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Member postings for John Bunting

Here is a list of all the postings John Bunting has made in our forums. Click on a thread name to jump to the thread.

Thread: Ohm's Law?
23/09/2008 23:37:00
Well folks, when I started this thread a month ago I had no idea it would lead to so much fascinating diversity. Special thanks to Peter for a brilliantly clear explanation of motor characteristics: sometimes it pays to wander off the point a bit.
Thread: Amps Drawn
20/09/2008 13:43:00

Thanks, Chris.

Luckily I had a decent ammeter for calibration, but many digital multimeters have a current range reading up to 10 amps; maybe not highly accurate, but probably good enough for this purpose.

Here's part of an article I sent to 'Electric Flight UK', the magazine of the British Electric Flight Association (Issue No. 63, Autumn 2000).

 We need to know the tapping point spacing as a function of the wire thickness being used for the shunt. In my scrap box, I found samples of half a dozen different sizes of single-strand copper wire and connected them, one at a time, into a simple circuit which passed a steady current of 3.0 Amps through the sample. The meter leads were fitted with miniature hook clips and these were clipped on to the sample and the spacing adjusted to give a reading of 3.0 millivolts. The distance between the clips was then measured. The thickness of each wire sample was also measured with a micrometer.

For a given resistance, 0.001 Ohms in this case, the length of a wire is proportional to its cross-sectional area which, in turn, is proportional to the square of the thickness. So the tapping point spacing, for any wire thickness, is given by the expression S=kt2 , where S is the spacing, t is the wire thickness and k is a constant which was found by dividing each value of S by the corresponding value of t2 and taking the mean of the results. The value of k obtained is 1117 if you are working in inches, or 440 if in metric. For example, suppose your wire is 0.048 inches thick; squaring this gives 0.0023 and multiplying by 1117 gives you the tapping point spacing, 2.57 inches. For the same wire, but working in metric, the thickness will be 0.1219cm, so t2 = 0.0149 and multiplying by 440 gives a spacing of 6.5cm, the metric equivalent of 2.57 inches.

Cut your shunt wire about an inch longer than the spacing distance, to allow half an inch at each end for the battery and motor connections. Also avoid large blobs of solder when attaching the tapping point wires.

As well as being useful on the test bench, a shunt could also be installed in a model as part of the wiring, making the tapping points accessible on the side of the fuselage. This would make it easy to compare motor currents when trying different propellers or making other changes to the system.

20/09/2008 00:22:00

In-flight power monitoring is great - if you can afford it and be bothered with it! - but a cheap and versatile method of measuring current that I've used is to make up a .001 ohm resistor, fitted in series with your battery, and measure the voltage across it when current is flowing, using a digital multi-meter (£5 or less, from Maplin) on the milli-volts range: thus a current of 10 amps would give a reading of 10 millivolts. 

The resistor is just a short length of thick copper wire, with tapping points of thinner wire soldered on, at the correct distance apart to give .001 ohms between them. I worked this out a few years ago, using several different sizes of wire, in series with a good quality ammeter as a standard, then plotting a graph and deriving an expression relating tapping-point spacing to wire thickness. As the resistance is so low, it could form part of the wiring in a model, with the tapping points accessible from the outside, so you can see the effect of using different props etc. 

I'll dig out the figures and post them here in a day or two, in case anyone is interested.

Thread: Grave lack of lift
19/09/2008 23:12:00

Hi Terence,

The tailplane area of 23 sq ins is just over 10% of the wing, which might be a bit too small; but of course the moment arm, the distance between wing and tail, also affects the stability.  As Simon and Alistair said, it might need a good heave to get it leaving your hand at flying speed. Do you have access to a good slope-soaring site? If so, with a good breeze up the slope, I don't see how it can fail to fly, even if only for a short hop. Once it's in the air, you should find how controllable it is, and what needs changing, if anything. Keep us informed!

John.

