Complete 5/5/18 DareDevil Built Thread, Air Brake Recovery Rocket

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BABAR

Builds Rockets for NASA
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DareDevilPad1 (2).jpgDareDevilDeploy1 (2).jpg

"Built" is not a typo, I don't have the patience to interrupt the build process with taking AND posting photos, I barely remember just to TAKE the photos, so it's already built. We will see how coherent my memory is.
Start with a 1/8" balsa sheet, 6 inches wide, at least 18 inches long (final length of the blades will be 18".)

Note: I will switch between metric and English frequently, just enough to annoy Everybody!

I recommend using a sharpie or other INK marker to draw the lines. Do NOT use a pencil. Using a pencil is likely to "score" the balsa (some of the lines you will NOT be cutting) and I have learned from experience this weakens the balsa significantly on this model.

At the top, mark a transverse line 2 inches from the top.

Do the same at 16" and at 18". The 2 inch segments will make up the hub section and tail sections of the Rotors.

Take your balsa strip and mark off a 2 inch wide margin at EACH end.

Your first template is a strip of cardboard, 6 inches wide by 1 inch long. (wide and long orientations relative to the where you will use this on the Balsa.)

Mark forward edge of your template at 48 mm, 70 mm, 118 mm, and 140 mm.

Mark the trail edge of your template at 13 mm, 35 mm, 83mm, 105mm.

for the record, I do it this way to get the maximum use of the balsa sheet. Yup, I'm cheap.

Use for forward edge of the template to mark the forward edge of the Balsa sheet and put the SAME marks on the 2" line.

Use the rear edge of the template to mark the 16" and 18" lines.

Connect lines between the front edge and the 2" line. These lines will be straight along the long axis of the rotors.

Connect the lines between the 16" and 18" lines. These lines will likewise be straight along the axis of the rotors.

NOW beginning on the right at the edge (you could call it the "0" mark) of the 2" line, draw a line (it will be slightly oblique to the long axis of the rotors) to the 13mm mark on the 16" line.

Next one goes from the 48mm mark on the 2" line to the 35mm line of the 16" line.

Next one goes from the 70mm mark on the 2" line to the 83mm mark on the 16" line.

Next one goes from the 118mm mark on the 2" line to the 105mm mark on the 16" line.

Next one goes from the 140mm mark on the 2" line to the left EDGE (you could call it 152mm, it is actually a bit longer but works okay for this rocket) of the 16" line.

If I have expressed this right and you did it right, you should have something that looks like the last picture here.

Do NOT start cutting yet!

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Okay, this build is not as slow as the Rube Goldberg Lander (which to my knowledge JAL never completed!), but taking me a while to get back to it.

Key feature on my Helicopter Rockets and AirBrakes is the combined Rotor Stop/Pull Band Displacers. From here on out I will just call them Rotor Stops.

the rotor stop concept is pretty self explanatory, rotor flips up at the attachment hinge and has to hit something (or pull on a tether) to stop the travel at a certain point.

The Pull Band Displacer requires a bit of explanation.

If the pull band is just attached at the nose, the initial direction of the pull (I think the physics term is "Moment") is perpendicular to the direction you need the rotor to initial move. You are pulling toward the nose, you need the rotor tip to go directly outward. The Pull Band displacer, by moving the direction of pull just a bit away from the body of the rocket, helps provide just enough displacement to help get the rotor started. Of course, once it starts, the angle of pull just keeps getting better.

When I first started with these, they were always breaking in one direction or another, because the force on the Rotor Stops comes from multiple directions. My solution to this was, initially, to do a two piece balsa play, with the grains oriented 90 degrees to each other. This worked.

My second problem was that the bands would slip off the edge of the Rotor stops. Initially I put half piece of launch lug on the edge, which worked, but broke off easily, lot of drag, just not great solutions. Then, when I was using only two pieces of balsa, I beveled each piece at the edge. Then glued them together as you see in diagram.

RotorStop.png

This worked, although the bevel was kind of thin, and had a tendency to snap (also needed to do grains 90 degrees to each other, but 45 degrees relative to long axis of the rocket, otherwise one side or the other had grain parallel to bevel and just snapped off.)

