P-51 Cruise Speed = 395 mph TAS

Originally Posted by wmaxt

I know thats the theory but it doesn't show in the numbers - it still takes virtualy the same hp/lbs to go the same speed.

Actually that's not true. The Spit IX with the same engine as the P-51B and a slightly lower coef. of drag (in the wind tunnel) is over 30 mph slower even though it weighs over 1500 lbs less.

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Lunatic

[wmaxt]

Originally Posted by RG_Lunatic

Actually that's not true. The Spit IX with the same engine as the P-51B and a slightly lower coef. of drag (in the wind tunnel) is over 30 mph slower even though it weighs over 1500 lbs less.

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Lunatic

The C/D numbers are as follows:

P-51 C/D = .0163 Lift/Drag = 14.6
P-38 C/D = .0268 Lift/Drag = 13.5


Reference http://www.hq.nasa.gov/office/pao/Hi...-468/ch5-3.htm
Table III shows a comparison of related numbers/specs. on a large number of aircraft

Why doesn't the P-51 give a better showing?

Originally Posted by wmaxt

The C/D numbers are as follows:

P-51 C/D = .0163 Lift/Drag = 14.6
P-38 C/D = .0268 Lift/Drag = 13.5


Reference http://www.hq.nasa.gov/office/pao/Hi...-468/ch5-3.htm
Table III shows a comparison of related numbers/specs. on a large number of aircraft

Why doesn't the P-51 give a better showing?

What do you mean? First, 0.0163 is quite significantly lower than 0.0268 (the Coef of drag for the two planes). Second, this probably does not account for radiator thrust. Third, remember that increasing speed results in a geometric increase in drag.

Just how is the P-51 failing to give "a better showing" ?

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Lunatic

[wmaxt]

Originally Posted by RG_Lunatic

What do you mean? First, 0.0163 is quite significantly lower than 0.0268 (the Coef of drag for the two planes). Second, this probably does not account for radiator thrust. Third, remember that increasing speed results in a geometric increase in drag.

Just how is the P-51 failing to give "a better showing" ?

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Lunatic

Exactly, Why isn't it 20-40 miles an hour faster?

[pasoleati]

Originally Posted by RG_Lunatic

There is clearly a difference. In the P-51B and very early D's, their were two settings, "auto-lean" and "auto-rich", just like on the P-38. Then something in the fuel regluation system was changed, and the new P-51's had a "RUN" setting that automatically adjusted the mixture control. Pilots just put the P-51D in "RUN" and forgot about mixture settings.

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Lunatic

Trust me, there were no changes in the mixture control device itself. In the P-51B the auto rich was rendered inoperative as it was found that auto-lean gave satisfactory operation at all powers. In the P-51D the quadrant was redesigned accordingly. In the Spit IX there was no mixture lever at all, only a slow running cut out to shut down the engine.

Another point of interest in the manual page you have is the low altitude max range power setting. 1600 rpm and 30 in Hg boost is not too efficient, i.e. Merlin´s highish minimum rpm is evident here. E.g. 1200 rpm coupled with up to 46 in boost would have been considerable more efficient. E.g. the R-1820 could be run down to 1200 rpm for long range cruising.

Actually, there is some reasonable evidence that the P-51 was actually faster than normally quoted. The P-51B's official top speed is 440 mph, but there were NACA tests giving 453 mph level speed performance. The conditions of the test often had a lot to do with the results.

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Lunatic