F4U Corsair Engine History
Pratt & Whitney R-2800 Double Wasp Engine
Excerpt from "Allied Aircraft Piston Engines of World War II" by Graham White, used with permission of the author. Copyright 1995 Society of Automotive Engineers. No part of this publication may be copied without the written permission of the publisher.
Photo courtesy of Aircraft Engine Historical Society at www.enginehistory.org.Although not built in the same numbers as the R-1830, the Pratt & Whitney R-2800 was the most significant aircraft engine built in the United States during World War II. Eighteen air-cooled cylinders were arranged in two rows, and total displacement was 2,804 cubic inches, (46 liters).
Cylinder bore was 5.75 inches and stroke was 6.0 inches. Several significant deviations were made in the design compared with previous Pratt & Whitney two-row radial philosophy. All bearings were plain, with the exception of the propeller thrust bearing, which was of the ball thrust type and the roller bearings supporting the rocker arms. In addition, the crankshaft was a three-piece built-up unit, allowing the use of one-piece master rods.
Prior to Pratt & Whitney developing the R-2800, the company was working on the R-2180, another two-row radial; however, that project was dropped in favor of the R-2600. When news came through that Wright Aeronautical was also developing an R-2600, Pratt & Whitney upped the stakes by increasing the displacement to 2,804 cubic inches. Design commenced on the R-2600 in August 1936; the decision was made to increase displacement to 2,804 cubic inches in March 1937. By 1940, the R-2800 was ready for production, rated at 1,850 horsepower. Design of the A- and B-series engines was fairly conventional, although several milestones were reached.
For the first time, a standard production Pratt & Whitney engine exceeded 100 horsepower per cylinder. Many engine designers during the 1930s felt 100 horsepower per cylinder could not be reached, never mind exceeded, primarily because of cooling problems and to a lesser extent fuel quantity. With the technology that existed when these statements were made, it was true. However, as casting, manufacturing and design techniques, and materials improved, allowing deeper and more closely pitched cooling fins along with improved baffle technology, the previously insurmountable cooling problems became less of a gating item to ultimate power.
At the same time cooling fin design was improving, methods were investigated to obtain the most mileage from the available cooling air. With the application of these new technologies, the R-2800 started out as a very good engine and ended up as arguably the finest piston engine ever. Crankcase design was similar to the R-1830, being a three-piece aluminum forging machined all over and bolted together. As previously mentioned, Pratt deviated from its former design philosophy of rolling-element bearings and used plain silver-lead-indium bearings for the crankshaft main and master rod bearings.
The three-piece crankshaft used splines and precision bolts to hold this heavily loaded component together. In an effort to dampen second-order vibratory modes caused by the eccentric masses of the master rod assemblies, two counterweights, geared to run at twice crank speed, coaxially but in the opposite direction to the crankshaft, were incorporated, one at the front and one at the rear of the crankcase.
The cam plates were also incorporated into the crankcase, one at the front and one at the rear. Following usual Pratt & Whitney practice, four lobe cams driven through 8:1 reduction gears were used, each cam plate featuring two tracks, one for exhaust and one for intake valves. Roller cam followers rode on the cam profiles. Cylinders were chrome molybdenum steel alloy forgings with aluminum cooling muffs shrunk onto the barrel, a first for Pratt & Whitney.
The bore was choked; that is, tapered in its diameter toward top dead center. The cast aluminum cylinder head was screwed and shrunk onto the barrel in the usual way. Cylinder head cooling fin design was similar to the R-1830 but considerably deeper, as manufacturing technology was continuously pushed as far as the existing state of the art would allow. Combustion chamber design again followed Pratt & Whitney's tried-and-proven philosophy of hemispherical with two inclined valves actuated by tubular pushrods actuating roller-bearing supported rocker arms. A pivoting pad on the valve end of the rocker arm ensured that full-face contact was always maintained between the rocker arm and the valve stem.
Valve clearance was set by fitting shims in the pushrods, which featured pressed-in hardened steel ball ends. Although a screw adjustment was provided on the pushrod end of the rocker arm, this was only for fine-tuning the clearance. Prior to valve clearance adjustment, the cam ring required centering; this was accomplished by depressing a valve in the opposite cylinder, a time-consuming and laborious process and another example of the meticulous work required of the unsung heroes of World War II, the ground support crew.
A ring of waisted studs secured each cylinder to the crankcase, and the cylinder was sealed with an O-ring at the cylinder barrel flange where it joined the crankcase. Pistons were aluminum forgings, heavily finned inside the skirt. Three compression rings and two oil-control rings were used with the second oil-control ring situated below the wrist pin. Two reliefs for valve clearance were machined into the crown.
A single-piece magnesium casting housed the reduction gearing, magneto drive, distributor (all R-2800 models featured nose-mounted magnetos and distributors), propeller governor drive, and a scavenge pump. Reduction gearing was planetary-supported in plain bearings; the more common ratios were 0.350:1, 0.450:1, 0.4:1, and 0.5:1.
Early production A- and B-series engines used SAE No. 50 spline propeller shafts. As the engine was developed and more horsepower was extracted, the C-series used the larger diameter SAE No. 60 spline propeller shaft.
The supercharger collector section, mounted at the rear of the crankcase with a ring of studs and nuts-another magnesium casting-contained the supercharger diffuser and nine outlets for the intake manifolds, which were fabricated from thin-wall steel tubing. A Y junction for each intake manifold branched out into two tubes, one feeding a front cylinder and the other feeding a rear cylinder. Six highly sophisticated engine mounts were bolted to the supercharger collector section.
