AEAJR

SELECTING ELECTRIC POWER SYSTEMS
Reference Resources

Electric airplanes can be lots of fun and very little work. There are lots of
RTF planes
that already have their power systems selected and installed. They require no
work at all on your part. That is about as easy as it gets.

If you like ARFs or kits, many come with motors and props standard. Some will
include a gearbox as well, if the designer considers that the best approach.
While these may not be the most powerful motors you might put on the plane, you
can be reasonably confident that they will fly the plane. The instructions will
normally recommend an appropriate sized electronic speed control, ESC, and
battery pack to use with that motor and propeller combination on that plane.
Just read the friendly manual, RTFM, and you can usually get the plane in the
air with little trouble. That is still pretty easy.

But what if you want to design a power system yourself. Perhaps you are
converting a glow or gas plane over to electric power. How would you go about
doing something like this. This is not a trivial task. There are many factors
to take into consideration and there are hundreds of motor, ESC and battery
pack options out there. How do you choose?

Or say you want something better or stronger than the motor that came with your
plane? And, of course, some ARFs and kits don't include motors. And finally
there is the fun of designing your own planes. Now you need to plan the power
systems.

If you are going to design your own systems you will need some help and some
knowledge. What I have compiled is a list of links that take you to resources
to help you. First I recommend you read this article for background
information. It will help you understand and use the rest of the resources
listed below.

Sizing Electric Power Systems -
http://www.ampaviators.com/index.php?option=com_content&task=view&id=41&Itemid=27

Remember that the system operates as a whole. If you have the right motor
with the right battery and put on the wrong prop, things can get pretty ugly
pretty fast.

I highly recommend you pick up a wattmeter. If you are going to be a designer
there are only two ways to know if you got it right. The first is to measure
the
amps, volts, watts being drawn by your new power system and making sure all
components are up to the effort. The other way is to see the plane fly badly,
watch your new motor burn up, your new battery pack fail or see the magic smoke
come out of your ESC. Personally I would rather you get a wattmeter. This
thread discusses their value and uses. Several are discussed.

WATTMETERS - Vital to the power system designer
http://www.wattflyer.com/forums/showthread.php?t=11863


Brushed vs. Brushless motors

Many RTF, ARF and Kits come with brushed motors. These are often identified by
a "speed:" designation such as speed 280, speed 370, speed 400, speed 600 and so
on. They work fine but are less powerful and less efficient than brushless
motors. If you are going to design your own power systems I suggest you go
directly to the brushless motors. Prices have come down a lot and quality, even
at the low end, is good. So leave the world of brushed motors behind and go
brushless. From this point forward I will only be looking at brushless systems.

Resources

Some manufacturers have done a good job of providing tables and charts that can
help you use their components to come up with the right system. Armed with the
knowledge from the first article you are now better equipped to use them. The
ones I have found most useful are next.

MAXX PRODUCTS

Maxx Products - How to Choose a Brushless Motor for an Airplane
pretty good tip sheet on coming up with a glow to electric power comparison.
http://www.maxxprod.com/mpi/tips3.html
HiMaxPower System Packaged sets and info chart
Motor, speed control and prop all matched up for you
http://www.maxxprod.com/mpi/mpi-264.html


ELECTRIFLY

Guide to Glow to Electric Conversions
http://www.electrifly.com/manuals/gpmz0010-glow-to-elec-conv.pdf
Electrifly Electric Motor Configuration Tool
It assumes you know where you are going. Read the article above and you will be
better prepared to use this too.
http://www.electrifly.com/config/
Brushed motor to Brushless Motor Conversion Chart
The recommendations will yield a significant power boost.
http://www.electrifly.com/powersystem/brushed-to-brushless.html
Chart of Sample Planes and Recommended Power Packages
http://www.electrifly.com/powersystem/specific-plane-conversions.html
Electrifly Brushless Brochure. Lots of useful info that you can print out.
It is only for Electrifly components, but it is well done.
http://www.electrifly.com/sellsheets/power-system-brochure.pdf

GWS

GWS provides excellent information about their brushless motors and how they
perform with different batteries, and props. Useful, once you understand the
watts/pound rules.
http://www.gwsus.com/english/product/auxcircuit/brushless.htm

Other Resources

A series of posts on electric power system basics
http://www.wattflyer.com/forums/showthread.php?t=1933
http://www.rcgroups.com/forums/showthread.php?t=417868

MotoCalc will tell you everything you need to know: Amps, Volts, Watts, RPM,
Thrust, Rate of Climb, and much more! It is a popular tool for predicting the
proper motor, prop, battery pack for electric planes. Read the first article
first and this will make a lot more sense.
http://www.motocalc.com/

Amps vs. Volts vs. C
http://www.ampaviators.com/index.php?option=com_content&task=view&id=37&Itemid=27

Low Voltage Cutoff
http://www.wattflyer.com/forums/showthread.php?t=3445

Milton

It is all about watts, Watts are power.

