P-51B vs. Spitfire Mk IX

Discussion in 'Aviation' started by davparlr, Oct 23, 2013.

  1. davparlr

    davparlr Well-Known Member

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    On another thread, now closed, there was a discussion going on relative to the aerodynamic performance of the Spitfire to the Mustang. To see the actual performance differences, I thought I would do an apple-to-apple comparison between the P-51B with a Merlin -7 engine and a Spitfire Mark IX with a similar Merlin 66 engine, both operating at 67” Hg boost (18.5 lb/sqin). I will use these references. This is only a speed and climb comparison. There or other variables I did not attempt to address such as roll rate, dive rates, etc.

    http://www.wwiiaircraftperformance.org/mustang/p51b-speed-wf.jpg
    http://www.wwiiaircraftperformance.org/mustang/p51b-climb-wf.jpg
    http://www.spitfireperformance.com/bs543climb.gif
    http://www.spitfireperformance.com/lfhfspeed.gif

    In order to do the apples-to-apples comparison, I had to adjust the P-51B’s climb data, both rate of climb and time to climb, to reflect the discrepancy in fuel quantity. The P-51B was tested with 256 U.S. gallons of fuel, the Spitfire was tested with 110 U.S. gallons of fuel. This is almost a 900 lbs difference. This roughly worked out to be about 500 f/m improvement. Airspeed data was insignificantly affected by weight difference and was not adjusted. A note here is that I did chart interpretation on small charts with poor resolution so there is an inherent data uncertainty. But it should be accurate enough for comparison purposes.

    Data will show Altitude(ft) versus airspeed(mph), rate of climb(ft per min) at every 5kft, and time to climb(min) at 10k ft, 20kft and 25kft. Also shown is the delta speed advantage for the Mustang and the delta climb advantage for the Spitfire.

    SL
    P-51 364 3900
    Spitfire 336 4600
    Delta P-51 speed + 28 mph
    Delta Spit climb +700 f/m

    5k ft
    P-51 394 4100
    Spitfire 358 4670
    Delta P-51 speed +32 mph
    Delta Spit climb +570 f/m

    10k ft
    P-51 408 3700 2.5 min
    Spitfire 380 4280 2.15 min
    Delta P-51 speed +28 mph
    Delta Spit climb +580 f/m

    15k ft
    P-51 422 3680
    Spitfire 383 3860
    Delta P-51 speed +39 mph
    Delta Spit climb +180 f/m

    20k ft
    P-51 425 3500 5 min
    Spitfire 399 3560 4.75 min
    Delta P-51 speed +26 mph
    Delta Spit climb +60 f/m

    25k ft
    P-51 425 2810 6.5 min
    Spitfire 395 2835 6.3 min
    Delta P-51 speed +30 mph
    Delta Spit climb +25 f/m

    A favorite myth about the P-51 was that it was fast and had great range but it was not a good climber. But, as can be seen from the above comparison, the Spit had noticeable advantage at low altitudes, 700 f/m (8 mph) at SL, but above 10k ft it held only a slight edge in climb over the Mustang.

    It appears that, roughly, an equally loaded P-51 taking off with a Spitfire will reach 25k ft altitude about 30 sec behind the Spitfire but with a delta speed capability of 30 mph. It seems to me that, once at altitude, the P-51 would have a much easier time chasing down an enemy and certainly more endurance to do so and that 30 sec advantage would quickly disappear.

    The aerodynamic efficiency of the P-51 aids it climb, being very similar to the Spitfire even though the Mustang has a 30% higher wing loading than the Spit. It helps in acceleration by having more available hp since it uses less hp than the Spit to maintain any given airspeed. And, of course aids in endurance and/or range just because it uses less fuel per mile than other aircraft. It sent many aerodynamic engineers, both allied and German, back to the drawing board to compete with, or fight with, this new fighter. It also had a marvelous British engine that could put that great design to good use. That same marvelous British engine could not give its best in any other airframe, including the Spitfire, since it had to use more of its power just to overcome drag.

    Now, the Spitfire, and also the Bf-109, were wondrous aircraft capable of great performance deserving all the accolades they got, and indeed, with upgraded engines stayed formidable for the duration of the war and more. But they were early 30s aircraft and the P-51 was a 1940 aircraft, and was basically a generation of aeronautical improvement different.

    There maybe some errors in these numbers. If so, I am sure they will be pointed out and I can adjust my numbers accordingly. I think in general this is pretty accurate.
     
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  2. JtD

    JtD Member

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    I like your approach, and I do agree that the "bad rate of climb" of the P-51 needs to be balanced against the range it had. Helps putting it into perspective. But I think you overestimated the impact of the 900lb reduced fuel load, in particular at higher altitudes. How did you arrive at this figure?
     
