Bending Plywood with Ammonia?

[PedroTheRocketNerd]

I am working on building a MPR (E-G) helicopter rocket, based around an LOC thick wall BT-80, with rotors made of 1/8" model airplane lite-ply. I am attempting to bend the rotors like those in Apogee's Gyro Chaser kit, by soaking them in cleaning ammonia from Home Depot to soften them and then bending them around a piece of 2" PVC. Has anyone ever tried anything like this? Can anybody please offer tips? I have had the rotors soaking for maybe an hour, and they are not nearly flexible enough. Is my method doomed, or should I leave them to soak longer so they get softer? Any and all advice is appreciated.

 

[dr wogz]

Plywood, by nature / definition isn't supposed to bend.

R/C plane 'lite ply' is usually just three layers, and is meant to bend into shape, not hold a shape. So you can bend it around wing ribs & formers. It's then glued in place.

Now, think of how some preformed plywood parts are made; think skate board decks. Layers of wood have glue applied, are then stacked together, then pressed into the shape they want, and then left in the press until the glue has cured. You then get a piece of plywood, in the shape / curvature you want, and no stresses in the ply wood trying to 'unbend' the piece.

I would suggest you ditch the plywood, and think about making your own "plywood in the shape you want". (Bending wood against the grain can be difficult.) Make one form as described in the Apogee video, then layer another layer of Balsa on top of that. Then add a third if you so desire. Alter the grain directions. Even a 30° degree offset can add strength.

Or, you can bend one piece to the desired curve, then laminate that curved piece with either paper, cloth & resin [glassing], or use Monokote to add some colour & strength (but Monokote can be quite tricky to use!) You can get 1/32" or even 1/64" plywood that is bendy. Skin your balsa form with one of these ultra thin plywoods to make a strong (and super tapered trailing edge!!) rotor blade.

Soaking balsa in really really hot water, with a splash of ammonia can also make it bendy & easily shaped.

Chances are, can probably get away with 3-5 layers of 1/32" balsa, varying the grain direction slightly, and won't need to soak it . The wood is soft enough to bend to the radius you want.

 

[PedroTheRocketNerd]

Thank you for your advice. My worry is about rotor strength when they hit the ground. Your balsa plywood idea seems like it would work and be strong enough; however, making 4 blades with 4 layers each seems like it would be tedious. Do you think a single piece of papered 1/4" balsa would bend if soaked, and would it be sturdy enough? If it won't, I will laminate 1/32 balsa. Also, how do you get the 1/32 balsa to hold its shape? You say you can bend it without breaking, but it seems like it would tend to flex back into its original shape. How do you prevent that?

 

[mcderek]

Chances are, can probably get away with 3-5 layers of 1/32" balsa, varying the grain direction slightly, and won't need to soak it . The wood is soft enough to bend to the radius you want.

Good advice. For bending you want A grain balsa. This link has the pertinent information.

Balsa Grain Classification

 

[mcderek]

Also, how do you get the 1/32 balsa to hold its shape? You say you can bend it without breaking, but it seems like it would tend to flex back into its original shape. How do you prevent that?

The same process used to mold balsa propellers should work. They are usually covered with light fiber glass. here is one way to do it:

An easier way to make balsa blades

 

[HVArcas]

awesome discussion, thanks for sharing

 

[PedroTheRocketNerd]

The same process used to mold balsa propellers should work. They are usually covered with light fiber glass. here is one way to do it:

An easier way to make balsa blades

Thank you, mcderek, for that link. It seems that, per the article you linked, I could shape 1/4 inch balsa using the method described there. Do you believe that would be strong enough to withstand landing (hitting the ground while spinning) if I papered it, or would I have to go with a multi piece laminated balsa rotor?

