Just two weeks ago I picked up a new RC toy, the Blade mQX quadchopper, and boy am I impressed! There hasn't been a day since that I haven't flown it and gained great sense of achievement from learning a new flight trick or improving my ability to hover it better.
I've had my eye on quadrotor choppers for quite a while, and since motors, batteries, power controllers and microcontroller tech. has gotten lightweight enough to make tiny choppers of all descriptions, the quadrotor design, imo, is amongst the most innovative. I'm somewhat amazed why the aerospace industry haven't started using Quadrotor designs for all uses.
There are a number of key advantages -

• All blades provide lift, unlike most other propeller VTOL designs, thereby providing great efficiency.
• dual contra-rotating blades provides natural flight stability.
• Computer controlled and gyro/accelerometer/magnetometer stabilised, allows great maneuverability, all without a complex heavy swash-plate mechanism.

So I found myself thinking how cool it would be to fly it at night, outside or in, and see its full orientation and direction, plus at the same time troll those alien conspiracy 'theorists' online - check out some of the YT comments on my vid below for a laugh!
Here's a really simple lighting kit you can build yourself, I'm using components I already had lying around but it's really easy to get hold of the bits and pieces off eBay and beyond. The circuit is probably the simplest that can be built, and because we're using a weak coin cell, you don't even need to provide resistance for the LEDs.

P.S. Ain't it stupid how all you need to do is describe a video as 'UFO sighting' and it gets more views in a day than some of your other vids get in a full year!

Step 1: Get the Parts Together

So you'll need -

• Some enameled copper wire, really thin stuff.
• Surface mount LEDs. Use the biggest SMD LEDs you can buy, I think mine are size 1206. They are very small and have small semicircular notches either side. Makes it easier to solder wire into it.
• Coin cell battery holder (not pictured here) and equivalent battery. It really doesn’t matter which you go for provided they are rated at 3v, pretty much all of 'em.

Tools needed -

• Soldering iron and some solder (not pictured).
• Wire clippers
• Helping Hands tool.
• Tweezers, sufficient for holding 1206 SMD LEDs securely.
• sandpaper, around 200+ grit.

Step 2: Make a 'unit Piece'.

You'll need to make four (for a quad design) of this item, instructions provided for just one, obviously -

1. Take a length of enameled wire, measure it around 2x the length of one boom of your quadcopter.
2. Fold it in half and kink the half way mark.
3. Create a loop on the folded end and put a screwdriver or something thru it.
4. Use your fingers and the screwdriver to spin the enameled wire to make a ‘twisted pair’. Thin enameled wire has a tendency to twist up nicely for only a few centimetres, so what I did was to create a 90 degree bend close to the working point and use the bend as a twisting point for another few centimetres. Repeat till the end.
5. Heat up your soldering iron. All soldering irons I've bought need to be kept at their minimum settings for decent soldering work, electric or gas powered, if your solder spits and bubbles on the iron, and smokes like a chimney, it's way too hot!
6. Once you have your twisted pair, snip the looped end to create a nice set of circuit wires.
7. Use sandpaper to scratch the enamel off the two wires two ends (four points) ready for soldering. Test the wire with a continuity sensor to ensure you have properly exposed the copper, repeat the sanding until you see zero (or thereabouts) resistance.
8. Tin all four ‘exposed’ ends and place the wires into your ‘helping hands’ tool. Splay the two ends out to allow for 'access' and have both wires pointing upwards or thereabouts.
9. Now it’s time to deal with a fiddly little SMD LED. Mine came in short strips of the wrapper, these originally come in massive spools but eBay sells small quantities cut off of the spools. Each LED is located in small plastic trays along the spool cuttings, if that makes any sense! Strip back the ‘foil’ to expose one LED in it’s tray, and pop it out into your hand. Put it carefully down somewhere you won’t lose it as it's around a millimetre in size!
10. Ensure your soldering iron is clean and freshly tinned. I use some toilet paper folded several times and wetted with tapwater to clean my iron.
11. Pick up the LED with the tweezers by the sides, ensuring you can still touch one solder contact point of the LED.
12. Manoeuvre the LED carefully into position, putting the semi-circular notch into one of the tinned wires and touch it with your hot soldering iron for a few seconds from the side, you may need to experiment to find the right amount of solder to leave on the iron tip for this job.
13. Remove the iron, leave the LED and wire in place until the solder cools and solidifies. The LED should now be very securely attached.
14. If you did this correctly, you should now be able to easily move the second wire into the other solder point of the LED. Hold it on, touching it with your tinned soldering iron and hold until hard. Tinning of the wires is critical for this process not only to see if you've removed enough enamel, but you also need it to be a very 'compliant surface' for accepting the solder - the LED is already solder compliant so the two join nicely.
15. Test the assembly out - touch the other side of the twisted pair (we'll call this end the 'contact end') against both contacts of your coin cell battery. Don’t hold it lit for too long. If it lights, success! Remember, they're polarised so if it doesn't light up, try flipping the battery over!

