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Help with electric choice


Old 01-16-2006, 05:07 PM
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Default Help with electric choice

I am in the process of building a 1/4 scale se5a from balsa usa. I am considering makeing it electric. Plane weight will be around 22 pounds with a 80" wing span. What would be recomended for electric flight? motor? battery types, size...? speed control?

I am new to electric so anything will help.

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Old 01-16-2006, 11:36 PM
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Default RE: Help with electric choice

by Ed Anderson
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This may get a little technical but I will try to keep it as simple as I can.
I will draw parallels to cars and bicycles in many places as most people can
relate to these and know at least a little about how they work. I will use
round numbers where I can and will use some high level examples. If you are
an engineer you will see that I am taking some liberties here for the sake of
simplicity. I will go through the parts of the power system, then, toward the
end, I will show you how we tie these all together to come up with a complete
power system.


I will be using the terms Volts, Amps and Watts throughout this discussion.
Let me define them.

Volts = the pressure at which the electric energy is being delivered - like
pounds per square inch or PSI in a fuel system or water from a garden hose.
Volts is about pressure, it says nothing about flow. You will see volts
abreviated as V.

Amps = the quantity or flow of electricty being delivered, like gallons per
minute in a fuel system or that same garden hose. Amps is about flow, it says
nothing about pressure. You will see amps abreviated as A.

Watts = V X A. This is a measure of the energy or power being delivered.
This is how we measure the ability of that electricity to do work, in our case
the work of turning a propeller to move our airplane through the air. Watts
is about both pressure and flow. This serves the same purspose as the
horsepower rating of your car's engine. In fact 746 watts = 1 horsepower. So
if you had an electric car, the strength of its motor could be reported in
either watts or horsepower. You will see watts abreviated as W.

If you want more depth on this, visit this thread.

MOTOR EFFICENCY - Brushed vs Brushless

Whether brushed or brushless, the motor's job is to convert electricty into
mechanical motion to turn the propeller to move air. Efficency is how we
measure how much of the power, the watts, that our battery delivers to the
motor is actually turned into useful work and how much is wasted as heat. A
higher efficency motor delivers more energy to the motor, and wastes less.

A typical brushed motor, say a speed 400, is only about 40-50% efficent. Only
about half the watts delivered to the motor actualy end up as useful work
turning the propeller. The rest is wasted. Motors that have a "speed"
designation, like speed 400, are brushed motors. There are other names for
brushed motors but the "speed" term is a common one. They are inexpensive and
they work. For example, you can buy a speed 400 motor and electronic speed
control, ESC, for $35. A comparable brushless motor/ESC combination would
typically cost 3 to 4 times
that much.

Brushless motors tend to be more efficent. They typically deliver 70-90% of
that input power to the propeller, Thus you get better performance per watt
with brushless motors. Seen a different way, if you use a brushless motor,
then, for the same flying performance you will use less energy which means you
battery will last longer. Or you can use a similar size and weight brushless
motor and get much higher performance because the motor turns more of the
watts from the battery into useful work of turning the propeller.

So, as with many decisions we make, this is a cost benefit decision. Am I
willing to pay more to get more? That is up to you.


Think of the battery as the fuel tank plus the fuel pump and a supercharger
all rolled into one. It feeds/pushes energy to the motor. So you have to
look at the battery and the motor as one unit when you are sizing power
systems for electic planes. In many cases we start with the battery when we
size our systems because the motor can't deliver the power to the prop if the
battery can't deliver the power to the motor.

The higher the voltage rating of the battery, the higher the pressure, like a
supercharger on a car engine. More pressure delivers more air/fuel misture to
the engine which allows the engine to produce more power to turn the wheels of
the car. Higher voltage pushes more electicity into the motor to produce more

Using our electric motors, a given motor may take 10 amps ( the quantity of
electricity flowing ) at 8.4 volts ( the pressure at which the electricty is
delivered) to spin a certain propeller. We would say that the battery is
delivering, or that the motor is drawing 84 watts, ie: 8.4V x 10A. If you
bump up the voltage to 9.6 volts, the battery can ram in more amps deliveing
more energy to the motor which will produce more power to the propeller. In
this example, if we move from an 8.4V battery pack to a 9.6V battery pack the
motor may now take 12 amps. This will typically spin the motor faster with
any given propeller or allow it to turn a larger propeller at the same speed.

