Mini A Heli modification: no burn string

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Amsterdam

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That one does have a piston mechanism similar to what I'm envisioning, would be interested to see exactly how it works. Anyone know?

I'll also note that it is quite similar in concept to a certain user's design that caused a bit of a brouhaha here recently.
I own one of these and made a quick sketch, it recovers slower than you’d expect as long as the hinges hold on,
F3339FCD-9D6B-4F4C-ABF3-A34969D5D354.jpeg
 

neil_w

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Interesting, works well if you actually have a motor mount. Does the mount get fed in from the top of the tube? Seems like that would be necessary to manage a sane gluing sequence.

I'm trying to decide if the vents in the top of the Mini A Heli will reduce the amount of push the ejection charge will exert on the nose. I'd think it would still be more than enough to shift the airframe a half inch. Will have to see.
 

MadRocketer

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I’m going with a baffle type system. With a back up length of Kevlar just in case. Going to count on friction and the length of the rod to keep it together. If it does turn into two pieces maybe it will pendulum in an interesting way.
1613237155742.jpeg
 

Amsterdam

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the Motor mount / piston is fed from the top, and I believe apogee uses two sections of airframe, not one long tube, just to makeit that much easier to drop the motor mount in.

i had a heli that used a short tethered motor mount, and it was not as balanced as my one piece models, the swinging would cause it to drift further too and it got treed this summer.
to sum it up, I prefer “solid” one piece designs.

below is a minimum diameter piston, the top half, orange coupler, and green piston are glued together and move as one.

i would vent the top of the model still to lighten the impact at Ejection


1CD0589D-EDF7-4164-9732-349CE908F3EE.jpeg
 

MadRocketer

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the Motor mount / piston is fed from the top, and I believe apogee uses two sections of airframe, not one long tube, just to makeit that much easier to drop the motor mount in.

i had a heli that used a short tethered motor mount, and it was not as balanced as my one piece models, the swinging would cause it to drift further too and it got treed this summer.
to sum it up, I prefer “solid” one piece designs.

below is a minimum diameter piston, the top half, orange coupler, and green piston are glued together and move as one.

i would vent the top of the model still to lighten the impact at Ejection


View attachment 450516
Dang, I like that. I’ll have to remember this when I build the next one! No trees where I fly, but still, the idea is to keep it in one piece!
 
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Lugnut56

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That one does have a piston mechanism similar to what I'm envisioning, would be interested to see exactly how it works. Anyone know?
I have one that has flown twice. Flew once on a 1/2A3-2T and once on an A3-4T. The fin mechanism worked as designed both times, but it didn't spin near as much as I was expecting (although it landed with no damage). Also the material for the fin "hinge" doesn't look it will survive a lot of flights.

Texas Twister (1).JPG
 

ShoelessJoe

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Thanks, that's exactly the sort of thing I was wondering about (although I was thinking hinge-on-top). Do you know if it was a good/reliable mechanism? They seem to have abandoned it.
Still have 2 of them, they are very reliable and meet the criteria you describe. Very little flight prep, and fun to fly.
 

icyclops

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Sorry, i did not see the motor size....so its small...no wonder it survived. I saw the image and thought it was 18mm-24mm. My mistake.
 

neil_w

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I've definitely settled on a mechanism similar to the Texas Twister for my Mini A Heli. I will split the tube, connect with a coupler glued only on one side. Then, a finagled mechanism on the outside will allow it to slide about 1/2" on ejection, which will release the ends of the rotors from hooks at the bottom of the body. I have a 3D-printed solution in mind, but won't finalize it until I've finished construction and can see how much clearance is under the rotors.

Probably going to just modify this one, rather than wait for a second build. Very easy to retrofit.
 

neil_w

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Here's what I'm thinking:
1613359130946.png

The slide is glued to the lower tube, the stops are glued to the upper tube. The couple is glued into the lower tube, and extends up into the upper tube. It's extremely simple, but should be effective and durable. As I mentioned, the only question is how much clearance there will be under the rotors; I don't want the slide to push the rotors out. In the pic above I have the wall thickness of the slide mechanism at 1.5 mm; the whole thing provides for about .5" of slide. I figure I'd use 1/4" deep hook on the rotors, so this slides twice as far as needed. Remember this is a BT5 we're talking about, so in reality those pieces are really small.

