Thrust is a force ie it shoves things along. Force equals Mass x Acceleration. The more force you apply the greater you can accelerate a mass. Alternatively, for the same rate of acceleration you could move a larger Mass. Energy is used when a force is applied to make something happen eg accelerate
If you achieve something by applying a force eg move an object then you have done 'Work'. Work equals the Force x Distance.
Power is the term used to describe how quickly Work is done ie Work divided by Time. Think of it as the rate the energy is used to do something.
Thrust will have been needed to accelerate to the speed but without drag the thrust will have to be removed to maintain the desired speed. Alternatively, if there is a counteracting force (Drag) the acceleration will reduce to zero ie maintain a constant speed, once the counteracting force is equal in magnitude to the 'desirable' force (Thrust).
Accelerating something is a way of transfering Energy into it. The faster the object goes, the more energy it now contains. The amount of Energy tranferred increases by the square of the speed ie double the object's speed and you quadruple the energy stored within the object. That's why even reducing a crash speed by a few knots makes a significant difference to the damage done. The energy has to go somewhere during the sudden stop. It gets used up in deforming the metal, deforming the passengers & other contents, noise, heat etc etc
The faster you can transfer this energy into the object - the more power you have - then the sooner the object will reach its final speed. Increased Power lets you do this by allowing you to increase the size of the Force used to propel the object.
With less power available you could only apply a smaller Force. You would eventually get to the target speed but it would take a longer time & greater distance.
Since Drag is an increasing factor eventually the two forces would balance and IAS (actually EAS but for simplicity's sake...) would be constant.
To go faster you would have to provide more power to supply the force needed to accelerate once more. Remember though that the final 10kt increase in IAS requires a LOT more power than the first 10kts (the energy proportional to the square of the speed again).
The thrust needed maintain speed is proportional to Drag. Drag, in turn, is related to IAS.
Power required to do provide this Thrust is proportional to TAS. At S.L. ISA, IAS is the same as TAS but as altitude is increased TAS increases for a constant IAS.
The result is that more power is needed to maintain a given IAS at altitude than when maintaining the same IAS at S.L.
Hope my ramblings haven't confused you further!
T.
Last edited by Tinstaafl; 19th June 2002 at 18:12.