RG_Lunatic said:
Just a few comments:
1) Turbochargers had a critical altitude as well, though it was typically set higher than for mechanical superchargers. The turbine could only turn so fast, I believe 29-31,000 RPM. Once it topped out, power started to fall with altitude. The P-38 stopped making WEP around 29,000 feet and the P-47 around 32,000 feet (varies some by version).
2) The fluid coupling on the Bf-109 was pretty good, but it also cost some performance. Fluid couplings always involve loss. It was probably superior to the 2-stage-2speed supercharger toward the middle of the ranges between the 4 critical altitudes of those systems, but not a lot. It did avoid the sudden "bump" from the gear changes. The Turbo-supercharger was clearly superior, which is why the German's kept trying to build one till the end of the war.
3) The top speed of the F4U-4 was left out of the plane speed list. It was 463 mph at 20,700 feet (much lower than the other fast planes) in the "clean" configuration (i.e. no capped pylons).
=S=
Lunatic
You are correct that the turbos limiting factor was turbine speed. All turbo systems used on US aircraft were the same from what Ive read being that they were Minneapolis-Honeywell units. I only know about the later electronic versions though, not about the earlier mechanical versions found on B-17's and B-24's. I also am not familiar with those of the P-38 aside from the fact that they were MH units, so operation should be fairly similar as far as regulation and governors.
In B-17's, B-24's and B-29's there was a Turbo Boost Selector with 10 positions. Number 8 gave takeoff power, to turn past that to 9 and 10 there was a cam that had to be moved on the control to access these settings. 9 and 10 was War-Emergency Power settings and could not be used for more than 5 minutes. However I have read of certain instances where they have been used much longer. I know of one where a B-17 ran 3 engines on WEP for over an hour and another where a B-29 ran 2 engines on WEP for about 3 hrs!!
At this #10 WE setting the wastegate at the end of the turbo was completly closed allowing all exhaust to be diverted through the exhaust turbine.
The turbo control on B-17s and B-24s used four trimming potentiometers that could be adjusted. The B-29 (and B-36 for that matter) had the same setup but 8 trimpots as each engine had 2 turbos. More on the B-36 in a minute.
Each turbo was controled by a Waste Gate Motor, a Pressuretrol, a Governor, an Amplifier for each engine and the Turbo Control mentioned above.
The Waste gate motor controlled the open/close of the exhaust waste gate, which in effect, caused the exhaust driven turbine to spin..the more the waste gate was closed...the more the exhaust was diverted through the turbine and the faster the exhaust turbine spun. The intake impeller was directly connected to the exhaust turbine and the faster this spun the more air was fed into the intake and raised manifold pressure.
The governor monitored shaft speed and kept the turbo from overspeeding. This was the final limiting factor for the turbo performance and could come into play at anytime. The pressuretrol was also part of the governor system in that as altitude increases, density of air decreases. With this decreased density the turbine has less resistance and can overspeed. So above 27000 feet the pressuretrol reduced manifold pressure by 1"Hg for every 1000 feet gained. This number is from an early unit however and as the war progressed better materials and design allowed higher altitudes to be reached while maintaining the same level of power. All this translates into is that the turbines were allowed to spin faster.
Most times as well these high setting wouldnt be used and the turbines really didnt have go turn so fast. Especially with bombers, they would go over target at a fairly slow speed compared to their maximum speed. One B-17 flight engineer/top turret gunner I know states that on bombing missions their cruise speed was set somewhere around 155~160 mph by the flight leader. B-29's were faster cruising around 220mph all the way to Japan and back.
Fighters like the P-38 and P-47 I would say would definatly be much harder on their turbos as during dogfighting there would be a much greater tendancy to use WEP as well as constantly varying throttle positions. This could be another reason to use lower turbine speeds with the fighters.
In all turbos were clearly superior in response and flexibility and as better design and materials were used even better performance could be achieved.
Anyway I said I was going to mention some stuff on the B-36 turbo system. This was a different setup as above 40000 feet one turbo would be shut off and the engine would run on a single turbo. This was done because at the very high altitudes the B-36 could reach exhaust flow and power became so low that the turbos fell below their efficiency range and harsh pulsations of power would result. Shutting off one turbo and redirecting all exhaust flow through one turbo would raise the turbine speed, improve efficiency and eliminate surging. Also the B-36 with the R-4360 engine didn't have a War-Emergency setting and also only had a Turbo Boost Selector with 7 positions. Takeoff power was max power and was obtained at #7! Some later B-36's had water injection, not for use in a combat setting, but to provide extra power for takeoff.
After checking the account I read you are correct.
