<< Page 17 Page 18: Page 19 >>   Airplane Trim As an airplane flies faster, the air makes it farther over the wing before it turns. This means the lift is generated further and further back on the wing. So say a plane was “balanced” when it took off, i.e. the center of lift was at the center of gravity, and no control inputs are needed for the plane to fly in a straight line. As the plane speeds up, the lift effectively moves back (relative to the center of gravity), so now the plane wants to go nose down. As this happens, the pilot would be required to apply a nose-up command (pulling on the stick or wheel). But it is undesirable for the pilot to have to pull on the stick during the whole flight. Trim is a way for the pilot to generate a constant nose-up command (or a constant command for the plane to roll or yaw in some way, if the plane is not “balanced” in those axes). Conventionally, this works one of two ways: Most often, a small plate (trim tab) on the elevator (or other control surface), like a mini-elevator, can be set to deflect down all the time, creating lift over itself and pushing the back of the elevator upwards, making the elevator push down and thus creating a slight nose-up force. In other words, the trim tab is deflected the opposite way from the way you want to adjust the control surface, since it will then push the control surface in the direction you want. The trim tabs are the thin moveable rectangles visible on the trailing edges of the control surfaces on this L39 and this T-6: Sometimes, the whole tail can be adjusted to deflect up (point slightly downwards) more, i.e. the whole tail turns in order to decrease its angle of incidence. This is like an all-moving tailplane, except it changes slowly and stays in place once set (and it still has elevators/elevons on it). Below, a B-52, a 737 and an A320, show markings that indicate their tails’ range of motion. I know this is complicated so let's go over it again. Say your airplane is trying to drop its nose, and you want to put in some nose-up trim. What you do is deflect the trim-tab on the elevators downwards. This will make the trim-tab create upwards lift and be pulled up by the air. The rising trim-tab will pull the back of the elevator up along with it. Now your elevator is naturally deflected up a little bit. This means your elevator will be pushing air up, and pushing the tail down, like a car spoiler. This will solve your nose-down problem since, when the tail is pushed down, the nose comes up. Above: The left wing and left horizontal tail of an airplane. Say the center of gravity is behind the center of lift (A). This will cause the airplane to want to go nose-up. The pilot can fix this by pulling on the stick, causing the elevator to deflect down so that the tail generates lift (B) and the plane balances out. But the pilot can’t hold that for hours (unless he has a fly-by-wire system and can re-calibrate the relationship between his inputs and the actuator settings). If his plane has an all-moving-tail-trim system like the ones at the end of the previous page (737, A320, B-52), then he can set the whole tail at a higher alpha (C). On most smaller aircraft, what a pilot would do is turn a knob that pushes up a small trim tab at the back of the elevator (D). This pushes down the back of the elevator (E), causing it to generate lift. This is how trim conventionally works. I said that trim works like this “conventionally” (on the second paragraph of the previous page, and in the caption above) because computer-controlled airplanes can have the elevators deflecting up continuously without the pilot pulling up continuously, because the relationships between pilot input and controlsurface position can be adjusted by the computer. So you don’t need a trim tab to push the control surface into place; you can have the computer ask the actuator to do it by means other than continuously pushing/pulling on the stick. And then there are the really fantastic cases; Extremely fast aircraft (namely the Concorde and SR-71) do not want to fly with the elevators sticking up all the time. At those speeds, “you might as well be flying with the airbrakes on”, a Lockheed test pilot once said. So what do they do? They have a “trim tank” at the back of the airplane, empty during take-off, and fuel is pumped into it to balance the airplane as the center of lift moves back. Some airliners, such as the DC-10, also have trim tanks (although the DC-10’s rearmost trim tanks, when used, can make the aircraft slightly unstable). As I’ve mentioned, trim is not only for nose-up. Damage in the airplane, engine failure on one side, offcenter weight, and other variables might make rudder trim or roll trim desirable. For example, if it so happens that a person on the left of a small airplane is heavier than the person on the right (or maybe the heavier bag was put on the left side, whatever), this might cause the plane to naturally roll left, and rightroll trim would be desired. Or if a four-engine airplane loses one of the right engines, it would tend to yaw to the right, and left-yaw trim would be a good idea. Most airplanes have trim tabs on the ailerons, elevators and rudder. Below: The SR-71, Concorde, and DC-10, which trim in pitch with the use of trim tanks and fuel pumping... and the F-16, whose computerized flight controls deflect the control surfaces for trim automatically, and hold them that way in addition to any input from the pilot.