Nerdy Derby

In August, Pete posted an innocuous message to the Milwaukee Makerspace mailing list: Now that Power Wheels is (mostly) over, we should turn our attention to this: I mean, we’d need space to do such a thing, but I think it would be awesomely cool. Who’s in?? Hmmm…  A ‘no rules’ pinewood derby?  I […]

In August, Pete posted an innocuous message to the Milwaukee Makerspace mailing list:

Now that Power Wheels is (mostly) over, we should turn our attention to this:
I mean, we'd need space to do such a thing, but I think it would be awesomely cool.
Who's in??

Hmmm…  A ‘no rules’ pinewood derby?  I was never in Boy Scouts, but racing cars certainly was appealing to any boy (and I raced many many Hot Wheels cars in my youth, with extravagantly banked and looped tracks, often borrowing construction elements from Lincoln Logs, Legos, Tinkertoys, and whatever else was at hand).  What better way to relive my childhood than to downhill race blocks of wood in the same fashion as I did with Hot Wheel cars with friends and cousins: without any rules.

Discussion on the mailing list quickly dove into the underlying physics, and how heavier cars will lose less of their overall potential energy to friction, which led many people to work on designs maximizing mass (and then resulted in questions on how to do lead casting – the competition was serious for the event).  My own thoughts were originally along this line as well, thinking of building a car out of steel plate with rails to keep it on the track.  But then I figured that just adding power to the car was really the best solution – the heaviest derby car still has only gravity to propel it, and a lightweight (though powered car) should certainly be able to best it.

Powering the wheels was the obvious method, but I had never messed with RC cars, slot cars, or any other powered car toys enough to know what sort of motor/transmission system they used.  The closest I had come was years ago when helping someone with concepts for a mousetrap racer.  My idea was to have a foam cone on the drive axle around which the string from the driving arm would be wrapped.  The string would first pull from the large end of the cone to maximize the torque and get off the starting line faster, and as the string unwound, it would be on successively smaller diameters to decrease torque and increase speed.  But how to determine the best shape for the cone, and what would be the best power source?  Rubber bands?  Torsion springs?  And how to make sure that power isn’t engaged before the starting gate drops?

I quickly discarded that line of thinking in favor of just brute force – stick a model rocket motor on the back of the car.  Of course, that would probably run afoul of the one rule imposed in an otherwise ‘no rules’ race – don’t damage the track.  While I could argue that scorch marks are merely cosmetic, it probably wouldn’t fly with race officials, and the open flame indoors may not be a great idea.  Plus, how to trigger it only after the start gate?  Someone else on the mailing list linked to information on CO2 powered derby cars, which are most certainly fast, but any bit of off-center thrust gives them a bad tendency to leave the track.  Given that there is a hump right in the middle of the official Nerdy Derby track, it was very likely that such a car would become airborne.

I then recalled a novel race car I had read about as a kid – the Chaparral 2J.  What made it so interesting was that it was built as the opposite of a hovercraft, so that a blower unit would actually evacuate air from underneath the car, keeping it glued to the road surface even when at low speed (compare to a modern F1 car that can generate tremendous downforce via aerodynamics, but only as a function of its speed).  Of course, the Chaparral 2J was banned from competition very quickly – sounds like just the thing for a ‘no rules’ race.  What to use for a fan suction system, then?  Well, a ducted fan seemed a logical choice – I hadn’t yet used one in any of my RC planes, but this seemed like an excellent opportunity to experiment.  If I angled the ducted fan back on the car, I could use it for thrust as well as forcing it down on the track.  With a propulsion method in mind, I briefly turned my attention to the wheels.  It sounded like plain old off-the-shelf pinewood derby wheels were being used by most other builders, but I figured disc style wheels with ball bearings could only help.