 

[dr wogz]

Pedro, i would approach this as described, 3-5 layers. it's going to take a bit of time & patience. And even a bit of experimentation

Exactly what I would do it this:

either:

As you initially describe, curve the sheets around the PVC form with hot water. Soak the wood, then wrap it around the form, then let dry. lay down the wood, then wrap it all in a few sheet of newspaper (to absorb the water) and finally in a few wraps of card stock / something stiff but bendy. Then secure it all with elastics to hold the shape. This will take a day or two to fully "cure". And you'll need any where from 12 - 20 curved sheets.

Then on the form, apply glue to one side of each piece, position on the form, and wrap with card-stock and then add the elastics again. then let cure. Use a sheet of waxed paper both under the stack, and on top of the stack, so the glue doesn't glue the stack to the form or the outer card-stock layer! I would be using yellow wood glue, maybe thinned a bit..

Remember, balsa is soft, and the elastics will leave dents / compression lines, hence why the card stock to distribute its force over a larger area.

Or, forgo the soaking step, and just go straight the glue step. if the glue is watery enough, it'll do the soaking for you. watering down the glue is the key here.. not too much, just enough

In either method, clamp the layers as tightly as you can on your form to induce the shape / curve you want. It'll take a bit of practice.

I would then glass these, or skin them with the ultra thin ply (use the ply for my first layer on the form) the 2nd / out layer would be added once I've sanded in / shaped my profile. My leading edge would be a strip of bass wood or maple, something hard.

i would not try to form a solid 1/4" sheet, way to thick. more layers, the better. easier too!

What shape are you after? Do you want airfoils? or just curvy blades? You say 1/4".. is that determined form something, or it "just sounds right"? 3 pieces of 1/16" balsa would work too..

I believe the biggest issue you will have; your weakest point will be where these blades attach to the air frame. This is where they will likely break. You've got a long leaver flapping about, and this leaver will easily snap a small pin or connection. Have you given any though as to how you will be attaching these?

 

[dr wogz]

Another thought for yu is to take a page form boat builder: strip planking / hull planking / cedar strip planking.

it's essentially like how a wooden barrel is made. you have a lot of log 'staves' that fit together to make a curve.

lay down a piece of paper on your form. then add your first stave. the keep adding to the desired width.

you'll need a balsa stripper, or will need to buy a lot of 1/4" x 1/4" sticks, and sand the required angle into each one so you get a nice glue surface & curve.

 

[PedroTheRocketNerd]

What shape are you after? Do you want airfoils? or just curvy blades? You say 1/4".. is that determined form something, or it "just sounds right"? 3 pieces of 1/16" balsa would work too..

I believe the biggest issue you will have; your weakest point will be where these blades attach to the air frame. This is where they will likely break. You've got a long leaver flapping about, and this leaver will easily snap a small pin or connection. Have you given any though as to how you will be attaching these?

The shape I am after is something akin to that described in Peak of Flight #342 (https://www.apogeerockets.com/education/downloads/Newsletter342.pdf). The bend around a mandrel creates the blade's airfoil. I say 1/4" balsa because on BABAR's Whopper Flopper Chopper, which flew on Estes E engines, he used a mix of 3/16 and 1/4 inch balsa. I figured that, because I am building something bigger but still in the MPR range, and because shaping the blades adds stiffness, 1/4" balsa would work well.
As far as blade attachment goes, I am not attaching them to the airframe. They are attached to a 3D printed hub that spins freely around a central shaft, which shaft is attached to the nose cone at the top and to the shock cord at the bottom. The hub is made of PETG, with 1/4" plywood arms that will attach to the blades. The arms will be attached to the hub by 1/8" steel pins, which they will pivot around to allow the blades to deploy. If my description seems unclear, look up the Apogee Gyro Chaser; my mechanism is based heavily on its.

 

[dr wogz]

Yeah, what I figured, a flat, but cambered airfoil. with a 1*/4" you can give the top-side a bit of a sanding, to give it some "airfoil" shape.

You might also think of running a piece of hardwood about 1/3 the length of add some stiffness and to add a hard mounting point for the attachment

 

[PedroTheRocketNerd]

Yeah, what I figured, a flat, but cambered airfoil. with a 1*/4" you can give the top-side a bit of a sanding, to give it some "airfoil" shape.

