Originally posted by F4UDOA:
Actually Gents,
Based on that chart the best numbers for performance are the lowest ratio's.
1. P51= .146
2. F4U= .156
What is being measured is the ability of the A/C to convert HP into staright ahead speed. It appears the US planes are more efficient at this.
This is an interesting exercise, but we need to look at power loading too. Of the major American fighters, the lowly FM-2 has the lowest power loading at sea level (5.94 lb/hp). The F6F-5 and the F4U-1D come next. However, once we get to 4,000 feet, the P-38L (6.21 lb/hp) has taken over the lead position, and maintains it through 30,000 feet. The P-47D comes in a solid second.
The P-51 lags well behind the P-38 in power loading at all altitudes, and suffers greatly in comparison above 25,000 feet.
The P-38L was the best accelerating fighter in U.S. service, even better than the P-47M.
Other American iron
1. P38L
Thrust = 2782
Thrust to weight = .159
2. P-47D-30
Thrust = 2261
Thrust to weight = .155
If you are considering thrust to weight ratios as important, you're correct. However, there is one factor that is missing. Drag coefficients. When these are factored in, you get something called the performance index. A performance index above 4.5 was considered very good. The performance index may be an excellent method for comparing aircraft in that it considers hp, thrust (and consequently, total drag), weight and the coefficient of drag. I don't know who came up with the Performance index, but it makes comparisons easy. In addition, if you know the weight, horsepower at best speed and that speed, you can estimate the coefficient of drag fairly accurately. This method was suggested to me by an engineer at Dryden, who claims that this is the great "de-liar" of performance numbers. I have tried it on several aircraft and it seems to bear out known performance numbers.
Perhaps, some of you kind folks will try it and see if works reliably on different aircraft than I used.
I am using the typical combat weight on max internal fuel. Based upon 3,200 hp, the P-38J should generate no more than 2,253 lbs of thrust. Factor in its weight of 17,699 lbs, and the P-38 should have a thrust to weight ratio of .127 lbs thrust per pound of weight.
Let's do the same for the P-51D, allowing for 1,280 hp @ 25,000 ft, where it makes its best speed (437 mph) This produces 879 lbs of thrust at its maximum speed. 879/10,176 = .086 or .086 lbs thrust per pound of weight.
How about the P-47D? 2,200 hp @ 32,000 ft produces a speed of 426 mph. This generates 1549 lbs of thrust or .107 lbs thrust per pound of weight.
Finally, let's plug in our old friend, the P-40. In this case, let's use the P-40F with its Merlin. With 1,065 hp @ 20,000 ft, we can calculate that it generates 878 lbs of thrust. This calculates to .101 lbs thrust per pound of weight. That's better than the Mustang. Odd isn't it? Well, let's figure in each plane's drag coefficient.
P-38J: .127/.0270 = 4.70 P.I.
P-51D: .086/.0176 = 4.88 P.I.
P-47D: .107/.0226 = 4.73 P.I.
P-40F: .101/.0242 = 4.17 P.I.
Let's do a few more.
P-63A: 1,072 hp @ 25K. 410 mph. 784 lbs thrust. .087 lbs thrust/lb. .087/.182 = 4.52 P.I.
F6F-3: 1,725 hp @ 20K. 376 mph. 1,376 lbs thrust. .112 lbs thrust/lb. .112/.0260 = 4.29 P.I.
F4U-1D: 1,795 hp @ 20K. 417 mph. 1,291 lbs thrust. .107 lbs thrust/lb. .107/.0239 = 4.47 P.I.
N1K2-J: 1,230 hp @ 18K. 369 mph. 1,000 lbs thrust. .112 lbs thrust/lb. .112/.0263 = 4.26 P.I.
F4F-3: 910 hp @ 21.1K. 330 mph. 827 lbs thrust. .103 lbsthrust/lb. .103/.253 = 407 P.I.
So, we have calculated the performance index for the above aircraft as follows:
P-38J: 4.70 (426 mph)
P-51D: 4.88 (437 mph)
P-47D: 4.73 (428 mph)
P-40F: 4.17 (364 mph)
P-63A: 4.52 (410 mph)
F6F-3: 4.29 (376 mph)*
F4U-1D: 4.47 (417 mph)*
N1K2-J: 4.26 (369 mph)*
F4F-3: 4.07 (330 mph)
*The Cdo for the F6F-3 and F4U-1D are an average of two different CDo numbers reported. The CDo used for the N1K2 is an estimate based upon known top speed and combat weights. It is probably very close.
My regards,
Widewing