Wind Tunnel

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.

Fixed Pitch
What becomes clear if we look at the characteristics of a fixed-pitch propeller being driven  by a piston engine is that the situation is similar to an automobile that has only one gear ratio in its transmission. The fixed-pitch propeller can efficiently deliver the rated power of the engine to the airplane at the airspeed and rpm at which it was designed to operate (its design point), but the thrust horsepower it can deliver falls off relatively quickly as the airspeed varies from the design point.

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.

Speed Variation
A fixed-pitch prop is sensitive to variations in airspeed. The greater the variation in airspeed over the mission, the harder it is to come up with an acceptable compromise pitch. Early airplanes were relatively slow and had a relatively  small speed range in takeoff, climb and cruise. The same is true for modern ultralights. Since these airplanes fly at nearly constant airspeed, a fixed-pitch propeller works well for them.

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.