ELECTRIC MOTOR SELECTION
GUIDELINES
Choosing the power plant for your new creation is one of the keys
between success and failure. The correct choices can allow your new
plane to have the performance you desire- an incorrect choice can
result in either a powered glide to the ground or an
overweight aircraft which is challenging, but not necessarily very
rewarding to fly.
There are three major components in an electric flight power system:
the battery, the propeller, and the motor/gearbox. The power system is
only as good as its weakest link, therefore, for optimum performance,
all components play a critical role.
In the design process of matching a power system to an aircraft, one of
the best ways to proceed is to determine the projected all up weight of
the aircraft, determine the watts needed, the propeller (or fan) and
finally the motor. Iterating the process until you come up with a
solution that hangs together is one of the fun challenges in e-flight.
There are 3 pieces of information you need to determine what battery
(group of cells) to use in a particular airplane:
1) The number of watts needed.
2) The weight of the battery
3) The size of the battery.
There are a variety of rules of thumb out there. For most reasonably
aerobatic sport aircraft applications- it’s not very hard- start at 100
watts/lb (when measured hot off the charger, i.e. within the first few
seconds of a running.) This will give performance comparable to glow
powered aircraft. Slower aircraft can get away with fewer watts/lb,
faster aircraft may need more. Old style pattern type performance
probably needs closer to 125-150 watts/lb- race airplanes- you’re on
your own.
Obviously figuring out your total watt requirements is something of an
iterative process. Starting with your empty weight, you get to add in
the weight of the batteries and everything else to get to your total
weight. Typically figure battery weight between 25-35% of an airplane’s
gross weight.
Nearly all e-planes will require a battery made up of two or more cells
(light indoor stuff is an exception.) This FAQ is intended to help you
choose what cells to use to make up your airborne battery.
At this point in time (November 2002) there are 5 broad classes of cell
types that can be used in e-flight. I suspect that this FAQ will become
rapidly dated, battery technology has been changing quickly over the
past decade, and this rate of change shows little signs of abatement.
While other areas of e-flight (motors, escs, radios) etc. have reached
some level of maturity, battery cell technology continues to evolve. It
is quite possible that fuel cells will render all our current
technology obsolete in less than a decade.
As noted above, the first question we need to answer when choosing a
battery pack for an airplane is: how many watts do we need? This is
where the 5 broad classes of battery cell types come in. Recall that
watts are simply amps x volts. In terms of battery cells, the weight of
the cell is related to the number of amps it can deliver- larger and
heavier cells can deliver more amps, and hence, more watts. The 5
classes of cells are:
1) Less than 8 amps (generally found in airplanes less than 1 lb.)
2) Between 10-15 amps (generally found in airplanes from 1 lb to 1.5 lb)
3) Between 22-28 amps (generally found in airplanes from 1.5-2.0 lbs)
4) Between 30-40 amps (generally found in airplanes 2 lbs plus)
5) Between 35-45 amps (generally found in airplanes 2.5 lbs plus)
This all seems so simple doesn’t it? Where lots of aspiring e-pilots
run into trouble is that battery manufacturers don’t label their cells
in this fashion. Well, why not? In e-flight, we abuse batteries, we
don’t use them within the operating parameters specified by the
manufacturer. Battery manufacturers sell batteries on the basis of
capacity first and foremost. Unfortunately, battery capacity changes as
a function of discharge rates- higher discharge rates will yield lower
capacity. Consequently, battery manufacturers don’t label their cells
as a 10 amp cell for example, they label cells as having 1020 mAH. For
e-flight purposes, we really need to identify the amp delivery
capabilities of a cell, much more than its capacity, so the
manufacturers rated capacity becomes an arbitrary designation for a
cell. Further complicating the issue- one manufacturers 1700 mAH cell
will deliver up to 15 amps, another manufacturers cell will handle 25
amps, and yet another cell will handle 40 amps. Consequently, you need
to be careful to identify a cell with information other than capacity-
often sizing codes are used. This doesn’t make things easier for the
e-flier, but that’s the way the battery industry operates. Since we are
not a significant market to these manufacturers, they’re not going to
listen to our requests, so this becomes something that we all must
learn to deal with. (Battery mfg aside- there is only one battery mfg
which has devoted significant resources to e-flight- SR Batteries,)
which manufactures both cells and assembles packs. SR cells for
e-flight are conveniently labelled Max cells and come in sizes which
roughly correspond to the classes below. Some of the newer li-poly mfg
such as Kokam and Thunder Power are also devoting some resources, but
neither of these firms has the track record of SR.)
