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Helicopter Recovery - Modifying a Cosmic Cobra

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  1. #1
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    Helicopter Recovery - Modifying a Cosmic Cobra

    I have (actually, it’s my daughter’s) an Estes Cosmic Cobra. I figure it’s probably about as simple as helicopter recovery can be, with the nose being the helicopter and the rest descending on parachute. That’s the theory anyway...it has yet to have a 100% successful flight. It’s been repaired/rebuilt after every attempt. Basically, we haven’t had any luck with getting the parachute to come out. The nose has come down occasionally by helicopter blade successfully. But the rocket body itself has either lawn darted or core sampled with every attempt as I can’t seem to pack the chute and blades together tight enough so that they can both deploy with the ejection charge.

    Last night it hit me…why can’t I make the Cosmic Cobra come down together and eliminate the parachute all together. I’ve seen a few posts in here where the builders wanted their rockets to descend nose first, or flat to protect fins on landing. And I remembered at least one or two descriptions of the shock cord actually running along the outside of the rocket. And with that I had my idea.

    It took me all of five minutes to do it. I drilled a hole in the tip of the nose cone. Then I took some Kevlar cord and threaded it through the hole, pulling the end out through the shoulder so I could tie a knot into it to keep it from slipping back through the hole. I tied a swivel to the other end of the cord and attached the swivel to the shock cord in the rocket’s airframe. Then, I cut a small rectangular section off the front end of the body tube, wide enough for the Kevlar and bit longer than the nose cone’s shoulder. Boom…I was done.

    The idea, is that when I prep the rocket, most of the shock cord is inside the airframe, but a section comes out along the cutout section, pulled tight up to the tip of the nose cone. No parachute, and the rocket launches normally. At ejection, the nose cone separates from the body, its blades deploy and it starts spinning but stays connected by the shock cord, and the entire rocket descends under the spinning nose. I didn't take any pictures last night when I did the work, but have attached a basic drawing showing how the rocket should look on ascent and descent.

    I have two concerns, one being that the blades were designed to only bring down the nose, and with the full rocket, it may not actually be able to slow the descent enough for a landing with no damage. I figure that the rocket has landed with no slowing for every launch of its career, so even not slowed enough is better than not slowed at all. Assuming everything works correctly, I’ll know after one launch how well the entire rocket is slowed by this design.

    The second concern is that the shock cord gets tangled up with the blades during deployment. I haven’t come up with a way to prevent this, I’m just hoping that the nose separates from the body fast enough that the cord is pulled tight before it starts spinning, thereby minimizing the chances. But again, with the blades out, even if they can’t spin properly because of a tangle, I’ll end up with at least tumble recovery, which probably won’t be enough to prevent damage, but it’ll be better than what we’ve had to date with it.

    Through my searches here on TRF, it seems that the better design is for a solid connection with the blades, as opposed to a flexible cord. But I think it's worth a try to see how it works. Aside from the two problems I've already thought of, are there other things I'm missing that I should address prior to the first launch?
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    Last edited by MrGneissGuy; 21st July 2011 at 05:39 PM.

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  2. #2
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    27th June 2011
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    All I did with mine is no put the parachute on.The first flight it lawn darted broke off the first 2 in. of the body so I ripped of the parachute a 4 perfect flights the body wiggles down fins first never damaged and a perfect heli recovery every time. I think that your idea will work and if it does than I want to try!
    I don't always fly rockets,... But when I do, I get them back. (The most interesting man in the world TV commercial voice)

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  3. #3
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    I built one just like you describe - I drilled a hold in the tip of the nose cone and tied a kevlar threat to it, running down the side of the nose cone and into the body tube. I also replaced that heavy fin can with a set of swept back balsa fins and a tailcone down to a 24mm motor mount (I am hoping to use it as a qualifying model for D Helicopter events). With an adapter, it is light enough to fly on B6 motors.

    Unfortunately, on the first test launch, the shock cord did, indeed, tangle with the rotor blades. I tried using a paper sleeve to enclose the blades and run the shock cord outside that, but it didn't help. I'm still studying the concept and will continue to experiment - hopefully I can get it to work.

    BTW, flying a stock Cosmic Cobra is not too difficult - I roll the chute into a spike with the shock cord wrapped around it a couple times, then fold the helicopter blades around the chute spike. Put some wadding in the model and then insert the blade/chute sandwich and let her rip. I've had no problems with deployment using this packing method.
    Greg Poehlein

    Member of Launch Crue - http://launchcrue.org/

    Hint #1: Do not use magician's flash paper for recovery wadding!