  3. davparlr

    davparlr Well-Known Member

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    A bit of chart interpretation using the following from "Spitfireperformance" showing the North American calculations of climb for the P-51D.

    http://www.wwiiaircraftperformance.org/mustang/na-46-130-chart.jpg

    It shows for the weight difference between 8000lb and 10000lbs, climb rate is close to being flat rated over altitude and reflects about 1000 f/m. So half that is 500 f/m, 450 may be more accurate but it is all in the ball park, maybe a bit less at 25k, still very close to the Spit.
     
  4. Hop

    Hop Member

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    Looking at the charts you linked, it says gross weight was 9,680 lbs for the P-51.

    From Mike's page, the test headed "Army Air Forces Material Command Wright Field, Dayton, Ohio 15 May 1944"

    P-51 Mustang Performance

    I can't quite square the weight, though. An earlier P-51B test, and a later British one, give the weight for the B as 9,200 lbs with ammunition but without the rear tank. 85 (US) gallons should weigh 510 - 550 lbs, which would put the weight up to something over 9,700 ls without the weight of the tank and extra piping.

    I'd prefer to see a P-51D comparison because the B had just over half the firepower of the Spitfire. If we're balancing range, shouldn't we balance armament as well?
     
  5. davparlr

    davparlr Well-Known Member

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    Going to "America's Hundred Thousand", which I consider to be the best source for weight in that it breaks it down by component, I get a P-51B fighter weight with 180 gal of gas of 9077 lbs. With a reduction of gas to the 110 gallon level (-420lbs), this weight becomes 8657 lbs. If I add two .50s and additional ammo as the P-51D has, the weight becomes 8983 lbs, or 1023 lbs less than the tested weight (there is probably some structural weight not accounted for). After going through all that, it seems the data is still pretty good.
     
  6. Hop

    Hop Member

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    Still not getting my head around the weights. If you go to this test, of the same aircraft:

    P-51B Performance Test

    If you then look at the climb performance at 9,335lbs:

    0 ft - 3,750 ft/min
    5000 ft - 3,900 ft/min
    10000ft - 3,300 ft/min
    15000ft - 3,100 ft/min
    20000ft - 3,100 ft/min
    25000ft - 2,300 ft/min

    The figures are in fairly close agreement with yours at low altitude, allowing for the slightly higher weight, but the discrepancy increases as the altitude increases (and climb rate drops).

    It seems to me the best way to adjust climb rate for weight difference is to use a percentage increase, rather than a flat figure for all altitudes.

    As to the weight of the P-51B, several reports on Mike's site give the figure as 9,200 or 9,205 lbs without fuselage tank. Reducing by 420 lbs to equalise fuel, adding 320 lbs for the extra armament puts it down to 9,100 lbs.

    There is one difference in method I can see between the US tests of the Mustang and British tests of the Spitfire. The Mustang notes "oil and coolant shutters auto" whereas the Spitfire tests say "radiator flaps open". It was standard practice for the RAF to test climbs with radiators forced fully open. In the tests of the a Spitfire at 25 lbs boost, that made a difference of over 400 ft/min at low level, 270 ft/min at 25,000 ft. In standard conditions the radiator flaps would be shut on the climb.
     
  7. drgondog

    drgondog Well-Known Member

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    Hop - all the figures for climb and speed are pre June 44 for the P-51B/D prior to 150 octane fuel and 72" Hg boost.

    The P-51 (all versions) will never turn with the Spit - and only the P-51H was close to the XIV in climb - although far superior in speed.
     
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  8. Aozora

    Aozora Well-Known Member

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    Could there be an error in the first report stating a weight of 9,680 lbs? A similar test run on a Mustang III at the A&AEE Mustang III Flight Trials shows a test weight of 9,200 lbs; unfortunately the report doesn't specify the fuel or ammunition load. The climb tests were done with the radiator and oil cooler flaps fully open.
     
  9. parsifal

    parsifal Well-Known Member

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    Off topic I know, so i do apologise, but I would really like two additional questions answered......the turn radius of each type at normal combat speeds 9which might be different for each type), and the weight of shell that each type could generate with a "standard armamanet load out.

    My hypothesis is that the P-51 will climb and dive as well or better than the Spit, but the spit will turn faster and/or tighter, and will have greater offensive firepower.

    The last supplementary question to answer might be the weight of each type deveoted to armour, which would give a suggestion to its relative strength or resistance to damage.
     
  10. OldSkeptic

    OldSkeptic Active Member

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    Yes Dave those numbers don't look right. Very important to compare like with like, such as the LF IX (Merlin 66) vs the -7 engined Mustabg (= the Merlin 66).

    The Mustang was a fair climber, good as the 190s, but the plaudits always go to the 109 and Spit. Climbing ability is a function of thrust loading and wing loading. The 109 did it by thrust loading, being so light, the Spit by wing loading.
    Now the Mustang had a worse thrust loading and a worse wing loading than the Spit.

