combat altitude in the ETO (1 Viewer)

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I forgot, the P-51 also had the advantage of a high cruise speed, as range degraded very little at max cont. power. (1120 hp at 20,300 ft) Giving 420 mph at 30,000ft with 1,700+ mi for the P-51B with 2x 150 gal tanks.
 
To give a better idea of the altitudes of the interceptions between fighters. The first couple runs of P-51B's (P51B-1NA and -5NA) had the 1650-3 Packard Merlin, this was called the high alt. version. This was changed from -10NA through the D models to the 1650-7 which was Medium Altitude rated.

Although pilots have said with engine replacements in the field that you could have either one in the later B's and D's. Later I can post the HP of each at critical alts. low and high blower from Gruenhaugen's book. ISBN 0-668-03912-4.
 
There is a slight difference in peak power between the -3 and -7, but the -7 had more power at the alt. that the engagments were taking place at. ie. the bomber alts and below.

Critical Altitudes

1650-3:

1380 BHP at 3000 RPM at 61" - T/O power

1330 BHP at 3000 RPM at 67" WEP at 23,000 ft. - high blower
1600 BHP at 3000 RPM at 67" WEP at 11,800 ft. - low blower

1210 BHP at 3000 RPM at 61" at 25,800 ft. - high blower
1490 BHP at 3000 RPM at 61" at 13,750 ft. - low blower

950 BHP at 2700 RPM at 46" at 29,500 ft. - high blower - normal power
1110 BHP at 2700 RPM at 46" at 17,400 ft. - low blower - normal power

1760-7

1490 BHP at 3000 RPM at 61" - T/O power

1505 BHP at 3000 RPM at 67" WEP at 19,300 ft. - high blower
1720 BHP at 3000 RPM at 67" WEP at 6,200 ft. - low blower

1370 BHP at 3000 RPM at 61" at 21,400 ft. - high blower
1590 BHP at 3000 RPM at 61" at 8,500 ft. - low blower

1065 BHP at 2700 RPM at 46" WEP at 23,000 ft. - high blower - normal power
1180 BHP at 2700 RPM at 46" WEP at 11,300 ft. - low blower - normal power

As you can see more power at mid to lower alts. where it was needed for combat. As was noted in a prior post most combats started at the bomber alts, then proceeded lower from that point.

Of course when the England based squads started using 100/150 fuel in June 1944 higher power was made till the alt. was reached that the supercharger couldn't supply the boost for 72" (USAF) or 80" (RAF).
 
Hi Madmax,

>As you can see more power at mid to lower alts. where it was needed for combat.

Hm, that raises an interesting question: Was the ideal "best altitude" the same for the Luftwaffe as it was for the USAAF? For the Luftwaffe, it would be advantageous to have a superior performance at an alttiude of the USAAF top cover or even above in order to break through the escort screen and get into a position to attack the bomber formations. If the USAAF on the other hand had planned to extract the heaviest possible toll from the Luftwaffe after the German fighters had already attacked the bombers, their idea of an optimum altitude might differ. (I'm not sure this was in fact the USAAF plan, but it sounds a bit like it from the above posts, and from what I've read, it often worked out like that regardless of the original intentions).

Of course, the "ideal best altitude" was also affected by technological factors (such as available engine technology) and by a certain amount of guesswork regarding the enemy's future moves (like introducing the B-29 to Europe, for example).

Regards,

Henning (HoHun)
 
And of course the DB-601/605's, despite being single stage supercharged had the excellent "soft clutched" variable speed superchargers, so power output was not stepped like most superchargers (do to shifting stages/speeds) and power output was pretty constant with the necessary slow power loss as alt increased all the way up to crit alt. (higher alt required more power for the supercharger to maintain boost)

They didn't have inter/aftercoolers though iirc, but since they operated at lower boost pressures than allied counterparts this wasn't as important.

This is similar to the characteristics of a turbochargers boost, though a turbo still consumes almost no power. (slight due to increased exhaust back pressure)
 
Henning,

(I'm not sure this was in fact the USAAF plan, but it sounds a bit like it from the above posts, and from what I've read, it often worked out like that regardless of the original intentions).

From everything I've also read, studied, and from talking to some WW2 fighter jocks I feel your assumption above is correct. I know everyone hates to hear about flight sims, BUT the S3's that are a player developement of the sim Warbirds has shown it's very hard to stop aircraft from getting to a group of bombers and making that first pass. Changing the normal procedure of close escort, so that some aircraft are 15-20+ mile out from the bomber stream is what broke Germany's defense. Until then they always got to the buffs and really at times even after the change they still managed to get to them before the fighters could react.

So I'd say they wanted to have the best performance under the bombers alt where the fights would end up after that first pass. I wouldn't think the German's were wanting performance at high alt except for the models with GM to bounce the escorts at their 25-29k flight alts. Without it they'd still be plenty fast (605 with the 603 supercharger version) to intercept bombers.


Kool Kitty,

You are correct in saying the excellent system of the variable speed supercharger in the 109. Less power used at low alts., but a single stage supercharger is still a single stage supercharger. Now if they had a 2 stage setup with the fluid coupling...that would've been the ticket. Without 2 stage the Germans had to rely on GM injection to get better high alt. performance. Of course using that had it's downfalls also. Much more engine wear, tear, and stresses.

Always a position of give and take when it comes to innovation. To get one thing something else has to be compromised!
 
True, but i'd immagine that arranging a system with both superchargers on fluid couplings might be a mit complex, probalby more than necessary. Adding a second stage with 1 or 2 speeds would have been practical and if only used at high alt in addition to the fluid coupled stage the stepped power changes would not be nearly as drastic as contemproaries. (though with the added compression of air, and the resulting air tep increase, an intercooler may be necessary)

And the location of the second stage is something else to consider, the German engines (Jumo 210/11/13 and 600/01/05/03 had sid mounted superchargers with the added bonus of facilitating a "motor cannon." Though 2-stages were possible as shown with the 213E/F, and an intercooler was used on the higher altitude E version, but omited on the F which was tuned for the med alt range)



But on the single stage itsself the limitations are mainly of compression ratio and mass flow, both effected by rpm limits, hence a arger compressor could adress this as well, without the added stage. Though there is a limit to a centrifugal compressor's compression abilities, in the case of the operating circumstances this doesn't go much over 2:1, though some could get closer to 3:1 but those levels are inefficient for this use and would sap a lot of power, a turbo may be more reasonable with them though.


Those used on the early jets though can manage 2.8 at the lower end (the HeS-3b managed this, the HeS-8 about 2.9) and the Welland 3.5, Derwent I 4.0, Derwent V 4.3, J31 4.5, some Nene's and J48's 5.5:1.

But again jut mentioning it for curiosity, as those kind of values are not practical for a supercharger. And for al practical purposes a second stage is needed to acheive higher pressures at reasonable efficiency.
 

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