Bird of Time

The Bird of Time is a beautiful and distinctive sailplane dating back to 1979.  The plane is nostalgia legal when built to plan, and has a reasonably wide performance envelope, especially for a R/E design.  Back in the early eighties, I wanted one but was flying the Maestro Megan and Viking at the time, and really was not doing enough in the hobby to justify building one.

Then the Dynaflite ARF Bird of Time was released in 2003, this piqued my interest in the plane again, but the issues associated with the poorly constructed spar system surfaced.  Since it became clear that there was no easy way to beef up the spar on the ARF model, I decided to build a Bird of Time from the kit.

The kit is a real bargain considering that you get pretty good balsa (at least in my kit), with beefy and hard stock on the high stress pieces like root ribs and nice light balsa sheets for the D-tube.  Looking at the spar system, it was clear that some improvement could be made, incorporating carbon spar caps and bias fiberglass on the D-tube leading edge. If you are going to modify the wing spar, you might as well do it right. I want to have a wing which is capable of close to 150 lbs. of winch line tension.  If this is to be the case, the joiner rod and wing mounting system will have to be improved also.

I am not going to put spoilers on my plane because the original Bird of Time did not have spoilers.

So this kit will be built according to plan but not LSF nostalgia legal with the following changes:

  1. The spar will have CF spar caps and will be wrapped with Kevlar.
  2. The joiner rod will be increased to a 0.500 carbon rod. Center dihedral is 5 degrees, poly break is unchanged.
  3. The wing will be made to bolt down, replacing the rubber band hold downs.
  4. The horizontal tail will be converted to a V-mount without joiners.
  5. The rudder hinge line will be made more vertical, tail moment is increased by 7%.
  6. The stab will have CF reinforcement for stiffness.
  7. A composite fuselage will be built to cut 8 ounces off the airframe.

This picture shown the center panel wing spar under construction just after being wrapped.

A 1/2" inside diameter joiner tube is inset into the spar web with the correct dihedral angle. The joiner rod is straight. A small amount of the joiner rod tip is beveled off the end to fit within the spar, otherwise the joiner rod will be too short.  There is a bursting load on the end of the joiner tube of 400 pounds. Before the LE sheeting is put into place, plywood shear webs will be added to the front and back of the spar on top of the Kevlar wrapping.

Here the panels are being joined, full size carbon carry through spar caps and crush braces are used at the poly joint. You can see that the outer panel only has CF spar caps to the 6th rib. The kit building sequence is out the window at this point.

The D-tube balsa skins are going to be a weak point of the wing structure, and the torsional stiffness needs to be improved.  Here the wing skins are being vacuum bagged on Mylars, and the foam form puts in the correct curvature for the ribs. The 1.4 oz./sq.yd. fiberglass is wet out on the Mylars, rolled to remove excess epoxy, and then stuck to the balsa. The balsa/FG skins need to be pre-curved so no bending is necessary when attached to the ribs.  Funny, the foam shuck is from the center panel of my Bubble Dancer, it had the right curvature for the Bird of Time ribs. The resulting skins have a beautiful smooth finish on them which will not spoil the wings when covered with Monokote.

Here's what the building board looks like when things are in construction. The building board is a full size sheet of drywall on a 12 foot slate snooker table.

Here is a picture of the V-mount and horizontal stab.  Essentially the same as the my Bubble Dancer mount.  The stab is stock except for the addition of CF on the main spar, and internals to mount the hinge system.

Here is the airframe requiring only a little finish sanding before covering. The original version has an angled rudder hinge line, I hated the turn response on old planes with this setup.  You get a bunch of pitch response whenever rudder control inputs are given. I made the hinge line much more vertical, and the rudder is hinged with full length Kevlar.

The composite fuselage is 2 layers of 1.7 oz/yd. Kevlar, uni-CF strips and 3 oz satin FG.  All this is vacuum bagged into the mold to reduce weight.  The rolled carbon boom is 4 layers of uni-CF with 1.4 oz FG on the bias.  The pod and boom weigh 108 grams (3.8 oz.!) before radio installation.

And the final result.

Weight is a very reasonable 38.8 ounces after final trimming with only 0.75 oz. of noseweight, right on target including all of the structural reinforcement. It would be easy to whack another 4 ounces off this plane with a lighter tail and smaller radio components. The only remaining improvement which I would consider is to cap the ribs with CF strips to increase the stiffness of the TE.

Photo by Martin Pilko

Flying

December 2003

There's a foot of snow on the ground and very little thermal activity with only 8 days to winter solstice.  I threw the plane in the yard a few times for trim and can get a couple of circles before catching it. In a fit of boredom, the winch was set up with a line of less than a 80 yards.  First launch and the plane is nose heavy, the climbout is too vertical and it pops off at 100 feet. Add some down trim and the full bore launches on my winch with a 100 foot zoom show that the spar and wing sheeting can take it. Float around for a while and set up for the hand catch, this plane is sloooww. Spiral stability at low speed is right on with the increased dihedral, the Bird rolls out a lower bank angles and is neutral at minimum thermalling speed. Imagine the transparent covering with the sun behind against the winter blue sky. After 4 flights I'm worn out from retrieving the chute in 18" of snow, flight tests will have to wait for springtime.

Summer 2004

I flew this plane for fun whenever the wind was light and the lift was weak. After balancing and getting used to the slow turn response of the rudder, it was easy to get flights over an hour while carrying on a conversation and having lunch.  Hand launches into thermals are pretty easy, it can get out about 1/3 of the time when the thermals are popping in the morning. With the stiff structure, descents from altitude are made by pointing plane straight down in a terminal dive followed by a bunch of loops which kills the speed immediately.  People still ask if the covering job is intentional. People still wonder how a BoT can be launched this hard. A 39 ounce floater with an ancient airfoil is still pretty hard to beat for fun flying,

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