[Courtesy of Pete Young [pwyoung at ix.netcom.com], December 1998 - as published Pete's RC Report Soaring Column]
The 108" span Bird of Time sailplane was designed by glider guru Dave Thornburg in the early '70s for F3B competition, and the plane did quite well in overseas competition in the hands of F3B Team Member Steve Work. 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-WW2 "Wolf" German-style wing planform came about because Dave liked the looks, not for any theoretical reasons. And the BOT 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 BOT, and his discussion on the importance of finding the right ballast for calm, as well as windy, conditions, was ahead of its time. It's perfectly easy to build a glider and get the CG in the right place with a little noseweight, 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 firmly 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 reengineered from the original kit release. I built two BOTs back in the late '70s and the primary differences between the old and the new BOTs, 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 those 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 for F3B competition, and the BOT 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'ed at 5.5 oz/ft2, unballasted), the BOT is a real "floater" 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 to 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 BOT in, perhaps too quickly, to "dork" into the landing circle, when I can slow the plane down on command.
My BOT construction started with the two-piece wings which are 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, 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 quickly 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.
I've built quite a number of D-tube glider wings over the years and I just didn't care for the wing construction sequence given in the instructions. I'm not saying that the instructions are wrong, it's just that building the wing in the suggested sequence has a high certainty to cause building and alignment problems and I have found that it's MUCH better to AVOID building problems than try to fix 'em after the fact. Following is a summary of the primary construction sequence items which I changed.
First, the leading and trailing edges are not attached to the wing ribs until fairly late in the building sequence - step 19 for the T.E.s, step 32 for the L.E.s. This means that the thin and fairly delicate wing ribs are first glued permanently to the wing spar, then the rib ends must be carefully trimmed to mate up to the T.E.s and L.E.s fairly late in construction when fine adjustments to the ribs' alignments will be impossible. And aside from being difficult to sand for precision, tight-fitting, T.E./L.E. joints at this point, the fragile ribs will be extremely vulnerable to handling damage. In my opinion, it's far better to attach the ribs to the T.E.s and L.E.s at the start of the wing building sequence when all the major components can be attached together "on the building board" rather than as suggested in the instructions.
Second, the wing tube for one wing half is firmly epoxied into place before construction of the second wing half (and installation of its mating wing tube) is started. Coupled with the fairly thin wing airfoil, this means that you'd have to be exceedingly lucky to get the proper wing alignments (dihedral angle, wing halves' mating angles) for the wing rod tubes to line up inside the spar boxes with this construction sequence - there are extremely small margins for error for the wing rods to seat properly in the spar boxes! It's far less risky to take both wing halves to a moderate degree of completion (top and bottom spars installed, but NOT all the center section ply braces) and install both wing tubes simultaneously (with the wing rod installed for alignments) into the wing root sections with the wing panels blocked up to the proper angles. This approach will give you several opportunities to make "best and final" adjustments to guarantee a) that the wing rod and its tubes fits perfectly into both wing halves and b) that the proper dihedral and wing mate angles are achieved prior to making permanent (and difficult to undo) center section bonds.
The last item is fairly minor compared to the first two but is worth mentioning. The wing main and tip panels' trailing edges, 1/4" x 1 1/8" rectangular balsa stock, have to be tapered sometime during construction and the instructions don't specify when this step should be done. I recommend that you taper the T.E. stock prior to starting construction of the wing panels - it's clearly easier to do this shaping "before" rather than "after" ! Note the short - approximately. 6" long - T.E. section at the polyhedral break - here it's best to do preliminary shaping first, then final sanding of this particular section after the T.E. sections are finally joined together.
With these details out of the way, the remainder of the wing construction is fairly straightforward. I decided to add spoilers for better sink rate control during landings, and added them to the main panels as shown in the pictures. Spoiler opening actuation is by string-and-tubes from a fuselage mounted servo, with the spoilers closed by light rubber bands. The curved wingtip outlines are made up of laminated pre-cut balsa sheet sanded to shape - this requires a little care but the final results are worth it. In order to get the 1/16" top and bottom sheeting to fit properly over all the wing ribs on both the top and bottom surfaces, it's important to carefully align the ribs during construction, another good reason to add the T.E.s and L.E.s earlier rather than later. The wing airfoil, incidentally, has quite a bit of "Phillips entry" which requires the leading edge to be elevated slightly from the building board during construction - do this step carefully to get uniform alignments as well as good fits to the ribs, as well as the top and bottom L.E. sheeting.
The two-piece horizontal stabilizers are very lightweight and are built up by bending 1/16" x 1/4" capstrips over a central spar, attaching to L.E.s and T.E.s jigged up from the building surface. The horizontal stabs don't have much structure, and precise fits and careful alignments are the keys to success here. Since warps could be induced if matching top and bottom capstrips were to be cut from 1/16" x 1/4" strip stock differing in stiffness, I made sure that I set aside pieces - cut from the same balsa strip - for matching top and bottom capstrips during construction and suggest you do the same. And rather than just epoxy the rear stab wire tube to the central spar, I wrapped it to the spar with some small diameter Kevlar - unwaxed dental floss will work just as well (if you do this, make sure you use UNUSED dental floss, Mr. Editor!)
