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Anonymous
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The chart above shows clearly that the P-51 could in fact maintain 395 mph TAS at 25,000 feet at a fuel consumpiton rate of 85 gph.
Now lets figure the range at this speed. Lets assume a fuel load of 239 gallons (94 gallons in each wing, 60 gallons in the fuselage tank). Lets assume 9 gallons are used in warm-up and takeoff (it took 23 gallons to climb to 10,000 feet), and that drop tanks are used from then until the start of our "combat cruise". This leaves 230 gallons, enough fuel for 2.7 hours of flight at that speed. 2.7 hr. x 395 mph = 1066 miles with no reserve.
More realistically, 395 mph would be maintained only over the area of threat. Lets say 1 hour at 395 mph fast cruise, consuming 85 gallons and leaving 145 gallons. The remaining cruise home would be made at 260 mph at 10,000 feet, consuming 47 gph, and that a 25 gallon reserve is desired, leaving 120 gallons of non-resever fuel, enough for 2.5 hours of flight. Total range = 395 miles + (2.5 hrs x 260) = 995 miles.
It took 36 gallons to climb a clean plane to 25,000 feet. If we include the climb, and then figure a 23 gallon reserve (to land on), this still leaves 180 gallons to cruise on. At 395 mph @ 25000 feet @ 85 gph, this still gives a range of about 830 miles plus about 30 miles covered in the climb. So the Baugher site is not all that far off wmax!
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Lunatic
Now lets figure the range at this speed. Lets assume a fuel load of 239 gallons (94 gallons in each wing, 60 gallons in the fuselage tank). Lets assume 9 gallons are used in warm-up and takeoff (it took 23 gallons to climb to 10,000 feet), and that drop tanks are used from then until the start of our "combat cruise". This leaves 230 gallons, enough fuel for 2.7 hours of flight at that speed. 2.7 hr. x 395 mph = 1066 miles with no reserve.
More realistically, 395 mph would be maintained only over the area of threat. Lets say 1 hour at 395 mph fast cruise, consuming 85 gallons and leaving 145 gallons. The remaining cruise home would be made at 260 mph at 10,000 feet, consuming 47 gph, and that a 25 gallon reserve is desired, leaving 120 gallons of non-resever fuel, enough for 2.5 hours of flight. Total range = 395 miles + (2.5 hrs x 260) = 995 miles.
It took 36 gallons to climb a clean plane to 25,000 feet. If we include the climb, and then figure a 23 gallon reserve (to land on), this still leaves 180 gallons to cruise on. At 395 mph @ 25000 feet @ 85 gph, this still gives a range of about 830 miles plus about 30 miles covered in the climb. So the Baugher site is not all that far off wmax!
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Lunatic
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Anonymous
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DAVIDICUS: I do not have the P-47D manual, I'm looking for it. I do have the P-47N and P-38H/J/L manuals, but neither include charts like those in the P-51D/K manual listed above.
wmax: Just for comparison, here is the power settings info from the P-38H/J/L manual. As you can see, there is no way the plane could maintain a high speed cruise comprable to that of the P-51 without a much more significant range penalty.
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Lunatic
Source:
AAF MANUAL 51-127-1
PILOT TRAINING MANUAL FOR THE LIGHTING P-38 (Revised Edition)
Aug. 1, 1945
Page 41
wmax: Just for comparison, here is the power settings info from the P-38H/J/L manual. As you can see, there is no way the plane could maintain a high speed cruise comprable to that of the P-51 without a much more significant range penalty.
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Lunatic
Source:
AAF MANUAL 51-127-1
PILOT TRAINING MANUAL FOR THE LIGHTING P-38 (Revised Edition)
Aug. 1, 1945
Page 41
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wmaxt
Staff Sergeant
RG_Lunatic said:
The "Smooth Page" numbers are more realistic, RG, except in theory.
Your numbers are and the 85gph at 395mph is also optimistic though at this point I have to accept that it maybe somewhere in between.
