SHANE Ingate wrote:
> Bill wrote:
>
>> Supercharged piston engine aircraft had short pipes for the same
>> reason that drag motors do--to get the exhaust gases out of the way
>> as fast as possible. they weren't looking for a few inches of vacuum
>> at the exhaust port to help scavenge the engine. some supercharged
>> motors have exhaust pressures in excess of 500 psi, whereas
>> conventional engines might reach 70 psi at the valve and average 10
>> psi in the pipe.
>
>
> Bill, I beg to disagree. Super/turbo/super-turbocharging on military
> aircraft was to help them operate at their ceilings. There is not a
> lot of
> air at 35,000 feet and even with boost, they never performed as well
> as at sea-level. I think it was Michael who suggested that weight
> was the reason for the short exhaust stacks, but I do not buy into that
> 100%. If longer primaries bought more hp, then the extra weight of long
> primaries would be easily compensated for. Remember, these motors
> (including the water-cooled vees of Merlin and Allison) were only turning
> 2,500 rpm at fast cruise. Propellor blade design was a much greater
> factor in determining 1940s performance than primary length.
>
Yeah, quite true. But, the Merlin engines, at least, were operating with
a helluva lot of boost. Earlier versions of the Merlin were rated at
60" Hg above atmospheric. When they wanted more power late in the war,
they just upped the boost (in fact, the 622 series used on commercial
aircraft after the war was, I believe, producing even more power than in
Spitfires and Hurricanes--those mostly used the V1650-7, rated at max.
60" Hg, while the V-1650-9 and later engines were limited at 66" Hg, I
think). Sort of like drag cars--want more power, just crank up the
boost regulation, so the stack length could be kept short--pipe tuning
just wasn't necessary.
As for high/low altitude performance, that's kind of true, but is
case-specific. The Allison 1710 had some problems with high-altitude
performance early on, which I think was related to the turbocharger
characteristics. The Merlin, however, used a crank-driven, two-stage
supercharger, and was barometrically-compensated, so the engine could
not obtain full boost at low altitude. I've forgotten the exact
altitude, but at around 20-25,000 ft., the first-stage boost was shunted
to the second stage in series, which, practically speaking, gave the
engine equal or better performance at altitude.
Cheers.
--
Michael D. Porter
Roswell, NM
Never let anyone drive you crazy when you know it's within walking distance....
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