Build Thread for Whopper Flopper Chopper

[BABAR]

Make 4 small wire “s-hooks” from paper clips. Attach one to each of the 4 pull loops at the end of the Stand-Off Adapters. Once you attach it, you can crimp the “s” closed so that it stays with the pull loop. It should still slide freely, but try to wrap it around enough so it stays on. Leave the front part of the “S” open to attach the rubber bands.

 

[BABAR]

Get 24 (yes twenty-four) #16 [2.5” x 1/16”) rubber bands (you can get a 1 Lb bag at Office Depot for $5.29. I don’t know how many are in a pound, but it is a lot.

https://www.officedepot.com/a/products/854866/Office-Depot-Brand-Rubber-Bands-16/.)

Alternatively, you can try using all those left over Estes shock cords that you replaced with Kevlar on your Estes kits.

Loop them in 8 groups of the three each with Larks-Head-Knots (picture here).

You will attach two sets to each of your 4 loops on the tip of the nose pyramid dowel (hey, I like redundancy.)

 

[BABAR]

You need a E9-4 motor, tape, a crochet hook (yes, a crochet hook), and two #16 rubber bands.)
Insert the E9-4 motor into the tail. If you DIDN’T put in an engine hook (I didn’t- but wished I did) you can friction fit and/or use a piece of tape on the outside to prevent engine ejection (I do both.)
Fold the Flop-Hinge closed (the Inner-Rotors folded flat against the Outer-Rotors).
Fold the Outer-Rotors downward to boost position. The engine should be just sticking out below the Stand-Off Adapter.
When you first do this, easier to have someone else hold the rocket.

 

[BABAR]

Use a large crochet hook; go through the hole of one Rotor-Fin, through the Burn Chamber, and out hole in the OPPOSITE Rotor Fin. Using your non-dominant hand (in my case, my left), loop a rubber band around your 4rth or 5th finger. Grab the free edge of the band with the crochet hook. Pull it back complete through the rocket. Loop this side through your index finger of you dominant hand. Pull the rubber band from both sides so the middle of the band is about in the middle of the rocket. Pull the loop on your dominant index finger through the middle of the Pull strings and over the motor. Make SURE that the rubber band goes over the tail edge of the Standoff Adapter and NOT in between the rotor fins (it will “want” to slip between the rotors.) Now take the other loop of the rubber band (on your non-dominant hand) and pull IT through the pull strings and over the Motor.

 

[BABAR]

These pics show placing the rubber band over the engine "peg", i.e., the protruding segment of the engine out the tail.

I used an extra large rubber band on some of the pics so you can see it better. The number 16 band doesn't show up as well in the pics.

You do have to thread the band THROUGH the Pull Loops as well.

All this seems tricky the first few times you do it, but it gets easier. Really!


I like the rubber bands as they provide a very "tight" pull that is hard to get (in my limited experience) with burn strings. Note that the rubber bands are pulled tightly across the "burn chamber" between the front of the motor and the nose cone deflector. When ejection fires, the rubber bands break and the loose ends are pulled out through the holes. The ejections gases continue forward and are deflected laterally by the nose cone with the mylar. I believe the deflected gases also add in rotor deployment, literally blowing the rotors outward.

 

[BABAR]

Turn the rocket 90 degrees along the long axis. Do the same thing with the second rubber band and the other two Rotor Fins. To avoid catching on the rubber band in place, I initially aim the crochet hook a little nose-ward as I insert it, and I keep the hook facing nose-ward. Again, you must thread the band through the middle of the pull strings, and you must make sure the band rests against the tail edge of the Stand-Off Adapter and NOT between the Rotor-Fins.


Notice the little strip of carbon fiber just tailward of the hole. This keeps the band from “cutting into” the balsa.

The second picture shows a WRONG placement of the rubber band, note it slips between two Rotor Fins. Use the crochet hook to pull it back over the true tail end of the rotor stop.

The third picture shows appropriate position.

 

[BABAR]

You will have 8 pull bands (2 on each of the nose dowel loops).

Pull these down over the S-Hooks. To keep from having too much un-matched pull in any one direction, I usually will go around the rocket, attaching 1 on each side, then go around again to attach the second on each side.