All aircraft Manufactures have been known to be optimistic NA too. The difference is that the fuel consumption of the P-38 was Proven time and again to be conservative.
I'll be back when I have more info - with all other performance figures so close and the P-38 being MORE efficent at economy cruise by 4/5%, a 30% difference at 390/395mph is not reasonable. NO one else considers these figures viable in combat situations.
For instance at mid weight (ave flying weight) the
P-51 @ 9,300lbs with 1,650 hp yeilds 5.6lbs/hp and 437mph at WEP. at economy cruise 4.9miles/gal.
P-38 @19,200 lbs with 3,450hp yeilds 5.56lbs/hp and 443mph at WEP. At economy cruise 5.1 miles/gal. The P-38 also accelerates .6 mile/sec faster.
Both aircraft had an AAF ETO combat range at 450 miles on internal fuel.
No engine can add that much power (Double? as required to raise speed from 230+/- to 390/395) on only 20% additional fuel.
In a comparison the P-38 is as/more efficent than the P-51 These numbers only work in a theroretical sense and only if the 85gph figure is correct.
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Anonymous
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wmax,
Do you see what it says in the lower left corner?
"DATA AS OF 8-20-44 based on: FLIGHT TESTS"
This is not "theoretical", it is factual, as determined by flight testing. This is from the USAAF pilots handbook and the page given is specifically for the use of pilots and commanders to plan their missions. The part marked in brown relates to walking through the process of such a mission plan which is done on the next page. There is a chart on the previous page to figure out how much fuel will be used in warmup and climb-out (23 gallons to 10,000 feet for the 325 mph cruise mission example). If the chart were "theoretical", it would be useless for planning missions, wouldn't it?
Read the notes at the top which explain how to use the chart. If you can get on MSN msgr and contact me ([email protected]), I'll send you the whole manual - it's about 45 mb. You'll see that this is not "theoretical", the whole manual is about what you can do safely and expect from the P-51D/K.
It's not the engine that makes the difference, it's the slick lines of the P-51 vs. the P-38, the laminar flow wings, and the radiator thrust system which cancles out over 12% of the planes drag at high speeds. When all is accounted for, the P-51 has less than half the drag of the P-38 to contend with at high speeds.
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Lunatic
Do you see what it says in the lower left corner?
"DATA AS OF 8-20-44 based on: FLIGHT TESTS"
This is not "theoretical", it is factual, as determined by flight testing. This is from the USAAF pilots handbook and the page given is specifically for the use of pilots and commanders to plan their missions. The part marked in brown relates to walking through the process of such a mission plan which is done on the next page. There is a chart on the previous page to figure out how much fuel will be used in warmup and climb-out (23 gallons to 10,000 feet for the 325 mph cruise mission example). If the chart were "theoretical", it would be useless for planning missions, wouldn't it?
Read the notes at the top which explain how to use the chart. If you can get on MSN msgr and contact me ([email protected]), I'll send you the whole manual - it's about 45 mb. You'll see that this is not "theoretical", the whole manual is about what you can do safely and expect from the P-51D/K.
It's not the engine that makes the difference, it's the slick lines of the P-51 vs. the P-38, the laminar flow wings, and the radiator thrust system which cancles out over 12% of the planes drag at high speeds. When all is accounted for, the P-51 has less than half the drag of the P-38 to contend with at high speeds.
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Lunatic
wmaxt
Staff Sergeant
RG_Lunatic said:
I don't have a modem that will handle that and I have one coming. Do you have the coefficent of drag for the two aircraft?
My problems with this are a) the speed/hp/lbs are almost identical showing that the aircraft are comparable. Drag increases 2x as fast as speed if the 38 were significantly worse it would take more horsepower or less speed than it does to fly at those speeds. If the P-51 was so slick or got so much thrust from the exaust/radiator (the P-38 got some too though the ammount was not specified) the P-51 would be 30 or 40 mph fasterthan it is. b) Twice as much horsepower takes twice as much fuel to produce. c) It's not mentioned as a normal tactic anywhere but here.