Thread: Inkpen
19/09/2008 01:24:00
I think Inkpen was a well-known site for full-size gliders back in the thirties. Never flown there myself, but used to fly a glider - free-flight, before the days of radio - many years ago at Cherhill, between Marlborough and Calne.  Anyone else flown there?
Thread: Grave lack of lift
19/09/2008 00:43:00

Hi Terence,

With that span and loading, I guess your model weighs a bit under a pound and has a wing area of about two square feet, and should have a fairly moderate flying speed, not difficult to hand-launch, even in calm conditions. The C of G position sounds about right, so if it persistently heads for the ground , with no tendency to stall, maybe it needs a bit more up elevator. I assume it's just two-channel, rudder and elevator, with no ailerons, and you say the balance is OK in both axes, so if it always drops the left wing, could there be a wing warp somewhere?

What was the original wingspan and area, before adding the centre-section? The larger wing area means that the tailplane is now a smaller percentage of the wing area, which may affect the longitudinal stability. Can you confirm the weight, and give us the wing and tailplane areas?

John.

Thread: Splitting servo rails............
13/09/2008 00:58:00

Yes, softwoods grow fast, hardwoods grow slow, but balsa, which is technically a hardwood, grows damn fast!

The hanging upside down thing reminds me of a gliding song from my days at Lasham:

"Guide me O thou great instructor

Pundit in this gliding game

We are flying upside down now

Does the stick still work the same?

Now the speed has dropped to zero

Everything is deathly still".

"Never mind boy, I have got her,

Upside down we'll soar the hill"

Thread: October issue feedback...
26/08/2008 23:19:00
Well, Kevin says, "In a DC circuit with the same resistance if the voltage (battery) source increases the current used will actually drop". That sounds rather like an error to me.
Thread: Ohm's Law?
26/08/2008 19:17:00
Who's seen the letter from Kevin Annells on page 53 of the October RCM&E? If you haven't, please have a look. Is it as wrong as It looks to me, or am I misunderstanding something?
Thread: Aviation Funnies
07/08/2008 00:43:00

A faintly amusing true story from my early working life in the Met. Office.

The time: 1949, plus or minus a year or so.

The place: the Met. Office at RAF Lyneham.

Enter the station CO, to speak to the senior Met Officer about the forecast for an impending York flight to Gibraltar. In the course of conversation, the CO asks, "What time does it get dark in Gibraltar?  "Hm, not sure, sir" says the Met officer. "Just a moment, I'll ask Maurice". Presses switch on Tannoy 'squawk box', communicating with Air Traffic Control on the floor above. "Maurice, what time does it get dark in Gibraltar?"  Maurice, a somewhat irreverent F/Lt., replies, "Haw, dunno; when the bloody sun goes down, I suppose". 

Thread: Free Flight Models
30/07/2008 01:11:00

John, I think the problem with gummed-up small diesels is that the piston-to-cylinder fit is so close that it is almost impossible for any thin fluid such as WD-40 or petrol to get between the two and soften the film of thickened old oil that is holding them together. I'm assuming that no corrosion has set in. My approach, for what it's worth, would be to apply some heat, for example by supporting the engine over a switched-on soldering iron or a light bulb, say  60 to 100 watts, taking care not to let the engine get much hotter than it would get during normal running.  Fit a prop, so that you have something to turn the crankshaft with, and use a cloth or glove to hold the engine itself. Squirt in a bit of WD-40, then try to turn the shaft, first one way then the other, but don't force it! You just have to use your judgement about how much torque you can apply without doing any damage. Once you notice even the slightest movement, you've won. Keep the engine hot and just keep rocking the shaft, both ways, gradually increasing the movement. This has worked for me with a couple of seized-up engines, but maybe someone else will come up with other ideas. Good luck!

