25.05.2015

The understanding and definition of Propeller.

I would like to start with the definition of the word
Propeller to which in the world of HoverpodĀ® is something that is often misused.
I have hear the driving force of the HoverpodĀ® called a Fan, Turbine and many
other terms but in truth it is a propulsion system and to that end defines it
as a Propeller.

The definition and description of the propeller as well as
this history of the propeller has a varied and interesting history and some of
which is defined below:

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
Wrights 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. The
Wrights introduced 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 (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 aluminum
blades for his 14 bis biplane. Some of his designs used a bent aluminum sheet
for blades, thus creating an airfoil shape. They were heavily under cambered,
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 aluminum in the construction of an air screw.

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 ‘air-screws’.

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.

Since the 1940s, propellers and prop fans with swept tips or
curved “scimitar-shaped” blades have been studied for use in
high-speed applications so as to delay the onset of shock waves, in similar
manner to wing sweep back, 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 prop fan because the propellers are
not mounted directly to the engine shaft but are driven through reduction
gearing.

Variable Pitch

The purpose of varying pitch angle with a variable-pitch
propeller is to maintain an optimal angle of attack (maximum lift to drag
ratio) 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 power plant. 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 home built
aircraft.

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 mufti-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 (although the rotation of
the engine itself does not reverse). 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 float planes out of confined docks. See also Thrust reversal

Team Principal smiley

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