    It has a drag advantage but that gap closes as height goes up, as drag reduces with height (because the temp of the air goes down). So climbing as fast at the higher altitudes as an equivalent, lighter, same powered, larger winged Spit .. can't see it. I need to check the numbers.
     
  11. Neil Stirling

    Neil Stirling Member

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    Spitfire and Mustang both fitted with the Rolls-Royce Merlin 70, propeller reduction gear 0.477:1 Radiator flaps open.

    R.A.F data cards.
    Merlin 70 1,710hp at 11,000ft and 1,475hp at 23,250ft
    V-1650-3 1,710hp at 11.000ft and 1,470hp at 23,000ft

    Spitfire IX BS.551 weight 7,470lbs. Tested at R.A.F Boscombe Down.

    Mustang A68-1001 weight 8550lbs. Tested at No.1 Aircraft Performance Unit. Laverton.

    Rate of climb.

    Spitfire Mustang

    Sea level. 4390ft/min 3950ft/min

    6000ft 4550ft/min 3950ft/min

    10000ft 4570ft/min 3950ft/min

    16000ft 4180ft/min 3250ft/min

    20000ft 3500ft/min 2975ft/min

    26000ft 3270ft/min 2550ft/min

    30000ft 2600ft/min 2000ft/min

    36000ft 1600ft/min 1225ft/min

    Neil.
     
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  12. davparlr

    davparlr Well-Known Member

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    #12 davparlr, Oct 24, 2013
    Last edited: Oct 24, 2013
    Good discussion all. I may have opinions that vary but I want my data right.



    Let me straighten up some of my confusion here. Using “AMT’ data, and with the fuselage tank and 180 gallons of gas, the fighter weight of the P-51B is 9332 lbs. (I forgot to add in the weight of the tank). That would mean a comparison weight due to reduction of gas of 8912 lbs, or 420 lbs less. So using your numbers, which look good to me and adding 420 lb reduction from P-51D chart climb data, I get:

    0 ft – 3,981 ft/min
    5000 ft – 4,131 ft/min
    10000ft – 3,531 ft/min
    15000ft - 3,331 ft/min
    20000ft – 3,331 ft/min
    25000ft – 2,510 ft/min

    This is for a standard P-51B with fuselage tank.

    Corrected for six .50s and ammo, the data you show above would be close.

    Somewhere in my brain a cell keeps trying to tell me that the British Mark III Mustangs did not have extended range tanks. If so the original numbers posted would still be valid for standard British Mark III Mustangs.

    It must be noted that there are two 44-1 fuel test showing the same plane at different times at different weight but with the same data points! I think the one with the weight of 9335 lbs is correct.

    The North American chart on climb verses weight performance does not support your comment. It shows a flat rate of increase due to weight reduction up to 22k where it starts to slightly converge with the higher weight.

    http://www.wwiiaircraftperformance.org/mustang/na-46-130-chart.jpg

    That is what I did
    "I would do an apple to apple comparison between the P-51B with a Merlin -7 engine and a Spitfire Mark IX with a similar Merlin 66 engine, both operating at 67” Hg boost (18.5 lb/sqin)."

    Where did they get a Mustang with a RR 70 engine in it? I thought the Packard Merlin -7 was similar to the RR Merlin 66, not the 70.

    I couldn’t find this test.
     
  13. Milosh

    Milosh Well-Known Member

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    Packard V1650-7 had the R-R designation Merlin 69.
     
  14. OldSkeptic

    OldSkeptic Active Member

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    Correct, the rear tanks were removed.
     
  15. OldSkeptic

    OldSkeptic Active Member

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    Note the ratio of Spit to Mustang:
    Spit P51-B Ratio
    Sea level 4390 3950 11%
    6000ft 4550 3950 15%
    10000ft 4570 3950 16%
    16000ft 4180 3250 29%
    20000ft 3500 2975 18%
    26000ft 3270 2550 28%
    30000ft 2600 2000 30%
    36000ft 1600 1225 31%

    You can see the Spit gets better relatively as the altitude goes up and the drag difference proportionally decreases.

    The exception is at the 2nd stage cut in.
    The reason is that before that point engine power is dropping, the thrust loading for both is dropping, but the Spit's lower wing loading is still enabling it to have a better climb rate. They are also getting into the altitudes where drag is dropping rapidly.
    The 2nd stage cuts in and the thrust loading for both increases, with the lower Mustang drag allowing it to close the gap a bit.

    I'd be interested to see the climb speeds and angles at the different altitudes. I suspect the Mustang is either (a) slowing down or (b) having to reduce its climb angle (possibly a bit of both of course).

