BACK TO BASICS
PROPS II: MORE ON FIXED-PITCH PROPS
Last months column discussed the substantial loss of takeoff performance created by fixed pitch props. Most wont permit takeoff RPM above 2300, which severely limits total horsepower output. If only they were built with flatter blade angles, fixed pitch props would permit higher takeoff RPM and significantly increased climb performance.
To understand the design considerations that limit this potential, it's necessary to look at the other end of propeller performance: CRUISE. Although flattening blade angle allows more RPM and better performance during takeoff and climb, it works directly against economical and efficient performance during cruise.
This stems from the fact that the prop, dependent like any other airfoil on angle of attack for its efficiency, is subject to a unique effect: its angle of attack doesn't stay constant as the airplane accelerates. The faster the aircraft speed, the lower the prop angle of attack.
A little imagination is required to understand the event. To start with, when airspeed is zero, relative wind over the prop results exclusively from propeller rotation; this produces an angle of attack that exactly matches the mechanical angle cast into the aluminum. The angle is most easily visualized by picturing the airplane from its right side, with the prop horizontal. Imagine looking directly from the right prop tip in toward the hub. The blade should appear as an airfoil with its chord line twisted several degrees counterclockwise from vertical.
At this point in its rotation, the blade is moving straight down, producing a relative wind equal and opposite to its motion. The angle between the blade chord and the relative wind is the propeller angle of attack; it is critical in determining the forward thrust produced by the prop. This angle changes as the airplane starts to move.
As the aircraft begins to move forward, its own relative wind acts with the air already flowing over the prop. Using the airplane you have already visualized, picture the propeller relative wind--an arrow coming from a point directly below the nose, then add the aircraft relative wind--an arrow from the right, pointing straight in toward the prop. The two vectors act together to create a new relative wind over the prop, an arrow twisted a few degrees counter-clockwise from the original. Angle of attack has decreased and will continue to do so as the plane accelerates.
The immediate result is familiar to every pilot who has flown with a fixed pitch propeller: the reduced angle of attack corresponds to a reduced load on the engine and shows up as an increase in RPM as the plane accelerates down the runway. For a while, this effect is beneficial: horsepower and thrust both go up as the RPM increases. However, a continued reduction cannot work forever. Airfoils have only one ideal angle of attack, and as airspeed increases to cruise, the propeller achieves and then passes this ideal. Once past, efficiency starts to drop off steeply.
To avoid an unacceptable loss of efficiency in the cruising range, fixed pitch props must be designed with enough mechanical angle from the very start to accommodate the angle of attack loss as the airplane accelerates to cruise speed. This, in turn, creates a dilemma for the prop designer: too much blade angle, as discussed last month, makes for low takeoff RPM and low horsepower. Too little angle, and cruise efficiency will be reduced unacceptably. All fixed pitch props deliver a compromise somewhere between these two extremes: just enough angle to give some cruise economy, but not enough to slow the engine down so much at low airspeeds that takeoff and climb don't work at all.
If your interest is primarily in takeoff and climb performance, a prop with a slightly lowered pitch angle is the answer, provided you are willing to make some sacrifices in cruise economy. These props, usually available for all aircraft types, are called "climb props." If optimized climb performance isn't a factor, a "cruise prop" (one with a little more blade angle) is the answer. Given today's high fuel costs, it's easy to guess which side of the scales manufacturers currently favor. In either case, the difference in actual blade angle between the two types is quite small, and prop performance still suffers from the basic compromise described above.
By now, I'm sure that the most reasonable approach to this entire area has occurred to you--why not make the pitch angle variable? Next months column starts in on that subject.