Battery Chemistry- there has been a great deal written about the type
of chemistry a cell uses to deliver its voltage. Let me point out that
the airplane does not know whether it has a nickel cadmium (nicad), a
nickel metal hydride (NiMH) or a lithium ion polymer (li-poly) cell
inside. We’ll cover the advantages and disadvantages of each down
below, but getting the correct amount of watts and weight is far more
important than which battery chemistry to choose. (Environmental Aside-
one of the attractions of e-flight is its non-polluting nature. It
should be pointed out that all batteries are not alike in terms of
environmental considerations. Nicads, which contain cadmium- a toxic
heavy metal- are far and away the most harmful to the environment and
need to be disposed of carefully. Radio Shack is serving as a
collection point for discarded nicads. NiMH cells are much less
problematic, and Li-polys even less so. Li-polys can apparently be
discarded with normal household trash, I’m uncertain as to the status
of NiMHs.)
About amp delivery- most sources estimate the amp delivery of a cell as
where it will still deliver close to its rated voltage. As the amount
of amps drawn out of a cell increases, the voltage the cell produces
drops. Since amp delivery can increase faster than voltage depression,
fliers interested in maximum performance strive to use higher amp
systems- as exemplified by F5B competitors who exceed 100 amp draws
from their batteries. For sport fliers however, the rated amp delivery
of a cell is often set at where the cells voltage hits 1.0V for either
nicad or NiMH, or 3.7V for Li-poly (a safer alternative is to use 3-5 x
rated capacity of the cell, C frequently written as 3-5C. This
technology is evolving rapidly- there are already some cells which are
rated at 5C continuous draws, and 10C for short time periods.) In
e-flight, there are 3 questions to ask about a cell:
1) How many amps can it deliver?
2) What does it weigh (and how large is it if battery space is limited)
3) For how long will it deliver this amperage? (Duration- often the
bugaboo of e-flight.)
While the first two questions are straightforward, the last one is not.
Since rated capacity falls as amperage draws increase, we should not
expect to get the manufacturers rated capacity of a cell under e-flight
demands. The difference between the rated capacity and the actual
capacity depends upon the usage of the cell. One thing to be careful of
is that a cell which has a high rated capacity, when used under
e-flight amperage loads, can fall dramatically. As an example, a CP
1300 mAH cell will deliver about the same amount of mAH as a 1700 mAH
4/5 AUP cell when used at amp draws exceeding 25 amps. Cells which have
high capacities, but relatively low amp delivery characteristics will
only be good for low powered duration applications.
Battery Chemistry General
Here are some guidelines which describe the overall characteristics of
cells based on users experience.
Nicads-often the highest energy output of any cell, robust, well proven
technology. Easy and fast to charge. Moderate temperature dependence.
(Memory is nonsense in R/C applications.) Disadvantages- in most
applications, the lowest duration of any cell type.
NiMH-some newer NiMH cells offer energy output comparable to nicad at
reasonable amp draws. For higher amp draw applications, most nicads are
superior. (Exception- largest NiMH cells may outperform nicads.)
Disadvantages- strong temperature dependence- NiMH cells like to be
warm. NiMH cells also lose charge rapidly, best to charge immediately
before flying. NiMH cells will generally not tolerate abuse as well as
nicads- more sensitive charging technology needed- total number of
cycles less than nicads.
Li-poly- can offer very high energy output and capacity. Low
temperature dependence, plus excellent charge retention. Disadvantages-
limited availability, slow charge rates, delicate. Still experimental
in applications, thus longevity is unknown.
