Electric Flight
- Can We Help Finding Us Starting up Club Rules Constitution Contact us IC Power Aircraft Gliders Electric Aircraft Helicopters
Electric Flying Model Aircraft...... A Short Introduction (By Murray Hackney)
Having started aircraft modelling again after a gap of 55 (!) years, I remembered trying to start a clapped out ED 2.46 racer diesel on a cold day, and decided I wanted no more of that. Electric power was just getting into its stride, and was a bit “iffy”. The rule was “build in lots of lightness“, power was limited and short. My trainer (Foam, to the amusement of all the pundits) started with a geared brushed motor and nicads. It worked, just, but lipo batteries and a brushless motor turned it into a pretty good, tough, waterproof (how do I know?) model.
Since then I have learned a lot - not enough, and all I can do is try to take some of the mystery out of electric flying.
I’m not going to go into the old cost arguments, nor the “wrong” noises. I have to admit I like an engine noise, especially a 4 stroke, but on balance, I have opted for E flight. Either you like it or you don’t, but it’s very noticeable that many of our members now have an electric model or three in their fleet!
The easiest way to get into this is to buy a set-up which has been proved commercially, but if you like to build, then read on……….Short descriptions follow………
MOTORS:
"Brushed” have brushes! They wear out, spark and give some radio interference. Being phased out more and more. Not too powerful without a gearbox, but the Electronic speed controllers (ESC) are cheap.
Brushless motors are 3phase, and really have only one moving part. “Outrunners” are a bit like the WW1 rotaries in that the armature is fixed, and the case revolves around it. They have lots of torque and can turn surprisingly large props. Suppliers have managed to create chaos by using different numbering systems - this makes it difficult to work out an I/C equivalent. Best ask, but Dualsky and some others give direct comparisons for I/C engines
Brushless “in runners” look almost like bog standard brushed - the inside turns this time, and they are mostly used for very high rev. applications, like pylon racers or ducted fans (EDF)
The KVA rating of a motor gives its preferred revs./ volt. EG a 1000 KVA motor on 14.8 volts will try to run at full throttle at 14,800 rpm. “Try” because this is its most efficient speed, and is governed by the no. of turns in the windings on the coils (Don’t worry about this, just accept!) Of course if you use too big a prop, then the motor will just pull more amps (and there are limits to this) to try to get the revs. Too small and it will reach the revs ok, but you will obviously get less power. Prop choice will come later.
ELECTRONIC SPEED CONTROLLER
This has a lot of work to do. Firstly, it has to convert DC from the battery into AC for the 3phase motor. (Thus two wires from the battery, and three to the motor.) It might be worth mentioning at this point that if you ever connect the ESC to the wrong polarity you will instantly fry it - go buy another! So always ensure a failsafe connection system - I use exclusively gold male and female which are impossible to get wrong unless soldered to the wrong wire in the first place!!! Then the ESC has to give a throttle effect basically by switching the power off and on rapidly - more on than off = more speed. They have soft starts, hard starts, freewheel or braked, etc. etc., some can be programmed with your TX, or you can get “programme cards”
Often they also have to tap off a suitable voltage to run the radio and servos (called Battery eliminating circuit (BEC) without the need for a separate flight battery, saving weight. It’s a busy little thing, and expect it to get hot. Too hot and it will give up- no radio, no servos- not good, so ensure plenty cooling air, with an attached heat sink if possible. Also I have a rule of my own- if they say it will take 40 amps and that’s what you need, buy a 60 amp. I always add 50% because I don’t believe suppliers’ claims!
If you see “OPTO” ESC, that means it controls the motor only, and you need a flight battery for radio and servos. This is usually cheaper and eliminates interference. Personally this is the way I go with heavier models - peace of mind……Incidentally, all ESC’s now have a fail safe, cutting all power to the motor on loss of signal.
BATTERIES - getting better and cheaper all the time. Most people know about Nicads and NMHs, so I will just do:- LITHIUM POLYMER First, a few figures:
VOLTS notional 3.7V per cell, chargeable up to 4.2, dischargeable to 3V. (Never let a lipo get below this, or its life will be short, or it might just go dead on you. It happened to me! (There are gismos on the market which will prevent this in flight.) EG. a 4 cell Lipo is 14.8V notional. (called a “4S“- don’t ask how come S stands for cells, maybe it originally came from a different language!) So a 4S2P is 4 cells X 2 packs in parallel.
CAPACITY. Usually given in MA hours, so a 4000 MAH battery should give 4 amps for an hour (they could have called it a 4 AH battery, which is more commonly used) or 8 amps for half an hour, or 40 amps for 1/10 of an hour, I.E. 6 minutes.
