Airborne Awesomosity

The Boeing XB-38 Watercooled Fortress

Note The Radiator Between the Engines

[image National Air Force Museum]

Towards the end of the war several unusual prototypes made their way off the drafting board.  This modification was yet another example of an aircraft answering a need that didn’t actually exist.



Side Profile Showing the Allison V-1710’s

[image National Air Force Museum]


During the second World War the US Air Force was afraid of  engine shortages and went back to Allison to give them an alternative yet again. The Allison V-1710 inline water cooled V-12 was being used in many US aircraft at the time and can be seen as the best domestic engine we had available. So it would make sense to put them onto the wings of the B-17 in case Wright couldn’t keep up with production of its own radial engines.


How Something With Clean Lines Can Look This Ungainly Is Beyond This Author

[image National Air Force Museum]

General characteristics

Crew: 10
Length: 74 ft 0 in (22.56 m)
Wingspan: 103 ft 11 in (31.67 m)
Height: 19 ft 2 in (5.84 m)
Wing area: 1,420 ft² (131.9 m²)
Empty weight: 34,750 lb (15,762 kg)
Loaded weight: 56,000 lb (25,401 kg)
Max. takeoff weight: 64,000 lb (29,030 kg)
Powerplant: 4 × Allison V-1710-97 turbosupercharged liquid-cooled V12 engines, 1,425 hp (1,063 kW) each

Maximum speed: 327 mph (284 knots, 526 km/h)
Cruise speed: 226 mph (197 knots, 364 km/h)
Range: 3,300 mi (2,870 nmi, 5,310 km)
Service ceiling: 29,600 ft (9,020 m)

The work for this project was handed over to Lockheed’s Vega subsidiary who were already building B-17’s at the time. When the prototype flew it showed improved performance over the standard radial engines.  The B-38 was 40mph an hour faster then the standard B-17.

The XB-38 would receive its name in honor of Lockheed’s own P-38 fighter but would end up as nothing more then an engineering exercise thanks in part to that very fighter. The Allison V-1710 was needed by to many American aircraft of the time and was needed for those aircraft.

Here You Can See The Distinct Lines Of The Allison’s


Leading to the obvious question of why they even tried this out. Also in combat these aircraft would have been more fragile then their air cooled cousins.  The Wright R-1820 was capable of continuing to run while missing major components. Something the Allison would never have been able to duplicate.


  • I'm amazed the performance was that much better. Water-cooling for aircraft isn't usually done for several reasons, not the least of which is the extra parasitic drag of radiators and the extra weight of the water cooling system. Assuming the Allison engines weigh less than the big Wrights, and that offsets the weight of the water cooling system, the loss in aerodynamic performance with the radiators tucked in the wing leading edges can't be negligible.

    • fodder650

      Well tomorrow I will be showing a more extreme example of this as well. Of course now you have me doing math in my head that is far better suited to you. Ok let's look at a few things I should have put in the post.

      Dry weight: 1,184 lb (537 kg)
      Length: 47.76 in (1,213 mm)
      Diameter: 54.25 in (1,378 mm)
      Power-to-weight ratio: 0.84 hp/lb (1.39 kW/kg)

      Allison's stats –
      Dry weight: 1,395 lb (633.5 k
      Width: 29.28 in (744 mm)
      Height: 37.65 in (958 mm)
      Power-to-weight ratio: 1.05 hp/lb (1.76 kW/kg)

      So actually the Allison was heavier. The Wright only nine cylinders where the V-1710 had twelve. Wiki doesn't mention the diameter of the Allison with all of its pieces in piece either. I have a hard time believing that the drag was really that much less.

      Ok now I'm confused and its your fault.

      • CaptianNemo2001

        Do you not see how streamlined the casing is on the Allison? Compared to the almost open casings on the R-1280. Also in terms of with the Allison is likely much narrower then then R-1280. P-T-W is debatable since both could be modified through the roof if needed,

        More drag on the engines. It also looks like something happened to the nose. Pic caption says damage from Cologne Germany. Basically what I am getting at is over all frontal area. And while less frontal area does not always mean less drag it is usually a good indication of the potential for less drag.
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        • Frontal area is a good indicator, however it isn't everything. The cowling on the Wright engines would minimize drag over those engines significantly. That's what it's there for.

          My biggest question has to do with the radiators on the XB-38. Radiators add a surprising amount of drag, and having them in the wing leading edge is surprising to me. That would reduce the effectiveness of the wing, I would think.

          Crap, now we need to build two scale models of these planes and rent some time in a wind tunnel. Fortunately, I know of a few wind tunnels.

          • CaptianNemo2001

            The radiators are embedded into the wing… Well, crap I cant remember if its the Oil coolers or water filled radiators that are in the wing. I want to say its the oil but I would need to check up on the XF-12 to find out which is which.
            Radiators do not have to add much drag at all. The trick is to get air into them by using the natural high pressure area on the wing which would limit drag…

            Or do it like the Daytona supercupe of the 1960's. Or better still modern cars where most of the air is sucked in from the underside to then pass through the radiator and cool the car.

            Back in a sec with the cooling XF-12 answers.

            Articles call them air intakes but then later contradict and say air is taken in through the tight engine cowlings. I'll find it eventually.

        • rusty

          Hate to have been sitting in that bombardier seat…yeesh..

          • CaptianNemo2001

            I hazard to guess that the bomber got clipped by a plane from above…

            Interesting how the seat is intact and the windshield is intact… yet the rest is more or less toast.