The horizontal stab is actuated by a 90 degree bellcrank located at the base of the fin assembly. Space is tight back there and the stab linkage should be carefully checked for binding before the bellcrank assembly is closed up for the last time - be sure to eliminate any interference back there throughout the full range of bellcrank motion. Don't ask me why I know how critical it is to check this before sealing it up - - -
The rudder outline is built up from three plies of pre-cut curved 1/8" balsa sections, then sanded to final tapered shape. I found that the lamination joints for all three plies of balsa fell on top of one another, thus causing possible failure points. I fixed these during construction by inserting short sections of carbon fiber tow over the splice joints. As built, the rudder turned out rather heavy, primarily due to the density of wood supplied. I therefore thinned the trailing edges rather aggressively to save as much weight as possible. I suspect that a lighter rudder could be made with lighter wood and an outline laminated around a curved form.
The fuselage is a basic box built out of balsa and ply, and in the stock configuration is wide enough to hold two standard servos side by side. The instructions suggest thinning the fuselage to reduce drag, and I took this suggestion and made the fuselage just wide enough to hold standard servos single-file. I also had an ulterior motive to save weight by reducing the top and bottom sheeting needed, compared to the stock fuselage. There's a lot of potential for weight savings by aggressively shaping the 3/8" balsa top and bottom sheeting, so get out your rough sandpaper, go outside the house, and round off the fuselage until you get the desired contours. In retrospect, I suppose I could have taken off more material, but it's awful hard to reapply balsa dust if you go too far, so I'm satisfied with the shaping that I accomplished.
To cover the flying surfaces, I used transparent red Monokote over the open sections and white Goldberg UltraCote over the sheeted wing leading edges, first prepping the wood with Coverite Balsarite to improve adhesion. Several coats of thinned nitrate dope were used to fill the wood grain in the fuselage, then Coverite Century 21 white primer, and Century 21's glossy white and red applied with spray cans.
To provide the muscle power to move the rudder and elevator, I installed two Airtronics 94102 standard servos in the forward fuselage and a HiTec micro servo in the mid-fuselage to actuate the spoilers. A Cermark 600 mah battery pack fits snugly into the nose, with an Airtronics 7 channel receiver placed behind the spoiler servo. Approximately 6 ounces of noseweight was needed to get the CG right, and the final flying weight came in at 54 ounces overall, yielding a 7.3 ounces/sq ft wing loading in the unballasted state. The boxtop label claims a 41 ounce flying weight - I suspect that a lightened rudder, as mentioned above, would help a lot to get flying weights down into that weight range. Let me hasten to add that 54 ounces is an excellent flying weight, in my opinion, for a 1000+ sq in glider and should not, by any stretch of the imagination, be considered "overweight". We're talking "floater" performance here, folks!
To date, I've put my Bird of Time through several flying sessions with all flights launched on 12 volt electric winches. I've successively moved the tow hook forward just a tad, and it presently is 3 1/2" inches from the L.E. with the CG located at the forward side of the wing spar. The plane's wings are long and flexible, with tip deflections clearly showing the effects of too vigorous winch loads. The glide is slow and majestic, yet with a fairly brisk top end speed when elevator trim is dialed in. The thermal performance and handling is excellent, and my BOT is a sheer pleasure to fly in calm to moderate wind conditions. You'll note from the photos that I positioned the spoilers fairly far outboard on the main wing panels - this was done to minimize the spoilers' turbulence over the elevators when spoilers are opened. My thinned fuselage is now too crowded to handle any ballast blocks inside the fuselage, so despite what I said earlier, I won't be able to do any ballast checks with my BOT - oh well, I like the appearance of my slightly anorexic fuselage and will plan on flying it for fun just as it is - so there!
Some lessons learned from flying the plane: you'll note the single row of rubber bands which hold down the wing. In order to keep the rubber bands from slipping between the wing halves, it's very helpful to put masking tape (vinyl tape will work, also) over the wing joint - this keeps the rubber bands on top of the wing, yet will release in case of a hard landing. Also, with a single row of rubber bands, the wing may "rock" sideways on the fuselage - a tendency aggravated in my case by the extra fuselage slimming which I incorporated. The fix for this was simple, merely to add 3/16" triangle stock to the fuselage sides to provide some added support.
Summing up, I found the Dynaflite Bird of Time to be a pleasurable project both to build and to fly - designed in the 1970's, it's loads of fun to fly in the 1990's! With the minor exceptions noted above, the plane is straightforward to assemble and would be perfectly suitable as a "second" plane for novice builders, in my opinion. Built and flown with care, the Bird Of Time should reward its builders with many hours of thermalling performance at moderate cost, and that's a pretty good deal these days!
Product Test Report Summary:
CHEERS: balsa die-cutting, final flying weight, flight performance
JEERS: Wing rod tubes mis-sized; wing construction sequence