Best method to bend balsa

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Just a followup: Much better results this time using A grade flexible/soft competition balsa and 1st wetting it down with an ammonia cleaning solution and letting it soak for 1/2 hour, then soaking it in 120° water for 1 1/2 hours. (See the pics of the previous attempts and my latest try).


The other thing I did differently was wrapping it around the outside of the form (5" OD PVC coupler) rather than trying to cram it on the inside of the form (which is only about 4 1/4" in diameter).


I also used flexible self-adhesive Ace bandages wound around the outside in spiral-fashion to hold it in place -- much better than closthespins. It holds tighter without leaving dent marks. I think I've also figured out that I'll start wrapping from the middle and work my way to the ends of the balsa strips, which are the most resistant to bending.


Thanks everyone for the all the helpful feedback. I hope my learning experience helps anyone else considering bending balsa strips the long way into really tight curves.

IMG_8687.jpgIMG_8689.jpg
 
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I use a method called planking, like the method of planking an old sailing ship. I use a balsa stripper, canted it on an angle and just start stripping anything from 1/4" to 3/8" pieces for covering round fuselages and engine nacelles on my RC planes. IMG_0011.jpg This is the stripper itself. IMG_0019.jpg These are the strips I made. IMG_0018.jpg These are what the ends look like. IMG_0020 (1).jpg These are the pieces being laid down on an engine nacelle. IMG_0034.jpg This is half the nacelle being filled with lightweight spackle. IMG_0037.jpg And the finished product after sanding prior to glassing. I also use CA glue for this. It speeds things up real quick.
 
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What I thought, you were trying to bend the wood against the grain, not with it. Very hard, as you've found out. I think we all assumed you were trying to bend it the other way; what is considered "normal". Boiling water next time. that helps a lot!

Can I ask why you wanted to bend it this way?
 
What I thought, you were trying to bend the wood against the grain, not with it. Very hard, as you've found out. I think we all assumed you were trying to bend it the other way; what is considered "normal". Boiling water next time. that helps a lot!

Can I ask why you wanted to bend it this way?

I'm trying to build a ring glider (annular wing glider) from old plans that appeared in a 2003 issue of the Sport Rocketry publication. It's pretty cool looking, but I've since come across some inaccuracies in dimensions and technique in the instructions. This is just one of them.


Anyway, I picked up some calculator paper rolls to use to roll paper rings if this doesn't work out. It seems far easier and less frustrating to go that route (though admittedly it won't look quite as cool as wooden rings).
 
What I thought, you were trying to bend the wood against the grain, not with it. Very hard, as you've found out. I think we all assumed you were trying to bend it the other way; what is considered "normal". Boiling water next time. that helps a lot!

Can I ask why you wanted to bend it this way?

BTW, I've been trying to find out if curving the rotor blades on a helicopter rocket has the same effect as airfoiling it? You're definitely a whole lot more knowledgeable about the sport than I am at this point, since I just got lured back into it after decades of being away from it.
 
1) I would go with stiffer paper, like card stock for making paper rings. Using the calculator paper will just come out heavy with all the glue being added ( no pun intended!) Cardstock, Bristol / poster board (be careful, as that stuff have a grain to it too!)

2) a thin paper wrapped around your form, then balsa wood added to it, then more paper around that to make a paper-balsa-paper sandwich (with glue) should be fairly strong for your intent. (with the balsa wood grain running parallel to the cylinder's axis)..

As for air foiling your rotor blades.. I have to ask. a helicopter rocket refers to what? helicopter recover from a standard rocket (uses blades instead of a streamer or parachute) or are we talking about a blade with a rocket motor at the tip to induce spin, therefore to induce flight..

