1. Consider a Jet engine thrust reverser, shown below. The density is 1.15kg/m^3

Question

1. Consider a Jet engine thrust reverser, shown below. The density is 1.15kg/m^3 and the inlet area is 1.2m^2. The pressure forces can be neglected.

a) When the reverse air flow rate is zero. What is the normal thrust?

I calculated this problem off the basis that my Vout would have to change if my reverse air flow rate is zero. However, my professors solution was based off using a Vout of 450m/s. My professor said that "Vout is the same in both situations, could possibly be regulated by some internal engine...." But is that even possible?? Vout HAS to change if there is an increase in pressure from sealing off the Vrev openings..

Mass flow rate in = mass flow rate out

MVin = 2Mvrev + Mvout

If MVin stays constant, then Mvout has to increase by 2Mvrev.

I am almost 99% positive that my professor is wrong on this. I got a very low score on this b/c I did not assume that Vout = 450m/s. I believe the only way Vout can stay at 450m/s is if Vin decreased. Which is possible. However, I dont believe that this problem can be analized with the given figures.

Explanation / Answer

Part B definitively tells you that it exits at 450 while reversing also, so obviously you cant say otherwise. Note that area isnt given for the reverser or outlet, so the transition from all forward to 80% reversed would be accomplished by opening the reverser area (which would start at 0) and closing the rear exit area. But knowing either area is unnecessary since you can find mass flow rate in, and know what percentages go which direction at what velocity.

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