Page a5
Trim and Holding the Yoke
Return to Whittsflying Home Page

Contents:

…Trim history; ...Trim; ...Trim Use; ...Cruise-Control; ...Tight Grip vs light touch; ...Trim instruction; ...Trimming Cessnas; ...Trim Exercises; …Emergency Trim Use; …

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 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 power steering 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 empenage 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 empenage. 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 climbout, 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.
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 Opinion
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 Opinion
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

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 downwload that overcomes the nose weight of an aircraft. Download is 'lift' of the tail surfaces directed opposite to the lift of the wing. Refer to the Flight Training Handbook, AC 61-21 Page 277.

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. Practice flying with just the rudder when copying the ATIS, using the sectional, or just for fun.

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.

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.

Trim instruction (Instructor)

The direction to move the trim and the proper instructional words to use are a common source of confusion. Raising the trim wheel lowers the nose. It is better to have the student think of touching the trim wheel as touching the tail of the aircraft. Moving the trim wheel up will cause air flow over (under) the elevator to move the tail up. Thus, raising the trim wheel raises the tail while lowering the nose. Lowering the trim wheel lowers the tail and raises the nose. By agreement, I make a practice of telling the student the direction to move the trim wheel. I also advise against pinching or flipping the wheel. I suggest use of the index finger on the knobs of the wheel. A full turn and count system seems to have been deliberately engineered by Cessna. All full movements of the trim should be with the forefinger getting the very top or bottom button of the trim wheel and moving it as far as possible. Always start at the very top button or the very bottom button with the finger tip. Grasping the trim wheel between the fingers prevents the full movement needed thus making necessary more trim adjustment. Pinching the wheel always leaves the turn one button short.

Trimming Cessnas (Instructor)

Cessna has engineered its trim so that certain changes in trim can be anticipated to correspond to flight path changes in different models. For example, in the C-150 from level cruise abeam the numbers, a power reduction to 1500 can be trimmed off by holding heading and altitude to 60 knots. It will take three full top-button to the full travel bottom to do this. Pinching between the buttons will leave you short. 10 degrees of flaps while holding sixty with the yoke can be re-trimmed to a 60 knot descent by undoing one of the previous three turns. Bottom button all the way to the top. Let go and if the nose begins to change pitch make the slight trim adjustment required. 10 more degrees of flap while holding 60 knots can be locked there by taking off another full turn of trim. Full flaps while holding sixty can be set by taking off the last turn. You, every student and pilot, should learn a count or feel system for applying and removing flaps. Learn to use the flaps without looking at the indicator during application. A four count works well for 10 degrees on a Cessna. Flaps that have indent settings will work only if calibrated for trim movement.

Cessna ruined a terrific engineering design when they build the C-152. The trim/flap ratios of the C-152 are there but not at 1 to 1. You can work out the procedure for stabilized airspeeds by using the suggested procedure of the C-150 and keeping track of the amount of trim required for each ten degrees of flap. It can be done but the neat engineering isn't there. Abeam the numbers the C-152's power should be set at 1600. By the 'key' position it will be at 1500.

Authorized Cessna dealers are required to teach flying the C-152 in such a manner as to not require trim except to the slightest degree in the pattern.. A C-152 with the trim set for climb on takeoff is supposed to fly slightly faster than Vy. On reaching 50 feet below pattern altitude the power is reduced to 2000 rpm which causes the nose to fall and the plane to fly level at 80 knots. On reaching the numbers the power is reduced to 1600 and 10-degrees of flaps are put in. The plane will descent at 70-knots. The turn to base is made as another 10-degrees are put in. The plane now will fly at 65-knots with no trim change required. The turn to final is made while adding flaps and the aircraft will fly at 60 knots. The plane is flown into the flare at 60-knots and allowed to decelerate for the landing with a pitch attitude that will not cover the far end of the runway as the throttle is gradually reduced for touchdown. Seems that the C-152 was the first aircraft designed around a specific method of teaching landings. Anton Flettner must be turning lengthwise in his grave.

I have given this procedure considerable thought and feel that it is designed for this specific type of aircraft. I question if any other aircraft type or manufacture could be flown in this manner. Cessna must be growing its own brand of instructor to fly this aircraft in this manner. I can only suspect that an ulterior commercial motive exists, and the hell with the consequences.

Once you trim and let go, the trim may not be capable of holding the aircraft where you place it. Many trim controls have a trim-speed band. This band is a range of speeds that the plane will fly at the same trim setting. This band is due to friction in the trim cable system. This friction does not become noticeable until some yoke maneuver is made that the yoke does not return to where it was. The new trimmed speed depends on where the friction resistance stops when pressure is released. The allowable FAR 23.173 trim-speed band is 10% either way from selected speed. I could not comfortably fly a plane with such a range.

Trim Exercises (Instructor)

To teach the effectiveness of trim try the following exercise. The instructor should demonstrate how to hold the nose in one fixed position while the student varies the trim wheel for several turns in each direction. From level cruise, have the student lock his left elbow on the door. First the instructor varies the trim up and down while the student holds the nose in a fixed position against all pressure. Then let the student move the trim. This clearly shows the control pressures as affected by trim. The function of trim on control pressure should be shown on initial climb-out on the very first lesson. Skill in moving the trim and keeping track of these movements is an essential skill for what is to follow.

An additional exercise, once some skill in leveling off is acquired, is to have the student trim the plane for level flight. Then watch the nose attitude change as the arms are held forward or back over the head. Show the student that, in a correctly trimmed aircraft, even head movement will have an effect. This is a good reason not to keep sectionals where you must look down to read them. Move the sectionals up to panel level. Further, if the plane is placed into a 30 degree bank with about 2/3 if a trim down (direction of wheel movement) the aircraft maintain both bank and altitude. These are good confidence maneuvers and illustrations of the aircraft stability and should be used to encourage the student to maintain a light, trim sensitive, touch on the controls. The student should be trained to keep track of trim turns, the indicator position, and verbalize trim movements.

The next element in maintaining speed is knowing where the trim is, in the first place, and knowing how much movement is required for each power setting. From a hands-off trimmed climb at 60 knots, the C-150 will be level with a full fingertip trim turn of the trim wheel bottom to top. The C-172 takes one and a third turns. If you pinch the wheel you will always come up short. Some fine adjustment may be needed for hands-off level flight but you will be very close. To initiate the speed reduction required at the numbers for landing, the trim is moved with full finger-tip turns top to bottom three times while holding heading and altitude. The C-150 is now hands off at 60 knots and the C-172 is hands off at 70 knots.

Because of patent laws the nose gear geometry of different manufacturers was forced to develop different mechanisms for flaps, gear, and trim. Pipers trim differently, the addition of flaps changes attitude and airspeed without requiring much trim change. Cessnas require trim change for changes in airspeed and flaps. The same factors exist in both cases. It is the interrelationships between the factors that require pilots to adapt their skill to the design requirements. The finger tip technique that works so well on the Cessna must be changed to the pinch and flip, palm roll, and electric assist of the Piper. In my mind, the greatest skill difference in making manufacturer type transition lies in the use of trim.

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.

Return to whittsflying Home Page
Continued on Airport Ground Procedures