RDT QUESTION

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GarydNB
Posts: 282
Joined: 15 Feb 2018, 23:12

RDT QUESTION

Post by GarydNB »

Hi experts,
There are several Radio Dethermaliser units around and one or two electronic timers called 'band burners' where after a pre-set time a small electric charge briefly heats a nichrome wire which ,in turn, burns through a small rubber band thereby releasing the tailplane/wing what have you.
I'm wondering if one of you clever chaps (not me, I'm an idiot) could devise a system whereby a radio command would do the same job as the pre-set timer and send a signal to briefly ignite a short length of wire?
It would have to be a one shot only because you wouldn't want to flatten the tiny (maybe 50mAh) lipo by inadvertently leaving your thumb on the button.
Just thought I'd ask the question..
Gary
jackdaw
Posts: 165
Joined: 16 Feb 2018, 20:30
Location: Wet and Windy North Wales

Re: RDT QUESTION

Post by jackdaw »

I believe PhilG has a system that he could advise you on. He'll no doubt see this post and give details.
Stew
Posts: 495
Joined: 02 Mar 2018, 10:21
Location: Staindrop, Darlington.

Re: RDT QUESTION

Post by Stew »

I'll vouch for the efficacy of Phil's RDT trip. Simple, small, and easy. Works beautifully, and avoids any rubber band burning.
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Phil_G
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Re: RDT QUESTION

Post by Phil_G »

The RDT's I've been doing for a while now use a conventional servo Gary, small as poss, say 1.5gm
Lemon receivers seem most popular and a small single lipo cell.
The device itself is embarrassingly simple but thats what FF'ers wanted, I offered timers and stuff but they particularly wanted the simplest most basic trigger mech so it could be ratified as acceptable in competition. The FAI rules I believe permit only a 'single, one-shot transmission to irreversibly trigger a dethermaliser' which is exactly what it does and no more :D
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GarydNB
Posts: 282
Joined: 15 Feb 2018, 23:12

Re: RDT QUESTION

Post by GarydNB »

Thanks guys,
I'm aware of the RDT gadget you make Phil, in fact you generously gave me one of them when I was last in the U.K. What I was thinking though, was something like shorting out a servo plug with a short length of nichrome wire to burn through a dental band. Reason being, no tiny servo to overload and no mousetrap to release the dt line and if they don't exist they can't weigh anything. I was mainly wondering if a receiver can do something like that without bursting into flames and the signal would have to be momentary and one shot for the very rasons you mention.
Gary
GarydNB
Posts: 282
Joined: 15 Feb 2018, 23:12

Re: RDT QUESTION

Post by GarydNB »

Thanks Phil,
This is what got me thinking....albeit his timer is set with little switches whereas I was asking if it could be activated by a radio signal. I hope it makes sense...





Gary
Martin
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Joined: 16 Feb 2018, 14:11
Location: Warwickshire

Re: RDT QUESTION

Post by Martin »

I wasn't aware of that system, but from the video it looks to work very well. At first, I thought the actual wire would have to melt - like a fuse - but now I see that the wire just melts through a rubber band, so the only thing you need for each flight is fresh rubber band and the 'fuse' wire itself should last indefinitely.

Judging from the size of the tiny LiPo and the thinness of the wire, I guess it's just a very brief burst of about one amp to melt the bands. The controller chip could use PWM to drive the power transistor if a 100% 'on' signal would make the wire get too hot and melt.

A radio control method has the disadvantage of the extra weight of a receiver, but the advantage of not needing to set the timer - and not 'wasting' a rubber band if the plane lands without the need of dethermalizing.
Stew
Posts: 495
Joined: 02 Mar 2018, 10:21
Location: Staindrop, Darlington.

Re: RDT QUESTION

Post by Stew »

I read somewhere about rubber DT bands being bad for wildlife, birds in particular, so I stopped using them for that reason. It may be just a rare chance of an animal being hurt by such a thing but anything for good karma.
I honestly believe the weight matters less than good air picking. I've had DT servos float off the bench... ;)
Twitcher
Posts: 53
Joined: 16 Feb 2018, 12:17

Re: RDT QUESTION

Post by Twitcher »

Just thought I'd mention that the DT method I use is extremely simple and although it's in a freeflight model and actuated by an electronic timer, it could just as easily be driven by a lightweight receiver.

There are lots of electronic timers available for freeflight, and many (all?) of them have at least dual outputs to allow control of a) an electric flight motor and b) a DT function, driving a tiny servo. The motor control allows pre-set speed and run duration to be controlled, whilst the DT servo is triggered after a suitable delay. I use a simple, very light, carbon pushrod 'twixt servo arm and the pop-up tailplane. It terminates in a short length of piano wire that at the tail end, holds down a small piano wire hoop glued into the tailplane trailing edge. When the DT servo is actuated, it drives to one extreme position, withdrawing the wire from the hoop and releasing the tailplane to its DT position. The tail tip-up is powered by elastic bands in the usual way.

Note that the pushrod is only ever under tension, so rigidity isn't required and it can therefore be very light. At the tail end of the fus, the piano wire runs in a bearing comprised of a short length of brass tube.

There are no sacrificial elastic bands to worry about. Re-setting is a simple matter of pulling the t/p back down to its flight position and re-setting the timer so that the servo returns to its "hold" position. That drives the pushrod back through the hold-down loop, and off you go again. I'd suggest using a 3-gram servo: you just need enough movement on the servo arm to ensure positive hold-down.

That's made it sound more complicated than really is, but I can post/send photos if necessary. The point is that the servo is only responding to a signal from the timer, and that signal could just as easily be generated by an r/c receiver.

Hope that helps.

David
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