Solution, which is what I use now, was a three piece ply. The outside pieces had a grain perpendicular to long axis of rocket AND to the bevel, so had strength in THAT direction. The inside piece had grain parallel to long axis of the rocket, so had strength in THAT direction. The inside piece is cut just a bit narrower, so when you glue it in between the others creates a channel. IMG_5454.jpgIMG_5407.jpgIMG_5411.jpgIMG_5412.jpgIMG_5451.jpgIMG_5452.jpg

In either case, you end up with a "trough" at the edge, which the rubber band drops into and holds it pretty well.

for glue options, that are advantages and disadvantages to wood glue and CA. Wood glue is great for strength, but tends to warp a lot. I have to immediately put the pieces between wax paper and stick them in a book. Thick CA actually works pretty well, and stiffens the wood as well, and doesn't tend to warp. Plus it is pretty quick. So that's what I go with most of the time.


As easy as it seems like it would be to keep the grain directions straight, I am probably up to about 50 variants of this, and I still tend to mess it between outside and inside, so now I just try to idiot proof it and label them when I cut them out.
 
These are not contest flyers, and certainly there are lots of sources of drag other than these Rotor Stops. Since they are however at the FORWARD end of the rocket, and therefore inversely affect stability (kind of like canards, I guess) because they are waaaaaay forward of the CG, I tend to AirFoil them (round the front and taper the back) at least a little bit, in theory it makes them less "draggy" and less likely to make the rocket unstable. Don't know if it makes that much difference. I make the fins a bit larger on my best "guestimate" of what it takes to counter the forward stops. Given the final rocket has a square body, these don't lend themselves well to Open Rocket or RockSim simulations. There is an article on ApogeeRockets on this, but it is beyond my patience and abilities!
 
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Okay, so Initial cuts on Rotors, you can see the lines for where the hub sections will be cut out at the forward end of each rotor.

Next is layout for cutting out the fins and the reinforcements. Grain direction for the fins is standard, grain will be parallel to forward edge of the fin. DareDevil uses a forward swept fin, part of this is style (I just think they look cooler), but there is a very PRACTICAL reason for the trail edge of the fin to have a forward sweep, which we will get to when we reach the rigging for flight section.
There is a lot of stress on the hub and forward sections of the rotors at deployment. Early models when I didn't recognize this suffered when the rotor stops just punched through either the hub or snapped through the blade because of the grain direction. For this reason, I reinforce these with a balsa ply with grain perpendicular. The reinforcements must be cut a little bit narrower than the outer hub and rotor width to allow leeway when you put the parts together.

I cut out a template that allows me to mark the end of the blade for the cut so the fin fits on just right. Also mark where the hole will be for the rotor retention bands.

I mark this hole with felt pen (need something dark that will how through masking tape.) I then wrap a strip of tape around the whole, to protect the balsa. I know it is inelegant, but a hole punch works really well to place just the right size hole through the balsa.
 
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Second pic just shows cutting off the hub from the front of the rotor. Save this piece, we will use it later.

Trick I came upon that makes attachment of the Rotor Stops a cinch, as well as improves the strength of the attachment.

At first I was just gluing the rotor stops on the outside of the Rotor. But it was always breaking off, plus had the problem that sometimes even if it didn't break off, it would just break or split through the grain of the rotor. Also tricky getting the alignment perfect, as mentioned these things aerodynamically are sort of like putting fins or canards on the front of the rocket. Put them on even a bit out of alignment and you really have a problem.

what I came up with was to mark and cut a notch in the front of the rotor. Assuming I mark it and cut it right, the Rotor Stop will just drop right into the notch and align itself.

Since the Rotor Stops are a ply of 3 piece of 1/16" balsa (1.5875 mm, for SNRKL and any other metric dudes and dudettes out there), comes out to be 3/16" (4.76 mm) wide.
 
