Propellers

Motion

At the rear of the 1903 Wright Flyer one finds a pair of pusher propellers. The propellers are long, thin, twisted pieces of wood which are spun at high speed. The propeller blade is shaped like an airfoil and there is a pressure difference created across the blade because of the motion of the spinning blade. The pressure difference causes large amounts of air to be accelerated through the plane of the propeller and the reaction of the vehicle to this motion generates a force called thrust. The thrust pushes the aircraft forward in accordance with Newton’s first law of motion.

Energy

The physics involved in the generation of thrust is introduced in middle school and studied in some detail in high school and college. To accelerate the air, we have to expend energy. The energy is generated as heat by the combustion of some fuel in an engine. The Wright brothers used a single, 12 horsepower, gasoline powered, internal combustion engine to turn their propellers. For comparison, a typical lawn mower engine is a 6 horsepower engine! Automobile engines are also internal combustion. The Wright brothers placed their engine next to the pilot and connected the propellers by “bicycle” chains.

High Efficiency of Speed

The use of high speed (~350 revolutions per minute), thin propellers on their aircraft was one of the major breakthroughs for the Wright brothers and allowed them to succeed where others failed. At the time, others employed low speed, thick bladed propellers, much like the blades of a wind mill. But the brothers correctly determined that high speed, thin propellers are more efficient than low speed thick blades. High efficiency means that they were able to convert more of the available energy of the engine into thrust. The brothers did not have much power to work with, so efficiency was very important. The brothers used two propellers rotating in opposite directions. This eliminated any gyroscopic forces which would have been generated by a single propeller and would have made the plane harder to control.

Propellers are still used today on general aviation and commuter aircraft and were the primary propulsion system for all aircraft until the development of jet engines during World War II.