AIRCRAFT PROPELLERS

The aircraft propeller functions to convert the powerplant shaft horsepower into propul­sive horsepower. The basic principles of pro­pulsion apply to the propeller in that thrust is produced by providing the airstream a mo­mentum change. The propeller achieves high propulsive efficiency by processing a relatively large mass flow of air and imparting a rela­tively small velocity change. The momentum change created by propeller is shown by the illustration of figure 2.18.

The action of the propeller can be idealized by the assumption that the rotating propeller is simply an actuating disc. As shown in fig­ure 2,18, the inflow approaching the propeller disc indicates converging streamlines with an increase in velocity and drop in pressure. The converging streamlines leaving the propeller disc indicate a drop in pressure and increase in velocity behind the propeller. The pressure change through the disc results from the distri­bution of thrust over the area of the propeller disc. In this idealized propeller disc, the pres­sure difference is uniformly distributed over the disc area but the actual case is rather different from this.

The final velocity of the propeller slipstream, Vt, is achieved some distance behind the pro­peller. Because of the nature of the flow pat­tern produced by the propeller, one half of the total velocity change is produced as the flow reaches the propeller disc. If the complete velocity increase amounts to la, the flow veloc­ity has increased by the amount a at the pro­peller disc. The propulsive efficiency, yp, of the ideal propeller could be expressed by the fol­lowing relationship:

Since the final velocity, V2, is the sum of total velocity change la and the initial velocity, Vl} the propulsive efficiency rearranges to a form identical to that for the turbojet.

2

So, the same relationship exists as with the turbojet engine in that high efficiency is de­veloped by producing thrust with th^ highest possible mass flow and smallest necessary velocity change.

The actual propeller must be evaluated in a more exact sense to appreciate the effect of nonuniform disc loading, propeller blade drag forces, interference flow between blades, etc. With these differences from the ideal propeller,

it is more appropriate to define propeller effi­ciency in the following manner:

r

Many different factors govern the efficiency of a propeller. Generally, a large diameter pro­peller favors a high propeller efficiency from the standpoint of large mass flow. However, a powerful adverse effect on propeller efficiency is produced by high tip speeds and compressi­bility effects. Of course, small diameter pro­pellers favor low tip speeds. In addition, the propeller and powerplant must be matched for compatibility of output and efficiency.

In order to appreciate some of the principal factors controlling the efficiency of a given propeller, figure 2.18 illustrates the distribu­tion of rotative velocity along the rotating propeller blade. These rotative velocities add to the local inflow velocities to produce a variation of resultant velocity and direction along the blade. The typical distribution of thrust along the propeller blade is shown with the predominating thrust being located on the outer portions of the blade. Note that the propeller producing thrust develops a tip vortex similar to the wing producing lift. Evidence of this vortex can be seen by the con­densation phenomenon occurring at this loca­tion under certain atmospheric conditions.

The component velocities at a given propeller blade section are shown by the diagram of figure 2.18. The inflow velocity adds vec­torially to the velocity due to rotation to pro­duce an inclination of the resultant wind with respect to the plane of rotation. This incli­nation is termed ф (phi), the effective pitch angle, and is a function of some proportion of the flight velocity, V, and the velocity due to rotation which is vnD at the tip. The pro­portions of these terms describe the propeller “advance ratio”, J.

where

J=propeller advance ratio V— flight velocity, ft. per sec.

»=propeller rotative speed, revolutions per sec.

D=propeller diameter, ft.

The propeller blade angle, 0 (beta), varies throughout the length of the blade but a representative value is measured at 75 percent of the blade length from the hub.

Note that the difference between the effec­tive pitch angle, Ф, and the blade angle, 0, determines an effective angle of attack for the propeller blade section. Since the angle of attack is the principal factor affecting the efficiency of an airfoil section, it is reasonable to make the analogy that the advance ratio, J, and blade angle, 0, are the principal factors affecting propeller efficiency. The perform­ance of a propeller is typified by the chart of figure 2.19 which illustrates the variation of propeller efficiency, i}p, with advance ratio, J, for various values of blade angle, 0. The value of rtp for each 0 increases with J until a peak is reached, then decreases. It is apparent that a fixed pitch propeller may be selected to provide suitable performance in a narrow range of advance ratio but efficiency would suffer considerably outside this range.

In order to provide high propeller efficiency through a wide range of operation, the pro­peller blade angle must be controllable. The most convenient means of controlling the propeller is the provision of a constant speed governing apparatus. The constant speed gov­erning feature is favorable from the standpoint of engine operation in that engine output and efficiency is positively controlled and governed.

The governing of the engine-propeller combi­nation will allow operation throughout a wide range of power and speed while maintaining efficient operation.

If the envelope of maximum propeller effi­ciency is available, the propulsive horsepower available will appear as shown in the second chart of figure 2.19- The propulsive power available, Pa, is the product of the propeller efficiency and applied shaft horsepower.

(ВНР)

The propellers used on most large reciprocating engines derive peak propeller efficiencies on the order of i7p=0.85 to 0.88. Of course, the peak values are designed to occur at some specific design condition. For example, the selection of a propeller for a long range transport would require matching of the engine-propeller com­bination for peak efficiency at cruise condition. On the other hand, selection of a propeller for a utility or liaison type airplane would require matching of the engine-propeller combination to achieve high propulsive power at low speed and high power for good takeoff and climb performance.

Several special considerations must be made for the application of aircraft propellers. In the event of a powerplant malfunction or failure, provision must be made to streamline the propeller blades and reduce drag so that flight may be continued on the remaining op­erating engines. This is accomplished by feathering the propeller blades which stops rotation and incurs a minimum of drag for the inoperative engine. The necessity for feather­ing is illustrated in figure 2.19 by the change in equivalent parasite area, Д/, with propeller blade angle, /3, of a typical installation. When the propeller blade angle is in the feathered position, the change in parasite drag is at a minimum and, in the case of a typical multi­engine aircraft, the added parasite drag from a single feathered propeller is a relatively small contribution to the airplane total drag.

At smaller blade angles near the flat pitch position, the drag added by the propeller is very large. At these small blade angles, the propeller windmilling at high RPM can create such a tremendous amount of drag that the airplane may be uncontrollable. The propel­ler windmilling at high speed in the low range of blade angles can produce an increase in para­site drag which may be as great as the parasite drag of the basic airplane. An indication of this powerful drag is seen by the helicopter in autorotation. The windmilling rotor is ca­pable of producing autorotation rates of descent which approach that of a parachute canopy with the identical disc area loading. Thus, the propeller windmilling at high speed and small blade angle can produce an effective dtag coefficient of the disc area which compares with that of a parachute canopy. The drag and yawing moment caused by loss of power at high engine-propeller speed is considerable aftd the transient yawing displacement of the aircraft may produce critical loads for the vertical tail. For this reason, automatic feathering may be a necessity rather than a luxury.

The large drag which can be produced by the rotating propeller can be utilized to im­prove the stopping performance of the air­plane. Rotation of the propeller blade to small positive values or negative values with applied power can produce large drag or re­verse thrust. Since the thrust capability of the propeller is quite high at low speeds, very high deceleration can be provided by reverse thrust alone.

The oprating limitations of the propeller are closely associated with those of the power- plant. Overspeed conditions are critical be­cause of the large centrifugal loads and blade twisting moments produced by an excessive rotative speed. In addition, the propeller blades will have various vibratory inodes and certain operating limitations may be necessary to prevent exciting resonant conditions.

PROPELLER EFFICIENCY Figure 2.19. Propeller Operation