# How airplane’s reverse trust makes sense? (On jet engines, not controllable pitch propeller ones)

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While I understand the mechanical part of the turbine which alternates the airway and redirects the sucked air back to the front via the reverser doors, physically it doesn’t make sense to me. Similarly with sea vessels that work with waterjet systems and reverse using a deflector which basically does the same thing (in marine propulsion systems).

The forces between the thrust from the propeller, and the deflector which the air hits should counteract each other. To me, it’s like you’re trying to stop a boat with sails, using a leaf-blower on board. The blower’s directed air hits the sails from the front, but the fan itself sucks air from the opposite direction (on a theoretical leaf blower that sucks air from the back, not the side).

Adding the forces from the propeller (which pulls the turbine) and the redirected air (which pushes the turbine back) should result in equilibrium (if not positive because the redirected air is going back to the front in an angle, not straight from the direction that was sucked).

This question arose after studying marine waterjet systems, and how they achieve reverse thrust, which reminded me of the airplane’s reverse system. Surely it is possible that I did not fully comprehend the principle of operation of both systems, or I wrongly thought they are similar, so please feel free to correct me if my understanding is incorrect 🙂

In: Physics

You have to consider the entirety of the airplane to be one singular object, and then think of only the final velocity of the air relative to its initial velocity; anything else simply cancels out. You could replace the reverse doors with a U-shaped pipe and nothing would change.

Effectively, what you are left with is the air intake (which causes a small amount of forwards thrust) and the reversed air exhaust (which causes a large amount of reverse thrust). The fact that the air has to make a U-turn doesn’t matter since the thrust of the engine (forwards) and the thrust against the reverse doors (backwards) effectively cancel out.

What you typed doesn’t make a whole lot of sense. Redirecting the exhaust forward redirects the thrust forward. It’s not any more complicated than that. You seem to think that engine air intake “pulls” the plane forward the same amount as the exhaust pushes it forward, but that’s not how it works. The thrust comes from the exhaust exiting the back of the engine pushing it forward.

> The forces between the thrust from the propeller, and the deflector which the air hits should counteract each other.

If the forces were “counteracting” each other then you would expect the air coming out of the deflector to not be moving. But that isn’t what a thrust reverser does, a thrust reverser reverses the direction the air is moving. Taking a volume of air from zero to velocity V is going to provide 1/2 the amount of ‘push’ that taking air from V to -V will provide.

Aircraft do not get “sucked” forward, nor do they get “sucked” upward. The pressure and force of the air movement coming out of the engines propel it forward. Blasting that air in any direction will propel it in the opposite direction. Similar to how the pressure on the bottom of the wings push the plane upwards. Your thinking is correct, you just have it backwards.

A jet engine isn’t the same as a leaf blower (or ducted fan), since fuel is added to the combustion chamber. This fuel is added to the compressed air and burnt causing the air to expand. The expanding air also creates a force pushing the jet forward.

But it’s not just the added fuel that causes thrust. It’s the acceleration of the air as well.

Think about trying to propel yourself forward by throwing ball backwards. You can reach forward and pick up a ball off of the ground, and then throw it backwards to accelerate yourself. And when you want to stop, throw the balls the other direction. The picking up the balls isn’t affecting your speed much, it’s mostly the throwing.

Same with a jet engine, while there is some suction, the amount the air is being accelerated into the engine is a lot less than the air is being accelerated out the back.

If I throw a ball then I transfer a certain amount of momentum to that ball. Specifically the mass of the ball multiplied by the change in its velocity (which is final velocity minus initial velocity of zero). If the ball then sticks to a wall then it transfers the same amount of momentum to the wall (because the magnitude of the change in velocity is the same).

If instead the ball bounces off the wall then the change in the ball’s velocity is double, so the change in momentum of the ball is double. Momentum is conserved, so the momentum transferred to the wall is also double.

ruster. The engine imparts some momentum to the air, but then twice as much momentum (or slightly less) is transferred to the thruster plates. So the net momentum imposed on the aircraft is backwards.

The deflector counteract the engines if the fluid move at a velocity of zero in the direction the engine accelerated it. That would require the fluid to change the direction by 90 degrees and symmetrically. If you change the direction so it moves in reverse the force to do that will be higher than what the engine applied. If perfect change direction 180 degrees the change of speed is double the speed the fluid exited the engine.

Let’s assume the fluid is not moving when it gets into the engine so simplify things.
You can look at the force but it is simpler to look at momentum,

p=m*v where p is the moment m is the mass and v is the velocity. Both p and v are vectors so let the positive direction be where the fluid motion with no reverse thrust.

The engine will accelerate the fluid to a speed v and the momentum is p.

When the reverse trust is activated ideally the fluid move in the other direction at the same speed. So the velocity is not -v and the momentum is -p

So the part that changes the direction of the flue change if from v to -v that is a change if -2v the same for the momentum p to -p is a difference if -2p.

So the engine will get a momentum of -p from the acceleration of the fluid initially. The momentum of the engine is the opposite of the momentum of the fluid because momentum always conserved, this is what Newton’s third law us about.

The part that changes the direction will get a momentum of 2p when that is the reverse of the fluid change of -2p.
The net result is -p from the engine and 2p from the reverses and the sum is -p+2p=p

So if you perfect change the direction you have the same force forward as you had backward before the change. In practice, a jet engine will have some moment to the side of the air so the force is less.
For water where the mass of the reverses is a problem like for a aircraft, you could have the same force in any direction.

I want to thank all of you for talking time to answer my question! With your explanations I did manage to understand how airplane’s reverse system works 🙂 I did use some wrong terminology there and completely overlooked the combustion process, where I only included the engine’s fan forces, but you guys were extremely helpful!

Follow the molecules; specifically the collisions. So long as they’re “in the system”, the force from the collisions is netting out to zero, like the air in a balloon. You’re interested in their delta-V from when they enter the system (first collision) to when they leave the system (last collision). That delta determines the net force each applied to the system. So a molecule enters the system at a very high speed (the engine is the frame of reference), but after it bounces off the deflector it leaves in the other direction. (Without the deflector it leaves in roughly the same direction it entered but faster.)