How to
choose a power set-up for your model
Using
this Guide
Although it’s possible to
accurately calculate every important bit of data to determine the optimum power
system for a model, most of us will prefer the "educated guess"
approach. Unfortunately there is some knowledge required to make an
"educated guess", and that is hopefully what this guide will provide.
For those completely new
to the world of aero modeling who also intend building a new model, I suggest
starting from the beginning of this guide. Even if it doesn't all make sense at
first, it will eventually. Some trial and error might not seem like the most
economical way of learning but it can sometimes be the most effective.
Many people will only want
to access certain information, so each topic has a heading. Look down the page
to see if you can find the information you need.
Index:
Weight Power and Dimensions
Inrunner or Outrunner?
Inrunners
Outrunners
kv
Electronic
Speed Controls (ESC's)
Cut off voltage
Pitch
and Pitch Speed
Safety
“C” rating
mAh
Wing loading is the loaded
weight of an aircraft divided by the area of the wing. It is broadly reflective
of the aircraft's lift-to-mass ratio, which affects its rate of climb,
load-carrying ability, and turn performance.
Many aero modelers try
their hardest to make models as light as possible. This is because a model with
a light wing loading is easier to fly as it has a slower stall speed. In a
banked turn an airplane is subjected to a gravitational force (G) which
increases its weight, the same as a weight on the end of some string gets
heavier if you spin it around like a lasso. The heavier the wing loading in a
banked turn, the higher the stall speed gets. Other factors that can affect the
stall speed of a model airplane are aerofoil shape and aspect ratio. The only
way of giving a heavy model a light wing loading is to increase the size of the
wings. So a model with a high wing loading will have a high stall speed, which
will become even higher in high G maneuvers. Because we land our models at or
slightly above their stall speed, a model with a high wing loading will have a
high landing speed which can call for some very good piloting skills.
Wing loading is usually
calculated in the form of oz/ft² or gr/dm²,1 dm² =
15.50003 inch². If you want to calculate the wing loading of your model, try
this calculator http://www.csd.net/~cgadd/eflight/calcs_wingload.htm
Volts,
Amps and Watts
The most important terms
you need to understand when choosing components for your electric model are
Volts, Amps and Watts.
Here is the
"Hydraulic analogy" from Wikipedia which explains these terms in a
simple way.
"The hydraulic
analogy is sometimes used to explain electric circuits by comparing them to
water-filled pipes, voltage is likened to water pressure - it determines how
fast the electrons will travel through the circuit. Current (in amperes), in
the same analogy, is a measure of the volume of water that flows past a given
point, the rate of which is determined by the voltage, and the total output
measured in watts. The equation that brings all three components together is:
volts × amperes = watts".
As we will see, the size
and efficiency of a motor and the load imposed on it by the propeller affects
the Volts and Amps. The idea is to choose a motor, battery, esc and propeller
combination that will fly your model in a desired manner within the
specifications of the components, preferably at around the peak efficiency of
the motor. This will be covered in more detail in the following sections.
Choosing
a Motor
Weight Power
and Dimensions
The most important thing
to keep in mind before choosing a motor is its weight and dimensions. We would
all agree that extra weight added to a model to achieve the correct centre of
gravity is undesirable. I personally would prefer to have a larger, heavier and
more powerful motor than a smaller less powerful motor and a lump of lead in my
models. Sometimes there is no choice but to use lead, but just don't forget
about the relationship between the weight of your motor and the centre of
gravity of your model. The dimensions of a motor are obviously important. Don't
buy it if it won't fit in your model.
You will want a level of
performance suitable for the type of model you are powering. A 3D model will
need thrust greater than 1:1, and a scale WW1 biplane will need considerably
less. Here is a table giving performance in Watts per pound. Remember that if
you are running your motor above its maximum efficiency the Watts per pound
rule won't be accurate, as a higher percentage of the Watts going into the
motor will be producing heat instead of power.
70-90 watts/lb. Trainers, gliders and slow flying
aerobatic models.
90-110 watts/lb. Sport aerobatic
and fast flying scale models.
110-130 watts/lb. Advanced
aerobatic and high speed models
130-150 watts/lb. Lightly
loaded 3D models and ducted fans.
150-200+ watts/lb. Unlimited
performance 3D models.
Inrunner or Outrunner?
