What's wrong with maths ?
Without a heart I couldn't think, but does my heart think ?
Thrust = force.
Take any geometry inlet you like (without the engine behind it), drop it into an airstream and what will happen ? Will it zoom off on its own ?
Take an undriven compressor stage of an engine and put it in the airstream. The airstream will drive it so it windmills. You could get electricity from that with a generator: it's taking energy from the air, slowing it down, and at the same time feeling the drag/push back.
In compressing a gas work is done on the gas be it by a fwd moving nozzle or driven turbine blades. In doing that work you expend energy. If all you have is kinetic energy you will slow down. Drag if you like, but not thrust.
The issue isn't 'maths' its 'concepts'; that's the source of the difficulty. Took the human race ~7000+ years to get to Newton's simple equations (which are actually definitions of the constructs of force and inertial mass), but once they were accepted look what happened in the next 300 years. Those concepts are deceptively simple looking: it's not human nature to think that way.
All these devices heat gas and then extract useful work from it. How that produces 'thrust' (ie a force) is easiest to see for a rocket motor with the distribution of pressure. However, perhaps imagine that the 'inside surface' - engine side- of the compressor blades and the 'inside' surface of the inlet are the surfaces upon which the pressure acts: the increased pressure produced by burning fuel in the gas and heating it, and the pressure produced by driving the compressor stage.
To say that x% of thrust comes from this and y % comes from that only makes sense if the thing is a whole: this and that wouldn't produce the thrust without everything else maintaining the pressure differences.
Forget jet engines and rockets. Get in your car and drive. Put your foot down and accelerate. How ? Must be a fwd acting force on the car. Assume there's no atmosphere (eg near Swindon). where did this force come from ? Must be tyres on road as there is nothing else, agreed ? Take your foot off and you decelerate. How ? Must be tyres on the road - agreed ? One scenario the tyres are a source of thrust, other they are a source of drag.
Hmmm. Given that to avoid slipping the rubber in contact with the road is moving at the same speed as the road, what mechanisms are these that produce thrust and drag ?
Think compression and tension in the tyre at the zone of contact (producing pressure at the interface).....
Last edited by Mr Optimistic; 12th Jul 2009 at 14:40.