Christchurch and District Model Flying Club
 

Diary of a flier, episode 16

I have taken the liberty of combining Leonard’s story with Brian Wiseman’s article on “Electrickery”. These appear separately in the paper edition, but here, Brian’s amusing take on a newcomer to quiet flight is given to that doyen of Model Shop proprieters, who in this story remains anonymous - Ed.

The story so far. B Leonard Wise is a learning to fly models, he and his son have started on his first kit. They each did half a wing and when completed discovered that that had both done the left wing.

I went for my flying lesson with Walter on Wednesday morning and told him about the mistake we had made with the wing, which had ruined the plane. He asked if the wing was tapered or the same width all along. The same width I replied. I have them in the Car. He looked at them and said we all do that some time or other. Usually when we are listening to the radio, and building at the same time. It is not a problem and he showed me how to turn them both the same way, take the wing tip off one end and put it on the other. You will also need to strengthen the main spar at the joint.Use a bit of initiative!

Won't that look botched up I asked. Why do you think we cover planes he replied, when it's covered and 50 feet in the air who will see it. (Walter sounds like a man after my own heart – Ed). Just then I noticed a very quiet plane taking off. I remarked that it was a quiet engine and Walter said that it was powered by an Electric motor and this was the future of the hobby. As I am new to the Hobby I thought that I should look into this and asked Walter. He said that he was an I/C man and would not change at his time of life. I should go to see the man in the Hobby shop. So I resolved to go, with some trepidation I might say, because his main joy in life seemed to be to confuse people like me. However there cannot be much room for confusion in a simple electric motor driving a plane, so I entered his shop.

Hello Squire he said as I entered. A form of greeting which I do not like. I am building a plane I told him, and am thinking of putting an electric motor in it. What do you have my good man?

(Health warning: if you know a lot about electric flight, this part of the story is not for you, as it will just make you cross and shake your head. If you don’t know much about electric flight, this is not for you either, as you will know less when you finish than you did before you started.  This article is for people like me who only know a little bit about electric flight. Our position will be unchanged at the end – Ed.)

Well he said, I am reminded of the Japanese Prisoner of War guard who lined up his charges and announced “You think I am stupid you think I know Bugger nothing, I tell you I know Bugger all.”

To make an electric plane go you need a Motor a Speed Controller and a Battery. To take the battery first there seems to be all sorts but the most popular are called Lipos. Lithium Polymer. Don't ask what Lithium Polomer is I don't know. They come in multiples of cells, from one to five. Each cell is 3.7 volts Why 3.7 volts? This is because elictrickery people are the only ones who can multiply and divide by 3.7 If you are good at sum like me you can work out that a 3 cell battery will be 11.1 volts. They also have a measure on them called milliamps, usually from 1100 amps up to 4,700 milliamps. Of course 4,700 milliamps is really 4.7 amps. but that is too simple. The amp or milliamp reading just tells you how much fuel there is in the tank. And so how much flying time you have.

Motors. These come in Brushless or not. Do not bother about the difference. The most fashionable now are brushless, which work on pulses and magnets. Now you and I know that electric motors are a tube with a propellor on the front. You will not believe this but some of them now have the front, or back stationary and the rest of the case spins. Why is this? You will see that the wires come from the stationary part of the motor and often have to pass the spinning case. This means that unless you are particularly clever the wire will chafe on the spinning bit and get cut through. Another bit of electrickery to mess you up.

You need to know how many watts the motor produces. Don't ask why. It is a measure of its power. As a rough guide, weigh the plane and you need a motor which produces 100 watts per pound.

So when you look at motor adverts the description will show the watts? Not on your nelly. They all have code numbers which mean nothing to the layman. I/C engines all have a number which shows the size of the engine and so gives an indication of the power, not so Electric. I notice that some of the adverts do give voltage now and the I/C equivalent. I expect these people will be drummed out of the institute of electrickery.

Each Brushless motor has three wires and you connect them to the Speed controller. There is no indication of which is which. If you get it wrong, (there is a fifty/fifty chance of that) the plane will fly backwards. This can be interesting but you need to be experienced to get it right. You can change the wires around until the prop goes the right way round.

Now Speed controllers. These are a very clever piece of kit. They act as a throttle control, can also act as a brake on the prop (See instructions in the box for how to do it.) Also those marked BEC can monitor the Battery and when it gets low it will switch off the motor and leave you enough power to control a glide. They again have code numbers, but almost always show amps. I said that the amps in the battery shows how much fuel you have in it, you need to know how fast you will be using it.

You take the wattage of the motor, Say 600 watts and divide it by the voltage of the battery 14.8 (say 15 no one can divide by 14.8) you get a figure of 40. This is theoretically the flow of current from the battery to the motor. So you need a Speed controller of 40 amps. Oh no you don't. When you open the throttle you will be drawing more than 40 amps. So you will need up to 60 amps, to be safe.

If you have a 40 amp one it will either catch fire or die. Supposing you have a battery which states it is 4000 milliamps, which is 4 amps really. (Note are they all have much more difficult figures just to confuse us). This means that you can draw 4 amps for an hour before it dies. So it you are drawing 40 amps you will get l/lOth of an hour Because I am good at sums this is 6 minutes. If you glide or use a little amount of throttle you will get more. If you roar about at full throttle you will get less, and end up with a very hot battery.

You will see Kv ratings for motors. This tells you the potential speed of the motor. Apparently you multiply the voltage of the battery by the Kv value of the motor and you get the maximum speed it will go. If you have too large a prop it will slow it down and the motor will get fed up and die., It is important to get the right prop. The wrong prop can mess up the whole system Your supplier will tell you the recommended speed controller and prop for your motor, you hope.

Well then what do you think of all that? I started off like the Jap guard knowing bugger nothing, now I feel that I know bugger all.

I remember coming in here once asking for fuel, I thought, a simple request and all I got was an incomprehensible jumble of figures. He was doing it again I could not make head nor tale of him. But being quick witted I realised that he was making all this up just to take the mickey out of me, so I decide to respond in kind.

Did you know, I said that the square on the hypotenuse of a triangle equals that sum of the squares on the other two sides? Well I had heard that a long time ago he said, but I do sell triangles if that is what you want. It was, I said, discovered by Pythagoras. Can't say I've met him he said, no one of that name had been in here unless he paid cash. I get the names from the Credit cards you see. Doesn't sound English does he?

He was Greek, I replied and lived over two thousand years ago. Didn't know they had model planes that long ago. I wonder if they flew Electric then.

Drawing myself up to my full height, I replied electricity was not discovered until recently There was no such thing is Pythagoras' s time. Pull the other one he said how would they have powered their television sets?

Not wishing to be outdone I said They powered them by special candles made from Bees Wax and Paraffin. So saying I swept out of the shop. I got in my car and drove off feeling pleased that I had won that encounter, but realising that I still knew nothing about electric aeroplanes.

(But a Bee’s Wax and Parrafin-powered model? That sounds suspiciously like infernal combustion to me - Ed)

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