 

[BABAR]

First pic shows again the attachment of the S-Hook.

Once they are all attached, I pull each pair outward and place it in the “trough” on the end of the Rotor-Stops.

I believe this provides a better "moment arm" to pull the Rotor Fins out at apogee.

 

[BABAR]

Here is what one rotor fin looks like when deployed. For scale, you have a one foot ruler and a 2 foot rocketeer

Also a shot looking "down the barrel" at the fully prepped rocket.

Last picture is staged. It didn't really land in the soccer goal, (it was close) but I put it in there to show all rotors deployed.

 

[BABAR]

You are now ready to put in an igniter and launch as you would any other rocket.

Rocket Length in Boost Configuration: 38.5”
Weight without engine: 300 grams.
Rotor Length: 60”
Engine E9-4

Average flight length, just under 60 seconds.

I launch three foot 3/16" rod and have not had any problems with stability.

Parts:
one sheet of 36" x 6" x 1/4" balsa
two sheets of 36" x 6" x 3/32" balsa
Duct Tape
Dental Floss
Kevlar Thread (for pull strings and nose dowel loops)
2 small paper clips to make s-hooks.
3/8 inch dowel 3 feet long.
BT-50 body tube
BT-50 nose cone (just about ANY shape nose cone will work)
Mylar Tape
Thrust Ring for D engine BT-50
E size engine hook recommended
4 short (1 cm) carbon fiber strips, 1/4" thick or less.
4 short (6 cm) carbon fiber rods, 1/8" is fine
Lots of #16 [2.5” x 1/16”) rubber bands (you can get a 1 Lb bag at Office Depot for $5.29. I don’t know how many are in a pound, but it is a lot. https://www.officedepot.com/a/products/854866/Office-Depot-Brand-Rubber-Bands-16/.) The rocket uses 24 for pull strings, which are re-usable (haven't had to replace them after 7 launches), and two "burn bands" are consumed at each launch.
Qualified Competition Rockets ejection protecting "plugs" for D-engine rockets for centering "rings" for the dowel. Note you can make centering rings out of lots of different materials, I just happened to have some of these.
Large Crochet Hook
3/16 inch launch lug, can cut them from a 1 inch segment.
Small piece of heavy card stock for centering squares.

Anyway, I had fun building and flying this. I'm sure there are lots of ways I could have made it better. I think this may upscale very well. If you build one or have ideas to make it better, please share.

Tom

 

[bradycros]

Thank you very much for posting all the information on your Whopper Flopper Chopper, I think it's really cool.

I have several projects that have the Spacely Cogs & Wigets already in motion. Those will keep me busy over the winter months.

However, you can be sure I'll be asking construction questions next spring.

Good build thread, thank you again for posting alot of useful information.

 

[geof]

I agree...thanks a lot for posting such detailed instructions. I'm going to start one this weekend. If I have any questions, I hope you don't mind if I contact you?

Geof

 

[BABAR]

Be glad to help.
I've gone back through these a few times, and I theeeenk:sigh: I found all the errors. I tried to emphasize the potential boo boos. Please make sure you think through it, I have a bit more respect for those who write rocket instructions now!

Also look for things that can be improved. I'm too lazy to do airfoiling (I was thinking about the Sandman, if you love to sand, there is a whole lot of lumber here to put you in paradise! with 4 60 inch rotors.)

Let me know how it comes out. Kind of fun to know someone else wants to try my design!

Tom

 

[geof]

How much BT50 do I need? I didn't see that specified above, but maybe I missed it. I'm rummaging through my stuff to see what rocket I can use to cannibalize enough tube.

G

 

[BABAR]

I used an 11 inch segment. Looking at the side view, you can probably get away with less. These is a segment that I cut out, I think it is easier to put the carbon fiber struts on with the tube in place, then cut the section out, then to try to line up two separate, shorter tubes.

 

[hcmbanjo]

Hi Tom,
Thanks for taking the time to do the build thread.
Great work on a fun model - this one is added to my to do list.
Duct tape on a helicopter, who would have thought?

 

[BABAR]

I'd consider it an honor. Please let me know how your flies.

 

[geof]

Okay, the clone build is underway....

On post #2, the right-hand template image, the rotor stop inner angle is marked as 15 deg, but you clearly mean 75 deg.

