Text: Air propulsion

An aircraft propulsion system generally consists of an aircraft engine and some means to generate thrust, such as a propeller or a propulsive nozzle.

An aircraft propulsion system must achieve two things. First, the thrust from the propulsion system must balance the drag of the airplane when the airplane is cruising. And second, the thrust from the propulsion system must exceed the drag of the airplane for the airplane to accelerate. In fact, the greater the difference between the thrust and the drag, called the excess thrust, the faster the airplane will accelerate.

Some aircraft, like airliners and cargo planes, spend most of their life in a cruise condition. For these airplanes, excess thrust is not as important as high engine efficiency and low fuel usage. Since thrust depends on both the amount of gas moved and the velocity, we can generate high thrust by accelerating a large mass of gas by a small amount, or by accelerating a small mass of gas by a large amount. Because of the aerodynamic efficiency of propellers and fans, it is more fuel efficient to accelerate a large mass by a small amount. That is why we find high bypass fans and turboprops on cargo planes and airliners.

Some aircraft, like fighter planes or experimental high speed aircraft require very high excess thrust to accelerate quickly and to overcome the high drag associated with high speeds. For these airplanes, engine efficiency is not as important as very high thrust. Modern military aircraft typically employ afterburners on a low bypass turbofan core. Future hypersonic aircraft will employ some type of ramjet or rocket propulsion.

Exercise 7. Fill in the missing words from the table and translate the text:

Third law, engine, Latin, working fluid, drive, gas turbines, machine, thrust, generated through, reaction, aircraft engine, mechanical power, forwards.

What is propulsion? The word is derived from two_____words: pro meaning before or____and pellere meaning to drive. Propulsion means to push forward or____ an object forward. A propulsion system is a____ that produces____ to push an object forward. On airplanes, thrust is usually_______ some application of Newton's_____ of action and______. A gas, or________, is accelerated by the______, and the reaction to this acceleration produces a force on the engine.

An _______ is the component of the propulsion system for an aircraft that generates _______ . Aircraft engines are almost always either lightweight piston engines or_________ .

Exercise 8. Match words with their definitions:

1. Thrust 2. Propulsion 3. Engine 4. Bypass 5 Drag 6. Propulsive nozzle

a) A machine that converts energy into mechanical force or motion b) A pipe or channel used to conduct gas or liquid around another pipe or a fixture. c) The force that propels an object in a given direction d) The aerodynamic force that opposes an aircraft's motion through the air. e) A driving or propelling force. f) A device designed to control the direction or characteristics of a fluid flow (especially to increase velocity) as it exits (or enters) an enclosed chamber or pipe via an orifice. (the exhaust duct of the engine)