|Contributed by: Fly RC Magazine | Published: September 2004 | Views: 38600 | Email this Article
review appears courtesy of Fly
by: Steven Horney
by: Walter Sidas
the SIG 4 Star 40 to E-Power
Simple guidelines from a pro
power has really taken the market by storm over the past
few years, but most of the attention has been going to park
flyers and indoor planes. As it turns out, electric power
works just as well for bigger planes, including the very
popular .40 size planes flown at club fields. This article
will give you basic guidelines for making a successful electric
conversion, and show you a good example system as well.
I’ll be converting my formerly glow-powered Sig 4 Star
40, a popular sport plane that makes a great electric plane,
but the tips presented here will apply to many other planes
of similar size as well.
Sig 4 Star 40
Wingspan: 59.75 in.
Wing area: 604 sq. in.
Length: 47 in.
Flying Weight: 6 lb., 2 oz (98 oz) with 16 cells
Wing loading: 23.4 oz/sq. ft. (16 cells)
AXI 4120/18 motor
Controller: Jeti 70-3 Opto
Cells: 16 Sanyo NiCd RC 2400mAh cells.
Prop: Graupner CAM 14x9.5 folder
Full Throttle Power: 38 amps, 608 watts; 6.2 W/oz.,
Multiplex Cockpit transmitter from Hitec RCD USA, Hitec
555 receiver, 3 standard size Airtronics servos, 4-cell
500A receiver pack
system and prop provided by Hobby Lobby International.)
the 4 Star 40 is a Great Conversion Candidate
any plane can be converted to electric power, but
some airplanes are definitely better candidates than
others. The biggest key is wing loading —how
much weight each square inch of the wing carries.
Since the electric power system will weigh more than
a comparable glow system (assuming you are using NiCd
batteries), it stands to reason that a glow airplane
with a relatively light wing loading will make a better
electric conversion. A wing loading of less than 23
oz./sq. ft. is desirable (that’s equivalent to
one pound or less per 100 square inches—a good
rule of thumb to remember).
years of experience, my Sig 4 Star 40 is no longer
a thing of beauty, but it is a rugged, everyday
flyer that performs superbly as an electric conversion.
I’ll work on the aesthetics of the motor mount
in the future, but for now it showcases another
attribute of electric power – no need for fuel
at this practically, if you fly with 16 2400mAh NiCd batteries
you’re adding approximately 1.5 pounds to the airframe.
Suppose your airplane has 600 square inches of wing area
and weighs 6 pounds (wing loading of 23 oz/sq. ft.). Adding
1.5 pounds to your airplane will give it a rather heavy
wing loading (29 oz./sq. ft.); it will probably have a
fairly fast landing speed and may tend to snap if slowed
too much. On the other hand, if the glow weight is listed
at 5 pounds or less and your plane has 700 square inches
of wing area, there’s plenty of potential for an
electric conversion (wing loading would increase from
16.4 to 21.4 oz./sq. ft.). When converting to e-power,
there will be some additional weight savings along the
way (no fuel tank or fuel, fuel proofing is not needed,
servos can often be smaller, motor mounts are lighter),
but simply adding battery weight to the glow weight is
a good means of making a quick estimate of a plane’s
suitability for electric power.
much power do I need and how do I calculate the power
Power estimation and motor selection are probably the
biggest sources of confusion for the electric neophyte,
especially with the bewildering ways of describing motors.
Part of the problem is that electric motors are generally
much more versatile than glow motors; a single brushless
motor can potentially be flown in a small aerobat on 7
cells, then pumped up to 24 cells with the right gearing
to give serious power to a .60 size airplane. Consequently,
it’s hard to recommend a single setup, since a variety
of approaches will work well. Rather than go into an exhaustive
study of all that’s involved in motor systems, I’ll
give you a few rules of thumb and then show you a power
system that works well.
When choosing an electric power system, we generally focus
on the batteries first, since the batteries actually provide
the power. The motor simply converts battery power to
rotary motion. Two rules of thumb have long been used
in electric circles. The first was proposed by Keith Shaw,
and it’s often referred to as the Watts per Pound
rule. Power is measured in watts (multiply voltage times
current in amps to get watts). Dividing by the total weight
of the plane will yield a W/lb. number. Basic flying generally
requires around 50 to 60 W/lb.; good aerobatics require
80-plus W/lb. Going over 100 W/lb. will provide some serious
For a good estimate, multiply the battery current by the
number of cells (1 volt per cell is a good estimate when
NiCd cells are under load) to obtain the watts being produced.
Of course, you’ll need an Astro Flight Whatt-meter
(or a voltmeter and a shunt) to measure current.
The system I’m recommending draws close to 40 amps
at full throttle.
A simplified rule of thumb was proposed by Matt Orme (formerly
of Aveox, now producing and marketing Razor Motors). According
to “Orme’s Law,” use 1 cell per 50 square
inches of wing area for trainer-like flying; 1 cell per
35 square inches for solid aerobatics. Although simple,
this rule works surprisingly well. [Editor's note:
these rules of thumb apply primarily to models powered
by glow engines in the .20- to .60-size range.]
up the 4 Star 40
me show you how I applied the above rules of thumb
to arrive at my current electric conversion setup
for the Sig 4 Star 40. Originally a glow airplane
powered by an OS .46, the only changes I made to my
4 Star 40 for electric power were removal of the fuel
tank and engine mount and the addition of the batteries
and electric power components.
