Electric Flying - Basic Concepts


Get Started In Electric Flying - An Update
By Terry Sullivan

Since Peter wrote his original article introducing electric flight, there have been a couple of major changes to the scene, and I thought a short update may be helpful.

The first major change has been the advent of a new more efficient type of motor. These new motors do away with the Carbon brushes used in the older type, so we call them brushless motors. Without the brushes rubbing they produce less friction and heat, and as a result convert more of the electrical energy they get from the battery into motive power for your model. This means more performance, better duration or both.

There are two types of brushless motor. The first is the "Outrunner" in which the shaft is bolted directly to the outer casing, which rotates on a fixed base. The second type is more like a conventional motor in that the shaft runs through the motor casing, which is fixed. For the sake of convenience, lets call this an "Inrunner".

The Outrunner type typically produces more Torque, whilst the Inrunner produces more revs. Thus the Outrunner can swing a larger prop more slowly, while an Inrunner uses a smaller prop at higher speeds.

An Outrunner is normally visibly quite different from a conventional motor, but an Inrunner may look exactly similar. There is, however, one infallible way of telling a brushless motor from a brushed motor - whilst the brushed motor has 2 leads going to the Speed controller, both types of brushless motors have three. This is because the brushless motors require 3-phase AC current. This is supplied by the Speed controller, which has a converter built in.
The result of this is that:


The other major advance has been in the area of battery technology. The need for light powerful batteries in mobile phones has driven the development of advanced batteries. There are three types - Lithium ion, lithium polymer and Saphion. The Saphion technology is still very new, so I will not get into that here. Both types of Lithium batteries have been around a little while now, though. As there properties are almost identical, we can discuss them together.

The major advantage of the Lithium battery is it's small size and lightweight - the Lithium battery I use in my GWS Formosa for over half an hour flights is smaller and lighter than the Receiver packs I use in my gliders! This means more performance for the model as it doesn't have to haul around a heavy battery. It is also easier to find a spot for in a small model! But in a model designed for a Nickel battery it may be difficult to set up the Centre of Gravity.
The major disadvantage of Lithium is that they are chemically unstable, and so need careful handling. The current that can be drawn from them is limited.

There is normally some indication of the safe load you can put on a lithium battery on the casing, expressed as a multiple of C. C is the capacity of the battery in amps, but we are accustomed to talking in milliamps, so I will continue to do so. 1 amp = 1000mah. If the safe drain on a battery is 10c then multiply the Milliamps of the battery by 10, and that is the safe figure for normal consumption. The battery may also say something like 10c continuous, 12c burst. This means that for a few seconds you can pull 12 times the battery milliamps.

So you have to be very careful in selecting a suitable battery for your motor. No use fitting a tiny battery to a big motor! If for example your motor pulls 30amps., not much point using a battery of 10c 2200mah. The maximum this is capable of pulling safely is 22amps. You would either need a 3000 mAh 10C battery or a 2200 14C.

This brings into question how we determine what current our model is pulling, and the simple answer is we need to test it with a wattmeter. These are not overly expensive, and often a club mate will have one you can borrow. If you are planning on getting into electric flight in a big way, they are very useful tools to have.

The other factor one has to take into account with Lithium batteries is that you have to use a dedicated charger, or at least one with Lithium capability. They need a very slow charge at a variable rate, so a special charger is needed. It is also beneficial to have an identical charge in all the cells; so many Lithium batteries have a balancing socket fitted. You then plug a cell balancer into the battery, and this equalizes the charge in each cell.

A normal NiMH or NiCad pack will be 7 or 8 cells each with a nominal 1.2v voltage, giving 8.4v or 9.6v respectively. The nominal charge of a lithium cell is3.7v, so we use either a 2 or 3 cell pack, 7.4v or 11.1v. Whilst discharging Nickel batteries fully is not serious, a lithium cell taken below 3v is likely to be damaged beyond repair. For this reason, many modern speed controllers are designed to be set up for lithium batteries, either by programming or by use of a jumper switch. If they are programmable, there are 2 methods, one is by giving a sequence of commands on the transmitter, and the other is by a connection to your PC. The former is fiddly and sometimes difficult, the latter necessitates the purchase of a PC interface -yet another bit of kit to buy!

One thing that puzzled me when I started looking at Lithium batteries was what do they mean by 3s1p? This turns out to be fairly straightforward, the S is for series and the P is for parallel. So a 3s1p is a normal 3-cell pack, 3s2p is a pair of 3 cell packs connected in parallel. This would still give 11.1v, but have twice the amperage for a longer flight

So there are numerous disadvantages to modernising your fleet. So why are so many experienced fliers doing just that? Well, there is a simple answer- more enjoyable flying! I have one model that has been used with a geared can motor and NiMh batteries and later upgraded to brushless and LiPo power.

On the original set-up it had good performance for 30 seconds, adequate performance for 5 minutes, then land quickly. On the new set-up it has toe-curling performance for 30 seconds, blistering performance for 20 minutes and very good performance for at least another 10 minutes. Was it worth the money? I'll say so!!!!