One of the most common mistakes in e-flight is to use a cell larger
than needed. There are two ways to improve performance in a given
airframe- add lightness, or add power. If your cells are heavier than
needed for a given amp draw, you’ve violated tenet 1), i.e. add
lightness. Always be very careful when using a cell larger than needed
in an application.
Now that we’ve got some of the basics under our belt, let’s get to some
more specific guidelines. Please note that these are guidelines- not
the ten commandments. I’m sure that there are exceptions to every rule,
but these guidelines are intended to get your thought processes moving
in a direction which has been successful for lots of airplanes. If you
find that your coming up with a power system which is very far away
from these guidelines- be careful, odds are there’s a problem
somewhere. Also note that these guidelines are not intended for
competition purposes- they’re intended for the average “sport” flier.
This guideline also separates cells into classes. There are advantages
and disadvantages to this approach. The advantages are that we get to
compare various types of cells for similar applications, i.e. an apples
to apples comparison. It does very little good to compare a lithium
polymer cell which is suitable to a parkflier to a CP 2400 nicad cell
which is utilized in 4 pound plus sport aircraft. The disadvantages are
that these classes are not recognized outside these guidelines, so if
you go to a hobby dealer, and say “I need a class 1 cell.” He’s liable
to look at you oddly and sidle carefully towards a phone. On the other
hand, if you can describe what characteristics a class 1 cell has, i.e.
battery packs less than 5 ounces- used in under 8 amp draw
applications, then you have a fighting prayer.
Class 1 cells (under 8 amps)
These are the cells which are used indoors and in parkfliers with total
watt applications at 60 watts or less. Aircraft which use these cells
can be up to 20 oz. or so. Battery packs in this class typically weigh
under 5 ounces and range from 2 to 10 cells.
This area of e-flight has seen a revolution in the past year.
Effectively Li-poly technology has taken over the indoor scene, and is
now rapidly making inroads into parkfliers. This class of cell has more
types of cell chemistry represented than any other with: nicad, NiMH,
Li-ion, Li-poly, and Li-Mn (Tadiran, fading from the scene- no longer
available.) Consequently, there is a big shake up going on with motors
and aircraft as well, since motors must match the characteristics of
the battery.
Li-poly’s have a dramatic advantage in this size range since they don’t
have a metal case. Consequently, they have a big weight advantage over
all other forms of battery technology. Li-poly cells are available in
capacities from 50 mAH to 2070 mAH, with more capacities being added on
a fairly regular basis. Li-poly’s have wonderful energy density in this
class of cells, with Li-polys being able to deliver more amps, at
lighter weight and for longer than other battery technologies. These
cells are not as temperature sensitive as other cell chemistries, plus
they retain their charge without loss for extended periods of time. (No
need to top up right before flying if charged the week before.)
Other properties- Li-poly cells deliver 4.2V without a load, which
drops to 3.7V under load. Consequently, a 2 cell Li-poly battery will
deliver 7.4V under load- which makes it roughly comparable to a 7 cell
nicad or an 8 cell NiMH. At this time, 2 cell configurations are very
popular, but 3 cell batteries should also prove to be a good match with
motors which perform best under high voltage/low amperage situations
(ex. Astro 010, Speed 280). These cells are limited to 3-5C in terms of
amp delivery.
Disadvantages- motors which perform best on less than 7V, i.e. HY-50F,
MG-1, HY-50D are poor matches for these cells. Also, pack assembly
appears to be a tricky business, and these cells are not as robust as
other cell types. These cells are still experimental, and longevity
remains an unknown.
Cost- since these cells are being used in consumer applications, costs
of smaller cells is falling rapidly. Already a 2 cell pack of 1020
Li-poly cells is cost competitive with NiMH or nicad.
Li-ion- these cells use similar chemistry to Li-poly, but wrapped in a
cylindrical metal jacket. Consequently, the weight of these cells is
higher than Li-poly for a given amp delivery. In terms of performance-
these cells have excellent duration, but do not deliver more amps than
nicad at a given weight. Again, used in consumer applications, so cost
is highly competitive.
Nicad-For many years the performance standard in battery technology. In
this size range, nicads will deliver more amps per ounce than NiMHs,
however, nicads do so at the expense of duration.