With the basics in mind, it was time to start designing.  Charles and Frankie were also building cars, so we met up at Frankie’s studio a few weeks ago for a day of design, construction, and the bull session that invariably occurs whenever the 3 of us get together.  Frankie was already well underway constructing his belly tanker design, and Charles had some solid design concepts sketched out for his own car.  I had only the vaguest notion of what I wanted to achieve and a copy of SolidWorks – okay, time to get designing.  I started out with the track itself, and drew up a section about a foot long.  Wheels around 1.25″ diameter felt about right, and I roughed out a model of the EDF unit that I had in mind for the car.  Throwing them all together netted me a skeleton of the main components.

Given that the car’s stance was pretty narrow, I figured adding a set of outrigger wheels would be a good idea – just in case the car lost its footing due to the EDF’s torque.

Most of the day was spent on hashing out the body itself.  What I originally envisioned was something sculpted, smoothly flowing and elegant yet powerful, like a Lola T70.  I may have fallen a bit short in that department.  I designed the parts to be laser cut out of 1/16″ plywood on one of the makerspace’s laser cutters, and figured that I’d somehow use 1/4″ aluminum tubing with 5mm fasteners for the axles (with appropriate flanged bearings pressed into the wheels).

I converted the models of the plywood pieces to be cut into DXF files and then arranged them together into a single panel that would span a 12″ wide piece of plywood.  The DXF for the sheet is here.  I stopped by the space on a Thursday night, but only the 25W laser was operational, and it was only able to be run at half power.  As it turned out, 12.5W was simply not enough – even after multiple passes, the poor little photons had managed to fling themselves no further than 1/3 of the way through the plywood.  I took the piece home, expecting that maybe I could cut the rest of the way through with a hobby knife and jeweler’s saw.

Fortunately, I didn’t have to resort to that, as a week later both lasers were fully functional, and Shane was just finishing up one of his nifty laser cut/engraved card boxes.  He helpfully assisted me with the finer points of the big 60W laser, and I was off and running.  Wow, what a difference!

Even through the protective window of the 60W unit, its vigor was unmistakable.  There was a little more charring than I would have liked, but I’m not about to complain – having it cut all the way through beats the heck out of cutting it by hand.  At home, I broke the pieces free.

Next step, I needed wheels.  I found a length of 1.25″ diameter plastic (some sort of phenolic, I can only assume), chucked it in the lathe, and started cutting.

I drilled a hole in the stock before parting it off to provide (hopefully) a decent diameter to press-fit the flanged bearings into.  My parting tool has a bad tendency to drift to the left as I part off stock, so I needed a way to true up the wheels to be entirely flat on each side.  I figured I’d try machining an expanding arbor to hold the wheels, as it’s been rattling around in a tool drawer for years, awaiting a purpose.

While it machined really nicely, it didn’t work out well in practice.  The plastic wheels still flexed quite a bit while being cut, and wanted to walk off the arbor as a result.  It was now getting down to the wire, and race day was approaching.  Scratch that – race day was here.  As of Saturday morning, I wasn’t sure if I’d be racing at all, but I’d certainly make a go of it.  After soldering bullet connectors onto the EDF unit, I was ready to glue together the shell of the car.

I used CA to bond the plywood together (having used copious amounts to seal/strengthen the parts themselves) and hot glue to mount the EDF unit.  While waiting for the CA to cure, I headed to the garage to do more work on the wheels.

Good old carpet tape worked well enough to machine the wheels to thickness – just go slow, take light cuts.

7 wheels (always make more than you need, because you just know you’ll mangle one during subsequent operations).  However, Friday had brought a potentially devastating bit of news – the raised center section of track was 1.65″ wide, and not the 1.5″ wide that I had designed to.  Much wailing and gnashing of teeth ensued.  Fortunately, I had designed the body after the wheels, and as such the wheels could actually be moved outside of the body with no ill effects. Realistically, this this even better, since having the wheels external would remove the need for outrigger wheels, and internal wiring wouldn’t have a chance to get tangled up in them.  Also fortunate was the fact that I am a horrific procrastinator, and since I hadn’t yet gotten to the stage of machining axles, I was free to modify my design as needed.

