Aircraft technical Basics: Introduction to Airplanes - Navy Training Courses Edition of 1944: Chapter 7 Propellers

CHAPTER 7 PROPELLERS

WHAT THEY ARE

An engine is installed in an airplane for one main purpose - to spin the PROPELLER. Don't for a moment get the idea that this is a menial or degrading task beneath the dignity of such a roaring giant as a modern airplane power plant. After all, the propeller is a mighty important piece of equipment itself. What's more, without teamwork between the engine and the propeller, your airplane would be as much a landlubber as a mummy in a museum.

If you take a look at the blades of an electric fan, you'll see a basic resemblance to the blades of a propeller. Notice that the fan blades CURVE slightly from edge to edge and are fastened to the hub of the fan at a SLANT. The curvature (or camber) of each blade makes it look very much like a THIN AIRPLANE WING. And the slant at which the blade is set into the rotating hub determines its "angle of attack."

But such statements indicate that propellers are like ROTATING AIRPLANE WINGS. And that's absolutely RIGHT !

When the engine turns a propeller, what happens? Simply this --

Relative motion is set up between the wing-like propeller blades and the air. These blades, which are really a series of airfoils, set up a "lift" action similar to that of a wing moving through the air. Of course, this lift is actually a PULL or a PUSH because the propeller blades operate in a direction at about RIGHT ANGLES to the wing.

Naturally, as the propeller pulls or pushes itself through the air, it also carries along anything that happens to be hitched onto it - such as the engine and the rest of the airplane. You can readily see how important it is to have propellers attached securely to their engines - and engines fastened firmly to the airplane.

Although the primary function of a propeller is to make the airplane move forward through the air, there are some important secondary functions. The propeller helps to set up the air blast required for cooling the engine cylinders or radiator. And in tractor-type airplanes (those which have their propellers in front of the wings) the propellers help to furnish a relative wind to assist the wings in lifting.

Propellers are made with from one to four blades. One-bladed propellers are rare, but have been tried experimentally. The two-bladed type is the most common except on large transports and bombers, which usually have three-bladed propellers. The first airplane propellers were made of wood, and even today wooden propellers are found on some small engines. Most airplanes of modern manufacture, however, are equipped with METAL propellers of either forged aluminum alloy or steel construction.

A propeller must be capable of withstanding severe stresses, which are greatest near the HUB.

These stresses are caused by CENTRIFUGAL FORCE, which tries to make a spinning blade pull away from the hub ; by TORQUE or twist ; and by the THRUST, which tends to bend the blade forward. Propellers must be rigid enough to prevent a type of vibration known as FLUTTERING, in which the ends of the blades twist back and forth rapidly. Fluttering is likely to weaken a blade, and can cause fatigue failures. In turn, failure of a propeller blade is likely to wreck the engine.

HOW THEY WORK

For reasons you have already learned, a turning propeller tends to move forward through the air. Its actual motion with respect to the air is somewhat like that of a screw being driven into a block of wood by means of a screwdriver. If the air were a solid medium like wood, the distance that a propeller would move ahead through the air during each revolution is called the GEOMETRIC PITCH of the propeller. Actually, since it is turning in air instead of wood, a propeller does not move ahead quite as far as it would in a solid medium. The distance it actually moves is known as the EFFECTIVE PITCH. The difference between geometric pitch and effective pitch is called the SLIP. Figure 25 diagrams the difference between geometric pitch and effective pitch.

Figure 25.-Geometric pitch vs. effective pitch.

One arm, or "limb," of a propeller, from the center to the tip, is called a BLADE. The surface of the blade which corresponds to the top surface of an airfoil (that is, the surface that first greets new air) is known as the BLADE BACK. The surface facing to the rear is the BLADE FACE. The central portion of the propeller which is fitted to the crankshaft and carries the blades is called the HUB. The thickened portion of the blade near the center of the propeller is referred to as the blade ROOT. The portion of the blade farthest from the ,hub is the TIP.

Figure 26 shows you a propeller with its parts labeled.

Figure 26.-Parts of the propeller.

PROPELLER TYPES

Many different types of metal propellers are in use today. The simplest form is the one-piece type, which is forged and machined out of a single piece of aluminum alloy. Another relatively simple type is made with a steel hub, having aluminum blades fastened to it.

More complex propellers incorporate mechanisms which make it possible to change or adjust the pitch of the blades. It has been found that, when an airplane is moving at relatively low speed during take-off and climb, the pitch of the blades should be LOWER (permitting more revolutions per minute) than during level cruising flight.

The TWO-POSITION CONTROLLABLE-PITCH PROPELLER was designed to accomplish shifts to high pitch during flight. Only high and low pitch adjustments are possible with this mechanism.

Often, of course, you want to use a pitch in between these two extremes. The CONSTANT SPEED PROPELLER makes it possible to obtain many pitch settings. Constant-speed propellers are operated either by electricity or by oil pressure and, within reasonable limits, keep the blades at the proper pitch angle for whatever maneuver the airplane is called upon to make. Some constant-speed propellers have a further refinement which permits alinement of the blades with the direction of flight, so that if one engine stops, you can prevent its propeller from "windmilling." Propellers with this feature are designated as FULL-FEATHERING propellers.