|Electric Flying - Basic Concepts - More Advice|
Q. So if I was to go electric what bits do I require to get the most flight time as well as being able to add undercarriage?
A. Okay, first things first. What length of flight time are you looking for? Bear in mind that, unless you time your flights, you almost certainly grossly over estimate the time you actually spend in the air.
We sampled over 130 flights at our club field, during an event at which people were trying for extended flight times and the mean flight time was calculated as almost exactly 10 minutes. Most flights at the field tend to be around the 8-minute mark, unless people are practicing a particular schedule.
If you are looking for a 10-minute flight then, with the right motors and props, an eight cell 3700NiMh sub-c pack will give you eight to ten minutes easily. If you really do want longer flights of say 15 minutes then you will need to look at LiPo technology.
Q. Can someone explain the difference in the types of leccy motors? It's all jibberish to me.
A. I'll just give a brief, simple explanation of the main differences.
There's obviously a lot of differences between individual motors, designed for different applications but this should at least get you started.
There are four main different types of electric motors used in electric flight. These are
Brushed ferrite motors
The simple brushed ferrite motors are the cheapest available. The electricity is supplied to the motor armature via carbon brushes and a commutator. That means that all of the "intelligence" required to run the motor is self-contained within the motor itself. It is self-timed. If you apply a direct current at the appropriate voltage to the motor then it will run. Efficiency of these is about 55% -ish - so for every 10 watts you put in, you get about 5.5 watts of useful work out of them. The rest is lost in heat.
That's the type of motor that the Flair Beaufighter was designed to use - a pair of Speed 600 motors, costing less than a tenner each. Some are a bit fancier, with ball raced shafts but all in all relatively cheap and cheerful.
For a higher cost the brushed cobalt motor tends to have higher quality construction, with powerful rare earth magnets, adjustable timing, higher quality ball raced drive shafts and carbon or occasionally a silver alloy brush. These work the same as the cheap can motors, but are adjustable and because of the better magnets and higher quality components their efficiency is generally better, typically something like 70% and some exotic brushed cobalt motors are up to 80% efficient. The upshot is that for the same wattage into the motor you get more useable power out.
Both sets of brushed motors contain all the brains needed to run the motor built into the motor itself, i the form of the brush gear and commutator. That means the controller only needs to be a relatively simple piece of kit, capable of turning down the current supplied to the motor to act as a throttle. Though in the simplest form this can be achieved with a wiper arm resistor driven by a servo the more usual method is using an electronic speed controller. That works by chopping up the current to the motor and essentially letting the motor be on for a very short time, then switching it off, then back on. It does this many times a second. Excess current is lost via the heat sink on the controller. it's a simple system.
Brushless motors have no brushes and no commutators. The motor itself is very simple, rally consisting of little more than the rotor, with the motor winding wire wound around the armature, the magnets, the shaft, bearings and case. The motor is dumb and if you just fed it with a direct current it wouldn't do anything in the way of rotating. Where as the brushed motor just has a positive and a negative lead, the brushless motor has three leads and it works by the controller applying current to those leads in turn, driving the motor round.
So all the brains in a brushless system are in the controller. It has to be smart. The controller feeds a little burst of current to two of the wires and determines, which way the motor wants to turn from that. Once the controller knows where the rotor is, in relation to the can, it then switches between pairs of wires, very quickly, dragging (or pushing) the rotor round.
The brushless motors are three phase motors and the current supplied to them is not the simple DC current that the brushed motor feeds on, it's a form of alternating current (or more accurately chopped DC).
There are two types of brushless motors-
In-runners have the shaft attached to the rotor and the magnets in the outside can of the motor does not move, it is fixed to the model in the same way as the brushed motors. The rotor and shaft rotates within the motor can.
Outrunners have the rotor fixed and the shaft attached to the can, such that the outside of the can, with the magnets, rotates with the shaft attached. The result is a greater mass is in motion than with an Inrunner and that has benefits in the higher torque available from an Outrunner motor.
Brushless motors can be as efficient as 85%-90% and thus represent a big increase in the available power out, for the same power IN. The extra torque from the Outrunner also means that it can turn a bigger prop than a similarly sized Inrunner and therefore outrunners are typically used without gearboxes, whilst Inrunners may be used direct drive for small props or via a gearbox for larger props.
A simple setup for the Flair Beaufighter would be a pair of Sp600 7.2v motors in parallel, at least a 60 amp brushed speed controller and 8x4 props on 8x3700NiMh batteries. You ought to be able to get about eight minutes from such a setup. You could alternatively use 14 cells and a series wiring setup for the motors. That would be a heavier setup, but with potentially longer flight times. You would want to keep the build weight down as much as possible with such a set-up.
For the Flair Beaufighter a very powerful system would be two medium sized Outrunners (something like a pair of AXI 2820/10s), which would be able to drive a pair of scale-profile 3 blade varioprops of about 10 inch diameter on 2x8 cells packs. That would give you bags of power, but you are getting a bit heavier- and also more expensive, since you would need to buy two 40 amp brushless controllers. The model has enough wing area to cope with that increased weight though.
If you want to fit a retractable undercarriage then you may need to bite the bullet and look at providing power from a LiPo pack, rather than NiMhs, simply to save the weight. You would need to carefully match the LiPo pack to the motors that were selected, in order to keep the current draw within limits. That needs a few calculations to be made, but a pair of something like 3s2p 4000mah Lipos capable of 10C ought to do the trick. The AXI outrunners with 9 or 10 inch, three blade varioprops would be drawing something in the region of 35 amps with suitable cells, depending on prop pitch.
Q. Also, do you have a flight battery to power the motors and a separate battery for servo's etc?
A. You have the choice. You could use the flight battery, in an 8-cell setup, with a controller fitted with an internal battery elimination circuit to power the radio. If you have a setup using more than 10 cells then you would want to either use a separate receiver battery, or a separate BEC unit, like an S-BEC or U-BEC. Personally I like to use a separate receiver battery in models with more than 10 cells, for my own peace of mind.
I've got both these kits, un-started in the cupboard, and when I get the time I'll run some numbers through Motocalc to look for suitable and effective power trains for them.
Who knows I might even start one of them to keep you company.
Hope that helps