Comments welcome.
 

Amsterdam

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Neil,

I like that a lot. keeping things external takes a lot of the hassle away and makes it very simple.
there is space between the rotors and the airframe, at 1.5mm I doubt this will interfere.

If i had to give one con it would be the added surface area and increased friction over an internal plunger design. But a BT5 even with vents, will pressurize rapidly at ejection, I'm certain it will be enough to overcome the teeny amount of added friction.

Very nice idea.
 

neil_w

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I like that a lot. keeping things external takes a lot of the hassle away and makes it very simple.
there is space between the rotors and the airframe, at 1.5mm I doubt this will interfere.

If i had to give one con it would be the added surface area and increased friction over an internal plunger design. But a BT5 even with vents, will pressurize rapidly at ejection, I'm certain it will be enough to overcome the teeny amount of added friction.
The slide mechanism should add *very* little friction, since it's sized slightly larger than the BT. The main thing is the coupler sliding on the tube, which I hope to make loose with CA and sanding.

I mostly had to go external because there's not much room to do anything inside a BT5. A side benefit is that the mechanism is completely shielded from ejection gases, so there shouldn't be a problem with build-up and clogging.

The slide will *mostly* be covered by the rotors on ascent, so shouldn't add too much drag. In any case, this is certainly not designed for optimum performance, but for ease of use.

We'll see how (if?) it works!
 

Amsterdam

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Ah! Yes on my first look I imagined the friction gain would come from between the stops and the slide, but I see now the coupler is what does all the work and the slide is mainly a guide, and holds the halves together.
very nice Neil!
 

neil_w

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So... @Michael L printed my slide pieces, and they look (in pictures) great. I should have them next week. Which is awesome.

However, a small wrinkle. Tonight I dry-fitted one of the rotors to see how much room was underneath them for the slide mechanism. I concluded that if I put it close to the front of the rocket, it'll be no problem, because the hinge mechanisms at the top hold the rotors slightly away from the body tube. Great. But then I noticed another thing... the launch lugs! Because the lugs are flush to the body tube, the slide is going to be in the way of the launch rod.

🤬

I've thought of several possible ways around this:
  1. Cut a slot in the slide where the rod goes. That was my first thought, but I don't like it at all, because (a) it'll be hard to cut without breaking the whole thing, and (b) it'll significantly weaken the part. So thumbs down on this approach.
  2. Install a second set of launch lugs, on very small standoffs. The slide is 1.5 mm thick, so a 1/16" standoff should be sufficient to clear it. This is plausible, although it adds a bunch of drag and would be a bit unsightly.
  3. Elevate the existing front lug 1/16", and put the slide in front of that lug. That will guarantee that the launch rod clears the slide. It will position the rod at a 0.5 degree angle off vertical, which seems not worth worrying about. BUT... will the misalignment between those 3/8" lugs be enough to create rod binding? Here's a picture, pretty close to scale:
    1613701846604.png
  4. Replace the front lug with a 3/16" lug. This will give the rod a bit of breathing room, it will likely rest against the slide, which is probably not a problem (small additional drag off the rod). With this approach I would be forced to do a more thorough removal of the existing front lug. Once again, a picture:
    1613701865936.png
So, definitely not #1, and I'd prefer to avoid #2 if either #3 or #4 are possible. The more I think about it, the more I think that #3 is very unlikely to create any binding. If I just put a rod through one lug, there's more than enough slop to handle the offset.

What do y'all think?
 

BABAR

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That one does have a piston mechanism similar to what I'm envisioning, would be interested to see exactly how it works. Anyone know?

I'll also note that it is quite similar in concept to a certain user's design that caused a bit of a brouhaha here recently.
Ironically I am currently building this.

bt 5 motor mount for the 13 mm motor

three BT5 to BT20 centering rings.

two on the BT5 piston, I had to sand them down on the OUTSIDE so they would slide easily INSIDE the BT20

one at the very tail end of the BT20. Had so sand down the INSIDE of this one to let the piston slide THROUGH it.

notice carefully the “windows”

at launch, the piston is fully inserted and the plastic doohickey holds the fins. The top end of the piston in forward (nose ward) of the windows.

ejection forces the piston rearward, releasing the fins and UNCOVERING the windows to vent the gas so it doesn’t rupture the body.
image.jpg
image.jpg
image.jpg
image.jpg
 
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BABAR

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So... @Michael L printed my slide pieces, and they look (in pictures) great. I should have them next week. Which is awesome.