Also in post #2, you offered an addendum with some angle adjustments. You have provided so many great up-close photos, but I'm having some difficulty seeing the "big picture" of what a completed rotor looks like.

Photo 1 in post #40 helps a bit. The 15 deg angle on the rotor stop determines the amount beyond "level" (i.e., parallel to the horizon) with which the (first segment of) the rotor rises as it extends outward from the body. So, you suggested flattening that angle. My understanding, therefore, is that if you choose a different angle, it does not affect any other angle or measurement in the design whatsoever. Is that right? If so, then I will try 8-10 deg perhaps.

(Edit: Figured this next part out: The other part of the addendum suggests reducing the "angles on the hinge". This one is not as clear to me. Is it correct that the "hinge angle" determines the angle of attack of the (first segment of) the blades?)

So, to reduce this attack angle to 10 deg means to change the angle in the left image in post #2. Are there any implications of this change for the subsequent build?

(Insertion: 10 deg noseward offset = 0.25in noseward, very nearly. Helps people without protractors or trig.)

If you have time to clarify, that would be great.

I'm also planning on airfoiling the blades a bit. My current plan is to do this before assembly, but leaving .5-1 inch of square profile at each end so that hinges and connections don't get all messed up. An alternative would be to do the sanding afterward, but I'm afraid that the whole thing will be too fragile at that point. Thoughts?

Geof

 

[geof]

Next slight unclarity. Post #2 says to use the template to cut the notch at the nose end. There is no measurement given. The template has the notch slightly off-center: 0.75 in from the lead edge to the tailward edge of the notch. If it were centered, the notch should be 1/16 further toward the tail edge. I'm going to make that revision unless you say otherwise.

Also in post #2, you say "Cut each section back to 31.5”. (Save the rest, you will use it for the Stand-Off Adapters.)". In post #3, you omit any indication of what piece of balsa is supposed to be taped to the outer rotor, except that you call it the "Hub-outside rotor". It took me a while (post #5 left diagram) to figure out, but this H-OR = S-OA = the remnant left over after making the cut in #2. Assuming that I am correct, I recommend that you rename this piece the "Hub Standoff" and add that clarification to post #3, and possibly add a copy of the #5 diagram to post #3.


G

By the way, with these questions (thanks!) and suggestions, I'm not trying to criticize your thread in the least! I think that your chopper is such a cool idea that I'm trying to help you edit out unclarities to produce the perfect build thread for posterity. Eventually, you may find a whole fleet of these craft in the field.

If you make changes, I'll delete these posts to keep the thread as clean as possible.

 

[gdjsky01]

Next slight unclarity.

By the way, with these questions (thanks!) and suggestions, I'm not trying to criticize your thread in the least! .

No no!!! You are doing all of us who come after you a great service!!! Hope you get prompt answers!
op:op:op:
:clap:

 

[geof]

Ok, I made my first mistake.

For the front hinge, post #3, I made two of them with the outside tape on the outside face and two with the outside tape on the inside face. I was careless.

Looking back at the instructions (which I should have done more & earlier!), I still found it a little confusing. Ultimately, I realized that the guidance is fine but not intuitive: it assumes that you are laying the blade perpendicular to the front edge of the table with the nose pointed away from you. Most people would work (I think) with the blade parallel to the edge of the table because that's the way desks/workbenches are shaped and also it enables them to have the nose closer to them. Thus (if I've got it right), one could say "Place the blade with the nose end to your right and the leading edge facing away from you." Then put the outer tape on top, etc.

I cut the bad two apart and redid them the other way. All fixed. I also put small drops of CA under the tape and lined every loose edge with CA. This type of hinge seems pretty bombproof.

G

 

[geof]

Post #7:

... and mark the Trail Edge 8.5” from the tail end. Connect the marks. Cut across this line. The longer section is the Rotor, which will be 31.5 inches long. This should match the dimensions on the diagram.

The mark should be at 8.25 if it is going to match the diagram and the other parts.

(BTW, I used 3/16 instead of 3/32 thick; the thinner stuff just seemed too fragile. We'll see later how this screws me up.)

 

[BABAR]

Geof

Sorry, been off-line for a bit.

