Model Forum / Radio Controlled / Air Models / July 2008

"What color of car will be exactly twice as fast as the car I have now"?

In other words, that's a nearly meaningless question.

The Kv number tells you how fast the motor will spin at a given voltage
when it is disconnected from a propeller or any other load.  It also
gives you a pretty good idea of the motor current/torque relationship
(on a properly specified industrial motor the relationship is exact, but
many of the model motor sellers don't seem to understand, or care, about
doing it right).

So two motors, each of which will spin at 12000 RPM when you connect
them to 12V will have the same Kv number.  But there's nothing to say
that one of them is as small as your little finger and suitable for
flying 8 oz models, and the other is as big as your house and suitable
for driving the Queen Mary.

You use the Kv number to help you figure out the right propeller/battery
combination.  For motor power look for something that gives you the
motor power output, and look for twice what you have now.

Or look for similar-technology motor with a Kv about the same as your
current motor and a max current about twice as much.  This will give you
a motor that _consumes_ twice as much power as your current motor, and
you'll just have to hope that the efficiency is about the same.

Since you have a brushed motor, you can play the efficiency game.
You'll be making some big guesses (I'm making big guesses!), but you're
probably running at an efficiency between 55 - 65%, so the typical Speed
400 input power of 70W will be getting you 40-45 watts at the propeller
shaft.  A brushless motor is more like 75-85% efficient in normal model
service, so if you just replaced a crappy brushed motor with a really
good brushless you could see as much as a 50% increase in power at the
propeller without _any_ change in the power from the flight battery.
Aim for the same power loss in the motor and you'll see a 3x increase in
power (but you'll pull twice the power from the battery).

I suggest you find a brushless motor/esc/LiPo battery combination that
weighs about the same as your current motor/esc/battery combination and
uses about the same diameter prop, or not too much bigger.  I suspect
you'll have all the power that you want at that point.

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)