variable-pitch propellers

[steelpillow]

The advantages of the variable-pitch propeller were recognised early on, and experiments began somewhere around the start of WWI. But the engineering proved a formidable challenge.

By 1925, in the UK Dr. Hele-Shaw and T. E. Beacham were developing a viable hydraulic actuation mechanism. The next year, Gloster Aircraft acquired the rights to their Hele-Shaw Beacham propellers. Prototypes demonstrated their potential, but the engine industry preferred to sell bigger, overpowered and more expensive (i.e. profitable) solutions. Only Japanese company Okura bought a license, in 1928.

In 1929 two significant events happened. One was the arrival of Ministry interest and trial orders at last, the other was the moment when Tom Hamilton visited the Gloster stand at that year's International Aero Exhibition at Olympia and, on seeing a propeller on display there, promptly visited the Gloster works. His company's US patent for the two-position propeller was lodged shortly afterwards; a few years later, de Havilland would license it back from him.

All this from Derek N. James; Gloster Aircraft Since 1917, Putnam, 1971, pp.16-7.

But the timing reported by James is somewhat invidious. The Seventh International Aero Exhibition ran from 16 to 27 July 2029. However US Patent 1,893,612 had already been filed on 25 May, 1929. Hamilton lodged their British application in 1930, UK Patent 355329A being granted in August of the next year, the timing of which may have misled James into believing it set the date of invention. Or was James perhaps referring to a subsequent Hamilton patent which adopted Hele-Shaw Beacham technology?

Either way, it seems that Hamilton and Hele-Shaw/Beacham were working in parallel along similar lines. One wonders who knew what about each other's work.

And here's another odd thing. Gloster continued to manufacture Hele-Shaw Beacham propellers for their own aircraft, and sold them elsewhere too; eventually the propeller business would morph into Rotol. But when in 1934 de Havilland sought a V-P prop for the nascent DH 88 Comet Racer, there is no record of them looking to their compatriots, only to the US and France. Why ever not?

 

[fortrena]

One could argue that the hydraulically-operated variable pitch propeller was mainly developed by two teams, one in the UK and the other in the US. The latter, led by Frank Walter Caldwell, chief engineer at Hamilton Standard Propeller, created the first variable pitch propeller of its type to reach production. If I understand correctly (not an engineer...), to attain the low blade angle, or fine pitch, required for takeoff, oil under pressure from the engine was allowed to flow through a collector ring into the propeller shaft to move a piston inside the hub in an outward direction, thus forcing the solid aluminum-alloy blades to change their pitch. For cruising, the oil was returned to the engine crankcase by the pull of counterweights, and the movement of the oil reversed the piston, returning the blades to the proper, high pitch setting. The manually-controlled Hamilton Standard two-position propeller, flight tested some time in 1932, insured the success of a new generation of American airliners, starting with the trend-setting Boeing Model 247. It should be noted, however, that while this technology was only introduced with the new and improved Model 247D which flew in 1934, the potential impact of the new propeller was such that Hamilton Standard Propeller, thanks to Caldwell, won the most coveted annual aviation award in the US, the Collier Trophy, in 1933.

Although flight tested as early as 1928 by Gloster Aircraft, the British Hele-Shaw Beacham hydraulically-operated propeller, developed by a First World War pilot from the Royal Flying Corps and later professor at the University of Liverpool, Dr. Henry Selby Hele-Shaw and a former Royal Army Ordnance Corps captain, Thomas Edward Beacham, received little support from the UK authorities until the second half of the 1930s. Gloster Aircraft or - as it was known in 1926 - Gloucestershire Aircraft was the only British aircraft to show much interest. Indeed, it bought the design and manufacturing rights of the Hele-Shaw Beacham propeller in 1926. If I understand correctly, it differed from its American rival in that it was the cylinder at the tip of the hub that moved, using oil under pressure from the engine; the piston inside the cylinder was fixed. Because of this, the Hele-Shaw Beacham propeller did not need the large external counterweights of the Hamilton Standard propeller.

Interestingly, it is said that Thomas Hamilton of Hamilton Aero, a subsidiary of the American conglomerate United Aircraft and Transport, was most impressed by a Hele-Shaw Beacham propeller he saw in the Gloster Aircraft stand at the 1929 International Aero Exhibition held in London. This interest may have played some small part in launching the research project that led to the creation of the Hamilton Standard variable pitch propeller.

