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- Since powered flight began, early last century, propeller design has evolved from wooden fixed-pitch propellers to modern alloy variable-pitch versions.
- Variable-pitch and constant-speed propellers confer a number of advantages in performance.
- Fixed-pitch propellers still have a place, as a budget-friendly option.
What are the various types of airplane propellers? At first glance they might look alike but there are key differences in design and construction.
The main categories of airplane propellers concern the pitch of the blades, which can be fixed or variable. Each category has subdivisions such as the material of construction and, for variable pitch propellers, the means by which the pitch is changed, and the reasons why.
As a full time pilot and avgeek with countless hours of stick time on propeller-driven airplanes, I’m interested in how they work, and the differences between the different types of propellers installed on them.
A fixed-pitch propeller is the simplest propeller type. The propeller blades remain at a fixed pitch that cannot change. To vary the thrust that a fixed-pitch propeller produces, the speed of rotation must be changed, by means of the engine’s throttle.
First, we need to understand how propeller blade pitch affects thrust. Because the blades of the propeller are angled, they bite into the air as the propeller turns, forcing air backward. Thanks to Newton’s Third Law of Motion, an equal and opposite reaction drives the airplane forward.
The pitch of a propeller blade is the angle between the propeller’s plane of rotation and the blade itself. A pitch of zero degrees means the propeller would simply slice through the air, blades parallel to the direction of rotation, and would not produce any forward thrust.
Conversely, a pitch of ninety degrees would mean the blades are parallel with the longitudinal axis of the fuselage. As the propeller turned, the blades would present their full flat surface area to the air, producing zero forward thrust. I will return to this when I discuss ‘feathering’ later.
The ideal pitch for a propeller to produce forward thrust is greater than zero and less than ninety degrees. Lower pitch angles are known as ‘fine’ pitch; greater pitch angles are known as ‘coarse’ pitch.
Now that we are familiar with the idea of pitch, we are better placed to understand the different types of airplane propeller.
The very first powered aircraft used fixed-pitch propellers. The Wright Brothers’ Wright Flyer was fitted with twin 8.5 foot diameter laminated spruce propellers, geared to rotate in opposite directions to minimize the effect of torque on the airframe.
Interestingly, the propellers on the Wright Flyer were approximately 95% as efficient as modern fixed-pitch propellers. The Wright Brothers correctly deduced that the ideal shape for a propeller blade is an airfoil, like an airplane wing, but with a more twisted shape.
The reason for the twisted shape of a propeller blade is that the speed of the blade tips through the air is much higher than the speed nearer the blade root. Therefore, the blade pitch needs to be higher near the central hub, and lower near the tip.
Wooden Fixed-Pitch Propellers
Nowadays, we rarely find wooden propellers outside of aircraft museums, although they are still made, to refurbish preserved airplanes, as part of home-build airplane kits, or as decorative ornaments for aviation enthusiasts.
In the early years of flight, until the 1940s, all propellers were made from wood, using a laminating technique where they were built, layer by layer, from wood that had been specially prepared. There were usually between five and nine layers, each around three quarters of an inch thick.
The best types of wood for propeller-making are black walnut, spruce, black cherry, sugar maple and yellow birch, on account of their rigidity, strength and resistance to warping.
As airplane design improved, engineers sought ways to enhance performance, and the limitations of fixed-pitch propellers became all too apparent. To make progress possible, it was necessary to transition from wood to metal as the preferred material.
Metal Fixed-Pitch Propellers
Spearheaded by development of military aircraft in the 1940s, aluminum alloy propellers began to replace wooden ones. The first examples were made from solid steel, which is strong and comparatively easy to fashion into shape, but confers a significant weight penalty.
Alloys of aluminum with zinc, copper, manganese, magnesium, silicon or chromium combine strength and durability with light weight, and are heat-treated to make them resistant to warping in extremes of heat and cold. These alloys soon replaced steel as the material of choice for metal airplane propellers.
Metal propeller blades have a thinner section than their wooden predecessors, because of the different properties of metal, notably its greater strength. This allowed aircraft designers to aim for faster propeller rotation speeds, to increase climb rate and cruise speed, and limitations soon began to appear.
For any fixed-pitch propeller, the only way to control the flow of energy delivered is to vary the speed of rotation. Pilots do this by advancing or retarding the throttle. In a Cessna 172, with its fixed-pitch, twin-blade metal propeller, adding power causes the aircraft to pitch up and climb.
The more power the pilot adds, the faster the propeller turns. Eventually, the tips of the propeller blades can exceed the speed of sound, and that causes problems. At blade speeds below Mach 1, air behaves as an elastic medium, and the turning blades compress the air, forcing it backward.
Once the speed of sound is passed, the air behaves differently. The air molecules slam abruptly into one another with no ‘springiness’ and the smooth airflow over the propeller blade stalls, leading to a loss of thrust, inefficient performance, increased noise and possible damage to the blade tips.
Designing a fixed-pitch propeller, an engineer must aim for the best compromise, taking into account take-off power, climb rate, efficiency in the cruise, and avoidance of blade stall by making sure the blade tips do not exceed Mach 1 in flight.
