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Southport, Manitoba, Canada
Steve Pomroy is a professional flight instructor and aviation writer. He has been teaching since 1995 and holds an Airline Transport Pilot License, Class 1 Instructor and Aerobatic Instructor Ratings, military QFI, and an undergraduate degree in Mechanical Engineering. He's written and published three flight training books through his company, SkyWriters Publishing, and has several other books under development. Steve currently teaches RCAF pilot candidates on their Primary Flight Training course.

Tuesday, May 3, 2011

Fly the Little Wing

One of the bloggers I read (Cockpit Conversation) has been writing lately about linguistics. So, in that theme, let's start today's post by discussing the etymology of 'Aileron'!

Aileron is yet another word brought to the aviation community by the French (some others: fuselage, empennage, decolage, nacelle, pitot, sacre bleu!, etc.) It originates around 1909, which was around the same time the aileron was actually invented. (The Wright Brothers used wing warping to control roll. The aileron was invented later by other innovators. But ultimately, the Wrights won a court ruling declaring that the aileron was included in their roll-control patent of wing warping.)

From www.etymonline.com:
1909, from Fr. aileron, altered (by influence of aile "wing"), from Fr. aleron "little wing," dim. of O.Fr. ele "wing" (12c.), from L. ala "wing" (see aisle).
Bottom line, Aileron means "Little Wing"—not to be confused with the music of Jimi Hendrix. Since an aileron is essentially a little wing that gets used to modify a big wing, the name seems appropriate.

During normal flight operations, ailerons are pretty simple. When deflected, they alter the camber of the wing and the orientation of the chord line. Because they deflect in opposite directions, they serve to control the rolling movement of the aircraft—one wing gains lift while the other loses lift, and the aircraft rolls toward the up-going aileron.

Unfortunately, things get a little more complicated during non-normal operations—particularly in and around the stall. The aileron can quickly become unreliable, or, more unexpectedly, reliably work backward. The response of ailerons to the stall can vary quite a bit, and is heavily influenced by the planform shape of the wing—which in turn influences whether or not the wingtip will stall under any given set of conditions. Consider the following examples of planform effect on stalling:
Rectangular Wings
Rectangular wings stall from the roots, and the stall progresses outward—resulting in good roll control (read "unstalled ailerons") throughout the flight envelope.
Swept Wings
Swept wings tend to stall at the tips first, leading to a whole host of problems (pitch up and deep stall to name two that may be topics of future posts). One of these problems is the reversal of the ailerons.
Tapered Wings
Tapered wings begin stalling at around the midpoint of the trailing edge. The stall then propagates forward and outward until the whole wing is stalled.
These effects are further complicated by washin/washout (twisting of the wing), "aerodynamic twist" (varying the airfoil section along the span), configuration (flap setting), and features such as vortex generators and stall strips.

If the wingtip, and therefore the aileron, is stalled, deflection of the aileron will have an effect opposite to the intent. The down-going aileron—which normally increases lift—will effectively increase the angle of attack of the wing it's attached to, and deepen the stall—resulting in a reduction of lift and increase in drag. Vice-versa for the upgoing aileron—lower angle of attack, reduced stall, more lift. The effect in this case will be for the aircraft to roll opposite the aileron input—the ailerons are reversed.

On the other hand, if the wingtip (and therefore the aileron) is unstalled—even if the inboard portion of the wing is fully stalled—the ailerons will work properly.

Somewhere between these two extremes, when the stall has progressed spanwise to cover part of the aileron, there is a point where the aileron will have no effect at all. Why? Part of the aileron (which is stalled) is reversed, while part of the aileron (which is unstalled) is not reversed. The countering effect neutralizes any roll input made by the pilot.

So we have three possible aileron behaviors in a stall: they can reverse, they can be ineffective, or they can work properly. Which of these behaviors shows up will depend on aircraft design features (planform shape being a major one) and the depth of the stall. This stall-depth dependency creates some uncertainty regarding aileron response in many aircraft types, and is ultimately the reason for the "neutralize ailerons" doctrine of stall recoveries.

If we knew, with certainty, that the ailerons would work properly, we could just use them as usual. If we knew, with certainty, that the ailerons would be neutralized, we could just ignore them and not spend so much time learning not to use them. If we knew, with certainty, that the ailerons would reverse, we could just use them backward.

For most aircraft types, the aileron response will depend heavily on how deep the stall is. As a result, the ailerons can be considered unreliable—we don't know how they'll respond until we try. Since inadvertent stalls occur at low altitude, and minimum-altitude recoveries are paramount, spending time experimenting is ill-advised. So neutralizing the ailerons for stall recoveries is the best course of action.

For several models of the Cessna Skyhawk (C-172), the manufacturer, via the flight manual, recommends using pro-spin aileron to help ensure a proper spin entry. This approach will only work if we can reliably say that the ailerons will not be stalled at the entry to a spin. The same manufacturer does not recommend using anti-spin (or pro-spin) aileron during a spin recovery. Why not? as the spin develops, the stall deepens. It's possible in this case for the wingtips to fully or partially stall, resulting in the neutralizing or reversal of the ailerons. Unless we can reliably predict the effect, any attempt to use the ailerons is an experiment. If we get it right, bonus! But, if we get it wrong, we'll chew up much more altitude on the recovery—Unbonus!

As a counter example, on the Grob G-120A, the manufacturer, via the flight manual, calls for a spin recovery with full aileron deflection into the spin. Why? The tapered wing is a hint. We can reliably predict that the wingtips (and therefore the ailerons) will be stalled during a spin. This means that we know that the ailerons will work backward, and aileron into the spin creates rolling and yawing moments out of the spin.

I'm reminded by this post of a former colleague of mine who had previously flown F-5's in the Air Force. He explained to me one day that the spin recovery in the F-5 above 20,000' included full aileron deflection into the spin (not surprising, the F-5 is a delta-wing aircraft—wing tips stall first). He went on to explain that the spin recovery below 20,000' included pulling on the ejection handle and hoping your parachute worked!

Happy Flying!

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