Pitch Trim Can Work For You or How Smoothly Can You Fly?

by Harold Green

An often used and frequently misused control in an airplane is the “pitch trim.” Frequently it is used just to relieve the pressure on the elevator control, or the force the pilot must exert to keep the plane level. This discussion is not intended to introduce new material, but simply to review what we all learned when we began our flight training.

When conducting flight reviews, instrument proficiency checks, or new customer checkouts, more often than not, the pitch trim is either ignored or not used to the extent it could be.

Using the natural stability of the airplane in combination with the pitch trim can permit much smoother flight control and reduced pilot workload. This is emphasized during instrument training and it is not at all unusual for new instrument pilots to be complemented by their usual passengers on how smooth they have become. This is invariably due to their use of the power/pitch control to let the airplane do what it was designed to do.

While virtually everyone is taught how to trim the airplane properly, this skill seems to deteriorate with time. It is not uncommon to see pilots – student or certificated – reach cruising altitude, throttle back and then immediately set the trim, hoping for hands off level flight. Then as the airplane speeds up they re-trim and re-trim and re-trim. Some, even for long periods of time, fly with their hands near the trim control. Since trim (really negative lift on the tail) is speed dependent, the trim cannot be finally set until the power, airspeed and pitch are stable. Then aggravating the situation, pilots often use trim to change pitch, which changes airspeed, which changes the trim speed point which causes them to change the trim and so on for long periods of time. A more effective technique after setting power to the desired level is to hold the pitch manually until the airspeed is stable, and then set the trim to eliminate the inevitable pressure on the elevator control. This will take a couple of tries until getting used to using the trim in this way. Once this technique is mastered, setting the pitch trim is no longer a long and tedious process.

We know that key elements of a stable airplane are the locations of center of lift, center of gravity and down force on the elevator. The center of lift, located behind the center of gravity, becomes the fulcrum of a teeter-totter with center of gravity at one end and downward, or negative, lift at the other.

The horizontal stabilizer produces downward lift to keep the nose at a pitch so that airflow over the horizontal stabilizer produces a downward force, which balances the aircraft pitch. This down lift is a function of airflow over the horizontal stabilizer. When we change the power setting, we change this situation, but the balance will stay essentially the same because the system must return to the stable point, which in turn means there is the same airflow over the horizontal stabilizer. This means that adding power, and hence increased airflow over the elevator, will cause the airplane to pitch up until the downward lift once again balances the force at the center of gravity. Reducing power will result in reduced airflow, and hence reduced downward lift, over the elevator until that downward lift returns to balance in a descent. If the trim is changed, the airspeed will change because the downward force and hence pitch of the airplane will change. The extent of power change, for which this holds true, is limited depending on the airplane.  However, the airplane can be trimmed at a wide range of airspeeds and over a reasonable range of power settings.

Naturally, the range of stability with power depends on the airplane design. In general, high-performance airplanes will be more sensitive and have a reduced range of stable power/airspeed. However, the basic principles apply to all because the airplane cannot be certificated if it is unstable and all stable airplanes will react as described.

This can be demonstrated by trimming the airplane for level flight at a fixed power setting and noting the airspeed. Once this is done, gradually change the power setting a few percent, and without changing the elevator force while maintaining heading with the rudder, watch what happens to the airplane.

If power was increased, the airplane will climb and the airspeed will stay the same. The rate of climb will be dependent upon how much power was added. If power was decreased, the airplane will descend and the airspeed will stay the same. The rate of descent will be dependent on how much you reduced the power.

NOTE: If the power change was too abrupt, the airspeed will fluctuate above and below the original airspeed, but will eventually return to the level flight airspeed for which it was trimmed. This works because, as described earlier, the downward lift produced by the horizontal stabilizer will produce the same force at the same airspeed. The airplane was balanced at the beginning and it will return to that balance and hence the same airspeed.

Also, note that large excursions of power may result in a change in airspeed because drag effects tend to be non-linear so these effects will cause the airspeed to vary from what you set. However, there is a large range of airspeeds about which you can trim the airplane.

In general, in most airplanes it is possible to set level hands off flight at the approach speed and maintain this speed while reducing power to produce the required approach descent rate. The airplane configuration with respect to gear, flaps, etc., must also be taken into account.

For example, when descending from cruise while properly trimmed, it is only necessary to reduce power. The airplane will then descend at the established cruise speed. Upon reaching desired altitude, it is only necessary to return power to the original setting. Also, when climbing to a new altitude, advancing the power will cause the airplane to climb at approximately the airspeed existing at the time of power advance. By adjusting the power in both cases, the rate of climb or descent can be set. This technique reduces pilot workload and encourages smooth control operation.

NOTE: As stated earlier, if the power adjustment is too great, the airspeed will not maintain its original value.

Instrument approaches become much smoother by setting power to produce the desired rate of descent. Once this is established, approaches become much simpler. Before the final approach, trim the airplane for level flight at the desired approach speed. At the final approach point, simply reduce the power to provide the necessary rate of descent and the plane slides down the glide-slope at the trimmed airspeed. If the desired descent rate is not achieved, all we need to do is adjust power to achieve it. Since our airspeed remains virtually constant, non-precision timed approaches become much easier. In some complex airplanes, it only becomes necessary to lower the gear to produce the desired rate of descent. The upshot of all this is that pilot workload is reduced dramatically.

There has always been spirited discussion whether airspeed or altitude is controlled by pitch or power. The truth is that depending on the flight regime, either one does either. If you don’t have enough power, pitching up will cause a stall. If you don’t hold pitch, power will cause a change in altitude and if you reduce power enough you will stall if you hold too high a pitch. After all, we are operating in three dimensions and energy control is the name of the game.

As a final note consider the following: Since by trimming the airplane to descend at a given airspeed will maintain a stable descent rate, should a VFR pilot inadvertently enter instrument meteorological conditions, a possible “out” – assuming the airplane is reasonably well trimmed – is to reduce power until the airplane begins a gradual descent, and then hold heading with the rudder while not touching the control wheel (or stick). Sit on your hands if necessary. The airplane will descend at a reasonable rate and by holding the heading, a graveyard spiral will be avoided. Simply by restoring power, the descent can be halted. In addition to the classic 180-degree turn, I teach this technique as a part of the three-hour instrument requirement for the Private Pilot Certificate.

EDITOR’S NOTE: Harold Green is a CFII at Morey Airplane Company at Middleton Municipal Airport – Morey Field in Middleton,  Wisconsin    (www.MoreyAirport.com).

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