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The Pratt & Whitney R-2800 Double Wasp is a twin-row, 18-cylinder, air-cooled radial aircraft engine with a displacement of 2,800 in³ (46 L), and is part of the long-lived Wasp family.
Contents
- Design and development
- Peacetime
- Variants
- Military
- Applications
- Engines on display
- Specifications R 2800 54
- General characteristics
- Components
- Performance
- References
The R-2800 is considered one of the premier radial piston engines ever designed and is notable for its widespread use in many important American aircraft during and after World War II. During the war years, Pratt & Whitney continued to develop new ideas to upgrade this already powerful workhorse, most notably water injection for takeoff in cargo and passenger planes and to give emergency power in combat.
Design and development
First run in 1937, the R-2800 was America's first 18-cylinder radial engine design. The Double Wasp was more powerful than the world's only other modern eighteen, the Gnome-Rhône 18L of 3,442 in³ (56.4 L). (The American Wright Duplex-Cyclone radial of 3,347 in³ (54.86 L) was also under development at the time, and promised to be more powerful than either the P&W or Gnome-Rhone radials.) The Double Wasp was much smaller in displacement than either of the other 18-cylinder designs, and heat dissipation was a greater problem. To enable more efficient cooling, the usual practice of casting or forging the cylinder head cooling fins that had been effective enough for other engine designs was discarded, and instead, much thinner and closer-pitched cooling fins were machined from the solid metal of the head forging. The fins were all cut at the same time by a gang of milling saws, automatically guided as it fed across the head in such a way that the bottom of the grooves rose and fell to make the roots of the fins follow the contour of the head, with the elaborate process substantially increasing the surface area of the fins. The twin distributors on the Double Wasp were prominently mounted on the upper surface of the forward gear reduction housing and almost always prominently visible within a cowling, with the conduits for the spark plug wires emerging from the distributors' cases either directly forward or directly behind them, or on the later C-series R-2800s with the two-piece gear reduction housings, on the "outboard" sides of the distributor casings.
When the R-2800 was introduced in 1939 it was capable of producing 2,000 hp (1,500 kW), for a specific power value of 0.71 hp/in³ (32.6 kW/L). The design of conventional air-cooled radial engines had become so scientific and systematic by then, that the Double Wasp was introduced with a smaller incremental power increase than was typical of earlier engines. Nevertheless, in 1941 the power output of production models increased to 2,100 hp (1,600 kW), and to 2,400 hp (1,800 kW) late in the war. Even more was coaxed from experimental models, with fan-cooled subtypes producing 2,800 hp (2,100 kW), but in general the R-2800 was a rather highly developed powerplant right from the beginning.
The R-2800 powered several types of fighters and medium bombers during the war, including the US Navy's Vought F4U Corsair, with the XF4U-1 first prototype Corsair becoming the first airframe to fly (as originally designed) with the Double Wasp in its XR-2800-4 prototype version on May 29, 1940, and the first single-engine US fighter plane to exceed 400 mph (640 km/h) in level flight during October 1940. The R-2800 also powered the Corsair's naval rival, the Grumman F6F Hellcat, the US Army Air Forces' Republic P-47 Thunderbolt (which uniquely, for single-engined aircraft, used a turbocharger for its Double Wasp's installation), the twin-engine Martin B-26 Marauder and Douglas A-26 Invader, as well as the first purpose-built twin-engine radar-equipped night fighter, the Northrop P-61 Black Widow. When the US entered the war in December 1941, designs advanced rapidly, and long-established engines such as the Wright Cyclone and Double Wasp were re-rated on fuel of much higher octane rating (anti-knock value) to give considerably more power. By 1944, versions of the R-2800 powering late-model P-47s (and other aircraft) had a rating (experimental) of 2,800 hp (2,100 kW) on 115-grade fuel with water injection.
After World War II, the engine was used in the Korean War, and surplus World War II aircraft powered by the Double Wasp served with other countries well past the Korean War, some being retired as late as the latter part of the 1960s when the aircraft were replaced.
Peacetime
Engines grow in power with development, but a major war demands the utmost performance from engines fitted to aircraft whose life in front-line service was unlikely to exceed 50 hours' flying, over a period of only a month or two. In peacetime however, the call was for reliability over a period of perhaps a dozen years, and the R-2800's reliability commended its use for long-range patrol aircraft and for the Douglas DC-6, Martin 4-0-4, and Convair 240 transports. The last two were twin-engine aircraft of size, passenger capacity, and high wing loading comparable to the DC-4 and the first Constellations.
Today, three-quarters of a century after the first prototype Double Wasp, it still flies in restored vintage warbird aircraft displayed at air shows — such as the over two dozen airworthy examples of the first airframe design it powered, and sees service worldwide on aircraft such as the Canadair CL-215 water-bomber. In addition, R-2800s continue to power DC-6 cargo and fuel-carrying aircraft in locations such as Alaska. A total of 125,334 R-2800 engines were produced between 1939 and 1960.
Variants
This is a list of representative R-2800 variants, describing some of the mechanical changes made during development of the Double-Wasp. Power ratings quoted are usually maximum "military" power that the engine could generate on takeoff and at altitude: 100 Octane fuel was used, unless otherwise noted.
The R-2800 was developed and modified into a basic sequence of subtypes, "A" through "E" series, each of which indicated major internal and external modifications and improvements, such that the "E" series engines had very few parts in common with the "A".
Data from White (Airlife) unless otherwise noted:
Military
The dash number for each military type (e.g.: -21) was allocated to identify the complete engine model in accordance with the specification under which the engine was manufactured, thus it did not necessarily indicate the sequence in which the engines were manufactured; for example: the -18W was a "C" series engine, built from 1945, whereas the -21 was a "B" series engine, built from 1943.
Until 1940 the armed forces adhered strictly to the convention that engines built for the Army Air Forces used odd numeric suffixes (e.g.: -5), while those built for the US Navy used even (e.g.: -8). After 1940, however, in the interests of standardization, engines were sometimes built to a joint Army-Navy contract, in which case the engines used a common numeric suffix (e.g.: the -10 was used by both Army and Naval aircraft.)
The suffix W e.g.: -10W denotes a sub-series modified to use A.D.I Anti-Detonant Injection or water injection equipment, using various mixes of water and methyl alcohol (CH3OH) injected into the carburetor to increase power for short periods: several models of R-2800s were fitted as standard with A.D.I and did not use the W suffix. Few commercial aircraft used water injection.
"A" Series:
"B" Series:
"C" Series
Applications
The following is a partial list of aircraft that were powered by the R-2800 (and a few prototypes that utilized it at one point):
Engines on display
[The R-2800 double wasp is on display at the Aerospace Discovery at the Florida Air Museum.]
Specifications (R-2800-54)
Data from FAA TCDS