I drilled out some pieces of 1/4″ thick wall aluminum tubing and tapped the ends for 5mm threads.  I also cut some spacers so that the bearings wouldn’t run against anything else.  I then started packing the inside with components – a 22A ESC, a 2.4 GHz receiver, and an 800mAh LiPo battery (all credit to Charles for suggesting that I stick the battery inside – I had planned on running it externally for ease of access until he convinced me otherwise).  Ideally the ESC and battery should have been rated higher, but for burst use (under 3 seconds from start to finish), these components should hold up fine.  Besides, there’s always the ‘epic fail’ prize category should something go wrong.  Internal accessibility was not considered at all in the design phase, so the right body panel is held on by the axle spacers and wheels.

So 30 minutes before heading out the door to Bucketworks to attend the Derby, this is what I had.  A radio that I was using when attempting to fly my latest RC plane on it’s first and last airborne adventure (maybe I shouldn’t have transferred the ‘Centrino Inside’ and ‘Windows Vista’ stickers from my laptop to it), and a car held together with hot glue, CA, a few pieces of fiberglass fabric, and medical tape.  Perhaps MacGyver would be proud.  Or appalled.  Eh, whatever.  The car was assigned #32 as a race sticker, and I had a glance at the competition, which looked tough.

Pete’s ‘Raster Mobile’ certainly had mass, and Brant’s ‘Safety Car’ had lots of lights.

Fortunately, Kevin B’s “car” was deemed by race judges as illegal.  However, I admired his extension of the rules.  “Roads? Where we’re going, we don’t need… roads.” Brew’s Dorito-laden vehicle was a crowd-pleaser when it invariably ejected salted snack chips at the end of a run.  Unfortunately, Jim’s ‘Sparky’ was having electrical issues and wasn’t running nearly as fast as it had been on test runs.

Brent’s ‘Pootystang’ really had me worried.  A full diameter propeller will easily best a ducted fan unit in thrust, and he looked to be using the same battery as I was.

Ed’s “Aghghhhh!” was the best (only?) 3D printed car in the race.  [edit – Pete’s ‘Great White’ was also printed – forgot about that one!]

Charles finally gave his (highly polished after 5 hours of work) car a name of ‘2 ways’.  I added ‘both of them wrong’ to it (not shown in photo).  In the photo, it appears to be heading the right way (right to left).  He claims that the front end is actually the right side of the photo, for which he deserves to be mocked endlessly.   I think Rose entered ‘5 Rings’ and Dillon had entered ‘Indus’ (which was based on a pull-back racer).  Since we were to name our own vehicles, I decided to call mine “Waste of a Perfectly Good Afternoon” (that’s all that it was until that point).

Fortunately, I was paired against Charles for the first 2 heats.  He gave me a sporting chance by running his car in reverse – he keeps claiming that’s the front end “like a Studebaker”, but I know better.  I applied the throttle just enough to keep my car in the lead (seriously, I was worried about something erupting into sparks and flame – this was a 22A ESC with an EDF unit that can draw 33A).  This got me into the finals (thanks, Charles!  Sucker…).  I raced Jim’s ‘I Win’ and Brent’s ‘Sneaky Weasle’, which was amazingly fast.  Every time, I throttled up just enough to ensure that I was in the lead, but no more – damage to the car upon contacting the deceleration zone (a chunk of open cell foam at the end of the track) had me worried, as one race broke the EDF unit free of the body, necessitating emergency hot glue gun surgery.  In the end, I was in the lead at the final race, and gave it full thrust from start to finish to see just how quick it could go – 1.595 seconds was the elapsed time.  Incredibly, it survived intact!  After that, it was requested to see if the car could run the track in reverse and actually make it up the hill.  Our intrepid race official quoted Doc Brown’s famous words, the crowd gave a countdown, and I floored the throttle, easily sending it off the far end (thankfully Jim was there to catch it).  After that, there were the awards.