Now you have an idea of
the weight and power you will need for your model, what sort of motor is best,
an inrunner or outrunner?
Both have their pros and
cons.
Inrunners
Inrunners are constructed
with the magnets attached directly to the shaft, which is surrounded by the
copper windings. Because the magnets are close to the shaft it spins very
quickly. This means they produce high rpm but low torque. This high rpm can be
converted into torque by using a gearbox.
Inrunners are more
efficient and powerful, but need a gearbox to drive large propellers. They
produce high revs per volt (Kv) compared to outrunners. For models requiring a
small prop running at high speed like a Zagi (wing), pylon racers and ducted
fans, inrunners without gearboxes are popular. Once a gearbox is used there are
even more pros and cons. Gearboxes are an extra expense, require maintenance
and can be noisy, but you will still get the best efficiency and power with a
geared inrunner spinning a large prop. This is the reason why all competitive
F5b models still use geared inrunners.
Outrunners
Outrunners are constructed
with the copper windings on the inside. The shaft is attached to a
"bell", or casing that contains the magnets, which spin around the
copper windings. Because the extra weight of the bell and magnets are further
out from the shaft it acts like a flywheel. Generally outrunners produce lower
RPM at higher torque than inrunners due to the way they are made. This enables
an outrunner to spin a larger prop without a gearbox.
This means no maintenance,
quieter operation and cheaper purchase price (no gearbox). These factors
outweigh the higher efficiency and power of the inrunner for most sport flyer's.
kv
kv is simply the revolutions per minute (rpm) an
electric motor will spin at per volt when under no load. You could think of
high and low kv like the difference between a high
performance 2 stroke racing motorcycle engine compared to a 4 stroke Harley Davidson
motorcycle engine. Just say they put out approximately the same horsepower, but
the 2 stroke does it at 11,000rpm and the 4 stroke does it at 3,000 rpm. The
same can usually be said for high and low kv electric
motors. Assuming the same voltage, a high kv inrunner
with a small diameter propeller would be perfect for a high speed model like a
pylon racer, and a low kv motor with a large diameter prop will be better for
power i.e. getting a sailplane to altitude, or 3D maneuvers like prop hanging.
Kv is determined by the number of winds or turns. This is the amount of times
the copper wire has been wound around each stator pole. More winds = low kv, less winds = high kv.
kv has two main implications.
A high kv
motor will spin faster than a low kv motor at the same voltage. This means you
may choose to use a high kv motor if you are limited
to a lower voltage battery pack. An example of this would be in 7 cell glider
competition (7 NiMh or NiCd cells at 1.2 volts a cell = 8.4 volts). A lower kv motor could not produce enough rpm at 8.4 volts to be
competitive, so a higher kv motor is used.
If you are not limited to
a particular voltage, a lower kv motor can be used at
higher rpm by using a higher voltage. Large outrunners with a kv of 200 to 300 are a good example of low kv high voltage
motors. Make sure you consider the voltage limit for any motor you are
considering.
Choosing an inrunner and a
gearbox isn't as complicated as it sounds, it is basically the same as choosing
an outrunner but you add the downshift of the gearbox to the calculation.
When choosing an inrunner
you usually have two sizes to choose from. I'll use Feigao for this example,
Feigao have their motors listed on their site Feigao.com with a complete set of
numbers on every motor and they even have suggestions for different set-ups.
Feigao call their smaller diameter motors (27.6mm) "380" and their
larger ones (36mm) "540". Both come in three different sizes, Small,
Large and X-Large. These are copies of Hacker inrunners, "380" for
the B40 size and "540" for the B50 size which makes it very easy to
find an successful set-up to copy if you search the
net. The ratios are the same for both Hacker and Feigao gearboxes 4.1:1 for
"380" size and 6.7:1 for "540" size.
But what does that 6.7:1 mean ? To get the kv of the prop
shaft with a gearbox, simply divide the kv of the motor by the ratio of the
gearbox. If we take the FG540-07S as an example, it's a 5070kv motor but using
a gearbox with a ratio of 6.7:1 you get a kv of
5070/6.7 = 757. This would fit a 3S setup for a hotliner perfectly. If you want
to use it to it's max amp
rating (93A) check that your Lipos are up to it
(Check C Rating in this Guide). Personally I go a little easier on the Feigao
motor/gearbox combinations when copying a Hacker set-up, as Hacker are a higher quality product.
