Why a flap-gap seal? One of the leading manufacturers of such aftermarket performance modifications claims that the gap creates a pathway for some high-pressure air traveling across the underside of the wing to bleed up into the low-pressure air flowing across the top of the wing. This creates drag, which degrades climb, cruise, and handling performance.
Closing that gap seals the pressure leak and, theoretically at least, boosts the airplane's performance. You might think that if you were able to fly each of these airplanes back-to-back, you'd see a distinctive difference in performance. Maybe; maybe not. Lots of things factor into performance including engine power, control surface rigging, aircraft weight and balance, and instrument errors.
An apples-to-apples comparison of a modified and an otherwise identical unmodified aircraft can be hard to come by. For example, the airplane shown with the gap seals also has been modified with the installation of a 180-hp engine, while the other still has a stock 145-, 150-, or 160-hp engine, depending on the model. Those extra horses will make much more of a difference in performance than the gap seals.
