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View Full Version : Old dog, new tricks. Help Please


Robert53
12-10-2005, 04:34 PM
Hello
I built my first RC plane in '73 so my questions aren't flight related. My questions are electrical. All this started from a trip to the local hobby shop. I had been out of RC for 8+ years. I found a Typhoon, COOL! Time passed and a Edge 540T found it's way into my home. Both RTF. Now it's time to start something on my own. The problem is the more I read the more confused I become. I'm sure you guys can straighten some of this out for me.

ESC - I see several that list programming options. Even the one installed in the Typhoon can be programmed. Here's the problem, what is the importance of some of these?
-Programmable brake - No problem
-Programmable low voltage cut off - Got it.
-Programmable timing - This is important? How do I know what to set it on? I see some motors say standard timing. Standard timing is?
-Programmable PVM - Huh? Frequency to motor? Again, how do I know what to set it on?

Motors -
-Is there a clue to usable RPM and prop size with the info usually found on a motor?
-The KV rating? This should tell me something. It seems the lower number would mean more torque?

How about something specific. I have ended up with a pair of Himaxx 2825's, one 2700 and one 3600. I understand they will need a gearbox. My questions are

-Can anyone give me an idea of what these motors can do? prop, at rpm, w/volts, @ Xamps, gear ratio?
-Suggestions on a gearbox? ratio? or range of ratios? A source for gearboxes?
-Suggestions on an ESC? China ESC's do what they claim?

When I dropped out of RC I left a 30 sized plane framed up. I think it is a midwest Sukoi. 45" wing span, it should turn out light, say 3.5-4lbs. One of the 2825's in this?

:rolleyes: I've been thinking the 2825-2700, 4:1 gearbox, 3S li-poly, I'm guessing a 12X8. RPM? Amps? no idea on these.
Maybe the 3600 on a 5:1, 3S?

Yes, I do tend to dive into the water, then look for rocks.:D

Thanks in advance for any help. And Happy Holidays

Robert

AEAJR
12-12-2005, 06:22 AM
I can't comment on the specific motors and such that you

SIZING POWER SYSTEMS FOR ELECTRIC AIRPLANES

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 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. 746 watts
equals 1 horsepower.

For those who have flown glow powered planes, this might be expressed like
this for a given airplane.

Use a .36 for casual flying
use a .46 for sport/aerobatics
use a .60 for pattern and mild 3D
and if you want to go all out, put in a .90

If you are trying to size a power system for your electric plane and you are
accostumed to glow engines, I suggest you use 75 watts as your minimum level
of performance as typical glow planes have higher power to weight ratios than
the "stock" set-ups that come with many electic planes.

These might not be the right glow motors for this application, but you get the
idea. It is just a way of matching up power to weight ratios to get a given
level of performance.


AN EXAMPLE!

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!
http://www.gwsus.com/english/product...tem/edp400.htm (http://www.gwsus.com/english/product...tem/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.

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
plane.

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.
http://www.gwsus.com/english/product...tem/edp400.htm (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

Brushed Motors
http://www.hobby-lobby.com/elecmot.htm (http://www.hobby-lobby.com/elecmot.htm)

Brushless Motors
http://www.hobby-lobby.com/brushless-motors.htm (http://www.hobby-lobby.com/brushless-motors.htm)

Battery Packs - NIMH
http://www.cheapbatterypacks.com/main.asp?pgid=packs&pid=AIR&sid=445976&ctype (http://www.cheapbatterypacks.com/main.asp?pgid=packs&pid=AIR&sid=445976&ctype)=
http://www.hobby-lobby.com/hydride.htm (http://www.hobby-lobby.com/hydride.htm)

Battery Packs - LiPo
http://www.cheapbatterypacks.com/main.asp?sid=445976&pgid=tp&sort=PL (http://www.cheapbatterypacks.com/main.asp?sid=445976&pgid=tp&sort=PL)
http://www.hobby-lobby.com/lithium-polymer.htm (http://www.hobby-lobby.com/lithium-polymer.htm)

Gearboxes - Speed 400 & 480 examples
http://www.hobby-lobby.com/gear400.htm (http://www.hobby-lobby.com/gear400.htm)
http://www.hobby-lobby.com/gear480.htm (http://www.hobby-lobby.com/gear480.htm)

Matt Kirsch
12-12-2005, 06:16 PM
Ed's got the general stuff out of the way, now let's look at some specifics.

