This is what Boeing calls the "Minimum Drag Trim Technique":
Autopilot engaged (preferred method):
Stabilize the airplane on a constant heading with HDG SEL
Trim the rudder in the direction of the down wing. Apply rudder trim incrementally, allowing the bank to stabilize after each trim input. Large trim inputs do not give repeatable results.
The airplane is properly trimmed when the bank angle is zero as displayed on the ADI sky pointer. If the airplane is properly rigged, this should result in an approximately neutral wheel.
Disengage the autopilot. With the autopilot disengaged, hold the wings level with the control wheel using the ADI for reference. Trim out any control wheel forces using aileron trim.
Note: Use of aileron trim with the autopilot engagaed is prohibited.
If desired, re-engage the autopilot.
Autopilot disengaged (alternate method):
Hold the wings level with the control wheel using the ADI sky pointer for reference.
Use rudder trim to correct (stop) heading drift.
For airplanes with IRS, display heading on the IRS overhead panel. Use the tenths of a degree window to zero heading drift.
Use aileron trim to remove any control wheel force.
If the control wheel is displaced more than approximately 1.6 units (10 degrees) spoiler deflection will be initiated and a significant increase in aerodynamic drag will result.
What bugs me is the part "If the airplane is properly rigged, this should result in an approximately neutral wheel", because most of the time when I trim the airplane (737NG) according to this procedure I get an approximately 0,5 to sometimes even 1 unit right wing down indication on the control wheel.
Where does that come from?
I have a theory, but I couldn't find anything backing up my woolgathering so far. Maybe somebody here will be able to shed some light on this. Here it comes:
The CFM 56-7B has a redline N1 of 5380 rpm, so I would think that during normal cruise those fans spin at around 4600 - 4800 rpm. Fan diameter is 1,54m, so if I recall this correctly from my physics classes decades ago there should be an enormous amount of torque on the wing. Both engines rotate in the same direction (clockwise if seen from behind), so the resulting turning tendency is opposite to that, making the airplane roll to the left. That left turning tendency has to be trimmed out with the aileron trim, resulting in that tiny but nevertheless appreciable right wing down indication on the control wheel.
Every pilot with a multi engine rating remembers the four factors which make one engine more critical than the other in case of a failure. Three of those factors do not apply to jet aircraft, but torque certainly does, or doesn't it? And since jet engines usually are not counterrotating, there is definitely an asymetric torque situation even with all engines operating normally.
I know for a fact that Boeing employs thousands of engineers who are at least a trillion times smarter than me, so why wouldn't they mention that?
Is there maybe some design feature that already counteracts this asymmetry? Are the low and high pressure shafts of the engine counterrotating? That might reduce some of the resulting torque, but not all of it. Hm...
Anybody?