The other day I finally cracked open the molds to see how things had turned out with my wing pod and tail skid.

A few air bubbles here, and a lot of weave texture showing through – this is no substitute for vacuum bagging, but will it at least be sufficient?

Oh noes!  Only two lightweight fiberglass layers were apparently not enough – while flexible enough to be removed from the core, the molded part just wasn’t strong enough to hold up to the tugging and pulling needed to free it.  On the plus side, the fact that it was able to be fully removed means that the waxing and PVA application was sufficient to keep from ruining the plug, so this was promising for the wing pod.

My ‘kinetic separation method’ for breaking the mold halves apart (whack it on the floor a few times) is still far from ideal, as another corner broke off of the mold.  I now see why people who know what they’re doing build in screwdriver slots so that the halves can be separated in a less destructive manner.  With the satisfying sound of PVA breaking away from a surface, the halves popped free.  The part was still adhered to the female mold half (as evidenced by the black center), but a little careful pulling finally extracted it.

Finally, a completed, intact part!  Now, would it fit…

Almost perfect!  There’s a tiny bit of side-to-side slop, but I’d say this is well more than “good enough”.  Now, to make 3 more!

One thing I’ve noticed about EZ-LAM 60 epoxy is that the maker wasn’t lying about not using it when the ambient temperature is under 65 °F.  It’s now been over 48 hours since I laid up the two pod molds, and the fiberglass/epoxy is still slightly pliable.  I’m sure in another week or so when standard late spring temperatures finally arrive the epoxy will fully cure in 24 hours when in my basement, but for the colder parts of the year I’ll need to use EZ-LAM 30 or just start experimenting with the West System hardeners.  As such, this is a quickie post on removing the weights from the Diamond 2500 powered sailplane.

I hate seeing RC planes come from the factory with a bunch of steel washers glued into the nose – if the plane has the center of gravity too far back, I’d rather add more fuel to the front (in the form of a bigger battery) than dead weight.  I think I know why manufacturers do this, however – they want to be absolutely certain that the plane is stable, even if it means reduced performance.  “A nose heavy plane flies poorly; A tail heavy plane flies once” is the adage I’ve heard a number of times.  As such, I used to fret about having too little weight in the nose, while now I find myself pushing the CG on my planes further and further back to improve the glide slope.

The Diamond 2500 has a pair of steel blocks glued into the nose under the plywood battery tray at the very front (visible just under the motor wires):

Unfortunately, the plywood tray can’t be removed to get at the weights, but with a flat bladed screwdriver and an assortment of picks, I was able to extract them from the rear of the cockpit area (fortunately they weren’t glued in very securely).

129 grams of dead weight!  Not only is there nose weight in the Diamond 2500, but there are wingtip weights as well!  Supposedly this is to reduce the roll rate, but with a big 2.5m wingspan, I can’t imagine that the roll rate is all that blazing in the first place.

The wingtip weights are glued in a little more securely than the nose weights, so I epoxied a steel rod to the weight to pull it out.  After removing the weights, I glued a small block of white foam in the cavities.

Every little bit helps, as I intend to put plenty of FPV gear on this airframe.  One final bit of weight reduction is the wing spar, which is a length of thick wall aluminum tubing (which slides into square steel tubes inside the wings – I’d love to remove them, but both are glued in quite securely).  The aluminum spar weighs in at 133.8g, but the Goodwinds 020979 carbon fiber tube (which is a perfect fit in both length and diameter) is a mere 58.9g.  All told, these weight reductions add up to nearly 9 ounces – that’s the weight of a 2500mAh 4S LiPo battery pack!

Addendum – 9OCT2015

I’m rather embarrassed to say that even though I posted this several years ago, I still have yet to actually fly the plane.  However, I did just get a very helpful message from Marc Merlin, who offers some great information regarding a newer offering of the plane, a better source of the carbon fiber spar, and a full-scale pilot’s take on doing FPV flights over Burning Man!  Thanks, Marc!

Howdy,

First thanks for your page.

I bought a BFG2600 since the diamond 2500 isn't for sale anymore, and that
one comes with small steel balls in the wings, and nothing in the front,
likely because the motor is a bit bigger.

It does however come with a horrible heavy steel rod, but when I tried to
replace it, Goodwinds 020979 doesn't work anymore as they have discontinued
metric sizes.

After much searching (really, this took a while), I finally found this:
http://www.rockwestcomposites.com/products/t-rnd-472
and they're a perfect fit.

They sell them in 2m length, but I ordered them pre-cut at 83cm, giving me 2
tubes plus a leftover for some other project.

Would you mind updating your blog post to point people to that tube instead
and save them the time I spent?

