Unless you’re someone we’d really like to get to know better, it’s likely you’re not flying around in your own personal jet. Which means you probably are flying behind, between, below or in front of at least one propeller. They’re marvelous devices, often subjected to substantial forces as they unceasingly (we hope) spin thousands of times per minute. They also can be a bit mysterious, even the fixed-pitch variety.
At its most basic, an aircraft’s propeller simply is a rotating airfoil used to displace air. Since it is an airfoil, the nasty characteristics we sometimes associate with a wing—including stalls and induced drag—accrue, right along with the good ones, like predictability. And as with a wing, understanding how they do what they do, along with how the more complicated versions work, plus their care and feeding always should be of interest.
How They work
As with so many other things, we have the Wright Brothers to thank for being the first to understand that a propeller ideal for their purposes also had to be an airfoil. Using an engine, motor or other means to rotate the propeller moves it through the air and generates lift, err, thrust. Meanwhile, Newtonian physics tell us every action has an opposite reaction. How that reaction is characterized also depends on the action, so a rotating propeller’s aerodynamics are different than those of a fixed wing, even though they may be attached to the same airplane at the same time. Figures 1 and 2 in the sidebar on the opposite page show a propeller airfoil’s interaction with the air it’s pushing.
But there’s a problem, one associated with the speed at which the propeller’s airfoil rotates: As one moves, say, a foot from a rotating propeller’s hub toward its tip, the airfoil’s speed through the relative wind is greater. At the hub, the airfoil is moving relatively slowly while it’s moving fastest at the tip. To maximize efficiency, the thickness and shape of the propeller’s airfoil changes as we move from the hub to the tip. This is illustrated in Figure 3, on the opposite page. Figure 4 demonstrates why. (Both of these diagrams are adapted from the FAA’s Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25A.) It’s this change in the airfoil’s speed through the oncoming air that accounts for what’s commonly called a propeller’s “twist.”
Meanwhile, propellers live and work in a nasty environment. Depending on their blades’ length and engine rpm, the tips easily can be in the transonic range, if not supersonic. Additionally, various forces, mostly absent from those acting on a wing, affect them instead. These forces can include thrust loads, which tend to bend the blades forward, and the torque bending force, created by air resistance and inertia, which tends to bend blades opposite the direction of rotation. As if that were not enough, there’s also centrifugal force, which wants to pull them out of the hub, and centrifugal twisting force, acting to twist them to a low pitch angle.
Blade Count, Shape And Size
Blade count is another characteristic of propellers. Most props on personal airplanes are of the two-, three- or four-blade variety, although there is such a thing as a single-blade propeller. The idea behind a single-blade prop is to eliminate potential interference from air disturbed by the other blade(s). A counterbalance is used to offset the blade’s mass. Modern incarnations of the single-blade prop can be found on motorgliders with retracting powertrains: A single-blade prop simplifies the engineering. While a single-blade prop does offer some efficiencies, those benefits are lost as engine power increases and larger blades are needed. That’s one reason some modern turboprops are using five- and even six-blade propellers.
According to a technical guide published by McCauley Propeller Systems, “Multi-blade props…produce higher, less objectionable sound frequency; reduced vibration; greater flywheel effect and improved aircraft performance.”
In recent years, so-called “scimitar” propellers—which employ curved blades—have become popular. An example of a prop with such blades is shown on the opposite page; it’s a Hartzell mounted on a Beechcraft King Air. Although the popularity of curved blades has grown in recent years, the basic design dates back at least to the era between the world wars. The blades are curved for the same reason a jet’s wings might be swept: to delay the onset of increased drag as the blade approaches the speed of sound. A curved blade design helps reduce noise while allowing the propeller to absorb as much power from the engine as possible.
Typically, as the number of blades increases, the prop’s diameter decreases. Again according to McCauley, “Propeller diameters are a function of engine and airframe limitations. Larger propeller diameters are preferred for low airspeed operation, while smaller diameters are best for high airspeeds. For example, the diameter of a fixed-pitch propeller is often large to favor low airspeed operation, while the blade size is small to favor higher airspeeds and faster turning at low airspeeds. The diameter and blade size of a constant-speed propeller is often larger (than a fixed-pitch), due to the variability of blade angles.”
