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

Question

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

Answer 1

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.