Airplanes are one of the most amazing devices on Earth. They are, quite literally, ships of the air—although they move through a fluid rather than floating on a fluid. That being the case, airplanes could actually be considered to be “submarines of the air” because of the way in which they effortlessly (more or less) slither through their chosen medium and are capable of three-dimensional movement at will. Like a submarine, they depend on only a few very basic physical laws to stay “afloat” and go where their masters direct them. One of those physical factors, which applies to so many aspects of the controls and lifting surfaces, is camber, or the curve that is built into each surface.
First, we have to recognize that an airplane actually doesn’t have wings, elevators, rudder, or flaps. In reality, it has nothing but a bunch of wings of various sizes that are pointed in different directions to accomplish different purposes.
The horizontal tail (stabilizer and elevator), for instance, is nothing but a small wing with a moveable surface attached. Picture the elevator the same as ailerons, but both sides move in the same direction and run the full length of the surface.
The same thing is true of the rudder. It’s just a short wing that’s oriented vertically and lifts 90 degrees to the horizontal plane of the wings (the real wings).
The really cool part of all these “wings” is that all of them work exactly the same way, including the flaps and ailerons. Virtually every part of the airplane’s control system uses the same principle to accomplish its purpose. As each surface is deflected it changes that surface’s camber, thereby changing—increasing or decreasing—the lift that surface generates in a given direction.
Think of a cross-section of the wing viewed from front to back, through the aileron, with the aileron in a neutral position. It’s the classic airfoil shape. Now, draw a line through the wing, from the leading edge to the trailing edge, that is centered between the top and bottom of the airfoil. That’s the camber line. It’s a graphical representation that gives a general idea of how much lift one airfoil will generate when compared to another.
The basic rule of thumb for camber line versus lift is that the more curve there is, the more lift an airfoil will generate. That’s one reason why a really fat wing generates more lift than a thin one. By "fat," we mean the thickness when compared to the width of the wing (in other words, its thickness as a percentage of the chord is higher). The thickness of a wing on a typical general aviation airplane generally runs 11 to 15 percent of the mean aerodynamic chord, give or take a point or two. Thinner wings (those with less camber) are faster, but they generate less lift for a given speed and angle of attack—while fat wings (with more camber) generate more lift.
Totally symmetrical wings, like those on many aerobatic aircraft, have a straight camber line because the top and bottom surfaces are curved the same; they depend on angle of attack to generate lift. Watch a symmetrical-winged Decathlon or Pitts fly the approach and notice how high the nose is when compared to other aircraft.
Incidentally, some will argue that deflecting a control surface creates downwash off of that surface, which increases the lift. Still others will see the increased area presented to the relative wind by a deflected surface as something that contributes a push in the right direction. Both schools of thought are valid, to a certain extent, especially with large deflections of tail surfaces. But at normal flight deflections, camber-line changes to the airfoil are what control the airplane.
So, now that we’ve agreed different camber lines generate different amounts of lift, let’s see how the application of that concept to common parts of the airplane lets us control the airplane.
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 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.
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