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Trim history
The trim tab or servo trim was invented by Anton Flettner, a German aeronautical engineer. He started work in 1905 for the Zeppelin Company. Died in 1962.

Trim
Most aircraft have single axis trim for the elevator. Airliners have three-axis trim for the elevator, rudder and ailerons. Trim is used to correct for any forces that might tend to counter your selected flight performance. Trim allows the pilot to relax. A pilot who cannot trim will be an exhausted pilot in a short time. It takes only a couple of flights for a pilot to realize the benefits of trim.  The best check for proper trim setting for any flight configuration is to let go of the yoke completely and see what the nose does.

The simplest elevator trim uses a wheel, lever, or crank to pull a cable or rod attached to a trim surface bell-crank. Other systems use a jackscrew and rod to set trim. Electric trim is best used for coarse settings. Only the coordination of eye and hand can correctly set fine trim settings. Using the trim control positions the trim and the aircraft for the desired attitude.

If an aircraft is improperly rigged trim is not the fix required. An aircraft that consistently flies one wing low need help only a mechanic can give. The aircraft wings have adjustments that can correct problems detected in using trim.

The controllable trim tabs are required on all aircraft. It is usually on only one side of the elevators since they are both on the same rod (Cessna). It is hinged and can be moved only by use of a cable system connected to the trim control in the cockpit. The direction the tab moves causes an opposite deflection of the control surface. The ground adjustable trim tab is a small surface on the trailing edge of a control surface, most often the rudder, that can be bent to set control forces at cruise speeds. The trim setting creates the aerodynamic forces required to keep the elevator and the airspeed in the desired position.

The three factors affecting trim are the center of gravity, airspeed and configuration (flaps/gear). The passenger load will affect the center of gravity and require unique takeoff and level flight trim settings. Each trim setting has a corresponding speed that the aircraft will seek and hold.

If you are holding any pressure on the yoke against the trim setting a moment of distraction will result in an airspeed change. A stabilized approach to landing is difficult, to impossible, if the aircraft is not well trimmed. The less skilled the pilot the more likely he is to neglect proper trim technique and attempt to maintain control by arm and hand pressures. Good technique requires that the pressures felt on the yoke be from pilot applied input. Any pressures applied otherwise are indicative of improper trim. Trim is the cruise control of flying. Not using trim is equivalent to being able to turn on/off power steering.

Trim makes it possible for the pilot to configure the aircraft to counteract and neutralize the normal nose heavy condition. There is a designed twisting along the longitudinal axis caused by a difference between the weight on the center of gravity and the lift acting through the center of pressure. If the pilot does not trim then control pressure must be held maintain the negative lift value of the horizontal stabilizer and elevator. Trim allows this control pressure to be adjusted for hands-off flight. In a trimmed condition the pilot can feel the control pressures required to a acquire a desired flight attitude. An improperly trimmed aircraft is constantly seeking to relieve any pilot induced control pressure.

The original design of the aircraft sets the shape, position, and size of flying surfaces and controls so that in cruise conditions these would provide least resistance and maximum speed. Outside of this condition a trim control was installed to maintain the aircraft stability required for climb, descent, landings and other flight speeds and configurations. On some aircraft the angle of incidence of the horizontal stabilizer can be changed by a trim control. This is more effective and efficient than a trim tab (Mooney). The stabilator is another way (Piper). It is an airfoil that in one piece acts as both stabilizer and elevator. The trim control of the stabilator acts as both a trim and anti-servo tab (power assist). The yoke applies control forces to the tab to move the entire control. No change in trim technique is required in either case.

Ideally an aircraft would have a three-axis trim; elevator, rudder, and aileron. Without such trim some aircraft just fly crooked. Fixed tabs on the rudder and adjustment screws on the wings can make semi-permanent or even permanent fixes to the aircraft trimmed condition in level cruise. A pilot can, with low-wing aircraft utilize fuel weight/consumption to adjust the aircraft 'trim'. Passenger seating can also make a difference.

