Does a Jet Engine Push or Pull?

If you have ever watched an airplane accelerate, you aren’t alone to marvel at the sheer force of the thrust that escapes its engines. A modern jet engine can produce tens of thousands of horsepowers. For instance, one engine of a Boeing 777 is capable of delivering over 30,000 horsepower! With all this power, it’s not strange to ask whether jet engines push or pull themselves through the skies

Jet Engines push an aircraft forward by combusting compressed air mixed with fuel, which escapes the engine at high velocity. Even if the jet engine pulls in air in the front, the propulsion gained is much smaller compared to that caused by the thrust itself. 

There is not a straight-forward answer to whether jet engines push or pull aircraft, so let us get into some details. Simply put, for a jet engine to combust gases and release them out through the back, it also must suck in air. And knowing this, it becomes natural to assume that a jet engine also pulls, which we just mentioned in the answer section. 

However, there are some more details to share, as the extent the pulling force contributes varies depending on the exact situation. 

Do Jet Engines Pull As Well?

Envision an airplane ready for take-off; as the pilot pushes the throttle levers forward to get maximum thrust from the engines, the engines start revving up. As more exhaust gases are released, the suction of air increases in the front of the engine.

Although the primary thrust is being generated through exhaust gases, the air being pulled-in also contributes to the total force enabling the forward motion of the aircraft.

However, when an aircraft is flying at very high airspeeds (nearing Mach 1), the air available to the engines at their inlet is more than the required amount. The suction of air, therefore, is not a contributing factor in the flight of aircraft or the thrust of the engines at higher airspeeds.

To help us understand this better, let us get into the basics of a jet engine and how it produces thrust!

How is thrust produced by a Jet Engine?

A jet engine produces thrust by compressing the sucked-in air, combusting it by adding fuel, and then letting the hot burnt gases out at high velocity through the exhaust cone.

These three operations are performed in three distinct parts of a jet engine, namely:

• Compressor Section: The compressor section contains the engine inlet and multiple compressor stages which compress the air gradually up-to the required density. You could say that it pushes the air into the engine, where the decreasing amount of space forces the air to take less space, effectively leading it to compress.
• Combustion Chamber: As its name suggests, this is the section where the fireworks happen. The compressed air from the compressor stage is mixed with jet fuel and then ignited in the combustion section. The burnt gases escape the combustion at a very high-temperature and velocity.
• The Turbine Section: The turbine section has multiple stages that harness some of the energy from the burnt gases coming from the combustion chamber before they are let out through a narrow exhaust cone. The turbine, therefore, drives the compressor as well using the energy from burnt gases.

The exhaust gases after passing through the turbines are released into the atmosphere from the aft of the jet engine via a narrow exhaust cone. The velocity of these exhaust gases generates an equal and opposite force, pushing the airplane forward.

We may have described the basis jet engine for you, but it does not end there. Modern commercial airliners use turbofan and turboprop engines. But what is the difference between the two?

What Happens If You Stand Behind a Running Jet Engine?

Well, you would simply get blown away, literally! The exhaust gases of a jet engine can not only throw an average human quite far away but also burn him while doing that. 

The exhaust of a jet engine normally has a temperate ranging from 500-900 Celcius (900-1700 Fahrenheit), which can easily burn the human skin.

Add to this that the air normally exits at a speed between 600-1000 km/h, (350-650 mph) and you quickly understand the danger! Therefore, each individual working on or near an aircraft must be aware of the safe distance that has to be maintained while a jet engine is running.

What if someone walks in front of a jet engine?

Jet engines have a significant suction power near the inlet even at idle power. Hence, whenever a jet engine is started, a clearance is always taken from the ground to ensure that no one is present around it.

At high-power, the force of suction at the engine inlet becomes so strong that it can pull-in humans from a distance of several meters. No wonder the presence of any personnel is highly prohibited in maintenance procedures, aerodrome regulations, and manufacturer manuals alike.

If you want to learn more about the dangers of working near jet engines, I recommend our article that explains how a human could possibly get sucked into a jet engine.

Do propellers push or pull an aircraft?

Having covered jet engines, let’s look closer at propellers and whether they push or pull. 

But first, we just need to understand the type of engine that typically powers the propeller. In fact, it’s very similar to a typical jet engine and is called a turboprop engine.

TurboFan and TurboProp (Propeller Aircraft): Main Differences!

• A turbofan engine is the technical name of the most common type of jet engine you see in large airliners. 
  
  

  Turbofan engines have a large ducted fan within the engine cowl (or engine inlet). Although this large fan is typically larger than the core engine, it is considered a part of the compressor. This large fan pushes air in the aft direction around the core of the jet engine, which mixes it with the exhaust gases at the and helps generate thrust.

  Modern turbofan engines have a high bypass ratio, which essentially means that a lot of air is let through the engine unburnt.  For example, the bypass ratio of a CFM56-5 engine is 5.5 (84.5:15.5) which means that 84.5% of the thrust is generated by the fan in the front part of the engine, and the remaining 15.5% is generated by the exhaust gases.

• A turboprop engine is a jet engine that’s used to power a large propeller outside the intake of the engine itself. This large propeller pushes the air in the aft direction which generates thrust for the aircraft.
  
  
  
  The propeller of a turboprop engine, unlike the fan of a turbo-fan engine, is not a part of the engine.
  
  On most commercial turboprop airplanes, the propeller has a variable pitch, which means that it can change the pitch to provide more or less thrust, whereas the fan inside a turbo-fan engine has a fixed pitch.

So, do propellers push or pull?

This is where it gets a little tricky. You may be quite right in assuming that the propellers pull an aircraft forward. But imagine what rotates the propeller in the first place: the turbine of an engine which is also releasing high-velocity gases in the aft direction to aid the propeller. So, one can safely say that propellers move the aircraft forward by pushing the air in the aft direction, but are helped marginally by the exhaust gases. 

In any case, the force of suction in front of an engine or the high velocity of air and exhaust gases at its aft can be quite dangerous. When a jet engine is running, we always see caution signs in the front and the aft prohibiting people from coming near it as we have covered earlier.

Summary and Conclusion

• Jet engines provide the most important force required for flight of an aircraft, the thrust.
• Primarily, thrust is generated when exhaust gases are released from the jet engine in the aft direction. These high-velocity gases generate an equal and opposite force that pushes the aircraft forward, enabling the wings to generate sufficient lift for a safe flight.
• To generate thrust, a jet engine must compress the incoming air, then combust it, pass it through the turbine and finally release it though a narrow exhaust cone. These functions are performed in three visibly distinct sections of an engine, the compressor, combustion section, and the turbine.
• Turbofan and turboprop engines are the most common types of jet engines being utilized in commercial aviation today.
• No matter which type of jet engine we may find on an aircraft, it always advisable to stay at a safe distance from it when it is operating.

Happy flying.