Aircraft technical Basics: Aircraft Propellers - Navy Training Courses Edition of 1945: Chapter 1: Background for Propellers

VITAL STATISTICS

A propeller converts the power of an engine into the pushing or pulling force which moves an airplane forward through the air. It's a precision made device. And it's tough! It has to be-to stand the punishment dished out by the engine and the elements.

Propeller and engine work together as a TEAM. Without the help of the other, neither can do its job. The main purpose of an engine is to spin the propeller. The main purpose of a propeller is to convert the engine's work into something useful.

Before you get into the subject of how a modern airplane propeller works, it would be well worth while to have a look at its principal parts. They're illustrated in figure 1, and include the following:

BLADE-One arm or "limb" of a propeller from the hub to the tip. Propellers may have from one to four blades.

BLADE BACK-The surface of the blade which can be seen by standing in front of the airplane.

BLADE FACE-The surface of the blade which can be seen by standing directly in back of the airplane.

HUB-The central portion which is fitted to the engine crankshaft and carries the blades.

SHANK-The thickened portion of the blade near the center of the propeller.

TIP-The portion of the blade farthest from the hub.

LEADING EDGE-The "cutting" edge, or forward edge of the blade that leads in the direction the propeller is turning. The other edge (rear edge) is called the TRAILING EDGE.

Figure 1.-Parts of the propeller.

Although most Navy airplanes don't have them, SPINNERS are sometimes used on propellers. They are cone-shaped streamlined cups or fairings which fit over the hubs of propellers and revolve with them.

You may run into an accessory for propellers called the CUFF. It is a sleeve slipped over the round portion at the shank of a metal propeller blade to continue its airfoil action down to the hub.

One-bladed propellers are uncommon, but have been tried experimentally. The three-bladed type is the most common in the Navy.

The first airplane propellers were made of wood, and even today wood 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.

If you take a look at the blades of an electric fan, you'll see that they have a basic resemblance to the blades of a propeller. The fan blades CURVE slightly from edge to edge and are fastened to the hub of the fan at a SLANT. The CAMBER (curvature) of each blade makes it  look in crosssection very much like a thin airplane wing. Look at a cross-section of the blade. The distance between the leading and trailing edges is the CHORD LINE, as illustrated in  figure 2. The acute angle between the chord and a plane perpendicular to the axis of rotation is the BLADE ANGLE.

The ANGLE of ATTACK Is the angle between the blade chord and the RELATIVE WIND. The direction of the relative wind depends in part on the forward speed of the airplane.

Figure 2-Chord line and blade angle.

HOW IT WORKS

Generally speaking, the airplane propeller is simply a series of rotating AIRFOILS. When the engine turns a propeller, relative motion is set up between the wing-like propeller blades and the air. These blades (think of them as airfoils) set up a "lift" action -or THRUST- similar to that of a wing- moving through the air. Of course this lift is actually a PULL or a PUSH because the propeller blade lift operates in a direction approximately at RIGHT ANGLES to the lift of the wing. The propeller thrust will pull the airplane along if the propeller is mounted ahead of the engine, or push it ahead if the propeller is at the rear. If the propeller is forward, the airplane is called a TRACTOR type. If aft, it is called a PUSHER.

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

Figure 3-propeller pitch and slip.

A turning propeller tends to move forward through the air. Each time it turns it gives a pull or push to the rest of the airplane attached to it. The faster it spins, the greater the pull or push. The total motion of the propeller with respect to the air is somewhat like that of a screw being driven into a block of wood by means of a screwdriver. The distance that a propeller would move ahead through the air during each revolution, if the air were a solid medium like wood, is called the GEOMETRIC PITCH of the propeller. Actually, since it is turning in air instead of wood, the propeller does not move ahead quite as far as it would in a solid medium. The distance it really moves forward is known as the EFFECTIVE PITCH. The difference between the geometric pitch and effective pitch is called the SLIP. In figure 3, you see this difference diagrammed.

Picture a propeller in motion at high speed. Its blades are practically invisible, but the effect it gives is that of a wheel. In f act, it resembles a giant pinwheel. In rotating, the blades whirl around the propeller hub just as the spokes of a wheel whirl around the wheel hub.

