A trim tab is a small, adjustable hinged surface on the trailing edge of the aileron, rudder, or elevator control surfaces. Trim tabs are labour saving devices that enable the pilot to release manual pressure on the primary controls.

Some airplanes have trim tabs on all three control surfaces that are adjustable from the cockpit; others have them only on the elevator and rudder; and some have them only on the elevator. Some trim tabs are the ground-adjustable type only.

The tab is moved in the direction opposite that of the primary control surface, to relieve pressure on the control wheel or rudder control. For example, consider the situation in which we wish to adjust the elevator trim for level flight. ("Level flight" is the attitude of the airplane that will maintain a constant altitude.) Assume that back pressure is required on the control wheel to maintain level flight and that we wish to adjust the elevator trim tab to relieve this pressure. Since we are holding back pressure, the elevator will be in the "up" position. The trim tab must then be adjusted downward so that the airflow striking the tab will hold the elevators in the desired position. Conversely, if forward pressure is being held, the elevators will be in the down position, so the tab must be moved upward to relieve this pressure. In this example, we are talking about the tab itself and not the cockpit control.

Rudder and aileron trim tabs operate on the same principle as the elevator trim tab to relieve pressure on the rudder pedals and sideward pressure on the control wheel, respectively.

The tabs are usually controlled by a wheel which is often situated on the floor between the two front seats. Some aircraft have the trim controlled by a small rocker switch on the control column. The aircraft should be trimmed after every change in attitude or power setting. It takes a little practice to  trim an aircraft, but in the end it is done unconsciously.

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 centre of gravity, airspeed and configuration (flaps/gear). The passenger load will affect the centre 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 centre of gravity and the lift acting through the centre 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 centre of lift as required to balance the ever-changing centre 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. 

Trim Use

Correct use of the trim requires that control pressures be applied to hold the desired flight attitude. Then the trim is adjusted to relieve present control pressures. Some initial change in trim should always be made since it reduces drag. If the aircraft is in an accelerating or decelerating mode anticipatory trim changes may be desired. Proper trim is a necessary part of flying from both operational and safety standpoints. The skill of the pilot is proportional to ability to trim.

Being able to trim the aircraft for any attitude requires that the pilot adjust the amount of download on the horizontal tail surfaces. It is this download that overcomes the nose weight of an aircraft. Download is 'lift' of the tail surfaces directed opposite to the lift of the wing.

The important thing in using trim is always to be able to keep track of where it is. This is the reason I urge you to use a finger tip rather than a pinch. The fuel/pilots location in the c-150/152 are so near the CG that the trim movement will be rather constant. Any variation will be corrected if everything is predicated on beginning at a constant. The constant that I use has always been: Level cruise at 2400 rpm and hands off.

This constant works just a well if using C-172 or C-182. The presence of a rear-seat passenger will be corrected for using this constant. Pipers trim differently. Flaps change pitch attitude significantly but require very little trim adjustment. As you know the indicator markings are often illegible or not calibrated. A slipping trim cable is a frequent problem.

Cruise-Control

Learning to trim for level flight requires that you think in terms of setting as many constants as possible for a given flight situation. First, get a constant level attitude. Using the nose/horizon reference is more difficult than using the wing. The wing level with the horizon works best with the high-wing types. Second, get a constant speed at cruise speed or lower. If you exceed cruise speed without reducing power your trim setting will set for the higher speed. You should practice reducing power to 75% power setting as cruise. 2450 rpm is a good set. Third, trim off the pressure.

Is their only one way to trim? No. With experience you may just give a few flips and make a fine adjustment as required. You can even make numerous small changes. Doing it differently does not make it wrong. There is no one way to do anything in flying. Different aircraft and different trim systems require different techniques. The aim of my following suggestions is that it gets the beginner into anticipating trim movements as may be required for every change of configuration. Trim then becomes another constant.

Trimming off pressure is a search for the trim position that allows the aircraft to be flown with only one finger and the thumb. Which ever one you are using to hold altitude tells you which way to move the trim. Most students tend to move the trim more than required. You might do well as a student to use half as much movement as you think is required. You are trimmed when both finger and thumb need only to lightly brush the yoke. Getting trimmed to this point makes flying enjoyable and relaxing. Unlike an automobile, a correctly trimmed airplane can be flown hands-off. Once this sense of 'feel' is acquired you will not want to fly any other way. Every pilot has a slightly different 'feel' of an aircraft so changing pilots usually involves changing trim.

Every student and pilot should use trim to create opportunities to fly with rudder. Training aircraft usually have a rudder tab that has been set by prior pilots so that very little rudder is required in straight-and-level cruise. You can make slight turns using just the rudder with little difficulty. Steeper turns with the rudder will cause a loss of altitude. Much of this altitude is regained when using hard rudder to level the wings.

Once an aircraft is trimmed for a particular airspeed in level flight, additional power or a reduction in power will cause the aircraft to climb and descend at that airspeed. You must exercise some yoke control and rudder to correct for any transitional oscillations. Trim remains the same. Trim is the cruise control of flying an aircraft. I very much recommend not changing trim when descending from cruise to pattern altitude. Descend by reducing power. Enter downwind at cruise speed until abeam the numbers. The deceleration in airspeed while holding altitude on downwind will allow you to trim for the approach speed while reaching the appropriate 'key' position for turning base.

Airplanes should be trimmed for every condition of flight except during times you may be turning or changing airspeeds. Flying an aircraft out of trim makes control difficult and wearisome. Initial trim settings should be just 'close'. Fine trim when the power and airspeed has stabilized. The check of trim setting is confirmed by letting go of the controls.

