Any interesting site to learn the basics about rc electronics?

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Alex - 14 Feb 2009 01:33 GMT

Hi all,

I am new to rc technology and this week I bought a Twister Bell 47G
Medevac heli. I also have realflight G4.5 and FMS as a simulator tool
with which I spent most of the time this week. I have flown the heli a
couple of times too and it feels great! It's hard but I keep it safe
and hovering low to the ground with the tail towards me until I feel
more confident. I am so much interested in the technology that I have
a few questions:

1. Does anyone know a good site where I can study about rc heli and
airplane electronics for beginners? I would like to learn about
servos, motors, batteries, mAh and all the things you find in my next
questions.

2. My Tx uses 8 Alkaline AA 1.5V batteries and in 3 days I had to buy
a new set already. Is there another way for not wasting batteries so
fast? Like using a different type or use rechargeables? I have 4 NiMH
batteries of 2700mAh and 1.2V for my digital camera, could I buy 4
more of these and use them for the Tx or does nobody do that since it
takes like 12hrs to reload them?

3. My Tx also has a small hole at the side that contains a small pin
where I can plug in a round female jack. It says "charge" with a
polarity icon under it. Can I use this to charge 8 rechargeable AA
batteries inside the Tx? I have a power adaptor from some household
appliance that I can plug into the wall (230V/50Hz) and it has a
switch that I can set to 12V and another switch where I can change the
polarity. Can this be used to charge? Or can I connect it to use the
Tx without batteries?

4. The heli uses 7,4V 800mAh 2-cell LiPO batteries. The instructions
say you may not discharge to more than 3V per cell or charge to more
than 4,3V per cell. Does that mean the voltage should always remain
between 6V-8,6V? I would think that if you have 0V, the battery is
empty but I'm sure that's not right. I used a multimeter and noticed
after charging the voltage is around 8,46V and right after flying
(when the LED flashes to tell me to remove the battery) it's around
6,8V. Then I noticed the voltage increases to about 7,03V before it
drops to 6,62V and this with nothing connected to it. Explanation?

5. I read a lot about battery capacity (mAh) on the net but it still
remains very fuzzy. I understand, the higher the number, the longer it
will last under the same load, the more current it can deliver but the
longer it takes to charge it etc. But what happens with the current
and voltage over time when you put it in a graph? Does for instance
800mAh mean that it can deliver a constant 800mA during the first hour
and then drop rapidly to zero? And why do rc batteries only last
around 10min? Do they have to deliver high currents and get drained
very fast? When I am flying the heli, does the battery deliver a
constant amperage to the systems? That would mean
800mAh * 60min/10min flying time=4800mA or 4.8A??

6. If you want to use a different battery, is voltage the only
requirement that needs to be the same? Can you use a 7,4V NiMH battery
instead of a 7,4V LiPo battery?

I know it's a lot of questions so thanks in advance for your help

Alex

Reply to this Message

Andreas Beck - 14 Feb 2009 09:58 GMT

> 2. My Tx uses 8 Alkaline AA 1.5V batteries and in 3 days I had to buy
> a new set already. Is there another way for not wasting batteries so
> fast? Like using a different type or use rechargeables?

Using rechargeables would be a good idea. The Tx seems to be designed to
handle that, as it has a recharge plug.

In general most devices will work with rechargeables, though some will
not like it, as they have a lower cell voltage (1.2V vs. 1.5V).

> I have 4 NiMH batteries of 2700mAh and 1.2V for my digital camera,
> could I buy 4 more of these and use them for the Tx or does nobody
> do that since it takes like 12hrs to reload them?

Buy 12 more and swap them when needed, if you think reloading them
overnight is not acceptable.

> 3. My Tx also has a small hole at the side that contains a small pin
> where I can plug in a round female jack. It says "charge" with a
> polarity icon under it. Can I use this to charge 8 rechargeable AA
> batteries inside the Tx?

Probably. The manual says nothing about it?

> I have a power adaptor from some household appliance that I can plug
> into the wall (230V/50Hz) and it has a switch that I can set to 12V
> and another switch where I can change the polarity. Can this be used
> to charge?