Absolutely no offense taken with any corrections or suggestions. I'll try to make the corrections.

I'm afraid I'm an engineer wanna-be, but not sure I have the discipline for it. I dreamed this up, did a rough draft of it, but a lot of things didn't fall together until I actually started building it. I'll have to start writing it down as I go (what a concept, real engineer/scientist.) I DID take the pictures as I built it, at least that helped

Okay, trying to answer your questions.

On post #2, the right-hand template image, the rotor stop inner angle is marked as 15 deg, but you clearly mean 75 deg. The 15 degree (or 75 degree angle) on the HINGE gives you the “force” that makes the rotors spin when the rotors deploy, the heavy engine combined with the rotor drag orients the helicopter “upright” (engine down), and the helicopter falls/descends, generating an air current perpendicular to the rotors. If you remember those Christmas decorations with the four candles and the rotor thing on top, the ascending heated air rotates the Angels or Santas or whatever to spin.

The Rotor stop angle gives the rotors a certain amount of dihedral with the tips higher than the hub (at least dihedral is what it is called in the airplane/glider world.) I originally picked 15 degrees cuz that's what I use for my gliders. The dihedral in gliders helps with stability (I think of it as the part of the design which helps orient the glider up side up and down side down in the glide phase.) The "cost" of the dihedral is less blade directly perpendicular to the descent path (or parallel to the horizon.) Just as in a glider, there is a small cost in lift (for 15 degrees, 1-cos15 is about 3.5%.) One of my reviewers suggested that having a flatter angle would give me more lift (although if I do the math, I think it comes out to about 2%, which doesn't sound like much. He probably knows more than I do, however.) On my Gyskelion (build thread pending) I DO think I need the extra dihedral because it is engine eject, with only the paper nose and balsa hub pulling the down side down. On the Whopper Flopper Chopper (WFC) I have a long dowel and a reeeeallly heavy engine casing, I could probably go with no dihedral at all. Note you CANNOT have a zero angle on the hinge as the rotors won’t rotate (yes, I know that is not COMPLETELY true, but we will consider it so for purposes of the WFC.)



1. if you choose a different angle, it does not affect any other angle or measurement in the design whatsoever. With my design I don't think changing the angle makes any difference in any remaining flight characteristics or otherwise changes the build. Partly because a lot of the recovery system is probably "air brake recovery", meaning rotor rotation on my design (with no airfoiling) is simply big honking blades coming down perpendicular to the descent path-- the rotation just keeps it perpendicular. As der Micromeister pointed out when I dared post in the competition section some questions (and showed my blatent ignorance, lol), competition choppers require careful airfoiling and balancing, and the degree of airfoiling is dependent on the rotation speed (and some say actually the optimal airfoil changes along the length of the rotor as you get further from the hub, as relative to the air stream the tip ends are going much faster than the hub ends.) At least at this point, all that is beyond my abilities. As a Sport Flyer, I actually think I got lucky, the length of flight for the WFC is just right to be fun and exciting for me, and I don't think there is a great risk of thermaling this puppy away. Which is probably a good thing, as people think a 12 inch rocket landing in their yard or pool is cute. A 10 foot monstrosity landing in the pool or the park tennis court is pushing it-- even when it lands as softly as this puppy does. But I digress…..

I'm also planning on airfoiling the blades a bit.

This may sound stupid of me, but probably important to decide your goals with the build. “Basic Sport Flying” or “Long Duration Flights.” I kind of see this also when people talk about tapering fins.

I don’t know yet whether this design will ever be a competition bird (although at least it meets criteria for competition, as opposed to Gyskelion which is engine eject=auto-DQ’d.) As I said, I like my 50 to 60 second flights with only a few hundred yard runs at most. So if your (very logical goal) is long flight times, I THEEENK airfoiling will help, although I have no idea how much. I do know with this much lumber it is going to be a LOT of work (somewhere there is an experienced BAR who keeps posting on his signature he loves to sand. THIS is the bird for him!) I do know you won’t hurt this bird with sanding, especially with the thicknesses of balsa that we are using here (and you are going up a bit on the inner rotor.) Definitely agree with sanding BEFORE you assemble, definitely don’t airfoil the hub, and probably wise to leave a cm on each side of each hinge un-foiled. I would also leave a trail edge thickness of at least 1/16 inch if you are planning on flying this regularly for sport, I think if you taper it any finer you probably will end up with a lot of knicks in only a few flights.

