How Pratt & Whitney Changed Aviation

How Pratt & Whitney Changed Aviation

In Engines, Videos by Paul ShillitoLeave a Comment


When the chief engineer for the German aero maker Blohm and Voss got hold of a recovered Pratt and Whitney R-2800 and saw that the lapping of the two halves for the crankcase was machined with such great precision that it not only didn’t need a sealing gasket, it also didn’t leak oil, something that was common in many British and Japanese aero engines, he said that if that’s what the Americans can mass produce then the war is already lost.

Pratt & Whitney built almost half of all the allied engines used in WW2, going from just 5000 a year at the beginning of the war to 50,000 a year at the end.  

The Pratt & Whitney R-2800 double wasp exemplified this ability of the US in particular to build powerful yet highly reliable engines in large numbers. The R-2800 was the latest in a line of ground-breaking radial engines but they all were derived from the company’s first engine design that changed the direction of aviation from the mid-1920s onward and would be pivotal during WW2.

This is the story of how the Pratt & Whitney WASP & it descendants took on the world.

The history of the aero-engine in the first half of the 20th century was dominated by two basic types, the air-cooled radial engine and the liquid-cooled inline engine.

The air-cooled radial was the first to be used on aircraft, it offered a light compact design that was cooled by the airflow from the propeller. However, internal combustion engines of the time were still a new technology and had a long way to go before all the bugs would be shaken out of them.

One of these bugs was the cooling issues on early air-cooled radials, there was a French water-cooled version called the Salmson 9Z water-cooled radial engine which was popular from 1908 up to 1920 and was licensed to both British and Russian engine makers but lost out to the air-cooled radial later.

To overcome the air-cooled problems, a radical idea was used to fix the end of the crankshaft to the frame of the aircraft and the propeller to the engine casing and allow the whole engine to spin and create its own airflow to cool it down and the fuel-air mixture was drawn through a hollow crankshaft. These engines became known as rotary engines because the whole engine rotated along with the propeller.

This worked well and was popular in WW1 fighters but as time went by the performance limitations of the rotary design, in particular getting enough air and fuel through the hollow crankshaft meant that in-line water-cooled engines became more powerful and popular.

It wasn’t until the early 1920s that cooling issues of radial engines made them more reliable. In 1920 the N.A.C.A, the predecessor to NASA  noted that air-cooled radials could offer an increase in power-to-weight ratio and reliability and in 1921 the US navy said that they would only purchase aircraft fitted with air-cooled radials, soon other navel air arms around the world followed their lead.

The development of radial engines was boosted by US navy funding of Charles Lawrance’s 1922 J-1 engine which used aluminium cylinders with steel liners to produce an engine that could run for 300 hours compared to the typical 50 hours of the time.

Afterwards, the Wright Aeronautical Corporation with the encouragement of the US Navy bought up Lawrance’s company to produce the J-5 Whirlwind which in 1925 was widely claimed as “the first truly reliable aircraft engine” and gave navy pilots the confidence to fly long distances over water, so much so that it was chosen to power Charles Lindbergh’s “Spirt of St Louis” on the first non-stop flight across the Atlantic, from Long island, New York to Paris, France.

Frederick Rentschler, an executive at Wright aero was convinced that air-cooled radials would be the future for aero engines being lighter and more powerful than the heavier water-cooled in-line engines and be more reliable in combat due to lack of liquid cooling that could be damaged or leak which could cause engine damage or failure. Whilst opponents claimed that their shape made them less aerodynamic and slower, Rentschler claimed that the lower weight and greater power would more than make up the difference.

Rentschler, whose family business made automobile engines and was expected to take it over was inlisted in 1917 as a first lieutenant in the aviation section of the U.S. Army Signal Corps. Here he worked on inspecting Hispano-Suiza water-cooled V8 aero engines built under license by the Wright-Martin Company, something which he found fascinating and noted that it wasn’t that different to the car engines his family made.