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"The Hole" at the back of the rotor will serve for positioning of the motor retention "burn bands". Lot of stress on this region. Currently what I do is cut plastic hotel key card into pieces that will brace this area. Problem I have been having is getting this to adhere to the balsa. I will need to post the results of my experiment back on the forum, but I tried attachment with Epoxy, Gorilla Glue (polyurethane), Thick CA, and Wood glue. Gorilla Glue worked best, but the expansion and bits sticking out over the sides was impractical. Epoxy worked well. Thick CA and Wood glue were horrible. So I currently go with epoxy. Being a Low Power Rocketry person, I don't like it, because I have to wear gloves, it stinks, I gotta mix it..... But it does work. I find it easier to punch the hole punch through the cards BEFORE I cut out the shapes. I use a small leather punch to put a small hole in the forward edge of the shape, this is where I will tie off the pull bands later. I glue the brace on the INSIDE of the blade (make sure you keep track which is the inside). I put Mylar tape over it, both the keep it in place as well as to protect it from the engine deployment gases.
 
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Cut the "edges" off the ends of the rotors where fins will be attached. I use wood glue for the attachment, double glue joint technique. Apply glue to one surface, touch the other surface and rub it around a bit, let both sit for about five minutes, and then stick them together. I then stick them between piece of wax paper and shove them in a book overnight to dry.

Once dry, I put a bit more glue on both sides of the joint to reinforce. Once that's dry, I sand off the joint smooth on both sides.
 
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As mentioned, when deployment activates, the rotors are pull up by the bands on the hinges until they hit the rotor stops. It is an abrupt impact and has been a failure point in the past, where the rotor stop just punches through the grain of the hub, or fails at its attachment to the rotor. I use a ply technique with another layer of balsa, in this case 1/8 inch like the rotor and hub itself. By orienting grain perpendicular it provides extra support. I think also the glue itself adds a lot of strength to the "ply" structure, maybe like papering fins. This also gives more surface area and a stronger attachment point for the tape hinges, which will be outlined below.

Because these pieces will fit together later, the reinforcement backings are cut two balsa thicknesses (2x1/8, so 1/5 inch) narrower than the hub and rotor pieces. Notice the grain direction is perpendicular to the grain of the hub and rotor pieces.

Also, because I tend to get mixed up at times, I mark the edge I want these pieces to go against. When I make these as helicopters (essentially identical design, only difference is that the hub/rotor cut is 10 or 15 degrees off perpendicular, to impart a spin), there is a "leading edge" of the rotor (NOT the same as the leading edge of the fin!) when this rotates. I mark the leading edge with a thick "sharpie" pen (kind of an oxymoron, I guess). I line up the edge of the backing with the "leading edge" of the rotor. Also keep track, make sure you are putting the reinforcement on the BACK of the rotor (same side as the plastic piece you put on earlier.)

The wood glue makes this warp like crazy, so I slather it on one piece, quickly rub the two pieces together to get full coverage, then stick them between two hotel key cards and hold them with clothes pins to keep the flat.
 
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I learned about the double tape hinge from a QCR rocket model ("Qualified Competition Rockets", unfortunately they have gone out of business, they had some really nice stuff.) It was a glider model. The double tape hinge worked, but it still wasn't very strong. I don't know if anyone else has tried this, but my standard now is a double tape hinge which then is literally sewn into the balsa. It's cheap, it's light, and once I stopped using cheap duct tape, it's close to unbreakable (the blade will fail before the hinge does.) Oh yeah, and there is something about using Duct Tape on a rocket that I just like.

Despite my pics, I am not trying to promote Gorilla brands, but I really find their duct tape is worth it compared just getting whatever is cheapest (which is what I usually do for everything else!)

First, line up the hub and Rotor Blade pieces. then "bend" them (place them together so the OUTSIDE surfaces of the hub and rotor blade are in contact. Cut a piece of duct tape 3 cm wide, the width will be the width of the inside reinforcement. Stick it over the pieces, and bend it back so the "flanges" cover the edges of the inside surfaces of the hub and blade as in pic 6. Push the tape in firmly.

Pic 7 is when you "open" the hinge, looking from the OUTSIDE. The pieces will tend to move apart, so you have to kind of "hold" them against each other for the next part.

Cut two pieces of duct tape, 2 cm (yes, I know I mix English and Metric) long and the width of each side of the notch. Holding the hub and rotor butted together to keep the previous hinge as "tight" as possible, put these two pieces of tape in place and push them down firmly.
 