Peak Efficiency is a very
good site where you can see how your intended setup will perform. Nothing is as
precise as your own measurements, but it gives you some idea of what to expect.
The graph isn't very easy to read but you'll get the hang of it if you read the
"how do I read this chart" first.
Electronic Speed Controls (ESC's)
There are two main types
of ESC, for brushed or brushless motors. You cannot use a brushed ESC with a
brushless motor or vice versa. Think of the features you will need like a brake
and soft start. You will need a brake if you are using a folding prop and a
soft start if you are using a gearbox and an on/off switch for a throttle.
These features can often be found on Radio Controlled Sailplanes. The most
important thing to consider when choosing an ESC is matching the ESC to your
motor. It is good to use an ESC rated at a higher amperage than you intend
running your motor at as an insurance against over stressing your ESC causing
failure and potential damage to your model. Often you will see a burst rating
for an ESC, meaning you can run the ESC at a maximum Amperage for a limited
time, and exceeding this limit is asking for trouble. Most sensible aero
modelers like to have an esc capable of 10 to 20% more Amps than they plan to
use depending on its quality. You will need a meter to measure the Amps and
Volts being generated by your power system to ensure you are not stressing the
battery, ESC or motor.
What is a bec? Bec is an
acronym for battery eliminating circuit. This device provides power for the
servos in your model. Many ESC's have a bec that can only handle a certain
number of servos at a given voltage. The higher the voltage you use the less servos
you can use. Using too many servos from the bec in your ESC will cause
overheating and failure of the bec. This will be catastrophic if your bec fails
in flight so how can you safely run more servos with your ESC? External becs, or Ubec's use power from your flight battery pack and
are a cheap way of safely using more servos than the bec in your ESC can
handle. A receiver battery pack is another way of supplying reliable power to
your servos without using the bec in your esc.
Cut off
voltage
Set the cut off voltage on
your ESC to 3 volts per cell to ensure you don't over discharge and damage your
lipo pack.
Prop selection
The propeller is the
component that puts a load on a power system. With the wrong prop you can
damage your battery, ESC and motor. Think of the prop like the gears in a car.
Some props are like first gear and the motor will have to work at high rpm to
go slowly. If you have driven a 4X4 you will know that this gives you power to
climb steep hills at low speed without stalling the engine. You could compare
this to prop hanging a 3D model where power is more important than speed. On
the other hand you might want to go fast. This will require a prop that is more
like the top gear in a car. It doesn't have the power to take off and climb a
steep hill at low speed, but once up to speed it can maintain that speed
comfortably. The numbers on a prop, say 10X4, give you the diameter and pitch.
In this case you would have a prop with a diameter of 10 inches and a pitch of
4. A 10X4 prop will give you more thrust at a lower speed like the 4X4 analogy
above. If you swapped it for a 10X7 prop you would have a higher top speed, but
your take off run would be longer. The extra load on the motor would also draw a higher Amperage.
Pitch and Pitch Speed
Pitch is the distance
(normally expressed in inches) that the propeller "cuts" through the
air in a single rotation assuming no slippage. To achieve pitch, the propeller
blades are angled to move air to create thrust. The angle of the blade
determines its pitch. Propeller blades are airfoils, just like the flying
surfaces on our models. When they have a higher angle of attack they create
more lift. In the case of propellers, a higher angle of attack (pitch) at a
given rpm will create greater thrust.
Pitch speed is the speed
at which the propeller pulls through the air. It is calculated by looking at
the pitch of the propeller, and the number of revolutions it performs in a unit
of time. Pitch speed does not consider slippage, drag and other forces that may
affect the aircraft.
With a high wing loading
you need a higher air speed to stay in the air. A higher pitch speed means
lower thrust > longer take off > high landing speed. You can get both
thrust and high air speed but it will be at a weight penalty as the power
needed to get thrust for a short take off will not be in proportion to the
power needed to stay airborne.
War birds are an often
examples of models with high power/high wing loading which are supposed to fly
fast, and especially in glow to electric conversions you will need to take the
wing loading into account.
Hotliners and F5b models
are one of the most extreme examples of high power/high wing loading. The more
extreme examples have such a high pitch speed a catapult is needed to get them
airborne because of the square (16x16) or over square (16x17) props they use in
order to get extreme high speed/climbs. In a perfect world (with zero airframe
drag and 100% prop efficiency) you can calculate the speed of your model from
RPM x pitch)/1056 = your speed in mph. For example 10000rpm x 7" pitch
/1056 =66mph or 105.6 km/h.
