As it turned out the model was not as difficult to build as it may look but it was
monotonous thanks to the contest between Mike and I to see who
could come up with the most tedious feature and then me deciding to incorporate
each of those ideas.
I think Thwing! is the most structurally attractive flying model aircraft I've
ever built so it was worth it. I'd conservatively estimate that I have
200+ hours in building the model. It took about 5 weeks to complete.
I could do much of anything I had to figure out how I was actually going to build a
wing that has only 2 ribs. I decided to support the root and tip ribs on a
sub-leading edge and sub-trailing edge.
The leading and trailing edges would be tall enough that they could suspend the
wing off the board with either side up. The edges would remain over size until
all the skins were glued on at which point it would be nearly impossible to warp
To accomplish all of this I had to ensure that the leading and trailing edges
were cut absolutely parallel and to the exact same height.
The sub-edges needed to be glued on extremely accurately on the centerlines
of the outer edges.
If all these things are accomplished then the wing will be as straight as the
board it is built on. The root and tip ribs were notched to
slide onto the sub-edges.
At the time I first had this idea I thought that it would work ok but that
I'd probably come up with a better way to build the wing. It actually turned out to be
the simplest method I could think of and it resulted in an extremely straight
wing. In other words I would do it again.
I thought that building the spars would be the bulk of the work and the rest
of the construction
would pretty much fall together afterward. Unfortunately the lattice
was more of a project than I expected.
I had to cut 3 sets of lattice before I had one that would work. The first set cocked while I was routing it which
resulted in the lattice spacing not being the same across the sheet. I
didn't notice that had happened until I started assembling it.
The next set was made from
balsa and I was afraid it wouldn't be strong enough. I feel the contest balsa could have been used
now that the wing is completed. Having never built a wing like this before I wasn't sure how much strength it
would have nor how much it needed to have.
I didn't want to take
chances so I ended up using medium balsa. All the lattice
used was cut from a single sheet of 3/32" x 4" x 48" sheet. The router did
not cut the joints cleanly. Before cutting the sheet into strips, I sanded
as much fuzz off as possible. That only took care of fuzz near the face of
the sheet. After the sheet was stripped I used a
needle file to
individually clean every joint.
Fitting the lattice skin to the wing turned out to be more difficult than I
thought it would be. That was because of the oversize leading edge which
did not allow me to drop on the over-size skin to mark it for trimming.
Because of the curve of the airfoil, the skin has to be larger than what is
shown in top view.
I made a cutting jig to trim the rear corner at the
root of the wing. The idea was to ensure all the skins had an identical
base point. Once that corner was established, the other edges of the skin
were trimmed to fit.
Overall I would say the weight of the plane is similar to what it would be
using more standard construction techniques. It looks a lot
better. On the other hand, the wing is torsionally the stiffest that I
have ever built and I can only attribute that to the lattice.
From a design and style standpoint, the item that gave us the most difficulty was the
vertical stabilizer. We knew we wanted a rudder because the
tends to drift in climbs and there is no way to correct it. What gave us trouble was finding a
fin/rudder shape that blended into the overall theme while having enough surface
We spent hours drawing fin and rudder shapes and just couldn't
come up with a shape we liked. In all cases the fin was shaped such that
the leading edge was vertical or rearward swept. The trailing edge was
always forward swept. Whenever we had a shape we liked, it didn't have
enough area. We contemplated adding a sub-fin but were concerned about it
being damaged in landings.
All the while we were designing a fin that would be
glued on so that its trailing edge was aligned with the elevon hinge line.
The bottom of the rudder would be cut at an angle to allow elevon movement.
I've never seen a plane with a
rudder like that that I've liked — it's just a
really ugly rudder shape.
In frustration I finally drew some very sharply raked lines for both the
leading and trailing edge. Both
of us looked at it and realized that we had something we were going to like.
I went to work on other things and left Mike to continue tweaking the lines and
working out the area.
Of course this fin design gave us a whole new problem — how
to hold it on. Gluing the fin to the upper deck sheeting
alone would be really precarious. There would be too much unsupported area
hanging off the rear of the wing. We had discussed adding a "stinger" a few weeks prior as strictly an ornamental thing.
Now it would have a purpose. Of course this added another new problem.
The elevons were already
built such that they would meet in the center of the wing with a minimal gap
between them. I ended up chopping off a good amount of area to make room
for the stinger. They still have plenty of area and are very effective
even though the loss of area was directly in the prop blast.
the model began coming together we became more anxious to see it completed in
the bare bones. It was obviously going to be very attractive.
In the mean time the leading and trailing edges could not be cut down and shaped
until after the lattice was glued on so we had to visualize almost to the very
end of construction. By that time the model was virtually finished.
One last very tedious item remained. I had to edge the lattice at the
root, trailing edge and wing tip. I just didn't trust the small amount of
contact area not to break loose in a high G maneuver. The edging entailed
cutting and fitting a whole bunch of tiny pieces and even tinier triangles.
Each one was hand fit.
At this point all that was left to do was make the cowl, finish the hatches,
finalize the motor mount, sand everything and then finish it.