NiMHs- still do not have the amp delivery of nicads in this size range-
consequently it is necessary to add an extra cell as shown in the
example below. NiMHs also have a strong temperature dependence- these
cells should be warm when used. Furthermore, NiMHs also lose charge the
quickest of any cell- topping off before flying is highly recommended.
Example- M-100 motor with stock prop
Typically draws about 4-4.5 amps with stock prop at 7.4V
Battery options-
a) 2 cell Li-poly-1020 mAH wt. 1.5 oz.
b) 7 cell nicad 350 mAH wt. 3 oz.
c) 8 cell NiMH 720 mAH wt. 3.4 oz.
Most widely used cells- below 8 oz airplanes- 8 cell 300 mAH NiMH,
above 10 oz- 8 cell 720 mAH NiMH.
Class 2
Cells from 10-15 amps. Typical battery packs weigh from 5-8 oz.
Aircraft which used these cells range from 16 oz to 28 oz. Typical watt
range from 75-150 watts from 7-10 cells.
There are two popular types of cells, the 600 mAH AE nicad, and the
1100 mAH NiMH cell manufactured by HE cells (plus Hobby People
equivalent- which may be the same cell in a different wrapper.)
Li-polys are poised to make an impact in this class of cells, but to
date, their impact has been minimal.
The 600 AE nicad has been a staple of the Speed 400 class of airplanes
for years. This cell weighs about 0.6 ounces, and will deliver up to 12
amps- occasionally more if pushed.
The 1100 HE cell is much newer. This NiMH technology delivers basically
the same watts per cell as nicads, thus an 8 cell pack of HE cells is
roughly equal in power to the 600 AE. The cell may be able to handle
somewhat higher amp draws as well- some users have reported good
results at 15 amps, which is somewhat better than 600 AE cells.
Duration is much improved compared to 600 AE cells, reports range from
33-50% increases. Note that when pushed to deliver more amps than 600
AE, cell deteriorates rapidly.
Other options- 1100 AAU nicad. Somewhat heavier than 600 AE, delivers
more watts however. Cell is somewhat delicate for a nicad, well suited
for duration applications.
1350 NiMH cells- slightly heavier than 1100 HE cells. Performance
comparison data lacking.
950 KAN cells. NiMH cells- reported to deliver more volts than either
the 600 AE or 1100 HE Cell when used very, very warm. Possibly more
robust than HE cell.
Older NiMH cells, such as the 1050 mAH cells should be used with
caution- these cells do not deliver the watts of a 600 AE and should be
used in lower amp draw applications.
Example
Speed 400 on a 5.5 x 4.5 prop draws about 12 amps on 7 volts. Battery
options, 7 cell 600 AE, 7 cell HE cell 1100, 7 cell 1100 AAU, 7 cell
1350 NiMH.
Class 3 Cells
Class 3 cells are a large jump in weight over the class 2 cells. On
average, these cells weigh about 1.2 oz. or double the weight of class
2 cells. However, these cells will also deliver 25 and occasionally 30
amps, which means that they deliver double the power as well. These
cells work best between 20 and 28 amp draws however. These cells can be
used in airplanes from 24 oz to 48 oz. and can deliver from 150 to 275
watts. Typically class 3 packs range from 7 (for a 6 cell pack) to 14
oz and packs run from 7 to 10 cells (6 cell packs are somewhat
uncommon, but can be useful.)
There are really only two choices at this amp draw- the CP 1300 nicad,
or the 4/5 AUP 1700 mAH NiMH cell. Both cells are function well at 25
amps or below, the CP 1300 may have a slight performance advantage at
higher amp draws. The 4/5 AUP NiMH cell does have a significant
duration advantage when used below 25 amps.
Example- Jeti 15-4, 8.5 x 6 prop, 8 cell 4/5 AUP 1700 NiMH battery- 8 V
at 25 amps- 200 watts.
