# Compressing the air thrusted by an aircraft's propeller through a cone like tube

If I had an aircraft that used propellers for movement and took the air from the front running it through a tube that gets very small towards the end, would it have any impact on the speed?

Would the aircraft get faster because the ejected air would be faster then when it got in, similar to a waterpipe that is half closed?

Would it be slower because the compressed air would act like a barrier for the new air that comes in?

Or would the aircraft have the same speed regardless of whether we run the air thrusted by the propeller through our cone or not?

Here is an image with what I mean:

## 2 Answers

It would be slower.

Compressing the air takes energy - that will slow the propeller, and the plane as a whole.

Imagine a water pump, which is pumping water out of a pipe. Now, if you put your finger over the end of the pipe, the water speeds up, but the pump impeller will slow down, and, as a result, the total mass-flow rate goes down.

The speed of the plane is linked to the mass-flow rate of air going out the back, since it is propelled by the equal/opposite reaction force according to Newton's Third Law.

• But wouldn't the propeller force the compressed air out through the back? If not, why do jet planes have this system at the exhaust pipe? Jun 7, 2018 at 22:29
• Jet engines use the compressed air to explode fuel more efficiently, and so the compression is actually useful to the engine - the majority of the thrust in a commercial jet engine comes from uncompressed air flowing around the central turboprop. In your example, the compression is not useful, and will cause losses. Jun 7, 2018 at 23:35

This is done in a jet engine, and partly, by the cowl around the front fan on a jet airliner engine, but on a large propeller, you could only do it slightly without slowing the airflow through the propeller. The extra weight would outweigh the benefits.