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
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.
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