The aircraft trim system is used to adjust the aerodynamic center of lift as required to balance the ever-changing center of gravity primarily along the longitudinal axis of the airplane. This relieves the pilot from having to maintain control pressures on the yoke. The pitch can be varied with the trim wheel to adjust for weight, configuration, speed and power. A pilot should be aware that any change in these factors will require a trim change.

The trim system usually consists of a cable from a moveable small surface on the empennage forward to the cockpit. The FARs require that a trim position indicator exist in the cockpit with a takeoff position especially marked and visible to the pilot. Mooney aircraft move the entire empennage. Some of the surfaces called trim tabs are fixed and can only be adjusted on the ground by bending. The trim system is not intended as a primary flight control. Remember, Trim effects will be reversed if the primary control is jammed. 

Opinion on Trim
Trimming the airplane is something that comes with practice. Like many others have said here, the best way to learn is to get the airplane relatively stabilized where you want it (climb, cruise, or decent) and trim the forces off. The best way to get it right on is when you think you've got it close, simply let go of the yoke. If the nose comes up or goes down, feed in a little more trim in the right direction. You should eventually be able to trim the airplane to be rock steady in any flight condition (smooth air assumed:)). In fact, you know the trim wheel on the 172 has little bumps on it just for grip? You should get to the point where moving the trim wheel one bump in either direction makes a noticeable difference to the trim.

I tend to be very active with the trim. In fact, I usually retrim for nearly everything I'm doing if I'm going to be doing it for more than a couple of minutes. For example, on climb out, I trim for the climb airspeed and let the airplane fly the climb. In cruise, I do the same. Also on approach, I retrim for each configuration to give me the decent rate I'm looking for. In this way, I'm only ever maneuvering the airplane away from the trimmed condition. In other words, on approach, I don't have to "fly" the airplane for the approach because the trim is taking that workload. That allows me to concentrate on making the correct turns, setting up the approach, watching out for traffic, configuring the airplane, watching my decent rate, all without having to manhandle the yoke. If I need more or less decent rate, I just move the trim a hair.

Opinion
The trim on the Piper Cub is a great little arrangement that uses a winding handle like a car window. It takes quite a few turns to trim from a climb to level flight, etc. but it also allows you to make really fine adjustments. In fact, when in the glide for approach, I may make adjustments to the trim that are all of about 1/8th of a turn or even less.
Shawn

Trim Opinion2
My instructor recommended the following and it works for me;-)
--First NEVER fly the plane with trim wheel, always establish your attitude (level, climb or descend) with Yoke and power
first.
--When you're stable trim off heavy yoke pressure first and resettle
--Then fine trim until you just have your hand on the yoke but you aren't inputting any effort at all.
--Let go for a second or two and check and check that it doesn't climb or descend is the final proof.
--Retrim for every change in attitude. I got to practice this in the circuit under the lashing tongue of my instructor... Trim
for climb out before and after raising the flaps, trim on crosswind after leveling, trim after every flap setting and throttle setting on downwind, base and final. You really get the hang of it then I can tell you!

Gene's addendum:
Should you ever be in a situation where the elevator is jammed and will not move, you should be aware that this causes the movement of the trim to give reverse effects in so far as directing the nose up or down.

Trim Opinion3
Trimming takes some practice but once mastered will make every aspect of your flying easier. First thing is you can't trim the plane until it is where you want it to be. If it's climbing you can't just throw in nose down trim, that would be trying to fly the plane with trim and even though that's not that uncommon, it won't work. You have to hold the plane in whatever attitude you are trying to hold, let the plane settle up, determine whether you are pushing/pulling on the yoke and then trim those forces off. Of course if the forces are excessive trim most of it away, wait for the plane to settle and then finish trimming. The test is being able to let go of the plane at any time and the plane is still doing what it was doing before you let it go. Students never seem to see the importance of trimmed, hands-off flight until they have to either triangulate their position during a lost procedure or divert to an airport other than the one flight planned. Trying to fold and unfold those damn sectionals without being able to take your hands off the yoke can be a handful. On my private checkride the DE expected me to let go of the yoke, check for traffic, draw and compute to my diversion, check for traffic, maintain a shallow turn as I held altitude and check for traffic.
Stephen Ames

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