Hi all, This has been bugging me all day, so I thought I'd seek the help and advice of the internet. When a plane is stationary, you’d have to say that its jet engines are doing no useful work. The engines apply a static force on the airframe and could be said to be running at zero efficiency (ignoring the fact that they generate shaft power and compressed air for the plane’s auxiliary systems). I guess this means that the hot exhaust gases ejected from the rear of the engine lose all of their energy through heating up the surrounding atmosphere (entirely plausible). Now consider a plane taxiing at a slow velocity at the same engine speed (rpm). The engines are doing useful work, and the propulsive power it is producing could be said to be the product of the plane’s velocity and frictional force the plane’s airframe sees; P=FV (in the tyre-tarmac interface and drag). What causes this dramatic increase in efficiency? Mechanically, the same scenario is easy to visualize and rationalize in a car. When the engine is decoupled from the drivetrain (in neutral), the car revs freely, and not much air and fuel (accelerator pedal excursion) is required to redline the car. Here, the work done by the engine is used purely to overcome piston friction and other mechanical losses. When the car is coupled to the drivetrain, much more air and fuel is needed to speed the engine up as the energy is consumed by accelerating the vehicle. From the jet engine’s point of view, the only thing that has changed is the velocity of the intake air. When this intake air is no longer stationary, the engine seems to do useful work. It is hard to visualize how this works. A simpler related problem involves the propulsion of an inflated balloon. If the balloon is held static while it deflates, all of its energy is lost in heat and noise. By simply allowing the balloon to be unrestrained, and hence achieve a non zero velocity relative to the surrounding air, the balloon seems to do useful work in the form of giving itself kinetic energy. How can simply allowing the balloon to be unrestrained so dramatically increase its propulsive efficiency? Imagine the scenario from the jet engine's or balloon's point of view. All that has changed is the ability to propel yourself forward, but you are still applying the same force you were applying when restrained. The implication is that when unrestrained, the environment or surroundings aren't as hot as a result of your jet engine or balloon exhaust, because some of this energy has gone into the plane's/balloon's inertia. I can accept this, but I don't see how restraint (or lack thereof) can cause the exhaust to reconfigure itself magically in this fashion. It would seem at first glance that this lack of restraint allows the jet stream to create less turbulence and more ordered work in the form of propulsive thrust.