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ELECTRIC DRIVE SYSTEMS
This brief introduction to electric drive systems is
intended to just wet your appetite. There is a wealth of knowledge out
there concerning electric models and their drive systems available on the
internet. Please consider the information presented here as merely
guidelines as usual there are exceptions to every rule. Things are
changing so fast in the electric flight world and in my opinion this makes
it all the more exciting!
Contrary to popular myth any model aircraft can be
made to fly with electric power. Unfortunately some planes are easier to
convert than others. Luckily for today’s electric enthusiast more new
aircraft designs are being created for electric than any other power
source. Also manufacturers and retailers are beginning to offer package
deals with an airplane and matching electric power system it’s even easier
than ever to make the switch.
Part of the beauty of electric modeling is the
ability to go as “deep into the details” as you want. You can keep it
simple and enjoy clean/quiet flight or you can design, build, experiment,
and tweak to your heart’s content. The sheer number of motor choices,
gear ratios, prop sizes, speed controllers, battery chemistries, battery
pack configurations, battery chargers…well you get the point. All of
these choices can make jumping into electrics seem very daunting. Luckily
electric designs are now exploding onto the market. The fastest growing
segment is the Park Flyer/Indoor Flyer. Park Flyers usually fall in the
10-20oz category while the indoor flyers typically range from several
grams to about 15 oz. depending on the size of the indoor area.
Some of the positives of
electrics:
- Clean, no mess
- Quiet, potential for
expanded venues: backyard, parks, indoor and fewer “lost flying sites”
due to noise issues.
- Small inexpensive
planes can be built from foam and can rapidly increase your flying
abilities, you aren’t as attached to a plane scratch built in 8 hours
with less than $5 dollars (or <$50 for most kits), smaller size means
easier to transport in smaller vehicles or bring more planes to the
field
- Ability to tweak to
your heart’s content or keep it simple, bolt on a prop, charge the
battery and fly
- Usually less
“fiddling” than with a nitro
And now for some
downsides:
- Entry cost will be
higher than for an equivalent size nitro aircraft
- More downtime while
waiting for batteries to charge (can be overcome with multiple battery
packs, but this adds to the entry cost)
- There is slightly
more work away from the field to charge the extras battery packs, nitro
guys just throw the TX and RX on the charger and go to bed, electrics
usually have to charge the TX and several flight packs, but you didn’t
have to clean the oily mess off your plane at the field either!
- Flight equipment
placement can be more critical, electric motors generated EMI
(Electromagnetic Interference) which can cause more radio glitches than
a nitro engine, keeping things properly spaced usually solves this
problem, this can be difficult however since many electric planes are
smaller than their glow counterparts
- The smaller
electrics are more affected by wind, the larger ones can actually be
better in the wind due to there higher overall weight when compared to
an equivalent nitro.
Selection of an electric
drive system (system is the key word since all of the components work
together to produce the thrust) involves choosing either a prepackaged
system or mix and match. In either case, all systems will have the
following components: prop (or fan if you’re going for an electric ducted
fan jet), motor, speed controller, battery. A gear drive will also be
necessary for many applications and these are usually included when
purchasing a motor or complete package.
Each of these components
will be presented in the following paragraphs in no particular order since
they are all an integral part of the system.
Props
Just as in a nitro bird,
prop selection is important and must be tailored to the type aircraft,
speed of aircraft, and flying style. When working with an electric
aircraft it is usually best to use a prop specially designed for
electrics. They will be lighter, and can therefore be spun with less
power and will have an airfoil designed for the type a specific type of
electric flight. For example, if a prop is for a slow flyer it will have
an undercambered airfoil shape that makes it more efficient at slow speeds
rotation speeds and slow flight speeds.
In the U.S. props are
specified using 2 numbers, e.g. 12x6, first number is the diameter in
inches and the second number is the “pitch” also in inches. This number
is the theoretical distance the prop will move forward with each
revolution assuming no slippage.
Some basic rules of
thumb (this applies to all props):
- Larger diameter =
greater potential thrust and more efficiency, results in a higher
current draw for the same gear ratio (shorter flight times), lower RPMs
if a higher gear ratio is used keep the current the same.