Although Rentschler went on to become the president of Wright Aero and had pushed them to make the J-5 Whirlwind, he believed that the company should invest in future radials but the board which was made up of mostly investment bankers with little aviation knowledge didn’t agree and wanted to continue on the safe path on making water-cooled inline engines which were now the norm, so Rentschler left to start his own engine business with Wright’s chief engineer George Mead and several other of Wright’s best employees.

Rentschler approached a friend of his fathers, James Cullen the president of machine-tool company Niles-Bement-Pond, who had a subsidiary, the Pratt & Whitney machine tool company which had spare capacity and funds after WW1 if the right idea came along and had a customer waiting.

Soon the Pratt & Whitney Aircraft Company was founded, half owned by Pratt and Whitney and half by Rentschler & Mead.

Meanwhile, the Navy had to work within the Washington naval treaty which limited the number of battleships each country could build but allowed them to convert normal ships into aircraft carriers and these would need planes with powerful reliable engines.

Around this time Chance Vought, founder and president of the aircraft maker Vought turned up at the navy almost begging for orders as his company was going through a tough time. The navy told him that if he wanted orders he should come back with something genuinely original and not the same old planes. Vought replied that if he could get hold of an engine that weighed 650 pounds and produced no less than 350 horsepower, he’d build a world-beater.

So when Rentschler approached the Navy a few months later with the idea of a lightweight powerful engine engine the long-running match between Vought and Pratt & Whitney started and made with the Vought O2U Corsair.

This also suited the Navy as Curtiss aeroplane and motor company and Wright Aero would go on to merge and form a new company, Curtiss Wright which would have had the monopoly in Navy aero engines without Pratt & Whitney.

The advantages that the big radials had against the big V-12s was that they didn’t need the heavy complex plumbing for the cooling system and the radiators which were susceptible to the jolts and bangs of heavy landings, especially on rolling ships at sea, something that could weaken the brazed joints in the radiators and loosen hose connections and cause them to leak, let alone taking a round of bullets through them. They had a greater power-to-weight ratio which meant faster climb rates and were easier to maintain, an ideal engine for carrier-based aircraft.

The first Pratt & Whitney Aircraft Company engine design was the R-1340 Wasp, a 22 litre 9 cylinder radial with many advanced features which included a rotary induction blower, a split counterbalanced crankshaft, symmetrical front and rear forged crankcases, pushrod housings and accessories arranged to allow maintenance without dismantling the engine.

The new engine was tested & rated by the Navy at 410hp and weighed in at under 650lbs or 294kg. The Navy liked it so much that they placed an order of 200 before the 6th engine had even been built.

During testing, it was found that the machining of the engine was so precise that they had to pull it apart to loosen up the piston rings to allow more oil to bypass them and lubricate the cylinder walls and it was this precision which Pratt & Whitney were world-renowned for that would become a hallmark of not only the Wasp but its later derivatives.

The Wasp name came from Rentschler’s wife, Faye after she heard the engine development team saying about the buzzing noise of the 9-cylinder engine was like a swarm of bees and what name could they call it, she suggested that it should be called the Wasp and the name stuck.

It wasn’t only the Navy which used the wasp, the Ford Trimotor was the first civilian airliner to use them but its use would go on to include executive transports, sport planes, racers, fighters, passenger transports, WWII training aircraft, agricultural aircraft and helicopters.

But the engineers at Pratt & Whitney didn’t sit on their laurels, Mead was already working on more powerful versions and by 1930 power was up to 600hp with the R-1340 dash 49.

As aircraft developed the power requirement went up too, so in 1929 they started experimenting with two rows of 7 cylinders, 14 in all with the second row offset to sit behind the gaps between the front row. This allowed it to increase capacity without increasing the frontal area and creating more drag. Smaller cylinders also allowed an increase in the crank speed and more but smaller power pulses made for smoother operation with increased engine life.

