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Author Topic: Dethermalizers and Timers  (Read 5162 times)
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Hepcat
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« on: June 10, 2010, 04:52:52 PM »

Questions on Dethermalizers and timers have been answered many times but are usually spread (lost!) over different build threads. If some of these ideas can be brought together in one place they will form a more useful reference. This post, for example, is in response to reply #321 in the ‘Went Flyin’/General Discussion/Outdoor Free Flight, who would think of looking there for D/T information. In reply #321 a flyer was holding a ‘Gollywock’ with a wound propeller, under her arm, while she adjusted the D/T fuze at the rear of the fuselage.

Early dethermalizers were usually tipping tail and it seemed logical to put the fuze at the tail end to burn through the hold down band. Also in those early days winding a rubber model was usually a two man job. I must have seen hundreds of photographs catching that tense moment where the flyer is holding the fuselage with one hand, the propeller of his fully wound motor in the other, and looking anxiously back at his helper trying to light the fuze. Nowadays almost everyone uses a stooge for winding, helpers are rare, and in any case many flyers prefer to check everything themselves; in these circumstances the rear end of the fuselage is not the most convenient place for the fuse.

Two more, minor, points about tail mounted fuses may as well be mentioned. The weight on the rear end is sometimes mentioned. As fuze only weighs about 0.15 g/inch I don’t think this will normally be a problem but if you want to try for about ten minutes in a fly off and still have a chance of retrieval then it may be worth a thought. Secondly, some models are sensitive to small amounts of rudder tab offset and in these cases several inches of fuze waving in the breeze at an odd angle may be significant.

I think any timing device, viscous, clockwork, fuze..., all need to be mounted where the flyer can easily see them whilst waiting to fly – which usually means somewhere near the CG. I give below a sketch of how I arrange it on one of my lightweight rubber models. The snuffer tube goes diagonally across one of the bottom longerons. Easy to get out old fuze and the end under the fuselage can be pulled to set the length of the burning end precisely at launch. Don’t forget that if you don’t have some fancy electric lighter that the safest and probably quickest way of lighting the fuze on the model is to have a length of fuze already burning and press the two together.

A further point; not exactly on dethermalizers but related because of the original post. Don’t ever try making adjustments to a model yourself whilst also holding a wound motor because sooner or later something is going to get broken. Most competition models already have methods for holding wound motors and we could discuss these in another thread but if you don’t have a propeller lock get expert help or unwind the motor. (Inexpert help will break your longerons or knock your fin off!)

John
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« Reply #1 on: June 10, 2010, 05:16:36 PM »

Good topic idea John

I like the diagonal tube. I always prefer to have the fuse burning well and adjust just before launch. Trying to light and make sure it's going well after you've decided to launch always seems a bit of a faff Tongue

I have got an old sweet tin with a snuffer tube in the lid that holds a length of DT fuse for lighting from, which is convenient and even if you forget about it in the heat of the moment so to speak - it'll soon stop Wink

A couple of other variations.
I got hold of some sheet of litho plate a few years back which I use instead of tin foil (heavy but nice to use). I sometimes run the tube in the fuselage right through from one side to the other so that there is a length of fuse ready to be pulled out as you wait for that elusive thermal (well it always seems elusive when you are holding a fully wound model!)

Cheers
Paul

ps - just bought some DT fuse Shocked... not such a cheap option as it used to be.
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« Reply #2 on: June 10, 2010, 11:39:54 PM »

Hi,

Interesting topic. I have tried most dethermalizers from parachutes to small self made timers. The parachute type give a wild ride down and not for the faint hearted.

Just on the subject of D/T fuses being expensive that Paul mentioned, I have always made my own. I just assumed that everybody new how to do it cos it is cheap and so simple.

All you need to do is buy some Potassium Nitrate. It comes as a white powder and here in Australia easily available from the local Chemist. One small jar has been lasting me for years. Then you go to a camping store or similar and buy a quantity of small diameter cotton lamp-wick. Sold by the length here.
You could even use round cotton shoe laces. Just alter the snuffer tube to suit.

Then you make up a super saturated solution of Potassium Nitrate by dissolving an amount in hot water and keeping it in a glass jar with a lid on. I used an ex jam jar.

To make your super custom made dethermalizer fuse just soak a length of the cotton wick in the solution in the jar. I leave it overnight. Take it out and hang it up to dry. You're set to go. You can keep using your saturated solution over and over again.

Now when you go to fly the latest rubber model you can say "Yep I carved the prop myself, even made the dethermalizer fuse, saved a fortune". Wow!  Exit stage left to thunderous applause.

Hope this helps if you have not heard of it before.

Cheers

Algy
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« Reply #3 on: June 11, 2010, 06:57:46 AM »

A nice thing is that if you feel that you've bought too much potassium nitrate, you can always make up some sugar rockets for rocket assist Grin.
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Hepcat
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« Reply #4 on: June 12, 2010, 12:26:08 PM »

Paul, I had a tobacco tin with a snuffer tube fixed on the side. The spare fuze was coiled up in the tin and was fed out through a hole in the side and then through the snuffer tube. I guess a lot of us used something similar. I wonder where all the tobacco tins came from because nobody I knew smoked a pipe!

Your mention of using litho plate instead of tinfoil to protect the fuselage around the snuffer tube reminds me of a tip that John O’Donnell gave me some years ago. Aluminium kitchen foil can be rolled up to make a snuffer tube, just like making a paper tube except that it is probably better glued with contact adhesive than balsa cement. They are actually easier to make than paper tubes, can be any diameter and very light if not many turns are used.

Yes, Algy, I do remember making fuze back in the 1940s. That was when I first heard the term ‘saturated solution’ which I understood to be where the water held the maximum amount of solids. I guess your ‘super saturated solution’ has the maximum solids that hot water will absorb. The recommendations we got from the magazines of the day was to use butcher’s string. This was quite soft and absorbed the saltpetre quite well but was too thin to make good fuze and when Kelly Lampwick came along, which needed no treatment and had a convenient red, helical stripe for measurement we took the easy course. I admire your dedication to the home brew.

John
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