Type of propellers螺旋桨类型

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航空

Type of propellers
In designing propellers, the maximum performance of the airplane for all
condition of operation from takeoff, climb, cruising, and high speed. The
propellers may be classified under eight general types as follows:
1. Fixed pitch: The propeller is made in one piece. Only one pitch
setting is possible and is usually two blades propeller and is often made of
wood or metal.
Wooden Propellers : Wooden propellers were used almost exclusively
on personal and business aircraft prior to World War II .A wood propeller is
not cut from a solid block but is built up of a number of seperate layers of
carefully selected .any types of wood have been used in making propellers,
but the most satisfactory are yellow birch, sugar mable, black cherry, and
black walnut. The use of lamination of wood will reduce the tendency for
propeller to warp. For standard one-piece wood propellers, from five to nine
seperate wood laminations about 3/4 in. thick are used.
Metal Propellers : During 1940 , solid steel propellers were made for
military use. Modern propellers are fabricated from high-strength , heattreated,
aluminum alloy by forging a single bar of aluminum alloy to the
required shape. Metal propellers is now extensively used in the construction
of propellers for all type of aircraft. The general appearance of the metal
propeller is similar to the wood propeller, except that the sections are
generally thinner.
2. Ground adjustable pitch: The pitch setting can be adjusted only
with tools on the ground before the engine is running. This type of
propellers usually has a split hub. The blade angle is specified by the aircraft
specifications. The adjustable - pitch feature permits compensation for the
location of the flying field at various altitudes and also for variations in the
characteristics of airplanes using the same engine. Setting the blade angles
by loosened the clamps and the blade is rotated to the desired angle and then
tighten the clamps.
3. Two-position : A propeller which can have its pitch changed from
one position to one other angle by the pilot while in flight.
4. Controllable pitch: The pilot can change the pitch of the propeller in
flight or while operating the engine by mean of a pitch changing mechanism
that may be operated by hydraulically.
5. Constant speed : The constant speed propeller utilizes a hydraulically
or electrically operated pitch changing mechanism which is controlled by
governor. The setting of the governor is adjusted by the pilot with the rpm
lever in the cockpit. During operation, the constant speed propeller will
automatically changs its blade angle to maintain a constant engine speed. If
engine power is increase, the blade angle is increased to make the propeller
absorb the additional power while the rpm remain constant. At the other
position, if the engine power is decreased, the blade angle will decrease to
make the blades take less bite of air to keep engine rpm remain constant.
The pilot select the engine speed required for any particular type of
operation.
6. Full Feathering : A constant speed propeller which has the ability to
turn edge to the wind and thereby eliminate drag and windmilling in the
event of engine failure. The term Feathering refers to the operation of
rotating the blades of the propeller to the wind position for the purpose of
stopping the rotation of the propeller to reduce drag. Therefore , a Feathered
blade is in an approximate in-line-of-flight position , streamlined with the
line of flight (turned the blades to a very high pitch). Feathering is necessary
when the engine fails or when it is desirable to shutoff an engine in flight.
7. Reversing : A constant speed propeller which has the ability to
assume a negative blade angle and produce a reversing thrust. When
propellers are reversed, their blades are rotated below their positive angle ,
that is, through flat pitch, until a negative blade angle is obtained in order to
produce a thrust acting in the opposite direction to the forward thrust .
Reverse propeller thrust is used where a large aircraft is landed, in reducing
the length of landing run.
8. Beta Control : A propeller which allows the manual repositioning of
the propeller blade angle beyond the normal low pitch stop. Used most often
in taxiing, where thrust is manually controlled by adjusting blade angle with
the power lever.

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