The new engine was the 30 litre R-1830 Twin wasp which had power outputs from 800hp to 1350hp. This would become the most widely used of any of the Pratt & Whitney radials and including license-built units had the biggest production run of any aircraft engine in the world with 173,618 engines produced up until 1951 and was used in a range of aircraft that included the Douglas DC-3 DST, B-24 Liberator, Grumman F4F Wildcat, Curtiss P-36 and Vickers Wellington to name but a few.

While the twin wasp had the largest production runs, the 2nd largest run and probably the most famous is one of its descendants the R-2800 Double Wasp.

Although this was a development of the twin-row radials the Double Wasp was now an 18-cylinder with two rows of nine cylinders and was a year ahead of the other big American 18-cylinder radial the Wright R-3350 Duplex Cyclone which was designed for the new super bomber program and ended up powering the B-29 Superfortress, the R-3350 being larger at 54.9 litres to the R-2800s 46 litres

However, for both engines, early cooling issues would be a problem and something which severely plagued the Wright R-3350 till the end of the war. The normal way to cool the cylinder head was with cooling fins which were either cast or forged along with the head but because of the tight spacing, getting enough surface area from cast fins was difficult to achieve, so Pratt & Whitney decided to ditch the cast fin idea and go with something more radial.

They forged the cylinder head as a larger part initially and used gangs of milling saws following a preset shape to machine all the fins from the solid metal forging in one operation saving a considerable amount of time.

This created many more thinner and closer-pitched cooling fins which gave each cylinder head a much greater surface area to dissipate the heat and solve the cooling problem.

By now the design and building of the big radials had been honed into a highly scientific process so when the first R-2800 ran in 1939 it produced 2000hp straight off the bat but over time they still managed to increase the power up to 2400hp and by the end of the war it was upto 2800hp on the experimental R-2800 dash 57 fan cooled version.

This combination of power and durability coupled with two stage , two speed superchargers gave the US Navy the edge over the Japanese in the Pacific with planes like the Vought F4U Corsair, Grumman F6F Hellcat and in Europe the Republic P-47 Thunderbolt, all of which were more than a match for either Japanese planes in the pacific or german planes in Europe.

There are many stories of Double Wasp powered aircraft coming back to base with cylinder heads shot off but so long as they had oil in them they could often still get the pilots back home.  

The only time the wasp series came under serious pressure was with R-4360 Wasp Major, which really pushed the radial design to its limits.

This was a 71.1 litre, 28-cylinder four-row radial engine pushing out 3000 hp in the initial models and upto 4300hp in later ones with dual turbos and supercharger, making it the largest and most powerful piston engine to be made in the US.

The cooling issues which had plagued the Wright R-3350 were evident in the wasp major though not to the extent that it caught fire and it was reliable in flight but its complex build made maintenance difficult and expensive, requiring frequent replacement of the cylinders. The extra cooling flaps that were required also added to the drag of the aircraft which like the R-3350 in the B-29 put more pressure on the engines at the most critical times.

The giant Convair B-36 Peacemaker strategic bomber was based around six of these massive engines and they were also used in a late model B-29D’s and the B-50, the post-war updated version of the B-29.

But this would the swan song of the big radials before the jets took over. The cost of the maintenance and the extra down time to complete it made aircraft using the Wasp Major expensive run.

In the commercial world the Boeing Stratocruiser which used four wasp majors was unprofitable without government subsidy and was quickly dropped when jet airliners became available but in all 18,697 were built from 1944 to 1955.

The wasp range of engines might not have been the most sophisticated or powerful per litre compared to the evermore exotic V-12s but they were some of the most important ever made in the lead up to and during WW2 and without them the US might not have had the upper hand in the Pacific, the place which really proved to the world just how good they were and that carrier power was now the way forward.

So I hope you enjoyed the video and if you did then please thumbs up, subscribe and share and thanks to all our patreon supporters for their help too.   

Paul Shillito
Creator and presenter of Curious Droid Youtube channel and website

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