For the latter, (which I believe you are trying to achieve) look at a propeller blade, which is essentially what you're creating. It has a twist and a airfoil. The airfoil creates the lift due to it's design. The angle or pitch also helps with the lift, but also adds to the drag. As for the twist; to be efficient, the same amount of lift should be expected along the length of the blade. So, a blade, spinning about a central point, the tip will be spinning faster than the root. (the tip travels a greater distance than a point, say, mid point of the blade for the same amount of time, therefore going faster.) So the tip really doesn't need a great pitch due to its speed to generate the same list as a point, say, a third the distance from the center of rotation. That point, a third the distance from the center of rotation, needs a greater airfoil and a greater pitch to achieve the same amount of lift as the tip. Now the real trick is figuring out how much 'pitch' you need along the length... I definitely don't know that answer, but there is math involved as well as experimentation..
 
1) I would go with stiffer paper, like card stock for making paper rings. Using the calculator paper will just come out heavy with all the glue being added ( no pun intended!) Cardstock, Bristol / poster board (be careful, as that stuff have a grain to it too!)

2) a thin paper wrapped around your form, then balsa wood added to it, then more paper around that to make a paper-balsa-paper sandwich (with glue) should be fairly strong for your intent. (with the balsa wood grain running parallel to the cylinder's axis)..

As for air foiling your rotor blades.. I have to ask. a helicopter rocket refers to what? helicopter recover from a standard rocket (uses blades instead of a streamer or parachute) or are we talking about a blade with a rocket motor at the tip to induce spin, therefore to induce flight..

For the latter, (which I believe you are trying to achieve) look at a propeller blade, which is essentially what you're creating. It has a twist and a airfoil. The airfoil creates the lift due to it's design. The angle or pitch also helps with the lift, but also adds to the drag. As for the twist; to be efficient, the same amount of lift should be expected along the length of the blade. So, a blade, spinning about a central point, the tip will be spinning faster than the root. (the tip travels a greater distance than a point, say, mid point of the blade for the same amount of time, therefore going faster.) So the tip really doesn't need a great pitch due to its speed to generate the same list as a point, say, a third the distance from the center of rotation. That point, a third the distance from the center of rotation, needs a greater airfoil and a greater pitch to achieve the same amount of lift as the tip. Now the real trick is figuring out how much 'pitch' you need along the length... I definitely don't know that answer, but there is math involved as well as experimentation..

The former actually...just a rocket with helicopter recovery, nothing as exotic as rocket-driven blades (monocopters).
eRockets Heli-Roctor has rotors that are scored so they can be bent along the cut into a vaguely curved shape, and Apogee's Rotary Revolution competition model has truly curved blades.
 
Bending balsa -- the final challenge


I need help/suggestions on how to join the two ends of my 1/16" 360° balsa ring.
Instructions specify a lap joint but there are two problems with this:


1. The overlap is 1/8" thick and creates a huge lump that not even careful sanding can remove.
2. It becomes tweaked in that area, making the ring no long symmetrical.


I'd like to butt the two ends and glue them together instead. The problem is that is seeps under the balsa and glues it to the form. Slipping a scrap of paper under it will mean it's visible once I remove the ring from the form (the ring will not be painted).


Any advice/suggestion would be enormously appreciated. This is all that stands in the way of moving forward on my build
 
One technique to try is to use little biscuits of balsa. Usually like a diamond shape. Cut half diamonds out of each side of the butt joint where they match up, glue the diamond biscuits to one side, then the other. They can be sanded down flush like inlays afterwards. They can also be just straight strips of balsa or fingers. Kind of like a dovetail joint, but on a straight butt joint. You might experiment with a few types. If the balsa ring was thicker you might be able to use hidden biscuits, but for that thin, not sure if you could hide them in the middle, like dowels. You could try very short thin toothpick tips. Then there is the glue dowel method, where you just poke needle holes in each side of the joint so glue can get in there and adds more contact surface, it makes a little hidden dowel of glue for a little extra strength, but that does not help you line the joint up.

Wax paper on the form would keep the glue from sticking.
 
One technique to try is to use little biscuits of balsa. Usually like a diamond shape. Cut half diamonds out of each side of the butt joint where they match up, glue the diamond biscuits to one side, then the other. They can be sanded down flush like inlays afterwards. They can also be just straight strips of balsa or fingers. Kind of like a dovetail joint, but on a straight butt joint. You might experiment with a few types. If the balsa ring was thicker you might be able to use hidden biscuits, but for that thin, not sure if you could hide them in the middle, like dowels. You could try very short thin toothpick tips. Then there is the glue dowel method, where you just poke needle holes in each side of the joint so glue can get in there and adds more contact surface, it makes a little hidden dowel of glue for a little extra strength, but that does not help you line the joint up.