Next slight unclarity. Post #2 says to use the template to cut the notch at the nose end. There is no measurement given.
My bad. That’s not a template, that’s just a diagram. Originally I had a template, but I didn’t bring it on the road with me, so I couldn’t post it and I forgot to correct it in the instructions. I have corrected the post and just given the location, the dimensions, and the diagram. Interestingly, this does not HAVE to be centered, it does absolutely have to be straight with the long axis of the rocket.


In post #3, you omit any indication of what piece of balsa is supposed to be taped to the outer rotor, except that you call it the "Hub-outside rotor".

Okay, understood. Corrected to say after the cut the Little Piece is the HUB, and the Big Piece is the OUTER ROTOR. I am going to stick with the term Stand-Off Adapter.

For the front hinge, post #3, I made two of them with the outside tape on the outside face and two with the outside tape on the inside face. I was careless.

You’re actually making me feel better. I’ve built around 30 helicopters using these tape hinges, and I’ve done what you did at least 10 times. I am afraid that in my case I always think of the nose as pointed away from me and the tail as toward me, and I diagram things and draw them that way. For me personally it is more intuitive, although I agree with you that when you are working with 30 plus inch rotors you are more likely to lay them sideways. My clarification for this was to place a black mark on the lead edge and try to refer to that in all my orientations (lead edge left, lead edge right, nose end away from you.) I thought about trying to label the surfaces inside and outside, but even that gets confusing, especially with the inner rotor, as the face that is truly “inside” at launch is contiguous at deployment with the outer rotor “outside” face. In fact, the OUTER ROTOR at launch is closer to the hub at deployment (and technically would be the “inner rotor”—vice versa for what I call the INNER ROTOR.)

I think I’m going to stay with marking the lead edge and trying to describe the orientations relative to that and the nose position. I have made an emphasis to the lead edge orientation in the instructions.

Originally Posted by BABAR
... and mark the Trail Edge 8.5” from the tail end. Connect the marks. Cut across this line. The longer section is the Rotor, which will be 31.5 inches long. This should match the dimensions on the diagram.
The mark should be at 8.25 if it is going to match the diagram and the other parts.


Oops. You are correct, and I have corrected the post. 5.5 plus the fin width of 2.75 = 8.25. Thanks!

(BTW, I used 3/16 instead of 3/32 thick; the thinner stuff just seemed too fragile. We'll see later how this screws me up.)

That MAAAAY be a problem. Because the Outer and Inner Rotors are concentric squares, the remaining internal diameter (the "lumen" of the square tube) will decrease if you increase the thickness of the Inner Rotor without increasing the WIDTH of the inner and outer rotors accordingly.) I'll try to post a pic in next post.

As for the reason I went with 3/32"--- As I was building, I was really getting concerned about weight, and I had never used an E engine before (I had originally planned on doing this on a D.) I suspect as you go over this you will come up with ways to save weight that will more than make up for the difference in thickness here. Your fins will also be stiffer, and therefore will flex less on descent, which is a plus for your change.

So from a STRUCTURAL standpoint upping to 3/16 should not be a problem, but from a DIMENSION standpoint it may leave the inside too crowded.

Tom

Honestly Geof, I definitely appreciate the critiques, and I apologize for my errors that messed you up!:bang:

Tom

 

[BABAR]

Here is the pic. Although exaggerated in the pic, you can see how increasing the thickness of the inner rotor without any other adjustment may compromise the interior space for the motor mount.

 

[BABAR]

One of the other reasons I did NOT airfoil was to obtain the tight "box" you see in the pics above. (okay, the real reason is I'm lazy.)

My reasoning (right or wrong) is that the "gaps" between the blades of most existing external blade choppers (e.g. RotaRoc and TiddlyWink) probably create a significant amount of drag. For WFC and Gyskelion, I was trying to create a true square (or triangular) "fuselage" or body tube.

In a Flop Rotor of this type however, only the OUTER ROTOR (the one on the outside on boost phase) contributes to the outer surface/circumference (square-umference?)

