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How Aircraft Propellers Work



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Aircraft propellers work on the same principle as the aerodynamic lift created by air passing over a wing. The cross section of a wing will show that the upper surface creates a drop in air pressure allowing air to pass quicker on the upper side as compared to the lower side. Thus causing lift to occur.

If you take a propeller and cut a cross section from it, you will find it is similar in shape to a cross section cut from a wing. This lift that occurs is translated into thrust as the direction of movement is horizontal rather then the vertical.

On smaller airplanes the pitch of the propeller may be fixed, or non-adjustable. On larger airplanes the pitch, or angle of attack, of the propeller can be adjusted to determine the most effective bite into the air. A blade that is in a neutral position is called a "feathered prop" indicating it's in a position that provides no thrust. If the engine driving a propeller is damaged, a feathered prop will prevent the propeller from turning the engine by force of the air passing through it. The engine can be shut down and the propeller will remain motionless.

Adjustable pitch propellers are useful for stopping an airplane. The propellers can be placed in "Reverse" pitch which will change the flow of air, or thrust from rearward to forward. This in effect acts like brakes, slowing and actually stopping the airplane as it lands. This maneuver takes place after the airplane has touched down and is rolling down the runway. It can also be used for maneuvering around the airport.

Helicopter blades also follow the same theory of lift, and as they are considered a vertical flight it's referred to as lift rather then thrust. Their propeller is tilted which in turns provides the thrust for horizontal movement.

Aircraft such as the V-22 Ospery the propeller transitions from lift to thrust as it moves from vertical lift to forward thrust when the propellers change position.

If you were able to skewer an airplane on a vertical pole, and spin the whole aircraft fast enough, the plane would lift vertically. The lift from the wings of the airplane would create similar lift as that produced by a propeller.

In jet passenger aircraft the thrust is developed by the jet engines, without the lift created by the wings they would be a very inefficient flying machine.

With jet fighter planes, the overwhelming quantity of thrust compensates for the general lack of lift created by the wings. Aerodynamic lift is limited in this instance to allow for speed and strength in maneuvering. Lack of lift is the reason they glide like a rock after engine failure.

The more lift created in a wing will allow it to glide further. Gliders have no engine and rely completely on lift generated by the wing to keep them aloft.

The rotors of a helicopter will generate enough lift that when allowed to free wheel, (auto-rotate) the decent can be controlled well enough to land the craft. The faster the blades spin, the slower the decent. Changing the pitch of the blade will allow the helicopter to flare or slow before landing to make a safe touch down.

When all is considered a propeller and a rotating wing are the same thing. Propellers are constructed to withstand the stress of high speed revolutions. If a wing was constructed in a similar way, it would create the same lift as a propeller. As is the case with helicopters, basically a wing used as a propeller.

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