The Sig 4 Star 40 has a listed glow weight of 4.75
pounds and a wingloading of 18.1 oz./sq. ft. Note
that the 4 Star 40’s listed weight gives a 1.25-pound
margin before hitting the magic 1 lb./100 sq. in.
wing loading rule.
plywood plate (with triangular reinforcement) glued
to the front of the cut-down “ears” on the
4 Star 40 serves as a mount for the AXI motor. Thin
washers keep the motor off the plate enough to prevent
rubbing of the shaft retaining clip.
is a very powerful motor that needs to be mounted
securely. The folding prop helps prevent damage to
the motor shaft in the event of a serious crash.
found that 16 cells provide a good balance between weight
and power in a plane the size of the 4 Star 40. It’s
also relatively easy to mount two 8-cell packs in the
fuselage. As you can see in the specifications, my 16-cell
setup brings the airplane weight to around 98 ounces,
or just over 6 pounds. Going with the 1 lb. per/100 sq.
inch rule, you can see that the 4 Star 40, with 604 square
inches, is right on the money. This particular plane will
fly well even with higher wing loadings, but it’s
a very sweet flyer at this weight. As for power, assuming
I’ve propped for 40 amps, that leaves me with around
640 watts and 107 W/lb., a very good number for powerful
aerobatics. Using Orme’s Law, I should be running
17 cells for great aerobatics, so I’m very close
to that rule of thumb as well. The end result? A well-mannered
airplane capable of great aerobatics!
aerobatic performance no longer equates to short flight
times as it once did. The use of Sanyo 2400mAh cells
yields around 8 minutes of good aerobatic performance,
with reasonable throttle control, in my 4 Star 40.
Few glow flyers keep their planes up more than 10
minutes (as shown by several informal measurements),
so practically speaking my 4 Star 40 offers glow performance
and flight times without the mess and hassle of running
a glow engine. Flight time can be estimated by multiplying
the amp hours of the battery (a 2400mAh battery is
2.4 amp–hours) by 60 minutes to get amp-minutes,
then dividing by the average current draw. In this
case, the 2400’s yield 144 amp-minutes. If the
average current draw is 18 amps, you’ll get 8
minutes of flight.
high torque design of the relatively light AXI 4120/18
motor allows it to put out a tremendous amount of
powered my 4 Star 40 with several electric power systems
over the past few years, all of which have worked very
well. My current configuration is powered by an AXI 4120/18
motor and Jeti Advance 70-3 Opto controller carried by
Hobby Lobby. Jeti controllers are simple to use and very
reliable, and the new Advance series goes a step further
with programmable timing, lower rpm “idle,”
and improved efficiency.
3/16-inch balsa doubler glued against the top of the
fuselage interior serves as a base for the batteries.
Two 8-cell 2400mAh battery packs are attached to each
other with Velcro, then attached to the base plate
with Velcro. A Velcro strap ensures everything stays
in place. The speed control wires run through the
firewall to the motor, while the controller itself
is allowed to “float” in the battery compartment.
Anderson Power Pole connectors ensure good power flow
between batteries and controller, plus they allow
the two battery packs to be joined in series to make
up a 16 cell pack.
motors have a huge amount of torque for their weight due
to their unique design. Instead of the typical brushless
motor design which places the magnets on the rotor and
spins them inside the windings on the housing, the AXI’s
have an “outrunner” design—the magnets
are mounted to the housing (on the inside wall), spinning
around the windings which are fixed on the inside. The
shaft runs through the middle of the windings and attaches
to the rotating housing at the back plate. These motors
are designed to spin very large props without a gearbox.
AXI motors must be mounted at the front of the motor (generally
behind a firewall or a plate), since the entire outside
housing rotates. Most new brushless motors, such as the
AXI I’m using in the 4 Star 40, are sensorless designs.
Reversing these motors is easy; just switch any two of
the three leads and the motor is reversed. Motor timing
and commutation are all performed in the controller.
Before we get into the details, let's look at the good news
in terms of flight performance. In the electric conversion
that I will describe, an AXI 4120/18 brushless motor spins
a Graupner CAM 14x9.5 prop at 7500 rpm and 38 amps on 16
cells, providing a huge amount of thrust. This system provides
about 4 minutes of full throttle power, but you can fly
up to 8 minutes with a mix of good aerobatics and cruising
flight. Takeoff is almost immediate, and the power available
will allow the 4 Star 40 to fly knife-edge, extended verticals,
and even perform torque rolls. It performs better than it
did with the glow engine in it!
Electric power isn’t for everyone or every plane, but
I think most pilots who opt to power their planes with electrons
will find the experience very satisfying. Current technology
places electric performance nearly on par with glow performance,
without the disadvantages of noise, fuel, starting hassles,
etc. What could be better than glow performance and electric
convenience in a .40 sized airplane?
article appears courtesy of Fly RC.
RC Magazine is published by Maplegate Media.
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