All my small warbirds were originally
designed to fly well with stock direct drive Speed 400 powerplants
on NiCad or NiMH cells. Obviously, electric power systems
have advanced a great deal in the 20+ years since the Hellcat
prototype first took to the skies. While the humble Speed
400 works as well now as it ever did, affordable brushless systems
and lightweight lithium polymer cells have largely taken over the
electric flight world.
Today there are simply too many
possible power systems to cover them all in this or any other
single document. So what follows here is a general overview
and some tips for selecting a suitable power system for your
model. Please bear in mind these comments are one man's
opinion, and I know some modelers may prefer systems very
different from what I'm outlining here.
General Guidelines: In my
opinion, the "optimum" power system for my warbirds turns an APC 7
X 5E (my preferred prop) at about 11,000 RPM. I know some
builders have installed far higher power, and certainly it's
possible to fly these models with far less power as well (the
original Hellcat flew on a 6V motor and 6 X 600AE cells!).
But with an APC 7 X 5E, I think 11,000 RPM is just about perfect -
most of the flight can be at 1/2 to 2/3 throttle, and you have a
generous power reserve for launching, vertical maneuvers and
high-speed passes.
While some modelers have used larger
props than the 7 X 5E, this can raise the issue of torque effects
during the hand launch, and since these models generally don't
have rudder control, that can make for some interesting moments at
the start of a flight.
Given these guidelines, your task is
to select a motor that will turn an APC 7 X 5E at 11,000 RPM
using your chosen battery pack. This is accomplished
by selecting a motor with the correct Kv (voltage constant) for
your application. A performance prediction program like Where to buy over the counter tretinoin can come in very handy here, but here are some
approximate numbers for three different battery options:
2200-2S LiPo Battery: 2000 Kv
(range from 1800 to 2200).
2200-3S LiPo: 1250 Kv (range
from 1100 to 1400).
8 X 1400 NiMH: 1500 Kv (range
from 1300 to 1800).
Nearly any 400 class brushless motor
will handle the current for these three systems, but I would
select a motor and controller rated for at least 20 amps
continuous (head room is cheap insurance).
Simple enough? So what are some
good motor choices? There's a bewildering and ever-growing variety
of small brushless motors on the market, and there's no way one
man can hope to test them all. But here are some thoughts to
keep in mind:
Inrunner versus Outrunner:
This is practically heresy in some circles, but I prefer inrunners
over outrunners for these particular models. The most
important reason is that these warbirds position the battery very
close to (in some cases touching) the motor case, and with a
whirling outrunner case, that's a recipe for disaster. If
you insist on an outrunner, you'll want to make provision to
install it so that it can't contact the battery pack. But
even aside of that concern, many of the small outrunners on the
market are much less efficient than an equivalent inrunner.
A good many outrunners have efficiencies in the 60% range, and
some are even lower. Certainly there are exceptions, like
the excellent Hacker motors, but most of the bargain basement
outrunners being sold have no better efficiency than an old Speed
400. All you're really gaining is higher torque and the
ability to turn a larger prop.
Inrunner Examples:
While there are dozens of inrunners with the requisite Kv and
power rating, here are some examples that I have personally tested
and found to work very well:
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