1 - Plan to use an elevator trim tab and small RAC servo.  What dimensions should I make the trim tab?  I know there are easier ways, but I am intrigued by the TruTrack auto pitch trim.

 

2 -  Should the elevator trailing edge have a flat surface - similar to the ailerons?  1/4 inch like the ailerons?

 

3 - Plans call for the counter balance ends to be 2 inches wide.  Is this correct and adequate?  Seems that they could be made 1/4 to 1/2 inch wider.

 

4 - My construction manual states the correct horizontal stabilizer incidence is  -1/2 to -1.0 degree.  Is this still the proper range?  If I could build with 100% accuracy, which would be best -1/2 or -1.0?

Ron,

 

1 & 3:  Of course, the addition of a trim tab will require more balance weight.  The benefit of making the horns slightly wider is that more of the forward loaded weight will have a longer balance arm thus actually reducing the total weight needed.  This is always a benefit in the small tail Lancair since any additional weight in the tail is generally undesirable. 

 

Frankly, I don't like "automatic" trim systems or trim tabs.  The loss of my autopilot during an IFR flight and the resulting departure from level flight due to out of trim conditions led me to change my procedure.  I now turn off the autopilot every 1/2 hour, re-trim and hand fly for 5 minutes to make sure my "scan" is still usable.  My situation could have been worse if I didn't feed the header tank equally (automatically) from both wing tanks.

 

BTW, some servo driven trim systems solve the runaway trim problem by using spring-biased linkages that are easily overidden.  Examples are Bonanza and Cirrus aircraft.

 

I can't suggest a tab size since I have no trim tabs.

 

2: I believe the flat aileron trailing edge is to supress "aileron snatch" and/or flutter while maintaining control effectivess - see any small jet aileron.  I don't believe there is any "elevator snatch" condition and I think there is a small cruise drag penalty for that sort of trailing edge.  IMO, I don't think there is any benefit to a flat elevator trailing edge.

 

4: Depends on your purpose and other variables.  Some racers believe that the negative angle of incidence should be reduced to lower the induced drag at race speeds and loading.  Homebuilt parts vary and loadings will also vary. 

 

If you can, go for some rides in similar Lancairs and note the displacement of the elevator horn at cruise as this may be a measure of the elevator forces needed to unload the tail (nose down trim).  That is, if the negative angle of incidence (NAOI) is too great or the CG is too far rearward, the "horn" may be exposed above the tail as the downward load is reduced on the tail.  Ask the builder if he remembers the NAOI.

 

On the other hand, if the NAOI is too small, full flap deployment (causing sigificant nose down pitch) may result in the elevator not having enough authority to overcome those forces.

 

Best angle?  ????  Accuracy?  ????

 

I can only relate my experience.  Early in the empennage construction (slow build kit), I made the wooden jig with -1/2 degree built in (for use with a bubble level).  Being a techno freak, I later obtained a digital level.  Even later, during the horizontal stabilizer install, I used the jig with the digital level erroneously set for -1/2 degree.  Yep, you guessed it - I ended up with the H-stab at -.9 degrees.  However, at cruise I see that the horn is pretty much level with the upper stab surface.  At race speeds I never look back, but I do run out of nose-down trim until most of the wing fuel is burned off.  Also, I do not have a problem with elevator authority at a full flap landing, single pilot and no fuel in the wings.  Note that my empty weight CG is .1" forward of Lancair's recommendation.