An aircraft propeller is a rotating wing. The engine turns it, and each blade pulls or pushes the airplane forward the way a wing makes lift. What separates one type from another is the blade angle, also called pitch, and whether it can change. A fixed-pitch propeller has one angle built in. An adjustable propeller can be set on the ground. A constant-speed propeller changes angle in flight under a governor, and feathering and reversing propellers can go to the two extremes.
This guide explains how a propeller works, what each type does, how the governor holds rpm steady, and how feathering, reverse thrust and the beta range work. It's built from the FAA's powerplant handbook for mechanics, its Pilot's Handbook of Aeronautical Knowledge and its Airplane Flying Handbook.
Key Takeaways
The FAA's pilot handbook says a fixed-pitch propeller's pitch is set by the manufacturer and can't be changed, so it's ideal for neither cruise nor climb.
A constant-speed propeller has a governor that adjusts blade angle automatically to hold the rpm the pilot selected. The pilot has two controls: the throttle sets power and the propeller control sets rpm.
Feathering turns the blades to approximately 90 degrees from the plane of rotation, nearly parallel to the airflow, so a dead engine's propeller makes as little drag as possible.
Reverse pitch moves the blades past flat pitch to make thrust opposite to the direction of flight. The engine doesn't turn backward.
Propeller aircraft are limited by blade tip speed. The FAA's powerplant handbook limits the speed of propeller-driven aircraft to approximately 400 mph.
How a Propeller Makes Thrust
The FAA says the propeller is a rotating airfoil, subject to induced drag, stalls and the other aerodynamic principles that apply to any airfoil. The thrust it makes depends on the shape of the airfoil, the angle of attack of the blade and the engine rpm.
The blade is twisted. The pilot handbook says the angle is greatest at the hub and smallest at the tip, because the tip travels faster than the part near the hub. Twisting the blade to match produces uniform lift along its length. A blade with one angle along its whole length would be inefficient: as airspeed rises, the part near the hub would have a negative angle of attack while the tip stalled.
Two angles matter, and the powerplant handbook keeps them apart. Blade angle is the angle between the blade's chord line and the plane of rotation. Angle of attack is the angle between the chord line and the relative wind, which comes from both the blade's rotation and the airplane's forward speed. The handbook says the most efficient angle of attack is a small one, from 2 to 4 degrees positive. The blade angle needed to keep it there rises as the airplane speeds up, because the relative wind changes direction. That one fact explains most of this guide.
Blade angle is the angle between the blade chord and the plane of rotation. A constant-speed propeller changes it in flight. A fixed-pitch propeller has one angle, set at the factory as a compromise between climb and cruise. PilotPassion diagram.
Blade angle also sets the load on the engine. The handbook explains that raising the blade angle increases the angle of attack and produces more lift and drag, which increases the horsepower needed to turn the propeller at a given rpm, so with the same engine power the propeller slows down. Lowering the angle lets it speed up. A pilot or a governor can therefore control engine rpm by changing blade angle.
Pitch and blade angle are close but not identical. The handbook defines geometric pitch as the distance a propeller would advance in one turn with no slippage, and effective pitch as the distance it actually advances. The difference is slip. Propeller efficiency, thrust horsepower divided by brake horsepower, runs from 50 to 87 percent depending on how much the propeller slips.
The Propeller Types at a Glance
Propeller types by how blade angle changes (FAA powerplant handbook and Pilot's Handbook of Aeronautical Knowledge)
Type
How pitch changes
Where it's used
Fixed-pitch
It doesn't. The blade angle is built in. Usually one piece of wood or aluminum alloy.
Aircraft of low power, speed, range or altitude. Many single-engine aircraft.
Ground-adjustable
Only on the ground, with the engine stopped, by loosening the clamps that hold the blades.
The FAA says it isn't often used on present-day aircraft.
Controllable-pitch
The pilot changes the blade angle directly while the propeller turns, either between set positions or over a range, depending on the design.
The handbook says these types aren't in wide use today.
Constant-speed
Automatically, through a governor that holds the rpm the pilot selected.
The most common type of adjustable-pitch propeller. Many light aircraft and larger turboprops.