You asked a question about the adjustable timing and PVM settings on more advanced brushless ESCs. Both these settings affect the proper operation and efficiency of the brushless motor being driven by the ESC.

Timing is the more common setting to find, and there are generally two settings: Soft is for conventional (inrunner) motors, while hard timing works best with outrunner motors. This timing is very similar to ignition timing on a gasoline engine; if the ESC doesn't "fire" at the right time, the motor will be down on power, and may not even run at all. Just like modern HEI, brushless timing is driven by electrical impulses generated by the motor, known as back EMF.

The PVM(?), or frequency indicates how many on/off cycles are performed every second. An ESC doesn't "vary voltage" like many people think; it's a simple on/off switch that simulates partial throttle settings by inserting rest periods in between tiny pulses of full throttle. Pulse, rest, rest, rest, pulse, rest, rest, rest... That's the basic idea of "1/4 throttle" for example. The higher the frequency, the more times per second the ESC pulses or rests. As I understand it, motors with more poles, like outrunners, need higher frequencies to run efficiently.

You also asked about determining usable prop size and RPM using the basic specificationss that are included with the motor. Unfortunately, without a motor calculator like MotoCalc, you can't determine that information strictly from the motor's specs. The math is quite complicated.

Some motor manufacturers, like AXi, include application charts that show their motors with various battery and propeller combinations. This makes it e-a-s-y easy to find a setup for your plane knowing only the Watts requirement and the prop clearance.

Some manufacturers include no prop/battery information at all. In these cases you've got three choices: 1. Avoid this manufacturer. 2. Jump on Wattflyer, RCU, and Ezone to see if anyone has a working combination. 3. Experiment with a pile of props and a Whattmeter.

Some manufacturers include enough information to make you dangerous. Often you'll see things like "2-3 LiPoly cells" and "9x3.8 to 11x3.8 props." You can extrapolate enough information from this to at least give you a place to start. Remember that as you go up in prop size, Amps increase, and that if you increase cell count (voltage), Amps also increase. So, if you plan on running with a 3S LiPoly, the highest-voltage battery in the list, you want to start with the smallest prop in the list and work your way up. A Whattmeter is a good thing to have here, so you can test to see if a particular battery/prop combination is exceeding the motor's maximum current rating.

Kv... Kv is about as close to a "displacement" rating as you're going to get with electric motors, with one caveat: The motors you are comparing must be similar in design (compare outrunner to outrunner), similar in size, and similar in weight. Kv is RPMs per Volt, so a motor with a Kv of 980 will TRY to turn the prop 980 times per minute, per Volt applied. On 10 Volts, the theoretical RPM would be 9800. Of course, between the props inertia and wind resistance, you won't hit the theoretical max, but it does give you something to compare similar motors with. A motor with a Kv of 980 won't be quite as "powerful" as a motor with a Kv of 1050, for example, using the same battery and prop because the faster the prop spins, the more power it requires. Conversely, the 1050 Kv motor will draw more Amps and produce shorter flight times in exchange for that additional power. Remember that there's no free lunch.

rahtware
12-21-2005, 08:18 AM
Another old nitro flyer here... I learned a lot by reading this thread, more then I can digest in one sitting!

I would like to add that planes designed for E-power fly fine on less power then a nitro flyer usually needs. In nitro I was on the bottom of the power range flying .25 engines. The E-powered planes I am flying sucessfully are in what would be the .10 and below range. My stryker with the stock motor (less power then an .049) flies like my .25 nitro powered stick; as fast, with a little less verticle, but a whole lot more forgiving.

AEAJR
12-21-2005, 12:02 PM
Larry makes a good point. E powered planes can be lighter.

Because e-power produces very little vibration as compared to an internal combustion engine, the aircraft structures can be lighter. Also, since there is no fuel or exhust resedue, there is no need to fuel proof things and you can use materials that might be adversely affected by fuel or exhaust. This again can allow the plane's structure to be lighter.

You can literally take a free flight plane made of frail sticks and mylar and put an electric motor on it, with very little change, and fly it indoors under full radio control. Mondern ESC, electronic speed contols can even be programmed for soft starting and soft acceloration to reduce the torque of a sudden change of power to the propeller.