I use this to decide what type set up to use

100 watts to pound for good heavy sports flying
125-135 good aerobatic
150 and up for aerobatic and 3D type plane

Now what makes watts, amps X volts = watts

I like large planes so lets look at a 20 pound all up weight sports plane

Looking for 2000 watts, Knowing that I want 2000 watts and saying I will run a 10S lipo ( higher voltage is better than higher amps)

With a good battery I should hold around 35 volts under a load so now I will put 35 volts into 2000 watts = 57.14 amps. That tells me I need a motor that will handle 35 volts and about 60 amps.

There is another thing to look at, You have watts in and watts out, Or the amount of fuel you put into the system to what kind of power you get out of it.

You will see different efficiency ratings, I have seen them that was rated as high as 92-95%. That is hog wash, There are not many out there that go above 85% most are around 80%

So lets go with 85%
If 746 watts = 1 HP

So I will be putting in 2000 watts divided by 746 = 2.68 HP
That is power in now for power out I will take that times 85% ( efficiency)
2.68 times 85% = 2.27 HP.

Milton

Razmo

This thread looks promising. I actually came here to post a question about "Understanding and choosing a power plant". Looks as if this may be the place to start...

Thanks,
Raz

AEAJR

Milton, great post!

Razmo

Quick question...

To confirm, when sizing up an appropriate powerplant using the 50,75,100, etc Watt guidline. I'm including motor, battery, esc and etc, YES? ("ALL UP WEIGHT")

Raz

Milton

Yes, All up wieght.
You will just have to guess at it with what you think you will use to set it up.
I am alway's a little over when I finish a set up.

Milton

AEAJR

This is not an extremely exact science. +/- 10% in weight is not going to matter. Even 20% if you are not tyring to size it too close. When in doubt go for a little higer power to weight ratio unless you are trying to go very light, like an indoor flyer.

Razmo

I'm questioning because I recently had a Husky model built for me that came out at 13oz *all up weight*. According to the formula, I need a 40watt motor.

The vendor suggested a Himax 30watt motor (designed for 11.1v) which is rated for models between 6 to 8 ounces. This what the model is currently equipped with. (I haven't flown her yet) She is a float plane.

I'm thinking I may be better off with the Himax 50watt motor (designed for 7.4v).

Raz

TRASHBUG

Personally, I'd figure a minimum 100 watts per pound for a float plane. It's tougher to get off the water than a blacktop parking lot. 13 oz plane = 80 watts.

LOL

skiman762

Doe that mean a 25oz 600watt wing is way over powered or just way too much fun :<

AEAJR

You have two choices.

Try what you have and see how it does or take it apart before you try it and never know.

I would try it and see, unless you plan to send it back to the builder for a redo.

These guidelines are very broad. After all, what is "scale flight" and what is "sport" and what is "agressive aerobatics"? It is all very subjective.

These are starting points. My Aerobird Challenger flies quite nicely at 48 watts per pound but it will never be a pattern plane. My Easy Glider Electric climbs very agressively at 90 watts per pound, but I usually climb at 3/4 throttle because it uses less power and is more efficient.

Your float plane may do fine at the 38 watts/pound of the current set-up. Try it!

A large influence will be the propeller. If you are propped for thrust vs speed, you may be perfectly balanced in your current configuration for scale flight with an occasional loop. After all, float planes are not generally flown through agressive patterns.

Fly it, you may like it!

AEAJR

I revised the example part of the original post, so I thought I would provide the updated part here.

AN EXAMPLE!

This should be fun. Let's see where these formulas 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 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 as a starting point. GWS has good charts!
http://www.gwsus.com/english/product/powersystem/edp400.htm

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 or
to do more low speed aerobatics

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.
http://www.gwsus.com/english/product/powersystem/eps400c.htm
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. Spinning a
bigger prop gives me more thrust but a lower top speed typically. This is a
common strategy for 3D planes.

