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.
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