Even though some experts in Europe remained doubtful, the introduction of the first practical variable pitch propellers, a hydraulic one by Hamilton Standard Propeller and an electric one by Curtiss Aeroplane Division of the Curtiss-Wright Corporation, did not go unnoticed. Helped along by the superb service record of its designs on the new American airliners, Hamilton Standard Propeller, for example, was able to conclude licensing agreements in at least nine countries during the 1930s. Among the famous names involved were de Havilland Aircraft in Great Britain, FIAT in Italy, Hispano-Suiza in France, Mitsui in Japan, and Junkers in Germany. In the case of de Havilland Aircraft, the purchase of the production rights within the British Empire - with the exception of Canada - had to be made using the company’s own limited funds, with some (minor?) encouragement from the British Air Ministry, because there was little interest shown by the Royal Air Force or the British aircraft industry.

This lack of interest may have been caused in part by the increased cost and greater weight of the new variable pitch propellers, especially on relatively small aircraft. Merlin-powered combat aircraft like the Fairey Battle, Hawker Hurricane and Supermarine Spitfire entered service in 1937-38 with two-blade fixed pitch propellers, for example. Mind you, the high efficiency and reliability of the geared engines made in the UK from 1928 or so onward may have played some role in delaying the wide scale introduction of variable pitch propellers in that country in the 1930s. Even though geared engines were somewhat heavier and more complex to build than direct-drive engines, the improved propeller efficiency was more than worth the effort. Heavy aircraft could fly farther, climb faster and take off from smaller fields.

 

[Nik]

"Why ever not?"
'Snobbery' ??
Snark:
Having seen corporate politics in action, another answer might be 'Not Invented Here'.
Literally, 'Corporate' would not give credence to any notion brewed lower down their tree, or by a 'lesser' rival...
It had to originate from them, or ride in via mega-buck 'Consultants'...
{ Face-Palm.}
Upside, it got me banned from our focus groups, as I'd damned too many wondrous notions that subsequently 'crashed & burned' for the very obvious reasons I'd given...
/

 

[fortrena]

If I may, Rotol Airscrews was registered in May 1937 by the largest aeroengine makers in he UK, Rolls-Royce and Bristol Aeroplane, to develop and produce the Hele-Shaw Beacham propeller design they had both been experimenting with. The project was originally the brainchild of one of the most brilliant engine designers of the age, Albert Hubert Roy Fedden of Bristol Aeroplane. Intrigued by the performance of the Hele-Shaw Beacham propeller, Fedden was able to engineer the purchase of its design and manufacturing rights from its owner, Hawker Aircraft, which had taken control of the rights from the original owner, Gloster Aircraft.

At the risk of stating the obvious, Rotol Airscrews’ name was an acronym, with the Ro coming from Rolls-Royce and the tol from Bristol, coined by the wife of the new company’s general manager.

In early designs, the hydraulic cylinder could be fully enclosed within the hub. Later propellers with a higher operating range - 35 degrees and more - needed a larger cylinder mounted externally. Some of these could be fitted with a feathering mechanism.

In 1937, soon after its incorporation, Rotol also took over the rights for the Curtiss Electric propeller that Bristol had purchased some time before. Only a few thousands of these were built during the Second World War. On the other hand, Rotol was said to have built 100 000 hydraulically-operated propellers by the end of the conflict.

Rotol built the blades of some of its early propellers out of solid magnesium bur soon realized that, although lighter than aluminum, this metal was quite vulnerable to nicking from rocks and other objects found on airfields - a conclusion shared by German makers.

Even though it later used some aluminum-alloy blades, Rotol relied largely on wooden blades during the Second World War, a choice that proved very useful early on given the potential risk of wartime shortage of aluminum alloys not to mention the insufficient press and hammer capacity in the UK to produce such blades in sufficient numbers. Ironically, wooden blades eventually proved as vulnerable to damage as magnesium blades

Rotol used three variants of improved or composite wood in its propellers. Two of them were derived from the patents acquired from Gustav Schwartz Propellerwerke and H. Heine Mobelfabrik Propellerwerke in Germany a few years before the start of the Second World War. The British blades produced with the help of these patents were known as Weybridge and Jablo blades. The latter were built by Jablo, a company founded in 1936 by pioneer German pilot and propeller manufacturer Bruno Jablonsky. Later that year, Jablo merged with Frederick Tibbenham, a well known furniture maker which had made thousands of propellers during the First World War, to form Ipswich Propellers. Another offshoot, Jablo Propellers, came into existence in early spring of 1937.