Common Problems- one of the most common misuses of batteries in
e-flight concerns these cells. Many people desiring more power for a
parkflier using an 8 cell 600 AE pack will substitute a 7 cell CP 1300
pack or 8 cell CP 1300 pack. They will complain that the added power
does not make up for the additional weight. Realistically, a
significant power increase from an 8 cell 600 AE pack is either a 9 or
10 cell pack, or a 6 cell CP 1300 or 4/5 AUP pack. Clearly, this
entails changes in prop and gearing, and rarely will a motor be happy
under these conditions. In general, these batteries do not belong in
airplanes which fly acceptably on 8 cell 600 AE packs.
Class 4 Batteries
Class 4 cells offer a 1/3rd increase in amp draw up to 40 amps at the
expense of a 25% weight increase. Typically, these cells can be used in
applications from 250 watts to 650 watts with 8 to 16 cells in a
battery. There are three types of cells in this class, the CP 1700
nicad (most popular) the 4/5 FAUP 1950 NiMH cell (not widely available
yet) and the 2000 mAH NiMH cell (older technology and fading from the
scene.) Aircraft which use these cells can range from 2 ¼ lbs to 7-8
lbs (or larger)
These cells are very versatile and can be used in relatively small
aircraft, to some of the larger sport e-powered aircraft. These cells
offer a significant performance boost over Class 3 cells around 28 amps
or so, and will function up to 40 amps.
The CP 1700 remains the most popular cell in this class. Compared to
the older technology 2000 NiMH cell it offers significantly higher watt
delivery per ounce over 25 amps. The older technology 2000 mAH NiMH
cell does have a duration advantage however, when used at 25 amps or
below. There is too little data to compare how well the 4/5 FAUP cell
will function when used at higher amp draw levels.
Example- Jeti 30/3 on 12 cell CP 1700, 9.5 x 7 prop, 12 V at 42 amps-
500 watts.
In most cases, the 10 cell CP 1700 has proved to be a better choice
than the 8 cell CP 2400.
Class 5 Batteries
These are the largest cells commonly used in E-flight- typically
weighing between 2.1 and 2.3 oz. However, most applications for these
cells limit amp draws to between 35 and 45 amps. Consequently, aircraft
powered with these cells do not necessarily have a performance
advantage over class 4 cells, although they generally have a duration
advantage. Class 5 cells come as either nicad or NiMH technology, and
the difference between the two can be hotly debated. These cells should
be used in applications from 350 watts to 1000 watts- in aircraft of 4
lbs to 12 lbs. Batteries consist of 10 to 24 cells (occasionally more).
There are several cell types available- two nicad- the RC 2400 and the
CP 2400, as well as several NiMH cells, 2600, 3000, 3300 mAH cells,
also available in high voltage (really high amperage) varieties. In
this class size, NiMH cells have comparable wattage output to nicad,
thus a 16 cell NiMH pack is comparable to a 16 cell nicad pack. All of
these cells have their adherents and meaningful comparisons are
difficult. Realistically, the CP 2400 still offers an excellent blend
of performance and low cost.
Common Problems
All too often, relatively new e-flyers will want to use these cells at
25 amps or so. In general, this is a mistake. At that low an amp draw,
some smaller and lighter NiMH cells will provide excellent duration at
good weight savings. These cells should not be used below 35 amps or so.
A note about cell counts
As noted earlier in this FAQ, for a given number of watts, a higher
voltage setup will prove to be more efficient than a lower voltage
setup. This should favor higher cell counts in larger aircraft. You may
note that a lot of aircraft stop at 16 cells though. This has more to
do with ESC technology than batteries or motors. ESCs that can handle
20 plus cells have much heftier price tags than ESCs limited to 16
cells. In many cases though, a higher number of smaller cells could
deliver better performance than a fewer number of larger cells, as long
as both cells are used at reasonable amp limits.
Propeller Selections
One of the most common mistakes in e-flight is to use the same
propeller that you would use on a glow powered model. Most of the time,
this is an error- occasionally a bad one. The reason is that most glow
powered aircraft use propellers sized for the engine, not the airframe,
and there are often terrible mismatches. These mismatches are a loss of
efficiency- not such a problem when you have a very powerful
lightweight powerplant, but when you are usinga somewhat heavier or
less powerful powerplant- you need to be careful about throwing away
energy. This added increase in propeller efficiency is why electric
airplanes can fly comparably to glow powered models on less input power.