Frankie is out in Denver at the moment speaking at a conference, so this award certainly went to his belly tanker.

My car not only won as the ‘fastest’, but also captured ‘the noisiest’ and ‘best name’ awards.  No trophies or cash prize – just bragging rights.  As such, I’ve been telling attractive females that I’m a 3-time auto racing champion, but when they learn the details, they tend to edge away nervously.  *shrug*

Successful pod covers

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 […]

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!

Shaving weight on the Diamond 2500

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 […]

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!


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:
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

and this is the trouble I got into with it


Fixing a major mistake

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 […]

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.

Reverse engineering for molds

On the Diamond 2500 powered sailplane, there are small ‘pods’ on the underside of the wings in front of the servos for the flaps and ailerons.  I’m guessing that these pods are intended to serve as some form of protection for the servo arm and linkage on landing, but the problem is that the pods […]

On the Diamond 2500 powered sailplane, there are small ‘pods’ on the underside of the wings in front of the servos for the flaps and ailerons.  I’m guessing that these pods are intended to serve as some form of protection for the servo arm and linkage on landing, but the problem is that the pods will then be torn to smithereens (being foam, just like the rest of the wing).  While my quest to protect these 4 measly foam bumps seems to be ever-increasing overkill, it’s turning out to be a fun project and I’m learning a number of new skills from it.

While there were several ways to approach this, I decided to try making conformal covers for these pods out of fiberglass.  I could have just applied fiberglass directly over the pods, but I wanted to try something a little more precise (and replaceable, though I don’t know why I’m clinging to that notion when the plane is likely to be damaged in far more horrific ways).  I’ve been watching Tom Siler’s work on building his own fully molded F3K competition planes, and his videos are fascinating. He uses Corian for his molds, as it machines really nicely and is easily sanded and polished to provide an excellent finish on the composite parts pulled from the molds. I don’t yet have a vacuum system to bag parts, but I figured if I made a 2-piece mold, I could perfectly form pod covers without needing any sort of vacuum.

First things first – I needed to model the pod in SolidWorks.  Normally I just grab my calipers, radius gauges and other measuring tools, but the pod had me stymied – it’s a more complex feature than I initially thought and isn’t as simple as a truncated swept profile.  What’s worse is that it’s located on an airfoil, so I don’t even have a flat plane to reference.  I started to consider making a rubber mold of the pod, then casting an epoxy plug from the mold, then digitizing the plug with the touchprobe I have for the Taig (but have yet to finish wiring up), but that was turning into quite a production.  I remembered that one of Frankie’s toys is a NextEngine scanner, which would be perfect for this application, so I took the wing along during one of our Zcorp hacking sessions.

First step was to position the wing in front of the scanner itself.  The base will automatically rotate in increments if needed, but I just needed a 1-pass scan.

Once in place, let the scanner rip – a few of the laser beams are visible sweeping over the scan area, and the monitor screen shows a rough pass of the scanned pod.

I brought the generated STL into MeshLab and did some minor cleanup before bringing it into SolidWorks.  SolidWorks actually has some impressive mesh-to-surface capabilities, but since I was working with a mesh with a few holes in it, it would have taken a bit of work to get usable output (and I didn’t see a way to define a symmetry plane, but maybe I didn’t look hard enough).

Instead, I did my own surfacing, which took me quite a while.  I’m not good at it, and I know some of my techniques are wrong, but the final output should serve its intended function.

After finishing the male side of the mold, I thickened the surface by 0.010″  (I figure that should be plenty of fiberglass) to create a solid and extracted the far surface as the female side of the mold.  I set the two halves side-by-side in an assembly and exported it to GibbsCAM.

Once in Gibbs, I created my toolpaths (this shows the paths for the second operation, which uses a 0.250″ ball end mill).  After posting the file, I was finally ready to start cutting material.

