The great Ohm's Law debate

[William Halgarth]

A speed controller does indeed  switch the curcuit on off very quickly, thus increasing resistance. I was trying to keep it SIMPLE. Quoting A level physics isnt going to help many people.

[Jim Kidd]

Thanks Guys

You've managed to make someone who was thinking of converting to a cleaner and more convenient way of powering model aircraft think 'Does anybody out there actually agree about anything?' After reading this mess of confusing 'I know more than you, mine's bigger than yours' stuff, I've had enough and am going to get the petrol can out. I think I will wait until the so-called experts in the field can tell the rest of us plebs what we need to spend our hard-earned cash on to fly a particular model, without first needing to go back to college for a degree in electronics. I like to fly aeroplanes and have absolutely no interest in variables, constants, ohms, watts or any other stuff. If you are into that then great - just give the rest of us a break and agree what engine/ ESC and Lipo is required for model normally powered by a particular sized I/C engine and we'll all be happy.

Jim

[Pete Rieden]

Well I'll admit it's been nearly 30 years since I did my A levels, but I'm pretty sure that the concepts of back-emf, and the generator-motor rules (left-hand rule and right-hand rule) were both covered at O-level. Certainly the idea that current passing through a resistance generates heat is covered, complete with the sums.

But if people don't want explanations I'll stop giving them.

PDR

[David Ashby - Moderator]

Jim, I've amended your post, please be aware of language on the forum, thank you.  I sympathise with your sentiment none the less.

I've no background in electromics at all and take my hat off to folks like Peter yet I get along fine in electric flight. Sure, there's a learning curve as there is in anything for those who want to explore but there's only the need to assimilate as much as you need. Once a few simple rules of thumb have been taken on board then you can stick with those, it all depends on what you want to fly of course.  

[Tim Kearsley]

I think this thread's a classic example of how things can mushroom from an original post, and drift off topic a bit as well!  To go back to the title of the thread, there can't be any debate about Ohm's Law - it's just that - a law which states the relationship between voltage, current and resistance.  Volts = Amps X Ohms, or if you prefer, Amps = Volts/Ohms or Ohms = Volts/Amps.  It really is that simple!  A very good analogy is a water tap connected to a hose.  The pressure of water at the tap is Volts, the diameter of the hose is Ohms and the amount of water that comes out the end of the hose is Amps.  If you turn the tap on harder, more water comes out.  If you increase the diameter of the hose (reduce the resistance) more water comes out.  Simple, isn't it?

Cheers,

Tim.

[Bruce Richards]

I believe that an accurate detailed explanation of electric systems is complex and involves lots of complex maths, far more complex than has been discussed here. But you do not need all this complexity as a few rules of thumb, some basic knowledge and some experimentation is all that's needed. The information you need is available in this forum and in many of the magazines. No one ever wants an accurate and detailed explanation of IC engines involving port geometry, compression ratios, thermal efficiency, calorific value of different fuel mixes etc. So what do you need to know?

1. If the current (Amps) in the system is too high then you will break one of the components. Never exceed the current rating of the ESC and try to be at least 25% within the rating. You can go up to the full current rating of the motor. Do not use a current that is more that about 10-15 times the C rating of your battery unless you want very short flights and/or short life for your batteries (you can use short bursts of high current for take off close to the max rating of your battery but be careful you do not exceed yoir ESC rating). You need to get a whatt meter to help with this. Current can be changed by changing the prop size. Bigger prop more amps smaller prop less amps.
2. 100w (amps X volts) per pound of model weight is good for most sports models. You can go a lot lower (60-80W/lb) for low wing loading slow flying models and a lot higher (250w/lb) for 3D and EDF. So total wattage is say 100 X model weight in pounds.
3. Using total wattage required decide on how many cells you require for your battery. I try to not draw more that 40-50A and would rather be in the 20-30A range. You decide on the current based on what batteries you intend to use. So divide your total wattage by say 30A to give you the voltage you want from your battery and this suggests the number off cells you will need.
4. Roughly decide what prop diameter is right for your model based on size and type of model. Look to see what size props other people use on this type of model either IC or electric.
5. Now for the motor. There are quite simple calculations to do pitch speed based on how fast you want your model to fly. But I normally say I want the prop for somewhere between 8000-13000rpm unless its very small (less than 5 inches) in which case I go up to 20000 or more RPM. Anyway having decided on your RPM for your expected prop size the divide the RPM by the volts to give you the RPM per volt (Kv) of your required motor. You motor will also need to handle the amps and volts you have decided upon above.

Now put it all together and test it with your amp meter and adjust the prop size until you get the amps and hence power you planned on in step 1.