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 1000 -1300 mah capacity. Some examples here: http://www.cheapbatterypacks.com/mai...ells&chem=NIMH

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 plane.

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

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


See, that was easy, right? ( well sorta but ....)


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 combination
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 different gear boxes. I can't hit my
power goals using 7.4V. Maybe we go back to direct drive.
http://www.gwsus.com/english/product...tem/edp400.htm

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.


REALITY CHECK!

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.

I often use the MaxxProd combos for reference. If you read the details on each
package they have wonderful information. And, the fact is that I generally go
with brushelss motors these days. Costs are reasonable and their higher efficency
gives me more performance and longer flight times.
http://www.maxxprod.com/mpi/mpi-264.html

Following the example above, the combo 10 on that page would be an excellent
fit for my 1.5 pound plane for sport flying.

The Combo 049 might be a good fit for a slow flyer. Either way the package
has all I need.

If I wanted the plane to have all out performance, the 15A or 19A package would
be my pick. Note that these would need either higher voltage or higher amperage
battery packs. The flyers/PDF for the packages make recommendations.

For those who like to be even more analytical about it, there are packages
like MotoCalc that will allow me to play with all sorts of combinations and
make suggestions on what I should use. There is a link for MotoCalc below.


SUMMARY

So, in these few paragraphs you have taken in a basic knowledge of how electric
power systems are sized, the factors that are considered an how to predict
the outcome. Simple, right?

Of course there is a lot more to know and time and experience will teach
you plenty, but with this basic understanding you are better prepared to
begin playing with the power systems you put in your planes.

Grumman

Hello!!! Can any suggest to me a battery for my new Multiplex Twin Jet, it will arrive with 2 X Permax 480 Brushed Motors. I have heard everything from the rated voltage of 7.2 volts all the way up to 12 volts for these motors and now I am a bit confused, how is it possible to run a 7.2 volt motor on 12 volts and not destroy it???
I will buy Li-Po or nimh I have several chargers on hand... The manufacturer suggest a 7.2 nimh 1700 battery I believe... I understand it will come with Gunther Props and I have also heard that they spin off at times so I ordered some APC 5 X 5.5's and colet type mounting units. I would like to get a little more duration and power from the battery choice so I am prepared to spend whatever it takes to power this new Jet properly.... If any of you guys have any good information and your suggestions would be of great assistance to me I will look forward to your advice...
Thank You in advance for your time...
Happy Flying!!!
Maurice

AEAJR

I am a very big beliver in reading the instructions first.
http://www.multiplexusa.com/product_fs.htm

According to the Multiplex those are 400s, not 480s. If you are upgrading them, you should say so.
http://www.multiplexusa.com/product_fs.htm

On page 8 the battery recommendation is for 7 cell Sub C cells. That means 8.4V and able to feed a lot of amps.

Assuming you plan to upgrade to 480s, they recommend you go to an 8 cell 9.6V pack

The first question is what kind of amperage are those motors going to pull? I am going to guess about 40 amps between the two motors.

So a 3 cell 2500 mah rated at 20C or higher should work fine. You could go to 3000 mah if you like. That will give you higher voltage and more capacity at lower weight. But the 3 cell Lipo might burn out the brushes on the motors.

If that happens, you go to twin brushless for REAL power and speed!

Grumman

Thank you for your reply ED. Here is the spec list from where I purchased the TwinJet...

BBK Twin-Jet 480 Racers Edition White Eagle

Multiplex
Product No.: 002344
Print product data sheet
Frage zum Produkt

If you can handle a TwinStar with confi dence, this model is your ideal fi rst step into the “world of jets”. Hand-launching is straightforward, and the model is easy to land. The Twin-Jet is also extremely robust due to the use of ELAPOR® foam, making it a good choice for the relatively inexperienced flyer. Its strengths lie in elegant low passes, rolls and loops, while the sound of the twin electric motors is very impressive. • Endless upgrade potential • Low-cost NiMH sub-C fl ight batteries • Final assembly in around two hours • Straightforward hand-launching - no assistant or catapult required • Reduced crash risk through the use of resilient ELAPOR® • Two Permax 480 motors, propellers and cable set included Contents: • Moulded ELAPOR® foam components • All small items required • Two Permax 480 motors and cable set • Two propellers • Decal sheet • Instructions Specifi cation: Wingspan: 910 mm Fuselage length: 802 mm Weight, standard: min. approx. 1000 g Weight, upgraded version: min. approx. 1100 g Wing area approx.: 25.5 dm² Wing loading: min. 36 g / dm² Functions: A, E, T (delta mixer required)

Thanks again Ed,
Maurice

AEAJR

Sorry I have not responded. Must have overlooked the notification.

