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Author Topic: electromagnets  (Read 2008 times)

tonyH

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electromagnets
« on: March 08, 2009, 10:01:48 PM »

Evenin' All,

I have a cunning plan that requires recovering something with a weight of about 7 oz from the water. I'm thinking about using an electromagnet but it's donkeys years since I used any physics brain power and I've forgtten all the Flemings, Maxwells and anything else to do with EMF. Can anyone help with a simple idea for how many turns of wire and what current may be called for at, say, 12 volts.

Thanks

Tony
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maninthestreet

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Re: electromagnets
« Reply #1 on: March 08, 2009, 11:25:02 PM »

why not just use an ordinary magnet??
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RickF

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Re: electromagnets
« Reply #2 on: March 08, 2009, 11:38:36 PM »

Yes, I have a set/kit consisting of a couple of mirrors, a telescopic handle and an extremely powerful magnet that will lift a couple of pounds, bought from our local cheapo shop for 1. If I tied the magnet on to a string it would do the job a treat - with no need for a battery!

Rick
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malcolmfrary

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Re: electromagnets
« Reply #3 on: March 09, 2009, 11:02:40 AM »

Silly question, but ......... Is the item steel? Or does it have steel in it?
A magnet from a dead speaker on a line should do a good job of picking up, putting down is another story.  The usual rule is figure out how much space is available, then see how many turns can be fitted in that space.  The magnetic force is a function of the number of turns times the current, the current is dependent on the resistance of the coil and the voltage applied.
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tonyH

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Re: electromagnets
« Reply #4 on: March 09, 2009, 02:28:04 PM »

Right,

I've been contemplating trying to catapult an electric powered Walrus off a 1:36 HMS Penelope but I was in the National Ralway Museum in York the other day and there is a model of the seaplane carrier HMS Eglantine which set me thinking on an easier project for combining ship and plane. The type of seaplane used, the Short 184, has a substantially bigger wing area and is less limited as far as the prop is concerned by being a 'puller' rather than a 'pusher' - more power - more lift.

I would need to be able to lift the plane off the deck, onto the water and release it and then lift off the water and re-stow. The 7oz is based on my Mk1 Walrus, plus a bit for the suction effect of the water when lifted but a bit more would be useful in helping the 'hook' to centre on the metal insert lifting point on the plane.

I reckon I could get away with a cylindrical magnet about 1/2" diameter by the same in length, or possibly slightly bigger, without it looking out of place BUT have no idea whether it is feasible or not in principle.

Thanks

Tony

As an aside, I've also been looking at docking a working 'blimp' on the same type of vessel and that would also use a similar setup.
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dreadnought72

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Re: electromagnets
« Reply #5 on: March 09, 2009, 04:22:14 PM »

Oooh - the 1911 Seaplane carrier HMS Engadine? Saw action at Jutland?

Do you have any photos of the model, please?

Andy
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tonyH

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Re: electromagnets
« Reply #6 on: March 09, 2009, 09:58:34 PM »

Thanks Andy for spotting the deliberate error. Serves me right for being bored at the office and not thinking straight.

Yes, HMS Engadine, as at the front at Jutland. No, no photos of the model. It's not the builders model and it's hard to tell how accurate it is.

Tony
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dreadnought72

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Re: electromagnets
« Reply #7 on: March 10, 2009, 09:18:25 AM »

She's a lovely vessel before the big shed was nailed on. I see Glasgow University Archives have a wealth of stuff on Denny, the makers, but I can't find an obvious plan there...maybe the RN took 'em when the ship was converted to a seaplane carrier?

Andy
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RickF

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Re: electromagnets
« Reply #8 on: March 10, 2009, 09:43:08 AM »

Don't know a great deal about Engadine - although somewhere in the pile of old model magazines I grandly refer to as an archive i think I remember a Model Boat article - but I believe she launched/recovered her aircraft with a crane near the stern, rather than with a boom, as shown below. However, the sling will be similar.

If you were to go down the electromagnetic road, you are probably going to be modelling something as big and ugly as a car-breaker's magnetic crane, if only to accommodate the iron core and windings. To look anything like a scale hoist, I think it will have to be a mechanical "hook and eye" arrangement - but how to make it work on the water will be a challenge. Good luck!

Rick
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malcolmfrary

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Re: electromagnets
« Reply #9 on: March 10, 2009, 11:05:50 AM »

To keep the mass of the steel on the plane down, it might be better to go the hook and catch way - possibly with a solenoid operated latch to ensure coupling and uncoupling when wanted.
Ghost did something similar for launching and retrieving a mini sub over the stern of his Nord Icelandia.
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RickF

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Re: electromagnets
« Reply #10 on: March 10, 2009, 11:55:49 AM »

Here's a slightly more detailed picture. HMS London hoisting aboard what I think is a Shorts seaplane during the 1912 trials

Rick
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tonyH

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Re: electromagnets
« Reply #11 on: March 10, 2009, 08:48:06 PM »

Evenin' again,

I did think of a hook release using a third 'string' but this creates a problem with recovery. The hook would have to be built quite heavy as well or the cable would not hang right and the release would not have a resistance to work against.

As far as electromagnets are concerned, I've found an 0.5" diameter and 1.5" length with a 4lb hold on 12v on one site or 17mm diam and 21mm length with a 7lb hold on another. Both take under 2 watts, so power is not an issue. I'll keep surfing to see if there's a readymade answer but, from the above, the lift/current relationship seems fine. I't may still be a case of a rusty nail and some wire!

Andy, you're right that she wasn't improved by the warehouse but, even if the York model has a bit of artistic licence, she doesn't look wrong in the same way that neither Nelson or Rodney look wrong.

Tony
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