Following on from my previous post looking at WW1 Aero Engines, attention now turns to some of those units used to power the interwar aircraft. These interwar power plants helped aviation design make the transition from the fragile biplane fighters of World War One, right through to the legendary Schneider racers.

Armstrong Siddeley

LYNX – 1920

First tested in 1920, the Lynx was a 7 cylinder radial engine, with the first example, the Lynx I, providing 150hp. Following the rotary engine’s success during the last decade, the move to a more reliable radial engine proved a big step up.

Logically it made sense that the Lynx initially served as a replacement for those ageing rotary options. A notable aircraft which made the move over to radial power with the Lynx was the Avro 504.

A number of new designs featured this engine as well, though the most prolific aircraft to make use of the Lynx would probably be the Avro Tutor, which first flew in 1929 with the Lynx IVC which provided 240hp for this initial trainer, which was designed to replace the 504 which the Lynx first powered.

The engine was produced in its many incarnations until 1939, during which time 6,000 examples had been produced.

Mongoose  – 1926

Introduced in 1926, the Mongoose was a five cylinder single row radial engine. The Mongoose had a number of unique features for the time, most noted of which was that the lower cylinder was placed in the vertical position. The Mongoose engine provided between 135 and 155hp

An airworthy example of this engine can be found on the Hawker Tomtit at the Shuttleworth Collection.

Cheetah – 

In 1935, Armstrong Siddeley first ran the engine that would take the company to war, the Cheetah. This engine was essentially a development of the Lynx, featuring larger bore cylinders. Later models were adapted to use a reduction gearbox as well as superchargers

The Cheetah’s main usage was on the Airspeed Oxford and Avro Anson during world war 2, these types were initially used as light bombers before performing most of the war as navigation and crew trainers. In the UK, Cheetahs can be seen powering both airworthy Ansons at Old Warden and Coventry.

The Cheetah also saw use in the pre-war Bristol Bulldog trainer.

Following on from their collection of “big Cat” radial engines they had been producing during the lead up to the war, Armstrong Siddley would go on to be a pioneer in turbo-prop aero engine design.

As can be seen, the best place to see these old Armstrong Siddeley designs running is Old Warden where an example of each is kept in airworthy condition.

Pratt & Whitney – 1925

WASP

The Pratt & Whitney Wasp radials are now synonymous with a number of american aircraft and the famous “Sound of Round”, the Wasp and the company made their first appearance in the aviation world in December 1925 with the R-1340. The first wasp was a single row nine-cylinder engine.

Named the 1340 due to having 1,340 cubic inches of displacement, the engine featured a new design of crankshaft and master rod, which had been shown to be a weak area in a number of other designs of the era.

Initially this new engine was tested by the US Navy, with the prototype providing 425hp on early runs, it easily passed these Navy tests in March 1926 and soon the Navy had ordered 200 units. This new design opened up a whole new range of performance capabilities for aircraft designers and the wasp would go on to be involved in a number of world records.
The 1340 began with a power output of 450hp, with the final versions putting out 600hp. The list of aircraft that have used the Wasp over the years could take up a whole website on its own, let alone a post! Notable highlights include the T-6/Harvard, P-26 Peashooter, Lockheed Vega, DHC Beaver, Ford Tri Motor and the Gee Bee racers just to name a few!

Its unsurprising, given around 100 aircraft designs making use of this debut design from P&W, that over 34,000 units were made during its production run, which, for a 1920s design, remarkably remained in production until 1960!

Bristol –

Cosmos Engineering 

Bristol’s Aero Engine division originally started as its own company – Cosmos Engineering, whose first design was known as the Cosmos Mercury, a two-row, 14 cylinder radial, launched in 1918.

The Cosmos Mercury saw little use as it turned out, but it did provide the basis for the 9 cylinder Jupiter, which fared better in the post war market.

Roy Fedden was responsible for the Jupiter’s design, which proved to be groundbreaking. It was the first engine to pass the Air Ministries full throttle test, the first to be equipped with automatic boost control and it became the first engine to see use on airliners.