[Myron Beaumont]

It does seem to me that "lectric " beginners like myself and many others of varying educational backgrounds have to come to terms with the fact that that modern day aeromodelling is inextricably tied up with an increased necessity to know whats going on to enable us all to obtain satisfaction/fun from all aspects of this ever increasing technology.  I applaud Peters explanations & obvious knowledge & understand the clear explanations & at the same time can understand perfectly well Jim Kidds sentiments . I feel the same actually .) However the challenge taken up to get to grips with electricity was my choice & had I known when I first started what I now know -I would never have got so involved with it .As a dedicated modeller I think I can say on behalf of us all that its important to get rid of ALL unknowns ,for piece of mind when flying  to be able to concentrate on the job in hand . I know that for the amount of money I've spent in one year on "lekky "stuff I could have got in to the LMA scenario ! (my ambition )

I feel quite capable of using fuel powered engines after 50 years of experience & am never happier than when using my own experience to overcome tuning problems etc. I just like to see things moving around  I can alter and adjust & dont mind oily hands  On the contrary !!

Just watched the video of the large aerobatic biplane on the other thread .Wow! smoke and all 

Beautiful !  Electric ?   Say no more !   (steps down from soap-box ready for flack ) 

[Tim Kearsley]

I think that's a very interesting post Myron.  It's interesting to me, because I'm kind of the opposite of you, in that I began my career (which is admittedly very short) with electric power, with which I immediately felt at ease.  I've just this last model branched out into I.C. and I feel not at all at ease!  I can therefore sympathise entirely with how you feel, albeit from the other side of the fence so to speak!

I still think both disciplines have their pros and cons and I don't see anything "wrong" with either.  The only thing I DON'T particularly like with glow engines is cleaning the oil off the model after a session.  But I do like to see a bit of smoke and hear a nice noise!

Cheers,

Tim.

[Myron Beaumont]

Thanks Tim   I suppose in a few years time the arguments will be fusion power plants (computer controlled) versus old fashioned lipos controlled by human brain power    Remember what happened to steam ??

[Steve Hargreaves - Moderator]

Myron, I think it all depends on how curious you are.

Yes you can get bogged down in loads of maths with electric flight if you want to but you certainly don't need to...learn a few simple ground rules as David sez & enjoy......after all IC engines utilise a load of maths & engineering know how in their design & we don't worry if timing the exhaust port to open a few degrees earlier would be better......we just get on & use them....same thing applies to sparky ppwer!!!!!!!

 Right I'll get me coat!!

[scott cuppello]

This is what makes me laugh, electric power [and let's get it right, we are talking about POWER TRAINS, aircraft are my main interest, not power trains]....are no more complicated than IC....despite the best efforts of people to apply electronics know how to it all....which at the end of the day, makes NO difference what-so-ever to powering a model using batteries....in the same way that knowing nothing about 2 stroke engines helps with IC......

 I spent 15 years of my previous life tuning 2 stroke engines, and could quite easily bore everybody stupid about it.....but what would be the point? Who cares? Nobody.....you buy an engine and sling it in, barring prop choice and carb settings [very simple compared to a multi-stage race carb].....and get on with it.....electric flight is no different, even EDF is pretty simple now......I wish [as I said right at the start] the simple explanations could be left alone for people to digest and put into practice.

[Former Member]

[This posting has been removed]

[Pete Rieden]

<sigh>

Scott,

You will have noticed that some of the replies on here don't wish to understand any deeper, whilst others are interested. Neither group is "right" or "wrong". As you say, it is quite practicable to own and operate a car for years without understanding anything about how it works - the vast majority of the public do this. Others have a curiosity and wish to understand a little more about how it works. Occaisionally the deeper understanding is of use, but mostly it is simply a matter of interest.

The same is true of model flying - some wish to simply go and fly using black boxes purchased from a shop, whereas others actually enjoy understanding what is going on inside the black boxes. Neither is "correct" - they are just different approaches. Those who prefer the deeper understanding ask questions to seek information, and enjoy discussion and debate. Those who don't want this level of understanding don't have to read (let alone participate in) these debates - it's not a legal requirement.

What *I* have trouble understanding is why you object to these discussions even taking place. You seem to go out of your way to heckle them, jeer at them and generally ridicule those who participate in them. What is it about such discussions that so offends you that you feel the need to work so hard to prevent OTHERS from doing something they enjoy and/or find useful?

I'm not having a pop - I'm genuinely bemused by your attitude.

PDR

[scott cuppello]

It's simple Peter, some do appreciate your opinion's, and some do not, and when they don't your response is "if people don't want explanations, I'll stop giving them", and that is about the most polite of your reposts thus far......you may be very good with electronics.....you are not very good at putting things in laymans terms....accept it based on the mixed reception.

When I offered the opinion that things need to be simplified, you simply attacked me as a simpleton......so, you are suprised that I think your high brow explanations....way over the heads of many....are simply not helpful? Perhaps if YOU were tolerant of people explaining things in simple terms....others would be tolerant of your technical explanations....room for both points of view I feel....