Re: Brushless kv number with double the power of a brushed 400?

On Mon, 21 Jul 2008 05:27:32 -0700, PeteOlcott wrote:

What is the brushless kv number that has about exactly twice the power
of a brushed 400 motor? I want to upgrade my GWS Slow Stick and exactly
double its power.

It doesn't work quite like that:

KV is a motor constant to use it multiply the battery voltage by the KV
figure to find out the max free running rpm of the motor i.e. no-load or
without a prop attached.

If you really want to double the power and not the RPM then you need to
look no further than the Wattage quoted on all electric numbers. You can
work this out:
Power (Watts) = Voltage (Volts) x Current (Amps)

If you can use a multimeter to measure the motor current of your existing
setup at full throttle then you have a power reading to start from.

But bear in mind that both the free-running rpm and the power vary with
voltage: i.e. the more volts the higher the power (up to a limit!) and the
free-running rpm. So choose your battery first and work back from that.

If going the LI-PO route the battery will have a C limit (1C, 3C, 10C
etc.) that will influence the choice of motor too. Multiply C by the
battery capacity (in mAH) to find the maximum current that the battery can
provide without loosing too much life or capacity.

e.g for a 3000mAH 10C battery it is capable of providing 30A

if this is a 2 cell series LI-PO battery that is then 7.4V and from above
(7.4 x 30) = 222 W

For a 1500KV motor on 7.4v that will be a max rpm of 11,100rpm WITHOUT A
PROP! I don't know how much a prop will drop this RPM figure as it varies
quite considerably depending on diameter, pitch and shape of the prop.
generally, smaller, thinner, lower pitch props run faster but are less
efficient.

There is a program out there on the Internet for working these things out
called Motorcalc? (am I right TNP?) Usually kept up to date with lists of
parts and their specifications and you put in your planes details
(battery, airframe weight etc.) and it gives recommendations for you.
Google should be able to point you too it.

(I'm doing this from memory so please excuse any imprecise figures)



--

Chris
.

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