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The hinges are sewn in with a needle and thread. Pics above. A more detailed description with better diagrams is here

https://www.rocketryforum.com/showthread.php?28887-Build-Thread-for-Whopper-Flopper-Chopper

Sounds crazy (yes, I reinforce duct tape to balsa using dental floss......), but it works. Probably cotton thread would work as well as floss, but the floss is pretty strong (I try to use unwaxed if I can find it, just because I think it glues in better with CA, but doesn't really matter.)

Keys to sewing in: Whenever you go through the middle of the hinge itself, always poke the needle through from the INSIDE so you are sure to go through the MIDDLE of the tape. Also make sure on the OUTSIDE that you don't go ACROSS the notch, you need to keep that clear. Otherwise the pattern you go through doesn't really matter.

Once you are done and tie it off, use thin CA to tack it into place.
 
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This is a non-essential part which however makes the rocket a little easier to rig for launch.

Cut four balsa strips from 1/8" balsa, each is 1/8 x 1/8 x 2 cm long.

On the INSIDE of the rotor (same side as the plastic key card thingy) measure about 10 cm up from the tail. Use a 1/8" piece of balsa to make a line along the TRAIL end of the rotor (same side as the fin sticks out from.) Glue a brace on the inside of each rotor, with the OUTSIDE edge of the brace along your line. When you rig the rocket for flight, this brace keeps the adjacent rotor in just the right position.l

Do not use this for the Helicopter version, it will just cause drag when the rotor spins. One the AirBrake version, since it doesn't spin (at least not intentionally) it has no effect post deployment. Since it is on the INSIDE of the rocket on boost, it doesn't cause any drag.
 
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I guess I need to call the plastic thingy something, so I will call it the Retention Band Reinforcer

Place rotor inside face up, tail toward you. Big hole will be for retention band. Smaller upper hole will be to attach the pull band string or hook.


Cut a 12 " piece of string (I use Kevlar thread, mainly because it doesn't unravel and is fairly strong. But any string will do.) Use a needle to poke this through the small hole at the top, then poke back through the balsa just above the Reinforcer. Tie a knot, leaving about 10 inches of thread. The knot is on the inside of the rotor. Use a needle to pass the long end of the thread through the small hole, so the 10" length is on the outside of the rotor.

Use mylar tape to cover the whole assembly, will protect it from engine gases. Use a Pencil to "re-poke" the big hole through the mylar. Press the mylar hard into the balsa, assembly, and the hole.

On the 10 inch string outside the rotor, measure off 8 inches from the hole and mark with a sharpie. You can see this mark just above my thumb on the picture with the ruler. bend the string at this point and tie a loop, with your mark at the apex of the loop. This will be your rotor end attachment for the rubber pull bands.
 
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Put a small tab of duct tape on the outside edge of the hub of the rotor. This will keep the hook from breaking through the wood grain on this side.

Poke a big needle hole through the hub just below this tape.

Use short wires to form the hooks. I found 100 Christmas tree ornament holder wires at Hobby Lobby on the clearance rack for 20 cents.

Poke the wire through as shown, wrap around, and form a hook on the OUTSIDE edge at the forward rim of the hub. This hook will stick through your Nose Pyramid and will be the forward/hub attachment for the pull rubber bands. Pictures came out in mixed up order, the final assembly should look like picture 2
 
This is where the advance planning/notch cutting pays off.IMG_5532.jpgIMG_5531.jpg

If you did airfoil the Rotor Stops, make sure you mark with is the forward end.

Put some wood glue in the notch.

Stick the Rotor Stop in the Notch and pull it out, giving it a few minutes to set.

Stick in back in.

The alignment should be dead on with the long axis of the rotor fin, since it drops into a preset notch (I guess this is like precut fin slots for through the wall fins.)
You can sight along the rotor to make sure the Rotor Stop sticks out straight laterally.

Here you can see where the reinforcement piece cut perpendicular to the rotor fin grain helps you out. It provides a base for attachment.

Once this dries, put a fillet on each side.
 
