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Propeller Propulsion
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Glenn
Research
Center
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At the rear of the
1903 Wright Flyer
one finds a pair of pusher propellers.
The propellers are long, thin, twisted pieces of wood which are spun at
high speed. The propeller blade is shaped like an
airfoil
and there is a
pressure
difference created across the blade because
of the
motion
of the spinning blade. The
pressure
difference causes large amounts of air to be accelerated through the plane
of the propeller and the
reaction
of the vehicle to this motion generates a force called
thrust.
The thrust pushes the aircraft forward in accordance with Newton's
first law of motion.
The physics involved in the generation of thrust is introduced in
middle school and studied in some detail in high school and college.
To accelerate the air, we have to expend energy. The energy is
generated as heat by the
combustion
of some fuel in an engine.
The Wright brothers used a single, 12 horsepower, gasoline powered,
internal combustion engine
to turn their propellers. For comparison, my lawn mower engine is a
6 horsepower engine! Automobile engines are also internal combustion
engines. The engine is placed next to the pilot and is connected to
the propellers by "bicycle" chains.
The use of high speed (~350 revolutions per minute),
thin propellers on their aircraft was one of
the major breakthroughs for the Wright brothers and allowed them to
succeed where others failed. At the time, others employed low speed, thick
bladed propellers, much like the blades of a wind mill. But the brothers
correctly determined that high speed, thin propellers are more efficient
than low speed thick blades. High efficiency means that they were able to
convert more of the available energy of the engine into thrust. The brothers
did not have much power to work with, so efficiency was very important.
The brothers
used two propellers rotating in opposite directions. This eliminated
any gyroscopic forces which would have been generated by a single propeller
and would have made the plane harder to control.
Propellers are still used today
on general aviation and commuter aircraft and were the primary
propulsion system for all aircraft until the development of
jet engines during World War II.
Navigation..
- Re-Living the Wright Way
- Beginner's Guide to Aeronautics
- NASA Home Page
- http://www.nasa.gov
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