Tuesday, December 11, 2018

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The Front Office | Stalls

by Chas Hines

The Front Office | December 2018

A stall is simply the loss of lift that occurs due to aerodynamic forces (or lack thereof) on the wing of an aircraft. Let’s define a couple of key concepts to more clearly illustrate what that means.

Relative Wind

As skydivers, we’re all familiar with the concept of relative wind: It’s the airflow we feel as we move through the atmosphere. With airplanes, the relative wind is what the airplane’s wing feels as it moves forward through the air. In the illustration, think of the relative wind as a line moving from left to right, with our airplane’s wing flying to the left. (We are looking at a cross-section of the wing on the left side of the airplane.) This relative wind moves around our wing—with the air that flows around the curved upper part of the wing traveling a longer distance and faster than the air on the bottom—before meeting at the trailing edge (back end) of the wing. This results in a relative difference in air pressure on the top and the bottom of the wing, with air pressure on the upper surface being lower. Without getting into too much detail, this—along with the use of an engine, propeller or shove down a very long hill to gain speed and lift—results in the wing being able to fly.

Chord Line and Angle of Attack

Looking at the second illustration of the wing, you can see that there is a line running from the leading edge to the trailing edge, which represents the general profile of the wing. This is called the chord line. If you compare the chord line of the wing to the relative wind, you’ll see it’s at an angle. This is called our angle of attack—the angle that the wind hits the wing.

If you look at the first two images, you can see that the air moves very neatly around the wing. If the angle of attack is shallow, the airflow over the wing is smooth. However, if the airplane slows or abruptly pitches up, the angle of attack increases. This is the normal function of the wing: The angle of attack increases slightly, and the air molecules move around the wing (albeit less conveniently) from front-to-back.

If this angle of attack increases (as in the third illustration), eventually the airflow begins to separate from the trailing edge of the wing and is no longer cleanly moving from front to back. This continues until the wing stalls and no longer can support itself in the air. The angle of attack at which this occurs is called the critical angle of attack. In other words, if the angle of attack exceeds this critical angle, even if only for a moment, the plane (or parachute) stalls.

Stalls

So, what do you do about this? If you stalled your wing while flying straight and level, all you need to do is reduce your angle of attack below the critical value. The airflow will once again begin to re-attach itself to the wing, and you will regain control. Here’s where the differences between airplanes and ram-air parachutes begin.

In a stalled condition on an airplane, eventually the wing will violently pitch down as it hunts for a lower angle of attack. If the pilot prevents this from happening, whether intentionally or not, one wing will eventually begin to develop lift and the other will not. The result of this is a spin.

If you were under a stalled parachute, you might experience some additional stress. Your ram-air canopy is aptly named for its necessity to be continuously inflated and pressurized by air coming in the leading edge. By entering a stall, you effectively remove the air that the canopy needs to fly. The result is that while it has aerodynamically stalled, it has also lost its key functional structure … it probably looks like anything but “square and full of air.”

So, how can you reduce your angle of attack in these situations? In most airplanes, since they are inherently designed to be flown forward (and not in a fiery spinning ball toward the ground), you can either push forward on the yoke (thereby lowering the nose and the resulting angle of attack) or even let go of the controls completely. Yes, you read that correctly. For most stalls in airplanes, if the pilot lets go of the controls, most of the time the plane will return to a non-stalled condition.

The same holds for a parachute, although “letting go of the controls” means pitching the wing down by releasing the rear-riser or brake inputs smoothly so the canopy will surge forward, begin to hunt for more air and gain speed. Ideally, this means you get full control of it again. However, if you held the stall for too long or the canopy managed to get misshapen during the stall or an abrupt recovery, you may have ruined your perfectly good canopy flight. Just hope you are high enough to get to your reserve if this happens!

Chas Hines | C-41147
FAA Certified Flight Instructor and Airline Transport Pilot 

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