Continuous Flying

I wanted to be able to just keep at the hovering practice, and having taken an electronics degree many years ago, decided that what I needed was a step-down regulator in the helicopter, run by a captive wire to a 12 volt car battery on the ground. 

A scan of the news groups suggested a typical current draw of some 2 amps. A quick scan of voltage regulator spec. sheets came up with a suitable low voltage-dropout regulator capable of handling up to 5 amps, and a very simple design was born (see circuit diagram below)

R1 was selected at 680 ohms and R2 at 100 ohms giving a design output voltage of 9.67 volts. The tantalum capacitors of 10uF and 22 uF are both rated at 16v (Observe Polarity!). The dropout voltage of the regulator is typically 370 millivolts and a maximum of 600 millivolts, so a voltage of at least 10.04 to 10.27 volts respectively is needed at the input to the finished unit which is mounted in place of the battery in the heli.

Whilst waiting for the Piccolo to arrive, I had already decided from reading the news-groups that a captive tether/power supply was the way to go, and had designed the above circuit and ordered the parts, which arrived today.

Construction was by soldering all components and leads directly to the pins of the voltage regulator, which was then mounted on a small finned heatsink rated at 7 degrees Centigrade per watt (which has proved to be total overkill - a much smaller one rated at 13 degrees C per watt would be more than good enough at 12 volts input and the given cable voltage drop). Total construction time was 20 minutes!

I changed the Piccolo power lead to a standard Futaba plug and socket arrangement for compatibility with every other connector I use, and used another Futaba plug and socket for the connection between the ground-based battery and the regulator unit in the heli .

Initial tests with a dummy resistive load of 4.7 ohms showed that the cable I had selected for the connection between ground and heli was of far too high a resistance, and at about 2 amps load, the voltage drop in the cable was far too high. I changed the cable for a 12 foot length of cable that had been supplied as speaker cable for a car audio system and tried again. This time everything was OK with only a 640 millivolt voltage drop along the 12 foot cable.

The Acid Test

Everything was setup in the helicopter and, with a training under-carriage made of 5mm carbon rods and practice golf balls, power was gradually applied. Instant Success!!! Now I could keep on practicing till I got the hang of it, which I did in about an hours flying, with a final non-stop hover time of just over 15 minutes before the concentration gave out.

Even better, despite flying in my lounge, which unfortunately has furniture and my F15 jet to get in the way, and only leaves me a space of some 8 by 6 feet to use, I didn't break anything!!!

Footnote

I had designed this circuit with a target current of 2 Amps as per various messages in the news groups. Measuring the actual consumption showed that this figure is wildly inaccurate. Actual current consumption at full throttle, no servos moving, with the equivalent of an 8-cell pack, is 5.1 Amps!  Those of you considering Lithium-Ion cells might need to take this into account, although I admit that actual hovering  only takes about 3.2 to 3.3 amps. At 5.1 amps I am losing 1.632 volts down the cable - so this cable is just about ideal to minimise dissipation in the regulator. Any higher cable resistance and you would have to increase the supply voltage above 12 volts or decrease the target output voltage.

I hope this rather long article enables others amongst you to follow this route and really speed up your learning progress. One point I must make is that if you have to ask how to build this very simple circuit, then you shouldn't be doing it - you could damage the electronics in the heli.

Copyright

This information is not copyrighted. Please feel free to use it in any way that you wish, except to claim that it is your work rather than mine (unless you want to be seen to be a complete idiot!).

Designed by Colin Dyckes - 17th February 2000