A Crash Resistant Mini Quad (Assembly - Part 2)

Introduction

The standard size of quads is rather large. The reason is that it used to be very hard to make small quads controllable. This has all changed with newer control boards, like the KK2 board, which sports auto leveling. This feature makes it possible to construct a small quad that is beginner friendly and very stable to fly.

Smaller quads have a lot going for them:

• The reduced size means reduced weight, which means less energy if you crash your quad. Less energy equals less damage. And believe me, if you are a quad beginner, you WILL crash your quad many, many times.
• Smaller motors are used, which weigh less and are more affordable.
• The HobbyKing 6A and 10A Plush ESCs are affordable,have the perfect size and only weigh in at a few grams. These are perfect in micro Quad applications. No reflashing is necessary.
• All the material for the small wooden quad frame and the other necessary parts like landing gears and mounting equipment will not set you back more than a few $. A larger quad needs more expensive materials and fixtures. This quickly becomes very expensive.
• If you want to go fast, the power to weight ratio is very good.
• Larger multi rotors are a pain to transport. There are delicate parts like antennas and propellers sticking out in all directions and they do not fit in any cases. The small quad I describe easily fits in a standard aluminium case, together with a transmitter and some batteries.

And naturally some disadvantages:

• If you want to carry a big camera and have perfectly smooth video, you want at least a full size quad. 
• If you want to fly FPV you will also want a larger, more docile platform.
• Small propellers are fragile. The ones I recommend will break easily. On the other hand nothing else on the quad is likely to break and each propeller costs less than $1.
• The small size will not impress your fellow quad fliers much. If you are in a size sensitive age or have other size related issues you might want to buy a more impressive machine

Needed Parts

Frame: I use wooden frames made out of 8 mm pine wood. The arms are very hard to break when they are as short as in this quad. To keep this article short I have made a separate article detailing the material and techniques used to construct the wooden frame. You can read the article here.

Controller Card: KK2 from HobbyKing. A brilliant board that is very user friendly, has got working auto leveling (with the newer firmware versions) and accelerometers as well as gyros. All at a good price.

Motor: Turnigy 1811 2900 kv brushless motor (HobbyKing).

ESC: Plush 6A (HobbyKing). A long time favorite in micro planes that work very well in quads to.

Receiver: LemonRX 6ch DSM2 Feather Weight receiver. Very light and well made DSM2 compatible receiver. Bought directly from LemonRX.com

Battery: Turnigy Nano-Tech 950 mAh 2s (HobbyKing)

1 female battery JST battery contact (Hobbyking)

4 short  male to male servo wires

12 pairs of micro golden bullet connectors for the motors. Can be skipped to save cost and weight if the motor wires are soldered directly to the wires from the ESCs. I prefer to use connectors since this will make it possible for the motor to separate at a crash without causing further damage.

4 plastic springs or a piece of rubber tube

4 20 mm wood screws

A few centimeters of 50 mm PVC piping.

4 5x3 propellers (buy a lot more, you will need them). I like the red and black ones that HobbyKing sell.

4 very small screws to fixate the propellers.

8 small zip ties

Build Process

Start by making the frame. That is detailed in part 1 of this article.

Solder the battery connector to the power distribution board. Use hot glue to attach the battery connector to the bottom of the mounting board. Finish by using a volt meter to check that there is not a short between the + and - battery poles. DO NOT SKIP THIS STEP OR YOU MIGHT FRY EXPENSIVE ELECTRONICS LATER ON.

Solder each of the ESCs to the power distribution board. Slide a piece of heat shrink tubing over each arm and ESC and heat shrink it to fixate the ESCs. You could also use zip ties. Take note that the ESCs will have different sides up like in this picture:

Cover the power distribution board with some kind of scrap foam 5 - 10 mm thick. I used double sided mounting tape towards the board and a thin foam piece over it. This foam protects the open leads and provides a cushion for the battery to rest against.

Cut two 8 mm slices of the PVC tube. Cut these in half to create four equal half circles. Boil these in water to make them soft and place them in a 30 mm wide form of some kind. Anything round will do. They will cool down and form 3/4 circles.

Solder the bullet connectors to the motor wires.

Extend or replace the motor control wires so they reach to about 5 cm from the end of the arm. Solder bullet connectors to the end of the lines and add shrink tube or electric tape to insulate them. Use hot glue to fasten the bullet connectors to the bottom of the frame arms. If you have painted the frame you must scrape off some color to get the glue to bite into the wood.

Drill a small hole in each propeller and screw in a small screw in each hole. The suggested motors have got indents in their 2 mm shafts. If you use another motor you should make some indentations in the shafts to allow the screws to lock the propellers in place.

Mount the motors and PVC landing gear using zip ties (RCExplorer style). Please take note of the zip tie dimensions. They are small on purpose. The intention is that they should break at a crash and spare the quad from further damage. Do not use larger zip ties or any other more fixed mounting technique. If you do, the energy of a crash will only be absorbed by the frame which will break much sooner than if the zip ties had given way. 

Fabricate control board mounts out of the four screws. The tip of each screw is fastened in a screwdriver. Then use a Dremel tool to file off the thread of the screws with the screwdriver running. The reason for removing the threads is to allow the spring to move without restrictions.

The result will look like the screw in the foreground:

In the pictures I have used steel springs to provide dampening. I strongly discourage from this based on my own experience; At the lower side of the board there are several naked tracks and solder points that may be shorted by the steel springs. This is something I found out the hard way...

Use plastic springs, if you can get hold of it, or pieces of thin rubber tube to replace the steel springs.

Route the wires from the ESCs through the hole in the middle of the control board and attach it to the right motor control contacts.
Connect a receiver using short servo wires. I prefer the LemonRX Feather Light receiver for this application.

This is the finished quad. Super clean and not an unnecessary gram of weight:

The total weight, with receiver and battery was spot on 200 grams. It is not as small as the micro toy quads, but it is built out of high quality, repairable components and it will easily fly regardless of wind. Neither is true for the micro toys.

If you want to shed some more grams you can buy the pinless version of the LemonRX 6ch receiver and solder it directly to the control board. This has the added benefit of making the quad even more crash resistant. 

My mini quad flies quite well and is very sturdy. Just like I wanted. I made a short indoor flight video. The quad is really meant for outdoor use. It is small, but it is way to big to fly in a small room. But it was raining outside, so indoors was the only option.