As another point of interest to anyone who has a Mills .75 or other small diesel, Alex Whittaker mentions, in his article in RCM&E, that their plastic tanks were prone to getting lost or damaged. I had this problem some years ago, and finally found that some of the hollow mushroom-shaped polythene 'corks' that are used in sparkling wine bottles can be adapted as a tank. Of course it has to be the type that is closed at the bottom, and if you then cut off the larger top part, what is left is almost exactly the right diameter to fit the aluminium top plate from the original Mills tank. I've used the same method to replace a Mills 1.3 tank, but in that case using the cut-off bottom of a small plastic bottle that just happened to be the right size.

Thread: Cover Art
29/07/2008 19:03:00
Been there, done that, Timbo. I emailed the Editor about two months ago, saying pretty much what I put on here.  No response so far!
29/07/2008 01:19:00
Much as I look forward to the arrival of RCM&E each month, I get a feeling of, "Oh no, not again!" when the cover photo is revealed. Every month, for ages, we've had an oblique, foreshortened close up nose shot, with a few inches of wing showing each side, maybe a bit of fin and tailplane away in the distance, and a blank monochrome background. For example, the Sea Fury on the latest issue:  the whole thing looks posed and lifeless, and about as interesting as the bottom end of a milk bottle. Just occasionally, it would be good to see a picture of a whole aeroplane - or even half an aeroplane. Meanwhile, tucked away at the back of the magazine, there are the 'Parting Shot' photographs, which are of consistent high quality and full of interest. Why not put something that good on the cover?
Thread: WW1 biplanes
21/05/2008 22:17:00

There's a good article on Biggles in the Review section of last Saturday's Daily Telegraph (17 May). In the nineteen-sixties, after a Unesco survey (!) found that he was the world's most popular schoolboy hero, Biggles ran foul of a few politically-correct goons in the race relations industry, one of whom called for public libraries to destroy their copies of Biggles books. In response, Neil Clark, the writer of the article, quotes this passage: '"While men are decent to me I try to be decent to them, regardless of race, colour, politics, creed or anything else". Hardly the credo of an Alf Garnett in flying goggles'.              

20/05/2008 19:07:00
I still have an APS 1/8 scale Sopwith Pup, originally a free-flight design, which I built in 1968 and flew with single-channel radio, escapements, and a Mk 1 Frog 100 engine turning a home-made scale-size prop, !2 x 4, at about 4000 rpm., giving just enough thrust to fly. I am just putting in relatively new radio, servos, and an AXI motor.
Thread: Subscription scam?
20/05/2008 00:17:00

1. May I suggest that it would make sorting and tracking down back issues of RCM&E easier if the year and month, and perhaps the volume and issue numbers, were always printed in the same place on the cover, preferably in the top left-hand corner. You could then find and see them easily, whether you keep your copies in a flat heap or standing up on a bookshelf. 

 2. Nearly all the cover photos look the same, for the past year or more: an oblique close-up nose shot with a bit of wing showing on each side, and a totally blank background. The 'Parting Shot' photos on the last page of the mag are far better. Why not use them on the cover?

Thread: WW1 biplanes
19/05/2008 23:08:00
Cecil Lewis, in his book "Saggitarius Rising", said that in his opinion the Sopwith Triplane was the most pleasant WW1 machine to fly, and that with suitable settings of the throttle and tailplane trim, it would go on looping indefinitely, hands off.
Thread: The February Grand Prize Draw
26/02/2008 19:05:00
Yes, I'd love a Tigger, one of the light aircraft I've had the pleasure of flying in, the others being an Aeronca C3, an Auster, and a B A Swallow.
Thread: What cars type and model do the aged flyers use to transport their models
25/02/2008 00:18:00
Mine's a '79 Mini Clubman Estate.  At my age (don't ask!), and flying small to medium-size models, usually not too far from home, it's fine. Plenty of room, with the back seats folded down. If necessary I can take out the front passenger seat as well in a couple of minutes, and make room for a wing or fuselage up to 2 metres long.  
Thread: Aviation Funnies
24/02/2008 23:00:00
Australian bush pilot to nervous passengers, flying in an elderly aircraft in moderate turbulence: "Dangerous? Nah, be OK as long as the termites are all holdin' hands".
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