What Is Aircraft Trim - If you reduce power for cruise, you'll need to trim the elevator up slightly to maintain level flight. The higher you fly, the lower your indicated airspeed will be (although your true airspeed may be higher), so the pitch trim setting in cruise flight will usually be more nose-up at higher altitudes and/or lower engine power.
As fuel burns off, the center of gravity may change, requiring you to re-trim the airplane slightly. As you should know, if the airplane is trimmed for an indicated airspeed (IAS) slower than your current IAS, it will tend to nose up.
What Is Aircraft Trim
If the airplane is trimmed for an IAS faster than your current IAS, it will tend to nose down. By definition, to "trim" an aircraft is to adjust the aerodynamic forces on the control surfaces so that the aircraft maintains the set attitude without any control input.
Go Around
While all axes of rotation are affected by aerodynamic forces, not all aircraft types are capable of being trimmed in all three axes. Virtually all aircraft designs incorporate some form of pitch axis trim and most have provisions of some description for trimming in the yaw axis.
Roll axis trim exists on many aircraft but it is the least frequently encountered installation of the three. There are several different types of trim systems in use and more than one type may be found on a given aircraft.
Note this is independent of density altitude. Airplane performance, including trim function, is a function of indicated airspeed. Indicated speeds for desired performance do not change with density altitude; it just takes a longer or faster run to get to a given indicated airspeed when the air is less dense.
When flying a conventional single, adding power causes more airflow over the trim tabs, making the trim "think" the airplane is flying faster. Hence most airplanes will pitch up with power applications, and conversely will pitch down with power reductions.
Not Primary
Twin-engine airplanes can put a great deal more wind over the tail, so a pitch change following a power adjustment may be even more dramatic. The T-tail craze of the late 1970s, which continues among some designers and their airplanes, was a successful attempt to reduce the natural response to power changes by getting the elevator and its trim tabs up and out of the propeller blast.
The NTSB tells us that loss of control accidents—mainly stalls—most frequently occur immediately after takeoff, during the first moments of a missed approach, or as the pilot is attempting a go-around (see the diagram above). These are all situations when the airplane's trim is most likely to be set to a position for the previous phase of flight but which is not correct for the new one.
It's vital to know that, even if you perform a go-around correctly, you'll probably need to push on the controls—hard—to avoid excessive pitch and angle of attack until you can re-trim. To level off, add power to return to the same setting it was at in cruise at the higher altitude.
Following the principle of trim, the nose will come up to a level attitude and the airplane will fly level at the trimmed IAS. At most you may need to make very minor adjustments to the trim from this on-speed target.
Final Approach
If the aircraft is trimmed, the flight controls are in a state where no force needs to be exerted in order to continue straight and level flight. Aerodynamic and gravitational moments about all three axes are nulled out.
You're coming down final approach. Flaps are full, retractable gear is down, and power is at its normal setting. Airspeed is spot on and you are trimmed, flying at a fairly high angle of attack. Suddenly a deer runs out onto the runway and you need to go around.
Assume one of these two scenarios: For example, a Beech Bonanza's takeoff trim-setting range may be from three to six degrees up, but if only the front seats are occupied and the pilot trims off pressure for final approach and during the flare, it will be at 19 to 21 degrees
up at the point the wheels touch the pavement. Why is this important? Because it leads to one of the deadliest scenarios in aviation: the go-around. Using trim makes it very easy to plan an en route descent: If you reduce power you'll get a corresponding rate of descent, because the airplane will seek its trimmed airspeed.
Takeoff Trim
In many piston-powered light airplanes, changing manifold pressure by one inch will cause a roughly 100 fpm change in vertical speed. – It initiates the trim change from cruise climb speed to cruise speed, allowing you to make smaller trim changes a bit at a time instead of having to command a big trim change all at once.
The diagram above illustrates how pitch trim settings can change during climb through the transition to level cruise flight. Cruise climb occurs at a target indicated airspeed, with a resulting trim setting. In most airplanes this will be a few degrees nose up on the trim indicator, if the indicator is calibrated in degrees or units of deflection from neutral, or at an identifiable number of "dots" or distance nose up from the takeoff setting in other airplanes
. For example, when I take off in a 300-hp Beech A36 Bonanza with flaps up, if I smartly rotate at the book liftoff speed, the takeoff trim setting causes the airplane to be accelerating through VX almost immediately, 50 feet above the runway.
If I have a taller obstacle further out and need to maintain a VX climb longer, I'll need to increase back pressure slightly to maintain VX in the initial climb. With the trim properly set for takeoff, however, I know the airplane will do pretty much what I want it to do initially after liftoff.
Effectively its a way to fine tune/change the control surface to elevate control forces. Since airplanes can be loaded differently and the Center Of Gravity moves as the aircraft burns fuel the control surfaces must be slightly adjusted for each flight/flight segment to achieve straight and level flight for a given configuration/load.
An airplane's pitch trim system is considered a secondary flight control. Other secondary controls include flaps, spoilers and wing leading edge devices. Of these, the pitch trim system is of special importance because its proper (or improper) setting has a direct impact on the operation of the primary flight controls: the elevator, aileron and rudder.
For our purposes, we'll focus on elevator trim (or pitch control, depending on the airplane's configuration). Your trim setting in climb is highly dependent upon the density altitude, the power setting and the rate of climb you use.
