as a closed-loop controller to vary
propeller pitch angle as required to
maintain the selected engine speed. In
most aircraft this system is hydraulic,
with engine oil serving as the hydraulic
fluid. However, electrically controlled
propellers were developed during World
War II and saw extensive use on military
aircraft, and have recently seen a
revival in use on homebuilt aircraft.
= Feathering=
On some variable-pitch propellers, the
blades can be rotated parallel to the
airflow to reduce drag in case of an
engine failure. This uses the term
feathering, borrowed from rowing. On
single-engined aircraft, whether a
powered glider or turbine-powered
aircraft, the effect is to increase the
gliding distance. On a multi-engine
aircraft, feathering the propeller on a
failed engine helps the aircraft to
maintain altitude with the reduced power
from the remaining engines.
Most feathering systems for
reciprocating engines sense a drop in
oil pressure and move the blades toward
the feather position, and require the
pilot to pull the propeller control back
to disengage the high-pitch stop pins
before the engine reaches idle RPM.
Turboprop control systems usually
utilize a negative torque sensor in the
reduction gearbox which moves the blades
toward feather when the engine is no
longer providing power to the propeller.
Depending on design, the pilot may have
to push a button to override the
high-pitch stops and complete the
feathering process, or the feathering
process may be totally automatic.
= Reverse pitch=
In some aircraft, such as the C-130
Hercules, the pilot can manually
override the constant-speed mechanism to
reverse the blade pitch angle, and thus
the thrust of the engine. This is used
to help slow the plane down after
landing in order to save wear on the
brakes and tires, but in some cases also
allows the aircraft to back up on its
own - this is particularly useful for
getting floatplanes out of confined
docks. See also Thrust reversal.
Contra-rotating propellers
Contra-rotating propellers use a second
propeller rotating in the opposite
direction immediately 'downstream' of
the main propeller so as to recover
energy lost in the swirling motion of
the air in the propeller slipstream.
Contra-rotation also increases power
without increasing propeller diameter
and provides a counter to the torque
effect of high-power piston engine as
well as the gyroscopic precession
effects, and of the slipstream swirl.
However, on small aircraft the added
cost, complexity, weight and noise of
the system rarely make it worthwhile.
Counter-rotating propellers
Counter-rotating propellers are
sometimes used on twin-, and other
multi-engine, propeller-driven aircraft.
The propellers of these wing-mounted
engines turn in opposite directions from
those on the other wing. Generally, the
propellers on both engines of most
conventional twin-engined aircraft spin
clockwise. Counter-rotating propellers
generally spin clockwise on the left
engine, and counter-clockwise on the
right. The advantage of counter-rotating
propellers is to balance out the effects
of torque and p-factor, eliminating the
problem of the critical engine. These
are sometimes referred to as "handed"
propellers since there are left hand and
right hand versions of each prop.
Aircraft fans
A fan is a propeller with a large number
of blades. A fan therefore produces a
lot of thrust for a given diameter but
the closeness of the blades means that
each strongly affects the flow around
the others. If the flow is supersonic,
this interference can be beneficial if
the flow can be compressed through a
series of shock waves rather than one.
By placing the fan within a shaped duct,
specific flow patterns can be created
depending on flight speed and engine
performance. As air enters the duct, its
speed is reduced while its pressure and
temperature increase. If the aircraft is
at a high subsonic speed this creates
two advantages: the air enters the fan
at a lower Mach speed; and the higher
temperature increases the local speed of
sound. While there is a loss in
efficiency as the fan is drawing on a
smaller area of the free stream and so
using less air, this is balanced by the
ducted fan retaining efficiency at
higher speeds where conventional
propeller efficiency would be poor. A
ducted fan or propeller also has certain
benefits at lower speeds but the duct
needs to be shaped in a different manner
than one for higher speed flight. More
air is taken in and the fan therefore
operates at an efficiency equivalent to
a larger un-ducted propeller. Noise is
also reduced by the ducting and should a
blade become detached the duct would
help contain the damage. However the
duct adds weight, cost, complexity and
drag.
See also
Advance ratio
Axial fan design
Helicopter rotor
List of aircraft propeller manufacturers
References
External links
Experimental Aircraft Propellers
Smithsonian National Air and Space
Museum's How Things Fly website
