Thrust
Thrust is the force which a plane generates to overcome drag and propel the plane in the direction of motion. A plane can generate thrust with its propeller, jet engine or rocket engine. All these engines use the the principle of Newton’s third of law of motion to produce thrust. In order to stay in flight, thrust must be greater or equal to the drag created by the plane. If the thrust is greater than the drag, then the plane will accelerate and vice versa. When a plane is in flight if the weight, lift, drag and thrust are balanced then the plane will cruise at constant velocity.
Jet Engine
A Jet engine takes in large quantity of air from the surrounding area with a compressor and compresses the air with the many blades of the engine. The air is then heated and mixed with jet fuel and then ignited by an electric spark. Subsequently the burning gas rapidly expands and bursts out the other end of the engine. The backwards push of burning gas in turn causes the plane to move forward in accordance with Newton’s third law (for every action there is a equal and opposite reaction.) That is, the force of the jet engine pushing backwards results in an opposite force which pushes the plane forward. This force is known as thrust.
Figure 7-Inside workings of a Jet Engine
Rocket Engine
A rocket engine uses the same principle to generate thrust. However, unlike a jet engine, rocket engines do not intake air from the surrounding area but instead mix the rocket fuel with an onboard supply of oxygen known as an oxidizer. As it is not dependent on air from the surrounding atmosphere a rocket engine is able to work in space, whereas a jet engine and propeller cannot. The oxygen and fuel ignite in the rocket engine, in turn combusting and producing a hot exhaust out through the nozzle, which produces thrust in the same way a jet engine does in accordance with Newton’s third law of motion.
Figure 8- Rocket Engine
Propeller
In a propeller engine system the fuel is mixed with air and then burned to produce gas which moves a piston. This piston is attached to a crankshaft which in turn turns a propeller. A propeller's blades have an airfoil shape like the planes wings, meaning that fundamentally act as group of spinning wings. Like a wing it produces lift (to see how lift is generated click here), but in a forward direction, producing thrust and pushing the plane forward.