Terrence,

 

Nice in concept, it just doesn't seem to work that way.  At least not in my plane.  Certainly speed changes affect lift (or negative lift) on all flying surfaces - ergo the non-constant trim.

 

Here is the most simple example. 

Climb to altitude x, level out, set cruise power and trim for level flight. 

Hah!

In smooth air, laminar flow may take as long as 10 minutes to reach its final state and during those ten minutes the speed increases, the nose pitches up and further nose down trim is required to maintain level flight until everything becomes stable.  Another way is to maintain climb power, level off and wait until the predicted cruise speed is reached, reduce to the known cruise power and then set the trim just once.

 

A stable instrument approach in an unstable air mass is a real challenge to keep the vertical guidance needle steady.  Any change in speed or pitch induced by either the pilot or the air certainly changes the descent profile quickly.  One cannot chase it with power.

 

Maybe I am missing the point but if I am trimmed for level flight and only reduce power (MAP), the hands off plane will take a while and then enter a descent with the rate increasing rapidly along with the speed - then, the airplane will eventually begin to climb and slow until descent begins anew - I have never had the patience to for these cycles to finally result in stable level flight at a lower altitude.  It is not diverging, but it does not seem to converge at any reasonable rate.    I.E. Stable flight is easily upset and that is my point about keeping the airspeed margin above stall greater than the commonly accepted multiple.  I am sure the AOA is changing also and I will attempt to record such indicated changes the next time I'm out.

 

Scott

 

ABout trimming, I think of it by imagining the CG and aerodynamic center stuck together, and then think what the tail is doing.  The tail is way back there, with a long moment arm and can do whatever it wants.  If it develops a little camber, it will swing like a rudder on a windmill until it isn;t being pushed one way or the other any more.... it goes to Zero force..  And its leverage forces the balanced wing to go to that moment arm's angle.  The ANGLE is the thing... and when you trim the tail to force the wing to that angle, it will hold it at that angle, no matter how fast or slow the air is going by.  

And that's why we change altitude with power, and leave the trim alone if we want to avoid having to retrim,  for the final indicated level-out speed.