 

[geof]

Thanks for all that.

Your concentric square diagram makes me worried. I hate repairing broken balsa, so I'm reluctant to go back to the thinner rotor. However, I don't see an obvious solution. Here are some ideas

• Taper the width of the inner rotor so that the outer end is thinner. This would allow the square to "mostly" close, with the ajar portion rearward. Messy.
• Reduce the width of the inner rotor uniformly. Loss in lift.
• Allow the outer square to be open at the corners. Draggy; might require some sort of jury-rigged adaptation.
• Drastically airfoil and hope that does the trick. Sounds dubious, but see below.
• Revert to the design-specified thickness. Fragile.

Let's give the airfoiling strategy (for fixing the misfit) another moment. My inner rotor thickness is double yours. Suppose that my airfoiling thins the trailing edge by 50%, leaving the last 3/16 of the trailing edge as thick as your un-airfoiled version. Wouldn't this solve the problem? From your diagram, it looks like it would work.

Your further ideas about this are welcome. I think I'll proceed far enough so that I can "dryfit" the parts and understand the magnitude of the problem before deciding. If worse comes to worse, I'll start over on the inner rotors.

I'm a little worried (with my heavier inner rotors and reduced dihedral) whether the copter will orient itself correctly at deployment. With your version, is it every doubtful, or is it always easy and immediate?

My goals are simple: sport flying and craftsmanship. I don't care at all how high it goes or how near to record duration it achieves. I want it to go up, deploy at an altitude where the crowd can see, spin nicely for the crowd's amusement, and land without breaking. Also, I enjoy clean and careful building of elegant, creative and/or difficult designs. (Hence the airfoiling: for "style points", not performance.)

Geof

By the way, for this sort of sanding I use a handheld mouse sander. This makes really quick work of the job...indeed you need to be careful about removing too much in a moment of carelessness.

 

[BABAR]

Thanks for all that.

Your concentric square diagram makes me worried. I hate repairing broken balsa, so I'm reluctant to go back to the thinner rotor. However, I don't see an obvious solution. Here are some ideas

• Taper the width of the inner rotor so that the outer end is thinner. This would allow the square to "mostly" close, with the ajar portion rearward. Messy.
• Reduce the width of the inner rotor uniformly. Loss in lift.
• Allow the outer square to be open at the corners. Draggy; might require some sort of jury-rigged adaptation.
• Drastically airfoil and hope that does the trick. Sounds dubious, but see below.
• Revert to the design-specified thickness. Fragile.

Let's give the airfoiling strategy (for fixing the misfit) another moment. My inner rotor thickness is double yours. Suppose that my airfoiling thins the trailing edge by 50%, leaving the last 3/16 of the trailing edge as thick as your un-airfoiled version. Wouldn't this solve the problem? From your diagram, it looks like it would work.

Your further ideas about this are welcome. I think I'll proceed far enough so that I can "dryfit" the parts and understand the magnitude of the problem before deciding. If worse comes to worse, I'll start over on the inner rotors.

I'm a little worried (with my heavier inner rotors and reduced dihedral) whether the copter will orient itself correctly at deployment. With your version, is it every doubtful, or is it always easy and immediate?

My goals are simple: sport flying and craftsmanship. I don't care at all how high it goes or how near to record duration it achieves. I want it to go up, deploy at an altitude where the crowd can see, spin nicely for the crowd's amusement, and land without breaking. Also, I enjoy clean and careful building of elegant, creative and/or difficult designs. (Hence the airfoiling: for "style points", not performance.)

Geof

By the way, for this sort of sanding I use a handheld mouse sander. This makes really quick work of the job...indeed you need to be careful about removing too much in a moment of carelessness.

Okayyyyyy

Above the motor mount, you can just shave back the width of the inner rotors (drop each one by 3/64s, roughly) and you should be able to fit here with no problem, as above the motor mount you don't need much room.

The upper portion of the motor mount ABOVE the burn chamber CAN be smaller, or even "scrunched" to fit. You can even use a smaller nose cone-- dropping down to 20 mm may work.