- Slower turning props
mean lower top end speed for a particular aircraft
- Higher pitch =
higher current draw assuming the same diameter, less static thrust
(poorer low speed performance, prop is stalled or bogged down at slow
speeds), more “dynamic” thrust (better top end performance, i.e. higher
top speed if the plane doesn’t have too much drag) A high pitch prop on
a slow flying plane is a poor combination, the prop never really gets to
strut it’s stuff.
- Lower pitch = lower
current draw assuming same diameter (therefore longer flight times),
better low end performance (better for 3D work or slow flyers), limits
the top end speed of the aircraft
If you beginning to see
a connection, all else being equal, a higher current draw results in a
shorter flight time, a lower current draw results in a longer flight
time. This is one of the governing rules of electric flight!
Gear Box
A Gear Box essentially
allows a particular size motor to drive much larger props than would be
possible otherwise. As we already know this means that thrust goes up,
efficiency goes up, and flying time goes up for the same size battery.
The gear box allows a proper match between the optimum propeller and the
motor. Gear boxes are available for all common motor sizes and the gear
ratios can usually be changed for a particular application. This means
that the same drive system could be used on a slow flying Cub, a 3D
aerobat, or removed completely for a direct drive pylon racer.
Motor
Several basic types are
common in todays electric power systems, brushed motors and brushless DC,
both types come in any size imaginable.
Brushed motors have been
used the longest in models and are therefore usually less expensive for a
given size than their brushless counterparts. Some of the most popular
sizes are 280, 400, and 500 can motors (also known as “05” can motors used
in RC cars). The number usually refers to the approximate length of the
can in tenths of millimeters with each larger size also having a larger
diameter and usually a standardized bolt pattern. Basic motors can
purchased from virtually all of the electric manufactures (GWS, Graupner..).
The crème of the crop in brushed motors is the Astro Cobalt series which
uses numbers similar to nitro engines (i.e. 10 20, 40, 60) to “size” its
motors (although they don’t exactly match up to the equivalent nitro
engine, they’re actually more powerful).
Sometimes the name
“Speed” is placed in front of the sizes but that’s just one manufacturers
nomenclature and really (Graupner). The can motors are all basically the
same, permanent magnets are mounted around the outside of the can, a
rotating armature with windings creates a magnetic field when voltage is
applied. There are brushes that transfer the current to the contacts (commutator)
on the rotating armature. It’s the commutator that is responsible for
switching the magnetic field at just the right time to keep the motor
spinning. Some motors have sealed cans where the brushes cannot be
replaced and are considered throwaways when the brushes wear out. Others
(usually 500 and up) can be purchased with replaceable brushes since they
are more expensive and more costly to just throw away! All of the motor
sizes are available with a different number of “turns” of wire around the
armature depending on the application. The number of turns sets one of
the most important parameters for an electric motor, it’s so important
that it’s everyday name is the “motor constant” or KV. It is
expressed in RPMs/Volt. Fewer turns on the armature means less torque but
a higher achievable speed therefore a higher KV. More turns
means more torque but a lower top end speed and lower KV.
In all cases a brushed
motor needs to be properly broken in for maximum power output and longest
life. This break-in process helps to contour the brushes to the
commutator for maximum current transfer and more importantly less arcing!
There are two main methods of break-in: dry and wet. The dry method
involves running the motor with no load using a reduced voltage. The
voltage should be DC (so don’t use your models speed controller) and about
¼ to ½ the normal running voltage. Some people use 2 AA alkaline
batteries in series to do the break-in. Run the motor without prop (or
gear box if possible) for 5-6 minutes at a time (shorter if the motor gets
hot, longer if the motor only gets warm). A usual dry break-in requires
about 1-1.5 hours of runtime. The break in process is finished when the
brushes make constant contact all along the width of the brush. If you
have a sealed can motor and cannot inspect the brushes, the break-in
should be extended a little longer to guarantee a good mate between the
brushes and commutator (Note: The extra time is due to the fact that the
sealed motors usually have harder brushes that take longer to break-in,
the plus side is that they usually last longer than softer brushes). The
wet break-in involves holding the brushed end of the motor in a bowl/cup
of water and running it for 30 seconds at a time. Change the water when
it turns slightly grey. Repeat until the brushes are properly seated
against the commuatator (usually within only a few minutes of running
time, i.e. the wet method is much faster). In either break-in method the
motor should be blown out with compressed air, sprayed with electric motor
cleaner and allowed to dry before being put into service. Proper break-in
helps to reduce arcing at the brush/commutator interface which helps to
extend the life of the brushes and commutator. All that arcing burns
little holes into the brushes! Also less arcing produces less
interference with the aircraft control signals (i.e. glitches).
