Turbofan question; high subsonic vs low supersonic bypass ratios...

Factual Questions
1

I’m going through some past papers, and I’ve come across a question that’s stumped me.

I’m good on gas turbine analysis, and quantatively working things out, but this question seems a bit vague to me:

Consider turbofans for high subsonic Mach number and low supersonic Mach number propulsion. Which would have the higher bypass ratio, and why?

Where the bypass ratio is Mdot[sub]bypass[/sub]/Mdot[sub]core[/sub]

I understand that high subsonic propulsion generally means civilian or cargo transport, where an effecient cruise is the most important part of engine efficiency. So a high-bypass ratio turbofan is ideal.

Also, low supersonic propulsion generally means military aircraft, and certainly in fighter aircraft a large radius-of-action is required and massive thrust suddenly required during dogfighting. A low-bypass ratio turbofan is ideal here, as it gives good fuel efficiency during cruise, and the response from the engines that is required when moving to unsteady flight.

So have I answered the question there? It seems to me that the question wants an answer based on speed alone, and not the types of aircraft associated. I’m not too sure what to put if that’s the case.

Gas turbine/Thermodynamics/Aeronautical dopers…can you help?

2

Mostly. Consider frontal area as well - power goes with its square. A fighter engine needs to spool up and down pretty quickly, so a lower-polar-moment rotor is an advantage.

The biggest factor, though, would be propulsive efficiency, which is just the ratio of the average exhaust velocity to the ambient air. Ideally, you’d like the exhaust to just sit there relative to the outside air, and any extra velocity is just wasted. A high-BPR engine has a lower average exhaust velocity and loses propulsive efficiency at higher speeds, where lower BPR’s are closer to optimum.

3

Thank you ElvisL1ves, I understand this properly now.

I had wondered if I’d posed too specific a geeky question, even for the dope, but this place is still here for me when it matters.

Onto the quantative stuff now, turbofan analysis using isentropic efficiencies and all that jazz. I prefer that stuff.