Thursday, February 01, 1990

Little Richard sang

Spinning, spinning, spinning,
Spinning like a spinning top!
Jenny, Jenny by Little Richard

The early days of airplane design were more hit-and-miss than well-planned science. A lot of effort went into trying to design a plane which was resistant to inexplicably falling out of the sky, until military necessity changed that focus. A combat pilot wants his plane to be easy and quick to maneuver. Stability is not his friend. Design focus shifted away from stability toward maneuverability. The distilled result was the "cruciform" aircraft, a creature we're quite familiar with, as that style dominates most aspects of aircraft design and has done so for a century.

One of the significant instabilities of the cruciform design is that it stalls when its AOA (angle of attack of the wing) exceeds a critical point. Once an aircraft has stalled, if the stall is not immediately corrected, the airplane will spin. Before Frederick Lindemann arrived on the scene, spins always resulted in a crash. Nobody knew how to recover from a spin.

So who the hell was Frederick Lindemann?

Like Niels Bohr, one of the fathers of quantum mechanics and a professional soccer player, and Niels' brother Harald, mathematician and Olympic athlete, Lindemann did not fall into the studious-geek vs. dumb-jock stereotype of the current day. He was a tennis player who could compete at Wimbledon and he was simultaneously a brilliant physicist. His family had moved to England from Germany and after the outbreak of WWI he went to work for the government at the Royal Aircraft Factory where he developed his mathematical model of how and why airplanes spin. Once he understood how and why they spin, he devised a method of recovering from a spin. Armed with his self-discovered knowledge, Lindemann got into a plane, climbed to altitude, and purposely entered a spin, a flight profile which heretofore had always always resulted in a fatal crash.

Lindemann's solution was correct. (But can you imagine his excitement and fear? He must have felt some fear, no matter how sure he was of his abstract mathematical model.) He controlled the spin and pulled out of it. He could enter AND EXIT a spin at will and any pilot who learned his technique could do so as well. This gave British pilots an important extra tool in their panoply of combat tactics. Let me explain.

In a dogfight, even if your airplane is superior to your enemy's in the abstract, you can still get caught in a bad position. Once your opponent is on your tail and you can't shake him, it's only a matter of time before he kills you. He is Scylla, reaching to gnaw on your bones, and sometimes, in your frantic attempts to avoid Scylla, you get sucked into the whirlpool of Charybdis, which for an aircraft is that fatal spin.

But! Oh and what a but it is! If you have the ability to recover from a spin, you can easily escape your enemy by purposefully choosing Charybdis (going into a spin). He will not follow you because he knows it's a fatal maneuver and he doesn't want to commit suicide. If he follows you down in a controlled flight mode, he'll be so far behind you that he no longer has a dominant position over you and the combat scenario is back to square one where you are no longer at his mercy.

It took German pilots quite a while to learn how to do spin recovery themselves and in that interval many British pilots saved themselves from certain death by spinning away from an enemy on their tail and using Lindemann's technique to recover from that spin in full control at a lower altitude away from their attacker.

Cool, huh?

But we are NOT combat pilots (or passengers) and the tendency of a cruciform aircraft to spin is more a weakness than a benefit to a noncombat pilot. Stall/spin during takeoffs and landings is the largest cause of crashes and deaths by far. Are there ways to improve that? Is it possible to eliminate that?

In a word, yes. In a cruciform design, the empennage, vertical and horizontal stabilizers, is commonly called the "tail" because it's stuck on the rear of the aircraft. Early experiments fooled around with the "canard" concept where the horizontal stabilizer is at the front of the aircraft but, as I said, this fell aside in preference to the more-aerobatic cruciform airframe. However, the canard has a number of advantages if you're not planning to be in a dogfight with it. The most significant advantage is that it is "stall-resistant," like a watch is "water-resistant" because claiming that you're stall*proof* or water*proof* in this litigious society is not sensible. Here's how that works.

The canard, in the front of the aircraft, is set at a steeper angle than the main wing. This means that when you get to that place in your performance envelope where you're about to stall, the canard stalls first, before the main wing can stall. When this happens, the front of the aircraft "bobs" down a bit as the canard stalls, returning the aircraft to a flying configuration before the main wing can stall. Practically speaking, you cannot stall the main wing.

In actuality, it's astonishing. During takeoff, you can pull the canard's stick all the way back and the aircraft will still continue to climb out, albeit a bit more slowly than if you were flying efficiently, while the nose just kind of bobs gently up and down, going in and out of stall. Pull the stick back in a Cessna during takeoff, and you and your passengers are burning wreckage scant moments later.

The great designer Burt Rutan is the guy we can thank for this. Burt is the guy who has single-handedly done most of the great aircraft design work in the last few decades. He is responsible for Voyager, nonrefuelled nonstop around the world, Global Flyer, solo nonrefuelled nonstop around the world, Proteus, high-altitude high-endurance records, and SpaceShipOne, being followed as I write by SpaceShipTwo and private commercial spaceflight. How cool is that? Tickets are currently selling for $200K. I'm saving up.

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