Rolls-Royce Merlin fluid drive supercharger

[qantasqf1]

I recently purchased a fascinating book called The Secret Horsepower Race written by Calum E Douglas which deals with the development of piston type aero engines before and during WW2. On pages 58-59 there’s a section that may interest some members of the forum and I’d like to share it.
Here’s the story.
The story concerns engineer Geoffrey Wilde who originally worked for Daimler, but was subsequently employed by Rolls-Royce in 1939. During his days at Daimler he had become conversant with the theory and application of the fluid flywheel and after starting at Rolls-Royce applied this knowledge to develop a similar fluid drive for the Merlin supercharger, superseding its 2-speed Farman type gear drive. The major issue was the fundamental difference between the two since the Merlin supercharger drive demands a variable speed output, unlike that of the more or less solid drive of the Daimler fluid flywheel. He took his idea to the chief of Merlin supercharger development Stanley Hooker (later Sir) who was enthusiastic and said to Wilde that anything that improved the supercharger was to be encouraged. Unfortunately, the Merlin’s 2 speed gear driven supercharger was already embedded in the Merlin design so the fluid drive project was stillborn since the cost of cutting over to it was prohibitive.
Although Geoffrey Wilde’s arrival at Rolls-Royce was too late to develop his fluid drive for the Merlin it is worth bearing in mind that had he started at Rolls-Royce a couple of years earlier there is every possibility it would have been adopted. It is interesting to note that the corresponding drive of the Daimler Benz engined Messerschmitt Bf 109 fighter used a fluid drive for the supercharger, having been developed earlier that decade.
Maddeningly, the book gives an illustration of Wilde’s proposed fluid drive, but there’s no description of how it works. Clearly its variable speed output was achieved by varying the coupling’s oil quantity, but how that crucial detail was achieved is not described.
Copyright prevents me from showing a graph comparing the theoretical performance of the 2 supercharger drive types, but it shows the fluid drive is superior to the mechanical drive at all altitudes and power requirements.

[Christopher Storey]

Very interesting indeed, although I think a fluid drive would have had a limited production lifespan because of the development in 1940-42 of the two stage ( i.e. 2 supercharger ) Merlin to give better high altitude performance . It is some while since I read Stanley Hooker's " Not much of an engineer" but I recall it was equally interesting on this very subject . Hooker's career was bookended by his work on the Merlin which increased its power by a factor of 2, and almost as importantly his rescue of the RB211 when he was persuaded to go back to RR from Bristol and make the ( then revolutionary ) 3 spool engine run properly

[Brian-H]

My interpretation of what Steve has written, is that the fluid drive would have been superior to a fixed single or double-speed gearbox drive.

The Farman drive is a bevel planetary type see here but the issue is that it only had 2 ratios rather than a continuously variable ratio. I note that Steve says that the German Daimler-Benz engines (mostly used in the ME 109) did in fact have a fluid variable-speed drive on its supercharger. Indeed, there seems to be a wealth of stuff out there about the RR Merlin and the Hurricane and Spitfire, but a dearth in comparison on the DB engines and the ME109. It might be that the numbers of Merlin engines produced were incredible

Rolls-Royce: Derby = 32,377
Rolls-Royce: Crewe = 26,065
Rolls-Royce: Glasgow =23,675
Ford Manchester= 30,428
Packard Motor Corp US = 55,523 (used in the P-51 Mustang)

My parents were born and lived near Trafford Park in the mid 1920s, and both ended up working at Metro-Vickers in the war where the Ford factory was used to produce the Merlins that went into the Lancasters made at MetroVicks and assembled in AVRO hangars.

Anyway, basically a variable speed fluid drive works by varying the amount of fluid in the drive e.g. if you empty half of the fluid out of your fluid flywheel you'll have to rev the engine higher to get the same road speed - but don't do it for long otherwise the fluid flywheel will overheat, and the bush will also wear out meaning that the two faces will start to rub.

I've found a couple of youtube videos on Voith variable speed fluid drives (those clever Germans again)
Voith eg1
Voith eg2

[qantasqf1]

Christopher, the graph I mentioned shows BHP v. altitude for the two types with common x and y axes. The Farman plots show a typical stepped line in MS and FS drives while the fluid drive is a smooth curve that shows significantly higher BHP at all points apart from the maximum BHP developed by the Farman drive, where both plots are equal. I would love to show this chart, but I don’t was to compromise the Forum by infringing copyright.

