# Mph jet engine reaches 2 atmosphere 29.4 psi

1. Mar 22, 2015

### gary350

As a jet engine travels through the sky and picks up speed ram air increases the air pressure at the air intalk of the engine. How many mph does the engine have to be going for ram air to 2 atmospheres = 29.4 psi?

At what speed in mph = 3 atmospheres?

4 atmospheres?

Is this a linear or nonlinear increase in air pressure vs mph?

2. Mar 23, 2015

### Staff: Mentor

3. Mar 23, 2015

Bernoulli's equation would predict a velocity faster than the speed of sound, it's safe to assume any such situation is compressible and therefore not subject to Bernoulli's equation. The real issue here is that there isn't nearly enough information to answer such a question without making some major assumptions.

If the compression is isentropic, then the flow would have to be moving at something like Mach 1.04 to achieve double the atmospheric pressure, Mach 1.37 to triple the pressure, and 1.56 to quadruple it. That's just form isentropic stagnation, though, and discounts the formation of shocks, which are quite likely to occur here.

4. Mar 23, 2015

### Staff: Mentor

Bernoulli's equation can include a term for compressibility and that's discussed in the wiki.

But yes, the bigger problem is what the highest pressure you see is before reaching supersonic or choked flow.

Either way though, IMO one should start at zero and work their way up on this issue. That at least partially answers the last question.

5. Mar 23, 2015

I edited my previous post to include the Mach numbers required to achieve those compressions assuming the flow is isentropic, and they are pretty clearly supersonic. The compression ratio to achieve sonic flow in air is $p/p_0 \leq 0.528$, and the OP is asking about 0.5, 0.33, and 0.25.