Exploring the flight envelope-The Hughes XF-11

By Willie Bodenstein

The Hughes XF-11 was a prototype military reconnaissance aircraft, designed and flown by Howard Hughes and built by Hughes Aircraft for the United States Army Air Forces.

Millionaire Howard Hughes, famous aviator, movie producer, and playboy in the 1930's envisioned a super-fast aircraft that could be a long distance recorder breaker. Development of the D-2 started in great secrecy. When war broke out he approached the War Department and pitched his plane as a bomber. The Army however, didn't like his use of a resin bonded plywood construction but was none the less impressed and they asked Hughes for a high-altitude photographic reconnaissance version built from aluminium.

The Army's specifications called for a fast, long-range, high-altitude photographic reconnaissance aircraft. Hughes took much of his D-2 design, its twin-boom, twin-engines and relatively small central crew nacelle and began work on a new aircraft, the XF-11. One hundred F-11s were ordered. However, only two prototypes as well as a mock were constructed.

In configuration the XF-11 resembled the World War II Lockheed P-38 Lightning, but was much larger and heavier. It was a tricycle-gear, twin-engine, twin-boom all-metal monoplane with a pressurized central crew nacelle powered by two Pratt & Whitney R-4360-31 28-cylinder radial engines. Each engine drove a pair of contra-rotating four-bladed, controllable-pitch propellers, which can increase performance and stability, at the cost of increased mechanical complexity.

The first prototype with Hughes at the controls crashed during its first flight in 1946. Hughes survived the crash and despite the fact that the construction contract was by then cancelled completed the second prototype and successfully flew it on 5 April 1947. The aircraft proved stable and controllable at high speed. It however, lacked low-speed stability.

Photo USAF/commons.wikimedia.org

When the US Army Air Forces (USAAF) evaluated it against the XF-12, testing revealed the XF-11 was harder to fly and maintain and projected that it would to be twice as expensive to build. Despite being the better performer the contract for the XF-12 was cancelled, the USAAF opting to order the RB-50 Superfortress, and Northrop F-15 Reporter instead. Both had similar long-range photo-reconnaissance capability and were available at a much lower cost.

The remarkably clean, low drag profile of the XF-11, with its long, straight wing perhaps inspired a similar design a decade later in the Lockheed U-2.

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