This is my BFG2600 BTW
http://marc.merlins.org/perso/rc/post_2015-08-15_BFG2600-FPV-build-with-Pixhawk-Ardupilot-3_3-_Diamond-2500-remake-from-Hobbyking_.html

and this is the trouble I got into with it
http://marc.merlins.org/perso/rc/post_2015-09-01_Flights-over-Burning-Man_-and-the-current-unfortunate-BMorg-and-FAA-policies.html

Cheers,
Marc

After sawing the halves of my pod mold apart, the first thing I did was to fit the two halves together to see just how much clearance there was (should be right about 0.010″).  Unfortunately, this is what I got:

The two halves should fit fully together with no gap, so I started trying to figure out what went wrong.  As it turned out, this was simply a result of picking precisely the wrong surfaces to be machined, and I wound up with the exact inverse of what I wanted.  I was guessing that I’d have to simply recut the molds (I say ‘simply’, but it would be kind of a pain), until I realized that I should be able to simply make molds from the molds, thus flipping the surfaces back to how they’re supposed to be by turning the male half into a female half and vise versa.  Copying molds in this way is not uncommon for composite work – a male master or ‘plug’ might have a number of molds pulled from it, with each mold being used to create dozens or perhaps hundreds of parts before it starts getting warped or damaged.  If another mold is needed, you simply cast a new one from the master.

The first step was to polish the surfaces (which I would have had to do without a screw-up anyhow), for which I wet sanded each half with 400 grit sandpaper to eliminate the ridges left from machining (I could have done a little better, as there are some ridges left, but I think it will certainly be good enough).  After that, I applied a coat of Partall #2 and buffed it off. After applying and buffing 4 more coats (to ensure that there are no missed spots – if the epoxy adheres to the mold surface itself, you have a ruined mold), I was ready to apply a coat of PVA. I diluted the PVA about halfway with distilled water and then brushed it on the surfaces with an acid brush, then set the parts aside to dry (you can see the green tint of the PVA pooling in low spots on the Corian mold halves).

Next I needed to dam up the sides of each half, so I used some sheet foam and hot glue, taking care to leave no gaps between the Corian and foam (lest epoxy leak out the side).

Then it was time to mix up some tooling resin, which is basically just epoxy mixed with graphite powder to provide a nice dark surface (which makes it easier to see when you have fiberglass properly wet out against the mold surface).  I used a little bit of West 404 filler as well to thicken the mix a little.

I carefully brushed the tooling coat over the entire mold surface and up the sides of the walls.  I then poured the remaining resin into each half, and took a blurry photo.

I tossed the halves into the oven for an hour on a very low temperature to help the epoxy ‘kick’ and start polymerizing (which is what actually causes it to harden).  With the tooling coat thickened, I was ready to fill in the remainder.  Since epoxy is expensive (and epoxy fillers aren’t cheap, though certainly less costly than the epoxy itself), I figured I’d try using aquarium gravel as an aggregate filler since it comes in relatively small bags for just a few bucks.  I mixed in a bit of 404 filler as well, though my mixes were a bit unbalanced – one half was gravel poor, while the other had an abundance.

The aquarium gravel turned out to be not as strong as I’d like (it chips and breaks easily), so perhaps I’ll just use sand as a filler in the future.  After letting the halves cure for a day, I used a utility knife to slice away the foam dams and a pick to dig out the hot glue.  In retrospect, I should have really used plasticine clay, as the hot glue was difficult to remove.  Separating the Corian master from the epoxy mold was simple, if brutal – just whack the block on a hard surface a few times until the epoxy half pops away (this was how I discovered that the aquarium gravel isn’t the greatest in a structural sense).

When I had a look at the molded halves, I was amazed at the level of detail captured by the resin – not only the minute milling ridges in the Corian and smoother areas where I had wet-sanded were visible, but even the edge of the puddle of dried PVA could be clearly seen.  I did a little more wet sanding on these halves, then applied 5 coats of wax, and then a coat of PVA (this time standing the mold halves up on end to keep the PVA from puddling).

Meanwhile, I realized I had neglected another part of the sailplane that will probably take a bit of wear from landings – the tail skid.  While there is a strip of plywood embedded, I’m sure large gouges in the foam will result the first time I miss a grassy landing strip and plow through a gravel driveway.  Rather than haul the fuse back over to Frankie’s studio to digitize the tail, I thought I’d try making a flexible mold with some OOMOO 25 silicone rubber that was past its shelf life – best to put it to use than throw it out.

I brushed it on the tail skid and threw in a few strips of fiberglass to give it a little more strength (a technique I recall seeing in a book long ago where strips of burlap were used to strengthen and stiffen a latex mold).  The silicone started setting up quickly, so I blobbed the remainder on and covered it with another piece of fiberglass.

Once cured, the silicone popped right off the tail skid.

I then made a plug in the silicone mold, using more aquarium gravel for bulk.

Just as with the foam, the silicone separated very easily from the cured epoxy.  Note that the texture of the original foam is perfectly captured.  The plug was thoroughly waxed, brushed with PVA, and set aside to dry.

