Last month I discussed theories attached to pitch and power management. As promised, this month's topic will be less theoretical (and more practical): how to trim for altitude control.
PLUS OR MINUS 100'
From the outset, IFR flight demands standards of altitude and heading precision beyond those generally called for under VFR. At the same time, attitude reference is limited to the flight instruments—a sort of double whammy that usually creates several hours of misery for beginning instrument students as they attempt to master the general skills of "attitude instrument flight."
Even after basic control skills appear to be formed, problems with altitude tend to crop up periodically in IFR training as new items are introduced to contend for students' attention. If you've ever found yourself mysteriously 100' high or low after trying to think out a tricky holding assignment or copy an airborne clearance, you recognize the problem.
When the problem can't be made to go away—that is, when altitude control continues to be a source of consistent difficulty, the culprit nearly always turns out to be incomplete or incorrect trim procedures.
BALANCE
Effective and efficient elevator trimming is close to an art form—one that seldom receives all the attention necessary for early mastery. To unlock the subject, some basic aerodynamic issues need to be fully understood.
To start with, all airplanes are designed to be nose-heavy—that is, loaded so the tail surfaces always have to produce "down force" to balance the aircraft and keep it keep from entering diving flight. This out-of-balance condition is achieved by assuring that the center of gravity (or "center of balance," as it is coming to be called), even at its most rearward legal limit is forward of the center of lift produced by the wings—a ready-made guarantee of a permanent nose-heavy condition.
It is also a guarantee of a certain amount of inefficiency: the down force required at the tail adds to the total weight supported by the wings. In other words, the wings always support more weight in flight than you can measure on the ramp. This added weight requires added lift, which requires added angle of attack, which adds more drag.
SPEED
Many pilots are completely unaware of these dynamics. Thanks to the elevator trim tab, the considerable left bicep strength required to keep your airplane from constantly nosing over into a dive can be eliminated altogether through simple manipulation of a wheel or button. Once you have trimmed, inflight elevator pressures appear to be neutral; in reality, the trim simply masks the fact that the elevator is always being held up in order to push the tail down and balance the aircraft around its lateral axis.
To do its job, the trim tab uses relative wind over the tail to force the trailing edge of the elevator up, in turn producing the downward "lift" necessary to balance nose-heaviness. If after initial trimming, the aircraft remains somewhat nose-heavy, the solution is simple: more tab deflection—when the opposite occurs, deflection can be removed.
All of this data leads to the conclusion that the major variable in determining how much trim tab you need at any given time is airspeed. At higher speeds, the tab and elevator gain authority and require less deflection. At lower speeds, as the aerodynamic authority of the tail surfaces diminishes, more and more tab and elevator deflection are required to hold the nose up.
These are essential facts necessary to understand how to approach efficient elevator trimming. Before proceeding any further, however, one more datum needs explanation: why make airplanes nose heavy to start out with? If the out-of-balance condition adds weight and drag, why not simply balance the airplane more perfectly from the very beginning? The gain in efficiency and airspeed would both be dramatic.
The sacrifice would primarily show up in lost pitch stability—a loss so significant as to make IFR flight impossible.
STABILITY
When you fly, you rely, probably more than you recognize, on inherent pitch stability--that is, on the nose of your airplane returning reliably to its original starting place after a disturbance. If some errant turbulence raises the nose, you count on your airplane to start back down again on its own. Similarly, if you push over into a dive, you know the nose will quickly start to push back at you, trying to raise up to and even beyond your original pitch attitude. This stabilizing influence, although often not quick enough to suit every circumstance, is an essential ingredient in sensible aircraft design. Without it, pitch and altitude control would demand constant unbroken vigilance—making it virtually impossible to do anything except manage the airplane. While that approach might conceivably work in VFR conditions, an aircraft that is not pitch stable in IFR flight would be a nightmare to fly.
Hmmm. Could we be talking about helicopters here? Part of the reason it took so long to achieve IFR capability in rotary wing aircraft was the need to develop autopilots capable of taming the somewhat unstable flight characteristics of the machines. Without that addition, single-pilot IFR flight was simply not practical.
Fortunately, in airplanes we've got built-in reliable pitch stability. It is all due to the nose-heavy design and the balancing trim tab: once an airplane has stabilized at a given airspeed, the tab and the elevator can be counted on to work together to resist deviations and preserve pitch attitude. Since the tail uses relative wind to create its constant down force, any increased airspeed resulting from descending flight immediately starts to push the nose back up to where it was. Similarly, in a climb, lost airspeed results in less tail authority, and nose-heaviness wins, pulling the nose back down toward the original attitude. It's a clever solution to pitch stability: deviations from the desired pitch attitude immediately cause airspeed changes which then act to correct the attitude.
