Moni S/N #053 was brought to flight worthy status in 2006 by EAA Chapter #36 volunteers in Maryland. Looks pretty, huh? It had a standard V-tail built to the original plans, but it was severely damaged in transit to Utah. It ain’t so pretty anymore.
But maybe this shipping damage was a blessing in disguise after all, because at the other end of the aircraft was something that looked rather absurd. Open the cowling and you’ll see a beautiful, shiny, super-lightweight KFM 107 2-stroke engine -- but looking completely out-of-place is a 36 pound bundle of steel bars strapped on top -- nothing but a heavy chunk of inert mass weighing about as much as the engine! At the time, this heavy ballast weight on the nose was considered the best “fix” to locate the aircraft CG correctly, but I figured that making the tail lighter is far more desirable than making the nose heavier. Because the tail has about three times the lever arm distance from the CG as the nose, for every pound removed from the tail, three pounds can and must be removed from the nose ballast, resulting in 4 pounds total weight savings for the whole aircraft. Being an aerospace engineer with 30 years of expertise in composite structures, I was hopelessly attracted to the idea of making the tail lighter with a 4:1 weight savings advantage, and became committed to replacing all that mangled metal with lighter composite tail feathers of my own design.
But weight & balance is not the whole story here. I read up on some flight reports that the Moni has nasty habit of suddenly “flying backwards and upside down” when executing a low speed slip maneuver... Whoa, Nellie! It sounds to me like the V-tail airfoils are stalling, and that’s not a good thing! So, I inspected and measured the V-tail surfaces, and it turns out that the V-tail “airfoil” is not much more than a 5-7% thick flat plate with a rounded leading edge and sharp upper surface hinge line. This configuration seems highly susceptible to leading edge stalls at relatively small angles of attack (like in a slip).
1/11/08 Update: Moni pilots who have experienced this slip-tuck phenomenon are annoyed that I called it a tail stall, and say it is not a sudden occurrence, but if your feet are slow to bring the rudder pedals back to neutral, the nose-down rotation gets worse even with full back stick.. essentially, you run out of "elevator".. so it may not be accurate to say it is a leading edge stall, but there is probably some type of flow separation along with loss of downward lift force generated by the tail at these maxed-out deflections. In any case, more effective tail surfaces would not be a bad thing.
So, it seems reasonable to switch to a thicker, less stall-prone airfoil section for the tail surfaces. I selected Dr. Richard Eppler’s e297, an 11.4% thick symmetrical airfoil section with a plain flap hinged at the 62% chord station. The new V-tail surfaces would have a root chord of 26 inches, a tip chord of 14 inches, and a half span of 40 inches, which is slightly greater in area than the standard Moni design. The plain flap hinges on centerline with a simple radius external surface. The new V-tail airfoils should be lighter, lower in drag, more stall-resistant, and more responsive.
But that’s STILL not the whole story. I read some other flight reports that although the Moni is very responsive to elevator pitch commands, it is a bit sluggish to respond to rudder commands. I figured that a small movable composite rudder surface would enhance the responsiveness to rudder input, and it could easily be installed to replace the fixed aluminum tailcone fairing mounted behind the aft bulkhead. All the swivels and control linkages for this mini-rudder are already there to steer the tailwheel anyway.
So that’s my whale of a tail about how I ended up going down the path of designing and building my own composite Y-tail for what will become the Mosi motorglider… following in the footsteps of the Lear Fan, Waiex, and Xenos aircraft.
Y-tail design details in the next post... Happy Perihelion Day!!
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