Author Topic: twin engines and counter-rotating props (Read 820 times)

63tb · « **on:** July 05, 2005, 12:51:33 PM »

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

nuchpatrick · « **Reply #1 on:** July 05, 2005, 01:02:17 PM »

I think its a factor of cost and design. More parts..ect..

Ecliptik · « **Reply #2 on:** July 05, 2005, 01:36:18 PM »

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.

63tb · « **Reply #3 on:** July 05, 2005, 01:42:05 PM »

Oh, I thought the engines were identical but the reduction gear boxes were geared in reverse.

63tb

Keiler · « **Reply #4 on:** July 05, 2005, 02:30:45 PM »

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

HoHun · « **Reply #5 on:** July 05, 2005, 03:00:04 PM »

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)

JB73 · « **Reply #6 on:** July 05, 2005, 04:02:33 PM »

couldn't you just make the engines the same except have the drive shaft go out opposite ends, then mounting one "backwards"?

Captain Virgil Hilts · « **Reply #7 on:** July 05, 2005, 04:02:39 PM »

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.

63tb · « **Reply #8 on:** July 05, 2005, 04:42:51 PM »

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

Captain Virgil Hilts · « **Reply #9 on:** July 05, 2005, 05:19:22 PM »

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.

Holden McGroin · « **Reply #10 on:** July 05, 2005, 06:47:22 PM »

Quote

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.

SMIDSY · « **Reply #11 on:** July 05, 2005, 07:16:29 PM »

the russian Tu-95 "Bear" used counter-rotating props very successfully. achieving speeds up to 925km/h.

Captain Virgil Hilts · « **Reply #12 on:** July 05, 2005, 07:37:44 PM »

Quote

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.

SMIDSY · « **Reply #13 on:** July 08, 2005, 07:11:32 AM »

oh, i miss understood what you guys were talking about. feel kinda stupid now.

Captain Virgil Hilts · « **Reply #14 on:** July 08, 2005, 05:44:26 PM »

Quote

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.