Discussion in 'General Chat' started by HippoCrushEverything, Mar 8, 2017.

1. #26
Power to weight ratio of electric engines vs turbines looks favourable to me atleast on small scale hobby engines, probably not on full size aircraft engines.
For example a 9kw jetcat gas turbine weighs 2.8kg and about 1.5kg in turbojet form. A tp power electric motor can deliver 9kw continuous, 18kw peak for 1.2kg, perhaps 1.6kg including controller. So the question would be if you could make a lightweight compressor stage that is closer to a ducted fan in terms of manufacturing difficulty and cost. And secondly if you could exploit the lack of a turbine to run a more efficient fuel air ratio or achieve a better compression ratio than the single stage radial compressors found on hobby jets.

2. #27
Assuming that there's infinite free electricity coming to the motors turning the compressor stages.

Still, how does one calculate thrust and propulsive efficiency for that sort of thing? Is thrust in this case simply mass flow of air * (jet exhaust-intake velocity) * mass flow of fuel * jet exhaust velocity? At what sort of airspeeds does pressure thrust come into play?

3. #28
Where did you get that 9kW figure? I thought their jets (and everyone else's) were rated by thrust.

4. #29
Last edited: Mar 11, 2017
Its simpler than that. Usually its just the mass flow rate times the velocity difference; the fuel flow rate is usually negligible compared to the air flow rate, so it's usually ignored. Even on full military power and afterburner, when an engine like the F16's GE F100 is sucking down 10,000 pounds of fuel an hour, the same engine is probably moving about a ton of air a second or more. Although if you wanted to include it, it would it would be a second term added to the first, not multiplied by: \dot{m_air}*(u_e - u_i) + \dot{m_fuel}*(u_e)

Pressure thrust doesn't come in according to airspeed, but exhaust speed. In an ideal turbojet, pressure thrust should be zero always, but that is seldom the case in practice. It has to do with the choking of a nozzle, or otherwise non-ideal expansion. In a compressible fluid like air, when you force a flow through a simple nozzle (just a contraction), the flow will eventually reach a point where increasing the back pressure does not increase the velocity of the fluid. This is called a choked flow, and occurs at a velocity of exactly mach Ma=1. It's worth noting that increasing the backpressure does increase the mass flow rate, just not the velocity. At this speed there is still unused pressure at the nozzle exit, which you have to include in your calculation of thrust. With a conventional nozzle, a jet will choke at an exhaust speed of Ma=1.

Avoiding nozzle choke is the reason for the use of the de Laval nozzle on high-speed jet engines like rockets, and some afterburning turbofans and turbojets. However, a de Laval nozzle can still suffer from non-ideal expansion when its operating well off of design-point. Air-breathing supersonic nozzles require variable geometry to cope with changes in airspeed, and the exact speed where the nozzle would choke would depend on how far the nozzle is able to actuate. For instance, the main aft leaves of an afterburning turbofan can usually either act as a traditional contracting nozzle (ignoring the converging-diverging section upstream), or as the aft-portion of the diverging section of a de Laval nozzle. How well these things deal with changes in airspeed is a closely guarded secret, however, so I wont wager a guess at numbers.

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5. #30
Nicely explained, didn't have to check wikipedia even once

You should be like a teacher or something

6. #31
Hey VanIce, any advances in fusion?

7. #32
Yes, he does his daily commute by a levitating tokamak.

8. #33

9. #34
Interesting.

10. #35
There could be major breakthroughs as early as 2015.