Since these designs are not contest flyers, just "for the fun of it" sport rockets, added weight is not much of an issue. Vast majority of mine are unpainted as for me, it is just the engineering fun of building and then seeing if they fly well. For the helicopters, lighter is better but if you want to add paint or tape, they will fly just fine. May come down a little faster, but won't make them more likely to break. for the Air Brake models, particularly this version which is an engine release model (you could say engine eject, but it doesn't really kick it out, just literally let's it go), the pattern of descent is such that the pull bands actually absorb just of the rotor momentum when the thing lands. In short, works fine with lots of paint or lots of tape (I like mylar tape over balsa, doesn't require all the filling and priming.) For the DareDevil model, I did put on a white paint base coat and then overlayed it with Red Mylar. But the pics I am posting are of an unpainted version.

In any case, it is a LOT easier to do your any decorating BEFORE you put the rotors together. Just make sure that you cover the contact edges of the hubs so you have bare balsa on these areas to glue the model together.
 
Great tip on cutting a notch for the rotor stops to prevent breakage/splitting of the rotor blades. I'm going to incorporate that notching method on the rotors of my "standard" heliroc that I'm finishing up.
 
Here's a thought; what if the rotor stops were attached to the hub instead of the rotor? The Hub can be more easily reinforced with cross bracing on the inside.
 
Not only do-able but has been tried. It does work.

I think about these rotor stops aerodynamically as canards. The further forward they are the more they detract from stability. Putting them on the rotors puts them a bit further back. Does it make much difference? I really don’t know.

Either way you need a backing on the “other” piece to reinforce it. Otherwise the rotor stop will punch right through it.

Oh yeah, the other reason maybe works better on the rotor I think the physics on the pull bands works better with placement on the rotor.

Lots of ways to skin this cat. Thanks for the comment!
 
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Okay, now hopefully the alignment of the back support braces helps out here.

Again double glue joint technique, wood glue.

Smear glue on the areas of attachment

Put the pieces together. I use electrical tape, allows me to make it really tight.

To make sure it the alignment is good, I go ahead and put an engine casing in place, as seen on the next post.
 
I haven't seen anyone else use this exact technique, it is now standard for most of my birds.

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This seems a bit tricky the first few times you do it, then it becomes really easy.

Crochet Hook is your essential tool.

Pass the hook from outside , going through hole in one rotor and then through the hole in the other rotor from the inside.

Wrap a #16 rubber band around third or fourth finger of the opposite hand, use the hook to grab it and pull it back through from the first side.

Now you have a loop of rubber band on opposite sides.

Slide the engine in the end of the rocket, put each loop and slip it over the ends of the engine.

Now you can see why I put a forward angle on the fins, it helps keep the band from slipping between the rotors.

Two rotors down.

Now rotate the rocket 90 degrees, do the same with the other two rotors. Need to use a bit of finesse pulling the second band though so you don't snag the first band.

Once in place, you have four loops (two for each band) around the engine. Holds the engine snugly in place against the rotors, holds the rotors tight.

the FORWARD ends of the bands are conveniently located directly in front of the engine. Yes I know they are only rubber bands, but the easily provide enough restraint to keep the motor from sliding forward at ignition (rubber band equivalent of an engine mount.)

When the ejection charge goes, flames are directly hitting on the bands, they burn away almost instantly, and the elastic force of the bands pull the bands away and release the rotors AND release the engine.

for now, anyway, this should keep all the rotors in perfect alignment while the glue dries on the hub pieces.
 
Never one to underestimate the building strengths of Dental Floss, I use it to provide further support to the hub.

Once the glue is dry, I use pliers and a thick needle to pass dental floss across each of the four lateral "angles" of the hub.

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Usually go around twice, tie it down, then use thin CA to glue it in.
 
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I have read that nose cones are often the most expensive part of any kit. Not a problem for this model.

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Measurements are as shown. Basically just measure the edge of the rotor on each side, add a mm, this is the outside edge.

I cover the outside edge with Mylar tape. Not required at all, I just do it because it adds a little strength, and a lot of color and flash (which can help when this puppy lands in the weeds, unpainted balsa doesn't show up that well, and most of my models are unpainted.)

Once cut out, use the dull edge of the knife to score the fold lines. Fold it up and tape the edge.

Punch the holes.

