Finally got the Viperjet ready after going over it and over it for a long time. With the kind assistance of one of the most experienced pilots in turbines at the controls for the test flight all went pretty smooth until about 3 minutes into the flight the vertical stab and rudder decided to depart the airplane. Still controllable, a flapless emergency landing was made. It actually flew for two laps around the pattern that way before landing. The seperation occured in straight and level flight at a very modest cruise speed. Power never exceeding 40% except for takeoff. No inverted or crazy stuff just plain gentle flying. No signs of flutter were ever evident and no gaps between the stab and rudder to create flutter. Controlled by a $100 JR 9411 servo with lots of torque and no slop in the linkage.
Cause is not known but the stab and rudder were located after the landing and all that is in it is a balsa spar. The stab sheared just above the carbon fiber reinforcement that is used on the interior sheeting. Nothing is used above that but balsa. Further examination leads to believe the spar failed but we'll never know for sure. Just want to get the word out about a possible weak point. A phone call to Cermark ordered a new vertical stab and thought it was going to be no charge....turned out to be $95 plus the $100 servo I lost in the field somewhere that could not be found. The new one has been opened on the bottom and reinforced the spar to sheeting seams with carbon and all weak areas strengthed with thin CA. Hopefully this will prevent another incident of the same. Fortunately I had the bolt on version of the stab that I had not glued and it was easily removable. However the new ones are now designed to be glued on. So I hope it holds together this time for a long time. For those of you still building or have the stab removable I would advise strengthening the spar of the vertical stab and just making everything as strong as you can including servo mounting and linkage as well.
Thank goodness the Viperjet has winglets and tail strakes along with the portion that was left, the airplane was still controllable. Many thanks to the skill of a well known test pilot as well. Without them it probably would have gone in.
Hence the name ....Black Cloud