You might also think of running a piece of hardwood about 1/3 the length of add some stiffness and to add a hard mounting point for the attachment

I like the idea of a hardwood spar. I will also likely taper the leading and trailing edges, simply to cut down on drag. I am currently continuing to soak my plywood blades in ammonia, just to see what happens, but if nothing comes of that in the next couple days (which based on your comments is a likely outcome), I will definitely do what you have recommended.

 

[dr wogz]

I had a friend who wanted a white jean jacket. So he left it in bleach overnight.

no more jean jacket.

Chances are the ammonia has broken down the wood fibers to the point of there not being any (or much) left.. And/or the glue will be compromised..

 

[PedroTheRocketNerd]

So the plywood blades are a lost cause? Then I will visit the hobby shop for some balsa.

 

[heada]

I was thinking that the ammonia might soften the glue between the plys but don't most plywoods use a urea resin glue? If so, I don't think the ammonia will do anything to that and all you might be doing is weakening the wood fibers.

Might look into door skins. They're 3 ply with the center ply being balsa and they're 1/8" thick and so should be fairly easy to bend on their own. Score the outer layer with a utility knife and should bend even better.

 

[PedroTheRocketNerd]

Door skins sound interesting. Where would I find them, a hardware store?

 

[Ronz Rocketz]

Thank you for your advice. My worry is about rotor strength when they hit the ground. Your balsa plywood idea seems like it would work and be strong enough; however, making 4 blades with 4 layers each seems like it would be tedious. Do you think a single piece of papered 1/4" balsa would bend if soaked, and would it be sturdy enough? If it won't, I will laminate 1/32 balsa. Also, how do you get the 1/32 balsa to hold its shape? You say you can bend it without breaking, but it seems like it would tend to flex back into its original shape. How do you prevent that?

Tim talks about the fragility of the blades and why helicopter rockets kinda suck. I'm still trying to get my Mini "A" Heli to work and I'm on my 4th attempt.

 

[rharshberger]

Door skins sound interesting. Where would I find them, a hardware store?

Door skins are not usually a very good quality ply, most are luan. Premade plywood is probably not a good choice for this project, making your own ply blades from balsa is probably much better.

 

[Ronz Rocketz]

Door skins are not usually a very good quality ply, most are luan. Premade plywood is probably not a good choice for this project, making your own ply blades from balsa is probably much better.

Plus sanding sealer works great in the porous balsa and doesn't warp the wood (more than you want it to)...

 

[heada]

Door skins are not usually a very good quality ply, most are luan. Premade plywood is probably not a good choice for this project, making your own ply blades from balsa is probably much better.

Agree wholeheartedly, just throwing it out there as a last resort option. Custom shaped laminate is the ideal.

You can get door skins/Luan at the bigger hardware stores like Home Depot, Menards or Lowe's.

 

[BABAR]

Is this for competition? If so, by all means go with bended blades, air foiling, the whole thing. This will likely get you maximum air time.

The DOWN side of this is that “this will likely get you maximum air time.” For a competition model, that’s what you want (unless it is windy, in which case the winning entry will be the one that came down just fast enough to recover and didn’t thermal away.). For a non-competition model, it means unless you have a big field and little wind, there is a good chance you will be saying “bye bye” on your first flight.

If you are flying for FUN (er.....ahem.... .I mean “Sport” for you so-called grown up rocket enthusiasts!) especially if your field isn’t huge and winds aren’t typically light, you can have a great time with perfectly flat, uncurled, unsanded blades, and save yourself a heck of a lot of work.

Given you are putting a lot of work into this, build and fly a flat bladed model and see how you like it’s performance.

One comment on the above posts and recently recommendations, at the point you get to 1/4” balsa and plying and glassing, I fear your addition of mas is going to far “outweigh” your improved efficiency.

Best wishes.

 

[OverTheTop]

making 4 blades with 4 layers each seems like it would be tedious.

