Flight Training - May 2011 - 28

VARIOUS 'WINGS'
Elevator

Flap Aileron

Rudder

AILERONS. Go back to the wing that we sectioned through the aileron and then drew a camber line. Now, let’s deflect that aileron down and redraw the camber line. Because the trailing edge is down, there is more curve, and the wing with the down aileron generates more lift than the opposite wing, which has its aileron angled upward. This is also why airshow performers can literally lose an aileron and still fly: with one aileron they can still generate more lift on one wing than the other and can make the airplane turn. The foregoing is also at the heart of adverse yaw: As soon as an aileron is deflected, it causes the wing on that side to generate more lift (down aileron) or less lift (up aileron). Unfortunately, increasing the lift on the outboard wing also increases the drag, hence the adverse yaw. The outboard wing is held back by the drag only as long as the aileron is down. Once in the turn and the ailerons are returned to neutral, the lift on the right and left is balanced because the camber lines, left and right, are the same. Balanced lift means no rolling action and no imbalanced drag, so no rudder is needed. If an aileron is deflected even a little, however, rudder is required. VERTICAL TAIL. Look straight down on the vertical tail from above and what do you have? Another wing, right? But this one has a really big aileron attached that is the rudder. When you step on the left rudder pedal, what happens? The rudder moves to the left, and that increases lift
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in the opposite direction. Since you’re in the process of rolling into a left turn, the increased drag on the outside wing is trying to drag the right wing tip back, which pushes the tail to the left (nose to the outside of the turn). Adding left rudder changes the rudder’s camber line with the high side of the curve—the top of a wing that’s pointing up—increases lift, and pushes the tail to the right. This counteracts the effect of the aileron trying to drag the right wing back and keeps the ball in the center by eliminating yaw. The fact that the airfoil used on the vertical tail is usually symmetrical, and that the rudder moves both directions, allows us to change the camber line in either direction—thereby allowing us to cancel out adverse yaw in both directions. HORIZONTAL TAIL. The horizontal tail works like the vertical tail, but up and down, rather than left and right. FLAPS. When it comes to flaps, it’s important to define what kind of flaps you’re talking about. Yes, the camber thing still applies, but different flaps use it in slightly different ways and to different degrees. Simple hinged flaps, such as those on a Piper Cherokee, do nothing more than increase the camber line of the wing, which increases the lift. This is especially true for the first 15 degrees or so of deflection. Past that, as the deflection is increased, they begin generating increasing amounts of drag.

Fowler flaps, like those on Cessna aircraft, are more efficient for a number of reasons—some not camber-related. A Fowler flap, by definition, moves back slightly as it goes down, which increases the wing area slightly. So, in addition to the effect on the camber line, increased lift also comes from increased area. Slotted Fowler flaps are much more efficient at changing the camber line because the slot between the wing and the flap lets air flow over the nose of the flap, which helps to keep the airflow attached to the flap at larger angles of deflection. The more a flap of any kind is deflected, the more the airflow tries to separate because it can’t make the corner. It’s at this point that a flap stops creating additional lift and begins to create only drag. Slotted Fowlers delay that separation to a greater degree of flap deflection, so camber line effects and overall lift linger much longer before converting to mostly drag. Aerodynamics is nowhere near as black and white as flight instructors and magazine stories portray it. In fact, a lot of very smart people still argue over what makes an airplane fly, so there are lots of gray areas. For those of us who aren’t engineers, if we ignore the complexities and focus on the changing of the camber line every time we move a flight control, we’ll have an improved understanding that will, at the very least, make us better pilots.
Budd Davisson is an aviation writer, photographer, and magazine editor. Visit his website (www.airbum.com).

http://www.airbum.com http://FLIGHTTRAINING.AOPA.ORG

Flight Training - May 2011

Table of Contents for the Digital Edition of Flight Training - May 2011