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by Pete Young

Bird of Time photo

The 108" wingspan of the Bird of Time Sailplane was designed by glider guru Dave Thornburg in the early 1970's for F3B competition, and the plane did quite well in overseas competition. In the original RCM construction article, Dave described the design evolution which led to the configuration first kitted by Mark's Models, which became Dynaflite several years ago. Dave designed the Bird of Time with a thin 9.5% airfoil for low drag in F3B speed and distance tasks, and paid particular attention to cleaning up the airframe to eliminate excess drag—still excellent design practice today, by the way! The distinctive pre-WWII "Wolf" German-style wing planform came about because Dave liked the looks, not for any theoretical reason. And the Bird of Time was designed with light extremities, light wing tip panels and tail surfaces to provide responsive handling at all speeds.

Dave put a lot of emphasis on the proper use of ballast to provide "best" performance for all gliders, not just the Bird of Time. His discussion on the importance of finding the right ballast for calm, as well as windy conditions, was ahead of its time. It's easy to build a glider and get the CG in the right place with a little nose weight, but it's quite a bit more work to make up ballast blocks of different weights and conduct methodical flight testing to find the "right" wing loadings for different weather conditions. And this, I believe, is a vastly neglected aspect of sailplane fine-tuning for best performance.

The subject of this month's product review, the "new" Dynaflite Bird of Time, has been re-engineered from the original kit release. I build two Bird of Time's back in the late 1970's, and the primary differences between the old and the new kits, as far as I can recall, are the substitution of balsa for plywood fuselage doublers, and replacement of an all sheet rudder with a built up version. And the assembly manual has been redone to provide additional details for novice builders. There may be more substantial changes in this Dynaflite re-release which I have not caught, but I'd have to do more background checking to determine what they are. I did note, however, that the die-cutting and wood material was excellent quality, a real plus for this "building intensive" kit.

Radio control sailplanes have changed quite a bit since the Bird of Time was first designed, and it is now more suited for sport thermal flying and the popular rudder/elevator "Nostalgia" competitions which have caught on with many soaring clubs. With its large wing area (over 1000 sq.in.) and light wing loading (spec'd at 5.5 oz.sq.ft., unballasted), the Bird of Time is a real "floater" and thermal hog. With the addition of spoilers, the 118" span Bird of Time should be quite competitive in club contests against other rudder/elevator sailplanes. The instructions don't provide any details on spoilers, but these are quite straightforward too install using material (nylon tubes, thin string, trailing edge stock) left over from other projects. I'd rather use spoilers than try to fly the Bird of Time in too quickly, only to 'dork' into the landing circle, when I can slow the plane down on command with spoilers.

My Bird of Time construction started with the two-piece wing which is joined by a 1/4" metal rod. Fiberglass arrow shaft sections, which fit into the wing halves to hold the wing rod, are supplied but unfortunately, the arrow shafts' inner diameter is quite a bit larger than the 1/4" metal rod, and so much so that there is quite a bit of side-to-side play when the wing rods are inserted into the arrow shafts. I tried to convince myself that I could wrap the wing rod with some material (masking tape, electrical tape?) To make up the difference, but in the end I decided that substituting 1/4" ID K&S brass tubing was the "right thing to do", and this indeed produced snug fits with the wing rods. I suggest you do the same. Using the arrow shafts will produce some rather undesirable stress concentrations unless the sloppy fits are corrected in some manner.

Reprinted with permission.
November, 1997 R/C Report
Editor: Gordon Banks

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