This is a page of general notes. The plan is not to go into too much detail here. Some topics are covered on other pages.
The goal for RC should be best possible reliability. This comes from choice of parts and installation.
Lithium Polymer. 2-cell, 3-cell, 4-cell, 6-cell etc. More cells = more volts which allows more efficient higher power delivery. ESC, motor and BEC must be rated to suit the cell count.
C Rating indicates the maximum recommended discharge rate – how much current can be drawn without excessively damaging the battery. Batteries deliver less of their rated capacity as the current draw increases.
Higher discharge rates (more current draw) and faster charging reduces the life of the battery. Internal degradation reduces the batteries ability to maintain the voltage while delivering current. The useful number of charge-discharge cycles is reduced.
Over-voltage and under-voltage are very bad for LiPo batteries. 3V minimum and 4.2V maximum per cell.
LiPo batteries age from the date of manufacture. Store in a cool place at a storage charge of about 40%.
Always with supervision. Half to 1C seems to be reliable and not too stressful on the battery. Over-voltage can cause failure with heat, smoke and flame. Lipo chargers use a smart CC-CV charge scheme.
Current rating: size to suit motor and prop. with a bit of safety margin.
Heat and cooling: transistor losses waste power and make the ESC hot. Too much heat can cause failure. Air-flow ventilation is usually required and always a good idea. See Watts below.
Frequency: changes the rate at which the ESC pulses the motor coils. Faster usually means less efficient ESC operation (more heat).
Timing adjustment: allows tuning for maximum power or maximum efficiency. Effectively adjusts the KV rating slightly. Moves the coil pulse in relation to the magnet position.
Most RC aircraft motors are rated by size and KV. Size is an indication of power, but bigger is not always more powerful. KV indicates no-load speed in revolutions per volt applied. The other important rating is power; how many watts can the motor handle.
Overloading a motor will make it draw more current, consume more power and get hotter, leading to failure.
Outside of motor stays still and inside rotates. Can be lighter and spin faster. prone to overheating.
Outside of motor rotates and inside stays still. Can produce more torque. Better cooling.
Failure and Testing
Other than mechanical failure, most motor failures usually mean overheated windings. This often causes shored turns, which can result in complete failure or unreliable operation, stuttering etc.
Measuring the winding resistance is not practical with a standard ohm-meter; the resistances are too low. The best test is to measure the inductance; special inductance meter.
Re-wind or replace.
Diameter and pitch apply load to the motor. More load means the motor must produce more torque, which draws more current and therefore more watts.
Matching prop. to motor to application is complex or a lot of trial-and-error. Generally, for higher air speed, use higher pitch.
BEC or UBEC
BEC = Battery Eliminator Circuit
These are voltage regulators that drop the main battery voltage down for the RC and other sensitive parts. There are two types, Linear and switching.
Linear operate like a variable resistor, produce less interference, but are much less efficient, meaning they get hot and waste power. These are still found in most smaller ESCs. Linear regulator chips will self protect by shutting down for a while when they get too hot. The problem is that they also turn off the receiver and servos.
Switching regulators shop up the power and transform it to the required voltage. The process produces electrical interference but can be very efficient, meaning much less heat to get rid of and therefore usually better reliability.
The risk with these is that the ESC gets hot and the linear BEC shuts down. This causes loss of RC control. Ensure there is enough cooling and that the peak servo load does not exceed the BEC rating. Heating is usually the limiting factor. Most linear BECs rated over 1A actually consist of 2 or more smaller regulators in parallel.
Yes, you can parallel linear ESC BECs. As if connecting 4 ESCs on a quad together. The total load is much less than the rating of one of the BECs. The higher voltage will supply most of the load.
But it’s not such a good idea to parallel switching BECs. Depending on the design, they may not play nice together.
RC – radio control
All the low voltage control gear. Usually requires a reasonably good 5V power.
Plug it in and it should work. Vibration isolate mounting. Do not damage or shorten the antennas.
