An aircraft propeller, alternatively called an airscrew, is an aerodynamic device that converts rotary motion into thrust. It transforms rotational energy to accelerate air and produce a low-pressure differential, thereby propelling aircraft. The propeller is a rotating wing made from wood, composite, or aluminum. It consists of several radial airfoil-section blades attached to a rotating power-driven hub that operates in undisturbed freestream flow. Widely used in aviation, the propeller delivers engine power efficiently and comes in fixed-pitch, variable-pitch, and constant-speed types; a fixed-pitch propeller does not adjust blade pitch, while the other types do.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What are propellers on aircraft?
A propeller is an aerodynamic device that converts rotational energy into propulsive force. Rotational energy is produced by a piston or gas-turbine engine or by an electric motor, and the resulting torque spins a wing with a twisted airfoil section around an axis perpendicular to the direction of motion of the aircraft. As the helical spiral rotates, it exerts linear thrust upon the working fluid-air and the pressure difference pushes the aircraft forward. Lift generated in the forward direction is referred to as thrust, and thrust is approximately perpendicular to the propeller's plane of rotation. Thus, air displacement produces forward thrust, and the propeller area swept out by blade length determines how much thrust can be generated.
What is the function of propellers in the aircraft?
The function of propellers in the aircraft is to propel the aircraft by converting engine power into thrust. This thrust helps aircraft move forward, countering drag and enabling flight. The primary purpose of a propeller is to keep the airplane moving forward. The propeller converts rotational energy produced by the gas turbine engine into propulsive force. As the propeller rotates, it accelerates air rearward, displacing the air and resulting in the aircraft being pushed forward.
The shape of an aircraft propeller is based on an airfoil comparable to a cross-section of an aircraft wing. This airfoil shape creates a pressure difference between upper and lower surfaces, generating lift in a direction that becomes thrust. The pressure difference creates thrust approximately perpendicular to the propeller's plane of rotation, pushing the aircraft forward.
The mechanism begins when the engine supplies brake horsepower through a rotating shaft. The propeller converts this brake horsepower into thrust horsepower, transforming rotational motion into a propulsive force. A propeller governor, a speed-sensing device, causes the propeller to maintain rpm, guaranteeing steady thrust. Whether the propeller operates as a tractor in undisturbed freestream flow or as a pusher in the aircraft's wake, the same principle applies: the swirling slipstream accelerates air rearward, producing forward thrust.
Advantages of the propeller propulsion system include simplicity, reliability, and efficiency at subsonic speeds. Propellers are powered by electric motors or human-powered aircraft using a bicycle gearing device, demonstrating flexibility. By creating thrust that counteracts drag, the propeller provides a method of propulsion, allowing wings to produce lift and sustain flight.
How do airplane propellers work?
An airplane propeller works like a rotating wing. Each blade has an airfoil shape that creates higher pressure on one surface and lower pressure on the other. Lift is directed forward as thrust. Propellers work by displacing the air, pulling it behind itself, and this displaced air results in the aircraft being pushed forward. At the same time the propeller works by creating a pressure lower in front and higher behind and the low-pressure differential pulls the airplane forward while the accelerated mass flow pushes from the rear.
In a single-engine plane the engine supplies brake horsepower through a rotating shaft to the hub. A propeller governor senses changes in engine RPM and adjusts oil pressure to change blade angle, keeping RPM constant by altering pitch. The pilot sets desired RPM with the propeller control lever and the governor moves oil back and forth through the propeller hub so the piston moves blade pitch. Low pitch gives more RPM for take-off WHILE high pitch gives less RPM for cruise. If the engine fails, the propeller can be feathered automatically which reduces drag.
The basic theory is captured by the actuator-disc idea: the propeller disk acts like an actuator disc that does work on the airflow, so thrust equals mass flow rate times velocity change. In a turboprop, a gas turbine engine drives the propeller through a reduction-gear assembly, but the aerodynamics remain the same - propeller blades produce forces that create thrust, and pressure difference creates thrust.
How do propellers make planes fly?
Propellers make planes fly by creating a pressure differential. The engine rotates the airfoils of the blades; each airfoil creates pressure difference between its faces. The low pressure pulls the airfoil forward while the high pressure pushes aft, so the pressure differential produces thrust. Because lift in forward direction is called thrust, the propeller thrust must be greater than drag for acceleration. Displaced air results in aircraft being pushed forward, and the machine continues to move forward as long as this thrust exceeds total drag.
What are 4 types of aircraft propellers?
The 4 types of aircraft propellers are listed below.
- Controllable-Pitch Propeller
- Fixed-pitch Propeller
- Constant-Speed Propeller
- Ground-Adjustable Propeller
Propellers are grouped as fixed pitch, ground adjustable, controllable pitch, and constant speed. Ground adjustable propellers are similar to fixed pitch units yet can be reset on the ground with tools before the engine runs, usually through a split hub. Controllable-pitch propellers allow the pilot to change blade angle in flight. Constant-speed propellers are a type of controllable-pitch propeller that use a governor to maintain constant RPM by automatically varying pitch, are heavier, and employ reverse pitch.