    Hint #2: Clean your shoes after flyin' in that cow pasture - that ain't no dirt clod on the sole!

  4. #4
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    We used to mod the old Turbocopter, which was basically a BT20 version of this rocket, in the exact same manner. Since that thing was so light, it would descend pretty much OK, if a bit 'wobbly' at times.

    Come to think of it, wasn't there an old Sprocketry mag article about this very thing at some point roughly 15 or so years back?

  5. #5
    Quote Originally Posted by gpoehlein View Post
    BTW, flying a stock Cosmic Cobra is not too difficult - I roll the chute into a spike with the shock cord wrapped around it a couple times, then fold the helicopter blades around the chute spike. Put some wadding in the model and then insert the blade/chute sandwich and let her rip. I've had no problems with deployment using this packing method.
    I've done this method as well and it works fine. The thing I made sure was just to 'spike' the chute, not fold it over. Deployed with no damage (which was a first...I had been encountering problems like the OP had with the Cosmic Cobra until I used this method).

    Also, my concern (besides the tangling) with your modification is that the helo part will spin, but the body will have too much mass to spin with it, resulting in the kelvar/shock cord wrapping up and either causing a tangle issue, or worse, the spinning blades can't overcome the inertia of the rocket body, and will eventually stop spinning...

    FC
    Last edited by FastCargo; 22nd July 2011 at 10:50 AM.
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  6. #6
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    Quote Originally Posted by FastCargo View Post
    Also, my concern (besides the tangling) with your modification is that the helo part will spin, but the body will have too much mass to spin with it, resulting in the kelvar/shock cord wrapping up and either causing a tangle issue, or worse, the spinning blades can't overcome the inertia of the rocket body, and will eventually stop spinning...

    FC
    That one I did think of. The shock cord is actually in two pieces. One permanently attached inside the airframe, the other to the nose cone. I'm using a swivel, which has a barrel (is that the correct term?) that spins to connect the two cords. Hopefully that will correct, or at least minimize this potential problem.

    I might try a low flight out back over the weekend to see how it does. But it's been so stinkin' hot I don't know.

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  7. #7
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    Quote Originally Posted by FastCargo View Post
    Also, my concern (besides the tangling) with your modification is that the helo part will spin, but the body will have too much mass to spin with it, resulting in the kelvar/shock cord wrapping up and either causing a tangle issue, or worse, the spinning blades can't overcome the inertia of the rocket body, and will eventually stop spinning...

    FC
    Yeah - I need to add a swivel or something like that to it - the kevlar shock cord kinda looked like the rubber band on a rubber band powered airplane that was wound up and ready to go!
    Greg Poehlein

    Member of Launch Crue - http://launchcrue.org/

    Hint #1: Do not use magician's flash paper for recovery wadding!

    Hint #2: Clean your shoes after flyin' in that cow pasture - that ain't no dirt clod on the sole!

  8. #8
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    Quote Originally Posted by AstronMike View Post
    We used to mod the old Turbocopter, which was basically a BT20 version of this rocket, in the exact same manner. Since that thing was so light, it would descend pretty much OK, if a bit 'wobbly' at times.
    That's what gave me the idea to try it - one of our club members used a Turbocopter with this mod at our last contest. Since I don't have a turbo copter...

    (Although I think I may have solved that little dilemma - I found a slingshot style freeflight copter with almost the exact same kind of blades as the Turbocopter I don't think it would be that tough to mount said blades to a BT-20 nose cone.)
    Greg Poehlein

    Member of Launch Crue - http://launchcrue.org/

    Hint #1: Do not use magician's flash paper for recovery wadding!

    Hint #2: Clean your shoes after flyin' in that cow pasture - that ain't no dirt clod on the sole!

  9. #9
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    Well I finally got a chance to try this out this past weekend, on a C6. I'll call it a partial success. The set up was the biggest headache. I had removed the rubber bands from the blades so they wouldn't get too stretched out, but they did anyway...by quite a bit. So I tried doubling them over, which worked for one. The other two broke. I eventually got the rubber bands/blades all set up and put it on the pad, rod angled so that it flew well away from any spectators, just in case.

    It flew well and the kevlar did not get tangled in the blades. But the blades did not seem to want to rotate at first. It looked like maybe the body was a little light and/or experiencing too much drag and didn't drop below the nose and blades initially. About halfway back to the ground, the body swung below and the blades started spinning. They spun like they were supposed to and it seemed the swivel did it's job as well.

    The descent was faster than I'd like even when it was spinning properly (blades too small for the weight I guess), but it landed with no damage.

    Dandelion Preservation and Appreciation Association

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