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That Knobby Little Wheel

Like most student pilots, I tended to fly with the type of casual touch that my primary instructor described with the term 'death grip.'Like most student pilots, I tended to fly with the type of casual touch that my primary instructor described with the term 'death grip.' Then, somewhere along the way came my first introduction to that little knurled wheel. 'Trim what?'

Think back to when you were a kid. When you built a balsa wood glider, what's the first thing you adjusted? You adjusted the balance and deflection of its extremities so it would fly the way you wanted it to, right? There was no little man in there jockeying the controls; it was strictly a hands-off affair. So it is with the real thing. Trim tabs are small adjustable or hinged surfaces on the trailing edges of the elevator, rudder, or even the ailerons that allow us to fly with a featherweight touch instead of arm-wrestling exertions. Trim is aviation's power steering. (Not only that, but unlike a car, it's almost like hands-off power steering!) It's not too surprising then that control surface trim appeared as early as it did, when the German aeronautical engineer Anton Flettner first applied it to zeppelins, around 1905!

THE BENEFITS OF BEING TRIMMED
Flying a trimmed airplane means: more relaxed flying and less fatigue. An out-of-trim airplane flies a touch more slowly, and uses more fuel. Learning how to adapt to differences between various trim mechanisms is one of the best ways to improve your ability to make type transitions further along in your flying career.

The name of the game is stability. It comes from the differential disbursement of an airplane's weight and the lift provided by its wings. The center of gravity lies in front of the center of lift, and the 'tail down' force that we learned about in ground school counteracts that (due to the lesser angle of incidence of the horizontal stabilizer, and described by the snooty term décalage). Because of the horizontal stabilizer's far greater distance from the center of gravity (its greater moment arm), it needs only to counter with a very few percent of the lift provided by the main wings.

Self-correcting stability: Any increase in airspeed yields more opposing moment, which brings the nose back up, decreasing airspeed. Likewise, a decrease in airspeed translates to less of that 'down force' and the airplane pitches down to 'try' to regain the lost speed. It is for this reason that Wolfgang Langewiesche spoke of an airplane's 'gaits.'

Translation: Even though you actually trim for an angle-of-attack, what that often translates to in a practical sense is trimming for a given airspeed. The airplane's fervent attempt to return to its trimmed angle of attack in fact is partially responsible for phugoids or 'porpoising.' (They're worst at a forward CG, while stability itself is worst at an aft CG.)

CONSTANT ADJUSTMENTS
You can trim for an airspeed (or a narrow range at least) but that only applies for a given configuration (flaps, power) and CG. I almost learned that lesson the hard way about eight years ago when I took my father and sister for a short hop in a C172 at Sussex Airport in New Jersey. They were in the back seat. I did my “flaps, trim, doors, transponder, strobes” pre-takeoff check and we were off -- but to my considerable alarm! The airplane wanted to climb at a ridiculously steep angle, and it took a moment for me to realize why. Though their combined weight wasn’t much over 300 pounds, my passengers caused a sizeable rearward shift in the aircraft’s center of gravity -- for which I had not trimmed. The lesson was a good one, but I could have done without the sudden heart stoppage...

BEHIND THE SCENES
Just like the empennage has a large moment arm behind the center of gravity, so do trim tabs, because tabs are mounted on the trailing edge of a control surface -- far from the hinge line. Even a relatively small tab has enough leverage to deflect the entire elevator (or rudder, or aileron). Aside from many early airplanes that were so slow and had such narrow ranges of speeds that they didn’t have or need trim, the simplest type of all is the fixed trim tab: you fly, you land, you tweak it, you go fly some more, etc. Then there is the adjustable trim tab, and in fact on some large airplanes it is the small “servo” tab connected to the primary control surfaces. Servo tabs generally deflect opposite to primary control surfaces and thus assist in moving them, lessening control forces -- especially at high speeds. Anti-servo tabs however add to the force required, but improve effectiveness. Other types of trim control are the adjustable stabilizer, like the window-crank type in a Cub, which raises or lowers the leading edge of the horizontal stabilizer. There are many more types, and many combinations.

TRIM CONSIDERATIONS:

  • Consider trimming for any attitude or flight configuration that you will be in for more than a short period of time. It reduces workload, both mental and physical. In calm air, you should be able to fly the airplane with two fingers.
  • Stay aware of trim position, and the consequences its current setting might render on a different flight configuration.
  • Anticipatory trim changes can be made with experience of course, but always allow the airplane time to “settle in.” Hold pressures for the attitude you want, then trim until that pressure is relieved.
  • Once you trim for a given airspeed in level flight, adding power will result in a climb at that airspeed; reducing power will produce a decent at that airspeed.
  • Never fly the airplane with the trim wheel alone. (One exception is if you have a control surface failure.)
  • The regulations (Part 23.677) require that trim controls have a “takeoff” position indicator, but that’s all. Become more aware of your trim tab and the flight characteristics you should expect for its current setting. It could help you someday, when your workload is high (like on an ILS).
  • If you have an electric trim, and it goes a bit batty while you're flying with an altitude hold autopilot, you could experience a violent pitch change when the autopilot disconnects. Include the trim indicator in your scan.
  • If you do have electric trim, part of your preflight should be to ensure that it can be deactivated while running, by hitting the power-off switch or pulling a breaker.
  • Be mindful when trimming during approaches, because it can involve a good deal of nose-up trim. If you have to go around, a nose high (landing attitude) trim setting can get you killed when you apply power and the trim setting causes the nose to rocket skyward as the wing stalls.
  • Don’t trim out back pressure during steep turns, because you won’t be as aware of how much control force you're applying and you could conceivably overstress the airframe or enter a stall if you roll out of it too quickly.
  • Check that tabs, cables, or push rods aren’t loose. (If they are, they can induce flutter.)
  • Unless you have an anti-servo type of stabilator trim, you can increase elevator effectiveness by applying trim in the opposite direction from elevator movement. This may come in handy in some extreme situations
And there you have it. Stay fit... and trim.

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