Use an odd number of layers so the blades don't warp with changes in moisture (most commercial plywood has an odd number of layers). Make sure the layers on each side are in the same alignment and orientation.

 

[Rktman]

Have you considered the alternative of making a partial cut in the rotor diagonally and then bending it to achieve the curvature? Sport models like eRocket's Heli-Roctor and Este's Mini A Heli (and even for some larger competition models with external rotors) employ this technique. It'll allow you to use whatever thickness of balsa or ply you choose. Some example pics below.



It sounds like your blades are external, so it's not as essential to curve them to get them to fit inside the BT. While it does streamline the rotors and reduce drag because curving gets them to conform closer to your body tube, if maximum performance is not your goal, then I'd suggest achieving the curve using the angled cut method. A "how-to" explanation can be found in #2 in this thread: https://www.rocketryforum.com/showthread.php?139554-Helicopter-recovery

 

[PedroTheRocketNerd]

Is this for competition? If so, by all means go with bended blades, air foiling, the whole thing. This will likely get you maximum air time.

The DOWN side of this is that “this will likely get you maximum air time.” For a competition model, that’s what you want (unless it is windy, in which case the winning entry will be the one that came down just fast enough to recover and didn’t thermal away.). For a non-competition model, it means unless you have a big field and little wind, there is a good chance you will be saying “bye bye” on your first flight.

If you are flying for FUN (er.....ahem.... .I mean “Sport” for you so-called grown up rocket enthusiasts!) especially if your field isn’t huge and winds aren’t typically light, you can have a great time with perfectly flat, uncurled, unsanded blades, and save yourself a heck of a lot of work.

Given you are putting a lot of work into this, build and fly a flat bladed model and see how you like it’s performance.

One comment on the above posts and recently recommendations, at the point you get to 1/4” balsa and plying and glassing, I fear your addition of mas is going to far “outweigh” your improved efficiency.

Best wishes.

I am doing curved internal rotors because this rocket is designed to fly on anything up to a 29/40-120 G reload. I would worry about aerodynamic stresses on the balsa helicopter blades if they were external. I am not plying or glassing; at this point, my plan is to do 3 layers of 1/16" balsa, with a slightly different grain angle on each layer. I will bend the blades around my mandrel first, before gluing them together.

Use an odd number of layers so the blades don't warp with changes in moisture (most commercial plywood has an odd number of layers). Make sure the layers on each side are in the same alignment and orientation.

That is my plan, more or less. As I said above, my blades will be 3-ply 1/16" balsa. However, balsa bends easier with the grain, so my grain isn't perpendicular layer to layer, instead differing by maybe 20-30 degrees.

Have you considered the alternative of making a partial cut in the rotor diagonally and then bending it to achieve the curvature? Sport models like eRocket's Heli-Roctor and Este's Mini A Heli (and even for some larger competition models with external rotors) employ this technique. It'll allow you to use whatever thickness of balsa or ply you choose. Some example pics below.
View attachment 418522
View attachment 418523

It sounds like your blades are external, so it's not as essential to curve them to get them to fit inside the BT. While it does streamline the rotors and reduce drag because curving gets them to conform closer to your body tube, if maximum performance is not your goal, then I'd suggest achieving the curve using the angled cut method. A "how-to" explanation can be found in #2 in this thread: https://www.rocketryforum.com/showthread.php?139554-Helicopter-recovery

These are internal rotors, so the curve is intended to get them to fit inside the BT, as well as creating an airfoil to get the rotors to spin up and create lift.

Thanks to everybody for all your great advice. The "balsa plywood" method is what I am going to use, with 3 layers of 1/16" balsa per blade. I will soak the balsa in hot water before bending the blades around my mandrel, then I will let them dry in shape before gluing them together. Does anybody see any problems with this method? I will report on how the test flight goes when it happens; my goal is sometime toward the end of June, as I have already got most of the rocket built. All I have left to do now is curving and gluing the blades, assembling the rotor hub, and 3D-printing the nose cone.