How the 747 Got Its Hump
In the evolution of the airplane, Darwinian principles have applied unevenly.
- By Bill Sweetman
- Air & Space magazine, May 2003
(Page 3 of 6)
Boeing’s first idea for a jet transport incorporated the swept wings and podded engines of its bomber predecessors, but it also shared a high wing. With the wing on top of the fuselage, the only place for the landing gear was in the belly, arranged like a skateboard’s wheels—ahead of and behind the airplane’s center of gravity. The tricycle gear on an airliner places the main gear close to the center of gravity, with just enough weight forward to ensure that the airplane stands lightly poised on its nosewheel. On takeoff, every airliner pivots (or “rotates”) on its main gear, noses up, and climbs away at an angle.
But because of the tandem bicycle landing gear arrangement on Boeing’s bombers, they can’t rotate to climb any more than your car could. To take off, the bombers’ wing has to be in a flying attitude to generate lift on the ground, and that’s how they’re built: with the wing at a positive angle of attack. They both land and take off with their fuselage—and landing gear—in a level attitude, levitating more than climbing, settling more than descending. On rollout after landing, the wings generate lift and the wheel brakes don’t work very well, so they need braking parachutes. This would not do for a transport. The 367-80, therefore, had a low wing to which was affixed the main wheels of a tricycle gear, which folded inward into the fuselage. So in the end, the bombers contributed only swept wings and pod engines.
Nobody knew at the time that the 367-80 layout would eventually dominate designs for all airliners. Boeing’s second all-new commercial jet, the 727, had three engines clustered in the aft fuselage and a T-shaped tail. It was a hot seller in the 1960s, as was the rear-engine Douglas DC-9. Great Britain and France produced rear-engine jets, including Britain’s 707-size Vickers VC-10 and France’s early Sud Caravelle. The 707 looked positively dated.
And yet today, the last of the paltry 54 VC-10s, converted into military tankers, roar and snort their way around the world. The last 727 rolled off the production line in 1984, replaced by the underwing-engine 757. Europe adopted the Boeing-style shape for the Airbus A300—and for the first time had an airplane that the world wanted to buy. Today, the Boeing 717—the re-badged tail end of the DC-9 line—is the last rear-engine, 100-plus-seat jet in production.
Underwing-engine airplanes evolved and became dominant because they were more efficient. An airplane’s wing is like a beam with the mass of the fuselage at its center; engines hanging at mid-span on the wing counteract some of the bending loads on the beam, so the wing can be built lighter. With the weight of the engines at the back, an airplane balances nicely with a shorter aft fuselage, so the horizontal stabilizer has less leverage to balance the airplane and must be bigger and heavier. British reports bragged that the VC-10’s all-moving tail was bigger than the wing of a Hunter fighter—but it weighed a ton.
Iguanas and Komodo dragons survive after allosaurs die out. Almost all business jets and most regional jets still have aft engines and high tails. In 1959, McDonnell actually did try to adapt the 707 shape to a small airplane; the result was the model 220 prototype, a competitor for an Air Force contract for a utility transport and trainer. It looks like it might be a 707’s awkward chick, a creature that has some growing to do before it can fly. The model 220 shows that 707-style underwing engines don’t adapt well to small sizes. You can scale down an airplane but not the air; the gap between the engine nacelle and the wing cannot shrink in proportion to the airplane. While the forward cabin door is high off the ground, the engines’ ground clearance is reduced and the inboard engine is in line with the bottom of the steps, perfectly positioned to eat the impatient CEO who won’t wait for the engines to shut down.
Military transport airplanes all look like Grandpa: the Lockheed C-130. It has a high-mounted wing, a stumpy-legged main landing gear in pods on the lower corner of the fuselage, and an upswept tail that incorporates a ramp door. The ramp can be lowered to the ground so that vehicles can drive in, it can be extended straight out to roll cargo pallets onto or off a truck, or it can be opened in flight to drop loads out the back by parachute.
For a brief period between World War II and the Korean War, it appeared that a very different configuration would reign, with a central pod fuselage and twin booms supporting the engines and tail—see the Fairchild C-82 Packet and C-119 Flying Boxcar in the United States and the Nord Noratlas and Armstrong Whitworth Argosy “Whistling Wheelbarrow” in Europe. But that species was doomed.