Wax paper on the form would keep the glue from sticking.

Does white glue really not stick to wax paper? That sounds like the solution I need!

When I tried to glue the butt joint before, it oozed under the balsa and glued it to the round PVC form underneath that I'm using. Grrrr another perfect ring ruined.
I want to keep the balsa on the form while gluing though because it keeps it perfectly round and aligned. So I'd still have the same problem of glue seepage down to the form if I used any kind of dovetailing method.
 
there is a small amount of adhesion between white glue and wax paper(uncreased) but can be peeled off readily.
Rex
 
Timing is the key, the glue will bond to the wood faster. If you let the wood joint set for a bit first, then you can remove it from the wax paper while it is still a little wet and wipe off the excess.
 
Or you an slip the ring out of the form and peel the paper from the interior of the ring.
 
I've been watching this thread with interest. I hope some nice pictures of the ring / glued ring will be forthcoming.

My attempts at fashioning thin, strong rings have been poor so far. Really interested in this.
 
If I may suggest, to glue the two ends of your ring. Instead of a butt joint, a lap joint will be stronger. 1) wrap / form your ring. ensure you have 1/2" - 1" overlap. 2) once dry & formed, sand a shallow angle into the underside of the top overlap. Sand a matching shallow angle in the top of the bottom overlap. When these two come together, you should have an angled lap joint where you but joint would be. more bonding surface..
 
I've been watching this thread with interest. I hope some nice pictures of the ring / glued ring will be forthcoming.

My attempts at fashioning thin, strong rings have been poor so far. Really interested in this.

Marc_G If things turn out as good as I hope they will, I will definitely provide pics and feedback (fingers crossed).
 
If I may suggest, to glue the two ends of your ring. Instead of a butt joint, a lap joint will be stronger. 1) wrap / form your ring. ensure you have 1/2" - 1" overlap. 2) once dry & formed, sand a shallow angle into the underside of the top overlap. Sand a matching shallow angle in the top of the bottom overlap. When these two come together, you should have an angled lap joint where you but joint would be. more bonding surface..

dr wogz Picking up more A grade pliable balsa today just in case the butt joint doesn't work out. I want to try presanding bevels into a lap joint version to see which method works out better and more consistently uniform in symmetry anyway. The fins will be glued to the inside of the ring (one directly over the joint) and I'll be applying Wood Hardener for strength so I'm sure things will be well supported and strong enough for C engines going either route.
 
First, your ring looks great. I really didn't think it was do-able. How much does it weigh?

BABAR BTW, I rolled a paper ring using thin calculator paper and it weighed a pretty shocking 0.4 oz. I should have used poster board, as it took 6 layers and it's still somewhat "wobbly". Anyway, a balsa ring looks way cooler if I can finally nail down a reliable technique.
 
BABAR BTW, I rolled a paper ring using thin calculator paper and it weighed a pretty shocking 0.4 oz. I should have used poster board, as it took 6 layers and it's still somewhat "wobbly". Anyway, a balsa ring looks way cooler if I can finally nail down a reliable technique.

Interesting.
I am guessing about 16 inches of balsa went into this radial wing. is that just the weight of the balsa strip you started with?
If you get wouldn't mind giving me the dimensions, I will weigh out an equivalent cut out from a Quaker Oats carton.
 
Interesting.
I am guessing about 16 inches of balsa went into this radial wing. is that just the weight of the balsa strip you started with?
If you get wouldn't mind giving me the dimensions, I will weigh out an equivalent cut out from a Quaker Oats carton.

BABAR Good guess. It's 17" to allow for the overlap. I also used A grade lightweight/flexible balsa.The Quaker Oats carton sounds like a great idea since it's already preformed into a ring. Let me know how it compares weight-wise.
 
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