Realistically, you need to size the propeller to the model without
using some preconceived notions. Full scale aircraft occasionally have
propellers that are up to 1/3rd the wingspan- check out the props on
WWII fighters for example. Other constraints such as landing gear, or
other clearances need to be looked at. You want to use the largest
propeller that can comfortably fit on your airframe- with one codacil-
pitch speed.
Pitch speed is how fast the propeller would move through the air if
there would be no airplane attached. We generally assume that a slick
airframe can actually get pretty close to this theoretical pitch speed,
but a draggy airframe cannot. Pitch speed is calculated by multiplying
rpm x the pitch in inches. (No the units don’t work- it’s a complete
kludge- but it turns out close enough.) As an example- a propeller with
a 6 in pitch turning 10,000 rpm has a pitch speed of 60 mph.
Nicely performing aircraft have a pitch speed which is 2.5x stall
speed. Most sport models in the 4-8 lb range stall around 20 mph, so a
pitch speed of 50 mph is a good minimum for a high wing sport
scale/trainer type airplane- closer to 60 mph for an aerobat or
warbird, and 70 plus mph for old style pattern.
As an example of prop sizing- I went from a 9 x 5 on a K + B 28 (no
tach) on a 500 sq in Corbin Super Ace (1930s high wing lightplane) to a
12 x 8 E prop turning about 6500 rpm. The electric version is slower,
not as maneuvrable, but flies in a more scale like manner- and has far
better climb at low airspeeds. The larger prop has the ability to pull
the airplane out of trouble.
I’ve also used comparably sized props on electric versions of airplanes
originally powered by glow. In general, the electric versions are a bit
slower, and lack some of the vertical performance of the glow powered
versions. However, if you go to a larger prop, you will not regain the
missing airspeed (larger props have lower pitch speeds) but you will
regain or conceivably surpass the glow powered versions vertical
capability.
Motor/Gearboxes/ESC
I’ve left the selection of motors till the end, because with electrics,
I find it’s more critical to know watts and propellers before looking
at motors/ESCs and gearboxes. Of course motor mfg would rather have you
pick their motor first, which may help explain some pretty funky
battery recommendations coming from them.
There are several types of motors in the market today- ferrite
(cheapest), cobalt (fading from the scene) and brushless (often my
recommendation). It’s important to realize that most of the time, what
you’re paying extra money for in a snazzier motor isn’t more power-
it’s lighter weight. In general, you can generally find a ferrite motor
that will turn the same prop the same rpm as a more expensive brushless
motor- but it can weigh 2x as much – or more.
Ferrites
Ferrite motors use relatively inexpensive ferrite (iron) permanent
magnets. These are the cheap can motors produced by Mabuchi for
applications such as automotive windows, wipers etc. Some folks have
made an art of the care and feeding of these motors, and there are
racing classes which use them.
Ferrite and cobalt motors use brushed ESCs- and the ESC’s are
interchangeable, as long as amp limits are obeyed. In anything larger
than Speed 400 applications (i.e. around 1 lb) most ferrite motors need
a gearbox for improved efficiency.
Cobalts
Cobalt motors use rare earth permanent magnets, which are much more
expensive than ferrite magnets. These motors come with ball bearings,
and are generally of greater construction quality than ferrite motors.
Cobalt motors are fading from the scene however, because relatively
inexpensive brushless motors have largely replaced them. In general,
the materials and construction of a cobalt motor and a brushless motor
are comparable, and since brushless motors have become more popular,
and are being made in greater numbers, the cost of a brushless motor is
frequently lower than a cobalt motor of comparable power. The major
advantage of a cobalt motor over a brushless motor is the ability to
utilize less expensive, brushed speed controls. This savings can be
negated by the necessity to use a gearbox for most applications.
Brushless motors
Brushless motors come in several flavors. There are high end, high
efficiency brushless motors, there are somewhat lower efficiency,
moderately priced brushless motors, and there are outrunner brushless
motors- i.e. rotating can.
Brushless motors, like the name implies, have eliminated the need for
brushes by turning the problem of motor construction on its head.