The simplest, lightest and least-expensive propellers are fixed-pitch: Their blade angles cannot be changed. As such, they’re a compromise between high-speed efficiency and low-speed power, and are designed to give best performance at a specific airspeed and rpm.
Although different fixed-pitch props used on the same airframe and engine combination will provide different performance, they usually are optimized for either takeoff and low-airspeed operations (a so-called “climb” prop) or for high-speed cruising (a “cruise” prop).
For example, a Cessna 172 Skyhawk with a cruise prop likely will have a lower static rpm and reduced takeoff and climb performance. However, it’ll generally demonstrate faster cruise speeds than an identical airplane with a climb prop.
Fixed-pitch props usually are manufactured from a solid piece of aluminum alloy, but also can be made from wood or composites. A wood prop can be quite attractive, but also introduces the effects of atmospheric moisture, which can be absorbed by the wood. A common effect of moisture absorption is to alter the torque of the bolts attaching a wooden prop to the engine. Meanwhile, many composite props are built using wood or metal cores to which the composite blades are fitted. They’re usually immune to the kind of moisture affecting a wood prop.
At a constant power setting, a fixed-pitch prop will increase or decrease rpm corresponding to changes in airspeed. That’s why a Skyhawk’s engine rpm increases when the nose is lowered, and decreases when it’s raised. To keep the engine’s rpm at the desired value in a descent, the pilot must reduce power with the throttle. If, however, the Skyhawk’s airspeed is kept constant, propeller and engine rpm change with the throttle setting.
Given the performance limitations of a fixed-pitch propeller, props were developed in which the blades can pivot in the hub, allowing their pitch to change. These come in two basic flavors: ground-adjustable and constant-speed.
The ground-adjustable prop is just what its name describes: The propeller’s pitch can be changed on the ground to meet operational needs. Typically, a series of bolts are loosened, the blades are rotated in the hub to the desired pitch, and the bolts are retightened. Ground-adjustable props are more complicated and expensive than their fixed-pitch brethren, but are popular on many modern light-sport aircraft, Experimentals and Part 103 ultralights.
The constant-speed propeller, meanwhile, is designed to maintain a specific rpm no matter the airplane’s airspeed or the engine’s power setting. The typical installation in a personal airplane uses engine oil at a stepped-up pressure to hydraulically rotate the blades in the hub to the desired pitch. Electrical mechanisms also can be used to perform the same task. The diagrams in the sidebar on the opposite page show how a typical non-feathering hydraulic prop uses engine oil from the propeller governor to change the blades’ position when increasing, decreasing or maintaining a pitch setting.
A constant-speed prop’s greater efficiency—when compared to a fixed-pitch prop—results from allowing the blade angle to change with flight conditions. Such a propeller is both heavier and more expensive to acquire and maintain than a fixed-pitch or ground-adjustable version, but the additional performance available usually makes the trade-off worth it.
Two other propeller features deserve mention here: feathering and reversing. Typically reserved for multi-engine airplanes, both are implemented more or less by allowing the blades a greater range of rotation in the hub, either hydraulically or mechanically. And we haven’t even touched on de- and anti-icing equipment, like electrically heated boots and TKS fluid slingers.
The modern airplane propeller is really a model of product evolution, and it continues to be perfected. Lightweight hubs and high-tech blade materials are all the rage, especially in multi-engine turboprops like a re-engined Lockheed C-130 Hercules or a Bombardier Q400. Many of those technologies are trickling down to products for Experimental designs; operators of certificated aircraft generally have to wait a bit to see them offered.
While the physical act of exchanging most props for another isn’t all that challenging to a good technician, doing the required testing and certification can be. But at the end of the day, your propeller is just another airfoil. It generates lift, albeit in a different plane, and its shape and configuration are optimized for its environment.