Douglas A/B-26 Invader

Propulsion - Civil versions

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Power plants used by the A/B-26 in research and development

The Hamilton Standard Hydromatic Propeller

Pratt and Whitney's R-2800
The Pratt & Whitney R-2800 "Double Wasp" was probably the most important aircraft piston engine ever produced. It powered more different types of aircraft than any other. In addition to its inestimable contribution to victory in World War II, it also helped establish postwar commercial aviation in classic prop liners such as the Martin 404 and Douglas DC-6. 
Pratt and Whitney's R-2800 cu in. (45.9 L) was America's first 18 cylinder radial, the Double Wasp. Much smaller than the world's only other modern eighteen, the Gnome-Rhone 18L of 3,442 cu in. (56.4 L), it was nevertheless more powerful, and heat dissipation was correspondingly more of a problem. This meant that, for the R-2800, the cast or forged cooling fins that had served so well in the past had to be discarded. The cooling fins needed were so thin and fine-pitched that they had to be machined from the solid metal of the head forging. All the fins were cut together. A gang of milling saws was automatically guided as it fed across the head so that the bottom of the grooves rose and fell to make the roots of the fins follow the contour of the head. The results were worth the trouble as it was a case of designing an engine component that could only be made by a new method and then keeping everything crossed until the new method proved to be practical. In addition to the new head design, the Double Wasp had probably the most scientific baffling yet to direct the flow of cooling air, more so even than the excellent arrangements on the Ranger inline air-cooled engines.

2,000 hp was obtained from the R-2800 with 1 hp/1.4 cu in. (43.6 hp/L) of displacement. In 1939, when the R-2800 was introduced, no other air-cooled engine came close to this figure, and even liquid-cooled ones barely matched it. The designing of conventional air-cooled radial engines had become so scientific and systematic by 1939 that the Double Wasp was introduced at a power rating that was not amenable to anything like the developmental power increases that had been common with earlier engines. It went to 2,100 hp in 1941 and to 2,400 late in the war, but that was all for production models. Experimental models, as always, were coaxed into giving more power, one fan-cooled subtype producing 2,800 hp, and considerably more (up to 3,600 hp) on dynamometers. Technicians at the Republic Aircraft Corporation ran the engine at extreme boost pressures at 3,600 hp for 250 hours without any failure using common 100 octane avgas, but in general, the R-2800 was a rather fully developed powerplant right from the beginning.

It was exclusively a powerplant for fighters and medium bombers during the war, being used in the Republic P-47 Thunderbolt, the Grumman F6F Hellcat, and the Vought F4U Corsair, and also in the Martin B-26 Marauder and Douglas A-26 Invader twin engine mediums. Post-war its reliability commended its use for long-range patrol planes and for the Douglas DC-6, Martin 404, and Convair transports. This last application is noteworthy, since these were twin-engine craft of size, passenger capacity, and high wing loading comparable with the DC-4 and the first Lockheed Constellations.

Two engines were all right for transports as with the Douglas DC-3's moderate wing loading, and the high wing loading of the Douglas DC-4 was safe enough when there were four engines, but all that weight with only two engines seemed like tempting fate. However, the Convair engineers knew what they were doing. (Those at Martin, and those who tested the Martin for government approval didn't—the Martin's wings failed from fatigue after a while.) The Convairs were just as good in their way as the four-engine transports. A well engineered installation and good controls were probably what made the difference.

When the USA went to war in December 1941, there were very quickly some major changes in philosophy. Such long-established engines as the Cyclone and Double Wasp were re-rated on fuel of much higher anti-knock value to give considerably more power. Perhaps the most outstanding example was the great R-2800 Double Wasp, which went into production in 1940 for the B-26 Marauder at 1,850 hp and by 1944 was in service in late model P-47 Thunderbolts (and other aircraft) at a rating of 2,800 (experimental) hp on 115-grade fuel with water injection. Of course, all engines naturally 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 time over a period of only a month or two.


R-2800 General characteristics


Type: 18-cylinder air-cooled two-row radial engine with water injection

  • Bore: 5.75 in (146 mm)
  • Stroke: 6 in (152 mm)
  • Displacement: 2,804 in³ (46 L)
  • Diameter: 52.8 in (1342 mm)
  • Dry weight: 2,360 lb (1,073 kg)


  • Valvetrain: Poppet, two valves per cylinder
  • Supercharger: Variable-speed (in F8F-2, unified with throttle via AEC automatic engine control), single stage single speed centrifugal type supercharger
  • Fuel system: One Stromberg injection carburetor
  • Fuel type: 100/130 octane gasoline
  • Cooling system: Air-cooled


  • Power output: 2,100 hp (1,567 kW)
  • Specific power: 0.75 hp/in³ (34.1 kW/L)
  • Power-to-weight ratio: 0.89 hp/lb (1.46 kW/kg)

Ignition System

The ignition system was a low tension, high altitude type, comprised essentially of one double tandem, low tension magneto, two distributors, thirty-six shielded ignition harnesses, eigthteen double high tension coils, shielded spark plugs, and one induction vibrator for each engine. The system is completely independent of the aircraft electrical system, with exception of the induction vibrator."

The distributors were those shaped like an inverted tapered drinking cup. The coils were bolted to the baffle plate on top of each cylinder. The Quickboost set has the wrong engine for the A-26K. On the upside, the engines are correct for the R2800-27 used in the earlier A-26B/C.

The Aires Late R-2800 is the best choice for the R-2800-52W. The Aires kit supplies the correct ingition parts. Aires obviously based their resin engine on the CB-16/17 series engines.


The A-26A R-2800-52W engine was the military version of the CB-17. The R-2800-52W had the bolted front case like that of the "C" series R-2800. The ignition was, quoting the "Dash -1":


  • Flight tests revealed excellent performance and handling, but there were problems with engine cooling which led to cowling changes and omission of the propeller spinners on production aircraft
  • The noticeable visable difference between the engines is the shape of the propeller reduction gear case.
  • The " B " having a single piece tapered bell shape, while the " C " series has a two piece gear case, the rear section being cylindrical with the forward part being nearly hemispherical.
  • The two sections are connected at the peripheral parting flange with around 36 attachment bolts. The Magneto / Distributors and Propeller Governors were different as well.

Propulsion - Civil


  • On Mark Marketeer - 2 No Pratt & Whitney R-2800-CB-16 / -17 Double Wasp
  • On Mark Marksman A - 2 No Pratt & Whitney R-2800-83AM3 Double Wasp
  • On Mark Marksman B - 2 No Pratt & Whitney R-2800-83AM4A Double Wasp
  • On Mark Marksman C - 2 No Pratt & Whitney R-2800-CB-16 / -17 Double Wasp
  • LeTourneau executive  - 2 No Pratt & Whitney R-2800-27/71 Double Wasp
  • Rock Island Monarch 26 - 2 No Pratt & Whitney R-2800-79 Double Wasp
  • Smith Tempo I - 2 No Pratt & Whitney R-2800 B-series Double Wasp
  • Smith Tempo II - 2 No Pratt & Whitney R-2800 C-series Double Wasp
  • Wold Corp. executive Invader - 2 No Pratt and Whitney R-2800-CB-16 Double Wasp


B Series R-2800


C Series R-2800