Brushless motors are actually built very differently than brushed
motors- they have the magnets attached to the shaft, and utilize
windings in the can of the motor. (Rotating can motors put the magnets
in the can and rotate that- which is connected to the prop.) By
alternating the electric field of the windings, the magnets on the
shaft rotate. There is no contact between the shaft and the windings
(brushed motors use brushes to carry electrical energy to the windings
on the shaft) which has eliminated most of the wear points of an
electric motor. The downside to this technology though is that the ESCs
are much more complex, and somewhat larger than the more conventional
brushed ESCs.
For some years, brushless motors were thought to be beyond the reach of
the average modeler in terms of price. However, a few years ago, the
introduction of moderately priced brushless motors by Jeti, along with
controllers has largely dispelled that perception.
High end brushless motors are produced by Kontronik, Hacker, Aveox,
Astro Flight (in smaller sizes) and Maxcim- high efficiency, typically
mated to a gearbox, and often an expensive solution.
Moderately priced brushless motors- typically not quite as finely made,
but still very sturdy- Jeti and Mega motors. The Jeti Phasor line comes
in 3 sizes- the 15 series, 30 series, and 45 series. The numbers refer
to the length of the motor in mm. Somewhat coincidentally, the numbers
roughly correspond to the power of a glow motor- the 15 is kind of a
lazy .15, the 30 would probably be a reasonable .30, and the 45 is a
decent .45. All of these motors have various windings- lower numbers
means that they spin a smaller prop. Higher number motors such as the
15/4, the 30/3, and the 45/3 all turn typical glow prop sizes, or
pretty close to it. In most cases, these motors will have comparable
performance to a glow motor, probably lacking a little in the top end
department, and a touch heavier when batteries are included. Mega
motors use more numbers, but are pretty comparable to the Jeti motors.
Mega motors are much happier with gearboxes though.
Outrunner motors
These motors allow the use of larger props than other motors without
the use of a gearbox. I don’t have much experience with these motors-
but that’s going to change soon. For applications where you need better
vertical than top end speed, these motors make more sense than the
commonly used Mega and Jeti brushless motors. Most common mfg is Axi.
ESCs
Most folks have their favorite brand of ESCs. I’ve used Jeti, Model
Motors, Kontonik (small sizes), FMA, and Castle Creations. I like doing
business with Castle Creations- so they’re in most of my airplanes.
I’ve also fried at least one ESC from all these mfg- Castle was the
nicest to deal with, although a little slow.
Gearboxes
Gearboxes allow you to correctly size the propeller to your airframe.
What I have found out about gearboxes- good ones can be nice to use,
but aren’t cheap-, and cheap ones used above 100 watts should be
adjusted with a sledgehammer. One of the most expensive commonly made
mistakes is “I got this motor on a good deal.” Often, a cheap motor
will need an expensive gearbox, which negates any savings. I find
gearboxes in general to be an annoyance- so I prefer going direct drive
whenever possible. On fast airplanes, this isn’t that much of a
problem- but slower airplanes really need gearboxes or outrunner motors.
Sticker Shock
If you’ve read this far, and are wondering how much more you’re going
to have to shell out for an electric power system than your glow motor-
you’re probably in for a shock. Electric power systems are definitely
more expensive than glow- there are no two ways around it. As an
example- a Jeti Phasor 30/3 with a 35A speed control and batteries is
in the $250 range. Note that this replaces the glow engine, fuel tank,
muffler, throttle servo, and receiver battery, but if somebody were to
say, that’s 2x a glow motor- I wouldn’t argue. However, the Jeti motor
is probably a life time investment- the motor should not wear out in
several thousand hours- the only moving parts are the bearings and the
shaft. (I have brushless motors with over a 100 flights on them- and
they sound the same as they did new.) Speed controls also can last a
long time if not abused, and neither of these components should need
maintenance. Batteries however, will need to be replaced.
Let me point out though- that what we’re really discussing is the
difference in cost between a $400 glow powered airplane, and a $500
electric powered airplane- yes it’s more expensive, but the whole
airplane is not 2x more. One other point- the airplane that you don’t
enjoy flying for whatever reason is a waste of money in my book.