Thrust: The Driving Force
Airplanes need thrust to provide the forward speed that the wings transform into lift.
Thrust is what we get when an engine takes in air and accelerates it. When the air gains velocity, it causes thrust. When thrust and drag are in balance, an airplane’s speed stays constant. When thrust is greater than drag, speed increases, and when thrust is less than drag, the plane slows down.
A propeller generates a thrust force by taking a relatively large amount of air and accelerating it by a small amount. A jet engine takes a relatively small amount of air and accelerates it a lot. Either way, the result is thrust.
Saving weight on an airplane is a constant consideration for airplane engineers. Even on large airplanes with very powerful engines, there is a limit to how much equipment can be earned. That’s because each model of airplane has a maximum weight it can carry safely. In addition to the basic weight of the empty airplane, you have to add the weight of fuel and the weight of passengers and cargo. So any luxury equipment, such as airconditioning units and on-board bathrooms, reduce the amount of fuel the plane can carry (and that means reduced flying time) as well as the passenger and cargo capacity. If you think it’s no fun flying in a modern jetliner, with no leg room, it’ll take some adjusting to get used to the Spartan conditions of a small airplane.
Jet engines, or to call them by their full name, turbojet engines, rely on the principle that high-pressure air shot out of one end of an engine creates a force in the opposite direction.
In a jet engine, normal atmospheric air is allowed in at one end of the engine, is compressed and mixed with fuel, and then ignited. The explosion causes the burning mixture of fuel and air to expand and shoot out of an exhaust pipe. Whichever direction the exhaust shoots out, a force is created in the opposite direction that can be harnessed to accelerate an airplane—or a speed boat or anything else that you attach the engine to, for that matter.
Since jets were first invented in the 1930s, they’ve grown to be a lot more powerful, a lot more reliable, and a lot quieter. They now produce so much power thrust that engineers are building larger airplanes that carry more than 500 people in luxurious comfort.