"Gross" and "Net" rotaries power outputs.

[Bletchley]

I have been chasing down the statement attributed to S.D. Heron by Andrew Nahum, to get to the primary source (I am always a little bit suspicious of secondary sources), and here it is! From Heron, S.D. History of the aircraft piston engine: a brief outline. Ethyl Corporation, 1961, p.8-9 (an elusive book, but worth getting hold of if you can...):

"The rotary engine builders objected strenuously to the power of their engines being determined on the dynamometer. They insisted that torque stands should be used. On a torque stand the very considerable windage loss due to rotating the cylinders was recorded as useful horsepower. The engine builders had some justification in their objection to the dynamometer since the engine was usually run in the open and the windage losses were significantly higher than when cowled in an airplane.
During World War I, I had the job at the Royal Aircraft Factory of determining the windage losses of the Bentley BR1 rotary engine, the work being done under the direction of A.H. Gibson. The BR1 was a 9-cylinder rotary with a cylinder capacity of 1055 cubic inches and a rated torque-stand horsepower of 150 at 1250 rpm. The engine was run on a dynamometer both in the open air and with a standard rotary cowl taken from an airplane. Windage horsepower was determined by motoring the engine with the pistons and connecting rods removed. The gross horsepower of the engine (dynamometer hp plus windage hp) was about 142. The windage horsepower was 24 with the engine in the open air and 16.5 when cowled. I have been able to find a report which I wrote dated April 18, 1918 covering this work."

Note that this date would mean that this engine was the lower compression AR1 prototype, not the high compression BR1 (the date matches the date of the engine test on the Air Board data sheet). William Morse (Rotary engines of World War One, Nelson & Saunders, 1987, p.53) states that this version of the BR1 was originally rated at 130 hp, not 150 hp.

I hope this helps

Bletchley

[Greybeard]

Thanks Bletch!

Surely it helps!

GB

[rc.gardner]

I have come across a Camel prop which might just have a bearing on this thread. It has standard Camel hub markings for the drawing number of AB644 except that the engine is described as 130 HP AIR.

I have not seen this description before and cannot think of any relevance to a rotary engine other than he-who-stamped-the-boss got his letters the wrong way round and it should be AR 1. This, I thought, could not be correct as the AR1 is quoted as 150hp but the thought expressed in this thread that it was 127hp is interesting. Also, is there any mileage in the thought that Bentley was tasked to produce a reliable version of the Clerget, so the initial intent would be to match the quoted 130hp, development soon raising the AR1's power to 150hp?

I would appreciate any thoughts.

With regards,

Bob

[Bletchley]

"Also, is there any mileage in the thought that Bentley was tasked to produce a reliable version of the Clerget, so the initial intent would be to match the quoted 130hp, development soon raising the AR1's power to 150hp?"

Yes, I think so. The big problem with the Clerget 9B was the obturator rings that were designed to distort or flex to follow the shape of the nickel-steel cylinders, as these were prone to distortion due to unequal cooling of the cylinder sides. This was particularly bad in the British licence built Clerget's made by Gwynnes, which had an effective time between overhauls of only 15 hours because of the poorly made obturator rings or cylinders (30-50 hours for French built engines), and a tendency to loose compression and therefore power between overhauls because of poorly fitting rings. W.O. Bentley was initially sent to Gwynnes by the Admiralty to sort out this problem, and made a number of improvements to the 9B, including fitting aluminium-alloy pistons and raising the compression ratio to create the 9BF. This helped reliability, as the aluminium alloy pistons would conduct heat away better than the old cast iron pistons, but the obturator rings remained as Bentley could not persuade Gwynnes to change out the steel cylinders for cast aluminium cylinders with shrunk-in cast iron liners. The Le Rhone rotaries had successfully avoided the use of obturator rings by using cast iron liners, and their rotaries were more reliable as a result. Having improved the reliability of the 9B, Bentley was given the go ahead by the Admiralty to move to Humbers where he could design his own rotary engine using both aluminium-alloy pistons and cast aluminium cylinders with cast iron liners. According to William Morse ('Rotary engines od World War One', p.53), the prototype was a "130 hp nine-cylinder rotary...first known as Admiralty Rotary No.1 (A.R.1) and subsequently as the Bentley Rotary No.1 (B.R.1)". This was flight tested in April 1917 (although it is listed, wrongly Greybeard thinks, and I agree, as a 150 hp engine), and S D Heron (who did the testing) states that it had an output of approx. 127 hp net (about the same as the Clerget 9B and less than the Clerget 9BF, the Gwynnes engine, that was tested in July 1917 and produced 148 hp). I am sure that it is this engine that was first fitted to the RNAS Camels in France and became operational in June 1917, as a Camel with the later (high compression) B.R.1, correctly rated at 150 hp and producing around 154-158 hp, was not flight tested until August 1917, and Bentley himself says that they were not operation until the autumn of 1917. It is complicated further, as there appears to have been no clear transition from the A.R.1 to the B.R.1, as production of the A.R.1 went hand-in-hand with development and the old name A.R.1 soon gave way to B.R.1 as original A.R.1 engines were modified in the field.

