Mini A Heli modification: no burn string

[vcp]

Well first I'm building it (and will fly it for a while) totally stock, then will consider whether it's practical to modify it or just build a new one with alternate mechanism.

I'm not convinced a piston per se is necessary. The ejection charge should push just fine on the nose cone, just need to implement the ability to have the two halves of the rocket slide apart about 1/4-1/2" without fully separating. An unglued coupler and a piece of internal Kevlar to allow movement but not separation, would do it, but exactly how to attach the Kevlar internally at the correct length is something I've not figured out yet. Doing it externally would be very easy, albeit unsightly and a bit draggy.

I was pondering some sort of 3D-printed thing, but haven't worked out the details yet.

I had the Mini A Heli worked up as a 3D print for NARAM60. Have to dig it up and get it posted.

 

[neil_w]

In the home stretch now. I was happy that when I went to check on the rocket today, there was no evidence of stray epoxy and the mechanism operated smoothly.

Next up: the hold-downs for the rotors. Here's one of them:

The curve in the base piece was formed in the usual way, with a piece of 220 grit paper wrapped around a spent motor.

Here they are on the rocket:

...and here's a pic showing a (dry fit) rotor in position:

All that's left to do now is glue in the rotors, and maybe clean up the Sharpie job on the front section of the rocket, which has suffered quite a bit from all the handling. Shoulda waited until after assembly for that.

 

[mbeels]

Yeah, I'm not looking for competition performance here, a bit of extra weight is fine. I want something as close to zero-prep as possible, to make it easier to get more flights in. Also I'm lazy.

I don't quite buy the "lazy" part, but I can definitely see the appeal in a rocket with near zero flight prep. No wadding, no parachute to fold, no stubborn shock cord to stuff into BT-20, etc... I'm starting to think about prepping rockets for Red Glare in 3 weeks, so a heli like this seems like the best thing since black powder.

 

[neil_w]

I don't quite buy the "lazy" part, but I can definitely see the appeal in a rocket with near zero flight prep.

There's a reason I bring my Quinstar and/or Cyclone to almost every launch...

 

[Alan R]

I launched a mostly 3D printed heli that simply used piston ejection. Very simple mechanism. The tip of the fins slip inside the NC, Ejection pushes NC out to release them. It flips over and comes down nose-first. However it lawn darted on me and shattered. I think the plastic parts were weak, the ejection charge or air stress snapped the hinges. And I need stronger fins. Those were shattered also.
I think the hinges broke at apogee and the wings shattered on impact. best guess.
It was one of my early 3d projects. I might print it again with better material.
Thingiverse heli rocket

 

[neil_w]

I am fully prepared for mine to blow apart on the first flight. Have been considering ways to improve it for v2.0 that will mitigate that risk. But who knows, maybe it'll hold together. It would be nice to get one good flight in.

 

[vcp]

I had the Mini A Heli worked up as a 3D print for NARAM60. Have to dig it up and get it posted.

stl files for the Mini A Heli parts I'd done. This was intended for a push off the nose/hinge section, with the blades clipped at the fin unit. The OpenSCAD code file is attached. I think it contains the code for all of these bits, but it's not formatted/commented for easy use. Fiddle with it at your own risk. Pics to follow.

 

[vcp]

This shows a blade slotting guide for cutting the slot for the swing arm, the swing arm itself, and guides for gluing/forming the blades.

Fin unit has integrated clips for the blade tips, and support for the fin trailing edge is included. I think I printed this in vase mode, ABS.

The hinge unit is printed upside-down. Support is included. The hinge pins are just a length of 1.75mm filament. Cut that bit of filament so that about a 1/8" length protrudes on each side of the hinge unit, then heat stake it with a soldering iron tip, to 'mushroom' it on each side. No glue in the hinge that way. I looked at those stacks of cardboard bits and just knew I'd make a sticky mess of it. This is clean and everything is aligned perfectly.

Nose section glued to top of hinge unit. Print vase mode.

 

[vcp]

BTW I did print all of those bits with pretty fair results, but never got around to attempting a flight. I've probably still got the prints around here somewhere; pics if I find them.

 

[neil_w]

Very nice, those clips are basically what I fashioned more crudely out of balsa.

 

[neil_w]

Finishing up:

I attached the rotors, and added a bit of CA coating a few places that are likely to get handled a lot. And, as promised, fixed up the Sharpie at the front:

Everything fits together nicely. Final weight is about 2.5g heavier than the facecard value, which makes sense for the stuff I added. If I were doing this again, I would have needed to add less stuff (like that ridiculous cardstock wrap at the front).

On minor annoyance that I discovered:

My hold-down clips cover part of the tail where tape is applied to hold in the motor. Only 1/4" is left exposed (now coated with CA, natch). I don't have any concern about it having enough contact area to hold, but I'll have to cut the tape narrow to fit it in there. Another fix to be made in a 2.0 version, should there ever be one.

