# Bristol Hercules. Why doesn't this engine get more respect?



## davebender (Sep 11, 2012)

Bristol Hercules - Wikipedia, the free encyclopedia
875 kg dry weight (Hercules II).
1,290 hp. 1939.
1,375 hp. Hercules II.
1,650 hp. Hercules IV.

57,400 total engines produced. Why doesn't this air cooled radial engine get more respect? It was reliable, available early on and had a decent power to weight ratio.


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## ShVAK (Sep 11, 2012)

How was its performance above 20,000 feet?


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## Jabberwocky (Sep 11, 2012)

It gets plenty of "respect", its just not a glamor-pus as it didn't power any single engine fighters during the war, and thus is not the object of online wang measuring contests.

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## Gixxerman (Sep 11, 2012)

I have a bit of an interest in the sleeve valve engine, a fascinating solution to some of the issues inherent in the poppet valved OHC/OHV engines.
Yes it can mean more weight and maintenance complexity (esp if units are not well used to it) but in a pure engineering sense it offers a lot of unique benefits.


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## buffnut453 (Sep 11, 2012)

Jabberwocky said:


> It gets plenty of "respect", its just not a glamor-pus as it didn't power any single engine fighters during the war, and thus is not the object of online wang measuring contests.



Is that an Imperial wang, metric wang or NATO Standard wang? 

Great post Jabberwocky - gave me a much-needed laugh!


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## nuuumannn (Sep 12, 2012)

> and thus is not the object of online wang measuring contests.


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## davebender (Sep 12, 2012)

Perhaps most important of all, it didn't power the P-51D or F4U. Both of those aircraft are media hogs (at least in the USA).

The Wright R-2600 engine doesn't get mentioned much either even though it's one of the most important WWII era aircraft engines with over 50,000 produced.


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## fastmongrel (Sep 12, 2012)

Simple it wasnt made by P&W. If P&W had made a cast iron single cylinder sidvalve engine with Hit Miss ignition internet fanbois would be calling it the greatest engine ever built 

I have often wondered if a folding wing Hercules engined Hurricane could have been a good naval fighter.


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## davebender (Sep 12, 2012)

The Hercules engine might be ideal for the Ju-87 dive bomber and Fw-190 fighter. Bramo or BMW would need to acquire a license agreement 1935 - 1937 during the Anglo-German détente. A whole lot quicker then designing the BMW801 from scratch.


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## Shortround6 (Sep 12, 2012)

Will you please get off this "license agreement" before an engine is even flown in a test mule aircraft nonsense. It goes both ways. Nobody was going to pay money for a license for a DB 601 in 1936-37 because as far as the world outside Germany was concerned the DB 601 was "vaporware" nobody had even seen one. 
Almost the the same with the Hercules, Bramo and BMW can go to Bristol in 1935-37 and say "here, take our money, now when can we start making engines" and Bristol will answer them "Oh, in about 3-5 years, we haven't figured out how to make them on a production basis ourselves yet, thanks for the money, when we figure out how to make the sleeves we will let you know". 
Bristol didn't figure out how to make the sleeves on a production basis until late 1939 or very earlier 1940.


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## Balljoint (Sep 12, 2012)

I’ve heard that the Lancaster was fitted with Merlins because an appropriate radial wasn’t available early on. But then I’ve also heard that some were later fitted with the Hercules only because Merlins were in short supply.

Perhaps both are true in that a radial would seemingly be the better choice for a bomber. But then the Merlin did so well changing wasn’t comfortable.


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## wuzak (Sep 12, 2012)

Balljoint said:


> I’ve heard that the Lancaster was fitted with Merlins because an appropriate radial wasn’t available early on. But then I’ve also heard that some were later fitted with the Hercules only because Merlins were in short supply.
> 
> Perhaps both are true in that a radial would seemingly be the better choice for a bomber. But then the Merlin did so well changing wasn’t comfortable.



The latter is certainly true. For a while Merlin supply looked to become critical - many of the RAF's major types used the Merlin.

Conversely, the Rolls-Royce designed engine module used on the Lancaster was originally developed for the Beaufighter in case Hercules production couldn't meet demand. It was, most likely, the availability of the engine module, which made the conversion easier, that led to the Merlin being chosen. 

The Halifax was designed aroudn Merlins when Handley Page changed to a 4 engine design. The Shorts Stirling was designed around the Hercules. These both predated the Lancaster conversion. So that would suggest that the Hercules was available to Avro for the Lancaster frm the start.


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## davebender (Sep 12, 2012)

RAF Bomber Command required so many engines that production cost matters. How did production cost of Bristol Hercules compare to production cost of RR Merlin?


