The Controllable-Pitch Propeller
As aircraft engines advanced in the 1920s, it became obvious that the key to getting full performance potential out of any engine was a propeller whose pitch could be changed in flight. In the United States, Frank Caldwell, the federal government’s chief propeller engineer (1917-1928) joined the Hamilton Standard Propeller Corporation in 1929 to develop a controllable-pitch propeller. His first design was a ground-adjustable metal propeller which allowed the mechanics/pilot to preset the propeller pitch for the desired efficiency of the aircraft while in flight. Later, Caldwell designed a hydraulically-actuated two-position design which provided efficiency at both takeoff and cruise, the two main operating regimes for the airplane.
Performance tests of the latter design revealed that Caldwell's invention maximized the performance of revolutionary aircraft such as the Boeing Model 247 and the Douglas DC-2. By the mid-1930s, controllable-pitch, variable-speed propellers were being manufactured worldwide. They contributed to the success of the early modern airliners.
The National Aeronautics Association recognized Caldwell and Hamilton Standard for their achievement by awarding them the 1933 Collier Trophy.
So how does it work?
The engineering and mechanics associated with propellers and airfoils is a complex a subject, and obviously one in which I am unqualified, but I think it is important to understand the principles of the propeller and its operation in order to appreciate the impact of Caldwell's invention on the development of aviation.
So, let's take a brief look....
The Propeller
A propeller is a rotating airfoil, and is subject to induced drag, stalls, and other
aerodynamic principles that apply to any airfoil. The propeller provides the
necessary thrust to pull, or in some cases push, the airplane through the air. This
is accomplished by using engine power to rotate the propeller which in turn generates
thrust in much the same way as a wing produces lift. The propeller has an angle of
attack which is the angle between the chord line of the propeller’s airfoil and its
relative wind (airflow opposite to the motion of the blade).
A propeller blade is twisted. The blade angle changes from the hub to the tip with
the greatest angle of incidence, or highest pitch, at the hub and the smallest at the tip
(see figure). The reason for the twist is to produce uniform lift from the hub to
the tip. As the blade rotates, there is a difference in the actual speed of the
various portions of the blade.
The tip of the blade travels faster than that part near the hub because the tip travels a
greater distance than the hub in the same length of time. Changing the angle of
incidence (pitch) from the hub to the tip to correspond with the speed produces uniform
lift throughout the length of the blade. If the propeller blade was designed with
the same angle of incidence throughout its entire length, it would be extremely
inefficient because as airspeed increases in flight, the portion near the hub would have a
negative angle of attack while the blade tip would be stalled.
Geometric Pitch is the distance in inches that the propeller would move forward in one
revolution if it were rotated in a solid medium so as not to be affected by slippage as it
is in the air. Effective Pitch is the actual distance it moves forward
through the air in one revolution. Propeller Slip is the difference between
the geometric pitch and effective pitch. Pitch is proportional to the blade angle
which is the angle between the chord line of the blade and the propeller’s plane of
rotation.
So...still with me? Now we move on to propeller types. Airplanes are equipped
with one of two types of propellers. The first is the fixed-pitch propeller, and the
other is the controllable-pitch or constant-speed propeller.
The Fixed-Pitch Propeller
In this type of propeller the blades are mounted directly onto the hub in a position
determined and "fixed" by the manufacturer and cannot be changed by the pilot --
hence the name. There are two types of fixed-pitch propellers; the climb
propeller and the cruise propeller. Whether the airplane has a climb or
cruise propeller installed depends upon its intended use.
The Climb Propeller -
This propeller has a lower pitch, therefore less drag. This results in the capability of higher rpm and more horsepower being developed by the engine. This increases performance during takeoffs and climbs, but decreases performance during cruising flight.
The Cruise Propeller -
This propelelr features a higher pitch, therefore more drag than that of the climb propeller. This results in lower rpm and less horsepower capability. This decreases performance during takeoffs and climbs, but increases efficiency during cruising flight.
The Controllable-Pitch Propeller
In this type of propeller the blades are mounted separately on the hub, each on one
axis of rotation, allowing a change of pitch in the blades. This arrangement
allows the pilot to change the pitch on the blades in flight; therefore, they are referred
to as controllable-pitch propellers. These propeller systems vary from a simple
two-position propeller to more complex automatic constant-speed propellers. The
number of pitch positions at which the propeller can be set may be limited, such as a
two-position propeller with only high or low pitch available. Many other propellers,
however, are variable pitch, and can be adjusted to any pitch angle between a minimum and
maximum pitch setting.
An airplane equipped with a controllable-pitch propeller has two controls:
(1) a throttle control which controls the power output of the engine which is registered on the manifold pressure gauge.
(2) a propeller control which regulates the engine rpm and in turn the propeller rpm. The rpm is registered on the tachometer.
The pilot can set the throttle control and propeller control at any desired manifold pressure and rpm setting within the engine operating limitation.
Within a given power setting, when using a constant-speed propeller, the pilot can set the propeller control to a given rpm and the propeller governor will automatically change the pitch (blade angle) to counteract any tendency for the engine to vary from this rpm. For example, if manifold pressure or engine power is increased, the propeller governor automatically increases the pitch of the blade (more propeller drag) to maintain the same rpm.
A controllable-pitch propeller permits the pilot to select the blade angle that will result in the most efficient performance for a particular flight condition. A low blade angle or decreased pitch, reduces the propeller drag and allows more engine power for takeoffs. After airspeed is attained during cruising flight, the propeller blade is changed to a higher angle or increased pitch. Consequently, the blade takes a larger bite of air at a lower power setting, and therefore increases the efficiency of the flight. This process is similar to shifting gears in an automobile from low gear to high gear.
Note:
In modern aircraft, pitch control is achieved automatically, and the propellers are
referred to as constant-speed propellers. As power requirements vary, the pitch
automatically changes, keeping the engine and the propeller operating at a constant rpm.
If the rpm rate increases, as in a dive, a governor on the hydraulic system changes
the blade pitch to a higher angle. This acts as a brake on the crankshaft. If
the rpm rate decreases, as in a climb, the blade pitch is lowered and the crankshaft rpm
can increase.
Well, there you have it. I hope this discussion helps to make the subject a little easier to understand. One can easily see how the introduction of the controllable-pitch propeller made such an important impact on the development of the airplane.
