P-51B vs. Spitfire Mk IX

[tomo pauk]

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.

If we check out the data cards for the Spit HF.VIII (Merlin 70, here) and Mustang III (V-1650-1, http://www.wwiiaircraftperformance.org/mustang/mustang-III-ads-3.jpg), we can see that, without ram, both engines are within 250 ft in full throttle height, 2nd gear, their power differing in 5 HP. They have the same power at FTH, 1st gear. Practically, the same engines?

The real-world difference occurred because of ram effect. The less draggy plane will be faster on same propulsive power, and greater speed will 'elevate' the FTH higher than lower speed. The FTH of the Mustang III was, as you note, as high as 29400 ft, vs. 27500 ft for the Spit HF.VIII. In other words, the Mustang III gained almost 6500 ft (23000 ft vs. 29400) due to it's low drag (and possibly longer/better intake?) and ram effect, the Spit only 4250 ft (23350 ft vs. 27500).

The ADS I've linked gives Mustang's maximum speed to occure at 28000 ft, making 450 mp/h there, vs. 442 mp/h at 29400 ft?

 

[davparlr]

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.

It is obvious that he Merlin 70 has a different supercharger profile than the -3. Does the 66 have a similar different supercharger profile than the -7, or is it similar. I have no idea. I thought the 66 and -7 were similar engines.

 

[wuzak]

It is obvious that he Merlin 70 has a different supercharger profile than the -3.

The same supercharger for both - 12" first stage, 10.1" second stage.

Supercharger gear ratios: 6.39:1 MS and 8.03:1 FS for both the 70 and the V-1650-3.

Does the 66 have a similar different supercharger profile than the -7, or is it similar. I have no idea. I thought the 66 and -7 were similar engines.

Again, the same supercharger as the -3 and 70 - just different gearing.


Supercharger gear ratios: 5.8:1 MS and 7.34:1 FS

 

[davparlr]

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 don't know how to answer this question. The North American chart is very clear that the rate of climb difference is basically constant from SL to 22k (FTH?) and I am sure they are not incompetent. Also, it does make sense that the percentage of improvement will go up with height. When the heavy aircraft reaches max altitude and is unable to climb, the lighter aircraft will still be able to climb, up to a higher max altitude, making the ratio infinite. So 200% is not unrealistic.

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.

I see your point and cannot answer except maybe the Mustang climb TAS airspeed is increasing at a faster rate than the Spitfire in this area which would improve climb difference significantly (velocity is squared in the lift formulas) and would also reduce induced drag. I don't know, I do not have enough data to determine the difference. Could be a chart error or mathematical error on my part.

 

[davparlr]

The same supercharger for both - 12" first stage, 10.1" second stage.

Supercharger gear ratios: 6.39:1 MS and 8.03:1 FS for both the 70 and the V-1650-3.

How can that be with different critical height? Something seems to be different. Maybe I don't understand.

 

[wuzak]

How can that be with different critical height? Something seems to be different. Maybe I don't understand.

Their FTH is the same, without ram air, as Tomo explained.

Ram air is dependent on speed, so the faster aircraft, the Mustang, gains more FTH than the Spitfire.

Lumsden has the V-1650-7 listed with the Merlin 66, 68 and 266 (US built 66). For some reason the FTH at +18psi boost is different between the 266 and the 66 (13,500ft vs 12,500ft). That I cannot explain.

Merlin 70
MS: 1700hp @ 10,500ft, +18psi boost
FS: 1475hp @ 22,500ft, +18psi boost

Merlin 66
MS: 1750hp @ 5250ft, +18psi boost
FS: 1625hp @ 12,500ft, +18psi boost

V-1650-3
MS: 1530hp @ 15,750ft, +16psi boost
FS: 1300hp @ 26,500ft, +16psi boost

V-1650-7 (M266)
MS: 1705hp @ 5,750ft, +18psi boost
FS: 1580hp @ 13,500ft, +18psi boost

 

[Aozora]

Here is a dataset from Australian tests showing the differences in performance between non-tropicalised and tropicalised Spitfire VCs: same engines but the tropical filter reduced the efficiency of the supercharger, partly due to the added drag but mainly due to the more restricted airflow:

 

[GregP]

Thanks for that V-G stuff, Bill. I'll look into doing it when I get the time.

I KNEW the Spitfire would out-turn and steady-state out-climb the P-51 in sustained rate, but figured the P-51 zoom climb would be about even with the Spitfire at equal velocities, assuming 280 mph+, for some thousands of feet.