Not having yet found any 1″ thick scrap Corian (everything I’ve gotten is 1/2″), I glued two pieces together.  The cold temperatures meant that the epoxy hadn’t fully cured after 24 hours, so I stuck it in front of a space heater for a day, and that firmed everything right up.

I drilled and counterbored mounting holes and then bolted the block to the tooling plate on the Taig.  This shows the results of the first pass, which was roughed with a 0.250″ flat end mill.  Note the curvature in the parting plane to match the airfoil surface.

This is the third and final pass, which used a 0.125″ ball end mill (and a generous amount of WD-40 as cutting fluid).

Once washed off, this is the result.  The pattern of the Corian makes it impossible to see any fine detail in the photo, but the surface finish is phenomenal – I used a 0.010″ stepover for the final pass (overkill, but it’s my CNC, so I’m not paying any extra for machine time) and it looks superb when you hold the machined surfaces up to the light.  All that remains now is to chop the two halves apart, then sand and polish the mold surfaces.

Just one word… Are you listening?… Composites.

‘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 […]

‘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).

Quick CNC work

I’m always impressed by Frankie’s ability to machine one-off parts on his Taig with minimal time spent on generating the toolpaths, which is something that I want to become better at. I’ll frequently over-think and puzzle over the CAM side so much that I wind up just bashing out parts manually on the Tree. However, […]

I’m always impressed by Frankie’s ability to machine one-off parts on his Taig with minimal time spent on generating the toolpaths, which is something that I want to become better at. I’ll frequently over-think and puzzle over the CAM side so much that I wind up just bashing out parts manually on the Tree. However, for a recent project I needed to do a lot of cutting in odd shapes that would be crazy to do manually for a single part, so I gritted my teeth and dove in.

As part of my growing RC aircraft addiction, I had purchased an Ikarus SU27-XXL kit as a fun ‘zoomy’ plane to advance beyond my trusty Slow Stick. My original plan was to use the brushed motor included with the kit – why discard a perfectly good motor, even though it may not be as powerful and efficient as a brushless one?  Well, a good reason is that brushed ESCs (electronic speed controller) are much more difficult to find these days than their brushless counterparts.  So I went brushless anyhow and purchased a motor and ESC.

Since the new 400 brushless was mounted at the base rather than the face, I couldn’t use the included light plywood motor mount.  I needed to build my own custom motor mount, and I happened to have some 3″x3″ squares of 1/16″ G-10 fiberglass sheet left over from a project that would make for very sturdy construction.  I drew up the needed parts in Cadkey and then tinkered with GibbsCAM at work to hopefully output usable toolpaths.  Fortunately, I’ve gotten much better in this regard, and the G-code worked out just fine.

I used a hunk of scrap polycarbonate bolted to the tooling plate on my Taig as a sacrificial base.  Frankie recommended using carpet tape to hold sheet stock flat for machining, and it worked like a charm.  I’m using a 1/32″ carbide cutter to do the milling – I think it cut through the sheet in 3 or 4 passes.  When done, the parts were easily pulled off of the base.

After removing the tape and adhesive with a ‘Goo Gone’ type of solvent, here’s the parts I had.  As it turned out, I could have skeletonized them far more than I did, and using 1/32″ G-10 may have been an even better material.  I did have to do a bit of filing by hand to make things fit – the original plywood parts were laser cut and so had perfect square cornered slots, which obviously can’t be done with a round endmill.

Glued together, it looks pretty good!

Fits beautifully on the plane and certainly looks like the beefiest part of the entire airframe.

