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Helicopter model rocket - bigbumble
April 8th, 2013
02:24 pm

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Helicopter model rocket
As I mentioned last week, I have been working on helicopter model rockets for teeny 1/4A3-3T model rocket engines. I test flew my first model last week. The blades deployed before landing, but just barely. I'm now working on a shorter model that I hope to test before the contest this weekend. Here is the model I tested with blades folded before launch.

Helicopter with folded long blades



Just barely visible is the white elastic thread that keeps the blue and yellow rubber bands from spreading the blades.

After the ejection charge blows through the little hole just above the fins and burns through the elastic thread, the blades spring out like this.

Helicopter Open long blades


I'm holding the model sideways/top down in the picture.

Now, for a close-up of the open blade/spinner hub assembly.

Helicopter Open closeup Long Blades

You can see the rod that passes through the spinner hub. The wooden nose cone is glued to the rod and nothing else. Everything below the nose cone spins freely on the black rod. Notice the wedges of balsa wood at the base of the blades where they are attached to the hinges. That is to give the blades an incline so they will spin around the hub. The black blobs are rubber containing flexible super glue.

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From:asg_qa_s7
Date:April 9th, 2013 01:59 am (UTC)
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Very cool! The photos lay it perfectly out.

I think you mentioned (or I read) that the delay allows the rocket to arc over quite a bit; does the ejection charge kick enough laterally through that little hole to help swing the rocket blades-up, or is it all rotor drag?

Anyhoo, I'll be waiting to hear the results.
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From:bigbumble
Date:April 9th, 2013 11:31 pm (UTC)
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There are actually two holes, one for the elastic thread to go in, and the other for the thread to come out. I actually considered making one hole smaller than the other to achieve the effect you mention, but perversely the small hole punch wouldn't fit in the tube to punch a hole.
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From:gfish
Date:April 9th, 2013 05:47 pm (UTC)
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I was working on an upscaled version of something like that, uh, far too many years ago: http://cyphertext.net/images/Erato/1030694607.jpg

It flew a couple times, and survived, but the blades never deployed very well. The top spun on a bearing, and once the blades were freed it spun up a *lot*. Enough to visibly slow the decent like it hit a layer of cotton candy, but not enough to let it flip over. (Or maybe the gyroscopic forces were actually working against it!) It lawn-darted every time, blades spinning madly the whole way down. I really wanted a way to deploy the blades with the hub locked, letting it tumble into rightside-up position, then release the hub to spin up and do its magic. But that's a lot harder to do mechanically, and I never got around to it. Fun project, though. I learned a lot about fiberglass layup making those foam-core blades!
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From:bigbumble
Date:April 9th, 2013 11:27 pm (UTC)
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Selecting a delay to deploy the blades at maximum altitude while the rocket is still pointing up combined with strong rubber bands to deploy the blades seems to be the best strategy.

Was you model a D engine cluster, or were you using composite engines?
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From:gfish
Date:April 10th, 2013 07:40 pm (UTC)
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It would have to be *exactly* at apogee, though. From what I remember seeing, I'd expect it to have continued on a ballistic trajectory even if the blades deployed while it was still pointing up.

I think it flew on a composite E.
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From:judifilksign
Date:April 9th, 2013 10:06 pm (UTC)
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Does it make it easier to track on the way down as well as gentling the landing?
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From:bigbumble
Date:April 9th, 2013 11:20 pm (UTC)
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Yes, it makes the model easier to follow. -Unless it hits an upward air current(a thermal) and drifts away never to be seen again.
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