In aeronautics, a propeller, also called an
airscrew, converts rotary motion from an engine
or other power source into a swirling slipstream
which pushes the propeller forwards or backwards.
It comprises a rotating power-driven hub,
to which are attached several radial airfoil-section
blades such that the whole assembly rotates
about a longitudinal axis. The blade pitch
may be fixed, manually variable to a few set
positions, or of the automatically-variable
"constant-speed" type.
The propeller attaches to the power source's
driveshaft either directly or through reduction
gearing. Propellers can be made from wood,
metal or composite materials.
Propellers are only suitable for use at subsonic
airspeeds mostly below about 480 mph (770
km/h; 420 kn), as above this speed the blade
tip speed approaches the speed of sound and
local supersonic flow causes high drag, noise
and propeller structural problems.
== History ==
The earliest references for vertical flight
came from China. Since around 400 BC, Chinese
children have played with bamboo flying toys.
This bamboo-copter is spun by rolling a stick
attached to a rotor between ones hands. The
spinning creates lift, and the toy flies when
released. The 4th-century AD Daoist book Baopuzi
by Ge Hong (抱朴子 "Master who Embraces
Simplicity") reportedly describes some of
the ideas inherent to rotary wing aircraft.Designs
similar to the Chinese helicopter toy appeared
in Renaissance paintings and other works.
It was not until the early 1480s, when Leonardo
da Vinci created a design for a machine that
could be described as an "aerial screw", that
any recorded advancement was made towards
vertical flight. His notes suggested that
he built small flying models, but there were
no indications for any provision to stop the
rotor from making the craft rotate. As scientific
knowledge increased and became more accepted,
man continued to pursue the idea of vertical
flight. Many of these later models and machines
would more closely resemble the ancient bamboo
flying top with spinning wings, rather than
Leonardo's screw.
In July 1754, Russian Mikhail Lomonosov had
developed a small coaxial modeled after the
Chinese top but powered by a wound-up spring
device and demonstrated it to the Russian
Academy of Sciences. It was powered by a spring,
and was suggested as a method to lift meteorological
instruments. In 1783, Christian de Launoy,
and his mechanic, Bienvenu, used a coaxial
version of the Chinese top in a model consisting
of contrarotating turkey flight feathers as
rotor blades, and in 1784, demonstrated it
to the French Academy of Sciences. A dirigible
airship was described by Jean Baptiste Marie
Meusnier presented in 1783. The drawings depict
a 260-foot-long (79 m) streamlined envelope
with internal ballonets that could be used
for regulating lift. The airship was designed
to be driven by three propellers. In 1784
Jean-Pierre Blanchard fitted a hand-powered
propeller to a balloon, the first recorded
means of propulsion carried aloft. Sir George
Cayley, influenced by a childhood fascination
with the Chinese flying top, developed a model
of feathers, similar to that of Launoy and
Bienvenu, but powered by rubber bands. By
the end of the century, he had progressed
to using sheets of tin for rotor blades and
springs for power. His writings on his experiments
and models would become influential on future
aviation pioneers.
William Bland sent designs for his "Atmotic
Airship" to the Great Exhibition held in London
in 1851, where a model was displayed. This
was an elongated balloon with a steam engine
driving twin propellers suspended underneath.
Alphonse Pénaud developed coaxial rotor model
helicopter toys in 1870, also powered by rubber
bands. In 1872 Dupuy de Lome launched a large
navigable balloon, which was driven by a large
propeller turned by eight men. Hiram Maxim
built a craft that weighed 3.5 tons, with
a 110-foot (34-meter) wingspan that was powered
by two 360-horsepower (270-kW) steam engines
driving two propellers. In 1894, his machine
was tested with overhead rails to prevent
it from rising. The test showed that it had
enough lift to take off. One of Pénaud's
toys, given as a gift by their father, inspired
the Wright brothers to pursue the dream of
flight. The twisted airfoil (aerofoil) 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 90% for a modern
(2010) small general aviation propeller, the
3-blade McCauley used on a Bonanza aircraft.
