Ask A Scientist
Question: How does a plane’s engine work?
Asked by: Gabrielle Mancini
School: Maine-Endwell Middle School
Teacher: Kevin Wagstaff
Career Interest: Aerospace engineer
Answer from Peter Huang, PhD
Assistant Professor, Binghamton University
Education: Bachelor of Arts in Physics, Cornell University; Doctor of Philosophy in Engineering, Brown University.
Research Areas: Fluid Mechanics, Heat Transfer, Nanotechnology and Biotechnology
Outside of military jets that use specialized engines, most modern designs of civilian aircraft engines belong to two major families: turboprop and turbofan. Turboprop engines can be identified with their exposed propellers and are usually used in smaller planes. Turbofans are the covered fan engine units found underneath of the wings of larger commercial jets, such as those made by Boeing and Airbus. While their appearances look dramatically different, their functioning principles are very similar. Their basic structures include a rotor with blades in the front (that is, either a propeller or a fan depending on the engine type), a combustion chamber and a turbine at the back, and a set of gears and shafts connecting the turbine and the rotor. During operation, airplane fuel is injected into the combustion chamber and ignited, causing a rapid expansion of the gas inside that turns the turbine. The rotational force of the turbine is then transferred back to the rotor through the shafts and the gears, turning the propeller or the fan in the front.
So how does the turning rotor move an airplane forward? The mechanism is the same as that of a ceiling fan. If you observe the ceiling fan, you will see that its blades are angled. When turning, the angled blades create a pressure difference between the top and the bottom of the ceiling fan, creating a downward draft that cools the room. Similarly, the angled blades on the propeller or the fan of a plane engine sucks in air from its front side and forcefully blows the air out in its rear, creating a strong thrust force that pushes the plane forward. Then perhaps you would ask how come the ceiling fan does not break the ceiling and fly off? This is because the ceiling fan does not rotate at a super fast speed that creates a force strong enough to support its own weight and damage the ceiling structure.
The turboprop and the turbofan engines do still have their performance differences due to their designs. The exposed propellers of the turboprops are very efficient at flight speeds below 450 mph and lower altitudes, making them a cost-effective choice for smaller regional and general aviation aircrafts. Turbofans, on the other hand, are the most efficient engines at higher speeds and are most suited for the long distance travel of large commercial airliners.