Hi Keith,
Please could you clear up some doubts in my mind?
Quote:
The first type is THP which is used to push the aircraft forward against the drag.
I thought we simply called that "thrust"?
OK, then I will refine the statement a little. THP is the power that we use to carry out the work of pushing the aircraft forward against the drag at our chosen aircraft speed.The second type is the power that is wasted in propelling the propwash rearwards.
Quote:
The second type is the power that is wasted in propelling the propwash rearwards.
I thought that is how we generate thrust - by accelerating a mass of air backwards - why would it be considered wasted if it generates something useful?
The only waste I can see is considering all of this with the brakes on, so there is no useful acceleration of the aircraft.
We often say that we create thrust by accelerating air rearwards. It would be more true to say that we create thrust by exerting a rearward force on the air. The air then exerts a forward force (the thrust) on our propeller in accordance with Newton's thrid law.
Unfortunately the air is a fluid, so it is not rigid enough to resist the force that we apply to it. This causes it to be accelerated rearwards. This in turn gives it rearward velocity. In giving it rearward velocity we have given it kinetic energy. This represents a waste of energy in that we are never going to get it back nor are we getting any benefit from it.
If you cannot see the difference then let's consider a man in a boat in a shallow canal. He has a long oar with which he can row the boat along or push it along by pushing against the bottom of the canal. The canal bottom and walls are lined with concrete.
If he pushes against the bottom of the canal, the canal bottom will resist this force so it will not move. Only the boat will move, so all of the energy that he uses goes into the boat to give it velocity. None of the energy is being wasted in moving the canal bottom rearward.
But if he chooses to row, then some of the energy will be wasted in accelerating water rearwards and some will go into moving the boat forward. This wasted energy is the equivalent of the energy that we waste when our propeller accelerates air rearwards.
Pushing against the bottom will clearly require less energy (and hence less power) than rowing.