Brushless DC motors come
in two basic flavors: internal rotors and external rotors (or “outrunners”).
In either case, no break in is required and no brushes need to be replaced
so the motors can last indefinitely. The bad news as that brushless
motors cost significantly (usually 2-3 times more) more than an equivalent
sealed can brushed motor. The plus side is that they are more efficient,
therefore they run cooler for the same power input or can produce more
power to the prop for the same amount of heat output. Ultimately it’s the
heat output that usually limits the maximum power of a motor.
Internal rotor brushless
motors can usually be found in the same basic sizes as brushed motors. A
partial list of manufacturers includes Aveox, Hacker, Jeti, Model Motors,
HiMaxx...(and is growing by the minute!) The main difference is that the
permanent magnets are mounted on the armature and the coils are arranged
around the outside of the can. Since the coils are not spinning there’s
no need for brushes. Finding a comparable internal rotor brushless
motor is most easily accomplished using the KV parameter and
the recommendations of the motor manufacturer, retailer, or someone else
who’s gone before.
External rotor brushless
motors (outrunners) have the coils situated on the inside of the motor
facing outward, the permanent magnets are arranged on the outer can. It
is actually the outer can that rotates on an outrunner. That’s right, the
shaft is actually attached to the outer can! Since the magnets are
further out on the motor, more torque is created therefore larger props
can be driven directly. In fact, almost all outrunner installations are
direct drive (except in electric helicopter installations, but again
that’s another discussion). Unfortunately do to their design there is
less standardization in size/part number and comparison between brushed or
internal rotor brushless. It’s best to again use the motor constant KV
and the manufacturers recommendations when choosing a motor.
Outrunner motors run about the same price as the internal rotor models and
manufacturers include AXI, Koehler, PJS… The good news there are some
alternatives for truly cheap external rotor brushless motors. These
motors are suitable for small aircraft around 10 oz. or so and are
available for free if you can find a suitable CD Rom drive from an old
computer. Luckily there are people on the internet that have obtained
large quantities and they can be had for $10-35 depending on the amount of
work you are willing do ($10 buys a motor kit that requires a little bit
of assembly while $35 gets you one ready to run). This attractive price
puts them in the same price range as sealed can brushed motors which is
awesome!
Speed Controller
The speed controller is
the device that feeds power from the battery to the motor. As its name
suggests it adjusts the speed of the motor. In the early days a servo arm
moved a switch to give 2 settings: off or full throttle. Then speed
controls progressed to a mechanical rheostat which was also controlled by
a servo to provide intermediate speeds. These rheostats would tend to
wear out over time. Today a totally Electronic Speed Control (ESC) hooks
directly into the throttle servo channel of the receiver so the weight of
the speed controller is offset by the loss of a throttle servo. Another
benefit of modern ESCs is the inclusion of a Battery Eliminator Circuit (BEC)
which replaces the receiver battery back by powering the onboard RX and
servos using the motor battery pack (again helping to reduce weight).
Unfortunately a different type of speed controller is required to drive
brushed motors and brushless motors. Both types of speed controllers are
available in any size to match any aircraft application and are usually
rated in Amps of current and also the numbers of battery cells that they
can be used with. Again manufacturers, retailers, or more experienced
electric modelers can aid in the selection of a proper speed controller.
Popular brands of brushed controllers are produced by GWS, Castle
Creations, Jeti… Brushed speed controllers are almost always less
expensive than the equivalent brushless controllers for a particular
application. As stated earlier Brushless controllers are required to
drive brushless motors. Castle Creations and Jeti are two of the most
popular brushless controller manufacturers.