Every control system has inherent frictions that tend to keep them in position. In some cases this internal aircraft factor may make an aircraft seem out of trim. Occasionally an aircraft may be affected by atmospheric conditions. In turbulence a tight grip will only accentuate the bumps. Single finger control is best in choppy conditions.

If your aircraft has rudder trim, you adjust it only after elevator trim has been fine-tuned. Rudder is trimmed in wings-level flight with a nose-on reference point. Use rudder pressure to maintain the reference point and then trim off the pressure. Confirm rudder trim setting by letting go of all the controls. Aileron trim, if there, is set much the same way.

Once an aircraft is completely trimmed it can be neatly controlled with small brief rudder input. Pitch changes can be controlled with VERY small power changes. Flying with just the rudder is a very useful experience. Even in instrument conditions the rudder can be used. Step on the high wing of the attitude indicator and the turn coordinator. Step on the heading desired of the heading indicator. Such flying removes flying as a problem part of the IFR equation.

Tight Grip vs light touch

The left hand has only two useable digits while flying. The forefinger is behind the yoke for back pressure and the thumb is for forward pressure. You cannot feel the pressures requiring trim if a heavier touch is used. Tension is the greatest single cause of a full tight grip. Note how a beginning driver grips the wheel. The sooner the student learns that a light touch with proper trim gives more positive control, the better. There is a safety factor in this. Any distraction or movement of the body will affect yoke pressure. This is especially true if the pressure is being held tightly against the trim. The pilot with a light touch can let go of the yoke and the plane will fly as trimmed. The tight grip increases fatigue as a factor. Easy to say; difficult to do. IFR pilots do it better with a light touch. A full grip on the yoke seems to result in inadvertent climbs and turns. Tension is the greatest single cause of the tight full grip on the yoke. The best analogy is the differences between student and experienced drivers in holding the steering wheel of a car.

One Finger and a Thumb Flying

Over controlling is a symptom. A student or pilot who is heavy, reactionary, or hesitant on the controls is not yet a believer. The proficient pilot has faith in the airplane’s ability to perform in a particular manner. All proficient flying is an act of faith just as is having the runway disappear during a landing.

Turbulence is one of the best opportunities for the pilot to see this. The natural, normal reaction of a student pilot in turbulence is to grip the yoke more firmly. This is what you do going over chuckholes in an automobile. In an airplane a firm grip gives you a two-for-one bump. A light touch will reduce the extent of light to moderate turbulence significantly.

With practice you can do it with your eyes closed. You will need to SEE only to confirm. Then just for fun you can move your arms forward and back to initiate shallow climbs and descents.

Emergency Trim Use

If the elevator is locked as with a control lock, the use of trim will be backwards. Any use of the trim will be as an elevator. To raise the nose of the aircraft you must raise the trim tab. Confusing but true.

You and Your Autopilot

Is the autopilot only an emergency device?
Is it an aid for the incompetent?
Is it a copilot to assist the pilot in need?
The autopilot has built-in complexity that is a potential hazard if not understood.
The autopilot is not a substitute for instrument competency.
Poor instrument skills and reliance on the autopilot can kill.
Some autopilots must be turned off rather than overridden.
Autopilots should be turned off in turbulence
Do not takeoff or land with the autopilot.
The autopilot probably has a minimum operating speed.
Altitude hold is not 'altitude return'.
It is up to the pilot to become an expert systems manager.
A pilot must be able to disconnect the autopilot without hesitation.
The autopilot can fly the approach while the pilot watches for the airport.
Autopilots do not have the capability to make a go-around or a missed departure procedure.
The autopilot allows a rested pilot to be there at the end of a long flight.
Cockpit housekeeping and planning is easier when on autopilot.
For every coupled approach you should make a hand-flown approach for skill maintenance.

Now back to Trim:
In the 150 we initially put in three turns of trim down. In the process of landing we took off three turns so the aircraft is trimmed for level. Therefore, before we takeoff again we want to put in one full turn down for takeoff and climb. This turn will give you very close to Vy climb. Hands-off check of trim but using right rudder.

On reaching altitude, lower the nose; hold altitude with pressure while taking off a full turn of trim for level cruise. Leave the power in until reaching over 80-knots, holding altitude to allow acceleration and then reduce the power to 2450 rpm. We are now back to our initial approach point. Hands-off.

Pinching Won't Work

Those who habitually pinch the trim wheel will always be applying too little trim to make this work.

You must learn to read the nose and anticipate its rise and fall as to how much pressure is required to stop at a
given airspeed.

Then, based upon the pressure you need to hold a given airspeed, make an adjustment of one or two buttons
using the fingertip.

Once you get close adjust accordingly.

Trim Factors

Stability of an aircraft may be judged by how well it holds a trimmed situation.
Static stability exists when an aircraft is trimmed for a condition and then moved will return as trimmed.
A statically stable aircraft is easier to fly.
A statically unstable aircraft is harder to fly.
A statically stable aircraft exists when the CG is forward of the wing lift.
the closer the centre of wing wing lift gets to the centre of gravity the more unstable the aircraft
FARs require aircraft to be statically stable with respect to speed changes
Lift is commonly close to 25% aft of wings chord from the leading edge.
Trim is used to adjust the tail to counter wings lift forward of the centre of gravity by giving down-load.
The forward C.G requires a tail download as any reduction in speed will lower the nose as a recovery
About one pound of pressure on the yoke is required for every six knots to prevent a change in airspeed
Friction of control and connections is a constant difference related to aircraft speed.
You must overcome this designed-in friction before the controls move
FRSR is a term meaning free return speed range
The FRSR is the range of speeds that can be stabilized with a single trim setting.
Aft CG conditions lower the FRSR speed ranges.