Feathering
A constant-speed propeller that can also turn the blades to about 90 degrees.
Multiengine aircraft, to cut drag after an engine failure.
Reversing
A controllable propeller whose blade angle can go negative.
Mainly turboprops, for braking after landing.
Fixed-Pitch Propellers
The pilot handbook says a fixed-pitch propeller's pitch is set by the manufacturer and can't be changed. It's efficient only at one combination of airspeed and rpm, so the setting is ideal for neither cruise nor climb and the airplane suffers a bit in each. The reward is low weight, simplicity and low cost, and the pilot never touches a propeller control.
The handbook describes two versions. A climb propeller has a lower pitch and so less drag, which means higher rpm and more horsepower during takeoff and climb but weaker cruise. A cruise propeller has a higher pitch and more drag, which gives lower rpm and less horsepower on takeoff and climb but better efficiency in cruise. Which one an airplane carries depends on its intended use.
With a fixed-pitch propeller the tachometer is the engine power indicator. The throttle sets rpm, and at a given altitude a higher tachometer reading means more power. The handbook warns the reading can mislead as altitude rises: 2,300 rpm at 5,000 feet produces less horsepower than 2,300 rpm at sea level because the air is thinner.
The powerplant handbook says fixed-pitch propellers are made of wood or aluminum alloy. A wooden one is built from five to nine glued layers of hardwood, and birch is the most widely used. Metal ones are now one-piece anodized aluminum alloy and are widely used on light aircraft.
Ground-Adjustable and Controllable-Pitch Propellers
An adjustable-pitch propeller was the forerunner of the constant-speed type. The pilot handbook says its blades can be adjusted on the ground with the engine not running but not in flight, and the powerplant handbook says a mechanic does it by loosening the clamping mechanism that holds the blades. The first systems offered only two pitch settings, low and high.
A controllable-pitch propeller can change blade angle while turning. The handbook is careful to say this isn't a constant-speed propeller: the pilot must change the angle directly, and it stays put until the pilot changes it again. The governor is the next step in the propeller's development.
Constant-Speed Propellers
The pilot handbook defines a constant-speed propeller as a controllable-pitch propeller whose pitch is automatically varied in flight by a governor maintaining constant rpm despite varying air loads. It's more efficient than other propellers because it lets the pilot select the most efficient engine rpm for the conditions, and it converts a high percentage of brake horsepower into thrust horsepower over a wide range of rpm and airspeed combinations.
Here's what the pilot does with it. There are two controls: the throttle controls power output, and the propeller control regulates engine rpm. On a piston engine, power is read on the manifold pressure gauge, which measures the absolute pressure of the fuel-air mixture in the intake manifold, and rpm on the tachometer. Turboprop power indications depend on the installation. The handbook gives the cruise example: after setting the desired rpm, an increase in airspeed or a decrease in propeller load makes the blade angle increase to hold that rpm, and a reduction in airspeed or an increase in load makes it decrease.
The powerplant handbook walks through the flight profile:
Takeoff. Maximum power and thrust are needed, so the propeller sits at a low blade angle. That keeps the angle of attack small and efficient and lets the propeller handle a smaller mass of air per revolution. The light load lets the engine turn at high rpm, with high slipstream velocity and low airplane speed giving maximum thrust.
After liftoff. As airspeed rises the propeller moves to a higher blade angle, which keeps the angle of attack small. The handbook says the higher angle handles more air per revolution, and that the resulting lower engine rpm reduces fuel consumption and engine wear while keeping thrust at a maximum. In steady flight the governor holds whatever rpm the pilot selected, so a lower rpm comes from selecting it.
Climb. The pilot reduces power by lowering manifold pressure and raising the blade angle to lower the rpm, so the propeller's torque matches the reduced engine power.
Cruise. Power is reduced again by lowering manifold pressure and increasing blade angle to decrease rpm.
The handbook limits the system. The propeller's constant-speed range is defined by its high and low pitch stops. While the blades are between them, rpm stays constant. If the blades reach a stop, rpm changes with airspeed and load just as it would with a fixed-pitch propeller.
