# Why is the force that moves an airplane forward called THRUST and not PULL?

As we know, there are four forces acting upon an airplane. Thrust ($$T$$), Drag ($$D$$), Lift ($$L$$), and weight ($$W$$).

Since most propeller-driven airplanes over history have had the propeller mounted in the front, the power or forces generated were to pull, not to push. So why is it called thrust (the meaning of which is equivalent to push), and not pull??

• How do you pull air? – jamesqf Dec 23 '18 at 19:45

## 2 Answers

Thrust is the force reaction from accelerating a gas from here to over there. If the element creating the thrust, a propeller, is in front of the engine, it's called a "tractor" configuration because it's pulling the engine, like a tractor (any normal front engine airplane). If the prop is at the back, or more correctly, behind the engine, it's a pusher configuration (A Lake Buccaneer amphibian is a pusher even though the engine is amidships, not at the tail, because the prop is on the back of the engine itself). Lots of other designs have put the prop all the way at the back.

In a jet engine, you don't really have an element pushing or pulling the engine itself because the thrust reaction is actually happening inside the engine, mostly in the area ahead the convergent nozzle of the tailpipe where the flow is forced to accelerate. So for jet engines it's just thrust.

Pushing and pulling should be considered in relation to the air, not the engine. For many means of propulsion, there simply is little in the way of any pulling going on. Best example of this is a rocket, all force comes from the push out the back. With jet engines, the inlet often creates drag, it does not suction the aircraft forward normally. Propellers are a bit more tricky in this regard when you examine the math, as the lack of mass addition or significant heating makes the "inlet" and "outlet" conditions conserve a lot better, but it is still the aircraft acting on the air, not the other way around, and the air that has been acted on normally ends up behind, not in front of the aircraft.

• Or, is it has relation to the previously using propeller, ship? – AirCraft Lover Dec 23 '18 at 10:16
• @AirCraftLover I don't understand the sentence you just wrote, but I'm guessing you meant to write: "Or, is it related to an older vehicle that has a propeller—a ship?" – Terran Swett Dec 24 '18 at 2:26
• If you think about it, suction is not pulling air, it is the pressure of the atmosphere pushing air into a place where you've created a partial vacuum. – jamesqf Dec 24 '18 at 5:57