However, a small wrinkle. Tonight I dry-fitted one of the rotors to see how much room was underneath them for the slide mechanism. I concluded that if I put it close to the front of the rocket, it'll be no problem, because the hinge mechanisms at the top hold the rotors slightly away from the body tube. Great. But then I noticed another thing... the launch lugs! Because the lugs are flush to the body tube, the slide is going to be in the way of the launch rod.

🤬

I've thought of several possible ways around this:
  1. Cut a slot in the slide where the rod goes. That was my first thought, but I don't like it at all, because (a) it'll be hard to cut without breaking the whole thing, and (b) it'll significantly weaken the part. So thumbs down on this approach.
  2. Install a second set of launch lugs, on very small standoffs. The slide is 1.5 mm thick, so a 1/16" standoff should be sufficient to clear it. This is plausible, although it adds a bunch of drag and would be a bit unsightly.
  3. Elevate the existing front lug 1/16", and put the slide in front of that lug. That will guarantee that the launch rod clears the slide. It will position the rod at a 0.5 degree angle off vertical, which seems not worth worrying about. BUT... will the misalignment between those 3/8" lugs be enough to create rod binding? Here's a picture, pretty close to scale:
    View attachment 451309
  4. Replace the front lug with a 3/16" lug. This will give the rod a bit of breathing room, it will likely rest against the slide, which is probably not a problem (small additional drag off the rod). With this approach I would be forced to do a more thorough removal of the existing front lug. Once again, a picture:
    View attachment 451310
So, definitely not #1, and I'd prefer to avoid #2 if either #3 or #4 are possible. The more I think about it, the more I think that #3 is very unlikely to create any binding. If I just put a rod through one lug, there's more than enough slop to handle the offset.

What do y'all think?
put the forward lug on the slide?

maybe integrate it into the plastic piece the next print rendition?

I don’t know where your fins are going (or if you are going to use an integrated fin rotor like Jim Flis’ Tidddlywink or most of my Helis and AirBrakes). But if you put the forward lug on the slide, possibly rear lug can be offset from the body tube slightly if you put it on a fin near the the body, with minimal drag penalty.
 

neil_w

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put the forward lug on the slide?
That's an interesting idea, although I don't think it really solves any problem here.

I don’t know where your fins are going (or if you are going to use an integrated fin rotor like Jim Flis’ Tidddlywink or most of my Helis and AirBrakes). But if you put the forward lug on the slide, possibly rear lug can be offset from the body tube slightly if you put it on a fin near the the body, with minimal drag penalty.
It's an Estes Mini A Heli. It's almost fully assembled including the launch lugs, which is the quandary. The rear lug, already filleted into a fin/body joint, ain't going anywhere.
ejection forces the piston rearward, releasing the fins and UNCOVERING the windows to vent the gas so it doesn’t rupture the body.
If I were starting over, I might well do it this way (have the slide open a vent). As it stands now, the Mini A Heli has three vents at the top of the body tube... I am hoping that the ejection blast still gives enough of a push to activate the slide... otherwise this is going to be a short-lived rocket. :)

In theory I could partially cover up the vents (wrap cardstock around them) to increase the strength of the push. But I just have no idea if it's necessary.
 

neil_w

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I fiddled with a 1/8" launch rod in the existing lugs, and found that there is *more* than enough slop to absorb the lug misalignment if I go with option #3. So that is what I shall do. Once I get the parts I'll document exactly what I do here, and sometime this year ( :rolleyes: ) will get to try it out.
 

neil_w

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After doing some more fiddling with the parts, I can see I've got big challenges ahead. This would have been a heck of a lot easier if I had planned it out before starting the build. Haven't given up yet, though. Further updates after I get the parts and do some experimenting.
 

