Features

October 2009 Issue

Managing Stability

Most of the airplanes we fly are stable, well-balanced machines when properly managed and flown. Changing that balance can be upsetting.

The balancing act aircraft designers must achieve amazes me. Examples include trading useful load for a strong airframe, cabin volume for reduced drag and high cruise speeds for low-speed handling. And whenever handling stands out as an issue, it’s generally balanced against whether the airplane also shines in its stability—how, for example, it stays in the attitude we establish and resists any temptation of responding to gusts. Or, when we purposely upset a trimmed attitude, how it naturally tries to return to that attitude. Sure, it’s likely to "hunt" its way back to trim, but hopefully for just a couple of cycles. The designer’s mastery always makes an airplane’s nimbleness seem that much more impressive—especially if it goes where, how and when you ask, and seeks to stay put in between. Stability counts. But it counts differently for different machine types. Some, like the NASA X-29 research vehicle pictured above, are naturally unstable, and able to sustain controlled flight only through computer processing. In the now-retired X-29’s case, computers continually adjusted the control surfaces up to 40 times each second. Others, like a jet transport, are optimized to resist any disturbance and can easily be flown with just two fingers. But any airplane can be made unstable, especially when we load it carelessly, or fly it beyond the regimes its designer intended. Where the average pilot should be concerned is in the many ways we can contribute to its loss of stability.

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