 

[Alan R]

Thanks for the build thread @neil_w I just finished mine the other day. First attempt at something like this so I enjoyed seeing how someone else might modify it. It's definitely not for fat fingered folks like me but I managed to complete it without too much hassle.

@vcp If I had started my build a week later, I might have given your parts a tryout.

 

[neil_w]

Here's the finished rocket in flight configuration:

Because the rotors are thin, and only held down at the tips, they bow out a bit under pull of the rubber bands. I checked this early in the build, and decided that it's OK, since the rocket isn't going to spend much time in this state.

I once again tested deployment by giving a stiff puff of air into the end of the tube, and it seems to work fine and dandy. Sorry, not enough hands to take a video.

It is now ready to fly.

 

[mbeels]

Alright, hope you manage to get a video (or at least pics) on the first flight. That's a fun one. And hope you get to launch sooner, rather than later.

You've got a fair bit more bowing out then I had, how much camber did the rotors take?

(P.S. Red Glare is in 2 weeks!)

 

[neil_w]

Alright, hope you manage to get a video (or at least pics) on the first flight. That's a fun one. And hope you get to launch sooner, rather than later.

Definitely. This one I'll be taking video on the maiden flight for sure.

You've got a fair bit more bowing out then I had, how much camber did the rotors take?

You mean more bowing when you hold the rotors down with your fingers? With the string tied up it shouldn't bow at all. That's one of the disadvantages of the hold-down-at-the-end scheme, but I couldn't think of any reasonable way to have the hold-downs operate in the middle of the rotors without making significant design changes. Maybe I can come up with something for v2.0.

My rotors took very little camber, although I bent them according to the directions. I'm fully prepared for it to behave more like one of @BABAR 's airbrake model than as a helicopter.

(P.S. Red Glare is in 2 weeks!)

Jealous.

 

[mbeels]

With the string tied up it shouldn't bow at all. That's one of the disadvantages of the hold-down-at-the-end scheme, but I couldn't think of any reasonable way to have the hold-downs operate in the middle of the rotors without making significant design changes.

Ah, yes. I've already forgotten the details of the stock build. You're right, with a string near the middle it will be pretty flat.

My rotors took very little camber, although I bent them according to the directions.

Mine were the same initially, I wasn't really satisfied with the lack of camber so I made deeper cuts (not just a score line) and reglued the rotors so that they held nearly the full camber of the cardstock template. It may not have been necessary, but it flew great (and heli'd off into the wild blue) on its first (and only) flight. This would be a model I'd rebuild.

 

[neil_w]

Mine were the same initially, I wasn't really satisfied with the lack of camber so I made deeper cuts (not just a score line) and reglued the rotors so that they held nearly the full camber of the cardstock template. It may not have been necessary, but it flew great (and heli'd off into the wild blue) on its first (and only) flight. This would be a model I'd rebuild.

I'm looking forward to flying it, and I would gladly build another if it doesn't come back.

One thing I'm curious about: why is the A10-3T the only recommended motor? It's .8 oz, should fly fine on a 1/2A3-2T, shoudn't it?

 

[mbeels]

One thing I'm curious about: why is the A10-3T the only recommended motor? It's .8 oz, should fly fine on a 1/2A3-2T, shoudn't it?

That's a good question, I have no idea. The 1/2A3 might improve your odds of a recovery on the first flight.

 

[Aeronerd]

Great solution. Real easy prep. Looking forward to the flight.

 

[BABAR]

May be a bit late for this. As I understand it, your piston at the front pulls the rotors forward releasing them from fixed position retainers at the base. Wouldn’t it be easier to have the piston at the REAR, pushing the rotor restrained BACKWARD and releasing the rotors that way? This is basically the Texas Twister approach. You can almost do it with near zero weight penalty. Cut the body tube just below the forward end of the motor casing. Attach your rotor retainers to this cut segment. Run a small segment of kevlar connecting the segments, you will need to tuck the slack somewhere on launch but many easy options.

at apogee, motor ejects with the the retainers, it will dangle from the Kevlar (a swivel wouldn’t hurt).

you really want to take weight off, remove the central segment of the body tube and replace it with a carbon fiber rod.

 

[neil_w]

May be a bit late for this. As I understand it, your piston at the front pulls the rotors forward releasing them from fixed position retainers at the base. Wouldn’t it be easier to have the piston at the REAR, pushing the rotor restrained BACKWARD and releasing the rotors that way? This is basically the Texas Twister approach. You can almost do it with near zero weight penalty. Cut the body tube just below the forward end of the motor casing. Attach your rotor retainers to this cut segment. Run a small segment of kevlar connecting the segments, you will need to tuck the slack somewhere on launch but many easy options.

There are two sections of the rocket: a front section that includes the rotors, and the rear section that includes the retainers. There is no meaningful distinction between saying that the front section moves forward, vs. saying the rear section moves backward. The two sections move away from each other. The only difference you're describing is having the separation between the two sections towards the back rather than towards the front. I chose to use the existing cut in the body rather than create a new one; that happened to be towards the front of the rocket.