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Author Topic: Glue Advice Needed  (Read 335 times)
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Brian Z
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« on: June 12, 2018, 05:40:07 PM »

I'm attempting to make my motor sticks lighter with the balsa I have available, so I took a piece of 1/16" and laser cut it with a triangle pattern.  I also cut two matching pieces without the triangles out of 1/32" balsa, and I stuck them together with 3M Super 77 spray adhesive.  It looked great and was lighter, but as I suspected it wasn't very stiff.  I built a plane out of it and it few in random directions as the motor stick actually twisted with the torque of the rubber, negating my horizontal stabilizer's angle with relationship to the wing for the first part of the flight, and I didn't wind it hard at all.  The 3M Super 77 never really "sets" so it's behaving like three pieces of wood next to each other rather than one single piece.  What kind of glue would you suggest for this task?  I only want to put the glue on the 1/16" piece for weight savings, but I can't just stick Titebond on there without it going into the triangle holes.  Maybe I should dab it on with a sponge.  This whole endeavor is probably a waste of time, and the weight of the glue will probably equal the missing triangles, but I would like to see it through and measure the final weight with glue so I can be sure.
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mkirda
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« Reply #1 on: June 12, 2018, 10:05:54 PM »

Thinned duco or thinned titebond. Not sure which would be lighter.
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calgoddard
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« Reply #2 on: June 12, 2018, 10:17:31 PM »

Brian -

I wrote my reply below before reading mkirda's reply.

Try a 70/30 mixture of acetone and Duco adhesive. The latter can be purchased at ACE Hardware and Walmart.  It is commonly used in indoor model airplane building. Coat each surface to be joined with the mixture using a small brush and then let these surfaces dry. Then coat one of the surfaces again and join the pieces. Hold them tightly together for at least five minutes using weights and a flat surface. This type of glue mixture adds very little weight as much of the acetone just evaporates as the glue cures.

You could also try a 50/50 mixture of Elmer's white glue and water.

For indoor duration model classes with a relatively high minimum weight, such as Wright Stuff, TSA and P-18, a solid balsa motor stick is just fine.  Select your solid balsa stick carefully for weight and stiffness. The latter can be tested with a crude flex meter.

If the minimum allowed weight is relatively low, then a rolled motor stick would be the choice, provided the rules allow the same.  If you are using the motor stick in a Wright Stuff model, you can roll a blank of 1/32 sheet balsa around a 1/4 inch mandrel, such as an arrow shaft.  The techniques for doing this are covered extensively on this web site.  Extra structure will be needed to reinforce a rolled motor stick of an indoor duration model at the locations where the prop shaft bearing and the rear motor hook are mounted. It is a lot of extra work to make a reinforced rolled motor stick and this extra work is not justified if the minimum weight is 7 g or 7.5 g.

A simple, light weight motor stick that resists both bending and twisting can be made by gluing a 1/8 x 1/8 stick to one of the taller sides of a 1/8 x 1/4 stick to produce an L-shaped cross-section.

The lightest models I have made are A-6 and Penny Plane models. I have not had the skill to build an F1D or F1L model. I really appreciate the patience and effort of those who build such fragile models.

I admire the ingenuity of your laminated motor stick design.  
« Last Edit: June 12, 2018, 10:29:46 PM by calgoddard » Logged
Hepcat
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« Reply #3 on: June 13, 2018, 07:57:21 AM »

Brian,
It may be cruel to say that your laser cut diamonds will be of no use (because the grain direction makes them weak and being in the centre of the stick they do little to resist torsion). For a stick to resist torsion it needs as large a cross section as is practical considering such things as weight and space.  A tube of some sort is usually preferred with a circular or square shape.  It may be your 1/16th wood would roll into a circular tube and a square one would be very easy to make. Don’t forget that a square tube made of longerons and spacers covered in tissue, like an outdoor model fuselage, would be light and strong.
As Cal Goddard has already said there is nothing better for glueing balsa than balsa cement thinned with acetone.
John.
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Crabby
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« Reply #4 on: June 13, 2018, 08:57:50 AM »

