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

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.