So, your prop may well be from this first operational batch of A.R.1 Camels

The puzzle for me, is why did the RNAS fit a 130 hp A.R.1 to their Camels in June/July 1917 when they already had the much improved British built Clerget 9BF (Gwynnes was under contract to supply engines to the Admiralty, not the RFC)? From the Air Board test figures that we have, this engine was clearly on a par, at least in power and performance, with the later high compression B.R.1 that would not be available for several months. I would guess that it was due to either production or to continuing reliability problems with the Gwynnes built Clerget 9BF, following Bentley's move to Humber.

Sorry for the long-winded reply. Essentially, yes, I agree, unless AIR means "air cooled"

[Bletchley]

Sorry, duplicate post

[rc.gardner]

Thank you Bletchley,

This erudite response adds substance to my musings!

I see mis-stamps fairly often on props, so it is certainly quite likely that AIR (which I have never seen before) was a new fangled name which the stamper sometimes couldn't quite master whilst chatting to his mate.

As to why the Navy preferred 130hp AR1's to 150hp Clerget 9 BF, my intuitive feeling is reliability. Soldiers and airmen want reliabilty. A weapon or aircraft which is unreliable is a disaster. If you squeeze the trigger, in any metaphorical sense, and it doesn't work, soldiers will go in search of something which does. Or sailors, in this case!

With thanks,

Bob

[Greybeard]

Bletchley wrote:

"The puzzle for me, is why did the RNAS fit a 130 hp A.R.1 to their Camels in June/July 1917 when they already had the much improved British built Clerget 9BF"

For me too. Also, since were built A LOT of them: from 1750 to 2350 according to sources; WHERE were they fitted?

GB

[Bletchley]

Bob, yes the RNAS appears to have had a preference for good reliability over performance (no doubt as a result of having to fly for long periods over water, where a forced landing is not much of an option), and Graham Mottram (in his booklet 'W.O. Bentley's aero-engines') states that "Reliability [with the B.R.1] was much improved. An early Bentley engine was flown for 100 hours with only minor adjustments, whereas Gnome and Clerget engines were being stripped down at 30-40 hours for major rebuilds". Gerard Hartmann, however, appears to suggest that the Clerget 9BF was at least more reliable than the French built 9B: "Les moteurs britanniques se montrent robusteset fiables, probablement superieurs aux moteurs montes chez Clerget-Blin a Lavallois": Le Clerget 130 ch

GB, my guess (and it is only that, I have no evidence yet to support it), is that the Clerget 9BF was at first the alternative powerplant to the B.R.1 for the RNAS Camels until April 1918, but when the RNAS and the RFC merged to form the RAF on 1st April 1918 the 9BF would then have become available to the former RFC Camel squadrons and would have been phased in alongside, or as replacements for, the RFC 9B Clerget Camels. If this is the case, then the performance of these RFC Camels would have been almost as good as the Bentley Camels in the summer of 1918, but as the Camel was already largely obsolete by then and mainly restricted to a ground support role this may not have been subsequently noted or much appreciated.

Although I have no evidence yet that former RFC units were drawing engines from RNAS stocks or contractors, I do have some evidence that former RNAS Camel squadrons were not restricted to drawing from the RNAS aircraft parks after April 1918: Christopher Draper, in his memoirs 'Naval Eight' records that after 'loosing' all their Bentley Camels in the Spring 1918 German offensive, they were replaced with Clerget Camels from 'the old RFC network'. He doesn't say if they were Clerget 9B or 9BF engines.

Bletchley

[Bletchley]

I have found this in Edward Kocent-Zielinski's 'Sopwith Camel' (Kagero, 2003):

"Gen. Trenchard (RFC Commander) ordered on Dec.9, 1917 that production machines be fitted with the lower-rated 9 cylinder Le Rhone 9J of 110 hp, which ensured a better rate of climb for the Camel than the Clerget 9B. Later production aircraft were fitted with Clerget 9Bfs...Later, all the earlier built Camels were to be re-engined, which at French units began only in April 1918."

There is no primary source referenced, but if true it would appear to support my guess that the Gwynnes 9BF from Admiralty stocks was phased in to use on the Clerget Camel squadrons after the formation of the RAF in April 1918, and this engine would probably have become the primary powerplant for those former RFC squadrons by the summer of 1918.

Bletchley

[Greybeard]

Originally Posted by Bletchley

There is no primary source referenced, but if true it would appear to support my guess that the Gwynnes 9BF from Admiralty stocks was phased in to use on the Clerget Camel squadrons after the formation of the RAF in April 1918, and this engine would probably have become the primary powerplant for those former RFC squadrons by the summer of 1918.

Bletchley

Thanks Bletchley, you're giving a significant contribution to rebuild history of Camel's powerplants while clarifying ways to measure rotaries power output.

I don't think, though, they really fitted 110 hp Le Rhone during winter 1917-18 since, although being about 66 lbs lighter than Clerget, it couldn't give better climb having 20 hp less... I believe they were indeed Le Rhone 9Jby, giving 140 hp, still called "110 hp" like they did with all Le Rhone 9J.

If so (please correct me if I'm wrong), the "big picture" about Camel's engines on the front should be: 130 hp Clerget9B until dec. '17, 110(140?) hp Le Rhone until april 1918, then 140 hp Clerget 9Bf for RAF ex-RFC and 150 hp Bentley BR1 for RAF ex-RNAS?

GB