C1: Going Tubular

This is the first of at least 3 projects I have planned for the winter. All designs are based on having a tubular body/frame. Once again, I’m going with a cylinder to reduce air resistance as much as possible. However, since the axis of the body is in the plane of movement most of the time (unlike the arms), resistance could be reduced further by going with a fairing shape where the top of the body stays rounded and the tail is tapered (like a teardrop). This might be something to revisit later on.

And yes, the front and back are made from plastic Easter eggs… If I see anything that might have a good application, I’ll use it.

Main Objectives:

1. Design a body in which the battery can be inserted/removed
2. See how well 6mm arms hold up – the carbon fibers run along the axis of the arm (not woven fiber) which makes it unbelievably strong. However, torsional stiffness suffers.
3. Create a symmetrical design so center of gravity, center of thrust, and center of pressure are all aligned and geometrically centered. Result was a “Z frame (see advantages of a Z frame)
4. Try out 3d flying (much later on in the project)

Update Jan. 3, 2017:

C1 is now on the back burner but the C2 is still alive (click here to jump to the C2 section). I was asking too much when cramming the ESC’s next to the battery. It was a tight fit, and apparently the battery must have pressed against one of the ESC’s too hard and caused a couple of capacitors to loosen and short out on each other. Once I revisit this frame, the plan is to mill out slots on the side of the body large enough for the speed controls to sit in. But for the time being, my attention has been turned to the C2 and the T150 (speed project).

Small Arms

These arms are small. They actually feel very strong when trying to bend inward towards the body, but with the pylon style motor mounts, they act like a torsion spring.  I still don’t think there will be any problems until I crash it. But who knows, maybe the springy motor mounts might act like a spring on a shock absorber? Probably just wishful thinking. One thing is for sure – as soon as I break an arm, I will move up to an 8mm arm.

Another issue with these arms is that I had a hard time pulling wire through them. I actually went to a solid wire so I could get just as much amp rating but in a smaller diameter wire. Even doing so, I had to lubricate the outside of the wire to pull them through.

Battery Modification

The first item that had to be addressed, was to fit the battery connectors (once connected) into a relatively small space. I had to cut the battery leads as short as possible and then solder the connector back on at a 90° angle. It was then secured to the battery with electrical tape.

A similar approach was taken with the quad side of the connector.

Once the battery is inside, it’s secured in place with a plate. A slot is cut in the top and bottom of the body. The bottom slot is left covered with tape. After the plate is slid through the slots, tape is placed over the top slot as well. Tape is good enough in a situation like this.

Electronics

Electronics list:

•  Although shown with the T2’s, I am going with the Emax red bottoms
• ESC’s are the Spedix ES 20 Lite. Although I had a rotten one in the 4 that I bought, I love this little ESC. When controlling the motors in Betaflight, I would say the Spedix are smoother than my Racerstar 30A V2’s (cicadas). A review of the Spedix ES 30 HV ESC is here
• Seriously Dodo flight controller
• Openlog Blackbox data recorder
• Crazepony 700TVL FPV Camera
• Vtx, OSD: unsure at this time

Keeping it Cool:

Unlike the C2, the ESC’s have no room to breathe. I assembled 2 ESC “modules” that sit on either side of the battery and making them as thin as possible. This enabled the battery to fit in, but just barely. I think the solution here is going to be two windows on either side of the body. I’ll then attach a heat sink on the ESC’s  and have the fins stick out the window (much like the V2/V3 project).

C2

This was being built in tandem with the C1. This is basically a “safer” version – not so fragile and cramped.

Differences from the C1:

• 8mm diameter arms
• Polycarbonate main body reinforced with carbon fiber
• Larger diameter body (50.8mm vs 47.6mm) which made a huge difference in terms of room for the electronics
• Bottom loading battery
• Rear arm is one piece since the battery does not load from the back
• Breakaway replaceable motor mounts to prevent arm breaks
• Overall much stronger construction

Frame Materials:

• 2 inch OD, 1-7/8 inch ID polycarbonate (found at Amazon)
• 1-7/8 inch OD, 1-3/4 inch ID carbon fiber (found at McMaster – expensive!)
• 8mm OD, 6mm ID carbon fiber (found at Amazon)
• 10mm OD, 8mm ID carbon fiber (found at Amazon)
• 10mm tube mounts (found at Ali Express)
• 2mm x 5mm aluminum (found at Mcmaster)
• 2 Large plastic Easter eggs (kids are best at finding these)

Total price: $111.13

Although the 1-3/4 inch carbon fiber is expensive, there is plenty left over for more projects. However, I found a much cheaper (and more practical) material to use instead of the polycarbonate/1-3/4 inch carbon fiber combo:

• 2 inch OD, 1.93 inch ID Aluminum Tubing (found at McMaster)

Price of frame materials using the aluminum tubing: $38.11. Best thing is that it ends up weighing roughly the same as the polycarbonate/carbon fiber combo.

I will be using the aluminum tubing on the next project, the C3 project, which will be weatherproof.

In order for the pylon mounts (10mm tube mounts) to work, small 5mm long pieces of the 10mm OD carbon fiber had to be cut to slide over the ends of the arms and epoxied into place. A more detailed explanation as to why I used a pylon design can be found here.

Breakaway mounts:

In the pursuit of an efficient frame, there is a trade off: durability. For the most part, the frame is extremely strong and the electronics are well protected (at least in this design), but the motors on these designs are extremely vulnerable. The arm mounts are the weakest part of the design and would like to have a way to prevent the mounts from being permanently damaged; a part that was easy and cheap to replace that would break and absorb the energy of an impact. Although I have only made 1 of these mounts, I decided to make the motor mounts (much like the mounts here on the SK2) out of 2mm thick by 5 mm wide aluminum plate (and I actually think this will not be weak enough…). The material is dirt cheap, can be cut with wire clippers, and is easy to drill. The idea is to be able to have a handful of spares on hand so that in the case of a crash, they could easily be replaced. It won’t take long to test this idea, I’m sure.

Battery Loading:

Battery loading is from the bottom. A rectangular cutout was made to the frame and the same modified batteries and connector that are used on the C1 are used here. The SK2 connector was also modified so all 3 frames can use the same modified batteries.

Update January 11, 2017:

Quick photo update. I only have to set up the Video and OSD. Hope to do a very quick test flight in the next week.

Update January 23, 2017:

Build is finally complete with all electronics in working order. It was quite difficult since I had USB port issues which I won’t elaborate on at the moment.

Symmetry:

I am now noticing that the awkward shape of the Racecraft props are upsetting the symmetry. In the photos, you can see that the center of thrust is now in front of the center of gravity. Although I don’t plan on regularly flying with these props, the center of thrust could be brought back by inverting the rear motor mount. If this isn’t enough, spacers could also be added under the front motors.

Final electronics list:

• Emax Skyline F3 flight controller (SP Racing F3) (found here)
• AKK CA20 600TVL FPV Cam (HS1177) (found here)
• Lemon Rx DSMX Satellite (found here)
• Spedix ES 20 ESC’s (found here)

• MP1584EN Adjustable voltage regulator (found here)

• Ublox M8N GPS Module (found here)
• Diatone SP3 Vtx/OSD (found here)
• Mini 6.5mm x 6.5mm Heat sinks to help cool Vtx (found here)
• 0.5mm thick thermal pad for ESC heat sinks (found here)
• 1.0mm x 10mm x 12mm aluminum for ESC heat sinks (found here)