Jim DeTar's Guardian (From the Fall 2006 Thermal Journal)

Flying Ace Club (FAC) free flight contests usually feature a large number of events, reflecting the many and varied types of aircraft and eras represented by the scale modeling enthusiasts' efforts. An incomplete list would include FAC Scale, Peanut Scale, Golden Age Civilian and Military, Modern Civilian and Military, Race Planes, WWI and WWII events. There are also non-scale endurance events such as Embryo Endurance, 2-Bit Rubber, Old Time Rubber Cabin and Stick. I have been seduced by enough of these different themes that eventually I found myself with a variety of aircraft and events in which to participate. That's the good news; the bad news is too many events and too little time to get all the flights in during the contest. I had to prioritize and plan my efforts. For whatever reason, I found that at a typical 2-Day contest I was too busy on Day 1, but somewhat slack on Day-2. Many times a "Modern Military" event for post 1945 military aircraft was included in the last day of a contest. Great, let's build a plane for that event, helping to balance things out. My choice was the Grumman "Guardian", an early 1950's Navy anti-submarine plane. With it's mid-wing configuration, generous nose moment and quite ample wing area and tail surfaces, it looked to be a good candidate for rubber power. Add to this the Robert Sweitzer Peanut Scale plan that had been published in the FAC Newsletter, and I had a natural starting point.

The peanut plan was enlarged to 21 inches wing span, and the fuselage construction was re-engineered to a box and former configuration. The original plan had separate, glue 'em to the fuselage sides wing panels which I felt should be changed to a stronger, one piece, straight through the fuselage wing. Past experience with other models had taught me that the separate wing panels, glued to the sides of the model were weak and subject to damage in anything but a perfect landing. Fortunately, before I went too far, I realized that putting the wing through the fuselage would leave no place for the rubber motor. Duhh!! The two separate wings turned out to be a blessing in disguise. More on this later. Also, that big vertical tail and 2 sub-fins looked great. The model was constructed and tests flights ensued. The Guardian flew nicely in a climbing left circling pattern under power, but when the power ran down, the plane slowed, dropped the right wing tip and went into a downward spiral to the ground. It was climbing left, but then almost stopped when attempting to transition into a right hand glide. The resulting stall, wing dip, and persistent downward spiral quickly ended the flight. Okay, let's add some left rudder. This got a left, left pattern, but the airplane could still get into the downward spiral. I went home thinking "spiral instability". I had what appeared to be sufficient dihedral in the wings, but hadn't I heard or read somewhere about a relationship between dihedral and vertical tail size? A little reading and research revealed that yes, that big vertical fin could be the culprit. It turns out, as no doubt many experienced flyers already know, that a too large vertical tail can result in spiral instability problems as much as a too small vertical tail can result in tail wagging "Dutch Roll". That big vertical tail and sub-fins apparently overwhelm the ability of the dihedral to keep the wings level. Seems somewhat counter intuitive, but unlike the horizontal stabilizer, in the case of the vertical fin, bigger is not necessarily better. So adjustments were made. A new, reduced size fin and sub-fins were constructed. An increase in dihedral could also help the problem, but build a new wing!!? Aha, those separate wing panels. By carefully loosening the glue joint attaching the wing panels to the fuselage sides, I was able to slip a 1/32" shim between the fuselage sides and the bottom on each wing panel root. Result: a few more degrees of dihedral. I know, this breaks the cardinal rule of making trim adjustments just one at a time, but it worked! The model now flies in a left-left pattern, with consistent times in the minute and a half range. It even managed a second place finish in the Modern Military event at the September FAC Outdoor Champs at Muncie. Gee it's great when things work out!! I have included a picture of the complete model with the modified tail and a picture that shows both vertical tails.

Jim DeTar