Joe,

     People try to sell spoilers for every aeronautical problem and for some it is a good addition, but with the yaw instability in the LIV you can see the problem just looking at similar airplanes.  You notice that most airplanes with the amount of horsepower that the LIV has have bigger vertical stabilizers and/ or a longer moment arm (more distance between the vertical stabilizer and CG.  My area of interest in College was low speed aerodynamics (below the sound barrier) one of the rules of thumb in aeronautical engineering is power is destabilizing. So you take a small stab and add a lot of power the outcome is very predictable.
    Additionally, this makes for problems with big power change as well an airplane that does not want to maintain yaw stability. The big power changes can be the most serious problems as a high speed abrupt loss of power the airplane well develop a very significant yaw, I would not let it go, but LIVPT acts like it would go sideways in the air at Vne.  Plus this instability adds significantly to stall control problems. 
       Now all of that said I have done a great deal of directional control flight testing for Piper and others and maybe incorrectly we tried VG's and it did not add significantly to the yaw stability. They did seem to improve rudder control with full application below 1.15 of stall, which is not a problem in the LIV. The LIV does not have a control problem but a lot of people do not realize that control and stability are two different things and are only slightly related. Anything that improves the handling qualities of an airplane I am in favor but in the case of VG's, on some airplanes they can be helpful at and close to the stall, but other than that they are really miniature speed brakes.  One of the reasons that the LIV performs so well is that it is really clean (low drag), look at speed brakes extended and you can see the resemblance to VG's.  Every airplane that I have tested with and without a significant number of VG's goes slower with VG's and this makes sense.
       There is no known case of VG's effectively adding wing area.  They are just a way to reenergize the boundary layer at high angles of attack and in some high speed applications they can help stabilize the standing mach wave as you approach the speed of sound.  If VG's would improve or effectively increase control surface size you would see most airplanes with VG's on the Vertical and Horizontal Stabs. Stability generally is not improved with VG's but sometimes control authority is improved
       Finally I have flown several airplanes with an added ventral fin and yaw stability problems are really improved.  I have not flown their VG installation so I cannot speak directly to that potential solution.

Jack Webb
L360, LIV

AeroSpace Engineer BSAE
FAA Designated Check airman for C421, C404 & C310
ATP, CFI Airplanes & Instruments, Multi, & Sea
Numerous Jet Type Ratings
Experienced Experimental Test Pilot both multi and single engine aircraft
Former Chief Engineering Test Pilot for a Major Manufacturer
Chairman Turning Point of Paulding County

Ohio

Joseph Trepicone wrote:

 

Clark and Jack,

 

I just spoke with Chris Skoppe regarding the Lancair’s stability in yaw.