What does an aircraft propeller assembly consist of?
An aircraft propeller consists of two or more blades and a central hub. The propeller hub contains both the blades and the mechanical means for holding them in position, and it connects to the engine through a propeller drive shaft and a gearbox. The main unit is the part of the propeller that is used to attach it to the engine shaft or gearbox. A rotating power-driven hub has attached several radial airfoil-section blades. Each blade is composed of an airfoil section and has a leading edge, a trailing edge, and a chord that joins the leading edge to the trailing edge. The whole assembly rotates about a longitudinal axis, and the spinner is a sleek aerodynamic unit designed to streamline airflow and minimize drag over the propeller hub.
The major subassemblies of the propeller assembly are the hub, the spinner, the pitch control unit, the propeller electronic control, the feather and unfeather valves, the low-pitch stop assembly, the beta feedback assembly, the overspeed governor, the torque motor, the barrel, the dome, the auxiliary pump, and the de-ice timer. The hydraulic counterweight propeller consists of a hub assembly, a blade assembly, a cylinder assembly, and a counterweight assembly; The counterweight assembly is attached to the blades and moves with them. The low-pitch stop assembly limits low pitch angle, the overspeed governor provides means to limit propeller speed, and the speed-sensitive governor provides means for the propeller to adjust itself automatically to maintain constant rotational speed. A constant-speed propeller automatically adjusts blade pitch using a governor, and the pitch control unit adjusts blade pitch. The auxiliary pump provides hydraulic pressure for blade pitch control. Feather and unfeather valves control feathering, de-icing boots prevent ice formation, and the de-ice timer controls de-icing. The propeller assembly is assembled from about 120 parts in the assembly area, and the engine-shaft-extension assembly is used with propellers that do not have feathering capabilities.
What material is used for aircraft propellers?
The materials used for aircraft propellers include wood, metal, and composite materials. Early propellers were made from wood, with yellow birch, black cherry, sugar maple, and black walnut commonly used for vintage designs. Mahogany was preferred through World War I, while oak, cherry, and ash were pressed into service during wartime shortages. Wooden fixed-pitch propellers are hardly found in modern aircraft, but when used they are reinforced with fabric or metal coverings and steel fittings to mount them on the propeller shaft.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
Aluminum has long been the metal of choice for creating airplane propellers. Aluminum propellers became common after World War II, when many of the earliest metal propellers were manufactured in one piece of forged Duralumin. Today, fixed-pitch propellers are made up of aluminum alloy forgings, and blades are made from high-strength aluminum alloy forgings or advanced composite materials. The metal material is treated to enhance its properties and make it resistant to immediate warping from heat and cold damage. For parts that are aluminum, they are tested through a unique process that uses black lights to see if there are any faults.
Composite propeller blades are constructed using aerospace-grade carbon fiber and Kevlar, materials that are up to ten times stronger than traditional spruce or beech wood cores. Hartzell structural composite blades are full composite, not composite over a wood core. They use a low-density foam core to support built-up layers of composite laminate and can be repaired and returned to service without adversely affecting airfoil shape. Aluminum alloys excel in terms of strength, lightness, durability, and cost, while titanium, although stronger and possessing better fatigue resistance, remains less common because aluminum is lighter and more economical.
The hub system of Hartzell propellers contains primary structural components of high-strength aluminum alloy or steel forgings, assuring reliable load-bearing capacity and long-term durability.
Why are aircraft propeller blades twisted?
Propeller blades are twisted because the blade tips travel faster than the center of the propeller. During one full rotation, the tip of the blade has to travel a much further distance than the blade root in the same amount of time. This difference in speed means the blade speed is much higher at the tip than at the root. To compensate for this speed difference and to produce uniform thrust along the length of the propeller blade, the blades are twisted. This twist produces a relatively uniform angle of attack across the entire propeller blade and prevents large angle-of-attack and pressure differences across the blade. The twisted shape ensures about the same angle of attack at all points along the edge, making the propeller efficient. Twist is built into the propeller blade to guarantee a more-or-less constant angle of attack along the length. The blade angle is greatest at the root and the least at the tip. The twist is necessary to extract the most performance out of the propeller.
How fast do plane propellers spin?
The speed at which propellers spin depends on diameter, engine power, and the presence of a reduction gearbox. On the Cessna 185 the 86-inch (218.44 cm) propeller is set to 2,850 rpm, producing a tip speed of 0.97 Mach on a 10°C (50°F) day. The Tu-95 uses contra-rotating propellers which spin at 750 rpm, so that, despite the 5.6 m (18.4 ft) diameter, the tip circumferential unit is only 220 m/s (722 ft/s), about Mach 0.67 at zero forward speed. When the aircraft cruises the vector sum reaches Mach 1.0. Propeller tip speed for a typical propeller is limited to 0.80-0.92 Mach because, when the airflow over the tip of the blade reaches its critical speed, shock waves form, drag and torque resistance increase rapidly, and noise rises sharply. A speed of Mach 1.01 in a dive was achieved by the McDonnell XF-88B experimental propeller-equipped aircraft, but efficiency fell to 78%.