The Jupiter proved to be a record breaker as well, when a Bristol Bloodhound powered by the unit completed an endurance flight of 225 hours and 54 minutes without any part failures or replacements.

2 years after the Jupiter first ran, Cosmos engineering went bankrupt, being bought out by Bristol in 1920 for £15,000. This purchase was largely due to the impressive nature of the Jupiter design.

Mercury – 1925

The Bristol Mercury was a nine cylinder single row radial engine. It was designed by Roy Fedden at Bristol Aeroplane Company. The Mercury was developed from the earlier Jupiter engine.

The Mercury went on to produce 800hp in its late production models.

Part of the genesis of the design was the introduction of superchargers within aviation. Redden wanted to use a small amount of this added boost at all times, therefore providing improved performance with a smaller engine.

The Mercury has a characteristic humming sound, especially when heard running on the two bladed props seen with the two airworthy Gladiators. The engine sounds so smooth and almost as if it is barely trying even at take-off power. Even when two are strapped together on the Bristol Blenheim the mercury sounds so smooth.

20,700 examples of the Mercury were built over its production run and it powered a huge range of aircraft. Again this 1920s design saw a long service life, powering a number of aircraft well into World War 2.

Pegasus 1932 

Another Bristol design which had a big impact on the build up to the Second World War was the Pegasus. Like the Mercury it would go on to power a number of important early war types. This engine started life as a replacement project for the succesful Jupiter, with the aim of improving the power to weight ratio. The Jupiter had a power output of 580hp, while the first Pegasus put out 635hp, an impressive jump for the same basic design. Late model pegusus’, through the help of a supercharger even broke the 1000hp barrier.

The array of aircraft which used this engine and would go on to make a big mark on the war is quite impressive. Perhaps the most famous application was the Fairey Swordfish, with honourable mentions going to the Vickers Wellington and Short Sunderland. like its older brother the Jupiter, Pegasus would go on to set a number of records both in height and distance during its 32,000 strength production run.

Bristol clearly left their mark on the world with their engine designs during the interwar years, in fact it was noted that the aero engine department were far more successful than the aircraft designers. The engines mentioned above would go on to provide a vital backbone to Britain as we went to war. As that war progressed, Bristol went on to create some of the most complex and impressive piston engines we have ever seen.

BMW 

VI – 1926 

A water cooled V12, was a 12 cylinder step up from the First World War IV engine. This engine powered a number of record breaking long-distance flights. This engine performed so well as it was capable of maintaining its performance event at high level. It actually was designed to put out its maximum power of 500hp at height rather than on the ground. An example of an airframe powered by the BMW powerplant in the HD37.

Rolls-Royce

Kestrel – 1926

In an era when radials seemed the flavour of the day, inline engines needed to make a return, Rolls Royce managed to make this happen.

This new design was set to build on the success of the Curtiss D-12 which had performed incredibly well and marked the first successful cast-block engines. Most early engine designs prior to the D-12 had cylinders which were created individually before being screwed together onto a crankcase.

A cast-block engine used a single piece of aluminium, out of which the cylinders were bored. This resulted in a lighter, simpler and more importantly, stronger engine. The UK government were so impressed with the D-12 that they asked Napier and Rolls Royce to try and design a British version.

In a twist of fate for Rolls Royce, one of Napier’s chief engineers, Arthur Rowledge had fallen out with the Napier management and made the move across to Rolls Royce. This move killed any real plans Napier had for their design.

Soon an initial design was completed which featured a supercharger, which was used at all altitudes, allowing this new design to outperform most contemporary units.

Cooling in inline aero engines has always been a chief concern, especially on the ground. With the Kestrel large leaps were made. The new design featured the revolutionary pressurised cooling system, this meant that the cooling system maintained its performance at higher altitudes as well as on the ground, as it raised the boiling point of the water to 150 degrees.

The engine was ready for production in 1927, with the first version putting out 450hp which was soon followed by the 1b version, with 525hp.

The most famous design of aircraft to make use of the Kestrel was of course the Hawker Hart family of biplanes, with the Hart, Audax, Demon, Hind, Fury and Nimrod all making use of this important engine. A number of airworthy examples can be found in the Hawker fighters based at Duxford and Old Warden.

