The value of a constant-speed prop.
Propellers have a large effect on the performance of piston-engine airplanes. One of the important decisions for a designer or builder is whether to use a fixed-pitch propeller or step up to the higher weight, cost and complexity of a constant-speed prop in order to improve performance. Last month we started our discussion of propellers with a look at how the propeller absorbs power from the engine and converts it to useful thrust to drive the airplane through the air. We saw how the power required to turn a propeller varies with airspeed and took a look at the thrust horsepower available from a fixed-pitch prop.
At airspeeds above the design point, the propeller cannot absorb the rated power of the engine at the rated rpm of the engine. Accordingly, the engine cannot be run at full throttle at these higher airspeeds without exceeding its rated operating rpm. To avoid over-speeding the engine we must reduce throttle, which decreases the power being delivered to the propeller, and hence decreases the thrust horsepower the propeller delivers to the airplane.
At airspeeds below the design point the situation is different. The power required to turn the propeller at a given rpm increases as airspeed decreases. Accordingly, at airspeeds below the design point, the engine does not have enough power to turn the prop at the engines rated rpm. The prop will govern the engine down to a lower rpm. At this lower shaftspeed, the engine will produce less than rated power, even at full throttle. The slower the airspeed is relative to the design point of the propeller, the more the prop holds down the engine rpm, and the lower the power delivered to the prop and, eventually, to the airplane.
Because of the effects just described, the choice of pitch for a fixed-pitch propeller tends to be a compromise. If we pitch the prop to get the best cruise performance, we wind up with a higher-pitch propeller that tends to lug the engine more at lower airspeeds, which hurts climb performance. On the other hand, if we choose a low-pitch prop that lets the engine deliver rated power at climb airspeeds, cruise performance will be severely reduced, because the engine will not be able to deliver anything close to rated power at cruise airspeed without over-speeding. For most airplanes with fixed-pitch props, the pitch is somewhat less than optimum for cruise but still higher than optimum for maximizing rate of climb. If the pitch is biased toward the high side to make the airplane faster, we call it a cruise prop, and if the pitch is biased down to emphasize climb performance we call it a climb prop.
As performance and cruise speed increase, the variation of airspeed between climb and cruise gets larger. At some point, the variation in airspeed between climb and cruise gets large enough that a single propeller pitch cannot adequately address the needs of the airplane over the entire speed range. By the early 1920s the builders of racing airplanes began to run into this problem. By the mid-20s the top speed of the fastest racing airplanes was more than 200 mph. To go that fast they had to have high-pitch propellers. These worked well at racing speeds, but they performed poorly on takeoff.
On some of these racers, the pitch angle of the prop blades was so high that the blades stalled when the airplane was moving slowly. In this condition, the engine could turn relatively fast, but the prop produced little thrust. It took a long roll for the airplane to get going fast enough to un-stall the prop blades. When this happened, the ability of the propeller to produce thrust increased, but the extra lift produced by the blades opposed the engines rotation enough to slow the engine and reduce power. The engine rpm would actually drop at this point in the takeoff roll, even though thrust would start to increase. Because these racers had high-power engines, they would eventually get off the ground, but their takeoff and climb performance was poor, and the takeoff roll was long in spite of their power.
Modern single-engine light airplanes span the speed range where the crossover between fixed-pitch propellers being adequate and being inadequate occurs. A trainer or light sport airplane has a small enough speed range that a fixed-pitch prop will work fine. On a Cessna 150, for example, cruise speed is only about 25% faster than best rate of climb speed.
Cleaner, high-power airplanes can easily have cruise speeds that are more than twice the best rate of climb speed. As the variation between cruise speed and climb speed increases, the severity of the compromise inherent in a fixed-pitch prop also increases. At some point, a variable-pitch prop becomes the better option.
What a constant-speed prop does is allow the engine to turn up to its rated rpm at any airspeed. The governor adjusts the pitch of the blades to keep the rpm constant. When the prop is set to maximum rpm, the engine turns at its rated rpm. If the throttle is wide open, this means that the engine is delivering its rated horsepower to the prop. It is this ability to get full power out of the engine at a wide range of airspeeds that is the primary advantage of a constant-speed prop.
To illustrate the difference between a fixed-pitch and constant-speed prop, lets look at Figure 1, (see PDF version) which shows the thrust horsepower available from an example propeller. This prop was designed to absorb 160 horsepower at 160 knots.
The first thing to note is that there is no difference between the performance of the fixed-pitch and constant-speed prop at the design point of 160 knots. If the two propellers have the same blade shape, they will be essentially the same, because the governor of the constant-speed prop will place the blades at the same pitch as the optimized fixed-pitch prop would have.
What is different is the performance of the two systems at airspeeds below the design point. The thrust horsepower available from the fixed-pitch prop falls sharply as airspeed is reduced. The constant-speed prop does much better at delivering power to the airframe at these lower airspeeds. By adjusting the pitch to keep the rpm at rated rpm, the governor allows the constant-speed prop to absorb the full rated power of the engine.
Notice that the performance of the constant-speed prop is not absolutely constant with airspeed; while the prop is getting full power form the engine, the propeller efficiency varies with airspeed. This effect is illustrated for our example prop in Figure 2 (see PDF version). At lower than design airspeed, propeller efficiency degrades for two reasons. First, the ideal efficiency of a perfect prop decreases with decreasing airspeed. Second, the blades are shaped to be efficient at the design point, and while the governor can change the pitch of the blades, it cant adjust the blade twist or planform.
Why Constant Speed?
This rate of climb improvement allows the operator of the airplane to use a propeller that delivers maximum cruise performance while retaining acceptable takeoff and climb performance. A fixed-pitch prop can deliver the same cruise performance only if the greatly reduced rate of climb is still acceptable. If it is not, then the operator will be forced to use a lower pitch prop that will compromise cruise performance to get an acceptable climb. As the top speed of the airplane increases, so does the magnitude of this compromise. When the performance of the airplane gets high enough, a constant speed prop becomes the best approach to using the cruise performance inherent in the design of the airframe, while still being able to take off in a reasonable distance and climb well.