Thursday, March 05, 2009

Great Moments in Aviation, Interlude

By 1910, when Alberto Santos-Dumont built the Demoiselle, the basic form for all airplanes to follow had been set. There would be a few variations, but from then on, most would follow the same basic plan.

The obvious question is: why? Why this shape, and not another? The answer is that, in engineering, form follows function. To illustrate this, we'll compare the standard layout with the Wright Flyer layout, which we saw last time.

The Wrights had decided that control was the central problem, and therefore built their fliers to be very maneuverable. An unfortunate consequence of this was that the Wright designs were all statically unstable. This is a good thing from a maneuverability standpoint. Since the vehicle doesn't really want to fly straight and level, it's always ready to respond to a pilot command to do something else. But if you do want to fly straight and level, it takes a lot of work to keep it that way. So, the Wright fliers were easy to steer, but hard to fly. (An unstable aircraft might well be impossible to steer or fly. It all depends on the details.)

The European pioneers, on the other hand, preferred designs that produced stable gliders. That is, they would glide straight and level without any need for a pilot on the loop. This greatly reduces a pilot's workload in straight and level flight. Although they were harder to steer, they were much easier to fly.

By 1910, designers knew well what features of an airplane's design conferred the stability that they desired. In the pitch axis, you design for static margin. In the roll axis, you design for dihedral. Passive stability is relatively easy to achieve in the yaw axis, with the placement of the vertical stabilizer. (In practice, yaw and roll axes tend to be tightly coupled, and you really can't analyze them separately. But these illustrations still suffice for a basic explanation.)

Static margin is simply the difference between the location of the plane's center of gravity and the location of the plane's center of lift. Designers like for this to be a small, negative number. The reason they want it negative is that a negative static margin confers resistance to pitch-up gusts. If you tilt the nose slightly upwards, and the CG is ahead of the center of lift, then the natural tendency of the nose will be to fall back to neutral. But because you've built in a nose-down torque, you need to design the horizontal tail to provide negative lift, so that there will be no tendency to rotate in the cruise configuration. This also provides stability if you experience a nose-down gust, since the horizontal tail will produce more lift as the nose points downward, providing a net nose-up torque, again tending to reset the pitch attitude to neutral.

Dihedral has to do with the tilt of the wings of the airplane. This is easiest to see in low-wing monoplanes.

This configuration benefits roll stability. With slightly upswept wings, a low-wing plane will tend to roll back to vertical if upset in roll to the left or the right. The lower wing will produce a greater vertical force than the higher wing, providing the restorative torque. This also induces a side force that the pilot will have to counteract with a bit of rudder. High-wing monoplanes generally have little actual upsweep to the wings. Since the center of gravity is usually below the wing plane in this case, there's a pendulum effect that basically does the same thing, restoring the airplane to the horizontal with little extra effort on the pilot's part.

The placement of the vertical tail is perhaps the simplest and most obvious thing. The vertical tail is in back for the same reason that fletching is on the back of an arrow: weathervane stability. If you tried to shoot an arrow backwards, what would happen? The very first gust that pulls it off-center will pull it farther and farther off-center, until it's flying with the fletching in back again. Once designers realized this fact, no one ever seriously contemplated doing it any other way.

In my opinion, the year 1910 brings to a close the first chapter in the history of aviation. It was, in a sense, the end of an era of innocence. The pioneers all looked forward to a peaceful future, one where ever-advancing airplanes would knit mankind's far-flung civilization ever closer together with bonds of peaceful travel and commerce. None of them foresaw what lay only four years in the future, nor would they have believed it had anyone told them.

In 1914, the airplane would go to war, and the second chapter of aviation's history would begin.

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