With the universal use of jet engines (either pure jets or prop jets) in all but the smallest of today’s aircraft, it’s easy to forget how large and complex piston engines actually were in their final iterations. So let’s take a look at the Pratt and Whitney R-4360 Wasp Major – amongst the biggest of them all.
Developed during World War II, the engine was released too late to be used in war service. However, it was fitted to the last generation of large piston-driven aircraft before technology moved onto jets.
The R-4360 Wasp Major was a 28 cylinder, supercharged radial engine. It was air-cooled and the four rows of cylinders were offset in a spiral pattern. It used two valves per cylinder and 56 spark plugs.
It had a bore of 146.05mm (5.75 inches), a stroke of 152.4mm (6 inches) and a displacement of 71.49 litres (4362.5 cubic inches). The compression ratio was 6.7:1 and it had a BMEP of 235 psi (16.2 Bar) – extraordinarily good.
It was just over 2.4 metres long, 1.4 metres in diameter and had a dry weight of 1755kg.
The connecting rods for each of the rows of seven cylinders used a master rod and six subsidiary rods. The forged steel crank had four throws and five bearings. The crank rotated in a crankcase comprising five sections, with the power sections made from forged aluminium and other sections magnesium castings.
The 14 inch supercharger was geared-up at a ratio of 6.374:1 and the propeller was geared down with a ratio of 0.375:1. A down-draught, four-barrel Stromberg pressure type carburettor was used. There was provision for water injection, which boosted take-off power by 250hp.
Power outputs varied from 3000 to 4000hp (2200kW to 2600kW) although one model is said to have developed 4300hp (3200kW) using dual turbochargers in addition to the supercharger.
The engine was produced between 1944 and 1955, with 18,697 built.
Design and development
Design work for the R-4360 started in November, 1940. A prototype engine, using parts derived from existing Pratt and Whitney engines plus a new cast steel crank and manufactured crankcases, ran in April 1941. By the end of 1941, 500 hours of full-scale operation, 176 hours of single-row operation and 1412 hours of single cylinder testing had been completed. The first flight of the engine was in May, 1942. By 1943, ten experimental engines were delivered for use in prototype aircraft.
The first engine released for use was the R-4360B – there was no ‘A’ version as the prototype exceeded the 2800hp output envisaged for an ‘A’ version.
During development, problems that needed to be overcome included piston ring sticking, and the need to replace the oil drain hoses with silicone so they could cope with the higher temperatures of the R-4360 when compared with earlier engines.
However, a more major change was removing of weight from the crankshaft: this was required because one part of the crank was stiffer than another, causing stress concentrations and resultant cracking. The fix involved the removal of material and the use of greater radii at the fillet edges.
Another change was to the oiling system, which was causing the failure of the front intermediate bearing when the engine was run on the test stand. The oil system improvement involved the use of air separators comprising small bleed holes.
Another oil system problem was that the dry-sumped system’s scavenge pumps were initially ineffective. The required changes were many: increased capacity of the scavenge pumps, adding extra stages or locations to the pumps, and revising pump inlets and adding air separator screens.
The oiling system changes were also responsible for a significant power gain. It was found that the R-4360 with a four-coupling variable supercharger drive developed around 200hp less than the same engine with a two-stage fixed supercharger drive – even at the same manifold pressure. The power difference was because the whirling couplings created such an oil mist that it could not be adequately scavenged from the compartment. The use of separator screens reduced the power loss from 200hp to less than 35hp.
During development, the intake pipes also needed considerable revision. Extraordinary as it may sound, the cylinders were “very flexible and under high power operation do considerable weaving, resulting in considerable stresses being imposed on the intake pipes”. Rubber couplings were fitted but these were not effective and had to be replaced with specially developed flexible steel couplings. Drains also needed to be fitted to the intake pipes to remove fuel that could pool in the intake; this fuel could cause the engine to ‘hydraulic’ if the throttle was suddenly opened.
These problems overcome, the engine successfully entered service. The long list of aircraft to which these engines were fitted is shown at the end of this article.
The incredible complexity of the engine belies its performance – its specific power (about 0.48kg per hp) and 235 psi BMEP are still very highly regarded, some 70+ years after its design. But in civil aviation use its application was short-lived; in military aviation, the passing of the KC-97 and C-97 series aircraft from the US Air Force in the late 1970s marked the end of its era.
Wright Turbo Compound R-3350 turbo compound
A competitor to the R-4360 Wasp Major for bragging rights as the most impressive piston aircraft engine ever is the Wright Turbo Compound R-3350 TC18.
This radial engine was smaller than the R-4360 Wasp Major, with ‘only’ 18 cylinders arranged in two rows and a displacement of 3347 cubic inches (54.86 litres).
However, its claim to fame was its use of three power recovery turbines spaced at 120 degree intervals around the ear of the engine. These were driven by exhaust gases, with the power transferred to the engine through fluid couplings.
This turbo-compounding added about 550 horsepower at take-off power and 240 horsepower at cruise settings.
Aircraft powered by the Pratt and Whitney R-4360 Wasp Major: