Originally posted by mold
Yes I believe we can, because these things should already be accounted for in the flat plate area. For our purposes, total drag is ultimately the only thing that matters--not Cd and not flat plate area, except when we can use such things to calculate total drag. FPA is just a ref number, and Cd is calculated from that. No extra information here, really.
My understanding is that FPA is nothing more than the calculated coefficient of drag X wing area. For the P-51D, this is .0176 X 233.19 = 4.14 FPA. Since the Merideth effect requires heat, and there is no heat without the engine at operating temperature, it is not accounted for in an FPA calculation.
There is another factor to be considered as well. As airspeeds rise above 0.6 Mach, drag rise increases dramatically. Dean offers an excellent example of the advantages associated with the Mustang's laminar flow wing. He compares the the drag rise between a conventional wing (a P-39N with NACA 0015 root airfoil and NACA 23009 wing tip airfoil) and that of the P-51D. As the P-39's speed rises above 0.62, drag rise begins and the Co reaches .050 at approximately Mach 0.77. On the other hand, the P-51 doesn't see a significant drag rise until well beyond Mach 0.7 and finally attains a maximum Co of 0.044 at Mach 0.83. So, when the P-39 has a Co of 0.050 at Mach 0.77, the Mustang's Co is just 0.0205 at that same speed.
We know that the Bf 109 used a conventional airfoil (anyone know which NACA profile was used? I do know that Kurt Tank used the same NACA profile for the 190 as Vought did for the F4U). Therefore, we can assume that the onset of significant drag rise would have occurred at a lower speed than would be seen with the P-51D. Precisely at what speed this happens I don't know, but it seems to me that the Co of the two aircraft will converge as the speeds increase and in all likelihood, the 109's will rise much higher than the Mustang's at Mach 0.7 and above.
Perhaps this is one clue as to why the P-51 attains similar max level speeds on much less horsepower.
As to the initial level acceleration debate; this can be calculated if you know the total thrust available and total drag. Dean gives us a useful explanation in his book (page 117-118).
The calculation is: Total thrust minus total drag divided by mass (weight/32.2). This provides an acceleration rate in feet per second per second, and you can divide that by 32.2 to obtain a constant G value.
Hypothetical example: You have a fighter weighing 8,000 lbs. It has 2,000 pounds of thrust and 750 pounds of drag.
So, 2,000 - 750 = 1,250 net pounds of thrust.
8,000/32.2 = 248.45
1,250/248.45 = 5.03 feet per second per second initial acceleration, or 5.03/32.2 = .156 G
So, if we know the total trust and total drag of the Bf 109K-4 and P-51D, we could calculate acceleration and end the debate. However, the caveat is that this calculation is only valid for speeds below the speed where dramatic drag rise is encountered because the total drag is rapidly increasing.
It seems to me that the 109 will have considerably better initial acceleration than the P-51D at low to moderate speeds. However, it also seems that this advantage probably disappears as speeds rise above Mach 0.6 and greater.
Well, those are my thoughts anyway..
My regards,
Widewing