Make four of these. You'll find the process is quick if you can efficiently get thru the process quickly and without trouble.

Step 3: Completing the Build. Final Assembly!

Once you have four tested LEDs on wires (LED Assemblies), simply take two sets, identify each + and - connections (these are LEDs and are polar sensitive!) wrap them together at the contact ends and solder them up. Test the two lights with the coin cell again.

Do the same with the remaining two LED assemblies.

Take one of the two LED pair assemblies and solder them to the battery contact points of the coin cell battery holder. Once secured, do the same with the other pair. If there's no stress on the wires, you'll find you can simply solder all points together without the wires splaying out and ruining the solder job.

Job Done! Test it out, you should know by now to put the battery into the holder!

Step 4: Attach to Your Quad.

It's now time to put it onto the quadchopper.
I simply twisted each LED assembly wire around each boom of the chopper, you'll need to remove the plastic canopy of the chopper to do this so be careful not to touch the chopper circuit board, the Blade mQX has some vulnerable looking thin wires and connectors on it's board.

Take the battery holder and use a finger to hold it down to the underside of the model, underneath the chopper's battery mount.
Run an LED assembly around one boom relatively tightly, when you reach the end simply point the LED upwards to illuminate the underside of the chopper blade.
Do the opposite side up the same way to help secure the battery holder to the underside, then do the other two LED assemblies to finish it off.

NOTE - I managed to block the slot where you put the chopper battery, as a tightly bound enameled wire totally obscured the E-Flite battery slot! Be sure to pay attention to where your wires are ending up so you don't do the same.

Step 5: You're Done!

Hopefully nothing has come loose while wiring it up and the LED kit still lights up!

I forgot to weigh it prior to 'installing' it on the quadchopper so I can only estimate the whole kit weighs less than 5 grams, it can't be much heavier, and so shouldn't present any difficulties with lift capabilities or stability.

UPDATE - It weighs only 3grams! I had a chance to weigh it after it needed dismantling after some crashes. I've now rewired it up and have sealed the collection of stray wiring (gathered up on the bottom of the battery holder) with some solid resin to prevent it getting damaged again.

I also do a bit of on-board flight recording by using one of those chinese 808 cameras off eBay (#16v2 with the 120 degree wide angle lens). I am literally velcro-ing it onto the canopy top, cam pointing forwards. The camera doesn't present any overwhelming lift difficulties with the Blade mQX so this kit won't do anything save for knock a few seconds off your flight time.

It's not easily removable without potentially damaging the lighting kit wires, but I leave it on as it really is un-noticeable to flight characteristics.

Well you may be thinking that I haven't included enough lights, and of course you're free to make more for your own if you feel the need, but I've found that even four illuminating the underside of the white blades, and seeing the LED light directly, allows for perfectly controllable flight - besides, the mQX is really small!
Our brains are very good at detecting patterns, specifically recognising angle, position and velocities given only a small subset of what you'd see in full daylight (point light sources and light reflection, so I didn't have any trouble flying it in complete darkness.

Of course feel free too to be a bit more creative than I have. Its perfectly reasonable to use differing colours of lights - if you fly your mQX in + configuration, use a different colour for the front LED might be a good idea. + config is where the chopper has one blade forward, backward to the port and starboard as opposed to the X config you see in the picture.

I also have an idea to include a small microcontroller to maybe flash the lights in a certain way, or even change their colours if you're using RGB LEDs, but I would only consider doing so if I were using more than four lights, as it could just become a hindrance if not enough lights are on at one time.
Be mindful of the extra weight. I'm almost certain an Arduino board will be too heavy for the mQX - instead base your design on a PIC or Atmel AVR chip with a very small etched board and keep the power supply light too. I'm thinking trying to run your microcontroller from a small 3.7v RC lithium ion battery - it's not a good idea sharing the power from the e-flite chopper battery as you don't want to cause problems for the flight electronics!

Overall, it's all about having fun!