Most early propellers were made of
laminated layers of wood carved into the proper airfoil shape by skilled
craftsmen. The shape of the propeller is defined by the manner in which it
operates. Since the portions of the propeller nearest the center turn the
slowest, the angle of the blade is the greatest. As the distance from the hub
of the blade increases the angle of the blade decreases since at the greater
distance the blade is turning faster. At the tip of the blade the angle of it
presents to the relative wind at almost zero.
Advances in engine design brought faster
aircraft. As aircraft advanced in design there grew a need for higher
performance propellers. Shortly wood was not a strong enough medium due to the
greater amounts of power available from piston engines. In order to harness
these larger engines metal propellers became more popular. Offering higher
strength, longer life, and an easier manufacturing process the metal propeller
virtually took over the market.
The new engines and aircraft were able to
fly at far greater speeds than would aircraft with old wooden fixed pitch
propellers. As aircraft speeds increased, the need emerged to allow the pitch
or angle of the blade to change. In comparing the controllable pitch propeller
to the operation of an automobile automatic transmission the engine is kept
operating at its best power range as the speed increases.
In most modern aircraft with controllable
propellers the operation is referred to as a constant speed propeller. Before take off the propeller control is set to a high pitch or
fine setting for the best climb. A propeller governor controls the speed
of rotation usually limiting the rpm to about 2,700. After take off, the prop
control is moved to slow the rotation to 2,500 rpm. During cruise the rpm of
the prop can be lowered more for best power, longest range, or quietest
operation.
One beneficial aspect to limiting the
speed of the propeller is to keep the tip of the propeller from going
transonic. The tip of a fast turning propeller will nearly reach and sometimes
exceed the speed of sound. The airfoil of the propeller is not designed to
produce lift at supersonic speeds. As the tip of the propeller goes transonic
or breaks the sound barrier the lift produced at the tip will be lost. Another
annoying aspect of high rpm is the loud rasping sound heard by those on the
ground as the tips go transonic.
Modern turboprop aircraft have wider and
shorter propellers. The propellers on these aircraft are more able to accept
the greater amount of horsepower available from turbine engines. The propellers
on turboprop aircraft turn much slower than do the propellers on piston powered
aircraft. The United States Navy uses a four-engine turboprop known as the P3
Orion for submarine hunting. In cruise the propellers of the P3 Orion turn at
only 1,200 rpm.
Even in most jet and fan jet engines the
use of the propeller is found. Within the turbine (jet) engine hundreds of
blades in an airfoil shape draw air into the engine. Several stages of blades
continue to compress the incoming air until the air reaches the combustion
chamber where fuel is injected to power the engine. The fan part of the fan jet
engine is a large set of blades at the front of the turbine. This large set of
blades is contained within a separate cowling and blow large volumes of air to
the rear therefore increasing the thrust produced.
It was once thought during the early days
of the jet, that propeller driven aircraft would become extinct. However, even
with the greater amounts of power that became available with the advent of the
turbine engine, the need for propeller driven aircraft remained strong due to
one important factor. At the slower speeds that most aircraft operate at (under
400 mph), the propeller is the most efficient means of producing thrust.
Propeller technology is advancing into the
jet age too. New designs known as unducted fanjet
engines are being tested for new generations of passenger aircraft. These
un-ducted fans will allow commercial passenger aircraft to fly as fast as
turbojets and fanjet engines but with a great savings in fuel usage.
Robert
Perry will answer all questions on matters aeronautical. Address your questions to "PLANE
TALK" care of this newspaper.