Tonight I finally attempted using both the wing pod and tail skid molds to make actual parts.  I used a layer of 3oz and a layer of 1.4oz glass for the wing pod and mashed the mold halves together (I should clamp them together, but they seemed to be sticking together quite well on their own).  The 3oz glass wasn’t draping well over the tail skid plug, so I abandoned that weight and went with a layer of 1.4oz. and a layer of 0.75oz.  Trying to fit the silicone mold over the fiberglass covered plug was a bit tricky, as the shape doesn’t ‘key’ together as well as it could, but I finally called it good enough and set it aside to cure.  In 2 or 3 days I’ll see if all this work has actually yielded anything useful when I crack open the molds.

‘Composites’ (which for a very long time I mistakenly equated with ‘carbon fiber’) were always one of those mysterious materials to me – rare, expensive, difficult to work with, but with incredible strength. Something of a real-world parallel to Mithril or Adamantium, if you will. It turns out that there’s nothing really magic about it (though production of carbon fibers is still difficult and expensive) when you consider that the hard fiberglass chairs in a school cafeteria are composite (the fiberglass being the base reinforcing material and the epoxy being the binding matrix).  I always wanted to learn more about composite fabrication methods, and lately I’ve been learning more than I perhaps wanted to.

It started with stumbling across a video of some East European F3K competitors having a bit of fun with their DLGs (F3K is a class of aeromodeling competition with hand launched RC gliders). I never had any sort of interest in gliders whatsoever – if it didn’t have a motor, what fun could it be? But after watching the video and seeing the incredible capabilities of a 1.5m airframe that you hurl into the air yourself, I was intrigued. I attended a DLG contest in September to learn more about this particular segment of RC flying, and was smitten with the graceful purity of unpowered flight.  I now understand what some people see in ballet – I simply had to experience it in a form I could comprehend.  After the contest, I decided that I’d like to give it a try myself. Fortunately, Steve Meyer, one of the fellows at the contest (a seasoned competitor who actually qualified for the USA 2011 team) had an old plane that he was willing to sell for a good price and a few weeks later I had my own DLG to try out. Launching it the first few times was equal parts terror and exhilaration, but I started to get the hang of things quickly enough and even managed to catch it a few times (outside square loops on the other hand, well, I’ll give that one a few years). I gained more valuable experience in RC flight during the first half hour with the DLG than I had all summer. I then began to fly it every chance I could get, though the creeping wind and colder weather kept thermals to a minimum. On the last reasonably warm weekend of November, I was flying at the local park, trying to put more power into my launches. When spinning around, my fingers slipped off the launch peg, and I felt more than heard a sickening crack as the plane made a beeline right for the nearest swingset. In retrospect, I had enough time to get my fingers on the sticks and pull up, but was shocked enough that all I could do was watch the impact.

Would you believe that this is what it looked like post-impact?  Incredibly, airframe damage was pretty minor – a few small creases on the wings, ailerons and horizontal stabilizer.  Really a tribute to the sturdy design of Dr. Mark Drela and Aradhana Singh Khalsa’s beautiful execution of it in kit form (and certainly Steve’s expert assembly of the Light Hawk II).  The right aileron servo was completely stripped, but I hoped that it wouldn’t be too horrific to replace and ordered a new Dymond D47 while I surveyed the rest of the damage.

This is the worst of it – a crease on the underside of the right wing (with a slightly smaller matching crease on the top surface), and the reflective tape removed to reveal the stripped servo.  The servo has been glued in place, so I’ll need to Drem-mill (Drem-mill?  Dremel? Get it? Eh?  Er, never mind…) it out of the pocket.  Fixing the crease itself would probably be easy, but fixing it well seems a more daunting task.  Steaming small dents out of the wing (which is a high strength blue foam covered in Kevlar) works quite well, I found – just pour boiling water over the dent, and the foam magically reverts to an undamaged state (or use an iron pressed against a wet paper towel on the wing).  These creases however, don’t steam out – trust me, I tried.  So I’m saving them for later while I get up to speed with the basics of composites.

This is the left wingtip, where the launch peg is located.  You grasp the peg with your index and middle fingers, whirl around, and release the plane (a left handed pilot would have the launch peg on the right wing).  I think the crease in the Kevlar skin just above the peg may be the result of the sickening crack I felt/heard (honestly, I though the peg itself had snapped off).  From what I’ve read, this is caused by swinging your arm downward on release (a natural movement, as it’s how you would throw a baseball).  This makes sense, as the buckling happened only on the underside of the wing.  My plan is to poke a few small holes into the crease, use a syringe to inject epoxy into the damaged area and clamp it to a flat surface while it cures.