MadRocketer

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After doing some more fiddling with the parts, I can see I've got big challenges ahead. This would have been a heck of a lot easier if I had planned it out before starting the build. Haven't given up yet, though. Further updates after I get the parts and do some experimenting.
I have found that this happens on every build I do. Even if I plan it all out, the finished product only generally resembles what the blueprint portrayed!
 

neil_w

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So, here we are. @Michael L has graciously printed my parts, some in polycarbonate blend and some in PETG. They look great.
a03CbsmYQ4WPcHjb4T4YgQ_thumb_12172.jpg

However, I decided that trusting the recoil from the ejection charge to activate the slide is not smart (remember that the front of the Mini A Heli is highly vented). I'm having my doubts about the whole thing, like whether the whole thing will blow apart on the first flight, but I'm going to soldier on anyway because it's fun. And so, I have to decide how to proceed.

I have concluded that the solution must involve some sort of slide-activated venting. Given the almost-finished state of the rocket, I can think of three ways to do this right now, and am completely stumped which would be best. I welcome any input.

In all three it is assumed for illustration purposes that the coupler is glued to the bottom/main body tube (left), and the front part slides forward on a coupler at ejection. The red piece is the existing 1" coupler glued into the bottom part of the BT. The green piece is a new 2" long coupler.
heli_venting.png

1) Use existing vents

There are already three rather large vents at the very front of the rocket. To use them, the slide coupler must be quite long, and extend the existing red coupler (which has 1/2" exposed) that is already glued to the bottom. Also, the rotor hinges obstruct the installation of the slide collar, so I would need to cut pieces out of it to install it. I cut up one of the pieces to see if I could do it, and how it looks. I could do it (wasn't too hard), and it is definitely a lot flimsier afterwards:

2iTX5MO9TDqb2M43hMNy1A_thumb_12173.jpg

Advantages:
  • Reuse existing slots, which vent into unobstructed area in front of the rotors and hinges
  • Reuse existing body tube cut (top and bottom are not yet glued together
Disadvantages:
  • Requires very long slide; attaching it to the existing one is a little janky.
  • 3D-printed slide is weakened by needing to cut pieces out of it. Could detach from BT by pulling away from it, whereas the fully intact collar could only slide off, which seems far less likely
2+3) Vent-in-body or Vent-in-coupler

Glue the top together, make a new BT cut. Cut new vents either (a) in the BT positioned above the coupler, or (b) in the coupler itself.

Advantages:
  • Single 2" coupler used for slide, very clean.
  • For vent-in-coupler, no exposed holes, when rocket is closed: even cleaner!
  • Slide collar stays intact for maximum strength
Disadvantages:
  • Requires a new BT cut.
  • For vent-in-body, cuts additional new holes in BT. Probably not a big deal.
  • For vent-in-coupler, holes in coupler weaken it. I'm not sure this is actually an issue; the coupler would be no more weakened than the body tube when holes are cut there.
  • Vents will need to be positioned carefully between the rotors. Ejection gases will likely blacken the edges of the rotors there. I'm not sure how serious this is.
    EDIT: probably not very. Toward the top of the body, there's quite a bit of space between the rotors (photo below is dry fit), considerably wider than the slots. Down by the fins it's a different story...
    D9l%IMLNTRq2ibimt55acQ_thumb_12176.jpg
What do you think?
 
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neil_w

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Well in the absence of any input I made a command decision to go with approach (1). If it blows apart on its first flight, so be it. In case anyone is still reading this, here's the process.

STEP 1: EXTEND THE COUPLER

Basically I needed a coupler for two couplers up front: the short red one, already glued into the main body, and a standard-issue brown 2" coupler from BMS. Fun fact: the ID of the two couplers is quite different. The red Estes coupler is somewhat thicker-walled. This means I need adapt between the two.

First I took the yellow space that comes with the kit, cut a slice out of it, and glued it into the red coupler:
iHIYQ0SCRjqi1btlavdu+A_thumb_12179.jpg

I could not immediately glue on the brown coupler because it would be too loose at the joint. So I glued a single layer of 110 lb cardstock around the yellow piece (not shown), which thickened it up just enough to have a good fit into the brown coupler, and voila:
N%mqSfg5TTuZJKgCMX4xCw_thumb_1217a.jpg

It actually seems pretty strong, not janky at all. Both the exposed red and brown coupler surfaces have been CAed and sanded down for easy and smooth sliding inside the upper BT5. It needs a bit more sanding to smooth out the joint, although it would be fine if I left it as is.

