Last year this time, a five-man team including me were working on a mini tricopter for a competition which later got replaced by the mini quadcopter for unexpected reasons. In the span of less than two months (Dec 2013-Jan 2014), a bunch of guys who had never before been around a multirotor and watched months of partial failure with the tricopter which barely lifted a foot off the ground and turned into a three bladed weapon of mass destruction, actually pulled off a working quadcopter that complied with the competition rules.
Honestly, trying to build a mini FPV multirotor with a flight time of 15 minutes is quite a challenge for any beginner. To build a mini quad that weighs in close to a kilogram to do 15 minute flights is out of question. But, we did manage to build a 330mm FPV quad that flew 10 minutes+ on a 3S 1Ah LiPo which is not really bad.
How we did it is not complicated as it is, but it took a lot of prototyping and design optimizations before we saw any real exciting number like 400g AUW and 10 minute flight times. I’m sure there are better contraptions than what we managed, but I’m going to share one crucial thing about our quadcopter, its frame, just in case someone out there is trying to build something quick & dirty similar.
The quadcopter’s frame was the centerpiece of this puzzle, it needed to be very light and very strong — classic engineering paradox — and the final product was a light 28g and about 50 crashes strong fiberglass frame. We improvised on most parts of the glassing process, but you don’t necessarily have to.
1. Mould and Plug : The first step to most fiberglassing techniques is to prepare a mould. Moulds are made using many techniques, you could choose to do it whatever way you find easy or optimum.
We improvised on this step by hacksawing through square wooden dowels to make the mould. The plug was pretty simple, we made it out of dense foam, cut them ~2mm bigger than the mould and when you squeeze it in, it’s going to sandwich the fiberglass nice and tight. It all looks a bit crude and unappealing, but it worked! In fact, I’m still using the same mould to this day and I’ve moulded over 20 frames in it, more than half of it were successful. But, if we had access to a CNC mill at that time, we would have taken advantage of it, obviously, It has to be better than wooden dowels.
2. Fiberglass : Fiberglass comes in many forms – chopped strands, chopped strand mat, woven cloth, etc. If you’re choosing a very thin material, you’ll need to add more layers of it to add strength. If you’re using a thick material, one or two layers should give you good strength. Personally, I used 360gsm woven cloth, I found two layers to be optimum.
3. Resin : Resin is basically the moulding element, a liquid, it’s what gives shape to the fiberglass. There are many articles on the internet that tell you how to properly mix a good batch of resin and what parameters have to consider like temperature and humidity, it’s always better if you contacted the manufacturer/seller and consider their recommendations.
4. Preparing the mould & plug for glassing : A mould is something you use to transfer a particular shape to another material and once the shape transfer is down, the mould has to come off. In order to facilitate the separation of the mould and the moulded product, a release agent must be used. There are specific types of mould releasing agents available, wax is by far the cheapest, safest and easiest to use. It’s always recommend to use a release wax that’s made for fiberglass moulds, because it’s nice and greasy, makes it easy to rub it in to every nook and cranny of the mould and the plug– very important! If the mould & plug are not waxed properly, releasing the fiberglass is going to be a hassle and it may never come off too.
5. Moulding : Once the mould is waxed, apply a generous coat of resin on the mould surface, make sure you coat it and not fill it up with resin! Lay the fiberglass on the resined mould and soothe it into the contours of the mould gently, apply a coat of resin over the fiberglass cloth, that’s one layer. If required, add another layer of fiberglass over it, coat it with resin and repeat the same for as many layers as you need. Personally, I used 360GSM woven fiberglass cloth and two layers were optimum. Once the glass is laid out and resined is applied evenly, it’s time to plug the mould. Gently push the plug/plugs into place without displacing the fiberglass too much, excess resin may ooze out from the mould. Place a heavy weight over the mould to make sure the plugs don’t spring out-of-place and leave it there over night/whatever curing time is recommended for your resin.
This is the first mould we made using 10mm square dowels and the plug using 10mm dense black
foam, it’s the same foam used to make shoe insoles.
This is a completed mould.
Mould and plugs waxed and prepped.
After coating a the mould surface with resin, lay the first layer of fiberglass and soother it along the contours of the mould.
Plugging the mould. Be careful in this step to prevent the plugs from displacing the fiberglass to much. Press them into the mould gently.
After leaving the mould to cure well, carefully ease the fiberglass it out of the mould with the plugs still in it. The fiberglass is not fully cured at this point, so it’s not as strong as it should be, pulling it out of the mould without the plugs in place can distort it.
Fully assembled quadcopter with APM 2.5 and Tarang 2.4GHz telemetry module.
Autonomous flight capable.
425g including FPV equipment and a 3S 1Ah Lipo pack.