Flaps are familiar to anyone who has flown in a jetliner and watched the wings closely during the approach to landing. There was a point at which the trailing edge of the wing seemed almost to come apart, as enormous slabs of metal moved backward and downward in a curve until the wing seemed to have nearly doubled its size. Those flaps, called Fowler flaps, are the most complex of all the varieties available to designers. Small airplanes use simpler hinged flaps that are much lighter than Fowler flaps. Remember, small airplanes can’t afford too much in the way of complex systems because of the weight restriction.
Simply put, flaps help an airplane fly slower. That’s why you see them used mostly during landing, when a pilot wants to slow the plane as much as possible. The slower the plane is flying when it touches down for landing, the less the brakes have to work to bring it to a stop, allowing the plane to land on a shorter runway and to touch down at a slower, safer speed.
Another major structural component of the airplane is the “empennage” (pronounced EM-puh-nazh). The word empennage comes from the French word for putting feathers on the end of an arrow. The empennage is largely responsible for stabilizing the plane in flight.
The primary surfaces making up the empennage of most planes are a horizontal stabilizer and a vertical stabilizer.
The Horizontal Stabilizer
The horizontal stabilizer resembles two miniature wings attached to the back end of the airplane. On the trailing edge of the horizontal stabilizers are hinged surfaces called “elevators,” which the pilot controls by pushing the control column forward and backward in the cockpit.
By a show of hands, how many believe the horizontal stabilizers produce upward lift just like the wings do? If you raised your hand, you’d be wrong. In fact, the horizontal stabilizers create a force that is downward in most flight conditions. We’ll examine why in the next chapter when we talk about the effects of weight on the way an airplane flies.
The vertical stabilizer helps keep the airplane from slipping through the air in a sort of sideways slouch. Think of what happens when your car door is slightly ajar while you’re driving down the highway. When you try to open it in order to pull it closed, you notice how difficult it is to open. As you already understand instinctively, the pressure of the blowing air against the surface of the door tends to keep it streamlined in a closed position. When you push it out into the air stream, the air pressure resists.
The same principle is at work on the vertical stabilizer. When some force pushes the airplane into a slight sideways angle to the wind, whether that force comes from an errant gust of wind or an intentional input by the pilot, the vertical stabilizer tends to push the tail back into line. The airplane is most “comfortable,” meaning its opposing forces are most in equilibrium, when the tail is exactly in line with the center of the propeller.
The empennage seems to have more than its fair share of surprises. First we learn that the horizontal stabilizer creates lift toward the ground instead of toward the sky. Now, we’ll see that the vertical stabilizer is attached to the airplane askew, and it’s done on purpose. During flight, and especially during a climb to a higher altitude, the airplane tends to turn toward the left. This is due to a complex combination of factors mostly connected to the torque produced by the engine and the peculiar aerodynamics of a propeller. (For more detail on torque, see Chapter 8, “How Airplanes Fly, Part 2: The Aerodynamics of Flight") One thing that engineers do to try to offset an airplane’s left-turning tendency is to attach the rudder to the vertical stabilizer at an angle that produces a permanent rightturning effect If you stand behind a small airplane on the airport ramp, you can see the very small offset angle.
Rudder pedals also serve as brake pedals on an airplane. When a pilot presses on the lower portion of the pedals, she controls the rudder surface. When she moves her feet up toward the top of the pedals, she is pressing the brakes. Brake pressure is used to slow the airplane after landing, but in most small – and medium-size airplanes, braking with one foot at a time also is how pilots steer on the ground.
Attached to the trailing edge of the vertical stabilizer is the rudder, a hinged surface that is controlled by the pilot using two foot pedals on the floor of the cockpit. These pedals are located roughly in the same position as the brake and accelerator pedals in a car, but are larger and sturdier.
Rudders play a large but mostly unnoticed role in making a flight comfortable. It is the sideways, fishtail motion of a plane that creates airsickness in passengers. A heavy-footed pilot who misuses the rudders can make passengers feel sick faster than a case of ptomaine poisoning. By the same token, a pilot with deft touch on the rudders can tame even the most unruly and turbulent atmosphere.