AOPA Pilot Magazine - May 2012 - 28

PROFICIENTPILOT

BY BARRY SCHIFF

Quit stalling around

ast December I mentioned that some airplanes, especially Beech Bonanzas, threaten to enter an over-thetop spin when stalled with full

is called “washing out” a wing and is characteristic of high-wing Cessnas. The wing twist (wash-out) of a Cessna 172, for example, is 3 degrees. When the inboard wing section of a Skyhawk has an angle of attack of 14 degrees, for example, the outer wing panel has an “alpha” of only 11 degrees, a scheme that forces a stall to begin near the wing root instead of the tip. It is not difficult, however, for a pilot to overcome a designer’s best efforts to provide benign power and flaps extended (“Proficient Pilot: The Nostall characteristics. All he need do is stall the airSpin Zone,” December 2011 AOPA Pilot). This is why, plane with full power and deployed flaps, what we Barry Schiff has I continued, Beech does not suggest using flaps for commonly refer to as a departure stall. If the engine been writing for takeoff. This caused a flurry of reaction from some is developing full or substantial power during stall AOPA Pilot for 49 members of the American Bonanza Society, stalentry, propwash helps to protect the inboard secyears. wart defenders of the venerable airplane. But before tions of the wings against stalling. Consequently, defending my comments about the Bonanza, it the airplane can be forced into a deeper stall that would be appropriate to review the effects of power involves greater (outboard) wing area. This partially and flaps on aircraft stall characteristics and behavior. explains why power-on stall speeds are lower than power-off A goal of those who design and manufacture GA aircraft is stall speeds. The outboard spread of the stall partially defeats to create an airplane with mild-mannered stall characteristics. some of the design features and can result in surprisingly strong This often is done by developing a wing such that a stall begins and rapid roll rates toward the wing most deeply involved in the near the aft, inboard portion of the wing and then spreads (or stall. Deploying flaps exacerbates the problem because flaps propagates) outboard and forward as the pilot continues to further protect the inboard wing sections against stalling. The stall is forced even farther outboard. increase the wing’s angle of attack and deepens the stall. This happens almost naturally when an airplane has rectA few decades ago I was involved in the forensic study of angular wings. The challenge increases for the designer when a tragic accident involving a heavily loaded Beech 35 V-tail wings are tapered or swept. In these cases, the stall tends to Bonanza that had departed California’s Big Bear Airport begin near the wing tips and propagates inboard. The problem toward the east on a day when density altitude on the ground with an “outboard” stall is that this can reduce aileron effective- approached 10,000 feet. Initial climb rate and angle were aneness during flight at minimum airspeeds. It also reduces roll mic, and the pilot apparently raised the nose in a misguided damping, the characteristic of an airplane to resist rolling. In attempt to increase both. Observers said that the airplane rolled other words, a wing-tip stall can induce a sharp roll and make rapidly into an inverted attitude before plummeting headlong it difficult for a pilot to maintain a wings-level attitude. into the ground from an estimated height of 100 feet. The horizontal tail surfaces are not behind the stalled porSubsequent flight testing confirmed that the flaps-down, tions of the wings during a tip stall. As a result, pilots might not power-on stall characteristics of the Bonanza can be too difficult be provided with the stall-warning buffet we are conditioned to for many pilots to handle, and that it would be wise for Beech expect. Also, a “self-correcting,” nose-down pitching moment not to recommend using flaps for a short- or soft-field takeoff. might not occur to assist in stall recovery. This is why checklists and pilot’s operating handbooks for the Designers go to great lengths to avoid these undesirable iconic Bonanza V35B (as well as the popular F33A straight-tail characteristics and employ a variety of aerodynamic tech- model) do not indicate the use of flaps for takeoff. Interestingly, niques to ensure that a stall begins to develop at the aft portion using flaps for takeoff in any Bonanza was never prohibited and of the wing root instead of at the tip. One trick of the trade is the mention of flaps for takeoff has been inconsistent between to install spanwise stall strips on the inboard leading edges of various models of the “short-wing” Bonanza. some wings. At large angles of attack, these sharp-edged strips Two lessons can be taken from this. The first is that a fullinterfere with airflow at the wings’ leading edges—they “trip” power, flaps-down stall in a Bonanza is more “adventurous” the air—and induce a stall to occur behind each such strip. than in any other four-place, high-performance single in which A more expensive technique is to vary airfoil shape in such a I have performed the same maneuver. The second is the absoway that the inboard airfoils (wing cross-sections) stall before lute necessity in any airplane to “maintain thy airspeed lest the the outboard airfoils do. A third and popular technique is to Earth shall arise and smite thee.” twist the wings slightly so that the inboard wing sections have larger angles of incidence than the outboard wing sections. This Visit the author’s website (www.barryschiff.com). 28 • MAY 2012

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http://www.barryschiff.com

AOPA Pilot Magazine - May 2012

Table of Contents for the Digital Edition of AOPA Pilot Magazine - May 2012