A Guide To Electrics for Nitro/Glow Heads

This one seems to come up a little bit as Nitro flyers look for a bit of assistance in understanding electrics. So, here's an attempt to provide some assistance. Now please understand - if you are looking for an exact answer to whether a KERJKER34 works with a 384HED you aren't going to find it here - all I'm trying to do is explain the concepts so you "get it".

Now, as a Nitro flyer you know almost everything you need to grasp electric flight - in many ways it isn't that different.

Before starting here go take a look at the Basic Electrics of Electric Flight. It may seem a little like goobeldy gook on the first read, but once you have read that come back here and hopefully by the end some of it will start making sense.

Okay, without further ado lets get into it.

Inter related nature of the Power System

In a nitro/glow model the power system is pretty much defined by one compoment. The engine - once you have defined that most other characteristics naturally flow from it. For example, while varying the fuel payload will change the flight performance of the model somewhat, it wont change the power output of the engine.

This is probably the biggest difference with electric powered flight. It is a combination of the battery, motor and propellor that you choose which defines the power output. So, you can't quite approach the problem from the "this is a .4 size model" perspective anymore (well, you can actually - eflite for example make a range of glow replacement engines - of course you need to choose the correct battery to team with it, otherwise you wont get the correct amount of power output).

How do they interrelate? Well, I'm glad you asked (hello to fans of the Curiosity Show).

The speed the motor revolves at is defined by:

1. The number of volts supplied by the battery.
2. The revs per minute per volt (or kv) rating of the motor.

The amps drawn by the system is defined by:

1. The size of the propellor (bigger prop - more amps, smaller prop - less amps).
2. The speed at which the propellor rotates (which was defined above).

Once you know the amp draw you need to:

1. Make sure every component is rated for at least that many amps.
   

Finally, the total power output of the system is defined by:

1. The input volts from the battery; multiplied by
   
2. The number of amps the propellor draws.

Now - on first glance that might seem a little tricky, but just go back and read through it once more. Each relationships is defined. Hopefully some of the explanations below will help further.

Infinite Possibilities

Okay - infinite is an awfully big number - probably not infinite, but there are a lot variations you can do with electric power. This flexibility often appears to be complexity to those first looking at electric power.

Understanding where a power system delivers

Watts = volts x amps - no doubt you are starting to get sick of seeing this, but you need to understand what this means in application to "get" electric flight.

Why? Why am I laboring the point so much? Because there are many ways in which you can get the power you need to fly your aircraft. And this is where the true elegance of electric powered flight comes through.

There are three ways you can vary the power output of an electric flight system:

If you keep your battery voltage and motor kv constant then:

• Increasing the prop size will deliver more power (by drawing more amps).
• Reducing the prop size will deliver less power (by drawing less amps).

If you keep the motor kv and prop constant then:

• Increasing the battery cell count will deliver more power (by providing more volts and drawing more amps {because the motor is spinning faster with higher voltage}).
• Reducing the battery cell count will deliver less power (by reducing the input volts and drawing less amps {because the motor is spinning slower with lower voltage}).

If you keep the battery and the prop constant then:

• Using a motor with a higher kv will deliver more power (by drawing more amps).
• Using a motor with a lower kv will deliver less power (by drawing less amps).
  

Let's say you decided you needed 150watts to make your plane fly the way you want it to (see the basic electrics of electric flight for info on figuring out the power requirements of your model based on its weight).

So how do you get to your power. The beauty of electric flight is that it can tailor to meet your other requirements with a great deal of flexibility.

For example, let's say you have a scale model and you don't have much room to spin a propellor when she is on her undercarriage. Starting from the prop size as the design input you could choose a motor and battery combination that will deliver the power you need.

Likewise you might have a 3d plane that needs lots of thrust - you will want to spin a big propellor - the biggest one you can. Once again - this can be a design input in choosing the other components.

Or maybe you just already have a battery and motor you would like to use. In this case, you can choose a prop that you know wont overdraw any of the components.

Defining Your Power System

When you are first getting into it, it probably pays not to get too adventurous. Many electric motor manufacturers will tell you a good combination of propeller and battery to use with their motor. Provided that none of their requirements are outside your plane's limits (so you can swing that size prop, the battery will fit etc) then that is probably the best way to start.

If your motor manufacturer doesn't give you the info then use online forums like www.rcuniverse.com or www.rcgroups.com - people here will be all to happy to tell you combos that work.

There some more great info on wattflyer on choosing power systems.

System Limits

Finally - I know it is alluded to elsewhere, but just to touch on it again - you can't just go on drawing more and more amps. Components are usually rated in terms of continuous amps they can provide (for batteries there is a C rating - multiply the battery's capacity by this to get its constant discharge limit - for example 2200mAh 10C battery is 22Amps maximum continuous).

If you can't measure your amp draw you need to be very careful about overtaxing your components. It is best to rely on tables provided by the manufacturer about combinations that work.

In terms of damage - it tends to be a little like this:

• Speed Controllers just tend to cut out when you try and put too many amps through them - the speed controller is safe - the rest of the model is in peril because you have no power.
• Motors will tend to take some abuse, but eventually the high temperatures will damage the magnets that make the motor work, and power versus input volts and amps will drop off.
• Batteries - LiPo and LiIon can be permanently damaged by even one session of overdrawing current from them - they are often the most expensive components and so they are probably the ones to be most careful with. NiMh and NiCd don't suffer from this but have their own issues.
  

Conclusion

There is still quite a bit more to know about this stuff, like going for pitch speed versus thrust etc. On the right hand side the RC articles link has some more stuff you might find handy.

Hopefully it is a little clearer now. If it is please feel free to drop me a line at ozrcboy@gmail.com or post a comment below. If its still all as clear as mud, or (hopefully not) you are more confused than when you started then please don't hesitate to drop me a line or post a comment - the question you ask might be the same one the other glow and nitro heads ask at the end of this - so share the confusion - we will probably all get something out of it.

Cheers,
Oz.