PROPELLER SELECTION

Propeller manufacturers offer propellers covering a range of diameters, pitch values, and solidities. The choice of these parameters can depend on considerations other than aerodynamic efficiency. For example, to keep the noise level of a propeller low, one may have to employ wide blades with low tip speeds. As another example, the propeller diameter is sometimes limited by ground clearance considerations or by the distance from a nacelle to the fuselage. The dynamics of the propeller must also be matched to the engine. The natural frequency of the first bending mode of a blade should not coincide with an impulse frequency from the engine. For example, a horizon­tally opposed, six-cylinder engine has three torsional peaks per revolution. If a propeller being driven by this engine has a natural frequency close to 3/rev, it can lead to excessive vibration and fatigue stresses.

Aerodynamically, one strives to select a propeller that provides a high efficiency for cruise and a high static thrust for takeoff. These two require­ments are easier to satisfy with a variable pitch propeller. A fixed pitch propeller is usually a compromise between these two operating regimes.

Given the results of a series of propeller tests, such as Figures 6.12 and

6.13, one can utilize these data to select the best propeller diameter and blade angle to match a given airplane-engine combination. One approach that is sometimes used is based on a Coefficient Cs, the speed power coefficient, defined by

(6.57)

Knowing Cp as a function of J, Cs can be calculated from

The advantage of Cs is that it does not contain the diameter in its definition.

Figure 6.21 presents / as a function of Cs for the same propeller for which Figures 6.12 and 6.13 hold. A maximum efficiency line is also shown in Figure 6.21. The use of this graph is best illustrated with an example. The problem will be to select the optimum diameter for this propeller if it is to be installed, on a Cherokee 180. Consider the selection of a propeller to absorb

75% of the maximum power of 180 bhp at 2500 rpm at standard sea level conditions. Using a value for / of 0.5 m2 (5.38 ft2) and an e of 0.6, CD can be calculated as a function of V. CT and CD are then related by (T = D).

Assuming a value for V of 130 mph leads to a Cs of 1.360. From the maximum efficiency line in Figure 6.21, a J of 0.76 and a /3 of 20° are obtained. These values in turn lead to a CT value of 0.0573, so obviously 130 mph will not be the trim speed for the optimum propeller at this power and rpm. By iteration, one obtains a trim speed of 132 mph and the following.

J = 0.76

В = 20°

CT = 0.0592 v = 0.84 D = 6.1 ft