# Relationship Between Aircraft Thrust and Jet Exhaust Velocity? [closed]

What is the relationship between aircraft thrust and jet exhaust velocity & thrust and mass flow.

Actually, my question is how to reduce or decrease jet noise without decreasing the thrust ?

• Welcome. You have two different questions, which are both broad. You should split your post into two different ones with appropriate titles and detail with type of engines your are asking for. There are already answers on the site, please explain what is missing, to get better answers, e.g. Why does the Boeing 787 engine nacelle exhaust have such an unusual shape?
– mins
Feb 18, 2016 at 7:45
• And, you question appears to be essentially a duplicate of your last one. It would fit with the flow of SE much better to have edited your original question, which would spawn a series of comments helping you refine it to get it reopened. Feb 18, 2016 at 13:58
• Though this post is a duplicate of the original, it does have an answer, as opposed to the (closed) original. Perhaps this question aught to be left open.
– J W
Feb 18, 2016 at 15:38

## 1 Answer

The thrust produced is propotional to the exhaust velocity and mass flow, in simple terms. The thrust equation can be written as,

$T$ = $\dot{m}_{e} v_{e} - \dot{m}_{o} v_{o} - (P_{e} - P_{o}) A_{e}$

where, $\dot{m}$ is the mass flow rate $v$ is the velocity, $P$ is the pressure and $A$ is the (exhaust) area. The subscripts 'o' and 'e' denote the freestream and exhaust values respectively.

There are multiple ways to go about noise reduction in aicraft engines, all with some kind of penalty- the Boeing method is to use chevron shaped exhausts (Interestingly, the type chosen is the one with lesser thrust penalty, not the most effective in noise reduction). However, Airbus has cited higher fuel consumption as the main reason for them not using it. Airbus uses acoustic liners in the engine to reduce noise:

The thrust reverser structure was acoustically treated along approximately half of its internal surface. It was clear that increasing this acoustic area would increase the acoustic sound absorption and hence decrease the overall measurable exhaust noise.