Sept 2003

Bring along any models/bits you’re bored with and see if you can’t pick up a good project or two for the winter building season. People have been known to sell more than they buy but that’s not really entering into the spirit of the thing.

A nice low-key competition that will make a change from assembling ARFs. You will be provided with some balsa (that’s a lightweight wood in case you have never seen it or perhaps forgotten it). Your challenge is to design and build a glider using the supplied materials. You’ll need to bring along a knife and some glue and preferably a mat to cut on. There should be time for a trimming session followed by the competition flights. Longest duration flight wins.

After the successful August show we are back to the normal format for this fly in. Flying from approx 10am to 5pm. Anyone with an "A" certificate is welcome to fly. There is no charge but pilots are expected to buy a ticket for Beale Park (which your family can use to have a look round the park once you’ve signed in).

I’m told that the events at Cholsey (July 26^th) and the Beale Park Show (August 2^nd/3^rd) were all successful. Unfortunately I was unable to attend them and I’m afraid that’s about all the information I have. With luck there will be some photos of the Beale Park show on the website by the time you read this.

I don’t know if you’ve been following the stories of all the latest battery types to hit the E-Flight scene. NiCd (Nickel-Cadmium) and NiMH (Nickel Metal Hydride) are now seriously old hat, though of course they’re what most people are still using.

The movers and shakers are now playing with lithium based technologies. Initially we had Lithium-ion batteries, which mainly looked like conventional batteries but had odd voltages 3V or 3.6V and were very fragile. At one point they were banned from being carried on US airlines because they had an unfortunate tendency to explode at the slightest provocation. They’re a bit better now but they still won’t give high currents and are very tricky to charge if you don’t want them shooting bits of themselves all over the place.

The really fashionable batteries at the moment are Lithium Polymer cells, shortened to "li-poly" or the rather unpleasant sounding "li-po". These are odd looking things if you’re used to the idea of batteries being metal cylinders with a contact at each end. As you can see below they look rather like little bags of stuff with a couple of terminals sticking out. Not surprising really because that’s exactly what they are. They are basically lithium ion cells with a gel electrolyte in a plastic bag instead of the more conventional rolled metal shell. The big advantage of these batteries over NiCd/NiMH is that they have much higher energy density. In other words they have very high capacity for their weight. They have a standard voltage of 3.7V (equivalent to 3 x NiCd cells) and can weigh as little as 1/3 as much for a given capacity. E.g. 3 x 3300mAh NiMH = 180g, 1 x 3300 Li-poly = 64g).

So far these nice little cells have mainly been used for small models, Speed 400 or below. For park flyers and the like it’s amazingly easy to put together a li-poly battery that will give you very long flight times. People are talking about 30-45 minutes from e.g. a GWS Tiger Moth as quite normal now. If like me you tend to get bored after say 10 minutes that can be 3, 4 or more flights without needing to recharge. So you may never need to charge batteries at the field again. No more lugging those heavy car batteries around just to run your charger.

And now these new cells are moving out of the small model arena. To give just one example, at the recent World Championship for F3A (Pattern) planes, Jason Shulman from the USA came 7^th with an electric plane using a Hacker brushless motor and Li-Poly cells (lots of them). Yes this equipment is expensive but bear in mind that we’re talking about the type of 2m wingspan pattern plane usually flown with something like a YS 1.40 on a pipe. Coming 7^th in the World Championship with an electric model is fairly impressive. The individual cells still aren’t able to put out very high currents but one of the advantages of li-poly cells is that it is easy to connect them in parallel for increased capacity and current. Unlike NiCd/NiMH batteries there are no problems charging these batteries when they are parallel connected. To get the power he needed Jason was using what is called a 10S3P pack, 3 parallel sets, each of 10 cells in series for a total capacity of almost 40V at over 6000mAh.

These cells are still quite expensive, especially the high capacity ones like Jason used. By my reckoning a pack like his would cost around £400 at today’s prices. So if you’re thinking of trying them you should bear in mind that li-poly batteries need special charging. Trying to charge them with a basic NiCd/NiMH charger will kill them in seconds. Li-poly cells don’t explode like the older lithium cells but they do "blow up" into interesting balloon shapes. At that point they are dead. The latest versions of many of the better chargers now have a li-poly charging mode (e.g. Schulze 330, Triton etc). If you have an older model some can be upgraded, e.g. the Schulze, others can’t so you’ll have to buy a new one. In fact the charging method for Li-Poly cells is fairly simple so if you don’t mind a bit of soldering you can easily make yourself a charger. See below for pointers to some web links containing for all the information you might need.