wuzak
Captain
Canopy and cooling designs.
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Do you have any examples?
I thought the XP-39 was already streamlined by the time of the XP-39B (lowered canopy)? As for cooling changes, what would you have recommended for the production P-39 variants?Canopy and cooling designs.
While I've touched on this before, how long did the 8.8 gears operate for? Do you know how much the 9.6 versions were shortened in comparison?I don't know exactly, but you have to think that they weren't expecting the gears to wear that quickly.
The 8.8 gears were fine. The early 9.6 gears wouldn't pass the standard US 150 hour test. These were the -59 and -61. In one example the 9.6 gears completed the test but failed during inspection after the test. Redesign of the case to make the gears wider cured this problem with the first examples of the -83 completed in late '42.While I've touched on this before, how long did the 8.8 gears operate for? Do you know how much the 9.6 versions were shortened in comparison?
By wider, you mean the teeth right?The 8.8 gears were fine. The early 9.6 gears wouldn't pass the standard US 150 hour test. These were the -59 and -61. In one example the 9.6 gears completed the test but failed during inspection after the test. Redesign of the case to make the gears wider cured this problem with the first examples of the -83 completed in late '42.
I would think the whole gear would be wider, certainly the teeth.By wider, you mean the teeth right?
With the knowledge of the time, would there have been anyway to sidestep the problem (i.e. other than a well-maintained crystal ball)?I would think the whole gear would be wider, certainly the teeth.
We're fond of hypothetical situations.You folks sure are over-working your brains trying to make a silk purse out of a hunk of 'meh'.
Hardly. Cost the AAF an extra 100 horsepower for the entire year of 1942. Doesn't sound like much, only added 800fpm climb and 20mph speed at 20000' in the P-39. Could have used that extra performance in New Guinea, Guadalcanal and North Africa.You folks sure are over-working your brains trying to make a silk purse out of a hunk of 'meh'. It didn't take the USAAC long to decide it wasn't worth the effort, I don't see why you don't do the same.
Vees for Victory blamed the excessive wear on manifold pressure. Until mid year 1942 the AAF was still clinging to the port backfire screens even though Allison told them repeatedly that they weren't needed with the new aluminum intake manifold that wouldn't catch fire like the magnesium intake manifold then in use. The port screens increased manifold pressure between the impeller and the screens but the screens reduced MP to the cylinders, effectively causing the supercharger gears to work harder than necessary. Earlier elimination of the screens and the freer airflow from the Impeller through the intake manifold to the cylinders would have reduced MP at the impeller and increased MP to the cylinders, which was the goal. The reduced MP at the impeller may have been enough to reduce wear on the 9.6 step up gears and allow the engine to pass it's 150 hour test. Spitballing here, but that's all I got.With the knowledge of the time, would there have been anyway to sidestep the problem (i.e. other than a well-maintained crystal ball)?
What pressures were the 9.6 V-1710's to run at?Vees for Victory blamed the excessive wear on manifold pressure.
Why were they so insistent on the backfire screens? Why did they believe they were needed when Allison told them they weren't?Until mid year 1942 the AAF was still clinging to the port backfire screens
Why were they so insistent on the backfire screens? Why did they believe they were needed when Allison told them they weren't?
44.5 military and 57.0 WEP.What pressures were the 9.6 V-1710's to run at?
Why were they so insistent on the backfire screens? Why did they believe they were needed when Allison told them they weren't?
I gave you bacon for the "hunk of 'meh'" phrase, man that got me laughing on what was starting out as a rather 'meh' morning, thanks.You folks sure are over-working your brains trying to make a silk purse out of a hunk of 'meh'. It didn't take the USAAC long to decide it wasn't worth the effort, I don't see why you don't do the same.
A backfire is caused by fuel burning in the exhaust system right? What causes this?Let's just say that Allison's reputation in late 1941 and early 1942 wasn't quite they may have wished for.
Several lost aircraft due to engine fires and back fires, a few more that needed rebuilding. About 170 engines in 1940 that needed new crankshafts and crankcases to meet the desired overhaul life. And a few other problems?
Is this the same as the 8.8?44.5 military and 57.0 WEP.
As I understand it a backfire is premature detonation of the fuel due to excessive MP or RPM. The backfire screens theoretically kept the detonation from getting back into the intake manifold which early on (pre mid '42) was made of magnesium and could actually catch fire.A backfire is caused by fuel burning in the exhaust system right? What causes this?
Is this the same as the 8.8?
Thanks for the excellent photo. I understand they also easily became clogged and had to be cleaned periodically. Clogged screens further restricted airflow. They were finally done away with in mid-'42.Backfire screens were not needed once a pilot learned how to start an Allison. I've been present or started myself more than 40 Allisons and only have seen a backfire twice, both times when a new owner was learning to start it. There was no damage either time.
This is an Allison backfire screen from my collection:
View attachment 587975
This screen was inserted into the intake manifold right at the intake valve and, as you can imagine, interfered with the intake flow to a not insignificant degree. The outside envelope fit perfectly into the intake manifold.
That's actually higher. It seems that it would be less likely to backfire. If S Shortround6 is correct that it had to do with starting, I'm curious what conditions tended to cause it to backfire on starting.8.8 geared Allisons were 51 for takeoff and 42 for military.