Roper quotes 90% for a propeller for a human-powered
aircraft.
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 (albeit not as efficient)
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 in 1906. 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.
Propeller research for National Advisory Committee
for Aeronautics (NACA) was directed by William
F. Durand from 1916. Parameters measured included
propeller efficiency, thrust developed, and
power absorbed. While a propeller may be tested
in a wind tunnel, its performance in free-flight
might differ. At the Langley Memorial Aeronautical
Laboratory, E. P. Leslie used Vought VE-7s
with Wright E-4 engines for data on free-flight,
while Durand used reduced size, with similar
shape, for wind tunnel data. Their results
were published in 1926 as NACA report #220.
== Theory and design of aircraft propellers
==
Lowry quotes a propeller efficiency of about
73.5% at cruise for a Cessna 172. This is
derived from his "Bootstrap approach" for
analyzing the performance of light general
aviation aircraft using fixed pitch or constant
speed propellers. The efficiency of the propeller
is influenced by the angle of attack (α).
This is defined as α = Φ - θ, where θ
is the helix angle (the angle between the
resultant relative velocity and the blade
rotation direction) 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. However, 'lift-and-drag'
is only one way to express the aerodynamic
force on the blades. To explain aircraft and
engine performance the same force is expressed
slightly differently in terms of thrust and
torque since the required output of the propeller
is thrust. Thrust and torque are the basis
of the definition for the efficiency of the
propeller as shown below. The advance ratio
of a propeller is similar to the angle of
attack of a wing.
A propeller's efficiency is determined by
η
=
propulsive power out
shaft power in
=
thrust
⋅
axial speed
resistance torque
⋅
rotational speed
.
{\displaystyle \eta ={\frac {\hbox{propulsive
power out}}{\hbox{shaft power in}}}={\frac
{{\hbox{thrust}}\cdot {\hbox{axial speed}}}{{\hbox{resistance
torque}}\cdot {\hbox{rotational speed}}}}.}
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, most 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.
The performance of a propeller suffers when
transonic flow first appears on the tips of
the blades. 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, the flow over the 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 (Scimitar
propeller); 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 (Mach 0.7–0.85)
is suitable for airliners, but the noise generated
is tremendous (see the Antonov An-70 and Tupolev
Tu-95 for examples of such a design).
=== Forces acting on a propeller ===
Forces acting on the blades of an aircraft
propeller include the following. Some of these
forces can be arranged to counteract each
other, reducing the overall mechanical stresses
imposed.
Thrust bending
Thrust loads on the blades, in reaction to
the force pushing the air backwards, act to
bend the blades forward. Blades are therefore
often raked forwards, such that the outward
centrifugal force of rotation acts to bend
them backwards, thus balancing out the bending
effects.
Centrifugal and aerodynamic twisting
A centrifugal twisting force is experienced
by any asymmetrical spinning object. In the
propeller it acts to twist the blades to a
fine pitch. The aerodynamic centre of pressure
is therefore usually arranged to be slightly
forward of its mechanical centreline, creating
a twisting moment towards coarse pitch and
counteracting the centrifugal moment. However
in a high-speed dive the aerodynamic force
can change significantly and the moments can
become unbalanced.
Centrifugal
The force felt by the blades acting to pull
them away from the hub when turning. It can
be arranged to help counteract the thrust
bending force, as described above.
Torque bending
Air resistance acting against the blades,
combined with inertial effects causes propeller
blades to bend away from the direction of
rotation.
Vibratory
Many types of disturbance set up vibratory
forces in blades. These include aerodynamic
excitation as the blades pass close to the
wing and fuselage. Piston engines introduce
torque impulses which may excite vibratory
modes of the blades and cause fatigue failures.
Torque impulses are not present when driven
by a gas turbine engine.
=== 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.