Battery
Finally the last piece
of the system, the battery (actually it’s a battery pack since most models
need more than one cell to properly power the model). Three main types of
chemistries are currently popular for model aircraft power: NiCd (Nickel
Cadmium), NiMH (Nickel Metal Hydride), and Lithium (both lithium ion and
lithium polymer, usually referred to as Li-Poly). Each type of chemistry
has it’s advantages and disadvantages. Cells are rated in “current x
time” capacity, i.e. “Amp hours” (AH) or “milliamp hours” (mAH). The cell
capacity is referred as “C”. For example a 1200mAH battery can
theoretically supply 1200mA for one hour, at this rate it is said to be
discharging at a “1C” rate. If we were to discharge it at higher rates,
say 2C (i.e. 2400mA) it would only last for 30 minutes, 3C (3600mA) it
would last for 20 minutes, 4C (4800mA) for 15 minutes, 5C (6000mA) for 12
minutes, 6C (7200mA) for 10 minutes, 10C (12000mA or 12Amps) for 6
minutes.
NiCd
Advantages:
can usually be rapid charged and peak charged, i.e. (2-3C rate)
which results in a 20-30 minute charge, cell voltage of 1.2V per cell
allows fine tuning of system performance by adding or subtracting a cell.
Packs are usually specified by the number of cells in series, type of
cell, and the total capacity of the pack (i.e. 8 AA cells, 1200maH)
Disadvantages: lowest power density of all cell technologies (i.e.
heavier), results in short flight times, cells cannot be connected in
parallel for increased current capacity. Larger cells are used for
increased current capacity. Packs need to be “cycled” routinely (i.e.
complete discharge to 0.9V per cell then slow charged) to prevent a
“memory” effect from being set up, the slow charging also has the added
benefit of “balancing” all of the cells in the pack (all cells are charged
to their maximum voltage).
NiMH
Advantages: higher power density than NiCd (about 50- 100% more)
lighter flight packs result in better performance with same flight time as
NiCd or similar performance as NiCd but with slightly longer flight times,
cell voltage of 1.2V per cell allows fine tuning of system performance by
adding or subtracting a cell. Packs are usually specified by the number
of cells in series and the total capacity of the pack (i.e. 8 cell AA,
1200maH). Virtually no “memory effect” like NiCd but occasionally need to
be slow charged to re-balance the pack.
Disadvantages: cannot be charged any faster than the cell capacity
(i.e. “1C”), using a peak charge algorithm results in around a 60 minute
fast charge, cells cannot be connected in parallel for increased current
capacity, larger cells are used instead.
Li-Poly (and Li-Ion)
Advantages:
extremely high power density, results in very lightweight packs for
greatly increased performance or similar weight packs with greatly
extended flight times, nominal voltage per cell of 3.7V results in fewer
cells in series to reach a required voltage. Cells can be paralleled to
increase current capacity to reach required current. Packs are specified
by the number of cells in series and parallel and the total current
capacity (i.e. 3s2P, 4200mA). Packs can be recharged when less than fully
discharged without fear of “memory effect”.
Disadvantages: can only be charged at 1C on a “non-peak” mode because
of the maximum voltage limitation per cell of 4.2V (or they may be
damaged), typically charge times for a completely discharged cell is
roughly 1.5 hours for full capacity (on the plus side if your pack has
extra capacity they can be charged to 80% of full capacity in about 1
hour), cells should not be discharged below 2.75V per cell under load (3V
no load) or they may be damaged, charging at rates greater than 1C or to
higher voltages than 4.2V per cell may cause these cells to catch fire, it
is extremely important to never leaves cells unattended while charging and
as an extra precaution they should be placed in a clay flower pot or other
fireproof container while charging.
Sanyo and Panasonic both
manufacture NiCd and NiMH cells in various shapes and sizes, AAA, AA and
Sub C sizes are very popular depending on the size of the aircraft. For
Lithium polymer cells Thunder Power imakes packs for all size aircraft and
Kokam and E-Tec also make excellent packs for smaller aircraft (park
flyers and smaller).
Each battery technology
requires a charger capable of charging that particular battery chemistry.
DO NOT CHARGE A BATTERY ON A CHARGER NOT DESIGNED TO CHARGE THAT
CHEMISTRY.
