By Terry Sullivan
Started In Electric Flying - An Update
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
The result of this is that:
YOU CANNOT USE A BRUSHED MOTOR SPEED CONTROLLER FOR A BRUSHLESS MOTOR
OR VICE VIRSA!
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
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
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!!!!