== Varying pitch ==
The purpose of varying pitch angle is to maintain
an optimal angle of attack for the propeller
blades, giving maximum efficiency throughout
the flight regime. The requirement for pitch
variation is shown by the propeller performance
during the Schneider Trophy competition in
1931. The Fairey Aviation Company fixed-pitch
propeller used was stalled on take-off up
to 160 mph on its way up to a top speed of
407.5 mph. The very wide speed range was achieved
because some of the usual requirements for
aircraft performance did not apply. There
was no compromise on top-speed efficiency,
the take-off distance was not restricted to
available runway length and there was no climb
requirement.For the highest possible speed
the highest possible propeller efficiency
is required at the high speed condition. As
pitch corresponds to airspeed a coarse pitch
is required. The variable pitch blades used
on the Tupolev Tu-95 propel it at a speed
exceeding the maximum once considered possible
for a propeller-driven aircraft using an exceptionally
coarse pitch.
=== Variable pitch ===
Early pitch control settings were pilot operated,
either with a small number of preset positions
or continuously 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 power plant.
Modern light aircraft and advanced homebuilt
aircraft sometimes have variable pitch (VP)
propellers. These tend to be electrically
operated and controlled manually or by computer.
The V-Prop is self-powering and self-governing.
A simpler version was the spring-loaded "two-speed"
VP prop, which was set to fine for takeoff,
and then triggered to coarse once in cruise,
the propeller then staying in coarse for the
remainder of the flight. An even simpler version
is the ground-adjustable propeller, which
may be adjusted on the ground, but is effectively
a fixed-pitch prop once airborne.
=== Constant speed ===
An improvement on the automatic type was the
constant-speed propeller. This type automatically
adjusts the blade pitch according to the engine
speed, thereby maintaining a constant engine
speed for any given manual control setting.
Constant-speed propellers allow the pilot
to set a rotational speed according to the
need 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 most variable-pitch propellers, the blades
can be rotated parallel to the airflow to
stop rotation of the propeller and reduce
drag when the engine fails or is deliberately
shut down. This is called feathering, a term
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 an inoperative engine reduces
drag, and helps the aircraft maintain speed
and altitude with the operative 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 ===
The propellers on some aircraft can operate
with a negative blade pitch angle, and thus
reverse the thrust from the propeller. This
is known as Beta Pitch. Reverse thrust is
used to help slow the aircraft after landing
and is particularly advantageous when landing
on a wet runway as wheel braking suffers reduced
effectiveness. In some cases reverse pitch
allows the aircraft to taxi in reverse – this
is particularly useful for getting floatplanes
out of confined docks. See also Thrust reversal.
== Counter-rotating propellers ==
Counter-rotating propellers are sometimes
used on twin-engine and multi-engine aircraft
with wing-mounted engines. These propellers
turn in opposite directions from their counterpart
on the other wing to balance out the torque
and p-factor effects. They are sometimes referred
to as "handed" propellers since there are
left hand and right hand versions of each
prop.
Generally, the propellers on both engines
of most conventional twin-engined aircraft
spin clockwise (as viewed from the rear of
the aircraft). To eliminate the critical engine
problem, counter-rotating propellers usually
spin "inwards" towards the fuselage – clockwise
on the left engine and counter-clockwise on
the right – but there are exceptions such
as the P-38 Lightning which spun "outwards"
away from the fuselage, and the Airbus A400
whose inboard and outboard engines turn in
opposite directions even on the same wing.
== Contra-rotating propeller ==
A contra-rotating propeller or contra-prop
places two counter-rotating propellers on
concentric drive shafts so that one sits immediately
'downstream' of the other propeller. This
provides the benefits of counter-rotating
propellers for a single powerplant. The forward
propeller provides the majority of the thrust,
while the rear propeller also recovers energy
lost in the swirling motion of the air in
the propeller slipstream. Contra-rotation
also increases the ability of a propeller
to absorb power from a given engine, without
increasing propeller diameter. However the
added cost, complexity, weight and noise of
the system rarely make it worthwhile and it
is only used on high-performance types where
ultimate performance is more important than
efficiency.
== 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 (to a certain
degree) drag.
== See also ==
Advance ratio
Axial fan design
Helicopter rotor
List of aircraft propeller manufacturers
Radial-lift rotors