So, what battery does the mfg suggest? Since the specs I found say it has 400s I am hesitant to respond. But again, based on my earlier post, if you assume 40 amps you will probably be fine.

What ESC do they recommend?

You ordered it, I presume you have it by now. What have you done with it? If you share the outcome here, others will benefit.

Grumman

Hello Ed!!! The Jet did arrive and it did come with Twin Permax 480 motors. I was reading some of the guys post in Pusher Delta's and decided to go with a Li-Po Pack...3200 3s2p Li-Po The Mfgr. suggest a NiMh 1700 pack... I have seen some clips with the plane flying with this pack and yes it flies fine but it's a bit slow... I have read a post where someone has the 480's with the 3s2p pack and all is fine.... I work on a ship and we just left the Porto Sole in Italy so I haven't had a moment to try anything, as soon as I can I will post all findings and readings...
Thank You Ed
Happy Flying
Mojo

AEAJR

Good thing it is foam, should it happen to land in the water. ;D

Grumman

That's Right Ed!!! I will check in as soon as I have some #'s to report...
Great Thread, lot's of good info...Thanks
Happy Flying
Mojo

arniep

For being a relative newbie, can anyone tell me what brushless motor would I use in place of a brushed Speed 400?

For example, the Overlord http://www.edgerc.com/overlord.htm says to use a speed 400 but I want to substitute a BL motor. Can anyone explain how I would find a suitable BL motor?

Thanks....arniep

AEAJR

First thing you do it read the article. Then you come to understand that watts per pound is a leading indicatior of how ot match your motor to your plane.

So, what watts per pound target are you looking for?

arniep

I think 75 watts per pound should do it. I am not a speed demon and actually enjoy slow flight with the ability to do some maneuvers.
Thanks...arniep

III Bones Air Force

Hello all: Great thread, my knowledge about power systems grew exponentially reading this thread. I have subscribed to this thread to see if I can find anwers to the following scenario:

I have scratch built a NASTY foamy with 34" wing span and total weight of 13.4 Oz. Electronics, battery and set up as follows:

Motor: BP 8Y brushless Rated at 10.9 Amp- see specs. at this link http://www.bphobbies.com/view.asp?id...97&pid=V615640

ECS: Castle Creation 18 AMP 2s, and 3S auto detect.

Battery: DN Power 900 MAH 2s Lipo (7.4 V) - Rated at 10C. I also have acquired but not yet used a 16 C 7.4 Thunder Power with 1320 mah.

My NASTY flies great with Rudder, Elevator, and aileron.

Propeller: 1041.

Great take offs and great handling, must always have throttle input to keep in air, otherwise it drops quicker than I would like. Maybe because it is flat.

Anyhow, the main problem I have with my set up is the flight duration. It is short, I have not exactly measured it since I just started flying it, but it is much less than the 8 - 12 minutes I was hoping for. I am guessing the battery's mah rating is low OR the voltage could be higher, or the Battery's C rating could use a boost. I do have MR Power Analyzer II that I just got. any help would be great on how I might be able to "fix" the flight duration on this set up and increase it. I know I must think in terms of the whole system, however, at this stage the only variables I am looking at "playing" with are: Battery, and/or the propeller.

Any help would be great. Thanks,

AEAJR

You must measure the voltage under load and the amperage being drawn by your set-up. A wattmeter is the most convenient way to do that. I don't know the MR Power Analyzer, but I presume that is what it is for.

Assming you are pulling 9 amps, the maximum your lipo pack is rated for, it should give you about 6 minutes at full throttle and the lipo will be working at its limit, which will not give you maximum lifespan for the pack.

Assuming 9 amps at 7.5V, that would be 67 watts or about 82 watts per pound. That should give you great performance.

If you are pulling 10 amps, you are overstressing the pack and will get less than 6 minutes. duration and very few flights before the battery degrades.

If you are going to design your own power systems you MUST measure it. You have the tool. Let us know what you get when you measure.

III Bones Air Force

Thanks Ed. I am planning on doing some measurments as soon as the temp climbs above 20. It's been brutal around here. I want to try and check the bench top measurmnents shortly before I take it out for a spin with the Thunder Power and again with the DN Power batteries.

The Medusa Research is a basic wattmeter that seems to work very well. Again thanks.