[Brian-H]

I found this graph freely available on the internet

I also found a website that is full of original documents on most WW2 aircraft and some of the engines
http://www.wwiiaircraftperformance.org/

That website has links to this website on the spitfire
http://www.spitfireperformance.com/spittest.html

One of the pages is a performance comparison of Spitfire Mk. I versus Me 109 E
It's a long read but very very interesting. Starts off with "The Spitfire first flew in March 1936. Entry into service was with No. 19 Squadron at Duxford in August 1938 while 18 more squadrons were equipped with Spitfires by the start of the Battle of Britain in July 1940. The first BF 109 prototype flew in September 1935, ironically powered by a Rolls-Royce Kestrel engine. The first production model, the BF 109B-1, was delivered in February 1937. The BF 109 E, the varient that saw action during the Battle of Britain, entered into service early in 1939. Given the multiple aircraft combat environment in which they fought, the performance of the Spitfire Mk I and the Messershmitt Bf-109E was sufficiently close that the results of combat would generally fall to initial position, numbers, tactics and pilot experience."

If you want to see a comparison of the Merlin III vs DB 601 A , use crrl-f and enter "1310" (without quotes) where you'll see a graph. In GB we used boost pressure in PSI whereas in DE they used ata (Atmosphäre, technisch, absolut, = atmosphere, technical, absolute or just atmosphere absolute). On the Merlin III the normal boost pressure was around 6 psi but the engine could be put into emergency boost of 12 psi for 5 minutes by using a boost lever In combat reports various terminologies were used "breaking the wire, pulling the plug, pulling the tit, pushing the throttle through the gate, boost overide, boost cut-out, Emergency, Emergency power"

One of the key developments was 100 octane fuel by the time of the Battle of Britain, which the Germans didn't have. A bit further down from the graphs, there's a quote
"It is an established fact that a difference of only 13 points in octane number made possible the defeat of the Luftwaffe by the R.A.F. in the Fall of 1940. This difference, slight as it seems, is sufficient to give a plane the vital "edge" in altitude, rate of climb and maneuverability that spells the difference between defeat and victory."
100 octane allowed the higher boost to be sustained for longer, and with other developments, 12 psi boost became more-or-less standard.

Here is a picture similar to the attachment above M Gear = Medium Speed gear, S = Full Speed gear. The MS gear gave higher boost at low altitudes, and the FS gear gave higher boost at altitudes over 6000 ft. You'll also see that these graphs are using 15, 25, and 28 PSI boost.

I presume that Steve's book contains this info and more, hopefully these graphs are similar to the ones in the book ? And if so, can you describe where the curve for the fluid drive would be on the attached graph ?

[qantasqf1]

Thanks Brian and for everyone else for the interest in my post. Brian, your graph shows the performance of the established 2-speed Merlin supercharger gear drive while the graph that I was referring to compared the proposed fluid drive versus the Merlin Farman 2-speed drive. Since the fluid drive never saw the light of day I would doubt that particular graph is available on the internet...l may be wrong! However, your graph shows the stepped lines of the Farman drive superchargers that I originally referred to. If you picture in your mind’s eye a rather graceful curve above the stepped lines showing higher BHP at equal altitudes you’ll get the picture.
Steve

[qantasqf1]

It’s like this. I managed to add my own plot to your picture. The heavy line is the fluid coupling type drive. Sorry about the rather unsteady line!

8D3FD912-3250-4A2C-9106-D9425F59887C.jpeg (25.59 KiB) Viewed 1052 times

[Brian-H]

Since the fluid drive never saw the light of day I would doubt that particular graph is available on the internet...l may be wrong!

I doubt if you are wrong. I myself certainly wouldn't even start to look for something like that, and even if I won the Euromillions I wouldn't pay a researcher to look (it might be in an obscure paper somewhere, or possibly in an online google book-reference). Regarding a google online book-reference, and out of interest anyway, what is the book's ISBN ?

[qantasqf1]

Hi Brian. The ISBN number is 978 1 911658 50 4

[Brian-H]

Thanks Steve, I might buy it - but it's out of stock at the moment

There's a facebook page by the author, created 13 August 2016 , loads of photos https://www.facebook.com/TheSecretHorse ... e_internal - several graphs but I didn't see the BHP vs altitude graph

The author was invited to give presentations at various establishments, including University of Modena 'Enzo Ferarri Department of Engineering' , Renault Sport Formula One Team in Paris , Mercedes AMG Petronas F1 Team at Brackley , and another that he doesn't name (at a guess, probably Ferrari themselves, not Honda)

Apparently the book represents 0.7% of his research