You still have to fit a 24mm motor plus the motor mount tube in the tail. This section is only a few inches long. You MAAAAY simply be able to sand this area flat, but that may be tricky. Also, if I remember (I'm at work and can't go into too much detail.) the notches for the fins to come out are only 1/8 inch. If you use thicker fins, you'll need to widen those notches a bit, although it won't be very much. Most of your rotor remains the same thickness, although the width of the rotor is less. Will probably drop your lift a bit. As I've emphasized, as a Sport Bird, I think there is plenty of lumber on this to provide a safe descent not matter what you do. As long as these rotors open, even if incompletely, I think this bird is safe. This is also why I am not sure with this bird that the effort of sanding is worth the improvement in performance UNLESS you are going to compete with it, and not sure this is a great competition design even if you perfectly airfoil and balance it (but it might be.....) Biggest concern for this one is a lawn dart. Knock on wood, I have never had a deployment failure with this (which I am extremely pleased with, as it is a complex mechanism) At 330 grams without engine, if this comes in ballistic it will not be pretty. But that's true of ANY rocket of this weight (for you medium and high power guys, I know, I'm a wimp, but 330 grams is big for me!)

Regarding your concern for the thickness of the inner rotors strength--- all I can say is I have yet to have one break. The "load" on these rotors concentrated right at the outer hinge. This area is reinforced by the standoff adapter (1/4" balsa) and the width of the fins. That areas is NOT going to break. The rest of the load is distributed evenly along the length of the rotors, and it isn't that much.

I think the long dowel "stem" on this rocket associated with a relatively heavy motor mount and engine casing are more than sufficient to orient this bird even without ANY of the dihedral (I would not have suggested it otherwise.) I.e., with that long a moment arm I think the mass of the engine casing will pull this tail down following deployment.

If you can't trim down the tail end, you will not have as tight a square and you will get some drag on boost. I don't think it is a game ender, but you might not get as much altitude. I theeeeeenk :eyeroll:the ejection delay will still be sufficient to deploy near apogee. Definitely stay with the 4 second delay and not the 6.

Probably goes without saying, but strongly recommend NAR restrictions of having few people around when you launch this as an experimental bird.

Any other opinions on this from the peanut gallery?

Hope this helps

Tom

 

[geof]

I airfoiled the rotors (very roughly). The photo shows how. The edge of the board is an improvised jig to control the depth of my sanding, and tape protects the other edge. After that, very light touchups give a rounder surface. By no means is this the best I can do, but with forty feet of edges, I figured I would do something quick and rough. It took me a bit more than an hour including a quick manual touch-up.

My inner rotors (outer after deployment) are 3/16 rather than 3/32, and my fins are 1/8. I also decided to use a different fin shape, just for kicks.
Tom and I are having some back-channel discussions about some details based on my modifications, but all is going reasonably well up to this point.

G

 

[geof]

Deleted. Correction made to instructions.

 

[geof]

Huge error :jaw:

Nobody's fault but my own.

Look at the photo. The outer rotor is on top and the inner rotor is on bottom (where outer/inner are in your terminology...backwards to me). The parts are aligned pretty much like they will be assembled, except that the top piece of wood is shifted back to see everything clearly. I was at the stage of installing the hinge here.

Both leading edges are facing the camera. It's a little hard to see, but both rotors have been airfoiled. For both rotors, the round part is on top and the flat part underneath. In other words, both rotors are oriented like each would work (separately) on an airplane taxiing towards us on the tabletop.

Now you probably see the disaster. When the inner rotor flops around, its airfoil will be upside down! Ugh. I resigned myself to starting completely over with the inner rotors, but I have an easier solution, I think.

I will take the inner rotor and chop it off just inward (leftward in the photo) of the fin. Then I will flip the inner rotor lengthwise so that the leading edge is unchanged but the wing is upside down now (which makes it right side up after deployment). Then I'll glue the two pieces back together. It might seem crazy, but wood glue is stronger than balsa, so the repair should be okay. There will be a bit of ugliness at the new seam, but I'll clean that up somehow after I see what I've got.

I had already done one of the hinges. (Indeed, testing this is how I discovered my mistake). However, my planned fix won't require any alteration of that.

This build would be a whole lot easier if I had a complete prototype in front of me, or the inventor looking over my shoulder. Actually, that's what he's doing as a cybercoach. Thanks, Tom!

Geof