That means the outer tips of the blades move faster than the parts of the blade near the hub. And by moving faster, the tips of the blades "bite" bigger chunks out of the air-have much greater PULL or traction.  Consequently, the blade doesn't need as great an angle of attack-or "twist"-at the tip as it does farther down to develop the same amount of pull. The twist of the blade is designed to provide more nearly uniform pull throughout the blade's length.

One of the main requirements of any propeller is an ability to withstand severe stresses. These stresses are greatest near the hub. The major ones and what they do to the propeller are pointed out in figure 4. Chief among them is CENTRIFUGAL FORCE, (A) which tries to make a spinning blade pull away from the hub. To prevent the blades from breaking into fragments or from flying off into space, the central portions are made thicker in cross-section.

There is also a THRUST BENDING FORCE, (B) that acts on the blades. A spinning propeller tries to forge ahead but is held back at the hub by the load of the airplane. The blade tips, which are thinner an d lighter than the blade shank, bend forward just as the tip of a thin stick bends when you wave it through the air.

The sum of these forces causing the blades to bend is carried at and near the hub. Hence, the section of the blade at the hub must be made thick and stocky.

Figure 4-Forces which act on propeller blades.

To some extent, centrifugal force and thrust bending force oppose each other. In other words, the centrifugal force tends to correct the bending force. But the propeller is right in the middle taking all the punishment.

Another bending force that in  ust be considered is known as the TORQUE BENDING FORCE, (C) or twisting force. The rotation of the propeller is caused by the turning force supplied by the engine crankshaft. Moved in a vacuum, the engine would have an easy time of it. But the blades of a propeller revolving through the air meet with mighty stiff resistance. This resistance results in a torque on the crankshaft-acting in the opposite direction to the torque provided by the engine.

Since every part of each blade takes some part in producing this torque, the full length of a blade is also subjected to a share of the load. Consequently a blade throughout its length tends to bend backward against the direction of rotation.

Another force tries to rotate the blades in the hub so that the blade angle will be increased. It's called AERODYNAMIC TWISTING FORCE. Notice (D) in figure 4. The point at which this force is exerted most strongly on the chord of the airfoil is known as the CENTER OF PRESSURE. Since under normal conditions, this center of pressure is nearer the leading edge of the propeller, the force tends to rotate the blade to a higher pitch. When the airplane goes into a dive, the center of pressure moves backwards and may even fall behind the center of rotation.

The CENTRIFUGAL TWISTING FORCE (E) on the blades is a tendency for the blades to twist into a low pitch. If you spin a watch on a string, you will notice that it will turn edgewise. This is because all parts of the watch try to remain in a plane perpendicular to the axis of rotation. Since the same thing happens to a propeller blade, it has a tendency to twist into low pitch.

Sometimes in the face of these forces, a propeller loses some of its rigidity. The result is a FLUTTER, a type of vibration in which the tips of the blades twist rapidly back and forth while the propeller is revolving. Fluttering causes a distinctive noise, which is often nearly drowned out by the exhaust noises. But propeller experts learn to spot the noise instantly. Fluttering will weaken the propeller and may even cause it to fail altogether.

WOOD PROPELLERS

Propellers made of wood were used on practically all airplanes in the early days of aviation. They were fairly satisfactory in most respects, because airplane engines weren't too powerful then, and practically all flying was done in fair weather. In the present day and age, however, powerful modern engines would tear wooden propellers apart in no time. You can't wait for sunny days to do your flying, nor can you pick your spots. Since wood propellers are relatively cheap and easily made, however, they are still used on many trainers.

Figure 5.-Wooden propeller.

In spite of the improvements made in airplanes during World War I, few changes were made up to that time on the original propeller developed by the Wright brothers. Ash, birch, cherry, mahogany, maple, spruce, walnut, white oak-many various kinds of wood were used. And solid wood remained the preferred propeller material until actual combat duty showed up some of its weaknesses.

You'll find today that the wood propeller is no longer carved from a single block of wood, but is now built up in layers-called LAMINATIONS-to give the propeller strength. This type of propeller is commonly used for testing engines in test stands.

Each lamination is usually about 3/4-inch thick. The wood layers are first coated with warm hide glue, then clamped together and kept under pressure until the glue sets. Next, the wood is roughed into shape, being checked with a jig or contour board by which all points on the blades can be measured from a reference plane.

