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One seriously LARGE engine


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#1 mariner

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Posted 19 November 2018 - 22:23

Way back in the 1940's Lycoming built the world's largest ever aircraft piston engine for the B 36. It was never used but it had many interesting features including variable valve timing to give high take - of power and cruise economy.

 

https://www.tested.c...-piston-engine/



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#2 gruntguru

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Posted 20 November 2018 - 22:12

Amazing! . . . and not just variable valve timing - it had two camshaft profiles - switchable by sliding the (9) camshafts along their axes.

 

On minor mistake in the article: Lycoming's design captures both negatives: the cross-sectional area of a radial and the plumbing needs of a liquid cooled engine. 

Although it had the cross sectional area of a radial, there were four rows, so the frontal area would have been quite low for an engine with 5,000 horsepower.



#3 Wuzak

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Posted 21 November 2018 - 01:10

Still, you could have 2 Griffons for each XR-7755 and have nearly 5000hp with less frontal area.

 

In fact, you could probably lay the Griffons on their side and be completely within the wing profile. 

 

The Bristol Brabazon I used 8 Bristol Centaurus (2500hp each), mounted in pairs, each couple geared together and driving the contra props. They were buried in the wing. Which is amazing, since teh Centaurus had a frontal area not much smaller than the XR-7755.



#4 Allan Lupton

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Posted 21 November 2018 - 11:10

As stated in the article, the XR7755 had a two-speed gearbox as part of the drive for the two contrarotating propellors. Hydraulically operated, it must have avoided the risk of missing the gear change which would have been a really frightening possibility.

Many high performance engines of the time had two-speed superchargers, which the XR7755 did not, but it did have a couple of turbosuperchargers as well as a mechanically driven one.



#5 gruntguru

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Posted 21 November 2018 - 22:02

I wonder what sort of gearbox that was? Surely some sort of constant mesh with clutch or brake band?



#6 GreenMachine

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Posted 22 November 2018 - 03:11

The gearbox intrigues me.  Why?  Traditionally, power is transmitted by adjusting boost, revs and pitch of the propeller.  I wonder if tip speed was an issue, as no doubt the propeller disc needed to be massive, and at max power the revs would have resulted in significant areas operating at speeds in the transonic and supersonic regions.  By allowing the engine revs to increase while not increasing propeller revs, the power produced could be transmitted more effectively - without trespassing into the poorly understood (for the time) area of supersonic airflow.

 

Looking at the reduction gearbox on the nose of the engine, I wonder if that really is the two-speed gearbox, or if that was an interim single speed box.



#7 GreenMachine

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Posted 22 November 2018 - 03:20

As stated in the article, the XR7755 had a two-speed gearbox as part of the drive for the two contrarotating propellors.


Allan, that is not how I interpret that reference - I think that it implies the opposite, particularly to the musing over 'what propeller' would be used. If contraprops were used, or intended, that would be (effectively) a non-issue.  The illustrations do not appear to me to show fitment for two shafts either.  However, as I speculated above, the initial configuration may have just been a single speed reduction box, maybe even optimised for running on a dyno - if ever it ran at all.



#8 Kelpiecross

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Posted 22 November 2018 - 05:23


I seem to recall reading about a proposed two-speed (two RPM) gearbox for the propeller on a Merlin/Griffon - don't know if it was ever built. Superchargers were finally 2-stage/3-speed.

#9 Wuzak

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Posted 23 November 2018 - 00:44

I seem to recall reading about a proposed two-speed (two RPM) gearbox for the propeller on a Merlin/Griffon - don't know if it was ever built. Superchargers were finally 2-stage/3-speed.

 

Not sure about 2 speed prop drives for the Merlin, but there were other engines under development that were proposed to use such a system.

 

The Wright R-2160 Tornado 42 cylinder liquid cooled radial engine was to use a 2 speed prop drive. The 2 speed prop drive wasn't actually developed, as Wright were still trying to get the Tornado to work, and it never took flight.

 

The Lycoming H-2470 was also proposed to use a 2 speed prop drive. That engine did actually fly in the XP-54 "Swoose Goose", but not with a 2 speed prop drive.



#10 Allan Lupton

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Posted 23 November 2018 - 08:55

Allan, that is not how I interpret that reference - I think that it implies the opposite, particularly to the musing over 'what propeller' would be used. If contraprops were used, or intended, that would be (effectively) a non-issue.  The illustrations do not appear to me to show fitment for two shafts either.  However, as I speculated above, the initial configuration may have just been a single speed reduction box, maybe even optimised for running on a dyno - if ever it ran at all.

I don't need to debate the issue as the two-speed gear is well documented and even that rather Gee-Whizz article clearly states The XR-7755 went one step further by having a two-speed gearbox between the engine and propeller.

All aeronautical piston engines needed reduction gearing to keep the propellor speed sensible and a choice of ratios could improve the effective performance in non-optimum conditions - e.g. hot and high take-offs.



#11 GreenMachine

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Posted 25 November 2018 - 09:00

Allan, apologies, I stand corrected.



#12 Kelpiecross

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Posted 26 November 2018 - 04:20

Not sure about 2 speed prop drives for the Merlin, but there were other engines under development that were proposed to use such a system.
 
The Wright R-2160 Tornado 42 cylinder liquid cooled radial engine was to use a 2 speed prop drive. The 2 speed prop drive wasn't actually developed, as Wright were still trying to get the Tornado to work, and it never took flight.
 
The Lycoming H-2470 was also proposed to use a 2 speed prop drive. That engine did actually fly in the XP-54 "Swoose Goose", but not with a 2 speed prop drive.


I think what I read was about Spitfire developments - so I assume that the engine was a Merlin/Griffon. It is not obvious to me under just what conditions a choice of propeller gear ratios would be an advantage. I vaguely remember that, like the multi-speed superchargers, the prop gears were for high altitude use.

#13 Greg Locock

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Posted 26 November 2018 - 05:43

Initially Spit had a fixed pitch prop, so a 2 speed gearbox would make the best of a bad job. Almost immediately this was replaced by a constant speed variable pitch prop.



#14 Wuzak

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Posted 26 November 2018 - 11:42

Initially Spit had a fixed pitch prop, so a 2 speed gearbox would make the best of a bad job. Almost immediately this was replaced by a constant speed variable pitch prop.

 

The prototype  and initial production Spitfires had a fixed pitch prop, but that was soon changed to a 2 position prop. These were changed to a constant speed prop in 1940, just before the Battle of Britain.

 

http://www.spitfirep...op-11july40.jpg

 

Britain was well behind on propeller development. 

 

de Havilland licence built Hamilton Standard propellers, while ROlls-ROyce and BrisTOL created a joint company ROTOL to develop propellers for their engines.



#15 Charlieman

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Posted 26 November 2018 - 14:12

The prototype  and initial production Spitfires had a fixed pitch prop, but that was soon changed to a 2 position prop. These were changed to a constant speed prop in 1940, just before the Battle of Britain.

In the 1960s and 1970s, few Battle of Britain planes were flying. At least one survivor was a hybrid with a constant speed prop. There were Hurricanes too...