For anyone wondering “why is it made out of Kevlar – you expecting to be shot at?  Har har!” – this is the reason.   The nose took the brunt of the impact from what I can tell, and you can see the cracks at the 4 corners of the rear cutout of the pod where the epoxy matrix entirely failed.  If this was just carbon fiber or fiberglass, I’d have a lot of pieces and fragments in the photo and would be looking for a whole new plane.  Carbon and glass are strong but brittle, while Kevlar is extremely tough and the fibers will remain intact to hold the airframe together even with such damage.  The nose is one of the most interesting parts of a DLG, if for nothing else than the sheer density of components.  The battery is crammed all the way forward with two small servos behind it (I don’t even want to think of the difficulty in getting the control rods connected), followed by a voltage regulator and low voltage beeper, with the radio receiver bringing up the rear.  There’s even a ballast tube further back sized for tungsten weights, but I haven’t even gotten to that point yet.  The red cord coming off the side of the nose is the charging plug, which you remove to turn on the electronics (this eliminates a standard power switch, saving precious grams).  Custom wing airfoils, specific weave patterns of fabrics, CNC made molds for airframe components – these are standard fare for top competitors, making DLGs truly the F1 cars of the air.

So as a new driver with a broken F1 car, it seemed prudent to start small on the repairs.  The vertical stabilizer and rudder (light blue) were unscathed, but the tailplane (in yellow-brown Kevlar) sustained a very slight crease just to its left of the centerline.  It was in good company – Steve had pointed out the damage on the right side and had explained to me how he fixed it with a little scrap of lightweight fiberglass cloth and CA (CA is short for short for cyanoacrylate, better known as ‘superglue’ for those who don’t frequent hobby shops or RCG).  I steamed out the crease as much as I could, and then applied a small rectangular patch of fiberglass cloth wetted with foam safe CA (regular CA will partially dissolve polystyrene foam, so you have to be a bit careful with adhesive choices).

I used a hex wrench set as a weight to keep the stabilizer flat as the CA cured (a piece of kitchen plastic wrap keeps it from bonding to the cutting mat).

The fix turned out quite well, and I had to position this shot just so in order to glint the light off of the repaired area.  The ever-so-slight delamination of the Kevlar from the foam core is no more, and the tailplane is ready for action.  Still, the rest of the airframe remains.  I decided that getting up to speed with epoxy and fiberglass would be worthwhile before diving into more involved repairs.

One aircraft I’m preparing for the upcoming flying season is a Diamond 2500, which is a giant (at least as far as I’m used to) 2.5m powered sailplane. I chose it because my 2m Radian has gotten a bit boring, and because I wanted a good platform for FPV flying. The Diamond 2500 has a generous amount of room in the fuselage for extra gear, and I’m thinking of making a replacement canopy/cover that has the camera and associated equipment attached (so I can easily swap back and forth between camera ship and sport flyer). So the first thing I tried was to just make a rough copy of the existing canopy.

I started with a piece of plastic wrap taped over the canopy (it easily peels away from cured epoxy) and then draped a piece of 1.4oz (the weight for a square yard of material) fiberglass cloth over the canopy and carefully wetted it out with laminating epoxy by using an acid brush.  Once that layer was done, I thought I’d be adventurous and put down a strip of Kevlar tape (which is just the name for a ribbon of woven material – there’s no adhesive involved), which I also wetted out.  Finally, I added one more piece of 1.4oz fiberglass and wetted that out, then covered the whole thing with another piece of plastic wrap (hoping that it would help squish down the layers).  Well, I now see the value in vacuum bagging, as the Kevlar strip has air inclusions all along it (you can see that it’s only partially wetted out in the center).  I was trying not to overdo the resin (very dry layups are the norm when building DLG gliders – every gram counts), but without bagging, there’s no good way to eliminate the trapped air without simply adding more epoxy.

Once I peeled off all the plastic wrap and trimmed the edges, it didn’t look that awful for a first attempt.  Rather floppy, though – a little less rigid than if it were made from blister pack plastic.  Still, I had absolutely zero concept previously of how a completed part would feel, so now I know that more layers are needed to get the rigidity I need for the part (and that skipping the Kevlar tape is a good idea).

Now that I have some bits of fiberglass, carbon fiber and Kevlar, I realize how useful these can be in simple household repairs.  Superglue and epoxy by themselves rarely hold up as well as hoped, but adding a bit of reinforcement can do wonders.  On a replacement rearview mirror I purchased for my truck, the plastic tip was broken off in the package.  I figured a superglue-only repair would likely fail in short order, so I used a few small pieces of Kevlar and carbon fiber tow sandwiched between patches of fiberglass to act as reinforcement.  Dousing them all with CA and letting it cure provided a reasonably stout repair that has held up just fine for several months (and will probably outlast the truck itself).

Ugly, but far more expedient than getting a replacement (and being on the backside of the part, nobody will see it anyhow).