NO! This is a power supply, not a charger. A charger will regulate the
current it pumps into the to-be-charged cells, measures their cell
voltage etc. and finishes loading at some point.

A power supply will simply try to push as much current into the attached
circuit as its resistance demands (and it can deliver).
Using that to charge Ni* cells usually results in more or less violent
destruction of either the cells or the charger or anything in between or
near.

> Or can I connect it to use the Tx without batteries?

This might work, depending on the setup of the Tx. However it might
cause radio interference, depending on the quality of the power supply.

> 4. The heli uses 7,4V 800mAh 2-cell LiPO batteries. The instructions
> say you may not discharge to more than 3V per cell or charge to more
> than 4,3V per cell. Does that mean the voltage should always remain
> between 6V-8,6V?

Yes.

> I would think that if you have 0V, the battery is empty

It is very empty then. "It is dead, Jim."

Discharging too far will destruct the cell.

> but I'm sure that's not right. I used a multimeter and noticed
> after charging the voltage is around 8,46V and right after flying
> (when the LED flashes to tell me to remove the battery) it's around
> 6,8V.

That's ok.

> Then I noticed the voltage increases to about 7,03V before it
> drops to 6,62V and this with nothing connected to it. Explanation?

The cell slowly creeping up in voltage is caused by its chemical
processes delivering a few more electrons slowly and when nothing is
connected to it, or something like a multimeter with a very high
resistance, the voltage will creep up slowly. Connecting the slightest
load will make that drop again.

This is why alkaline battery testers apply a moderate load, as checking
them with a high-R multimeter would still show 1.5V even for pretty
depleted cells.

> 5. I read a lot about battery capacity (mAh) on the net but it still
> remains very fuzzy. I understand, the higher the number, the longer it
> will last under the same load, the more current it can deliver but the
> longer it takes to charge it etc.

It has nothing to do with the current it will deliver.

It say how much current it can deliver over time.

If a battery has 2800mAh, it can provide 2800mA for an hour or
1000mA for 2.8 hours etc.

The current it can deliver is given a "C" factor - see below.

> But what happens with the current and voltage over time when you put
> it in a graph?

Current depends on the load. The voltage will drop from 4.3 to 3.3V over
the discharge of the abovementioned capacity (or rather the real
capacity the cell currently has).

> Does for instance 800mAh mean that it can deliver a constant 800mA
> during the first hour and then drop rapidly to zero?

This depends on your load. If the load counteracts the dropping voltage
by changing its resistance something like that will happen.

If not, you will notice the drop in voltage causing less current to flow
and thus decreasing overall power consumed.

> And why do rc batteries only last around 10min?

Because Helicopters are hungry beasts.

> Do they have to deliver high currents and get drained very fast?

Yep.

> When I am flying the heli, does the battery deliver a
> constant amperage to the systems?

No. It delivers what the system currently demands.

> That would mean 800mAh * 60min/10min flying time=4800mA or 4.8A??

Yep. At around 8V, that means a mere 40W. Compare to any household
appliance which doesn't even have to fly.

> 6. If you want to use a different battery, is voltage the only
> requirement that needs to be the same? Can you use a 7,4V NiMH battery
> instead of a 7,4V LiPo battery?

Usually not. It will be too heavy. NiMH should suffice from the internal
resistance PoV, which might be a problem for other battery technologies.

The battery must have:
- the right voltage
- a low enough inner resistance
- the ability to deliver enough current. This is usually measured in "C"
I.e. 20C means it can deliver 20x the Capacity in mAs as mA.
Or to say it in another way: It can be discharged fully in 1/20th of
an hour without damage.

However using a "foreign" battery technology will eventually cause
problems with stuff like the undervoltage-detection, as other
technologies have other characteristics.

CU, Andy

Reply to this Message

Alex - 15 Feb 2009 19:39 GMT

> > 3. My Tx also has a small hole at the side that contains a small pin
> > where I can plug in a round female jack. It says "charge" with a
> > polarity icon under it. Can I use this to charge 8 rechargeable AA
> > batteries inside the Tx?
>
> Probably. The manual says nothing about it?