Support Equipment
Which brings me to
support equipment, Astro Flight makes an excellent inline meter called the
Super Whattmeter (yes I know Astro Flight can’t spell!) that measures the
battery voltage, battery current and battery capacity while running on the
bench. This tool can help to fine tune a system. Change a gear ratio,
increase/decrease prop size, change batteries cell count or type and see
the results. Testing like this can help set the maximum current draw so
as not to damage the motor, battery or speed controller. A good charger
is vitally important. A multi-chemistry charger that supports all of the
battery types you’ll be using is recommended. A programmable charger that
can fast charge and slow charge a pack is a necessity for “balancing” a
pack (i.e. resetting all of the cells to their maximum voltage). Fast
charging tends to unbalance the cells over time and an occasional slow
charge will bring the cells back to the same voltage. Orbit makes an
excellent (but somewhat expensive) line of chargers called the Microlader
series that will charge everything (NiCd, NiMH, Lithium, Lead acid). The
Great Planes Triton is also a good choice but it doesn’t allow charging of
large capacity lithium packs. Hobbico makes a nice field charger that
does NiCd, NiMH, and Lithium but is intended for smaller capacity packs.
2A charge current limit for NiCd/NiMH and 1A for lithium. It actually has
two chargers in one so you can charge two packs at once which is very
handy. It will also charge your transmitter and receiver battery packs.
Finally Motocalc is an indispensable tool in designing drive systems from
scratch or making modifications to an existing system. You can simulate
combinations of props, gearboxes, motors, speed controllers, batteries,
and even airframes until you achieve the results you desire.
Drive System Examples
A good rule of thumb for
estimating the electric power required to fly a typical airplane
(extremely small or extremely large aircraft will have different
requirements due to the scaling properties, but that’s yet another
discussion!):
- 50 W/lb. =
Trainer/slow flyer
- 75-100 W/lb. = sport
aerobatics
- 100-150+ W/lb. = 3D
aerobatics
Usually the more power
the better, but these are good starting points. Taking the estimated
all-up weight of the plane and multiplying it by one of these factors can
give an estimate to the total input power required to fly the aircraft.
For example, a 6lb. 3D aircraft like the Hangar 9 Funtana 40 would
theoretically take 750Watts of power to perform 3D aerobatic maneuvers
(6lb. x 125W/lb.= 750W). Since Power (in Watts) is the Voltage times the
Current (in amps) we could estimate that if we used a 3S, 11.1V
lithium-polymer battery pack it would have to produce 67Amps for short
periods while performing 3D maneuvers. This is a fairly high current
draw. In order to reduce the current to something more manageable say
30-45 amps we can increase the number of cells in series in the pack to
say 5S, 18.5V results in a maximum estimated current of around 40Amps.
Less current will be used when just flying around or setting up for your
next killer maneuver. Knowing the required current can help us pick the
max current capability of the battery pack (40 Amps), the current handling
capacity of the speed controller (40+ Amps), and also the power output
capacity of our projected motor (750W). A good choice would be the Hacker
B50L or C50L series of brushless motors. The B50L or C50L series of
motors can produce the required power and are recommended by Hacker for
“40” size 3D aircraft. For the battery, a typical Thunder Power 5S,
2100mAH pack can support 10C continuous discharge rates we would need a
minimum of 2 cells in parallel or a 5S3P, 4200mAH pack. In practice,
going to the next cell size up (in this case adding another set of cells
in parallel to create a 5S3P, 6300mAH pack) is preferable when on the
upper edge of performance. This insures that the battery will not
overheat and it also lengthens the flight times. 6300mAH pack discharged
at 40A peak and say 32A continuous will result in an estimated 5C
discharge rate which will provide for 12minute flight times. A good speed
controller would be the Castle Creations Phoenix line. The Phoenix 45
would be well suited since it is meant for brushless motors, can provide
for 45A continuous current with bursts to 60A, and it is capable of
running with our 5cell, nominal 18.5V lithium battery pack. If you feel
that 40A is too close to the limits of the Phoenix 45, the Phoenix 60
would be even better, it’s rated for 60A continuous and 80A burst!
Knowing that this
aircraft will spend a lot of its time performing 3D maneuvers like
hovering or harriers we want it to excel at slow speed flight and it’s
drive system should be tailored for that purpose. Therefore we will pick
a large diameter prop with a low pitch. A large gear ratio like the
Hacker 6.7:1 will be required to drive the large prop. We could buy
several sizes of props and try them on the bench with the Super Whattmeter
and select the one which pulls around 40-45A on the bench or we could plug
these components into Motocalc and see what it recommends. Motocalc says
that an APC 17x9 electric prop would draw 42.8Amps at full throttle on the
bench. When the plane is actually in motion the prop will “unload” and
the current will descrease. The average current will always be less than
the peak current since the plane usually doesn’t spend it’s time tied down
on the ground at full throttle! For example at 35mph the current at full
throttle would drop to an estimated 23.3A which results in a 16 minute
flight. Even if the plane is hovering (the same as flying at zero mph)
only 80% throttle is required to lift the weight of the aircraft.