Unfortunately, she would never look this good again…  On the final flight (just after I had moved up to a larger prop that finally provided the performance I wanted), I pulled out of a fast low level loop right into a tree, and the brittle Depron foam snapped all over.  The motor and mount tore free from the plane and was unscathed, however, so I’ll be dropping the unit into a scratchbuilt MiG-29 made out of pink sheet foam from the home improvement store.  I have a feeling that the motor mount will easily outlast that airframe as well…

Yeah, like I need another hobby…

It seems that, as a boy growing up in the 80s, I was not alone in having an interest in R/C airplanes.  R/C anything, really (where R/C meant ‘radio control’, not the laughable ‘remote control’, which was toy company code for ‘has a 10 foot wire between the controller and vehicle, and anybody who buys […]


It seems that, as a boy growing up in the 80s, I was not alone in having an interest in R/C airplanes.  R/C anything, really (where R/C meant ‘radio control’, not the laughable ‘remote control’, which was toy company code for ‘has a 10 foot wire between the controller and vehicle, and anybody who buys it is a sucker’) – cars, boats, tanks, airplanes, helicopters, whatever, but particularly R/C airplanes.  I did get a control line P-51 as a present one year, but it never took to the sky, perhaps as part of my disdain for any sort of tether between plane and operator.  Looked good hanging from the ceiling, though, where it perennially engaged in a static dogfight of sheer will against a plastic model UH-1 Huey – first to fall due to weight of accumulated dust lost air superiority.

I had several issues of R/C Modeler, which was like crack cocaine for young imaginations.  Within the tattered and well thumbed pages were some articles (filled with technical jargon of unknown meaning) interspersed between glorious advertisements for incredible flying machines (and cars and boats, too – the ‘glass filled nylon’ bit in the Tamiya Hornet ad confused me for a while until I learned about fiberglass composites).  I recall ducted fan scale models of the fighter jets that graced posters plastered around my room, brightly colored aerobatic planes (which I didn’t pay much attention to – anything displaying a propeller rather than an afterburner didn’t merit much thought), and quite possibly the coolest thing ever, a scale R/C version of Airwolf, the baddest-ass helicopter EVER (although Blue Thunder runs a close second).

All of this airborne fantasy had one big drawback, however – price. I understood the hobby well enough to know that you had to have a lot of gear – an engine, radio, receiver, servos…   …oh, and the airplane itself.  I also understood the hobby well enough to know that crashes were a royal pain when repairs were needed.  Building a fuselage for a free-flight plane taught me that repairing cracked balsa gets old really fast, and the stuff will invariably warp on you.  Over the years, I’d thumb though other R/C magazines at the library, and I’d go to a few of the charity exhibitions held by the local R/C club to get a taste.  I even modified a ‘buddy box’ so I could play with an R/C Simulator on my computer. Eventually, it all sort of drifted out of consciousness.

Fast forward to this past Labor Day, where I went to a party hosted by my friends Chuck and Molly, with lots of kids in attendance. At one point, we all headed down to the local park to launch some model rockets, courtesy of Chuck’s sister. Chuck also brought along his son’s little 2-channel Air Hogs type of R/C airplane. Due to the slight wind, none of us could keep the plane in the air for more than a few seconds, but I was fascinated by its simplicity and manufacturability (i.e. low cost). I had a hunch that while I hadn’t been paying attention, R/C airplanes suddenly had become far more affordable than in my youth.

Sure enough, after a bit of web searching, it became apparent that advances in battery technology coupled with sturdy, lightweight foam and cheap outsourced manufacturing had yielded the ideal starter R/C aircraft, at a price that even a global economic meltdown couldn’t make unattractive. There were a number of such RTF trainers I found, but the least expensive (hey, ‘affordable’ is no reason to stop being a cheapskate) looked to be the Firebird Phantom. Several of us took the plunge, and we ordered 4 of them and a bunch of extra batteries. While waiting for them to arrive, I tried to consume as much information on them as I could, and I watched Dave Herbert’s Youtube videos on the plane (I figure if a guy who has been in the hobby for over 3 decades thinks it’s a good starter plane, I can feel confident about the purchase).

