Can thrust be produced only using the forward speed of a nozzle?

Question

Why can't a nozzle that is already in relative motion to the air produce thrust by speeding up the air that passes through inside it?

So let's pretend you're in a glider with two converging nozzles (without fans or burning fuel), one attached to each wing. Shouldn't the air passing through speed up due to continuity, and provide a forward force on the plane?

Obviously this does not work in real life so I would like to know why. Is it because the bottom an top surfaces essentially function as a diffuser, which slow the air around?

Answer 1

Something has to provide the energy that is needed to accelerate the air.

In the simplest combustion engines this is the combustion heat. Air expands when heated and when the route out the back is the easiest to take, it will flow that way. More complicated engines use this heat energy to raise the pressure of the air inside the nozzle so there is excess pressure energy that can be converted into speed.

In your converging nozzle there is no excess energy available. In order to stream out the back, the air must have at least ambient pressure when leaving the nozzle. In order to speed up, the pressure ahead of the nozzle must be higher than ambient. But since the air enters the converging nozzle already with ambient pressure and flight speed, it cannot accelerate any more. Instead, the face of the nozzle will block all the air that will not fit through the smaller exit cross section, forcing it to flow around your nozzles. This is almost as effective for producing drag as putting a solid disk into the air stream.

Answer 2

It does not work for a couple of reasons.

1. friction on the surfaces creates extra drag
2. a diffuser will lower the pressure, and you're not adding energy to the stream. As a consequence the air going through the diffuser is not capable of overcoming the backpressure exerted by the sorrounding air, leading to a lot of turbulence and no thrust.

So you're left with a dead weight that increases drag and does not offer any benefit.

If it would work, you would have created energy from nothing, and perpetual motion machines cannot exist.