A Crash Resistant Mini Quad (The Frame - Part 1)

Mini quad frames built out of glass fibre PCB boards are all the rave for small quads. An example of this is the Turnigy Micro Quad. HobbyKing sells these frames for a low price and they are also available as more or less completed quads. Here is a product photo from HobbyKings site:

The idea has several merits:

• You do not need a separate power distribution board since the cupper tracks are printed directly to the PCB.
• The build is super simple since the parts can be assembled using some CA glue and a screwdriver
• No motor control wires are needed since tracks for this is printed on the PCB.

Sadly there is a big disadvantage. Even mild crashes will break off the motor mounts, which are hard to repair. The Turnigy Micro Quad is especially bad in this respect since some genius has decided to place a big hole for the motor wires right at the weakest point of the arm. Completely insane!

Besides breaking off the motor mount from the arm, there is a real risk that the cupper tracks are ripped out. The motor connectors that are soldered to these tracks will probably be damaged in the process.

An even harder crash will break off the arms at their bases, causing even more damage to the ESC that are soldered at this point.

Here is a picture I have borrowed of the result of a crash. In a few weeks of flying my own Turnigy Micro Quad I managed to crash it just as bad or worse many, many times over:

 (Source Duane Degn on Parallax INC Forums)

After repairing the PCB boards a number of times and having to deal with an increasingly fragile frame that became more and more out of alignment I decided that it was time to do something drastic. I had a brand new replacement frame that I bought as a spare. But replacing the broken frame with a new seemed pointless since I would just end up repairing it instead of flying the quad.

I decided to make the most crash resistant mini quad frame possible. Something with much better landing gear, something with crash resistant arms, something that would come apart rather than break into pieces at a crash, something with not one extra gram of weight.

Something like this:

After some research I choose to use 8 mm (that is roughly 1/4 inch) pine wood spars for the arms and a plywood piece as mounting plate. I considered more exclusive materials like carbon and different plastics, but they are a lot harder to work with and the lower weight is negated by the added weight of the necessary mounting fixtures. In comparison wood glue will make a perfect fixture for the arms to the plate. The weigh of the glue is next to nothing.

My experience with the Turnigy Micro quad is that it is fun to fly, but so unstable that you need to tune it down a lot to get it flyable. After some consideration I decided to make the new frame a bit larger. The diameter of the quad, from arm tip to arm tip is 33 cm. After flying it a lot I am very happy with the size. The somewhat larger frame is much more flyable, but still lightweight enough.

To make the frame I started by cutting cut two 33 cm pieces of wood spar. In the middle of each spar I made two slots 8 mm apart with a saw. I then chiseled out the wood in between the slots. After some filing the pieces fit together to form a perfect cross. If done properly, this technique makes for a very strong construction.

I decided to make the mounting plate circular. A square mounting plate is easier to fabricate and will actually save a few grams, since it can be made exactly as small as the controller card, but I wanted to be able to use both KK1 and KK2 controller cards. The KK1 card must be mounted with a corner facing forward, while the KK2 board must be mounted with a side facing forward. If you know that you will never want to switch controller (or switch between a + configuration quad and a X configuration quad), you can go ahead and make a square mounting board.

If you happen to have a circle cutter for wood, like the one below, you can use it to cut out the mounting board:

I had no such luxury, so I made a template jig out of a discarded credit card blank. I drilled three holes. One at the side, one 17 mm away and the third 20 mm further off. I then pushed a small nail through the first hole and gently hammered it into the plywood. The second and third holes were used to draw perfect circles with a pen.

To cut out the outer circle I used an electric jig saw. A small hand saw would also have worked. In order to make the edge round and smooth I drove a larger nail through the mark left by the small nail and fastened the nail in a drill. This setup made for a low tech wood lathe. A file and some sand paper was all that was needed to make a perfect circle out of the somewhat jagged piece of plywood. 

To make the inner hole I used the drill and made holes along the inner rim of the circle. I then cut out the middle piece and used the drill with a sanding bit to make a nice and even hole. You could use a round file to do the same thing.This is the finished plate:

The plate was then simply glued to the wood cross using regular wood glue. I also applied glue in the centre between the two wood spars. I put a weight on the frame and waited a while for it to dry. If you are in a hurry you can use CA glue instead.

To make the cross of the frame even more rigid, I made a cross shaped power distribution board out of PCB experimental board material and glued the board to the bottom of the wood cross. A small power distribution board eliminates the need for a battery harness. This is a good thing since a harness would be a tight fit and add unnecessary weight to the small quad frame. Remember the design goal; Not one gram extra.

The PCB cross was cut with a Dremel tool:

I soldered four pieces of cupper wire to the board to prepare for installing ESCs and a battery connector later. Two of the wires were connected by a solder bridge in the middle. The other two were connected by a jumper wire.

The completed cross shaped board was glued in place using CA glue.

A lot of people who build wood frames simple leave them without any kind of surface protection. After a while the wood looks dirty and worn which I think looks tacky. In sharp contrast to carbon, it is very easy to paint wood. I used an airbrush to paint the frame in a nice pattern and cover it with some clear coating to make it resistant to wear and tear. To avoid contact problems I masked off the outer edges of the power distribution board. You can naturally use any wood paint and a regular brush for this. Here are some pictures of a finished frame. This particular frame has pre drilled holes for different board configurations.

This completed frame weighed in at 37 grams. It will be the base of a very lightweight quad.

The last step is to make hooks for rubber bands that can hold the battery in place. I simply made small hooks by bending two nails and inserting them through holes near the outer rim of the board. The nails wee secured in place by some hot glue. I think epoxy would work better, but I was in a hurry.

After a number of crashes I can confirm that the frame is very crash resistant. In the worst crash I had it tumbled down from 10 meters straight into the ground, landing square on an arm. There was no damage at all. I had to replace a zip tie, and that was the full extent of the damage. I do not recommend turning off the throttle in mid air to test if the quad recovers before it hits the ground. It obviously did not from 10 meters.

Propellers will however break all the time, just as with all quads.

In my next article I detail how to make and mount the rest of the needed components. The result will be a  small yet highly crash resistant quad.