However, if you bump up the pressure too much, you can break something.
Putting a big supercharger on an engine that is not designed for it will break
parts of the engine. Too much voltage can over power your electric motor and
damage it. So there is a balance that has to be struck. Different motors can
take different amounts of power, watts, volts X amps, without damage. For
example, a speed 400 motor might be fine taking 10 amps at 9.6 volts or 96
watts. However a speed 280 motor will have a short life with the same
combination of
volts and amps.

If you match the right battery with the right motor, you get good performance
without damage to the motor. In many cases airplane designers will design
planes around a specific motor battery combination so that they match the size
and weight of the plane to the power system for good performance.


Propellers are sized by diamater and pitch.

The diamater of the propeller determines the volume of air the propeller will
move, producing thrust, or pushing force. Roughly speaking the diamater of
the propeller will have the biggest impact on the size and weight of the plane
that we can fly. Larger, heavier planes will typically fly better with larger
diamater propellers.

Pitch refers to the angle of the propeller blade and refers to the distance
the propeller would move forward if there were no slippage in the air. So a 7
inch pitch propeller would move forward 7 inches per rotation, if there were
no slippage in the air. If we combine pitch with the rotational speed of the
propeller we can calculate the pitch "speed" of the propeller. So, at 10000
reveloutions per minute, that prop would move 7000 inches forward 70,000 inche
per minute. If we do the math, that comes out to a little over 66 miles per

By changing the diamater and the pitch of the propeller we can have a similar
effect to changing the gears in your car or a bicycle. It will be harder for
your motor to turn a 9X7 propeller than an 8X7 propeller. And it would be
harder to turn a 9X7 propeller than a 9X6 propeller. The larger, steeper
pitched propellers will require more energy, more watts, more horsepower, to
turn them. Therefore we need to balance the diamater and pitch with the power
or wattage of the motor/battery system. Fortunately we don't actually have to
do this as motor manufacturers will often publish suggested propellers to use
with a given motor/battery combination. We can use these as our starting
point. If we want we can try different propellers that are near these
specifications to see how they work with our airplane.


While unusual on glow or gas planes, gearboxes are common on electric planes.
Their primary function is similar to the transmission on a car. The greater
the gear ratio, the higher the numerical value, the slower the propeller will
turn but the larger the propeller we can turn. So you can use a gearbox to
help provide more thrust so you can fly larger planes with a given motor.
However you will be turning the propeller slower so the plane will not go as

With direct drive, that is when the propeller is directly attched to the motor
shaft, we are running in high gear ( no gear reduction). Like pulling your
car away from the light in high gear. Assuming the motor doesn't stall,
acceloration will be slow, but over time you will hit a high top end!
Typically direct dirve propellers on a given motor will have a smaller

With the geared motor, it would be like pulling away from the green light in
first gear - tons of low end power and lots of acceloration, but your top
speed is reduced.

So, by matching up the right gear ratios made up of the propeller and,
optionally, a gearbox we can adjust the kind of performance we can get out of
a given battery/motor combination.


The simplest approach I have seen to figuring power systems in electrics is
input watts per pound of "all up" airplane weight. The following guidelines
were developed before brushless motors were common but it seems to hold pretty
well so we will use it regardless of what kind of motor is being used.

50 watts per pound = Casual/scale flying

75 watts per pound = Sport flying and sport aerobatics

100 watts per pound = agressive aerobatics and mild 3D

150 watts per pound = all out performance.

Remember that Watts = Volts X Amps. This is a power measuremet.
In case you were wondering, 746 watts equals 1 horsepower, .


This should be fun. Let's see where these forumlas take us! We will use a 24
ounce, 1.5 pound plane as our example. If we want basic flight you will need
50 watts per pound or about 75 watts input to your motor for this 1.5 pound
plane. That is, 50 watts per pound X 1.5 pounds = 75 watts needed for basic
flying performance. If you want a little more spirited plane, we could use 75
watts X 1.5 pounds which is about about 112.5 watts.

Lets use 100 watts as the total target, just to be simple, shall we? I am
going to use a lot of round numbers here. I hope you can follow.

The Battery:

If we use an 8 cell NiMh battery pack at 9.6 V it will have to deliver 10.4
amps to hit our 100 watts input target ( 100/9.6 = 10.41amps) If my battery
pack cells are NiMh cells that are rated at 10C then I need an 8 cell pack
rated at 1100 mah to be able to deliver 11 amps. Sounds about right.

Now I select a motor that can handle 100 watts or about 10.4 amps at 9.6
Volts. From experience we know this could be a speed 400, a speed 480 or some
kind of a brushless motor.

We now need a propeller that will cause the motor to draw about 100 watts. I
don't know off the top of my head what that would be. I would go to some mfg
chart - GWS has good charts!