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## Shortround6 (Sep 12, 2012)

I am not sure anybody has any really good figures. Some writers have talked about the sleeve valve engines costing 2-3 times per hp what the Merlin or American radials cost but have never shown an numbers to back it up, Bristol spent Millions (in 1930s pound Stirling) to develop the sleeve valve engines and nearly bankrupted the company. Do you count the research and development costs or just the manufacturing costs? I am not sure about war time engines but post war Hercules engines in civil service achieved some remarkable times between overhauls. 

In the late 30s or early 40s you could find photos showing the number of parts for a Bristol sleeve valve cylinder and a Bristol 4 valve Cylinder from a Mercury or Pegasus and the poppet valve cylinder had a bunch more parts, what was not shown was the several dozen gears in the Hercules sleeve valve drive system. Is it harder to make rocker arms and valve springs or gears??? 

The arguments go both ways but without some actual facts it is really hard to say.


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## nuuumannn (Sep 15, 2012)

I guess the answer to Dave's original thread title depends entirely on your perspective; from a British point of view, the Hercules gets tons of respect and is quite rightly regarded as one of the most successful wartime and post-war piston aero engines built. I can't offer facts and figures regarding reliability, but I know engineers who worked on them on aircraft like the Short Solent flying boats with our national airline in the 1960s and the Bristol Freighter, examples of which were flying here in New Zealand until the late 1980s. I also know of a chap who has carried out a ground up restoration on two to running condition to power one of the ex-Safe Air Bristols. None of these guys have ever commented on difficulty in comparison to other piston engines.

The Herc was - along with the other Bristol sleeve valve engines, something of a work of art. Their concept was quite different at the time and although tolerances of the sleeves to distorsion were small, in terms of maintenance complexity and reliability, certainly did not prove insurmountable. If they were that bad, then the Herc would not have been applied in the large number of different types it was used in.

Sure, the guys on the hangar floor would have moaned about the problems with sleeve valve engines when they first appeared, but over time, with maintenance frequency, engineers get used to a certain way of doing something and take things in their stride. The fact that Hercs were still in scheduled operations into the 1980s isn't just a testament to the fact that New Zealand is something of a home to old aircraft operating past their sell by date )), but to the engineers and equipment they service.

A beautiful sectioned example in Scotland







Life is a pair of Bristols!








> I’ve heard that the Lancaster was fitted with Merlins because an appropriate radial wasn’t available early on.



Well, sort of; as early as April 1937 Roy Chadwick had raised the proposal of a four engined Manchester powered by Hercules', but many in the Air Ministry thought the Manchester had no further development potential; the only way the Lanc came about was through Avro pushing that Manchester production lines could be adapted relatively quickly.

Early discussions before the Manchester was built as early as 1937 stated that the prototype was to be powered by the Hercules, the HP.56, which was the twin engined forerunner to the Halifax to be powered, like the Manchester by the Vulture, was at one stage considered for the Hercules, although four Merlins were chosen because of lack of availability of the Hercules.

When exactly the four engined Manchester got its Merlins, I'm not sure, but it seemed the logical choice as the Halifax was being used as the benchmark for performance and capability, although the Halifax I was an utter dog. Ironically it wasn't until the Hali III that the Handley Page bomber got performance and capability near the Lancaster's!


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## fastmongrel (Sep 16, 2012)

Shortround6 said:


> In the late 30s or early 40s you could find photos showing the number of parts for a Bristol sleeve valve cylinder and a Bristol 4 valve Cylinder from a Mercury or Pegasus and the poppet valve cylinder had a bunch more parts, what was not shown was the several dozen gears in the Hercules sleeve valve drive system. Is it harder to make rocker arms and valve springs or gears???



Making gears is relatively straightforward once you have the machinery set up. A 1940s gear cutter will churn out precison gears 24/7 as long as it is fed with forged and machined blanks, the machine is automatic once the blanks are in place. The Germans thought the Hercules was a very expensive job but possibly this is because they were always chronically short of gear cutting machinery and had no experience of sleeve valve manufacturing on a production line. 

I am no expert in manufacturing but just looking at a late radial compared to an equivalent late inline there are a heck of a lot of extra parts in the radial. Just manufacturing say 14 individual cylinders + 14 heads compared to 2 blocks + 2 cylinder heads whilst its not a simple 7 to 1 components ratio it surely must be more time and money intensive. 

It would be interesting to see how much Packards price per horsepower with the V1650 compares to Wrights R2600. Both manufactured on similar state of the art lines minus the extra expense that Bristol went through developing the sleeve valve.


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