Top-end speed-more of it-is an obvious goal of many modifications, but certainly not all. Going slower is an objective sought by some modifiers. Why go slower? Pilots who fly in and out of short, unimproved, and obstacle-challenged strips want to fly as slowly as is safely possible. The same goes for low-altitude maneuvering-the slower you can fly, the better the safety margin.
Two important issues that must be addressed by modifiers looking to improve low-speed performance are improving controllability at low speeds and reducing stall speeds. One way to achieve the former is with the use of stall fences.
A stall fence is a strip of aluminum (typically) mounted chordwise (from the leading edge of the wing to the trailing edge) ahead of the flaps or ailerons and perpendicular to the surface of the wing. Its function is to block the flow of air from the wing root (where it meets the fuselage) outboard toward the tip.
Most general aviation wings are designed so that an aerodynamic stall begins at the wing root rather than the tips. Having the inboard portion of the wing stall first gives the pilot ample warning that a full stall is about to occur because turbulent air from the stalled root tumbles over the horizontal stabilizer and elevator, causing the yoke to begin shaking.
If the pilot delays recovery, the stall progresses outward toward the wing tips, rendering the ailerons ineffective. A stall fence simply blocks that spanwise flow of turbulent air, thereby allowing the ailerons to continue to function for roll control. The bottom line: Stall fences enhance aileron effectiveness (roll control) at low takeoff and landing approach airspeeds.
Other aerodynamic add-ons that pad low-speed margins include vortex generators (see "What It Looks Like: Vortex Generators," August AOPA Flight Training), aileron gap seals, "drooping" ailerons, leading edge cuffs, and special wing tips. We'll take a look at those in upcoming issues.
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