Upon receiving the package, I knew that the first thing to do was to strengthen the wing, so out came the strapping and packing tape. I even removed the stickers on the wing to reduce weight (a bit silly in retrospect, like being horribly out of shape, deciding you want to compete in the Ironman, and shaving your legs to give you a 0.6 second edge during the swimming phase of the event). I then charged up all 3 of my batteries, and eagerly awaited the next day.   After work, I excitedly rushed home, grabbed the plane and batteries, and headed over to the park we had been at on Labor Day.  I found an unused ball diamond in the park (crucial, as any witnesses to what was about to be attempted would undoubtedly result in exponential embarrassment) and quickly assembled my aircraft.  Despite the overwhelming possibility of failure, I hurled my charge into the air, then furiously worked the controls to adjust throttle, pitch and yaw.  Though my attempts were valiant, ‘soaring”, ‘majestic’, and ‘skyward’ were not adjectives applicable to the flight that followed.  After an ‘air time’ that generally requires the precision of an atomic clock to accurately relay the brevity of, the craft was reunited with terra quite firma.  Foam is nothing if not resilient, so second and third attempts were quickly mounted.  This third flight (I am of the opinion that any object, be it a tossed coin or a hummingbird, not touching a static, grounded item, may be considered ‘in flight’; issues of control, intent, and the ever-pesky ‘lift’ nothwithstanding) met with ‘arboreal interference’.  After throwing increasingly large sticks at the restraining limbs, I rooted around in the brush to find a 15 foot branch suitable for extracting the plane.  I was back in the air in no time (and nose-down in the ground in even less).  My final flight was perhaps the most dramatic, culminating in two full loops interrupted once more by tree branches.  This proved a disastrous end – though the plane miraculously managed to escape from the branches, the propeller was nowhere to be found.  Thus ended flying for the day, as well as the week.

After the disappointment wore off, I went back to the web to watch more videos and research what others had done with the plane.  I found that there were 3 things that had contributed to a less-than successful outing: Wind, space, and wing.  Though the wind was pretty low that day, it appeared that ‘dead still’ air was really what I wanted as a raw beginner without an experienced pilot to help me.  Additionally, more space (free of aircraft eating trees) was needed to allow for more altitude, larger turns, and simply much wider error margins.  Finally, a number of people commented that the stock wing was rather ‘fast’, and that a larger wing would slow down the plane and make it much more docile.

At the local hobby store, I looked at the selection of foam wings, and found a ‘Sky Fly’ wing for about $12 that had a generous surface area, more dihedral than the stock wing, and simply seemed a bit sturdier than the stock unit.  I then cut out the rear center of the wing to fit it to the Phantom.  Still without a prop, I did a number of hand tossed glides with the new wing to see how it ‘felt’ and whether it was too nose heavy.  I did some more launches with the stock wing and thought I could see a bit of difference – the stock wing felt ‘twitchier’ than the big one, and was more prone to rolling.  I also took the opportunity to tear out the ACT sensors, as the prevailing opinion seemed to be that ACT caused more problems than it solved.

At home, I started looking into better flying sites.  There are at least two local R/C clubs, but in order to use their fields you have to be an AMA member ($50/year) in addition to being a club member (another $50/year), and abide by 2 pages worth of rules and regulations.  I wasn’t interested in shelling out almost twice the cost of the airplane itself just for the privilege of flying it, so I spent time with Google maps to look for nearby wide open areas.  Local parks were the first place I looked, and I first checked the ball field where I had made my first attempts to see roughly how big the place was to serve as a baseline comparison.  Unfortunately, though parks are generally quite spacious, they tend to have a great many trees.  A local factory had a sizable vacant weedy lot nearby, and I wondered about the local soccer park as well.  I started a placemark map which I then shared with Chuck and Jared and we noted our discovery of possible sites with each other.  One mysterious location was what looked to be an enormous (32 acres, I’d later find) wide open field, just a few minutes from home.  Switching to Google street view, I ‘drove’ to a point where this field actually ran all the way up to the road, and I noticed what appeared to be an informational sign about the area.  Street view unfortunately doesn’t have a high enough resolution to read the signage, so I drove over for a look.  As I had guessed, the sign was indeed about the site, which turned out to be a floodwater retention basin operated by the city sewerage district.  I placed a call to the district office to find out if it was okay to fly an R/C plane there, and eagerly awaited a response.  A helpful gentleman gave me a call back a few hours later after checking with their legal department and said it was fine by them as long as I wasn’t a nuisance, and “hope you have fun!”

