by Brett_Henderson » Wed Nov 19, 2008 8:00 pm
Trimming is getting the airplane to do what you want it to do, without you having to hold control input. You can trim for straight and level flight.. you can trim for a climb.. you can trim for a descent... you can even trim to hold a turn.
I remember a bi-annual flight review, when the CFI showed me how de does steep turns. He'd roll into the bank (C172) and throw in a few whips of the trim-wheel (nose up) to keep from having to hold back-pressure through the turn. Elevator trim for a turn 8-)
ANYway, no matter what the axis (pitch, bank or yaw), the trim (if available) is to simply relieve you the pilot of having to maintain pressure on a control.
Most C172s only have elevator trim. A few have rudder trim..Most PA-28s have both elevator and rudder trim.. I've yet to fly a small, single-engine airplane that was equiped with aileron trim.
Elevator trim is pretty straight forward. Like most segments of a flight, you're trimming (pitching) for an airspeed first. If you try to trim for a specific pitch angle, or vertical speed... you'll end up trimming in circles. Takeoffs, climbs, cruises and descents all have specific power settings, then it's up to you to pitch for the target airspeed.. and THEN use the trim wheel so that you can let go of the yoke and maintain that speed.
For example... the teaching method for transitioning from a climb to level flight is, "Pitch Power Trim". PITCH to stop the climb.. As airpeed nears cruise, reduce POWER to cruise setting.. and then TRIM until you can let go of the yoke and nothing happens.
Takeoffs are similar, but in a different order (because obviously you have to add takeoff power before anything else can happen)... But the theory is the same. After rotating, you PITCH for best climb airspeed, and then TRIM so that you maintain that airspeed, even if you let go of the yoke... and so on.
Rudder and aileron trim are a goofy mix of co-dependancy. Use of one usually requires us of the other. Bank induces yaw and yaw induces bank. However, to simply compensate for an unbalanced load (a lone pilot in a small airplane.. or more fuel in one wing, etc.), just a tad of rudder trim will do the trick because the unbalanced load takes care of the rudder induced bank. You can't use only aileron trim for lateral load compensation, because that aileron deflection would cause yawing, and of course some rudder input (or trim) would be needed. This is why you'll find plenty of aircraft with just elevator and rudder trim. But few (if any) that have just elevator and aileron trim.
OK.. my long-winded explanation aside.. just think of trim (any axis) as I mentioned it earlier. Trim to get the airplane to do what you want, hands off.