Slide it over the hub, use Mylar tape to hold it in place, also covers up the dental floss bracing. The pull band wires obviously stick through the holes.
 
didn't take as many pictures as a I should have.IMG_5565.jpgIMG_5566.jpgIMG_5564.jpg

I use an 18 inch rule, line up inside edge along the Rotor Stop edge in front, and the pull band hole in the back.

Use the lateral edge to mark a line on the fin. This gives you enough "stand off" for the lug to keep it away from the rigging and the taper of the rocket. Note: this works fine for air brake recovery rockets. If you build this as a heli, you need to put two wire "loop" guides on (usually I make these out of paper clips.) Reason is that if the rotors are supposed to actually rotate, the launch lug causes a lot of drag. But I digress.

Okay, now you have the line for the lug. I don't have pictures, but I use a "cheater" technique to make it easy to get perfect alignment for fins on body tubes, and works well with the lug here.

Using a 1/16" balsa sheet, cut out a "stick" 1/16" x 1/16" x the length of your lug.

Pretty easy to glue this on with wood glue exactly along the line you drew. Because it is so small and light, you can pretty much stick it on and it won't move.

Once this dries, it provides both alignment and a bit more surface area to attach the launch lug.

as I said, I use this a lot on my rockets for line up and attaching the fins. Works pretty well. Might be something to try if you have a cub scout troop and want to build rockets, the troop leader can mark the tubes and put these on before build day, Perfect fin alignment every time, and makes for a stronger joint.
 
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I usually don't use these, but for a beginner they may be nice.

I can stick a toothpick peg in a couple inches above the burn band hold. Makes rigging a little easier for flight.
 
Hopefully this is a little clearer than the earlier rigging I described.IMG_5573.jpgIMG_5586.jpgIMG_5581.jpgIMG_5574.jpgIMG_5588.jpgIMG_5585.jpgIMG_5575.jpgIMG_5584.jpgIMG_5580.jpg

This uses the pegs you may have attached. If you don't use them, just wrap the loop around the hand NOT holding the crochet hook.

Loop one end of the band around the peg or a finger.

Pass the Hook from the opposite side through the rocket and then out through the hole of the peg rotor.

Grab the band and pull it through.

Pass a finger through the loop.

Pull the loop over the engine.

Go back to the side with the peg and pull that loop down over the engine.

Rotate 90 degrees and do the same with the other peg and rotors.
 
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I have read that nose cones are often the most expensive part of any kit. Not a problem for this model.

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Measurements are as shown. Basically just measure the edge of the rotor on each side, add a mm, this is the outside edge.

I cover the outside edge with Mylar tape. Not required at all, I just do it because it adds a little strength, and a lot of color and flash (which can help when this puppy lands in the weeds, unpainted balsa doesn't show up that well, and most of my models are unpainted.)

Once cut out, use the dull edge of the knife to score the fold lines. Fold it up and tape the edge.

Punch the holes.

Slide it over the hub, use Mylar tape to hold it in place, also covers up the dental floss bracing. The pull band wires obviously stick through the holes.

Where do you get your Mylar tape? None of the hobby shops or craft stores here carry it. Even Amazon doesn't specifically carry Mylar, just other tapes like "Silver foil tape" or "Metalized Polyester" or "High Temp Stainless Steel foil tape" which is probably not the same thing.
 
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Put one #16 band through each of the loops on the pull band strings.

forward end is looped over the hook on the hub.

When you are ready for flight, pull the band over the Rotor Stop/Pull Band Displacer until if falls into the notch.

Throw in an ignitor and fly.
 
Something occurred to me while I was working on the rotor stops for my own "traditional" helicopter: what if you 3D printed a flat plate with the rotor incorporated into it, then glued the root edge of the rotor to the bottom of this piece? Another plastic piece would also be attached to the hub to protect IT from the impact of the rotor stop. Not only would you eliminate the rotor and hub splitting problem, if you incorporated predrilled holes in the plastic pieces you could continue to use your floss and duct tape hinging method.

I suppose an alternative to 3D printing would be to fabricate these pieces yourself using lightweight sheet plastic or fiberglass sheet. Thin ply would probably also work. Because of your rotor size (especially with your larger helirocs) the impact of them opening is quite significant so the idea is to protect the balsa from that impact by letting the plastic or fiberglass take the punishment.
 
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