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Saturday, Jul 11, 2009
Planes without wings and tails, flying lawn mowers and other aerodynamic amazements.
Dec 3, 2004 | "It remains a fundamental aerodynamic axiom, by the way, that a plane will not stay in the air without its tail." That's a line that appeared in this space on Nov. 19, and that, as these things go, wrought a flurry of protests.
"I have several things I would like to point out relative to this statement," says Steve Willie, writing from Olympia, Wash. Letters beginning like this have a way of putting me quickly to sleep, the subtleties of aerodynamic theory not being among the topics that get my turbines turning.
"Several recent aircraft designs," writes Willie, "don't have a tail." He cites so-called flying-wing concepts like that of the B-2 bomber and the tailless X-45, an unmanned combat air vehicle, or UCAV. Several of the Wright brothers' experimental prototypes also were built without tails.
The key being, they weren't supposed to have them. What got this discussion running was the crash of American Airlines Flight 587, an Airbus A300 that parted company with its tail after a crew member overreacted to a wake turbulence encounter. The blueprints of a B-2 or a Wright brothers box kite from 1902 simply don't necessitate a vertical stabilizer. (That's the more technical term for tail, and I'll be using the terms interchangeably from this point.) The A300, like virtually every modern airliner, is a different story, and should that stabilizer be abruptly separated from the rest of the plane, well, my initial statement stands.
But wait a minute, and hang on tight ...
"Any aircraft with digital flight controls, including large commercial airliners, can theoretically be programmed to fly without a vertical stabilizer. I cannot guarantee that existing onboard computers would be sufficiently powerful to make the necessary control corrections, but recent simulations suggest that differential thrust could provide any missing directional stability. The plane cannot be allowed to bank, because any non-symmetric aileron input will result in uncontrollable adverse yaw, which needs rudder input and cannot be corrected quickly enough by differential thrust."
That's Willie talking again. I have no idea if this is true, or even what he's saying, exactly, except to know that it can't possibly apply to an airliner's day-to-day operations. In the interest of saving coffee, I won't be looking into it.
I should have learned my lesson. In one of the very first Ask the Pilot question-and-answer sessions, a seemingly straightforward illumination of how a plane gets off the ground brought forth a veritable pounding of wait-a-minutes from annoyed readers. My explanation, so went the dissenting letters, was overly bogged down in the principles of Daniel Bernoulli, the 18th century Swiss mathematician whose experiments with velocity and pressure form a staple of Aviation 101: high pressure beneath a wing, low pressure above. (See Chapter 1 of my book -- that's the boring chapter -- for a slightly more nuanced look at this.)
But while the laws of Bernoulli are not in dispute, theirs is only half the story. Or more, or less, depending whom you ask and how much they happen to care. "Bernoulli is so old school," countered one flight instructor. The simple matter of deflection, he and others insist -- the act of an airfoil pushing against the oncoming current of air -- is the more "true" reason a plane stays aloft. If nothing else this is more intuitively graspable, easily demonstrated by angling your hand out the window of a speeding car, your arm held aloft by an ample molecular cushion.
