January 2014

Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.

Issue link: http://experimenter.epubxp.com/i/247918

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Page 37 of 38

FLIGHT TESTING TECHNIQUES Stall-Proof Airplane? How's your nose wheel? BY ED KOLANO BELTS TIGHT, LOOSE ARTICLES stowed, clearing turn complete, airplane trimmed, power set. You slowly apply increasing back-stick, keeping the deceleration to a nicely controlled 1 to 3 knots per second, feet dancing lightly on the pedals to keep the slip/skid ball centered. More pull, slower speed, more pull, even slower now. Then nothing. The stick is all the way back, but the airplane doesn't stall. Eureka! The perfect pilot-proof airplane. Maybe. Maybe not. Wing rock, yaw excursions, power effects, entry rate, stall warning, control effectiveness—all important, but let's save them for another time. For now, let's assume the airplane is rock steady with full aft-stick. An airplane that remains unstalled with full back-stick applied is said to be elevator limited. Full trailing-edge-up elevator produces the minimum flying speed but not a sufficiently high angle of attack to stall the wing. Might sound good up and away, but this seeming safety feature can have an effect on your landing. In a traditional configuration airplane, the air flowing over the horizontal tail has first encountered the wing. A lifting wing creates vortices or air rushing around the wingtips from the higher pressure beneath the wing toward the lower pressure above the wing. This swirl imparts a downward component to the air leaving the wing's trailing edge, creating a downwash. The angle between this downwash and the horizontal tail's chord line is the tail's angle of attack. When the airplane descends to within about one wingspan of the runway, ground effect begins, and the lower the 38 Vol.3 No.1 / January 2014 plane gets, the more pronounced the ground effect becomes. Ground effect has several…well…effects on the lift and drag of an airplane. For example, the closer the wing is to the runway, the less room there is for the wingtip vortices. Because those vortices are a by-product of lift creation, decreasing them results in significantly less induced drag. Inhibiting these vortices has the same effect as increasing the wing's aspect ratio, which means more lift in ground effect than outside ground effect for the same angle of attack. Both of these effects contribute toward floating down the runway. Carrying a few extra knots into the flare can result in a much longer landing than expected. The physical presence of the runway also redirects the remaining downwash coming off the wing. The runway prevents the normal downward flow and forces it more parallel to the runway. Less downwash means less angle of attack at the horizontal tail. In other words, the deeper you descend into ground effect, the more trailing-edge-up elevator you need. Now let's get back to our elevator-limited airplane. If the plane can't be stalled with full aft-stick outside ground effect, it surely won't stall in ground effect where the full aft-stick angle of attack of the tail is less. So-called full-stall landings would be impossible in this airplane. That means faster touchdown speeds and longer landings than might be aerodynamically achievable if the elevator were rigged differently or if the incidence of the wing and tail were different. That's the good news. A little less good news is the fact that this plane is essentially still flying when it lands. That is, if the wing was Photography by Jim Koepnick

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