Gary,

 

My 320 uses spring bias systems for the aileron, rudder and elevator.  The purpose was to save the addition of balancing weight for trim tabs.The aileron and rudder use electric MAC servos to bias the springs while the elevator uses the Reichel wheel.  Among other airplanes that use such systems is the Cirrus that uses spring "cartridges" for aileron trim.

 

For the 300 series Lancair the elevator spring system is interesting.  Say at high cruise or race speeds one is trimmed nose down where the down spring is under compression and the up spring is more relaxed.  Hand pushing the nose further down requires additional force (increased force on control surfaces because of increased speed) without much resistance from the up spring since it is somewhat unsprung.  On the other hand, overcoming the down trim by pulling back on the stick means fighting the already compressed down trim spring (the Reichel gears will unwind if enough force is applied).  At either a high speed or slow flight regime where extensive trimming occurs there seems to be good stability under the spring system.  Well actually if the controls are light and mushy at very slow speeds then they are light and mushy no matter what system is used. 

 

At design cruise speeds and normal weights the trim mechanism is fairly balanced (elevator in trail) and the stick forces needed to move the aircraft pitch away from level is at its lightest (most responsive) since both up and down springs are at their most relaxed extension.

 

It is useful in the 300 series to begin to use flaps (take them out of reflex) once below 160 KIAS (helps keep the nose down, uh, a change in trim) and because of the flap position induced pitch change, indeed one must change the trim by dialing in nose up.  This would be true whether springs or tabs are used.  In the 300 series use of flaps and elevator trim must be integrated by the pilot to keep the airplane in a trimmed condition and on top of the beach ball.

 

Side note: In the Skymaster I flew, one needed to emphatically change the elevator trim (tabs) as flaps were deployed.  The trim was so dramatic that if the flaps were commanded to retract, a linkage system unwound the elevator trim automatically so the pilot didn't have to fight what would become a severely out of trim condition.  In fact when I take off with flaps 10 in the Lancair and elevator trim neutral, as I raise the flaps after gear up, I must indeed add nose down trim to keep the plane from pitching up (of course, that is for my normal 135 KIAS climb out).

 

Now, if one slows a 300 Lancair from cruise at 175 KIAS to 120 KIAS without a flap change, believe me that the nose will come up to give the wing enough AOA to maintain sufficient lift.  One cannot trim the nose pitch angle down because the AOA increase is needed unless the wing is changed by removing the reflex - that brings the nose down and usually requires nose up trim because it is such a strong pitch force. 

 

So, I would like to hear from 300 series pilots using elevator trim tabs to see if the auto trimming relative to speed changes actually happens.

 

Interesting in your comment about rudder trim in that the 300 series AOA increase at slow down (power reduction) or change to climb pitch requires a rudder trim change to keep the ball centered.  No problem as the feet have to occasionally do something until the rudder trim  push button finger catches up.  It is things like this that keep us wee Lancair pilots alert. 

 

Scott Krueger   

 

I'm not sure of the origination of the original post copied below, but I thought I would make a comment.  The spring bias method of trim certainly has the benefits described, but it cannot be assumed to be "equivalent" to a standard trim tab.  An elevator with a trim tab will "fly" hands-off at the same angle regardless of speed while the spring bias will allow it to go to a more neutral position as the speed increases.  If the elevator is biased up it will go up more as speed goes down and if it is trimmed down it will go down more as speed is reduced.  Is this good or bad?  I don't know.  I worry about the low-speed regime where the elevator is biased in the up direction.  As speed is reduced the spring will increase the up elevator position, reducing the amount of nose-up force required at the stick (compared to a trim tab in a fixed position).  The spring bias also increases the stick forces required to move from the trimmed position.  Like I say, I don't know of a spring bias system is better or worse, but it is certainly different in terms of pitch stability.  That being typed I have flown many hours in Cessnas with spring-bias rudder trim and it didn't seem to be a bother.

Gary Casey

ES #157

Previous post:

 The earlier LNC2 was designed with a trim system that used a spring bias applied to the elevator pushrod to control the overall elevator position.  It was controlled by a small cruciform trim lever that moved the biasing springs forward and aft.  Dick Reichel invented his trim wheel to replace the lever, allow finer and more precise trim adjustments, and to provide folks with the same sort of trim wheel that they had flown with in any number of certified aircraft.  

Trimming the location of the entire control surface is probably aerodynamically preferable (less drag) to moving a trim tab to use the airflow to move the control surface, and the removal of the trim tab eliminates its weight and complexity, as well as those of the servo motor and linkages... ie, less counterweight required for balance.  There are probably other aero issues, but I leave those things to folks more knowledgable than I in these matters.