Funbuild 5 - 42" DeHavilland DH-89 Dragon Rapide kit by Dumas
I'm building the Tritle designed, 42" DeHavilland DH-89 Dragon Rapide kit by Dumas.
This kit has been on my shelf for three years now but a clear workbench and the funbuild contest is good reason to get it going.
There have been several good build logs covering the Rapide including the prototype work done by Pat.
All three logs are quite good but I'll do what I can to add to the knowledge base and I have a few hacks in mind so I'll document those as well.
The Rapide is an interesting design that combines the draggy elements of a fixed gear biplane with a slender high aspect ratio wing, as exemplified by these two pictures:
Wings panels rough framed
In the interest of full disclosure I actually started on the kit over two years ago.
I got as far as the rough framing of the wing panels when some medical issues kept me away from my bench until this fall.
Then two weeks ago I formed the bowed outlines for the tail feathers, but had to again put the kit aside to repair another plane.
But the bench is clear now and it's time to take my hand off of the mouse and get going.
I made a copy of he elevator outline so as to make two forms so that I could do both halves at once and give the wood and glue time to dry.
The titebond has had two weeks to cure so I reckon that it's time to pull the outlines off of the forms and finish the surfaces.
About those bowed outlines...
This is the third model that I've built that uses them.
I've studied Pat's tips on making them,
and have seen many threads with hints here on the groups.
Despite that, I've yet to master them, in part maybe because I know that, within reason, the unintended cracks can be fixed fairly easily.
Most importantly, they'll look just fine after being covered.
I've hesitated over posting about this, it's not easy to admit that such a simple skill eludes me, but I reckon that there be others in the same boat, so here's proof that you're not alone.
Next time I'll try thinner strips for the lamination.
Anyway, attached are a few pics covering my tribulations in bowed outlines.
On a related note.
The tailfeathers are nearly complete!.
All that is left is to cut the surfaces apart and bevel the hinge spars before covering.
I mixed up the construction sequence a bit and started off by cutting the hinges into the spars and then building the surfaces around the hinged spars (hinges not glued yet).
The idea was to make it easier to sand the stab/elev and fin/rudder assemblies in one piece.
I've found that once cut apart the pieces (particularly the elevator) are a bit fragile when being sanded separately.
It worked well.
As a bonus the hinge slots were easier to cut into the bare spars.
I'm going to wait until after the tailfeathers have been trial fit on the fuse before cutting them apart.
CF landing gear
Something that I've had good results with on recent builds has been to replace the steel music wire used for the main landing gear with carbon fiber rod.
The CF fabbed gear saves about a quarter of an ounce, and is quite strong and flexible.
On the Rapide the gear is hidden by the engine/gear nacelles and so the looks/design of the gear isn't an issue.
The gear legs are 0.050" (1.2 mm).
Balsa wedges join the legs.
Dental floss and thin CA strengthen the joint.
The gear legs slide into holes in the 1/8" lite ply gear mounting plate. The 'vee' that the legs make is wider than the space between the mount holes so the gear is a spring fit.
They are not fixed yet, I'll wait until the nacelle/gear fairings have been fitted to the wing so the wheel position can be adjusted if need be.
Got the wings completed (well a little touch up finish sanding left but that can wait).
The razor plane made quick work of the job of shaping the leading edges.
If you've never used one, you should.
The Rapide's leading edge tapers rather gracefully, the pictures don't really do them justice.
On to the fuselage.
I've built about 15 or 16 balsa RC airplanes in the past 31 years and have always used CA.
Rarely have I bothered pinning parts - just hold them in place, hit with a drop of glue and move on.
However, in the preamble of the instruction book the designer suggest that "CAB or cellulose acetate butyrate cements ... for this size of aircraft ... are very appropriate because of their lightweight and flexibility."
To me the biggest issue with CA is it's (lack of) sand-ability. I've learned to mitigate this by hitting every joint with a light sanding as soon as the CA is applied. That helps but it's still easy to end up with glue high spots after final sanding.
Some complain about the weight but hey, I don't use that much and I still have a stash of 'hot stuff' tubing that makes it pretty easy to apply sparingly, just where needed.
And, since it kicks just about instantaneously, it's pretty unforgiving, so you had better be sure of the fit and alignment before gluing.
So for this model, I went to the LHS and got a tube of Ambroid CAB cement to try.
First off, the fumes are strong (why do you think that they it 'dope') and some sort of ventilation is a good idea.
Beyond that however, it's easy to use and does indeed sand well.
Minor gaps can be filled with a little Ambroid and then sanding before it drys.
It's not instant and doesn't wick like CA but grabs pretty fast if pre-applied and allowed to partially dry before assembly.
Rather than fit and glue on the go (like when using CA) I adjusted my building technique for the Ambroid by cutting and pinning in place every stick for the fuselage before any glue was applied. In fact, I cut the sticks for both fuselage halves at the same time.
After all the sticks had been cut, fit, and pinned in place, enough pins were pulled to remove the shorter sticks and allow glue to be applied.
Ambroid was applied to all of the mating surfaces. By the the time I got to the last stick, the glue on the first had partially dried.
Add a touch of fresh glue and then join the pieces and pin in place.
The seemingly tedious work of cutting all the sticks first was rewarded by how fast the complete structure got assembled. This also minimizes the amount of exposure to the dope fumes.
The Ambroid has plenty working time so it was easy to get every thing just so.
This is all probably old news to many of you but if you haven't tried Ambroid and want another adhesive option you should really give it a try.
Anyway, back to the build.
The fuselage is complete.
The fuselage construction is pretty straightforward up until the installation of the forward formers.
The curves in the DH-89 fuse require some pretty aggressive bending of the sides to achieve the shape.
Perhaps the wood that I got was stiffer than the prototype (or maybe I just suck at bending wood) but the bends gave me some trouble.
No way was I going to be able to have enough control bending things by hand so I got out a 'Jorgy' wood clamp to assist in massaging the front of the fuse into position. The clamp made it easy to dial things in and get the formers glued up.
But damn if the fuse sides didn't bend symmetrically. I reckon that there was enough difference in the stiffness in the wood of the two sides to screw up the curves.
So, back in the clamps, with shims in the right places to even up the curves, and then a misting of water and then left to dry overnight.
Fortunately, after drying overnight the fuse came out straight and true with all of the bends in the right places.
Next up is to trial fit the wings and tail feathers and then on to the motor mounts and nacelles.
When I was building models as a teenager back in the '70s, Ambroid and epoxy were the only glues I used. They were pretty much all that was available, but hey, they worked. When I took up the hobby again ten years ago, I was amazed at how CA had changed the building process. Like you, I use it almost exclusively, with Titebond and occaisionally epoxy playing a supporting role. I've often thought of picking up a tube of Ambroid for nostalgia's sake, but am starting to realize it would be perfect for a couple of special jobs, like planking. We'll have to see if I can keep resisting...
The Rapide is really shaping up. Keep up the good work!
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