Not able to stand the wait for a replacement prop for much longer, I ‘borrowed’ the prop from Jared’s plane, as I had yet to deliver it to him.  I also opted to move the control rods down to the position closest to the control surfaces for maximum movement (this luckily turned out to work very well with the large wing).  Around 7pm, I drove to a side street bordering the field with the plane, Sky Fly wing, and freshly charged batteries.  I was amazed at the field seeing it from this vantage point – if I didn’t know any better, I’d swear that it had been designed for R/C flying.  A berm surrounded the basin itself, with a mowed path running around the perimeter.  The basin was covered in thigh-high grasses, making for soft landings all around.  And I had the entire place to myself.

With the wind at a standstill, I revved the motor, threw the plane into the air…   …and it was a completely different airplane from the one a few days before.  I could actually control it this time, making slow circles around the field.  Most importantly, it was enjoyable rather than aggravating.  By the time I was on the third battery, I was feeling very confident, making flyby passes and enjoying the realization of a long-desired experience.

I had a few more flights (and certainly more crashes) with the big wing, and then finally broke the boom, which seems to be an expected occurrence with this particular model.  But the other half of flying is ‘fixing’, and I have a feeling that the only R/C planes without battle damage are the ones that have hardly been flown.  The problem with the boom is that there is a slot cut into the top of it just forward of the tail where the control rods exit.  This is a very clean design, but the cut significantly weakens the (extremely thinwalled) carbon fiber tube.  Although there’s a plastic stiffener glued to the boom around the slot area, it is of little help.  I found that 5/32″ thinwall brass tubing slipped inside the boom perfectly, so I epoxied in a piece of about 3 inches to join the pieces back together.  I then glued another piece of carbon fiber tubing along the top (covering the slot).  I went a little crazy and also wrapped some kevlar line through epoxy around the front and rear of the added tube.  I don’t expect any further breakage.

The strapping tape is there just to hold the antenna wire - it has nothing to do with the boom repair.
Control rods now exit the fuselage just above the boom.

With the slot now covered, I had to route the control rods through a hole drilled through the back of the fuselage just above the boom and through the top carbon fiber tube.  This kept them as low as possible, which is needed now that I have added a larger prop (some forum posts had said that the Phantom has a 2.3mm motor shaft, but this was not the case on mine and I needed a 2mm adapter instead).

Antenna now runs out the front of the nose.

I tried to snake the antenna back through the boom, but just couldn’t get it (how the manufacturer managed to get it and the two control rods through the boom is beyond me).  However, since I had a handy hole in the bottom of the nose as a result of removing the ACT sensors, I just ran the antenna wire through it and used tape to keep the antenna running along the outside of the boom.

As a result of the added weight, I had to rebalance the plane a bit.  I started by adding some noseweight (a screwdriver tip taped inside the canopy), which worked well, but with the noseweight, repair weight, and weight of the large wing, it struggled for altitude.  I had better results by simply shifting around where the battery sits, moving it forward into the nose.  After a bit of flying in this manner, I was ready to move back to the stock wing.  The big wing had given me some much needed confidence and I was able to fly fairly happily with the stock wing, noting that the added maneuverability came at the cost of dropping out of the sky during very tight turns.

It’s not always a special feeling to fulfil a childhood desire. But in this case, it was.