What the Governor Does
The propeller governor is an engine rpm-sensing device with a high-pressure oil pump. It takes oil from the engine's lubricating system and boosts its pressure, and the powerplant handbook gives approximately 300 psi as an example. A set of flyweights spins with the engine, and a speeder spring pushes against them. The tension on that spring, set by the propeller control in the flight deck, decides the rpm. A pilot valve moved by the flyweights directs oil to the propeller's pitch-changing piston or lets it drain away.
Three governor conditions (FAA powerplant handbook, in the typical design where governor oil moves the blades to low pitch)
Condition
What the flyweights do
What the governor does
On-speed
Their force balances the speeder spring.
The pilot valve sends no oil to or from the propeller, and blade angle stays put.
Underspeed
Rpm is below the selected value, so the flyweights tilt inward and the spring wins.
Oil flows to the propeller piston and the blades move toward a lower pitch, which lets rpm rise.
Overspeed
Rpm is above the selected value, so the flyweights fly outward and lift the pilot valve.
Oil drains from the piston and the blades move toward a higher pitch, which loads the engine and lowers rpm.
Two limits come with it. The handbook says the speeder spring's governing range is limited to about 200 rpm, beyond which the governor can't maintain the set rpm. And a governor is only as good as its oil: the engine's oil supply feeds it.
Which Way Does the Oil Push?
This is where the FAA's own handbooks need careful reading. The powerplant handbook says a governor normally provides oil to the propeller for low pitch, with centrifugal flyweights on the blades and springs, and sometimes air pressure, driving the blades toward high pitch. It says one model of nonfeathering Hartzell propeller works the opposite way, with governor oil moving the blades to high pitch and the blades' own centrifugal twisting moment pulling them to low pitch. The Airplane Flying Handbook, though, says the constant-speed propellers on almost all single-engine airplanes are the nonfeathering, oil-pressure-to-increase-pitch design, and the propellers on most multiengine airplanes are the full-feathering, oil-pressure-to-decrease-pitch design. The two descriptions of how common each design is don't match, so the honest reading is that both designs exist and the aircraft's own manual says which one it has.
The same set of forces acts in either design, but their directions and roles vary with the propeller. The powerplant handbook lists seven, among them centrifugal twisting force toward low pitch, blade flyweights and aerodynamic twisting force toward high pitch, and governor oil pressure, which it calls the strongest.
Feathering Propellers
The powerplant handbook says feathering propellers must be used on multiengine aircraft to reduce propeller drag to a minimum when an engine fails. A feathering propeller is a constant-speed propeller with a mechanism to take the pitch to approximately 90 degrees. With the blades parallel to the line of flight, drag is greatly reduced, the propeller stops turning and little or no windmilling occurs.
The Airplane Flying Handbook gives the reason it matters: parasite drag from a propeller changes with blade angle, and in the feathered position it's at its minimum. Depending on single-engine performance, feathering often permits continued flight to a suitable airport. Until it's feathered, the handbook says, a failed engine's propeller may windmill, producing a great deal of drag and adding to the yawing tendency.
How a feathering propeller feathers is neat. In the multiengine design, increased oil pressure drives the blades toward low pitch, away from feather, so a constant supply of high-pressure engine oil is, in effect, all that keeps the blades from feathering, according to the Airplane Flying Handbook. Bring the propeller control fully aft and the governor dumps its oil. The counterweights, plus a spring or high-pressure air in the propeller dome, drive the blades to feather, and the process may take up to 10 seconds. For the Hartzell system it describes, the powerplant handbook gives 3 to 10 seconds as usual. A side effect is a built-in safety: if oil pressure fails, the propeller feathers by itself.
The same design raises a problem at shutdown. The blades would feather every time an engine stops as oil pressure falls to zero, so small anti-feathering latches or lock pins engage below about 800 rpm and hold the blades at low pitch. The Airplane Flying Handbook says that if a propeller is to be feathered it has to be done before rpm decays below about 800. The powerplant handbook says the point of the latches is to avoid excess load on the engine at the next start.