To get 3M Super 77 to bond permanently, you have to use it like contact cement. That is, coat all surfaces that meet then let dry. Then LIGHTLY spray said surfaces and join. Roll with hand roller, or weight down. I would not be using it on balsa when there are cements that are specifically designed for the purpose!
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Brian Z
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« Reply #5 on: June 13, 2018, 10:21:47 AM »

Thanks everybody for the advice.  I don't really expect this to work better than a single stick of quality balsa, but I've got this laser cutter, and a local hobby shop that specializes in home built RC airplanes that has literal barrels of old 1/16" balsa on discount.  They're not amazing, with slight defects here and there, and the balsa density varies wildly in each piece, but for as cheap as they are I can pick and choose where I cut my parts.  Plus, I'm on summer vacation (I'm a high school computer aided drafting teacher) and my wife is out of town. 

What I should be working on is an all-plastic propeller with a 3D printed hub and spars and helical bucket blades designed with that spreadsheet and cut from a thin plastic cup I get frozen custard in.  That, and experimenting with the flaring Ikara props I now have.
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Brian Z
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« Reply #6 on: June 13, 2018, 03:17:06 PM »

Well I went in to school and designed and 3D printed a hub pretty much like an Ikara has, and cut my bucket props out of a plastic cup using the data from the bucket prop spreadsheet.  Unfortunately none of my glues will stick to the plastic cup blades.  Its recycle symbol is #5, so that makes it polypropylene.  The Mountain Dew bottle I used last time worked like a champ, but it's not tapered and it's a bit thick.  I'm not sure what my next step will be on this.

I bought some Duco glue and made a new motor stick/fuselage, and the glue held great and it was MUCH stiffer than the first try.  Unfortunately, it wasn't as stiff as a single piece of balsa, nor was it much lighter.  It was also difficult to get that last coat of Duco on fast enough for it to be wet when I put the parts together. 

In conclusion, not much worked out well today, but I learned a few things.
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ceandra
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« Reply #7 on: June 13, 2018, 03:41:44 PM »

Brian:

1. I would make the spar portion of the hub a bit longer for a glue joint along a larger portion of the blade.
2. Try sanding the blade at the glue joint a bit, then using CA (thin). That worked for us
3. If the cup material still won't glue, try the red plastic Solo Beer Cups. Sanded lightly that worked just fine with CA

I am assuming the glue is sticking fine to the printed part. we used carbon for our spars.

Chuck
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Crabby
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« Reply #8 on: June 13, 2018, 03:53:17 PM »

Work Duco just like contact cement. Practice makes perfect.
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« Reply #9 on: June 13, 2018, 04:04:29 PM »

Just my 2-pence worth.

To me all balsa cement like Duco, UHU which are diluite with fast evaporating thinner are not the ideal glue for large surfaces. Balsa cement is perfect for spot gluing, not for large surfaces because it does not give you time to spread, position and weigh down the pieces to be glued. For flat, large surfaces you need a slow drying glue. Aliphatic or PVA come to mind, maybe thinned with more water to keep it light. But also with low viscosity epoxy you can have light and excellent results.

Contact glues like 3M 77 remains always rubbery and the pieces can move one against the other.

Urs
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Olbill
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« Reply #10 on: June 13, 2018, 06:09:20 PM »

I agree with Urs that Ambroid and Duco are bad choices for gluing large areas. I wouldn't even attempt to do what you are doing with either of those glues. I think the best two bets are a solid stick with careful wood selection or a rolled stick.
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Crtomir
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« Reply #11 on: June 17, 2018, 10:37:07 AM »

Well I went in to school and designed and 3D printed a hub pretty much like an Ikara has, and cut my bucket props out of a plastic cup using the data from the bucket prop spreadsheet.  Unfortunately none of my glues will stick to the plastic cup blades.  Its recycle symbol is #5, so that makes it polypropylene.  The Mountain Dew bottle I used last time worked like a champ, but it's not tapered and it's a bit thick.  I'm not sure what my next step will be on this.

We tried to do the same thing:  build our own props from plastic cups.  We had the same issue with the glue not working.  I tried CA glue and model cement and Duco and others, but nothing held the plastic.  Finally, I found Loctite Plastics Bonder.  This worked like a charm.   You have this felt tip pen activator that you rub on the surface first. Then you apply the bonder, which feels like thick CA glue.  It requires 1-2 minutes of holding time before it's set, but it works well.  You should try it.


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