Modern receivers operate internally at 3.3V and have their own internal 3.3V regulator. Therefore most will be happy enough on 4 to 6 Volts. Below 4V you risk the receiver restarting and temporarily loosing connection, above 6V you risk damaging the internal voltage regulator and input capacitor.
Modern receivers typically output servo PPM signals at 3.3V maximum, some top out at 2.5V. Most modern servos and controllers are OK with this but sometimes a signal buffer may be needed.
Older analogue or newer digital. Unless you are after precision, it doesn’t really matter. Digital servos are becoming more common, but can draw very high peak currents which can cause interference and erratic operation. See Filtering below.
Some servos will operate from a 6V or even higher supply voltage.
For a lot of RC flying efficiency is not a requirement or issue. Just going really fast is fun, but doesn’t last long. Ducted fans are not very efficient; high speed, low torque, high power draw.
For multi-rotors carrying cameras and duration fixed-wing flying, higher efficiency is an advantage in more flight time or greater safety margin.
- lower current = lower losses
- more cells = higher voltage and lower current
- larger props are more efficient
- match prop to motor at best rpm
- minimise weight
Electricity – Volts – Amps – Watts – Heat
Volts: like pressure in a pipe.
Amps: like diameter of a pipe.
Watts: how much work can be done. More watts relates to more pulling power or more speed.
Heat: watts = heat, which kills electronics
Watts is the key to performance and reliability. More watts means more performance. But more watts usually means more losses, more waste heat where you don’t want it and worse reliability.
Heat has to go somewhere. 1 watt of applied heat will melt solder on a circuit board if it can’t be dissipated. Heat can be radiated or conducted away. Heat-sinks conduct heat away from the transistors and then to passing air. Wrapping it in heat-shrink reduces its ability to transfer heat out to the air. The mass of the heat-sink then determines how long it can cope before overheating.
A motor drawing 20A from a 3-cell LiPo is drawing about 200 watts from the battery. The ESC may be passing 90% of that to the motor (if you are lucky), leaving the ESC to dissipate 20 watts. Add to that the BEC delivering 5V at 1A to the RC with servos moving. 11V – 5V = 6V * 1A = 6 watts. Therefore the ESC has to dissipate 26 watts.
The same system operating from a 6-cell LiPo would be drawing 10A from the battery. The lower current would means lower losses in the ESC and less heat. You would have to use a Switching-BEC to cope with 22V reduction to 5V. Overall losses in the ESC are likely to be under 10 watts and the overall system reliability improved and flight time greatly improved.
Current passing through a wire produces a volt-drop. How much is determined by the current and size of the wire. Wiring can be heavy and difficult to hide, so you can remove what is not needed.
Because of the short wire lengths in most models wiring doesn’t have to be as heavy as most think. 0.5mm to 1mm wiring will do for most smaller models. Batteries like the typical 2200 3-cell are supplied with wires that are far too large.
Connectors have resistance and therefore volt-drop and power loss. Just use connectors that are suitably rated and keep them in good condition. repeated use will increase the connection resistance.
You can measure low resistances by passing a known current through the connection and measuring the voltage across it. The resistance in ohms is then V/I (I is amps).
Noise Filters – Filtering
Electrical noise is generated by ESCs, UBECs, power supplies servos and more recently, cameras and video transmitters.
For most park and club flying this is not a problem. But for FPV and expensive or the more dangerous models, it should be considered. Every potential reliability reduction should be addressed.
Ferrite cores on leads and capacitors across 5V power supplies can help greatly.
This is a more complex topic.
LEDs are current driven, not voltage driven. This means you control the current through the LED, not the voltage applied. As voltage applied increases, at some point current will quickly increase leading to failure. A resistor is always connected in series to limit the current.
When a current passes through a LED it emits coloured light, typically infra-red, red, yellow, green, blue or UV, but not white. White LEDs consist of a blue LED exciting phosphors that make the LED look yellow when off. The phosphors determine warm, daylight, cool, etc.
The voltage that you measure across the LED is a side effect of the colour, not an indication of the voltage that should be applied. Current driven, remember.
Bright LEDs get hot. A 1 watt LED dissipates about 1 watt of heat. This requires a good heat-sink or good air-flow.