ABSOLUTE BEGINNERS GUIDE TO ELECTRIC FLIGHT
by Dave Brown
So you thought you would like to try electric flight. May-be you have a tempting suitable field nearby. Perhaps you fancy the convenience of that electric assisted glider but you are a bit baffled when it comes to electrics. Well not to worry it doesn’t have to get complicated in the early stages unless you want it to.
I have tried to put together a very basic guide to electric flight with some hints and tips based on my own experiences that hopefully you may find useful. I’m no expert but I have had some good results and gained considerable experience along the way.
Firstly you need to decide on the model. There is a huge selection available. This area must now be one of the largest sections of the RC aero market. Once you have selected something suitable, assuming this is your first attempt at electrics its probably best to go with the recommended motor and battery setup. Or check out some of the magazine reviews as they now seem to offer advice on alternative 'hotter' setups with the same or similar models.
Here is a list of the basic items needed you will require with some simplified descriptions.
Batteries
A battery is made from a group of cells which are connected together to form a battery of the required voltage and capacity.
Basically there are three types in common use:-
Nickel-cadmium. (Nicads). 1.2 volts/cell. These generally are able to take a lot of punishment, i.e. able to withstand high rates of charge and discharge and are the cheapest.
Nickel-Metal-Hydride (NiMH). 1.2 volts/cell. Are similar in size and weight to Nicads yet offer a greater capacity, however their disadvantages are a higher price and they require a lower charging rate. Also earlier batteries were not suitable for high discharge; this is no longer the case. (I have found the 1050ma/h ‘Kan’ cells form Overlander to be excellent)
Lithium-polymer (Li-Po) 3.7 volts/cell. These cells have transformed electric flight,
They are much lighter and offer high current drains for a given capacity. They do however have some major draw backs. They require specialised chargers and balancers and must not be discharged below a certain voltage. They have been known to ignite when recharged in cases where they have been ill-treated. They are also much more fragile, and need some form of protection in the event of a prang, and are much more expensive.
Top Tips.
· Always follow the manufactures recommendations when charging any battery - it is usually printed on the cells. Most advise a slow first charge for any new battery.
· Always allow the battery to cool completely after use before recharging.
· Nicads and NiMhs seem to give best performance ‘hot off the charger’.
· Replace the ‘Tamiya’ type connector supplied with some cells as standard with the better quality soldered on ‘Corally’ style.
· Cover the exposed male connector with a short piece of silicon fuel tube to prevent any risk of a short circuit.
Chargers A wide choice is available. If you decide to go with Nicads or NiMH then as a minimum requirement it should have a ‘delta peak’ cut off system. This will automatically detect when the battery is fully charged and then drop the charging rate to a trickle. (The cheap timed cut off chargers are best left alone). If you feel you will only dabble with electrics then one of the less expensive chargers should serve you well enough, but if you are serious then it is probably worth investing in one of the programmable computer type.
I use the Pro-peak mercury, round £60. It charges, discharges and cycles, NiCad’s, NiMH, Li-Po, Li–ions and lead acid batteries at rates of 100mA to 5A, so it can also be used to fast charge and cycle the radio TX & RX and flight box etc.
If you go the Li-Po route then in addition to a suitable charger it is wise to use a cell balancer, this connects between the charger and battery and will automatically balance the individual cells as they are charged, this helps to eliminate the dangers associated with these batteries, will extend their life, and give better performance overall.
Motor / Gearbox
There are many factors involved in the choice of motor/gearbox for any given model, far beyond the scope of this article. So as mentioned previously it’s probably best to use the recommended setup.
Brushed motors are generally cheaper but are less efficient and can tend to generate ‘electrical noise’ due to brush sparking resulting in interference.
A reduction gearbox is used where a larger lower revving propeller would achieve a better flight performance. Generally this would be the case with a larger, slower and lightly loaded model.
The brushless motors commonly used tend to be purpose made and more expensive but are more efficient when used with the correct battery and propeller. They also seem to give fewer problems with interference.
Top Tips
· Always fit suppressors to brushed motors, to help eliminate interference.
· Also it is best to ‘run in’ a brushed motor. This is achieved by running the motor for a minimum of 2-3 hours at low speed with no load to allow brushes to ‘bed in’ to the contour of the commutator. This will prolong the life by reducing sparking when run at full speed and load.
Electronic Speed Controller (ESC)
This replaces the throttle servo and in most cases also supplies the power for the receiver and remaining servos eliminating the need for a separate RX battery. Power to the motor is cut off when the battery approaches the recommended minimum voltage whilst still powering the radio. Closing and re-opening the throttle allows the motor to restart. Careful use of the remaining power available should allow for a safe controlled landing.
For this system to work correctly the ESC must recognise the battery type and voltage. In some cases this is automatic but others need to be programmed. Some are only suitable for a specific battery type.
Top Tip
If using an auto setup ESC always ensure the battery connected is fully charged.
Interference and Safety
Electric models can be prone to self generated interference but in my experience using a better quality receiver and taking the simple precautions noted both above and below will eliminate most problems.
Top Tips
· I now wrap all receivers both electric and I.C. with a layer of tin foil after insulating any exposed connectors with a good layer of insulation tape. This will also ensure nothing comes unplugged. Follow this by another layer of tape to keep everything in place. This not only seems to reduce problems but is also supposed to eliminate troubles caused from micro-wave transmissions.
· Keep all wiring, motor leads, servo extensions, etc, as short as practical.
· Keep the receiver as far away from the ESC as practical.
· The aerial should be routed away from the ESC and any other wiring.
· When Carrying out the range check get a helper to hold the model and run the motor up slowly through the whole speed range while the control surfaces are checked. Interference is sometimes only generated at certain motor speeds.
· When flying with others it would be a good idea to have an ‘informed’ helper retrieve your electric model from the strip. It is bad practice to wander under the flight line of another model with your transmitter switched on.
· Until the battery has been disconnected the model should be considered a potential danger. Unlike a conventional I.C. powered model the motor would restart if the throttle is accidentally opened or would likely be affected similarly if the transmitter were to be switched off.
So that’s probably one of the shortest introductions to electric flight, hopefully not too boring and might be of some benefit to any one considering a new challenge.
Happy Landings!