Thrust is the force that moves an aircraft through the air. And thrust is generated by the propulsion system of the aircraft. Different types of propulsion systems develop thrust in different ways, although thrust is usually generated through some application of Newton's Third Law (action <-> reaction). A general derivation of the thrust equation shows that the amount of thrust generated depends on the gas flow through the propulsion system and the exit velocity of the gas.
Propeller-Produced Thrust
For the forty years following the Wright brothers first flight, airplanes used internal combustion engines to turn propellers to generate thrust. 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. 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. You can learn more about the aircraft by visiting the NASA Dryden picture gallery (http://www.dfrc.nasa.gov/gallery/photo/NIX/unique-aircraft.html).
Propulsion Systems:
Propellers:
Go to...
byTom
Benson
Please send suggestions/corrections to: benson@grc.nasa.gov