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Gluing ply doublers


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1 hour ago, Nigel R said:

How does pva dry in the middle of that surface area?

Many years ago I glued some doublers with PVA.  The plane met its end about 18 months later.  When I broke the fus. up for binning, one of the doublers split away, exposing uncured white glue at the centre!  I put his down to my use of cyano around the joint edges, effectively sealing it.  (This was not the cause of the crash, BTW!)

 

I still use pva, but thinly and evenly applied, and well slid around to get the joint to 'suck down'.  I may still tack with cyano, but leave  the joint edges mostly open so the pva can cure.

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Personally I’ve never had a bent or warped doubler construction but then, as Mike says, I apply it in thin layers. Another ‘trick’ I use is to run a covering iron over the ply after an hour or so. I first tried this when I discovered that an edge hadn’t stuck down so tried the covering iron and was pleasantly surprised at how well it worked!

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Thixofix used to be ideal for glueing doublers as it spreads like butter and repositioning was possible.  I dont think it's sold now.    As an alternative I have used Glue Film which is used in veneering and marquetry - needs heat from a covering iron rather like Solarfilm etc.    

You could use PVA and let it dry on one surface and then join the ply and balsa using heat from the iron.  

It is debatable whether we really need such large areas of 1/32 ply on models - maybe needed with glow but perhaps not on electric.  Perhaps half the normal height may be sufficient -say just a strip joining wing seat to u/c mount and then to the motor bulkhead might be enough if there is a lite ply tray to take the Lipo.    Saves weight and expensive ply. 

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3 hours ago, kc said:

is debatable whether we really need such large areas of 1/32 ply on models

 

I think it's as much convenience as anything. Manually cutting big holes to reduce surface area is a pain although it can be done. That said the doubler is usually in front of the cg so perhaps it makes little difference at the end of the day. 

 

With laser cutting making doublers with holes in is a piece of cake. 

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Thanks for the input fellas.

 

It makes sense that any water-based glue will soak much faster into the more porous balsa as opposed to the plywood, which is both of denser grain and has an impermeable barrier before the next ply.  Then, as the glue dries through evaporation, the stage is set for a differential shrinkage!  (A parallel is when veneering grown-up wood over a ply or MDF base:  balancing veneers are always laid on the opposite side for exactly this reason, even though the base is always a much thicker and therefore technically more rigid partner.)

 

I've dug out an old tin of Evostick contact adhesive... 🙂

 

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Old Bank card to spread it thin, jobs a good un.

 

This the chilli breeze? I think the doubler is quite narrow in this design? if so it shouldn't add that much weight. 

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20 hours ago, Andy Stephenson said:

I always substitute 1/64" where 1/32" ply doublers are specified even if it's part of a kit. No structural failures so far.

Brian Taylor never used ply doublers in his 10cc glow powered warbirds, the only ply he recommended was for the bulkhead and u/c mounts he only used hard balsa for dihedral reinforcing. 

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The Chilli Breeze plan shows 1/16" sides, a 1/32" ply doubler, and an additional 3/32" short doubler at the wing-seat - which all seems an entirely appropriate assemblage, especially for IC.

 

By contrast, my Gangster 63 Lite only had - as designed - 3/32" sides and nothing else except a short 3/32" balsa doubler between the ply firewall and the lite-ply tank former.  I beefed this up with three extra 3/32" pieces as far back as just aft of the former above the TE, especially as I felt the wing would be happier with a thicker seating etc.

 

DSC_0509.thumb.jpeg.73d448af89cf0bcf5c6a4eaad2dc91a6.jpeg

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9 hours ago, Eric Robson said:

Brian Taylor never used ply doublers in his 10cc glow powered warbirds, the only ply he recommended was for the bulkhead and u/c mounts he only used hard balsa for dihedral 

 

Were they intended for week in week or flying from grass patches though?

 

A small amount of thin ply can work wonders in making a strong box from firewall back to wing saddle. As noted it doesn't have to be the full height of the fuselage.

 

As an alternative i have used diagonal grain balsa. Or fairly chunky triangular stringers in the corners of the fuselage. 

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I may be wrong, but a Brian Taylor 10 cc 2 stroke shoved out a bit over a horse. So that is a cooking 40 in todays money..

He was a hero for light. But ultimately minimal structure will hunt out weak links. 
Nowadays, look at ARTF crash scenes. 

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12 hours ago, Nigel R said:

 

Were they intended for week in week or flying from grass patches though?

 

A small amount of thin ply can work wonders in making a strong box from firewall back to wing saddle. As noted it doesn't have to be the full height of the fuselage.

 

As an alternative i   1/16"used diagonal grain balsa. Or fairly chunky triangular stringers in the corners of the fuselage. 

I will find out this year Nigel when I fly my Tempest, the previous one lasted a while on grass but was badly damaged by a sick engine it would have been rebuilt but I was moving out of the area. 

Don, the under powered Merco did not help but the half a pound of lead did look for an escape route on impact. What ever engine was fitted the need to balance all gives extra strain on any airframe particularly the short nose variety.

I do use ply doublers when called for but on some of the early scale plans mainly C.A.P where the used 1/8" ply I would use a sandwich of 1/16" balsa and 1/16" ply. 

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23 hours ago, Eric Robson said:

Brian Taylor never used ply doublers in his 10cc glow powered warbirds, the only ply he recommended was for the bulkhead and u/c mounts he only used hard balsa for dihedral reinforcing. 

Brian Taylor's designs are up there with the very best available, I've built his Mk 1 Spit........however, the designs are light and designed mainly for competition use back in the day and not IMHO for everyday knockabout flying, with engines that when the original plans were published were smaller and less powerful than what we'd consider today. I know of one BT Spit that was built structurally as per plan and suffered a wing spar failure because of a heavier modern epoxy glass paint job and a decent 120 four stroke motor. The wing failed during a loop. The replacement wing (fuselage somehow survived for repair) was built with hardwood and strengthened spars rather than mainly 1/4 balsa and now flys very well through the full repertoire of Spitfire aerobatics.

Just my opinion, but with many designs that originated decades ago one has to take the best parts, but accept that changes may have to be made to make them practical flyers in 2022. Horses for courses.

 

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