Without speed reduction gearboxes, propeller tips on large turboprops travel at speeds in excess of 1,000 miles per hour (1,609 kilometers per hour). Gearboxes allow the engine to spin at high speed while keeping the propeller tips subsonic. Maximum performance is reached at a propeller tip speed between 0.80 and 0.92 Mach. Above this limit the sharp increase in noise and structural stress makes higher rpm undesirable. Propeller blade leading edges are sharp enough to cut, so sharp edges are sometimes sanded off. Trailing edges are sanded because they are easily damaged, although aerodynamically a sharp trailing edge is acceptable.
Why are airplane propellers placed at the front?
Early airplanes tried both back and front arrangements, but designers soon discovered that front facing engines face far less turbulence than rear facing propellers. A rear facing propeller operates in the wake of wings, fuselage and control surfaces. That turbulent flow behind the engine creates drag and lowers thrust. By moving the propeller ahead of everything else, the whole aircraft rides in the smooth, undisturbed stream the blade itself generates, so difference in air pressure pushes the airplane forward with less wasted energy. The reduction of drag is substantial, and being pushed forward from behind was found to affect efficiency unfavourably.
Forward propeller placement improves safety and maintenance. An aircraft with front facing engines does not have the problem of objects like dirt and rocks being caught in the propeller. Prop wash is not kicking up stones into the fuselage, so the engine stays cleaner and the airframe suffers less abrasion. Regulating temperature is much easier with a front facing configuration because the slipstream blows directly through cowling inlets, so cylinders cool evenly without extra scoops that add surface area.
A front-mounted propeller provides lift even at zero speed by accelerating air over the wing roots, boosting take-off acceleration. After touchdown, the pilot can use reverse thrust. Because propeller pitch can be reversed so the blades bite outward, that opposite thrust reduces ground roll after landing without heavy brakes. Front facing engines are not as noisy for occupants, while the pilot ejection path is clear, since pilot being forced to eject from a plane with rear facing engines is generally deemed unsafe.
Do all planes have propellers?
No, not all planes have propellers. While many aircraft still spin propellers, the majority of commercial and military aircraft are powered by jet engines. Most wide-body commercial airlines have jet engines, and these turbine-powered airplanes dominate long-haul passenger and heavy-cargo markets.
Yet other airplanes have propellers. Most general aviation or private airplanes are powered by propellers and internal combustion engines. Many weather planes still rely on propellers - turboprops - because turboprops are most efficient at mid altitudes and airspeeds and can operate from shorter runways than a jet. All branches of the United States military operate aircraft with propellers. The C-130, for example, uses propellers instead of jets. Some airplanes use both propellers and jet engines, combining the fuel-saving benefits of turboprops on short hops with jet thrust for higher or faster flight.
Propeller planes are much cheaper to build and operate than a turbine-powered airplane, and they serve regional routes, flight-training fleets, policing, surveying, and agricultural spraying. Many unmanned aerial vehicles use propellers for the same cost and simplicity advantages. Thus, although not every airplane carries propellers, propulsion by propeller remains an enduring, practical choice across civil, military, and unmanned aviation.
What are the latest plane propeller designs?
Today's propeller designs concentrate on aerodynamic efficiency, weight reduction, cabin comfort and noise suppression, areas where even small gains translate directly into lower fuel burn and longer airframe life. Hartzell Propeller's Top Prop STC program allows owners to upgrade current aircraft with the latest propeller designs: swept-tip scimitar blades, blended airfoils and dependable carbon-fiber composite materials whose wider yet thinner airfoil sections still bear the structural loads required for optimum aircraft operations. Nickel erosion mesh and uniform strength are incorporated to provide unmatched durability.
Hartzell's carbon-blended airfoil is now available in a two-blade propeller with carbon fiber blades developed to mimic the current RV-standard two-blade metal propeller yet deliver more thrust with better acceleration and climb performance. A key trend is the shift toward propellers with smaller diameters and more blades. 2-blade through 6-blade hubs are offered and the higher blade count is enabled by lower weight and lower moment of inertia which also reduces vibration. Adding more blades further helps reduce noise because three smaller pulses per revolution replace the single large pulse of a two-blade propeller, an important consideration when airport noise regulations tighten.
Latest new developments are props for the Beechcraft Denali single-engine turboprop and Cessna SkyCourier utility twin. Rapid delivery of new propellers leads operators to explore 4- and 3-blade Hartzell options specifically for reduced noise emission. Engineers employ advanced computational fluid dynamics simulations and wind tunnel testing to analyze different design iterations, while swept leading edges minimize compressibility effects.
Recovery of the same material is coupled to the gas turbine following three formulas that match the propeller to the engine and airframe mission. From Piper to Daher TBM, Cessna to Pilatus, Mooney to Beechcraft, more than four hundred blade designs are on the shelf and the Top Prop STC program supports most makes and models.
