Plane & Pilot
Tuesday, October 25, 2011

Angle Of The Wing

Angle-of-attack indicators, coming to a glass panel near you

An airspeed indicator assumes a cumulative error that's inversely proportional to speed. In other words, error increases as speed decreases. Finally, when an airplane is flying very close to stall, an ASI may be practically useless, a quivering needle bobbing near the bottom of the gauge, reading anywhere from five to 30 knots slow. Conversely, angle-of-attack indicators provide a continuous readout of margin above stall right down to the actual event.

Technically, AOA is the angle between the airfoil chord line and the wing's direction of motion relative to the air. In simpler terms, you could say AOA is the angle between where the wing is pointed and where it's actually going.

Contrary to what you might imagine, AOA instruments go back about as far as possible in aviation history. The original 1903 Wright Flyer had only one "flight instrument," if you want to call it that. It was a primitive form of angle-of-attack indicator, little more than a stick with a piece of yarn attached, mounted on the "aeroplane's" nose.

The men generally credited with the first powered flight used this device to determine the Flyer's attitude with reference to the relative wind. There was no airspeed indicator. Wilbur and Orville referred to their primitive AOA indicator to monitor the airplane's proximity to stall. This was essentially pure flight, little more than modestly powered hang gliding, so instruments were an unnecessary redundancy.

Alpha Systems offers a variety of displays to track angle of attack.
Considering what the Wrights went through to achieve their goal, pure flight was a worthwhile accomplishment in 1903, but it's hardly good enough if you're trying to operate an airplane a century later. Even modern airspeed indicators don't tell the whole story.

Angle of attack does, and it's pretty much immutable. Whether you're flying right side up, upside down, straight up, straight down or any attitude in between, angle of attack ignores airspeed altogether—true, calibrated, indicated, groundspeed or any other kind—and precisely defines the wings' margin above stall. An angle-of-attack indicator is a virtually foolproof device for determining your attitude with reference to the critical stall angle of attack.

Higher angles of attack result in higher lift but also generate more drag. Airplanes trimmed for cruise flight present their lowest angle of attack to the relative wind and therefore their lowest drag profile. All other factors being equal (which almost never happens), this results in the best possible cruise speed.

Pull the airplane into a climb, and speed bleeds off as AOA and drag increase. When the wing reaches the critical AOA, about 15-18 degrees on most general aviation airfoils, the airflow detaches from the wing's top surface, and the wing stalls. On aircraft designed with positive stability, the wing will typically pitch forward in an attempt to continue flying.


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