Now if this all seems a
little too difficult, checking with the aircraft manufacture you can find
their recommendations for equipment. The manual I have lists a Hacker
B50-10L motor, a 5S4P lithium polymer pack, and a 17x10 electric prop.
All very similar to the ones we chose. (Note, since the printing of the
Funtana manual I have, the Hacker C50 series was introduced and has a
slightly stronger armature design and an integral heatsink, although an
optional heatsink can be purchased for the B50 and is highly recommended
when producing 750W in a cowled aircraft like the Funtana. You’ll also
note the manufacturer suggested 5S4P lithium polymer pack is slightly
larger than the one we specd at 5S3P. This is due to the fact that since
the manual was printed the Generation 2 Thunder Power lithium cells we
chose have hit the market and can support the 10-15C discharge rate listed
in the example. Previous generations could only produce around 6C
continuously which is why the extra cell was needed. They also recommend
a 17x10 electric prop which will provide for a slightly higher top end
speed at the expense of a slightly higher current draw. Low end
performance will be identical to the 17x9). Both props will get the plane
into the air and let you wring the plane out, final selection is
definitely personal preference based on flying style). Whew, That’s a
lot of information in one sitting, I need a break!
Good Luck, hope this
helps get you interested in trying electrics. If so try checking out some
of the websites out there on the Internet dedicated to the advancement of
electric RC modeling.
Electric RC Links
(just a sample of what’s out there!)
Discussion Groups
www.rcgroups.com – General discussion forums for all things RC!
Check out the electric forums especially the Foamies group. There are
links there to free downloadable plans for planes that can be built at
home for next to nothing! Also many of the foam kits currently being
manufactured are built and test flown by fellow modelers and “reviewed” in
the forum. Also interesting are the 3D and the Electric helicopter
forums.
www.rcuniverse.com – More discussion groups!
Electric
Suppliers
www.hobby-lobby.com – Has many electric ARF and kits, large
selection of motors, speed controllers, props, some batteries, misc.
electric accessories.
www.aeromicro.com – Good for motors, batteries, speed
controllers, servos for Park/Indoor/Slowflyers.
www.radicalrc.com – Electric kits, motors, batteries, speed
controllers, props for Park/Indoor/Slowflyers, Orbit chargers, connectors.
www.towerhobbies.com – No explanation needed. They have
everything Glow but are playing catchup in the electric arena. Not bad
for ARFs, kits, motor combos, speed controllers, battery chargers,
connectors.
www.maxxprod.com – Misc. electric accessories, motors, speed
controllers, props, good source for custom made NiCd and NiMH battery
packs.
www.tppacks.com – Thunder Power Lithium Polymer packs.
www.helihobby.com – Site for Electric (and Glow) helicopters
kits, parts, motors, speed controllers, batteries
www.fxaeromodel.com – Site for Electric helicopter kits, parts,
motors, speed controllers, batteries.
www.fmadirect.com – North American distributor for Kokam Lithium
Polymer cells and FMA Radio equipment like micro receivers.
www.gws.com.tw/english/english.htm - Grand Wing Servo Tech.,
makers of many Park Flyer aircraft kits, inexpensive micro servos, brushed
motors/gear boxes, speed controllers.
www.castlecreations.com – Maker of the Pixie and Pegasus Brushed
speed controller lines and the Phoenix Brushless speed controller family.
www.nikitisaircraft.com – Club member Ed Nobel’s website for
aerobatic foam models.
www.foamyfactory.com – Download some plans (both free and for
purchase) for easy to build foam models. Lots of info on build and
outfitting aerobatic foam aircraft.
www.3dfoamy.com – Tons of foam aerobatic models in kit form
plus lots of information on building and outfitting.
www.quicktechhobby.com – Source for Tanic Lithium Polymer battery
packs and other supplies.
www.espritmodels.com – Source for all sizes of Thunder Power
Lithium Polymer battery packs and other modeling supplies (motors, kits,
speed controllers)
www.nesail.com – Northeast Sail. Sailplanes, electric aircraft
kits and ARFs both foam and traditional built up, speed controllers,
motors, combos, batteries.
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