Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: 63tb on July 05, 2005, 12:51:33 PM
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It's a slow day at work so I thought I'd ask a question that's puzzled me for a while.
I've seen it mentioned here quite a few times that one of the advantages to the P-38 design was that its props spin in opposite directions. If this is a design advantage, why don't all twin engine types use this?
63tb
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I think its a factor of cost and design. More parts..ect..
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What nuchpatrick said. The left and right engines on the P-38 had to be assembled seperately. This means almost twice the manufacturing overhead for the extra production lines. Maintenance is also worse as many parts cannot be interchanged between the two engines.
From a pure performance standpoint though, there isn't any reason you'd ever want a multiengined plane without counter rotating props.
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Oh, I thought the engines were identical but the reduction gear boxes were geared in reverse.
63tb
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Dont know, but from an engineering standpoint it would be aweful to make a completely mirrored angine.
I also think there is an exchange gearbox, one for left, one for right.
Anyway, thats a guess from my side.
If they really have mirrored engines in the 38, I would sign a paper approving hospitalization in a pyschiatry for both the engineers and their boss ;)
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Hi 63tb,
>Oh, I thought the engines were identical but the reduction gear boxes were geared in reverse.
I believe that was the more common way to do it, but the Allisons appear to have been complete mirror images indeed.
The German DB603 was developed to be right-hand or left-hand configurable, but apparently the advantages were found to be marginal so this feature was not used in practice. Of course, the German war industry was under much heavier strain than the US industry, so they were probably more conservative about increasing production complexity.
Regards,
Henning (HoHun)
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couldn't you just make the engines the same except have the drive shaft go out opposite ends, then mounting one "backwards"?
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With the Allisons, you swapped the crank around in the block (end for end), among other things, and rewired the mags. It didn't require different parts, just assembling them different.
The gearbox for the props was a problem though. Early gearboxes were the same, and because of the helical cut on the gears, there were problems with internal thrust wear.
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Wouldn't designing a "reverse" gearbox be a lot cheaper than making an engine that could be "reversed"? Could it have been something to do with the way the gears needed to be tooled?
Also what about 4 engined planes? Is there an advantage to have two on one wing spin opposite from two on the other? Did anyone do that?
63tb
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The Allison was originally designed to power Navy airships, and as such, was originally designed to run either way.
Since it was designed to run both ways to begin with, there was no need to come up with a reverse gearbox.
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Originally posted by 63tb
Wouldn't designing a "reverse" gearbox be a lot cheaper than making an engine that could be "reversed"? Could it have been something to do with the way the gears needed to be tooled?
Also what about 4 engined planes? Is there an advantage to have two on one wing spin opposite from two on the other? Did anyone do that?
63tb
A reversing gearbox means one more gear, one more shaft, more bearings, etc. that = more weight. more weight = bad; less weight = good.
As for four engines, the assymetric thrust of an inside engine going out and flying on an outside engine is probably of a larger concern.
If all engines are identical, manitenance, warehousing, cannibalization, and parts supply chain are greatly simplified.
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the russian Tu-95 "Bear" used counter-rotating props very successfully. achieving speeds up to 925km/h.
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Originally posted by SMIDSY
the russian Tu-95 "Bear" used counter-rotating props very successfully. achieving speeds up to 925km/h.
I think you mean contra rotating props, in the case of the Russian Bear.
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oh, i miss understood what you guys were talking about. feel kinda stupid now.
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Originally posted by SMIDSY
oh, i miss understood what you guys were talking about. feel kinda stupid now.
No need to feel stupid, it was an easy honest mistake some pretty experienced people make.
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Originally posted by Ecliptik
This means almost twice the manufacturing overhead for the extra production lines. Maintenance is also worse as many parts cannot be interchanged between the two engines.
That statement is completely false. Listen to Virgil, Allisons's are just assembled differently, but use common parts.
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Virgil misses working on Allisons terribly. I love those big sweethearts. When I heard both Glacier Girl and Porky II it brought back memories of working on those engines. About two years ago, I got lucky, and there was a killer Allison powered tractor on TV, and I cranked up the home theater. That one was direct injected with a custom intake and a BIG turbo. There was an Allison power unlimited hydro that actually won a race a year or so ago, it was the 1st piston engined boat to beat a turbine boat in about 12-15 years. Sounded great with the system cranked wide open.
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I was under the impresion that the allison engines the P-38 used were identical. The gear boxes (which could be unbolted from the engines) were the only thing different. to make the left engine into the right engine or the other way around, the only thing needed to be changed was the gear box. I believe the crank shaft in the left and right engines spun in the same direction.
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Nope. Like Virgil said, they just swapped the camshaft end for end, reset the mags, and likely redid the pumps and alternaters and such. But the engines litterally turned in opposite directions.
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yep I see my error now.
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Hey Virgil, you know if Allison even makes those engines any more? Seems like they would, what with racers and pullers wanting to get their hands on them.
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Originally posted by Tails
Hey Virgil, you know if Allison even makes those engines any more? Seems like they would, what with racers and pullers wanting to get their hands on them.
No, they haven't been built in decades. Everyone has been running surplus and scavenged stuff for all this time.
Allison is now, and has been, the truck transmission division of GM for 30+ years.
As opposed to 25 years ago when I was working on them, there are practically no Allisons left in pulling or boat racing. The turbines took over unlimited hydroplane racing, and tractor pulling is mostly dominated by the big block Chevy in a confirugration similar to NHRA Top Alcohol Dragster engines.
Oddly enough, I think Oliver and Eagle make connecting rods, Ferrea makes valves, and a couple of the big name racing piston companies make pistons. Other than that, plus overhaul parts, nothing new is available for the most part. That's why an overhaul costs $25K.
Every once in a while, you'll see some NOS surplus engines creep into the market. They bring more money than you can dream of.
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Well, I know some part of Allison still makes aviation engines, what with the A-250 on the turbine side, and the opposed type engines Piper likes to use. Maybe, at least, someone can get the castings off these guys (assuming they still exsist). Hate to see such a neat 'little' engine fade away once all the blocks wear out.
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I think GM sold off the Allison aircraft section.
I seriously doubt any of the molds are left.
It was a very complex engine when you think about when it was designed.
Reproducing it today, even if you have all of the dies and molds, would cost about $500K a copy. That is to produce a complete new Allison V-1710, without any used parts. That would not buy you the turbocharger for the P-38.
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The Allison that you knew and love is actually owned by Rolls Royce today. When we were restoring a P-39D, it had a V-1710-30, problem was of course finding the erection manuals and all that fun stuff. But I was in Champaign Illinois at the time and a very helpful RR rep based in Indianapolis was awesome enough to help go through the archives they had there and find everything needed.
The initial phone call went something to the effect of "I have a very strange request to ask, so don't fall out of your chair. (Ok) I need get some documentation on an engine that you guys havn't made in over 60 years." (extended silence)
Anyways, great engine all around - bulletproof compared to the Merlin.
Wolf
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thought they just injected the gas at BDC and it ran backwards? ..:rolleyes: