strengthening balsa

[Andrew Smith 5]

Hi I am going to start to assemble my first plane, Eflite Sukhoi. I have read about weakness in motor box housing, and undercarriage , and have heard of people reinforcing with with diluted epoxy and brush on, is this the best method? And if so what is best to mix with the epoxy and what ratio. Any advice on this would be greatly appreciated or any other assembly tips would great. Thanks Andy

[Martin Harris - Moderator]

Diluted epoxy makes a great fuel proofer but hardly necessary in your case!  Epoxy resin adds virtually no strength on its own.

 

 What you could do is use carbon fibre tows, cloth, Kevlar or glass cloth to reinforce the structure using epoxy to bind it andfill the weave.  Failing (or complimenting) that then strategic balsa fillets can add a lot of strength for very little weight.

 

Thin cyano is useful for hardening balsa - soaked in to screw holes (after unscrewing the screw from the hole!) or into the grain and you can use it with baking soda or microballoons to form fillets. Watch out for the fumes though!

Edited By Martin Harris on 27/12/2010 16:22:44

[Biggles' Elder Brother - Moderator]

Hi Andrew,

 

martin has it down pat. Epoxy on its own wont help much. Unless the design is terribly flawed (which given Eflite's reputation isn't very likely) my guess is that carbon weave etc would be a tad overkill.

 

I would guess your best bet would be to take a good look at the components in question and imagine the loads on them. How are they going to deform? Twisting? Bending? Shearing? Once you have that sorted, the name of the game is to add some stiffeners or fillets to the part to resist that particular deformation. Triangular section hard balsa epoxyed in is your best bet for this job.

 

With undercarriages be a little careful. Its possible to make them too strong! Then, in the event of a hard landing, instead of the undercarriage retainer coming free (something you can fix easily) you end up with whole underneath of the fuselage ripping out - not nearly so easily fixed!

 

Final point to think on. Forces are like electricity. Electricity needs a wire to flow along. Forces need material to flow along just the same. Your aim when adding strength should be to provide a path for the forces to flow along so they can be absorbed by a major structural component like the firewall, a strong ply former or a harwood wing spar etc.

 

BEB

Edited By Biggles' Elder Brother - Moderator on 27/12/2010 16:27:11

[Andrew Smith 5]

Thanks for clearing that up for me Martin, I'm new to balsa construction, think I will use the ca route just to strengthen any joints I can see. Biggles thanks also for reply can see perfectly now what your getting at great explanation, agree on the quality of my Eflite plane does seem sturdy, too much trawling I think, In my manual for on the section Gluing the ca hinges in, it says wipe any excess away with paper towels, of my experiencethe paper towels leave fibres wherever the glue was am I correct? Thanks again

Edited By Andrew Smith 5 on 27/12/2010 16:49:27

[Martin Harris - Moderator]

A very light and quick wipe will do the job.  CA cures in the presence of moisture/absence of air so wiping it away quickly works OK - practice it on some scrap material (like a piece of covering, plastic bag etc.) first.

Edited By Martin Harris on 27/12/2010 16:55:53