Question About Estes Mini "A" Heli

[mh9162013]

I'm thinking about building this kit and have zero plans of building it stock. At a minimum, I will build it so the tail/fin section on the bottom disconnects to release the blades, a la @Alan R (see his neat mod(s) here).

In addition to this mod, I'd like to build this so it uses 18mm engines. Why? Well, because I like the challenge, I prefer 18mm engines so I can use a screw-on motor retainer, I'll have a wider selection of motors to choose from, and the added weight on the bottom might make an inverted recovery less likely.

So my question is: Is there an obvious reason as to why building the Estes Mini "A" Heli with a BT-20 main body tube (and accompanying BT-20 nose cone) instead of the provided BT-5 tube won't work? After reading some threads and seeing some builds, I can't find one. The only potential issue I can think of is that I might need to increase the size of the fins and the spin recovery might be faster/harder due to the increased weight (so durability might be a long-term concern).

 

[neil_w]

In addition to this mod, I'd like to build this so it uses 18mm engines. Why? Well, because I like the challenge, I prefer 18mm engines so I can use a screw-on motor retainer, I'll have a wider selection of motors to choose from,

Normally with these sorts of rockets weight minimization is a primary goal. A screw-on motor retainer would seem to go against that. Of course you can do that on your own model if you like.

But if it's a minimum diameter rocket (BT20 with 18mm mount, how exactly do you plan to manage the retainer?

and the added weight on the bottom might make an inverted recovery less likely.

Does it? That sounds a bit like pendulum fallacy to me. Or maybe not. Eliminating inverted recovery is a worthy objective, though..

So my question is: Is there an obvious reason as to why building the Estes Mini "A" Heli with a BT-20 main body tube (and accompanying BT-20 nose cone) instead of the provided BT-5 tube won't work? After reading some threads and seeing some builds, I can't find one. The only potential issue I can think of is that I might need to increase the size of the fins and the spin recovery might be faster/harder due to the increased weight (so durability might be a long-term concern).

To me it seems you're just building an upscale at that point. New tube, new fins, new nose cone. The only thing that you could probably reuse are the hinges, which admittedly re pretty nice.

 

[mh9162013]

But if it's a minimum diameter rocket (BT20 with 18mm mount, how exactly do you plan to manage the retainer?

It would look a bit like the DARC-1 Estes rocket.

As for the reduced inverted recovery chance...it's just a thought and I could very well be wrong.

 

[kjohnson]

Building it with BT20 you're basically making a Rotaroc https://www.nar.org/wp-content/uploads/2014/05/Rotaroc-BC-HD-Gassaway.pdf
There was a plan in the early 2000's that used the drop tail to release the blades- Chicago Chopper I think it was called.

 

[BEC]

I have done a drop tail variant (inspired by the Semroc Heli-roctor https://www.erockets.biz/semroc-flying-model-rocket-kit-heli-roctor-kc-02/) and will be building at least one more that way for NARAM this year (along with a few other minor tweaks). I'll be glad to do away with the burn string. This year it's 1/2A, so what I built was actually a shortened model as well to save mass. It worked pretty well.

I've also thought of building one with BT-20, seeing as next year's NRC events include B helicopter duration. For A and under, the reduced mass and drag of using BT-5 would be clearly better for maximizing boost height.

 

[Woody's Workshop]

Why not build the fin can like a booster stage?
Just stage it down to the BT-5 and it all drops away at ejection.

 

[Art Upton]

Why not build the fin can like a booster stage?
Just stage it down to the BT-5 and it all drops away at ejection.

Then it is not legal for competition, no parts can fall off.

But for plain fun, sounds like it would be fun !

 

[mh9162013]

Why not build the fin can like a booster stage?
Just stage it down to the BT-5 and it all drops away at ejection.

While competition rules don't matter to me (for now), I'd prefer to avoid having any parts fall away such that I have two or more parts/components to keep track of.

 

[kjohnson]

FWIW I used to build HD models with a sliding piston with hold downs for the rotor tips. The ejection charge would move the motor mount down about 1/4" to release the blades. The thought process behind that was to eliminate burn threads, make the smallest movement possible to change the configuration, and avoid tangles, fouling and cord breakage of drop can designs.

Reminder here for me to look for a prior thread on this design.

kj

 

[neil_w]

FWIW I used to build HD models with a sliding piston with hold downs for the rotor tips. The ejection charge would move the motor mount down about 1/4" to release the blades. The thought process behind that was to eliminate burn threads, make the smallest movement possible to change the configuration, and avoid tangles, fouling and cord breakage of drop can designs.

I'm a big fan of eliminating the burn string, just to make prep simpler. My mod to the Mini A Heli rocket was a bit complex but works well. If designing from scratch a sliding motor mount sounds like a good approach, although if it's a minimum diameter rocket (as the Mini A Heli) is then there's no motor mount so slide, and you'd have to (I think?) rely on just shifting the motor, which is certainly doable. Would be interested to see your designs.

 

[kjohnson]

Of for sure, these had BT20 bodies with a BT5 motor mount to make the sliding mount work. To make it work at BT5 for the main tube, you'd need a section of something like 10.5mm tube and centering rings.

I checked and didn't see a post about it, but I know I've drawn up the action before. Here is a new version.
The ejection charge fires and pushes the motor mount down releasing the blades. The forward ring glued to the motor mount slides back uncovering the gas port, venting the ejection charge. Prep is insert the motor and tape on, fold the blades down, slide the motor mount up and engage the hold downs.

kj

 

[ReynoldsSlumber]

Or the best (or worst?) of both worlds: strings released by nose cone ejection. Can't say I'd recommend this method, as one would have to gather all three strings and hold them each at the right tension while sliding the nose cone on. Though it may be better than a burn string.

 

[neil_w]

Of for sure, these had BT20 bodies with a BT5 motor mount to make the sliding mount work. To make it work at BT5 for the main tube, you'd need a section of something like 10.5mm tube and centering rings.

I checked and didn't see a post about it, but I know I've drawn up the action before.

kj

Nice and simple. Maybe a little tricky to assemble in the first place (making sure the glue goes only where it's supposed to) but very nice if you're not MD.

 

[kjohnson]

Nice and simple. Maybe a little tricky to assemble in the first place (making sure the glue goes only where it's supposed to) but very nice if you're not MD.

Normally I would glue the upper ring to the end of a longer piece of BT5 tube and let that dry. Then glue the lower ring in the tube and let that dry. Drop the long tube in from the top of the body tube and feed it through the lower ring, then cut the motor mount tube to length.

kj

 

[BEC]

I just used a short length of BMS BT-5+ as an external coupler for my drop tail mod of the Mini-A. I intend to build another before NARAM that minimizes the distance it moves.

 

[dpower]

FWIW, the Mini-A-Heli is quite competitive in NRC. I've even flown it on 1/4A motors, with a piston. Using 18mm adds a lot of mass with the motor casing, and will substantially reduce performance on the way down. Never had a problem with the burn string, and it keeps it light. Might be better to upscale the whole thing for good helicopter action on B/C motors.