I see that if I use a direct drive speed 400 with a 5X4.3 prop at 9.6V then
the motor will draw about 12.4 amps or about 119 watts. This would be a good
candidate motor/prop for the plane using a 9.6V pack that can put out 12.4 or
more amps. This would be a set-up for a fast plane as that motor will spin
that small prop very fast.

However maybe I don't want such a fast plane but one with a really good climb
and lots of low end pull to help out a new pilot who is in training.

I can also use a speed 400 with a 2.38 gearbox and run it at 9.6V spinning a
9X7 prop and run at about 12.8 amps for 120 watts. The larger prop will give
this plane a strong climb, but since the prop speed has been reduced by 2.38
times, it won't be as fast. Spining a bigger prop gives me more thrust but a
lower top speed typically.

Back to battery packs and motors

So if I shop for a 9.6V pack to be able to handle about 15-20 amps, I should
do just fine and not over stress the batteries. In NiMh that would probably
be a 2/3 or 4/5 A pack of about 1100 -1500 mah capacity, depending on the
quality of the cells.

We view the battery and motor as a linked unit with a target power profile, in
this case about 100 watts. We use the prop and gearbox, if any, to produce the
manner in which we want to deliver that power to the air to pull/push the

If this is a pusher, I may not have clearence to spin that big prop so I have
to go for the smaller but faster prop combo.

If this is a puller, then I can choose my prop by grond clearence or some
other criteria and match a gear box to it.

See, that was easy, right? But we are not done! Oh no!

I could try to do it with a 2 cell lithium pack rated 7.4V. To get 100 watts I
now need a pack that can deliver 13.5 amps and a motor/prop combinatin that
will draw that much. So if I have 10 C rated lithiums, then the pack better be
at least 1350 mah. Probably use a 1500 mah pack to be safe.

Well, when I look at the chart for the geared speed 400 I see that, regardless
of prop, at 7.4V I am not going to have enough voltage ( pressure) to push 13
amps into this motor. So the 2 cell lithium won't meet my performance goal of
100 watts+ per pound using this gear box.

If I go back to the charts and look at a differnet gear boxes I can't hit my
power goals using 7.4V. Maybe we go back to direct drive.

We see that the best I can get this speed 400 to do is a total of 70 watts at
7.2V ( close enough ) so I can't hit my power goals using a speed 400 at this

voltage. but 70 watts would be about 48 watts per pound so I could have a
flyable plane, but not an aerobatic plane using this two cell pack.


Now, in fact that is NOT how I would do this. I would decide on the watt
target, go to the chart, find a combo that meets my goals, then select a
battery that will meet the demand and see if my weight comes up at the target
I set. A little tuning and I come up with a workable combo

Brushed Motors

Brushless Motors

Battery Packs - NIMH

Battery Packs - LiPo

Gearboxes - Speed 400 & 480 examples
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Old 01-16-2006, 11:50 PM
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Default RE: Help with electric choice

I am way out of my league here, but let me play with this a bit, just for fun. Please don't take any of this to the store and buy. But this may give you some ideas.

Based on what I posted above, a 22 pound plane X 75 W/pound = about 1650 watts to get into the sport plane power band or about 2 horse power.

I have not done a plane like that but will toss out some ideas. Others with more experience can tune this.

Perhaps something from Actro: Actro 40 will do 1700 but you would be right at the top of its scale
Motor and speed controller would be around $530


Maybe something like a pair of AXI 1520s in a belt or gear drive arrangement


Hacker B50 or C 50 series, again, you may wish to look at a pair in a belt arrangemet, especially if you are going to be going into high perfoormance aerobatics.

If we look at about a 60 amp range, you are talking about 10S2P , or 8S3P type Lipo packs to hit the power range you need.

If you want to go NIMH, you will need about 30-32 C cells that can handle 50+ amps to hit the wattage and account for the extra weight.

Have not taken flight times into account here, just wattage. If you are trying to go competitoin pattern or 3D I would expect you would have to go to twin motors for sure.

This is all very very rough and again, I have never approached anything this large. I am just working the numbers But this might be a starting point for order of magnitude. This ain't gonna be cheap! For a plane of that size, unless you are driven to electric, fuel power is still the most economical approach by a long shot.

For example, if you use make up a 10S 3P 6000 Mah pack of 2 of the 5S3P packs shown here from Thunder power cells, will run about $700

9S2P 8000 MAh packs made up of six 3 cell 4000 mah packs of these would be about $700

However this is just for estimation. You would probably have custom packs made for a plane like that.
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