No, I couldn't find anything.

> > Or can I connect it to use the Tx without batteries?
>
> This might work, depending on the setup of the Tx. However it might
> cause radio interference, depending on the quality of the power supply.

Can I use the power adapter for this purpose? It converts 230V AC 50Hz
into 12V DC. I just tried this but with no success. I tried the
polarity switch in both positions. The transmitter appears dead with
no batteries and power cable connected.

> > Then I noticed the voltage increases to about 7,03V before it
> > drops to 6,62V and this with nothing connected to it. Explanation?
[quoted text clipped - 4 lines]
> resistance, the voltage will creep up slowly. Connecting the slightest
> load will make that drop again.

The first part I understand but that it drops again with the slightest
load was not the case since there was at all times nothing connected
to the bat. I simply measured voltage and then left the battery alone
for 20 sec, measured again and waited 20sec with nothing connected
etc. Voltage was increasing until a peak of around 7.03V before it
dropped again. Nothing was connected.

> This is why alkaline battery testers apply a moderate load, as checking
> them with a high-R multimeter would still show 1.5V even for pretty
> depleted cells.

I didn't know this. So when I want to measure the voltage of an
alkaline battery I should not measure this directly from pole to pole
at the battery because it will always appear to be full?

> The battery must have:
> - the right voltage
[quoted text clipped - 3 lines]
> Or to say it in another way: It can be discharged fully in 1/20th of
> an hour without damage.

So that's why RC models need special batteries like NiMH or LiPo
because ordinary alkaline batteries can never develop the same amount
of current due to chemical limitations?

I remember a year ago buying 1.2V AA size NiMH rechargeable batteries
of 2000mAh for my digital camera was a big deal but now they even have
the same batteries at 2700mAh. The technology seems to be continuously
improving. Is that the same for RC model batteries? Will we be able to
fly for 30min instead of 10 with the same size batteries that we have
now? Just wondering.

Thanks for the help, I really appreciate it. "Dr McCoy to the bridge,
there seems to be intelligent life forms out here"

Reply to this Message

Nothome - 14 Feb 2009 12:45 GMT

>1. Does anyone know a good site where I can study about rc heli and
>airplane electronics for beginners? I would like to learn about
>servos, motors, batteries, mAh and all the things you find in my next
>questions.

There are several places on the Internet. A couple of my favorite
places for learning are:

www.helifreak.com

www.rcgroups.com

Bob

Reply to this Message

Alex - 15 Feb 2009 19:59 GMT

> >1. Does anyone know a good site where I can study about rc heli and
> >airplane electronics for beginners? I would like to learn about
[quoted text clipped - 9 lines]
>
> Bob

Thanks for the sites. Do you by any chance know if there exists an
online training site about the general basics of rc modeling? Like a
coursebook. Or maybe if you're an instructor, which book would you
recommend your students to buy when they come to your model flying
club? Then I can check if I can order it on an online site. Some
examples of my favorite sites (not about rc modeling though, but the
first 2 are about theory of flight):

http://www.av8n.com/how/

http://www.auf.asn.au/groundschool/contents.html

http://www.karbosguide.com/books/pcarchitecture/start.htm

Reply to this Message

Larry Farrell - 15 Feb 2009 20:22 GMT

[snip]

> Thanks for the sites. Do you by any chance know if there exists an
> online training site about the general basics of rc modeling? Like a
[quoted text clipped - 9 lines]
>
> http://www.karbosguide.com/books/pcarchitecture/start.htm

I suggest

http://www.masportaviator.com/surestartguide.asp

Reply to this Message

Alex - 17 Feb 2009 16:12 GMT

> [snip]
>
[quoted text clipped - 15 lines]
>
> http://www.masportaviator.com/surestartguide.asp

Thanks, that looks like a good site to start with!

Reply to this Message

Model Forum / Radio Controlled / Helicopters / February 2009

Any interesting site to learn the basics about rc electronics?