A controversial use for the Rolls Royce engine was when it was used in the prototype ME109 in 1935. The engine as also used the in the prototype JU87 Stuka.

The excellent design features laid down in 1927 meant that the engine had plenty of life in it, allowing it to be developed right through to the second world war. Powering a huge number of designs.

The “R” -1929

The story of this engine and the aircraft which it powered are outlined in great detail in my post on RJ MItchell’s Racing designs from earlier this year.

The R engine came about after the team being the Supermarine S.5 realised they had used all the potential that the Napier Lion engine had to offer. This engine built on an earlier Rolls Royce design, the Buzzard, and featured a number of improvements.

Visually there were a number of changes made in order to reduce the engines size when in situ, this included reshaping the reduction gear housing, camshaft and rocker covers.

The new engine flew for the first time, along with the S.6 at Calshot on the 4th August 1929. Ultimately the R would go on to power the Supermarine racer to victory in 1929 and a world speed record later that year.

A number of sources cite that the R was the stepping stone to the Merlin, it is likely that the Griffon was more the direct development of the R. The Griffon was in fact a de-rated version of the racing engine. The Merlin did take on some of the design features however.

de Havilland 

Gipsy – 1927

It’s hard to take a look interwar aviation without de Havilland appearing somewhere along the line. As well as launching the revolutionary run of Moths, the Hatfield based firm were also responsible for one of the great light aircraft power plants of the century, the Gipsy.

Early versions of the DH.60 Moths had been powered by the ADC Cirrus, Frank Halford had the task of designing a suitable replacement in 1927.

The Cirrus had been born out of the Renault 8 engine from World War 1, of which Geoffrey de Havilland was a huge fan. The Cirrus ended up being a 4 cylinder design which made use of the Renault’s cylinders and other components along with some car engine parts. The result was an engine which could put out 60hp, not much but very impressive compared to those their competitors were running.

The initial design of the Cirrus had been developed in 1925, by 1927 spares of Renault 8s were starting to run out, meaning that the parts needed to build the Cirrus were proving hard to come by. This is what led to de Havilland requesting a new design be built to match the Cirrus’ performance.

The new engine was developed as part of the design for the DH.71 racing Tiger Moth, this engine could produce 135hp, though it was downrated to 100hp for production. This would be the design which would bear the name “Gipsy”.  The new design was similar to the Cirrus in that it was four cylinder, in-line and air cooled.

As any fan of de Havilland aircraft will be aware, those early DH60 moths, using the cirrus or the Gipsy have the cylinders protruding out the top of the cowling, obstructing the pilots view. This was an important issue to be addressed in the Gipsy Major.

Pilots had claimed that the Gipsy engine could quite happily run inverted without issue, aside from fuel tank and carburetor being inverted. This was theory soon tested by Halford, with a  new test rig created. Sure enough the engine worked in that configuration and soon the Gipsy III was released, an inverted version of the engine.

The Major was a step up in terms of power, with 130hp available, thanks to slightly large bores. This engine would also become the basis for the 200hp Gispy Six. The Six came about as de Havilland wanted to make use of Hydraulic variable pitch propellor ready for the MacRobertson Race in 1934. Time wasn’t on their side however and a much simpler variable Prop mechanism ended up being fitted to the Comet.

The engines the Comet used for the race, the Gipsy Six R, generated 223hp, following the success of the aircraft and engine in the race a 200hp Gipsy Six was placed into production.

The final pre-war entry from de Havilland was the Gipsy Queen, which first ran in 1936. The Queen was a 9 litre development of the Gipsy Six designed for Military use, though production began in the late 30s it  went on until 1950. These 208hp engines went on to power de Havilland executive aircraft such as the Dove, Heron and Rapide.

de Havilland’s engine designs without doubt revolutionised the light aircraft scene, with these designs being used for countless classic aircraft of the era.

