Learn how airplanes generate thrust with propellers and jet engines, why accelerating air backwards moves the aircraft forwards, and what changes thrust.
Airplanes generate thrust by accelerating air rearwards. Propellers, ducted fans and jet engines do this in different ways, but the result is the same: the rearward change in the air’s momentum produces an equal forward force on the aircraft. That forward force is thrust.
In our Aviation & Real-World Flying coverage, momentum is the most useful model. A propulsion system gives a mass of air more rearward momentum, while the equal and opposite reaction acts through the engine mounts on the airframe.
What actually creates aircraft thrust?
The forward force comes from changing the momentum of air flowing through the propulsion system. Thrust increases when the system moves a greater mass of air, gives that air a larger rearward velocity change, or does both.
This follows Newton’s laws of motion. The engine exerts a rearward force on the air, and the air exerts an equal forward force on the engine. In a jet, any difference between nozzle-exit pressure and ambient pressure can also contribute to total thrust.
An airplane can produce static thrust while standing still because its propeller or engine accelerates air that was initially stationary relative to the aircraft. It does not need to push against the runway.
Propeller thrust versus jet thrust
The main difference is how engine energy reaches the surrounding air.
| Propulsion system | How energy reaches the air | Main thrust source |
|---|---|---|
| Piston or electric propeller | A shaft turns the propeller | Rearward propeller slipstream |
| Turboprop | A gas turbine drives a propeller through a gearbox | Mostly the propeller, with some residual exhaust thrust |
| Turbofan | The turbine drives a large fan and compressors | Bypass fan flow plus core exhaust |
| Turbojet | The engine accelerates air through its core and nozzle | High-velocity exhaust |
Our explanation of how the principal aircraft engine types convert stored energy into propulsion covers the complete energy path for each system.
How does a propeller generate thrust?
A propeller generates thrust by using rotating aerofoil-shaped blades to create a pressure difference and accelerate air rearwards. Each blade meets the air at an angle of attack determined by blade pitch, rotational speed and the airplane’s forward speed.
The engine supplies torque, but the propeller converts that shaft power into thrust. Pitch matters: a blade angle that is too low may not absorb the available power efficiently, while excessive blade angle can stall part of the blade. Constant-speed propellers automatically vary pitch to hold the selected RPM within their governing range.
See our closer explanation of propeller blade forces, pitch and the resulting slipstream for the aerodynamics behind this process.
How does a jet engine generate thrust?
A jet engine produces thrust by taking in air and ejecting it rearwards with greater momentum. The compressor raises the air pressure, fuel adds energy during combustion, and the turbine extracts enough energy to drive the compressor and, in a turbofan, the fan.
The remaining energy accelerates the flow through the exhaust nozzle. In a high-bypass turbofan, most thrust normally comes from moving a large mass of relatively cool bypass air, rather than from the hot core exhaust alone. Our guide to the compressor, combustor, turbine, fan and nozzle airflow explains that cycle in detail.
Is engine power the same as thrust?
No: power is the rate of doing work, while thrust is a force. Jet engines are normally rated in newtons or pounds-force of thrust; piston engines and turboprops are commonly rated by brake or shaft power.
A more powerful engine can permit greater thrust, but the conversion depends on propeller efficiency, blade pitch, airspeed and atmospheric conditions. At non-zero flight speed, the useful propulsive power delivered to the airplane is approximately thrust × true airspeed. At a standstill that value is zero even though static thrust exists, because the engine’s energy is being put into the slipstream rather than moving the airplane.
This is why a shaft-power figure cannot be compared numerically with a jet-thrust figure. Our guide to bhp, shp and thrust measurements shows which rating belongs to each engine type.
How does thrust balance drag during flight?
Thrust does not have to exceed drag throughout the flight; the required balance depends on whether the airplane is accelerating, climbing, descending or maintaining speed.
| Flight condition | Force balance along the flight path |
|---|---|
| Take-off roll | Thrust must exceed aerodynamic drag and rolling resistance for the airplane to accelerate. |
| Steady level flight | Thrust equals drag. |
| Steady climb | Thrust balances drag plus the rearward component of weight along the climb path. |
| Steady descent | Thrust may be lower than drag because a forward component of weight supplies the difference. |
| Power-off glide | Engine thrust is zero or negligible; the component of weight along the glide path balances drag. |
| Reverse thrust after landing | Redirected airflow creates a rearward force; the engine itself does not rotate backwards. |
Thrust need not exceed the airplane’s total weight for an ordinary climb. Only the force components along the actual flight path should be compared.
Why does the same throttle setting produce different thrust?
Available thrust changes with airspeed, air density, temperature, propeller pitch and engine operating limits. A throttle or thrust-lever position is a command, not a universal percentage of maximum thrust.
- Altitude and temperature: Hotter or less dense air reduces mass flow and usually reduces maximum available power or thrust. Turbocharging and engine-control systems can maintain a selected output only within their operating limits.
- Airspeed: Propeller efficiency and blade angle of attack change as forward speed changes. Jet net thrust is also affected by inlet momentum, ram pressure and engine design.
- Propeller configuration: RPM, pitch, diameter and blade condition determine how effectively shaft power becomes thrust.
- Engine limits: Temperature, pressure, torque and rotational-speed limits can prevent an engine from producing the same output in every condition.
A mistake we see constantly in flight simulation is treating 50% controller travel as 50% thrust. Use the aircraft’s proper indications instead: manifold pressure and RPM in many piston aircraft, torque and propeller RPM in turboprops, or N1, EPR and other specified engine parameters in jets.
Can an airplane fly without thrust?
Yes, an airplane can glide without engine thrust because gravity supplies the energy needed to overcome drag. The airplane descends as gravitational potential energy is converted into motion through the air.
The wings still produce lift, but they are not acting as a propulsion system. A sailplane can remain aloft or climb in rising air, while a powered airplane after engine failure can glide only within the performance limits set by its lift-to-drag ratio, configuration and windmilling-propeller drag.