The concept isn't perfect—it isn't an autopilot. For it to work, the wings need to be near level, and even then, several oscillations are always required for an aircraft to return to the original starting point entirely on its own. Nevertheless, pitch stability is a valuable ally—one that would be extremely hard to live without.
AIRSPEED AGAIN
With all this said and done, it should be clear that in contrast to appearance, you don't technically trim for a particular pitch attitude. Rather, you trim for an airspeed. For any given trim setting, there is one and only one airspeed that will produce the rush of air over the tail needed to develop an equal and opposite balancing force to the nose-heaviness of your aircraft. At that speed, pitch attitude will stabilize, and if you don't like the attitude that results, you have to change either the tab setting or the power.
As an example, if your intent is level flight and you find yourself instead in a descent, you have two choices: raise the nose, slow down and add trim to stabilize at the lower speed, or leave the trim alone, raise the nose and add power so as to achieve level flight without a speed loss. In either case, the eventual product is the same: a desirable pitch attitude arrived at through an appropriate combination of airspeed and trim deflection.
These options should make clear the two most important data for effective trimming:
1. Power management must be very precise.
2. Trimming cannot be completed until airspeed has stabilized for a given power setting.
To put these principles into practical application, it is best to examine four different trim situations.
Climb entry. When you enter a prolonged climb, performance efficiency is likely to demand a lower airspeed than cruise. Technically, it is this fact, not your raised nose, that brings about a need to change the trim setting. The entry process demands that you first lift the nose to the desired pitch attitude, add power, then start rolling in trim to attack the inevitable feel of nose-heaviness that comes with the slowing airspeed. Only when you have stabilized at your target climb speed can you stop trimming—until that point, a series of small adjustments will continue to be necessary in order to accommodate the reducing airflow over the tail.
Climb recovery. More challenging is recovery from climb, challenging because it generally takes longer to regain cruise airspeed than it does to lose it. As you push over from a climb to level flight, trim must be removed to accommodate your slow increase in speed. Throughout the acceleration, it will help to remember that trim is a process, not an event.
In instrument flight, you have four sources for flight information as you attempt to level off: the attitude indicator, the VSI, the altimeter and the "feel " of the yoke that develops as you struggle to keep the first three indications the way you want them. Any temptation to quit trimming early—that is, before the airspeed has completely stabilized at the cruise value will be unsuccessful. Additionally, when you reduce power out of the climb, you must take great care to assure a precise setting—any power more or less than you eventually want will end up requiring further change. This in turn will change your speed and necessitate another round of trim adjustments..
Descent--entry and recovery. Entries into and recoveries from descents are mercifully much easier than climb transitions, provided you are content to descend at approximately the same speed you use for cruise. In an ideal world, you should be able to descend and level off without any trim activity at all: simply reduce power and lower the nose to go down, then precisely reapply power as you raise the nose to level off. With no change in airspeed, there should be no need to trim. Any carelessness in the reapplication of power, however, will inevitably spark a need to retrim after level-off.
Level flight deviations. Level flight involves perhaps the hardest task of all—finding a trim setting that will keep altitude constant without lots of attention as you cruise the airways. The key is airspeed control: it helps to remember that airplanes are hardest to fly with precision during airspeed changes—so avoid them whenever possible. In light aircraft, if you are as little as 50' low it is generally a mistake to attempt to regain altitude by simply raising the nose—the accompanying loss of airspeed will put you even more out of trim that you were at the start, and doom you to several minutes maintaining back pressure and trimming as you reaccelerate to cruise speed after leveling off. Throughout this period, you will find it difficult to get on with all the other tasks that demand your attention, and any distraction from altitude control will almost inevitably find you sinking back down to where you started, all efforts wasted.
Instead, try adding power as you climb to keep airspeed constant, then reduce it again as you level off. The only trimming required will be the correction needed before you first lost the altitude—ideally, a fairly brief and minor adjustment.
Precisely the same approach to maintaining cruise speed works well for small descents: reduce power to avoid gaining speed during the altitude correction, then reapply upon level-off.
If level flight altitude control and trimming have been troubling you, these procedures will go a long way to reducing your workload. In aircraft heavier and faster than the average light IFR trainer, 50' deviations will not require this procedure, but 100' corrections may. The same principles work in every airplane—it is simply a matter of experimenting to find out how much altitude you can change without a significant airspeed change. For anything greater, make sure you add or reduce power as you make your corrections.
Next month: "trim, faith, and black magic."
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