To allow for further aging and glue setting, this rough propeller is laid aside for about 10 days. Then, by hand working, it is finished to exact dimensions and contours, tested, and checked. Finally it is dressed with several coats of spar varnish, or with hot linseed oil, and rubbed down with fine sandpaper.

For protection, the tips of the blades and the leading edges are covered with thin sheets of brass or copper, or a special fabric. This reduces the danger of damage from the impact of sleet, hail , or pebbles. Wood propellers also are waterproofed by means of an acetate solution.

A metal sleeve is inserted in the hole at the center of the hub. The inside of this sleeve is machined to fit the splines on the end of the crankshaft. One end of the sleeve is made with a broad flange that presses against one side of the propeller. An equally broad washer fits over the other end and presses against that side of the hub. Bolts passed through the flange, the hub and the washer serve to hold the three parts together. The bolts also serve to transmit the turning force of the crankshaft to the propeller.

Wood propellers have several structural disadvantages. Their blades are subject to warping and are easily nicked and scraped. Their protective sheathing sometimes comes loose. The laminations may separate, especially under tropical weather conditions. And, of course, wood shears off easily, and can seldom be repaired satisfactorily when damaged.

METAL PROPELLERS

The majority of Navy airplane propellers are made either of aluminum alloy or of steel. Early versions of the aluminum-alloy propeller were forged from a single piece of metal. They were shaped much like a wood propeller, but with thinner sections, especially at the hub and blade roots. A splined steel bushing by which the propeller was attached to the crankshaft was pressed or screwed into the hub. Most modern aluminum.alloy propellers, however, are made with detachable and adjustable blades, with a high-strength alloy-steel hub.

Figure 6.-Metal propeller.

Aluminum alloy is excellent propeller material. When slightly bent, it can be straightened cold. If severely bent, the blade can be annealed, straightened and re-heat-treated. If the leading edges of the tips are slightly torn or ripped, the blades can usually be trimmed down.

While aluminum alloy is resistant to the normal damage done by rain, hail, or salt water spray, all metal blades should be wiped off with an oily rag after exposure to salt water. This will help check corrosion.

The metal. propeller, besides being stronger and more durable than the one made of wood, gives better performance. And because it is heavier, it has a fly-wheel effect on the end of the crankshaft, which results in a smoother-running engine.

TYPES OF PROPELLERS

Many different types of propellers are in use today. They fall into four classes-fixed pitch, adjustable pitch, two-position controllable, and automatic propellers.

Fixed pitch propellers aren't used widely, most of them being on small, light airplanes. They are made of one piece of wood or metal, and the pitch of their blades cannot be changed. Adjustable pitch propellers have blades that can be rotated in the hub by loosening the clamping rings while the airplane is on the ground, but they cannot be adjusted during flight. The two-position controllable and the automatic propellers are the types you'll work with most frequently. As their names imply, they are made to provide changes in the pitch of the blades.

Modern Navy propellers include the TWO-POSITION CONTROLLABLE, THE CONSTANT-SPEED, the HYDROMATIC FULL-FEATHERING, and the ELECTRIC. The constant speed, hydromatic and electric types are automatic propellers.

The two-position controllable is in a class by itself. It has two pitch settings and is controlled by the pilot. The automatic types have any number of pitch settings and are controlled either by the pilot or by a governor control unit.

It has been found that, when an airplane is moving at relatively low speed during a 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 propeller was designed to accomplish shifts to low pitch during flight. The pitch of the blades is changed by a hydraulic device, which rotates the blades in the hub. ONLY HIGH- AND LOW-PITCH ADJUSTMENTS ARE POSSIBLE WITH THIS MECHANISM. Often, of course, it's desirable to use a pitch in between these two extremes, and you can obtain these intermediate pitches if your airplane is equipped with an automatic propeller.

Constant-speed propellers are operated either by oil pressure or by electricity and, within rea sonable 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," or turning over from the force of the wind. Propellers with this feature are designated as full-feathering propellers.

Various differences of control, operation, construction, and performance exist among all types of propellers. But despite differences in detail, there are certain family characteristics which ALL have in common.