    As the altitude goes up even more, power is dropping, drag differential is dropping and the Spits wing loading advantage dominates more and more. It will either be able to maintain a faster climb speed and/or angle than the P-51B.
     
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  16. GregP

    GregP Well-Known Member

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    Anybody have a V-n / V-G diagram for both? If so, the turn radius at verious speeds can be calculated.

    I have seen a V-n diagram for the P-51 at 8,000 pounds, not the weight of interest ... but the model (B, D, ect.) is not identified.

    I have not seen a V-n diagram for a Spitfire.
     
  17. drgondog

    drgondog Well-Known Member

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    the V-n diagram is calculated as a f(G) for 8000 pounds at max CL for SL. So, you can also calculate the Spit V-n diagram if you wish. Ditto for turn radius calcs..

    Simple answer the Spit has higher CL and lower wing loading so it is going to out turn any Mustang. as the engine is the same (pre XIV), for same boost the Spit will outclimb the Mustang A/B/C/D but perhaps not the P-51H. Although the Mustang has less profile drag - it is not enough difference to offset the Gross Weight difference..
     
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  18. davparlr

    davparlr Well-Known Member

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    I have some problems with this comparison. You used Neil Stirling data which is problematic to me. He compares an RR Merlin 70 engine to a Packard Merlin -3 engine. I don’t know if these two engines are equivalent. I think the -3 is equivalent to the Merlin 63 but I have no idea what the 70 is. Anyway, I compared the Merlin 66 to the -7. If we compare the Spitfire to the British Mark III Mustang, which does not have the long range tank, and with only 110 gallons of fuel. This would give the Mustang a weight of 8657 lbs (9335-120 (smaller fuel load)-255 (no extended range tank)). Your chart should look like this.

    Sea level 4600 4125 12%
    6000ft 4690 4223 11%
    10000ft 4280 3673 17%
    16000ft 3860 3473 11%
    20000ft 3560 3473 0%
    26000ft 2700 2550 6%
    30000ft 2600 1840 41%
    36000ft 1600 906 76%

    Interesting results which shows that you are correct for altitudes above 26k ft. Up to that the Spit is still always better but not over 600 f/m. As drgondog has said, the Spitfire will climb better and turn better than the Mustang, and as configured here, has better firepower. However, the Mustang must be admired by being so close in performance yet weighing a thousand pounds more, which certainly explains the desire in lightening the Mustang which eventually led to the P-51H (I wonder what the Spitfire numbers would be if I added a 1000 lbs to the plane. Nope, I ain't goin' there).

    I agree that if you need to intercept a bomber at altitude and do yanking and banking, it would be hard to beat the Spitfire. If you wanted to reach out and touch the enemy at a distance, it is just about impossible to beat the Mustang.
     
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  19. Aozora

    Aozora Well-Known Member

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    The Merlin 70 was the high altitude rated version with a Full Supercharger Height of 27,400 ft; used in the H.F Mk VII, VIII and IX: Spitfire LF HF Mk IX Test

    the V1650-3 had a High Blower Critical altitude of 29,400 ft: P-51B Performance Test so Neil's calculations are as valid as those of the Merlin 66 vs V-1650-7.
     
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  20. Hop

    Hop Member

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    #20 Hop, Oct 25, 2013
    Last edited: Oct 25, 2013
    Neil's figures are for a P-51 fitted with a Merlin 70. I believe the RAAF were concerned post war that they wouldn't be able to source spare engines from the US and so investigated the idea of using RR engines in their Mustangs.

    I found another test that might throw some light on the differences in climb at various altitudes. It's a British test on a Spitfire V with and without a 90 gallon drop tank.

    Spitfire Mk.VB (Tropical) AB.320 Report

    The drop tank equipped plane was 790 lbs, 11.8% heavier. Of course there is extra drag as well which complicates the picture.

    The climb rate difference between the two tests (with and without drop tank, all figures in ft/min):

    alt - without - with - diff - percent
    1000 - 2590 - 2115 - 475 - 18%
    5000 - 2610 - 2125 - 485 - 19%
    10000 - 2640 - 2135 - 505 - 19%
    15000 - 2540 - 2020 - 520 - 20%
    20000 - 1900 - 1440 - 460 - 24%
    26000 - 1400 - 1090 -310 - 22%


    The gap is clearly closing as the climb rate decreases. Whilst the percentage difference changes, it does so over a fairly narrow range.

    It just seems wrong to me to add a fixed figure across the range. 500 ft/min extra at 4,000 ft a minute isn't much. But when climb rate has dropped to 250 ft/min, all of a sudden it's a 200% increase.

    I also can't see how the Mustang can narrow the gap as altitude increases. Induced drag will increase with altitude, parasitic drag will decrease. The Mustang had less parasitic drag but more induced, so the Spitfire should have an increasing advantage with altitude.
     
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