Today we see electric planes that weigh under an ounce that can fly in your living room due to extremely light wing loading and, of course, no exhaust to upset the spouse of the house. :o

Typically, when you see new versions of planes that have been redone for e power, they are lighter. That is because much of the reinforcement of the original design is there to resist the vibration caused by the engine. Even if the plane uses heavy NiCd batteries, and the all up flying weight of the plane is higher than the glow planes, the structural reinforcement needed for vibration is reduced.

Even the foam padding that the typical nitro flyer puts around the receiver is there to isolate the electronics from the vibration of the engine. It is quite common to leave this off on e planes. Oh, there is no doubt that the padding helps in a crash, but from a day to day flying point of view, the foam is not needed to isolate the flight electronics from engine vibration.

Doesn't make one better, just different.

I am also told that one of the reasons that gearboxes are so common on e planes and so rare on glow planes is vibration. The smooth power delivered by the e motors lends itself to lightweight gearboxes while the bang bang pulse of a glow engine would require a much more robust gearbox structure. It is just simpler to put a bigger glow motor in, in most cases.

With the electics you have more options available, so there are more options to consider. With flexability comes choice and choice can lead to confusion. However those choices open new opportunities, so it is all good.

AEAJR
12-21-2005, 12:27 PM
How about something specific. I have ended up with a pair of Himaxx 2825's, one 2700 and one 3600. I understand they will need a gearbox. My questions are

-Can anyone give me an idea of what these motors can do? prop, at rpm, w/volts, @ Xamps, gear ratio?
-Suggestions on a gearbox? ratio? or range of ratios? A source for gearboxes?
-Suggestions on an ESC? China ESC's do what they claim?

When I dropped out of RC I left a 30 sized plane framed up. I think it is a midwest Sukoi. 45" wing span, it should turn out light, say 3.5-4lbs. One of the 2825's in this?

:rolleyes: I've been thinking the 2825-2700, 4:1 gearbox, 3S li-poly, I'm guessing a 12X8. RPM? Amps? no idea on these.
Maybe the 3600 on a 5:1, 3S?

Yes, I do tend to dive into the water, then look for rocks.:D

Thanks in advance for any help. And Happy Holidays

Robert

Dont' know if this will be helpful but it might put you on track to find more info about those motors.

There is a discussion about these motors at:
http://www.rcgroups.com/forums/archive/index.php/t-208073

Parts and accesories
http://www.grandrc.com/motors__gearboxes

http://www.backyardflying.com/securestore/c232484p16523266.2.html

Robert53
12-21-2005, 01:56 PM
[quote=rahtware;30747]Another old nitro flyer here... I learned a lot by reading this thread, more then I can digest in one sitting!


Larry

Amen! And the older I get, the more I read, and thus, the more I forget.
I've been reading everywhere. I was surprised how many forums are on Eflight. Now comes the part of trying to use it.

BTW, left off a little info in my post. I do have a couple of RTF's. I have a Typhoon (local HS), good plane after rebalance. And an Edge 540T (from china,ebay), this one just plain sucks. All due to constuction.
I know this is how I was infected with this electric bug. Hopefully it won't be fatal. (married)

Thanks Larry

Robert53
12-21-2005, 02:07 PM
[quote=AEAJR;28065]I can't comment on the specific motors and such that you

SIZING POWER SYSTEMS FOR ELECTRIC AIRPLANES

Ed

Read this before, Read it again, and will read it several more times. I've been to some, maybe most of the sites. You also brought up things that I haven't thought of. I hope to know enough about this to answer a question one day.:D
I have come up with a starting point.:) When I, at last, get everything together I hope it all works.

Ed, you helped alot, thanks

Robert

Robert53
12-21-2005, 02:21 PM
Some manufacturers include no prop/battery information at all. In these cases you've got three choices: 1. Avoid this manufacturer. 2. Jump on Wattflyer, RCU, and Ezone to see if anyone has a working combination. 3. Experiment with a pile of props and a Whattmeter.

Matt

I thought I posted this a few minutes ago but I don't see it.

To the above - or the manufacture places it on an out of the way page on their site.

You cleared up several things, and started a few new trains of thought. I'm not too sure about the KV/displacement. I've been concidering KV as a prop size indicator. Thanks for all the help.

Thanks again
Robert