Depending on the process, the wooden blades used by Rotol were protected by a bronze gauze covered by synthetic resin or by a thin layer of cellulose-based material. The latter could be used alone or in conjunction with fabric or a bronze gauze. As was the case with most wooden propellers dating from the First World War, the blades’ leading edge was in most cases protected by a metal sheathing of some sort. In case of accident, such blades tended to shatter and, as such, (greatly?) reduced the risk of secondary damage to the engines, their mountings and the propeller hubs. As well, wooden blades weighed less and were said to be easier to repair.

 

[publiusr]

We have heard about wing warping….but with materials research, could a propeller grow…change shape in flight?

 

[fortrena]

The following may be of interest. If I translate this text correctly, the Great War era SMAM propeller was made of thin strips of wood glued at right angle to the axis at the level of the hub before being twisted in a special mold. As the speed of rotation increased, the blades bent at the tip and twisted in their mid sections.

 

[chrismcd3]

If I may, Rotol Airscrews was registered in May 1937 by the largest aeroengine makers in he UK, Rolls-Royce and Bristol Aeroplane, to develop and produce the Hele-Shaw Beacham propeller design they had both been experimenting with. The project was originally the brainchild of one of the most brilliant engine designers of the age, Albert Hubert Roy Fedden of Bristol Aeroplane. Intrigued by the performance of the Hele-Shaw Beacham propeller, Fedden was able to engineer the purchase of its design and manufacturing rights from its owner, Hawker Aircraft, which had taken control of the rights from the original owner, Gloster Aircraft.

Ath the risk of stating the obvious, Rotol Airscrews’ name was an acronym, with the Ro coming from Rolls-Royce and the tol from Bristol, coined by the wife of the new company’s general manager.

In early designs, the hydraulic cylinder could be fully enclosed within the hub. Later propellers with a higher operating range - 35 degrees and more - needed a larger cylinder mounted externally. Some of these could be fitted with a feathering mechanism.

In 1937, soon after its incorporation, Rotol also took over the rights for the Curtiss Electric propeller that Bristol had purchased some time before. Only a few thousands of these were built during the Second World War. On the other hand, Rotol was said to have built 100 000 hydraulically-operated propellers by the end of the conflict.

Rotol built the blades of some of its early propellers out of solid magnesium bur soon realized that, although lighter than aluminum, this metal was quite vulnerable to nicking from rocks and other objects found on airfields - a conclusion shared by German makers.

Even though it later used some aluminum-alloy blades, Rotol relied largely on wooden blades during the Second World War, a choice that proved very useful early on given the potential risk of wartime shortage of aluminum alloys not to mention the insufficient press and hammer capacity in the UK to produce such blades in sufficient numbers. Ironically, wooden blades eventually proved as vulnerable to damage as magnesium blades

Rotol used three variants of improved or composite wood in its propellers. Two of them were derived from the patents acquired from Gustav Schwartz Propellerwerke and H. Heine Mobelfabrik Propellerwerke in Germany a few years before the start of the Second World War. The British blades produced with the help of these patents were known as Weybridge and Jablo blades. The latter were built by Jablo, a company founded in 1936 by pioneer German pilot and propeller manufacturer Bruno Jablonsky. Later that year, Jablo merged with Frederick Tibbenham, a well known furniture maker which had made thousands of propellers during the First World War, to form Ipswich Propellers. Another offshoot, Jablo Propellers, came into existence in early spring of 1937.

Depending on the process, the wooden blades used by Rotol were protected by a bronze gauze covered by synthetic resin or by a thin layer of cellulose-based material. The latter could be used alone or in conjunction with fabric or a bronze gauze. As was the case with most wooden propellers dating from the First World War, the blades’ leading edge was in most cases protected by a metal sheathing of some sort. In case of accident, such blades tended to shatter and, as such, (greatly?) reduced the risk of secondary damage to the engines, their mountings and the propeller hubs. As well, wooden blades weighed less and were said to be easier to repair.

It has also been said that Fedden bullied both Bristol and Rolls into creating Rotol dangerously late!

 

[Scott Kenny]

The WW2 Hydromatic constant-speed prop design basically isn't used anymore, unless you're talking about a warbird that came with it from the factory. It's heavy and significantly more complex than it has to be, using hydraulics on both sides of a piston in the prop hub to hold the prop at any given angle, with the regulating pressure using a separate centrifugal governor.

The more modern design deletes half the hydraulics from the prop by using a spring to push the prop to minimum pitch, and places the centrifugal counterweights on the prop blades themselves.

We have heard about wing warping….but with materials research, could a propeller grow…change shape in flight?

Props do grow a little in flight from the centrifugal force, but yes you could make one twist as RPM changes.