Unfeathering needs oil pressure back in the hub. The simplest way is to start the engine so the governor can pump oil to the propeller, and some aircraft add an accumulator that stores a reserve of oil under pressure and releases it when the propeller control leaves the feather position. The Airplane Flying Handbook says to follow the flight manual's procedure, and adds that manufacturers strongly discourage feathering and starting a feathered reciprocating engine on the ground because of the stress and vibration.
Reversing Propellers and the Beta Range
Turboprop pilots use two more positions. The Airplane Flying Handbook describes the full set: forward pitch makes forward thrust, with higher angles needed at higher airplane speeds. Flat pitch is the blade position with minimum resistance to rotation and no net thrust. Feather is the highest pitch angle obtainable and makes no forward thrust. In reverse pitch the blade angle sits on the other side of flat pitch, so air is pushed away from the airplane instead of drawn over it, and the result is braking. Reverse pitch doesn't mean reverse rotation: the engine and propeller turn the same way, and the engine delivers power the same.
Typical propeller blade positions from reverse through feather, from the FAA powerplant handbook (Figure 7-3). Real figures vary by propeller. Original diagram for PilotPassion.
The powerplant handbook says reverse is used after landing, when the landing gear is on the runway, to slow the airplane quickly and reduce brake wear, and it adds that engine power is applied to increase the braking thrust as the blades move into negative pitch. The Airplane Flying Handbook adds that reverse is used for backing away from obstacles while taxiing and for controlling taxi speed.
On a turboprop the flight range is the alpha range. For any power lever position forward of flight idle the propeller governor picks the blade angle that keeps the engine on speed. Aft of flight idle the power lever directly controls the blade angle, and that's the beta range, running from flight idle to maximum reverse. The handbook says operating in the beta range and with reverse thrust takes specific techniques that depend on the airplane, and that pilots must know them.
Two details from the Airplane Flying Handbook show how much a turboprop depends on its propeller. The propeller of a typical turboprop provides roughly 90 percent of its total thrust at sea level on a standard day. And the handbook warns that large propellers spinning rapidly in low pitch create considerable drag, so landing some turboprops and piston twins with the engines idled too soon can result in a hard, premature touchdown. For these airplanes it may be preferable to hold power through the flare and touchdown.
Tractor, Pusher and Blade Count
The powerplant handbook defines a tractor propeller as one mounted in front of its supporting structure and a pusher as one mounted behind it. Most aircraft use tractors, which turn in relatively undisturbed air and so see lower stresses. Pushers are used more on seaplanes and amphibians, and on land aircraft they're mounted above and behind the wings because wheels throw up rocks and gravel. Most propellers have two blades, but increases in engine power have led to four- and six-bladed propellers of large diameter.
What a Propeller Does to the Airplane
A turning propeller pushes on more than the air in front of it. The pilot handbook lists four elements of what pilots call torque, the left-turning tendency of the airplane, and it says most U.S.-built engines turn the propeller clockwise as viewed from the pilot's seat.
The four elements of torque (FAA Pilot's Handbook of Aeronautical Knowledge, Chapter 5)
Element
What happens
When it matters most
Torque reaction
Newton's third law: as the engine and propeller turn one way, an equal force tries to rotate the aircraft the other way. In flight this rolls the airplane. On the takeoff roll it puts more weight on the left main wheel, adding a left-turning moment.
Takeoff roll. The pilot corrects with rudder or rudder trim.
Corkscrew slipstream
The propeller gives the slipstream a spiraling rotation that strikes the vertical tail and causes a yawing moment. It also makes a rolling moment to the right.
High propeller speed and low forward speed, as on takeoff and approaches to power-on stalls.
Gyroscopic action
A force applied to the rim of the spinning propeller takes effect 90 degrees ahead in the direction of rotation. Any yaw makes a pitching moment, and any pitch makes a yawing moment.
Most noticeable in tailwheel aircraft when the tail is raised on takeoff.
Asymmetric loading (P-factor)
At a high angle of attack the downward-moving blade takes a bigger bite than the upward-moving one, moving the center of thrust to the right and yawing the nose left.
High angle of attack with power on.
The handbook adds that these elements vary with the flight situation and that the pilot is responsible for applying corrective action with the flight controls at all times.