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wmaxt said:
No, I've not been able to find coef's of drag for the P-38.
wmaxt said:
The P-38 got very little thrust from the radiator system. It has no pre-radiator expansion chamber, no post-radiator compression chamber, and no pressure controlled thrust nozzle.
The radiator thrust from the P-51, increases with the square of the speed, just like drag. So at any speed, the radiator thrust system cancles out about 90-100% (maybe even more) of the cooling system drag, which makes up about 12-14% of the total drag of the P-51.
The P-51 is not 30-40 mph faster than it is because to increase the speed by 10% requires 19% more power. Look at the Spit IX (the one with the equivalent engine and blower) vs. the P-51B. They have the same engine, the Spit has a lower coef. of drag in the wind tunnel, but the P-51 is about 32 mph faster.
Also, I think the mixture control system on the P-51D was superior to that on the P-38. You put it on "run" and it set the mixture to whatever setting was best. On the P-38 you ran lean and could not maintain high speeds, or you ran rich and it ate fuel like crazy.
As for it not being mentioned anywhere, read pilot accounts and take not of their initial speeds when they sighted the enemy. The are always up near 400 mph. It was just normal on the P-51 to fly like that.
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Lunatic
The key here is indeed the engine. The reason is not exactly as Lunatic (wwbic at AGW´s forum?). While the P-51D has better mixture control, it has nothing to with NAA people. The key difference is that while the Merlin´s max. continuous power is the same as its max. weak mixture power, Allison´s weak and rich mixture continuous ratings are wholly different. Simply take the fuel flow of both at their max continuous powers and see the difference. The Merlin simply runs at leaner mixture at higher powers (i.e. from about 60% up).
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pasoleati said:
That is certainly a good part of it. The P-51D had an improved fuel mixture system which automatically chose the leanest mixture setting which would not cause pre-ignition. But it is also a cleaner design, and the radiator provides thrust - both give it a futher advantage over the P-38 as it takes less power to sustain a given rpm.
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Lunatic
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pasoleati said:
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
wmaxt
Staff Sergeant
The P-51D had an improved fuel mixture system which automatically chose the leanest mixture setting which would not cause pre-ignition. But it is also a cleaner design, and the radiator provides thrust - both give it a futher advantage over the P-38 as it takes less power to sustain a given rpm.
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Lunatic[/quote]
The fact that an engine could be run leaner can be a significant factor in fuel consumption.
So can airframe efficency, However the power loading is only .04 lbs/hp different and at that loading the P-38 is a hair faster. The extra thrust/more efficent airframe Would be shown in etiher a higher speed or a higher lbs/hp ratio. It's Not There. I still want a coefficent of drag (C/D ratio) to compare.
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Lunatic[/quote]
The fact that an engine could be run leaner can be a significant factor in fuel consumption.
So can airframe efficency, However the power loading is only .04 lbs/hp different and at that loading the P-38 is a hair faster. The extra thrust/more efficent airframe Would be shown in etiher a higher speed or a higher lbs/hp ratio. It's Not There. I still want a coefficent of drag (C/D ratio) to compare.
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wmaxt said:
The radiator thrust is totally missing form the power ratings. This cancels approximately 10-12% of the total drag, which is very substantial. The (near) laminar flow wings also produce less drag than the P-38 wings.
I've not found any coef. of drag info of any reliablility on the P-51, P-38, or in fact many other planes. I think I might be able to find the coef. of drag figures for the wings of the two planes - I'll look and see if I've saved those .pdf's (if not then forget it, it takes hours to search each .pdf on the NACA site, which is the only way you can find anything this old).
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Lunatic
wmaxt
Staff Sergeant
RG_Lunatic said:
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.
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wmaxt said:
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
Staff Sergeant
RG_Lunatic said:
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/History/SP-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?
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wmaxt said:
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
Staff Sergeant
RG_Lunatic said:
Exactly, Why isn't it 20-40 miles an hour faster?
RG_Lunatic said:
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.
