Aircraft technical Basics: TM 1-412, Aircraft Propellers, 1941: I. General

SECTION I. GENERAL

1. General.-a. Power to drive the airplane through the air is furnished by the engine, the brake horsepower of which is transformed into thrust by the propeller. The propeller may be described as a twisted airfoil of irregular plan form. In order to analyze the blade element, each blade is divided into 6-inch sections and each section is set at the proper angle to the relative air (fig. 1). The sections near the tip of the propeller travel at a higher peripheral speed than those near the hub, consequently the blade angles become less as the tip is approached. The sections from 12 to 18 inches from the hub are thick in order to give strength to the propeller, and as a result, deliver little or no thrust. In general, each section is so designed and set at such an angle that when the propeller is being operated at a given rotative and forward speed, the best efficiency of each section will be obtained.

b. The angle at which each section meets the air in flight is much less than the actual blade angle of each section. This is due to the fact that the propeller is moving forward as well as rotating (fig. 2). The section G which is R distance from the hub, moves in one revolution 2πR distance and in N revolutions it moves 2pRN distance. This distance is shown graphically by the line ac. While the propeller is revolving N times, it is also moving forward a distance represented by be. The blade angle of this section is the angle D, while the angle at which the section meets the air, or its angle of attack, is represented by the angle E. Under normal flight conditions this is usually about 2°, but in a power dive it is possible for the airplane to obtain a speed, due to acceleration by the force of gravity, which is greater than the propeller tends to produce. When this occurs, the propeller is actually holding the airplane back and the (1) condition shown in figure 3 is approached. In this case the angle of attack E of the propeller section G is negative and no thrust is produced. In the opposite, as in a steep climb with the forward speed reduced but the rotative speed held constant, the angle of attack becomes greater until the efficiency of the propeller is very low (fig. 3(2) ).

c. Fixed pitch propellers are designed to have best efficiency at one rotative and forward speed. In other words, they are designed to fit a set condition of both airplane and engine speeds and any change in these conditions results in lowering the efficiency. Since the advent of the controllable propeller, on which the blade angle may be set or automatically changed to a new set of conditions, this lowering of efficiency had been greatly reduced.

2. Terms.-Some of the principal propeller terms used through-out this manual are as follows:

a. Blade angle.-The angle between the chord of a section of it propeller blade and a plane perpendicular to the axis of rotation. The blade angles for different airplane and engine combinations are specified in Air Corps Technical Orders. Deviation from these settings as much as 1° above to 1/2° below may be authorized. One degree change in blade angle will affect the engine r. p. m. between 70 to 100 r. p. m. ; on geared engines this will vary with the gear ratio.

b. Blade back.-The cambered or curved side of a propeller blade, similar to the upper surface of an airfoil section.

c. Blade face.-The flat side of a propeller blade, similar to the lower surface of an airfoil section.

d. Blade root.-The portion of the blade located in the hub.

e. Effective pitch.-The actual distance a propeller blade moves forward in one revolution in the air. Although this may be given in feet, it is usually computed in percent.

f. Feathering.-The term "feathering" designates the operation of rotating propeller blades beyond the highest angle required in normal flying to an approximate in line of flight position which prevents the propeller from "windmilling" in flight with the engine power completely off.

g. Geometrical pitch.-The distance a propeller blade would move forward in a solid medium in one revolution. This may be calculated by multiplying the tangent of the blade angle by 2p r, r being the radius of the blade station at which it is computed.

h. Slip.-The difference between effective pitch and geometrical pitch. This is usually expressed in percent.

i. Track.-The relationship of like points on all blades of a propeller, normally along the blade center lines, to a plane perpendicular to the axis of rotation.

3. Types.-There are three general types of propellers : fixed pitch, adjustable pitch, and controllable pitch.

a. The fixed pitch type is manufactured in one piece; no adjustment of the pitch can be made. It may be of wood or metal and its use at this time is limited to engines of relatively low power.

b. The adjustable pitch type has a split hub which permits the adjustment of the blades on the ground. The propeller is removed from the engine when this adjustment is made. Two or more blades may be used; they are usually of metal but may also be of other materials.

c. The controllable pitch type permits adjustment of the blade angle during operation of the engine in the air or on the ground. Two or more blades may be used. The mechanism for controlling the blade angle may be mechanical, hydraulic, or electrical.

4. Stresses and vibration.-a. There are three general types of stresses induced in a propeller : bending, tensile, and torsional.

(1) The bending stresses which are induced by the thrust forces that tend to bend the blade forward are the most pronounced. Other bending stresses caused by air drag on the blade are negligible.

(2) The tensile stresses are caused by centrifugal force.

(3) The torsional stresses are due to the forces which tend to twist the blade.

b. Fatigue due to vibration is the greatest cause of propeller failures. Vibrations can be set up by certain irregularities of air flow such as might be caused by a coolant radiator placed too close to the plane of propeller rotation. The main cause of propeller vibration is the engine power impulses. Vibration, if continued at the natural frequency of the propeller, will cause failure in a few hours' operation. Each engine has a critical range of operation for each type of propeller with which it is combined. Continued engine operation in this critical range must be avoided. Dynamic balancing of crankshafts and flexible drive couplings between the engine and propeller have greatly reduced propeller vibration and subsequent failures due to fatigue.

5. Advantages of the controllable propeller. a. The primary purpose of a controllable propeller is to permit the engine to develop full-rated power; second, to permit the propeller blades to operate at the most advantageous blade angle; and third, to permit readjustment of the blade angle to the particular power and altitude conditions. Summarizing the utility of the controllable propeller, a gain in performance is always obtainable when operating at any flight condition other than the one for which the blade angle of a noncontrollable propeller is set.

b. Where high efficiency of the engine and airplane is desired, the controllable propeller is a necessity. In the case of larger airplanes at take-off using noncontrollable propellers, the length of run would be great, and as long as the distance of take-off is limited to the extent of the airdrome, the controllable propeller is invaluable. The advantages in climb and cruising are just as pronounced.

c. Noncontrollable or ground adjustable propellers are designed and set so that the engine turns at its rated speed in normal level flight. When the propeller is designed and used in this way, its characteristics are such that it holds the engine to about 80 percent of its normal r. p. m. and consequently about 80 percent of its normal power output during take-off. Thus an engine normally rated at 800 hp. would develop only about 640 hp. at the time of take-off. During climb, this same propeller will hold the engine about 85 percent to 90 percent of its rated speed so that the horsepower output of the engine would be around 720. This loss of engine power is avoided by the use of the controllable propeller, since the blades can be adjusted to as low a pitch as is necessary to allow the engine to develop its full-rated horsepower.

d. In general, engines have a certain maximum safe speed for cruising and a certain safe manifold pressure at that speed of rotation. However, at high altitudes, the lower density of the air causes the propeller to allow the engine to turn at a higher r. p. m. than at lower altitudes. In a controllable propeller, the pitch may be adjusted to a higher angle to compensate for this difference in air density.

e. Some types of controllable propellers incorporate a feathering feature. In the feathered position they act as brakes to stop the engine rotation and at the same time offer the least possible drag on the air-plane. The ability to stop an engine from rotating in case of an engine failure on multiengine airplanes is, from the safety standpoint, the greatest asset of the feathering feature. In addition, flight tests with bimotored airplanes with propellers which can be feathered have shown a definite improvement in all phases of single-engine performance (fig.4).

FIGURE 1.-Typical propeller blade.

FIGURE 2.-Blade angle and angle of attack in normal night.

FIGURE 3.-Blade angle and angIe of attack in a dive and in a steep climb

FIGURE 4.-Comparative performance with propeller feathered on dead engine.