GrooveMonkey wrote:
The light blue section on the right is what Pratt & Whitney refers to as a free turbine. There's no mechanical coupling, just air.
I understand what you're saying about energy loss, but if the work being done is the maximum that can be done/extracted using a given technology, then wouldn't that be the usable HP output. You could say total HP and usable HP.
I know there's no mechanical coupling.
But the problem is that all you are measuring is the power delivered by the shaft, the jet does it's own thing.
Consider the exhaust gases coming out of there.
All the force with which they exit the engine to do bupkiss, would in a jet engine be forcing your plane, car or rubber ducky along.
With power=p, work=w and time=t, p=w/t.
In an engine driving a shaft for power-output, w=t*r, with t=torque and r=radians, for the rotation of the shaft.
Otherwise, work can be expressed as w=F*d, where F ist a force (the thrust of the jet engine) and d is the displacement vector. Or distance for you and me.
So, for a jet engine that stands still, w=F*0=0.
That brings us to p=0/t=0.
But with a jet engine that generates 1 Newton of force through it's thrust, while moving at 1 meter/second, you get
w=1N*1m=1Nm, p=1Nm/1s=1 watt.
The same engine, generating the same force at 20 m/s would generate w=1N*20m=20Nm, p=20Nm/1s=20 watt.
Because a shaft that just spins and only works the innards of the engine does no work for you, so all the spinning innards are inconsequential to the engines actual power output (well, not inconsequential as such, just from the point of reference where you want to know what the engine as a whole does), and an engine that sits still does no work and the power you get from something you drive off the echaust gasses of that engine depends as much on the nature of what you blow at as it does on the engine.
I know I'm picking nits here, but I get a massive headache whenever I hear some TV "science presenter" talking about a reaction engine like a jet or a rocket having so-and-so-many horsepower, likewise when I read it. Just because we once used engines that actually did work (as in doing work, rather than just working) without producing thrust, and the rotating shaft is what most of us are familiar with as the source of power, everything must be expressed as such, when that is NOT a valid, scientific comparison.
I wait for the day when one of those greased, handmade-suit wearing dildos actually gets orders from his editor to challenge the audience's ability to understand something more complicated than an egg-whisk. Because I think quite many will, and look at the world in a slightly different way because of it.
If there were absolutely anything to be afraid of, don't you think I would have worn pants?
I said I have a big stick.
