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Picture of the propeller powered P-51 and a wind tunnel test.

Thrust is the force which moves any aircraft through the air. Thrust is generated by the propulsion system of the aircraft. Different propulsion systems develop thrust in different ways, but all thrust is generated through some application of Newton's third law of motion. For every action there is an equal and opposite reaction. In any propulsion system, a working fluid is accelerated by the system and the reaction to this acceleration produces a force on the system. A general derivation of the thrust equation shows that the amount of thrust generated depends on the mass flow through the engine and the change in velocity of the gas going through the propulsion system.

Propeller-Produced Thrust

For the forty years following the first flight of the Wright brothers, airplanes used internal combustion engines to turn propellers to generate thrust. Today, most general aviation or private airplanes are still powered by propellers and internal combustion engines, much like your automobile engine. The engine takes air from the surroundings, mixes it with fuel, burns the fuel to release the energy in the fuel, and uses the heated gas exhaust to move a piston which is attached to a crankshaft. In the automobile, the shaft is used to turn the wheels of the car. In an airplane, the shaft is connected to a propeller.

Propellers as Airfoils

On this slide, we show pictures of a P-51 propeller-powered airplane from World War II and a propeller being tested in a NASA Glenn wind tunnel. The details of propeller propulsion are very complex, but we can learn some of the fundamentals by using a simple momentum theory. The details are complex because the propeller acts like a rotating wing creating a lift force by moving through the air. For a propeller-powered aircraft, the gas that is accelerated, or the working fluid, is the surrounding air that passes through the propeller. The air that is used for combustion in the engine provides very little thrust. Propellers can have from 2 to 6 blades. As shown in the wind tunnel picture, the blades are usually long and thin. A cut through the blade perpendicular to the long dimension will give an airfoil shape. Because the blades rotate, the tips moves faster than the hub. So to make the propeller efficient, the blades are usually twisted from hub to tip. The angle of attack of the airfoils at the tip is lower than at the hub.

Other Engines Drive Propellers

As noted, the engine used in the P-51 was an internal combustion engine. After World War II, as jet engines gained popularity, aerodynamicists used jet engines to turn the propellers on some aircraft. This propulsion system is called a turboprop. A C-130 transport plane is a turboprop aircraft. Its main thrust comes from the propellers, but the propellers are turned by turbine engines. The human-powered aircraft of the mid 80's were also propeller-powered, but the "engine" was provided by a human using a bicycle gearing device. Currently NASA is flying a solar-powered, electric engine aircraft that also uses propellers. Propeller-powered aircraft are very efficient for low speed flight. But as the speed of the aircraft increases, regions of supersonic flow, with associated performance losses due to shock waves, occur on the propeller. Propellers are not used on high speed aircraft.

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Editor: Nancy Hall
NASA Official: Nancy Hall
Last Updated: May 13 2021

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