Blade Tip Speed, Ice and Damage
Tip speed is the propeller's limit. The powerplant handbook says excessive blade tip speed may cause poor blade efficiency and also fluttering and vibration, and because propeller speed is limited, the speed of a propeller-driven airplane is limited to approximately 400 mph. The arithmetic, for a made-up example that's ours rather than the handbook's: a propeller 75 inches in diameter turning 2,700 rpm has a tip that travels about 523 knots from rotation alone, which is about 79 percent of the 661-knot speed of sound at sea level that the pilot handbook gives for a standard day. Forward speed adds to it.
Ice is a hazard for propellers because it distorts the blade's airfoil and cuts efficiency, and the handbook says ice usually collects unevenly, which unbalances the propeller and causes destructive vibration. Electric deicing uses heating elements on the blades with power passed through slip rings. A timer applies it to blades or sections in sequence, typically for 15 to 30 seconds with a complete cycle of 2 minutes, and the handbook says the system should be used only while the propeller is turning, apart from short test periods, so the elements don't overheat. Older fluid anti-icing systems pumped alcohol along the blade, and the handbook says they aren't used on modern aircraft. Our icing guide covers the rest of the ice protection picture.
The blade itself takes a beating. The handbook warns that nicks or scratches may cause very serious consequences, because stress concentrates at the bottom of a nick and cracks can lead to failure, and it lists a transverse crack of any size in an aluminum blade as cause for rejection. For composite blades it describes a coin-tap test, where a hollow or dead sound suggests a debond or delamination. A vibration at only one rpm range, such as 2,200 to 2,350 rpm, isn't normally a propeller problem but a poor engine-propeller match.
Propellers are one part of the powerplant. For the engines that turn them, see our guide to aircraft engine types, and for the electrical side of propeller deicing, our electrical systems guide.
Frequently Asked Questions
What are the types of aircraft propellers?
Fixed-pitch, ground-adjustable, controllable-pitch and constant-speed propellers, with feathering and reversing as extra capabilities on constant-speed types. The FAA's pilot handbook says small aircraft use either fixed-pitch or adjustable-pitch propellers.
What does propeller pitch mean?
Pitch is closely related to blade angle, the angle between the blade's chord line and the plane of rotation. The FAA's powerplant handbook also defines geometric pitch as the distance a propeller would advance in one turn with no slippage.
What's the difference between fixed-pitch and constant-speed propellers?
A fixed-pitch propeller has one blade angle, set by the manufacturer. A constant-speed propeller uses a governor to change the blade angle automatically and hold the rpm the pilot selected.
What's a climb propeller versus a cruise propeller?
Both are fixed-pitch. The FAA says a climb propeller has a lower pitch, so more rpm and horsepower on takeoff and climb, and a cruise propeller has a higher pitch, so better efficiency in cruise.
What does a propeller governor do?
It senses engine rpm with flyweights and a speeder spring, and sends oil to the propeller or lets it drain, so the blade angle changes and rpm stays near the value the pilot selected.
What does it mean to feather a propeller?
Turning the blades to about 90 degrees so they're parallel to the airflow. The propeller stops and drag is greatly reduced, which is why multiengine aircraft use it after an engine failure.
What's reverse pitch?
A blade angle on the other side of flat pitch, which pushes air away from the airplane and makes braking thrust. The engine doesn't turn backward.
What's the beta range on a turboprop?
Power lever positions from flight idle to maximum reverse, where the power lever directly controls blade angle instead of the governor. Forward of flight idle is the governing, or alpha, range.
Is an adjustable-pitch propeller the same as a constant-speed propeller?
Not exactly. The FAA says a ground-adjustable propeller is set with the engine stopped, and a controllable-pitch propeller is changed directly by the pilot. The pilot handbook notes the term adjustable-pitch can also cover constant-speed propellers, which a governor changes automatically.
What are propellers made of?
Wood, aluminum alloy and composites. The FAA's powerplant handbook says wooden ones are layered hardwood, most often birch, and that metal fixed-pitch propellers are one-piece anodized aluminum alloy.
