Electrics of Electric Flight Explained – A Beginner’s Guide To RC Planes Electrics
I’m trying to write down all the good stuff about the electrical powered flight which other people have taught me so that it might be useful to others coming into the hobby. So without further ado:
When it comes to our electric flight there are four things we typically think about:
As you probably know there are other electrical things that you might normally measure, like resistance etc, but we don’t normally need to worry about them for electric flight.
The easiest way to think about all these things is to imagine electricity as water.
Voltage is electrical “pressure”. It is measured in volts (v). Thinking of it like water, voltage is the number of meters of pressure you have – so if the reservoir is 50 vertical meters above you, you have 50 meters of pressure.
Current is electrical “flow”. It is measured in amps (A). Thinking of it like water we would measure it in something like liters per minute.
Power is the combination of voltage and current (power = volts x current). We measure it in Watts (w). This is easy to imagine with water as well. Think of one of those huge water wheels – the kind that were used to power sawmills in times gone by. Now imagine hitting it with a super soaker water pistol. Even though the water is at very high pressure, there is very low flow, and so the super soaker will probably not generate enough power to turn the wheel. Now imagine the gently babbling stream that feeds the wheel, and under the force of almost no pressure, but with a high enough flow rate, generates enough power to turn the wheel. Finally, imagine the firehose – the best of both worlds – high pressure and high flow rate – it would probably make the wheel spin quite quickly.
Capacity is a measure of how long you can draw a specified current from a battery. It is measured in Amp Hours (Ah), or more commonly for the scale of equipment used for electric flight, mill-Amp Hours (mAh). Using the water analogy this is simply how many liters you have in your reservoir. It is a little more complicated for electrical power and we will talk about it a bit later.
How Much Power Do You Need to Fly?
To figure out the power you need to fly a model depends on the weight of the model, and the type of model it is, as well as what you want from it.
In one of those quaint exposures of the inadequacies of the Imperial measures system which the US still cherishes this is normally expressed as Watts (a metric unit) per pound (an imperial unit). For those that want to work with a measurement system that makes sense, one pound equals approximately 450g for the numbers below.
- 50-70 Watts per 450g – Minimum for reasonable performance flight. Slow flyers and slow park flyers
- 70-90 Watts per 450g – Slow flying scale models, Trainers.
- 90-120 Watts per 450g – Sports aerobatic. Fast scale models.
- 120-150 Watts per 450g – Advanced aerobats. High-Speed Models. Excellent Vertical performance
- 150+ Watts – Very High Speed, Unlimited Vertical Performance.
Note – You must include the weight of all the plane’s components in your calculations – anything that leaves the ground with the plane needs to be included – batteries, the engine, speed controller, etc.
So, if you have a 900g delta wing, that you want to have unlimited vertical performance, you are going to have to try and generate 300w (900/450 = 2, 2 x 150 = 300).
If you have a slow flying scale plane that weights 350g then you need to try and generate a minimum of 54 watts (350/450 = 0.77, 0.77 x 70 = 54).
Understanding the Limits of Your Equipment
Most electrical equipment will have limits on the amount of current it can handle, as well as sometimes the number of volts it can handle. Some equipment also states a power limit as well.
Batteries, and particularly the Lithium Polymer type, are rated in C for the amount of current they can discharge. So, if you have an 800mAh 20C battery the maximum current you can draw from it is 16A (20 x 0.8=16). With the battery’s volts in hand (say a 3s 800mAh rate at 20C) you can generate the maximum power this battery can provide – 16A at 11.1v = 177watts. Batteries may have a burst rate, and a continuous rate – so 15C at burst, 10C continuous. Using the 800mAh battery again you might be able to draw 12A in burst, but only 8A continuously.
Speed Controllers are often rated by the amount of voltage, and current they can handle. The amount of current that is drawn through the speed controller depends on the engine. In general you need to make sure your speed controller can handle at least as much, and ideally a little more current and power than the engine. Obviously, your speed controller needs to be rated at the voltage for the battery – it will not reduce voltage either (there isn’t room for a transformer there).
Engines are usually rated at the maximum current draw they can handle. They will often have a burst and continuous rating. Sometimes engines are also rated for the maximum power they can handle. For example, an engine might say 18A or 200watts. This engine could handle a three cell LiPo (11.1v) @ 18 A = 198watts, but couldn’t handle a 4 cell LiPo (14.8v) @ 18A (266watts). However, if you restricted the throttle so that the current never got above 13.5 A you could use a 14.8-volt battery with the motor (provided the motor can handle 4 cell LiPos).
How Much Current Does An Engine Draw
The current an engine draws depends on the propellor it spins and gearing. Generally, if you buy a new engine information on propellor combinations, and how much current they draw will be included.
If it isn’t, and you can’t find it on the Internet, or you want to experiment with a different propeller then you really need a way to measure the current flow to make sure the engine is not drawing too much current for either the battery, the speed controller, or the motor.
If you want to measure your current draw you will probably find that most cheap multimeters will only do 2 or 3 amps. I use a clamp meter, where the clamp is placed around the positive lead from the battery, and the current is measured through magnetic inductance. This has the big plus of being a lot less hassle (because you don’t have to connect the meter in series) and a lot safer (as you aren’t messing around with bare wires). Can strongly recommend a clamp meter if you are into this stuff.
Propellors with a larger diameter will draw more amps because they are moving more air. Propellors with a more aggressive pitch will draw more amps to a point, although the best pitch for a propellor is normally determined by how fast the engine spins (the kV rating for brushless engines – 1000 of rpm per volt).
There are two ways to reduce the amps a system draws – reduce the prop size, or limit the throttle throw if you have a computer radio.
A note on props
Props have two ratings, and by now you have no doubt figured out the first number is the diameter in inches. The second number is the pitch. What this number actually represents is the number of inches that the propeller would advance through the air in one rotation assuming no slippage.
Choice of propellors can significantly change the way an aircraft behaves. For example. A big propellor will give your aircraft a lot of thrust, and allow it to reach top speed very quickly, but top speed will be quite limited. A smaller prop will take longer to accelerate but will have a higher top speed. Which prop you need depends on the application. For a 3D model typically you are after thrust and quick acceleration. If you are building a warbird, you will probably favor higher speed at the cost of acceleration.
A few more thoughts on batteries
Flight times and capacity
If you know how many amps your model draws whilst “cruising” it is pretty easy to estimate an approximate flight time. For example, if you have an 800mAh, which draws 8A while cruising you will have an approximate flight time of 6 minutes (800/8000(8A)=0.1 of an hour, or 6 minutes).
Our model of imagining a battery as a reservoir of water holds pretty well for a lot of examples, but not under all circumstances. For example, given two batteries – a 2 cell 1200mAh LiPo, or a 3 cell 800mAh LiPo, which would provide the longest flight time.
The answer is perhaps not as simple as you might think. Because the 3 cell has higher voltage you do not need to draw as much current to achieve the same power.
Let’s say you need 30watts to cruise your light parkflyer:
- For the 3 cell: power = volts x current therefore 30 = 11.1 x A, A =30/11.1, A=2.7
- For the 2 cell: power = volts x current therefore 30=7.4 x A, A=30/7.4, A=4.1
So, flight durations are as follows:
- 3s 800mAh: (800/2700=0.3 of an hour, or about 18 minutes)
- 2s 1200mAh: (1200/4100=0.3 of an hour, or about 18 minutes)
So, even though the 2 cell has higher capacity, because the current draw is so much higher to provide the same power, it ends up both these batteries have about the same flight time.
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