What specs/parameters do you need to know to calculate excess power?
Power Available = Thrust X Velocity (TAS)
Power Required = Drag X Velocity
Excess power = Pa - Pr
So, you need the ability to accurately predict thrust from stall speed to maximum speed (a tricky proposition with propellor driven aircraft, especially since thrust changes with altitude, either through changes in engine power or propeller efficiency). You also need to do an accurate model of total drag from stall speed to maximum speed. This is easier to do, but you need to include zero-lift drag, and drag due to lift (and compressibility drag if it rears its ugly head). Drag also changes with altitude due to changes in dynamic pressure, and the parasitic drag and induced drag components change with speed.
So, even if you're merely doing a comparison in level flight, you can imagine how complex this estimation can be. Introduce changing altitudes, and it gets even more complex, since you have to factor in some way to account for changes due to differences in dynamic pressure and engine power and do it repeatedly over some sort of time span.
This is why a lot of people fall victim to over-simplifying by using such metrics as wing-loading and power-loading--because to do the actual analysis required is very time consuming, complex, and sometimes beyond the grasp of most folks aerodynamics knowledge. The best model I've seen on these boards was from DTango, and his response to that epic Brewster thread last year. He mathematically estimated the zoom climb of the Brewster using a Pr/Pa model and posted an Excel graph in the thread. He actually was kind enough to send me the actual spreadsheet used to generate the graph, and it was stunning. Huge spreadsheet involving multiple variables broken down into each second of about a two minute period. Even as detailed as it was, Tango himself will tell you it was a mere estimation--an accurate estimation, but an estimation. I myself have generated a spreadsheet that I can use that will tell me Cdo and Cdi at a given condition for each aircraft in-game, but its dependent on determining an accurate stall speed for each aircraft in that specific configuration (i.e. 25% fuel, 50% fuel, etc.) and does not yet correct for compressibility drag. This was not easy to do, still needs some refinement, and the testing time in-game to determine the stall speed is very time consuming. To do something like this on paper alone (i.e. analyzing an aircraft based purely on its aerodynamic metrics without the ability to test in-game) would take even longer. Just creating thrust curves for each aircraft is very daunting, since propeller efficiency can be such a huge obstacle to model.
This is why everyone on the board should be thankful when guys like Badboy or Tango take the time to actually graph this stuff out, or provide us with tools like the Bootstrap Calculator to do some casual analysis with. Its also why I am continuously impressed with HTCs ability to model all of these very complex calculations in-game. And finally, its the only true reality check you can get to combat all the anecdotal crap that gets slung about on these boards, and from other sources.
Ultimately, there's not one simple equation that takes all of these variables into consideration--its a combination of a lot of formulas and will still only give you an estimation. I don't know if that answers your question or not...