STEP 2: ADJUST THE FRONT LAUNCH LUG

As previously discussed, the launch rod need to be angled ever so slightly away from the body to clear the slide. So I sliced off the old front lug, leaving the base intact:
bXUUXFoHS3GsdBE5VdLdJQ_thumb_1217b.jpg

Then I glued in a layer of cardstock to raise it just a bit more, and then glued in a new lug into this nice little bed:
PMo9LepyRM2T7K7HELOIqw_thumb_1217d.jpg

I tested with a launch rod and it fits fine and slides easily without bending (slide piece is just dry-fit on there for now):
j5wBn6jpRv+COPmKfzTLdw_thumb_1217c.jpg

Sitting horizontally like that, the rod rests on the slide, but it actually has a fair bit of wiggle room available, so I think it'll be just fine. If the rod touches the slide a bit during launch, that's no biggie anyway; just don't want there to be obvious binding or friction from it.

To be continued...
 

neil_w

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1 was the choice I would have recommended. You extended the coupler really well!
Yeah that part came out better than my thought. All my concern now boils down to whether it will hold together. If I built from scratch, knowing what I know now, I'm pretty sure I could do it successfully. This particular build... well it might not have a very long life, let's put it that way.

Tonight I glued on the slide. I designed some grooves into the inner glue surface to give good surface for the epoxy to grab. It was specifically designed to work with a fully intact ring, so I don't know if it will be as effective as intended in this mode. Here's a close up, along with a good view of my fingerprints:
VERqRVyHRNeRloM1Y43RbA_thumb_12181.jpg

I blobbed some BSI 15-minute epoxy onto the upper body tube and slide it on. I coupled a piece of scrap tube into it, to ensure proper alignment.
rF6U3eM4SlK0ZFG7u+dW7w_thumb_12183.jpg

I kept rotating the tube periodically while the epoxy was curing, to ensure that a stray bit of epoxy didn't glue the whole thing together.

I did not mean to glue it so far above the edge as I did. But since I did, I will take the opportunity to blog some more epoxy into that area to (maybe?) secure it a little better.

I should have made the slide longer; I don't know why I was so chintzy with it. When finished it will have about 3/8" of travel, which is less than I intended. That means the vents will be a bit less than 3/8" long when fully open. I hope that's enough.

Already it is clear what I should have done differently:
  1. Should have extended the front body tube about 1/4" below the hinge mechanisms. This would have allowed the forward ring of the slide to remain fully intact, and mount just below them. That would be much stronger.
  2. Should have made the slide longer, so it had more travel.
Oh well, next time....
 

neil_w

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More progress:

I needed to cover the upper part of the slot, so it would be completely covered before the ejection charge. Add this to the list of things that wouldn't have been necessary if I had properly planned in advance. I applied a two-layer wrap of 110 lb cardstock, making sure the front and rear edges were sealed with small fillets. The cover overlaps the extended couple ever so slightly, so it'll get started moving before the vent starts opening up:
nebdZhaiScG0y4b9Xoq4xA_thumb_12188.jpg

Yeah, it's not exactly beautiful. I confirmed it seemed to work OK by giving some very sharp puffs of air from the bottom of the body tube. It opens. I expect the ejection charge to be somewhat stronger than my own breath, so I should be in good shape here.

Next came the squirreliest part of the whole operation: gluing in the stops. These have to be installed between the ribs of the slide, in exactly the correct position, without any stray glue to gum up the works. Also, This is where the front and back become permanently attached; no turning back now.

This was an insanely fiddly tweezer-based procedure. If I ever build another one of these I will need to find a better way to do this part. Building a low-profile slide mechanism on the outside of a body is hard. But it seems to have worked OK.
TZfzLeUjSG+9aMMSrLyjSQ_thumb_1218b.jpg

And here is a demonstration showing the slide and the vent in operation.
 

mbeels

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Oh, this is interesting, how did I miss this? Cool idea!
 
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