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The Bv 138 attacked convoys, resupplied U-boats, and swept for mines mostly in Scandinavian waters. (NASM (SI Neg. #00010700))

Fork-tailed Devils and Flying Shoes

What does the Northrop P-61 have in common with Burt Rutan's SpaceShipOne?

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They look odd. Even though aircraft with twin tail booms have appeared in every era of aviation history—from the early designs of the Farman and Voisin brothers to Adam Aircraft’s turbofan twin awaiting FAA certification—each one looks like a fix to something broken, an escape route from a corner into which a designer has painted himself.

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Why do it? Why shorten and interrupt the fuselage and connect the wings to the tail assembly with two tubes? There are as many reasons as there are twin-boom designs. Booms have been used to shave weight, stiffen structure, give fighter pilots better aim, improve the efficiency of propulsion systems, reduce parasitic drag, and expedite the loading of munitions or cargo—sometimes all on the same airframe. Examining the various rationales for the configuration is a good way to understand just how many trade-offs aeronautical engineers have to make to build an airplane that will not only get off the ground but do something useful after it does. As this collection shows, twin-boom aircraft have been useful at almost everything the more familiar single-tail-boom airplanes do. The ones shown are merely a sample of hundreds of designs. (For more twin tail-boom airplanes and photos of aircraft mentioned but not pictured here, visit www.airspacemag.com.)

Twin-boom designs are still making history. Burt Rutan’s SpaceShipOne, the first private craft to take an astronaut into space, and its dashing launch platform White Knight both have twin tail booms. And if the movies are any guide, the configuration will be flying long into the future: In Ridley Scott’s 1979 science fiction classic Alien, the vehicle transporting man’s greatest threat is a giant spacecraft with twin booms.

SON OF A HOMEBUILT
The North American OV-10 Bronco owes its successes during the Vietnam War to the tenacity of two Marine Corps majors, K.P. Rice and Bill “Flameout” Bennett. In 1961 no one was interested in a small, short-takeoff-and-landing aircraft designed to support troops who were fighting guerrillas. But Rice and Bennett knew what was needed and built it in Rice’s garage. It was going to “dive bomb like a Stuka or an SBD, maneuver like an SNJ/AT-6, and be as fast and strong as a Corsair,” Rice remembers saying to anyone who’d listen. Obsessed with installing a 106-mm automatic recoilless rifle on the centerline, they added twin booms to lift the empennage out of the back blast. The Lockheed P-38, Rice says, had already demonstrated that placing guns on the centerline increases the shooter’s accuracy. Why the 106 rifle? “We were going to hit targets,” Rice says, “not surround them with bombs.” Twin booms also enhanced survivability, because redundant control systems were in the booms, widely separated. The military services came around to the need for a counter-insurgency aircraft, but in designing one, they increased its size, weight, and variety of missions—and dropped the 106. Still, the Bronco in its final form was a potent warplane. Kit Lavell, a pilot with a Navy OV-10 squadron known as the Black Ponies, says proof of the aircraft’s effectiveness in supporting SEALs and riverine forces was that “the enemy prepared crude handbills with the shape of the plane depicted on them and prescribed tactics to shoot down the Bronco.

CENTERLINE THRUST
Adam Aircraft expects the Federal Aviation Administration to certify its new push-pull, twin-boom business aircraft, the twin-engine A500, within the next few months. It’s a new airplane but an old idea. The concept of centerline thrust took shape most recently in the Cessna 336.

When it debuted in 1964, the 336 crowned years of development toward a simple, low-cost aircraft offering the safety of two engines without the conventional twin’s tendency to yaw if one engine fails. Twin tail booms made room for the second engine at the rear of the fuselage, and the two engines provided centerline thrust and a comfortable ride. A year later Cessna brought out the 337 Skymaster, which had more powerful engines and retractable landing gear. It was a controversial airplane. Despite the safety-oriented sales pitch, the Skymaster’s accident rate was higher than that of conventional twins. Hangar wags referred to it not as the Skymaster but the Mixmaster. But in 1966 the Air Force liked the 337’s high-wing visibility and ability to use short, rough fields, and it bought the twin as a forward air control craft. More than 500 of the 337s were fitted with rocket pods and smoke generators and shipped to Southeast Asia as Air Force O-2s. Author and O-2 pilot Robert Mikesh says the O-2 “felt civilian” and recalls “one ham-handed guy literally pulling the door handle off,” but gives it “a 9 on a scale of 10” for mission suitability.

TWIN BOOMS WITH SEA LEGS
Making headlines around the world—and earning propaganda dividends for Italy’s Fascist government—for a series of transatlantic flights in the 1920s and ’30s, the unique Savoia-Marchetti S.55 flying boat had two engines, two hulls, two tail fins (and three rudders), and of course two booms. The cockpit fit into the center section of a thick wing supporting engines with pusher and tractor props mounted in tandem on centerline pylons. Designed as a torpedo bomber and minelayer but seeing no action in World War II, the S.55 is best remembered for a July 1933 stunt when 24 of them flew from Rome to Chicago. Led by Italy’s flamboyant young air force general, Italo Balbo, the formation arrived for the World’s Fair in a little over 48 hours. (According to National Air and Space Museum archivist Brian Nicklas, the performance so impressed the world that for years any large formation of aircraft was referred to as “a Balbo of airplanes.”)

In the following decade, a twin-boom amphibian, the single-hull, tri-motor Blohm und Voss Bv 138, nicknamed the Flying Shoe, attacked Allied ships in the north Atlantic and distinguished itself by engaging in a dogfight with a Consolidated Catalina flying boat—and winning. Bv 138s menaced Allied convoys, but by late 1942 they were being quickly rendered obsolete by the deployment of Allied aircraft carriers and their fighters. On May 1, 1945, one of the few remaining Shoes alighted under fire on a Berlin lake. Its mission was to pick up and deliver two envelopes, but the pilot ignored the order and instead picked up 10 wounded and delivered them to Copenhagen, Denmark. In those envelopes, it was later found, was Adolph Hitler’s last will and testament.

A BUNCH OF GHOULS
When the de Havilland D.H.100 Vampire was designed in 1942, the turbojet was an immature technology. The jet pipe had to be kept short to limit loss of thrust from the anemic Goblin engine. So the engine was installed behind a short, molded-plywood cockpit. Shaving weight, skinny twin booms were employed to lift the tail assembly clear of the exhaust. Empty, the airplane weighed a mere 6,372 pounds.

The Vampire appeared too late to joust with the Me 262, but in 1945 it became the first fighter to exceed 500 mph and the first jet to land on an aircraft carrier.

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