SEN 863 - 21 Mar 2004
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- Category: Archive 2004
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SCAT Electronic News 21 MARCH issue 863
Table of Contents
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Rules for F1Q electric power - Linkosalo
America's Cup Results 2004 - to date
Mikey the Moocher - Roberts
Wing spar widths...F1P Update - Lorbiecki
Lost Hills web site - Coleman and Heinrich
Trailer for sale - Campbell
Jnr Team T - Deloach & Skyprincess
Monster HLG
Don't Forget Next Weekend
Rules for F1Q electric power
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Author : This email address is being protected from spambots. You need JavaScript enabled to view it.
While I'm writing this I have not heard the outcome of CIAM March meeting.
Following are some ideas conserning the suggested electric free flight
class F1Q. My concern is how to restrain the motor power to reasonable
levels. The reasoning is a bit long, so I give my suggestion first:
Require a standard resistor installed in series to the power leads between
battery and motor. The idea is not mine, it came from Reijo Liljedahl.
So to the reasoning. I think there should be no limitations to the motor.
In true FAI spirit the motor should be open to developments and
inventions. There is no simple way as there is in other motor classes
(rubber weight in F1B, engine displacement in F1C) to limit the power of
electric motor, therefore no arbitrary limits here. Especially there must
not be rules limiting the motor to a certain brand or make, or limiting
the price of the motor or rules like that. They are more or less
impossible to reinforce. Banning gears is not wise, as for climbing a
small, fast-spinning prop is rather inefficient.
There should be no limits to the battery chemistry. Especially no
requirements to stick to NiCads. These are old techonology, cadmium is a
severe environmental thread which is likely to be banned in EU in the near
future. Even though there have been some raports of lithium batteries
causing fires, these have been due to mistreating the battery ususally
during charge. If proper methods for charging lithium batteries are used
they are as safe as any type of rechargeable cell. Limiting the maximum
voltage and weight of the battery is sufficient.
With the above rules the factor that is limiting output power of the power
train is mostly the internal resistance of the battery. Each make of
battery cells have a certain maximum current that they can provide, going
beyond that the voltage of the battery drops, and power output suffers
seriously. Thus for any given battery you can find the maximum power that
you can get out the battery. So far so good, that would work beautifully
to restrain the power of a model. The problem here is that the development
of batteries, especially lithium ones, is at very fast pahse at the
moment, due to strong demand of various electric gadgets (and indoor
RC) to get more power out of smaller cells. Thus a few years ago you could
draw 2C out of cellular phone lithium batteries (2C means twice the
capacity, i.e. out of 1000mAh battery you got 2 amps of current) before
the call voltage dropped to unusable levels. Today the value is around 5C,
with some cells already giving 8C, and there seems no levelling out of the
development. So without any further limits we should expect increase of
output power of electric models something like 20% per year. Not a good
situation...
Ok, here comes the serial resistor. From ohms law we know that voltage
equals current times resistance. Power, again is voltage times current.
With the above-mentioned rules the voltage of the battery is limited, so
power depends on the current. The current going through the circuitry
depends on the resistance. This consists of motor resistance and battery
internal resistance. The problem comes from the latter decreasing with
cell