This means a fixed-pitch propeller will rarely spin at its optimal rotation speed to maximize efficiency and reduce fuel burn. Faster does not always mean more power - the Wright brothers discovered that, and they deliberately geared their propellers to turn quite slowly, with a gear reduction of 23:8.
The main advantage of fixed-pitch propellers today is their low cost, which is why airplanes are still made with that kind of propeller, as there is still a demand for them at the budget-friendly end of the market.
To maintain a propeller’s rotation speed at the sweet spot, delivering the best, most efficient performance, we need to be able to vary the pitch. It was because of this that the variable pitch propeller was born.
A variable-pitch propeller has blades whose angle can be adjusted, at the point where the root of each blade is joined to the central propeller hub. Rather than a single piece of aluminum alloy, a variable-pitch propeller is constructed in sections that can move relative to one another.
A ground-adjustable propeller can, as the name suggests, be adjusted while the airplane is on the ground. Clamps hold each blade firmly in place. A mechanic can slacken the clamps and adjust the blade angle, ensuring each blade is set to the same angle, then tighten the clamps again.
With a ground-adjustable propeller, the aircraft can be configured for different loads, various flight applications such as long cruises or airshow aerobatics, or extreme barometric pressures, such as when operating from high-altitude airfields. However, there is no way for a pilot to adjust this type of propeller in flight.
On airplanes with controllable-pitch propellers, the pilot can manually change the blade pitch using a lever in the cockpit. In this way, the pilot can maintain the rotation speed of the propeller within its optimal range, delivering best performance and fuel burn efficiency.
In the initial climb, for example, the pilot might set a larger blade angle, which prevents the propeller overspeeding at a high throttle setting, when a lot of energy from fuel needs to be converted into kinetic and potential energy. In the cruise, a lower blade angle confers better fuel burn.
Controllable-pitch propellers can also be set to a negative blade angle. In that configuration, the blades actually push air forward as the propeller rotates, slowing the airplane down. Needless to say, this is not done in flight, but on the runway after landing, to reduce speed before exiting the runway.
Controllable-pitch propellers are common on multi-engine aircraft. If an engine were to fail in flight, the pilot would shut it down, and fly on the remaining engine(s). While flying with an engine out, we do not want unnecessary drag to limit our range, preventing us safely reaching an airfield.
To reduce drag, the pilot can feather the propeller blades of the defective engine. When feathered, the blades are at ninety degrees, presenting their whole surface to the air in the plane of rotation. As the airplane flies, air slips past the feathered blades with little resistance.
While feathered, the chord line of each blade is almost parallel to the flow of oncoming air, so the blades are presenting the least possible obstruction to the air.
This prevents the shut-down engine from windmilling, and allows the flight to continue with minimum drag and adverse yaw. In some aircraft, the propeller blades are controlled by an automatic system of springs and hydraulics which will automatically feather the blades when necessary, reducing pilot workload in an abnormal situation.
Another type of controllable-pitch propeller is the two-position propeller. While not infinitely changeable like a true variable-pitch propeller, a two-position propeller can be switched between its two positions, in flight, by the pilot.
A constant-speed propeller is controlled automatically by a propeller governor, which is a system of springs, pistons and hydraulic cylinders inside the propeller’s central hub. The governor varies the angle of the propeller blades by rotating the blades slightly at their root.
Unlike a controllable-pitch propeller, there is no need for the pilot to take any direct action to change the blade angle. The governor takes care of it all, freeing the pilot to focus on other things.
The governor’s mechanism is driven by oil under pressure, which comes directly from the engine’s oil lubrication system. If the oil pressure is high, the angle of the blades can change quickly.
When the governor senses the engine rpm accelerating, it increases the blade angle, causing more air resistance on each blade as it takes a deeper bite out of the air. If the engine slows down, the governor flattens the propeller blades sufficiently to regain the optimum rotation speed.
Keeping the propeller at its optimum speed is helpful. It reduces wear on the blade tips, which never reach Mach 1. It maximizes fuel efficiency, allowing greater range. It improves performance, because the engine speed (i.e. the rotation speed of the propeller shaft) can stay where the power is.
Think about your car. As you accelerate through the gears, you notice there is an ideal range of engine revs per minute where you get the most forceful acceleration. If the engine is running too slowly, you need to change down to a lower gear to find the power.
It is the same in an airplane. When climbing or accelerating, you need the best power you can get, and therefore you need the engine to be turning at the right rate. A constant-speed propeller facilitates this very nicely.
For example, the Cessna 182’s constant-speed propeller is typically maintained at 2,600 rpm. If the pilot wishes to climb, they advance the throttle and the engine manifold pressure rises. Thanks to the governor, the propeller blades increase pitch and the engine delivers more thrust at the same rotation speed.
To descend, the reverse happens. The pilot retards the throttle, leading to reduce manifold pressure. The governor senses this and reduces blade pitch, delivering less thrust. In the cruise, the governor sets the blade pitch to deliver just the right amount of thrust to overcome drag and maintain airspeed.