Curtiss

The development of these Curtiss engines can be traced back to 1915, in the midst of World War 1, when the Hispano-Suiza 150hp engine first appeared on the scene, demonstrating a stark improvement from the engines which had gone before. Curtiss soon had developed an 8 Cylinder VX aero engine, but management wanted more out of the firm, they enlisted the help of Charles B. Kirkham who was tasked with creating an aero engine of 300hp or more.

Kirkham achieved this by enlarging the VX-V8 to a V12, which would go on to be the first V12 engine built by the frm, known as the V4, it would go on to produce 400hp. Ultimately these efforts were initially wasted as the Liberty powerplant got the contract instead. A later effort, which Kirkham designed from scratch after studying the Hispano for some time, became known as the AB. Once again though, this engine was thwarted at the last minute, though it produced 300hp, Hispano had developed a similar engine themselves.

This is where Kirkham arrived at the K-12, he expected this design to produce 400hp at 2,500 RPM, which at the time was an astonishing speed for the end of WW1!

Kirkham left the company early in 1919, owing to rising tensions between the two parties.

After a few years passed Arthur Nutt eventually took over the reigns at Curtiss, it is he who would create the masterpiece that was the D-12.

The D-12 came about as a development of the C-12, which like a number of these Curtiss V12s put out 400hp at 2,250 RPM. This design was much lighter than earlier efforts. An issue found with the C-12 was unreliability due to the complex gearing used between the engine and propeller.

The C-12 project proved to be a costly one for Curtiss, they ended up with an unreliable engine and no market in which to sell it, as Liberty engines were far cheaper at the time.

Nutt suggested that rather than abandoning the project, the gearing could simply be removed, resulting in a slightly less powerful engine. Though less powerful the engine was made considerably lighter than those being made by Wright and Liberty. After many hours of testing and tweaking with the US Navy, the direct drive “CD-12”, as it became known could produce 405hp at 2,000 RPM. This engine took Curtiss and the Navy to victory in the 1921 Pulitzer Trophy.

Having now proven itself as a viable design, the CD-12 was lined up for production, though it was soon suggested that a number of improvements should be carried out prior to this stage. By the time these were complete it was essentially a new design, which would become known as the D-12.

Orders came in quickly from the Navy, with the first production examples generating 350hp. As well as gaining Curtiss a further victory in the Pulitzer trophy of 1922, the D-12 also led to the Schneider Trophy gold in 1923.

This engine was the first successful cast block engine, so succesful in fact that the British government decided that British manufacturers must make a copy. Fairey imported 50 D-12s in 1926, simply renaming them the Fairey Felix. The D-12 is also where Rolls Royce took many of the ideas for the Kesterel

With a capacity of 18.8 litres, this water cooled V12 was the inspiration for most of the V12 inline aero engines which would follow and become the backbone of fighting forces around the world.

The original D-12, designed in 1921 produced 443hp and powered the Curtiss CR-3, one of the company’s Schneider race winners. In the aftermath of Curtiss’ imposing display at the 1923 race, the D-12 was cited as being essential to their success.

Curtiss Conqueror – 1926

It makes sense to tie these two great Curtiss designs together in this piece, though three years apart together they really did set the tone for inline aero engines for years to come.

The Conqueror could put out 675hp, compared to the D-12’s 443.

The design work for the Conqueror, or V-1570, to give it the full title, began in 1924, this engine featured pressurised liquid cooling, which would later be added to the Rolls Royce Kestrel design. The basic design was modified over the years as more and more supercharger technology allowed power increases until the engine started to succumb to a number of overheating issues, leading to the military cutting funds in 1932.

It isn’t hard to see though, with the two engines above, that Curtiss, of all people, laid the foundations for the big V12s that would become synonymous with Second World War fighters.

New Generation

Just as the Fist World War had seen favour change from rotary to inline power, the interwar years saw the radial take charge, with many of the interwar designs going on to power countless aircraft through to the 1950s. The groundbreaking work carried out by Curtiss and subsequentley Rolls-Royce resulted in a new generation of aero engines laying the groundwork for the iconic Alison and Rolls-Royce V12s that would go on to power an inconic era of military aircraft.

The next post in this series will take a look at how piston aero engines developed throughout WW2 as piston engine design reached its peak.