Originally posted by HoHun
Hi Badboy,
The document you mentioned may have some significance, but I've got to admit that I don't get it :-)
Regards,
Henning (HoHun)
Hi HoHun
Firstly, let me apologize for the following verbosity… I know I could explain this to guys like yourself and HiTech much more briefly than I’m going to, but I’m going to take the opportunity to get back to basics in the hope I can also open the subject to some of our less technically minded friends. Here goes…
It seems to me as though there are two different discussions here, one regarding best climb rate, and the other about best energy transfer rate, and they are absolutely not the same thing. Let me see if I can explain it a little better. Being in the best possible situation for an engagement, isn’t just about getting as high as possible in the shortest time, if it was, we could simply use our maximum climb rate to get as high as possible as quickly as possible and the problem would be solved. However, there are four vitally important energy related characteristics that need to be optimized to give a pilot the best possible chance of winning a fight, of which the climb rate solution only nets two of them, so in effect, solving the best climb rate problem only gets the job half done. The theory for finding the schedule for best energy transfer depends on optimizing the Es and Ps at the same time, so I’d better explain those terms:
Specific Energy:
Total energy by itself is not an accurate measure of maneuverability because of the inertia associated with weight. A B-17 weighing 40,000lbs may have more energy than a P-51 at 10,000lbs but the P-51 is clearly more maneuverable. A more accurate indication of maneuverability is the total energy divided by the gross weight to determine the energy per pound of weight, this is called the Specific Energy and is given by the expression Es = V^2/2g + h where V is the speed, g is gravity and h is the altitude, and is measured in units of length (feet say). Notice that the velocity is a significant part of this expression, having the best Es is not just about getting as much altitude as possible in the shortest time, it is about getting the best combination of speed and altitude. But that’s not all because getting to a fight with more altitude than your opponent isn’t any good if you don’t have the maneuverability to capitalize on it, which leads us to:
Specific Excess Power
The rate of change of energy with respect to time is called power, but once again, our B-17 with its four magnificent engines has more power than our P-51, but it still isn’t more maneuverable, so once again we are primarily interested in specific power, which is power divided by weight. Of course we are also only interested in the useful power, free to be used to maneuver, and this is called the excess power, that power left over once drag has been overcome. So we arrive at the specific excess power, or Ps and it is given by the expression Ps = V/g dV/dt + dh/dt which is the velocity divided by gravity, times the rate of change of velocity with time, plus the rate of change of altitude with time.
So, putting all this together, we need to find a way of optimizing four terms all at once, they are the four terms that comprise the Es and Ps expressions above. We need to find the best possible combination of velocity, rate of change of velocity, altitude and rate of change of altitude, all at the same time! It is the fighter pilots equivalent of nirvana, the mother of all sweet spots!
By climbing to an engagement at the best climb rate, you are only optimizing two of those terms, the altitude and the rate of change of altitude, but that’s only half the story… it simply won’t cut it. If you ignore the other two terms, you may arrive at the fight with a little more altitude, and a better climb rate, but you simply won’t be in the best energy situation for the ensuing engagement, you can only find that sweet spot by optimizing the Es and Ps together and those considerations push up the airspeed beyond that required for the best climb rate alone… That’s what I’ve tried to illustrate in the diagram I posted, I have never been talking about just getting as high as possible as quickly as possible, even though that can be a simple and effective strategy to use in the arena if you have a plane that can do it really well (the G10 springs to mind) but in terms of correct air combat, it simply isn’t the right thing to do, as almost any current fighter pilot will confirm.
Hope that helps…
Badboy