DISCHARGE RATE. Another confusion ! Called the “C” rating (“D” rating for discharge would make more sense!) If you see a LIPO. Without a C rating, don’t buy it! A “C” rating of 10 means that a 2 AH LIPO can be run at 20 amps
(2X 10), and a 20 C 4 AH will give 80 amps.
WATTMETER.
Connected in series with your battery, this will give you the amps, watts and voltage while ground running. Useful as you can swap around props to give you the max. amps and watts, (watts =Amps x Voltage) (and a good judge of max. thrust), always bearing in mind you must never exceed the “C” rate as above. Also note that a static run will always pull more current than when flying as the prop “unloads” due to its speed through the air, so that’s another safety margin. (UNLESS you go in for prop hanging, which is pretty much the same as a static test!)
PROPS.
Again, use recommended, or test fly until you get a good match. “Electric only” props are quite cheap, and it’s worth having a few around for testing - do not use them on I.C. engines! Sometimes you have to use an I.C. prop for scale effect, and that’s OK, if a little less efficient. An “out runner” motor will turn a surprisingly larger prop than its I/C. equivalent, but the same general rules apply- more pitch for speed models, less pitch for sport and scale.
CHARGERS.
Yes, you have to charge your batteries! There are many chargers on the market which have facility for Nicads, Nmhs, Lipo's, and the new kid on the block, Li FE or A123 batteries. These are just coming on the market, not much known about them, dearer, so they will reduce in price, and I have no experience with them.
The main thing is, LIPO charger only for LIPO's. The charger starts at a low amperage, and slowly works up to max of one “C”( which means 4 amps for a 4000 MAH battery, 2amps for a 2000 MAH, and so on), and down slowly again at the end. (Lipo's now available which can be charged at up to 5C, which sounds fast, but I personally stick to 2C max, which extends battery life.......anyway a 6400MAH lipo at 5C would need a charger pushing out 32 amps, which is way beyond most!!!) It will not push the voltage of the battery above 4.2V, otherwise it is likely to catch fire or explode. Yes, really! Also, always use a charger with a balancing facility or a linked balancer. This is to prevent one or more cells in the battery getting “out of line” with others. This used to happen, and the result is one cell may be charged above 4.2 v (fire again!) or discharged below 3 v (dead!) Always charge Lipo's in a fire bag or pyrex closed dish, and don’t go off and forget them - sheds, models and cars have been written off!
MATCHING SET UPS
In a nutshell, this is the secret. Prop, ESC, Battery all must be capable of giving the required power. So what is the required power? Here again “those who know” manage to confuse things by using a mix of imperial and metric. They say an approximate guide is : Light wing loading , 50 watts per lb.! You can convert one or other if you like, but I have not found anyone quoting Watts per Kilo or HP per lb!!
100- 150 w/lb for scale and sports models, and anything above that for lively aerobatic flying. Of course this a fairly broad range, and if you can get the equivalent I.C. figures, then that’s as good a guide as any.
As an example, my Airsail Chipmunk weighs 8lbs. with battery and the recommendation was 60 2 stroke or 70 4 stroke. Equivalent Dualsky motor is XM4260CA 4 , The ESC is an opto 100amp, the battery is a 20C 6400 mah. 4S 2P. Anyone who has seen this fly can vouch for the adequate power. It flies for 10 min's safely and I use a dedicated 5 cell radio/ servo battery for the radio and servos as per I.C.
SAFETY
Watch out for glitches - Range test all controls at full throttle properly restrained as usual. Any fluttering of controls might be fixed by keeping the ESC as far from the Rx as possible, and keeping ESC to motor wires as short as poss. 2.4Gh radios have been reported to eliminate glitches, and I certainly have had none since changing.
The propellers on E models can damage you every bit as much as I/C. Never go near a prop with battery attached - it may just start up with a spurious radio signal - I/C engines don’t do that, you can hear whether they are running or not! Also, E props are sharper, and the power is such that the motor won’t stall, it will just keep chewing away at your fingers until you notice…….
Carry the model out to the runway as if you had an I/C engine running. Getting the throttle stick caught in your jacket pocket could give you a fright or worse. Watch out for battery fade - (you ought to have the timer worked out), and don’t believe anyone who says you won’t get dead sticks. Trouble is there is no sudden noise reduction, and it can take a second or two to realise that the model has become a glider.
INDOOR FLYING
This now commonplace, and if indoor grabs you, then almost all the models and electric set ups are available ready to go. So that removes the need to say much about it. Still possible to design your own of course, but you’ll be hard pushed to match the AUW. of the ½ oz. “Vapour” !!
If the weather goes downhill any further, we may all be indoors!