Whatever speed for which you trim in climb, however, except for in the lowest-powered training airplanes there will be a large difference between the cruise-climb indicated airspeed and the indicated speed in cruise flight. This brings me to something I find to be little understood among even experienced pilots, but that has a significant impact on precision and the resulting performance when leveling off from climb into cruise: since you are trimming for an indicated airspeed, there will be a gradual
but dynamic change in trim required to keep the airplane precisely on your desired altitude. An airplane will seek the indicated airspeed for which it is trimmed. We might call this “the principle of trim,” and it is the basis for pitch stability and airplane control.
If you understand this basis, you can predict an airplane's response to just about any change in power, control input, autopilot command and even wind shear. In my mind from growing up watching hundreds of hours of Seconds from Disaster / Air Crash Investigation, trimming an aircraft is turning a dial that changes the airplanes "forward" direction.
So maybe it changes the control surfaces so that the plane is flying straight forward even though the rudder for example is at some deflection angle that isn't 0. We all should know by now to adjust attitude with the pitch control, then trim off the pressure so the elevator stays where you want.
But according to the NTSB, the most common fatal crash scenario directly results from inadequate pilot knowledge of and response (or lack of response) to the principle of trim. What is this principle of trim? How can he use this understanding to anticipate airplane control requirements, and to avoid the most frequent cause of fatal accidents?
When the deer runs onto the runway, you apply go-around power. This causes the airplane to pitch up. If you don't push, hard, against the nose-up trim, the angle of attack may become critical, and the airplane may stall.
The trim is generally adjusted any time the aircraft transitions flight phase (climb to cruise, cruise to decent etc) or when the power setting is changed or periodically in cruise if the CG is greatly affected.
For example if you climb out at full power, reach your cruising altitude of 3000 ft. push the nose over and pull the power back to your cruise power setting you may find you are still applying forward control pressure to keep the airplane from climbing.
In this case you would trim off the control pressure by rolling the wheel forward until you no longer needed to apply control pressure to keep the plane level. At this point you could fly "hands off".
On final approach with the runway in sight, you'll usually extend full flaps and slow the airplane to final approach speed. With flap extension and power reduction, in most aircraft, you'll need to re-trim the airplane nose up in order to maintain hands-off flying.
In light training airplanes this often puts the trim approximately in the takeoff position. But in larger or heavier airplanes, especially those with long cabins and therefore wide ranges of possible CG loadings, the trim setting on final approach may be well outside the safe takeoff range.
Like any other control surface, a trim tab does its job based on the airflow across it. Increase the air flow and it will have greater authority—it will exert greater force on the primary control surface.
Decrease the airflow and the tab has less authority, moving the primary control surface less. Changing the power setting, or adding or reducing drag (changing flaps and retractable landing gear position), changes the airflow across the trim tabs, driving a performance change.
The one time most Pilot's Operating Handbooks (POHs) or other manuals specify a specific trim setting is at takeoff. The relevant checklist will direct your attention to some sort of mark or pointer indicating the pitch trim is set appropriately before takeoff.
What does the takeoff trim setting give you? Elevator trim frees the pilot from exerting constant pressure on the pitch controls. Instead, the pilot adjusts a longitudinal trim control (often in the form of a wheel) to cancel out control forces for a
given airspeed/weight distribution. Typically, when this trim control (wheel or lever) is rotated or moved forward, the nose is held down; conversely, if the trim control is moved back, the tail becomes "heavy". Many newer aircraft, especially jet aircraft, have electric
trim controls. If the aircraft was in straight and level flight and the trim changes because of a change in any of the above, it can be compensated for by the other factors. Elevator deflection is fastest but requires a pilot force which is uncomfortable for longer periods, that is why the forces can be trimmed out to make the new stick position the neutral force one.
We're taught about trimming early in our flying career, and many of us get pretty good at using trim to fly accurately and smoothly. The deeper principles of trim operation to reduce workload, predict airplane performance and avoid the most common cause of fatal aviation crashes often isn't taught at all.
In your surprise at seeing the deer on the runway you instinctively pull back on the controls—what I call “the panic pull.” Airspeed declines and angle of attack increases. You then think about what you have done, and you add full throttle an instant later.
The increase in power pitches the airplane's nose even higher, increasing the angle of attack to the critical point. If you don't push the nose down now, hard, you may stall, with not enough altitude to recover.
Upon reaching your cruise altitude, lower the nose to level flight and allow the airplane to accelerate. As indicated airspeed increases, the airplane—following the principle of trim—will pitch up to try to slow down to the airspeed for which it is trimmed.
You'll have to apply nose-down pressure as speed builds, trimming off the pressure as you go. It won't all happen at once...the airplane accelerates, you hold forward control pressure and trim a little nose down to relieve that pressure;
the airplane accelerates some more, you hold more forward pressure and trim the nose down more; the airplane accelerates more yet and you hold more forward pressure and trim even more nose down...all the way until the airplane stabilizes on cruise indicated airspeed.
If you're flying an airplane with that characteristic, and want to descend at 500 fpm while maintaining your current indicated airspeed, then reduce manifold pressure by five inches. The airplane will nose down to remain on speed, and by doing so it will make your desired descent.
Adjust the throttle as needed to keep the manifold pressure constant during your descent and the vertical speed and IAS will also remain constant.
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