As in the R-1830, considerable design effort was spent on shock mounting and damping, resulting in a beautiful piece of engineering. For shock absorption, a circular block of rubber was contained in a two-piece steel forging screwed together set at a dynafocal angle. The damping was accomplished by a clutch arrangement that was similar to an automobile clutch. Pressure was achieved with a diaphragm spring. Each of the six mounts was bolted to the main aircraft engine mount, which was typically a chrome molybdenum steel alloy tubular weldment terminating at four points at the aircraft firewall. The intermediate rear section contained the supercharger, usually a single-stage unit that could be single speed or two speed. Two-stage, intercooled engines were developed for the Navy. For carburetion, the ubiquitous but beautifully engineered Bendix injection carburetor was mounted on top, although a few models featured updraft carburetion for applications such as the Vought F4U Corsair. Early A- and B-series engines used the Bendix PT-13 three-barrel carburetor, and the later C-series engines used the improved PR-58.
R-2800 supercharger impellers featured an innovative fuel distribution system. Fuel was injected from the fuel feed valve into a "slinger", a sleeve around the impeller shaft that butted up against he rotating inlet guide vanes. Radially opposed holes around the slinger sprayed the fuel into the impeller eye. The impeller also allowed fuel into radially drilled galleries in the impeller. The galleries, one between each vane, exited at approximately three-quarters the diameter of the impeller. Consequently, considerable centrifugal force was imparted to the fuel, ensuring good mixture distribution and reducing the possibility for induction icing.
At the rear of the engine the supercharger drive gears, clutch, accessory drives for generator, gun synchronizers, tachometer generator, vacuum pump hydraulic pump, and starter were housed in yet another magnesium casting. In addition to the drives already mentioned, the rear section also contained the two-speed supercharger selector valve assembly, the oil filter, the main oil pressure oil pump, the fuel pump, and the rear scavenge pump.
Performance As in the case of the R-1830, the R-2800 enjoyed a long production life and consequently saw significant development and increase in power. Early production examples were rated at 1,800 horsepower at 2,600 rpm. Power was soon increased to 2,000 horsepower at 2,700 rpm.
When the R-2800 was pushed to 2,000 horsepower, the limitations of the cast cylinder head were felt. Additionally, the highly stressed internal parts needed strengthening. Consequently, the definitive C-series resulted.
The entire engine required redesigning, and the only commonality was the 5.75-inch bore and 6.0 inch stroke. Although the crankcase remained a three-piece, bolted-together aluminum forging, it saw considerable redesign to strengthen it. Cylinder design was totally revamped. A forged cylinder head now replaced the previously cast cylinder head, allowing deeper, closer-pitched cooling fins. Ganged slitting saws following cams created the correct depth and pitch for the fins.
C-series R-2800s were the first Pratt & Whitney production engines with a forged head. The cylinder barrel was still a chrome molybdenum steel alloy forging retaining an aluminum cooling muff shrunk onto the barrel but with more cooling surface area. These modifications substantially increased the cooling fin area and heat rejection capability of the engine. Master rod and link rod strength was increased.
In addition, the rod ratio was increased, that is, the ratio of the center distance between the wrist pin center and the big end center compared to the stroke. This resulted in longer connecting rods. Crankshaft design was totally revamped, although the three-plain-bearing, three-piece concept was retained. The method of retaining the three-piece crankshaft was revised by using face splines at the joints instead of the previously used male/female splines in the A- and B-series engines.
Nose case strengthening again resulted in a total redesign. A two-piece magnesium casting replaced the former one-piece component. The planetary reduction gearing featured far more pinions to distribute the greater loads now imposed on them due to the increase in horsepower. The SAE No. 50 propeller shaft spline was replaced by the No. 60 spline. Refinements were made to the supercharger, and the PT-13 series Bendix injection carburetor was replaced by the PR-58 series of carburetor. The result of all of these modifications was an engine capable of 2,500 horsepower at 2,800 rpm dry and 2,800 horsepower with ADI.
Weights varied from 2,150 pounds for the earlier single-stage, single-speed engines to 2,560 pounds for the later models with auxiliary supercharging installed in F4Us and F6Fs. Some experimental models with contra-rotating propellers weighed in at a porky 2,650 pounds.
Manufacture: With the R-2800, Pratt & Whitney had a world-beater; consequently, numerous subcontractors were employed. The Ford Motor Company was one of the prime subcontractors; in addition, Nash and Chevrolet also manufactured the R-2800.
Construction of a new Pratt & Whitney manufacturing facility in Kansas City, Missouri, was started in July 1942. Although it was initially intended for the production of the B-series R-2800, a gamble was taken that the totally revamped C-series would be successful, and the decision was made to tool-up immediately for the C-series even though it was still under development at the time. Despite several setbacks, such as the diverting of a number of critically needed Sundstrand center less grinders to England for Napier Sabre production, which Pratt & Whitney rather uncharitably referred to as "Britain's great white hope," the Missouri facility was soon manufacturing the definitive C-series R-2800.
In retrospect it can be seen that the decision to start immediately with the C-series engine was correct, even though at the time it was a totally unknown quantity.
When chosen as a subcontractor, Ford naturally sent top manufacturing engineers to East Hartford in order to understand the magnitude of the project. To Pratt & Whitney's surprise, Ford made an exact copy of the East Hartford manufacturing facility in Dearborn. Although Ford built numerous dash numbers of the R-2800, indeed Ford was the only manufacturer of some models, they were all B-series engines.
Summary : Although the R-2800 did not excel in any one area-it did not have the best specific power, lowest specific weight, lowest specific fuel consumption, and so forth-the combination of the right attributes, particularly its reliability, rugged construction, and ability to absorb incredible battle damage made it well suited to the demanding environment in which it operated. Cases have been documented where complete cylinders were shot off, and yet the engine continued to produce sufficient power for a very grateful pilot, enabling him to make it back to base.
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