Pitch speed isn't only
about wing loading it's also about what you want to do with your model, as I
wrote above about hotliners and F5b. With an already light model or of moderate
weight you can determine the behavior from the choice of prop > pitch speed.
Without the need of changing anything (keeping the same amps) you can take a
GWS Formosa II with a 10x5 from being a sporty low wing aerobatic trainer to a
fast aerobatic plane with a 9x6. As a general rule 1" pitch relates to
1" of diameter, if you step up 1" in pitch you need to step down
1" in diameter to keep the same amp draw.
With more normal kind of
planes we usually use a prop with the proportion of 1:2 i.e. 10x5, 11x5.5, 12x6
and so on as it is most effective (from what I heard). A High wing trainer
could very well use a more square prop like 9x7 instead of 11x5.5, it'll still
have a high lift and once airborne you can throttle down, the higher pitch will
give it airspeed and you'll get long flying times with low amps, perfect for
photography or video.
As a rule of thumb, you
want to have a static pitch speed within the 2.5 to 3 times the stall speed. So
if your plane stalls at 15 mph in level flight you would like a static pitch
speed between 37.5 to 45 mph.
For a particular motor, I
know from testing that with a 12x6" propeller the motor is running at 7165
RPM. Each revolution pulls the plane forward 6". So my plane would be
making 6" x 7165 RPM or 42,990 inches per minute. Dividing by 12"
gives me 3,582.5 feet per minute. Multiplying my 60 minutes gives me 214,950
feet per hour. Dividing by 5280 feet gives me 40.7 miles per hour. The plane I
has a calculated stall speed of 14 mph. 40.7 divided by 14 equals 2.9. This
ratio falls within the desired 2.5 to 3 ratio of pitch speed to stall speed,
which is good.
To select a motor you may
have to work back-wards from prop diameter. The plane I have can take a
12" prop. I like to get the largest diameter prop that will fit.
Lipo packs
Safety
Always use your lipo packs
safely.
See this link for safety issues
concerning the safe use of Lithium Polymer batteries:
http://www.rcgroups.com/forums/showthread.php?t=209187
“C” rating
Broadly speaking, the
"C" rating is a guide to how much current it is safe to draw from
your battery. It's expressed in terms of the capacity or C. Beware that
constant discharging of your lipo pack at its maximum C rating will almost
definitely shorten its life. Depending on the quality of your pack, it is much
wiser to keep your current draw to about 10 C with short bursts up to 20 C if
you want your pack to last. The easier you are on your pack the longer it will
last.
A 2200mAh 10C battery is
rated to be discharge at up to 22A (10 x 2200mA/1000) and the same size 12C
battery would be good for 26.4A (12 x 2200mA/1000).
The internal resistance in
higher C rated packs is lower, meaning that the voltage drop found in higher C
packs is not as pronounced giving higher voltage under load and slightly more
power.
Also be aware that lipo
manufacturers often put an overly optimistic C rating on their packs. Unless
you see independent test results you trust for lipo packs, use them at about 75%
of the stated C rating and you should get many more cycles from them.
mAh
mAh is an acronym for Milliamp Hour, which is how much
current a battery will discharge over a period of one hour. Higher numbers here
reflect a long battery runtime and or higher storage capacity. For example a
2000 mAh pack will sustain a 2000 milliamp (2 amp) draw for one hour before
dropping to a voltage level that is considered discharged. A 1700 will sustain
a 1700 mAh (1.7 amps) draw for one hour. 1000 mAh is equal to a 1 Amp Hour (AH)
rating.
Like the C rating, the mAh
rating also determines the maximum current that can be drawn from a pack as can
be seen in the calculation in the C Rating section above. For example if you have
three 11.1 Volt 10C packs, one rated at 1000 mAh, one rated at 1700 mAh and the
other at 2000 mAh, we can determine that it is safe to draw the following
amperage from these packs. Multiply the C rating by the mAh rating and divide
by 1000 to convert milliamps to Amps:
10 X 1000 mA/1000 = 10 Amps
10 X 1700 mA/1000 = 17 Amps
10 X 2000 mA/1000 = 20 Amps