So, just as in the case of a fixed-pitch, variable speed propeller, the pilot still controls power using the throttle, even though the engine speed and propeller speed do not change when a constant-speed propeller is used.
Earlier, I mentioned the Wright brothers’ solution to the effect of torque caused by a propeller’s rotation. Torque is a twisting force on the airframe. As a propeller turns relative to the rest of the aircraft, a reaction force makes the aircraft want to roll in the opposite direction.
The Wright brothers resolved the issue by having two propellers, rotating in opposite directions, canceling out the torque. Such an arrangement is called counter rotating propellers, because the propellers are separate and do not share a common axis.
Another solution - used on single-engine airplanes like the Rolls-Royce Griffon powered P-51 Mustang, and on multi-engine planes such as the Russian Tu-95 Bear military reconnaissance aircraft - is to use opposite-rotating propellers on the same axis.
In an aircraft with opposite-rotating propellers, a pair of propellers is mounted coaxially on the same engine, rotating in opposite directions, canceling the effect of torque. Opposite rotating propellers present an engineering challenge, requiring planetary gears like those used in an automobile’s automatic transmission, or spur gears.
With a single propeller, especially at low airspeed, the blades can waste energy by producing unwanted rotational or tangential air flow. When this disturbed air travels backward and strikes the airplane’s vertical stabilizer, it can cause a yawing effect that the pilot has to counter using rudder inputs.
Having another propeller, on the same engine, rotating in the opposite direction, compensates for this effect and removes the spinning air flow. Air moves straight backward instead of spinning. Fuel efficiency is increased by between 6% and 16%.
Opposite rotating propellers have some disadvantages. They produce more noise than single propellers, which limits their use on passenger aircraft. The system of gears is heavy, which limits range and payload, and adversely affects performance.
Composite materials - combinations of different materials whose properties complement each other - are becoming more and more widely used in aviation. For example, the Boeing 787 Dreamliner is 50% composite by weight.
Modern propellers can be made of composite materials. The first composite propellers were designed by Hartzell Aviation over seventy years ago, but only now are they beginning to find use in the general aviation sector.
By combining materials such as carbon fiber, fiberglass and Kevlar, it is possible to achieve standards of strength and durability that surpass the properties of any single material. Moreover, composite materials are lighter than metals, and that is always good news for an airplane.
Since the materials are light, propellers can be made with additional blades, without detracting significantly from aircraft performance. Extra blades increase the propeller’s efficiency. The longevity of the materials means composite propellers can be certified for unlimited life.
Composite propellers used to be prohibitively expensive to produce. However, much progress has been made and they are now far more affordable. Some manufacturers use a wooden core with composite material added; others make fully composite airplane propellers.
A test-club propeller is used on the ground, to run in a new piston engine before its first flight. Like the engine in a new car, an airplane’s engine needs to run gently for a time, before being subjected to the full performance curve, to allow the parts to bed in.
The test-club propeller keeps the engine at its optimal rpm throughout the running-in period. Before the aircraft flies, the test-club propeller is removed, and is replaced with the actual propeller.
The usual position for an airplane’s propeller is at the front of the engine that drives it. This allows the propeller to force air through the engine’s radiator, keeping it cool. However, there are some outside-the-box designs that place the propeller elsewhere.
Take the Rutan Long-EZ as an example. On this distinctive little two-seater aircraft, you will find the propeller at the very back of the fuselage. To make that possible, twin vertical stabilizers and rudders are placed at the wing tips.
The Long-EZ uses a 2-blade, fixed-pitch wooden pusher propeller, so called because it pushes the airplane along, rather than pulling it as a conventionally-mounted tractor configuration propeller does.
It’s worth noting that the Wright Flyer had a pusher configuration, with the twin counter-rotating propellers facing backward.
The pusher configuration has advantages, including improved visibility for the pilot, as the wing roots are located well aft of the cockpit. Pusher aircraft generally perform better in crosswinds and are less prone to weathercocking - an effect that makes tractor-configured airplanes want to turn into the wind.
Pushers are largely immune from the prop wash effect, discussed earlier in the section on opposite-rotating propellers, that can require pilots of tractor-configured airplanes to apply rudder to counteract.
There are safety advantages too. Failure of the engine or propeller in flight is less likely to cause penetration of the cabin environment and endanger those inside, since both the engine and propeller are behind them.
There are also disadvantages to the pusher configuration. Air flows along the fuselage before reaching the propeller, which may make it turbulent, reducing the propeller’s efficiency. Also, to avoid the risk of the propeller striking the ground on rotation for take-off, the blade length has to be limited.
The center of gravity of a pusher airplane is further aft than in tractor configuration, because the weight of the engine is at the back. When the pilot is not in the cockpit to provide weight at the front, the airplane may tend to sit on its tail.
Pusher propellers are generally between 2% and 15% less efficient than tractor-configuration propellers, and they tend to produce more noise. In icing conditions, pieces of ice shed from the wings can strike the propeller and cause blade damage.
Airplane propellers have been around for well over a century and will no doubt continue to form a vital part of aircraft design for many years to come. There are sure to be future advances in propeller efficiency and construction.