Wing attachment fittings

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AncientAviation

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Apr 24, 2020
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49
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England
Greetings all...
The job this week is to drill then ream the ali wing attachment fittings - and then secure them to the inner and outer spars with the right dihedral built in.

The wing joint is made up of 8x pairs of 2 ali bars - 4 on the outer spar (top, bottom, front & back) and 4 on the inner spar.

The plans essentially say glue the fitting to the spar. No mention of painting or treating the fittings.
Also, it is not clear if the 2 fittings should be glued together and then the whole unit glued to the spar or if only the ali fitting that touches the spar should be glued to help secure it and block out the moisture.
I like the idea of gluing the two fittings together first as it could then help with drilling and reaming a consistent hole placement through both...

I have some primer and Duralac anti corrosive jointing compound but not sure if they are needed.

A local inspector I spoke to on the phone was horrified that I would consider gluing the metal fitting to the wood and wasn't sure why there would be 2 as opposed to 1 beefy fitting....

(as an aside, the plans do say attach the fittings then drill them but if I mess the drilling / reaming process up it can be an expensive mistake...prefer to predrill then glue them)

So.... Any words of wisdom on best practice / sensible way of dealing with this would be welcome thank you....
 

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TFF

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Double shear is the normal way. Usually there is one strap on one side of the spar and a book end duplicate on the other. Glue is probably one corrosion and rot protection between the wood and metal straps. Also it helps hold it in place while you drill. Pre drill the metal straps, then you can do the wood part.
 

Pops

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How to drill the holes straight when installing wing fittings on a spar when the wing is built.

Make a "U" shaped saddle from scrap steel, weld a tube across the opening with the ID of the dia of the hole you will be drilling ( say 1/4". Cut the center of the tube out. C- clamp the fittings to the spar and put the "U" saddle over the spar with the round end of a 1/4" drill bit into the tube and into the hole in your fitting.
Drill from the other side with another 1/4" dia drill bit . That's one drill bit to line up the back side and drill from the other side.

DSCF0024[2].jpg
 

Victor Bravo

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Jul 30, 2014
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KWHP, Los Angeles CA, USA
If you want to do the best, safest job possible, find the correct "conversion coating" for the aluminum and do that treatment before you glue the metal to the wood. If you find the right process, it turns the aluminum surface into something that LIKES to bond/glue with epoxy, as opposed to plain aluminum which HATES to bond with anything.

Someone here on HBA will be able to figure out which exact conversion process... it's not just the regular "Alodine". There are conversion systems that are specifically for bonding aluminum into honeycomb, aerospace structural bonding, etc. If it is possible, take the time to find/source/acquire these chemicals (or find someplace that does aluminum bonding and have them throw your parts in with their next batch).

In conjunction with the bolts, this chemical process followed by glue will give you the very best, safest, most secure attachment. May be more than is really needed, but may be well worth the extra effort.
 

AncientAviation

Well-Known Member
Joined
Apr 24, 2020
Messages
49
Location
England
How to drill the holes straight when installing wing fittings on a spar when the wing is built.

Make a "U" shaped saddle from scrap steel, weld a tube across the opening with the ID of the dia of the hole you will be drilling ( say 1/4". Cut the center of the tube out. C- clamp the fittings to the spar and put the "U" saddle over the spar with the round end of a 1/4" drill bit into the tube and into the hole in your fitting.
Drill from the other side with another 1/4" dia drill bit . That's one drill bit to line up the back side and drill from the other side.

View attachment 119165
I remember seeing this picture somewhere ages ago and throught that it was super, super clever.... thanks....
 

AncientAviation

Well-Known Member
Joined
Apr 24, 2020
Messages
49
Location
England
If you want to do the best, safest job possible, find the correct "conversion coating" for the aluminum and do that treatment before you glue the metal to the wood. If you find the right process, it turns the aluminum surface into something that LIKES to bond/glue with epoxy, as opposed to plain aluminum which HATES to bond with anything.

Someone here on HBA will be able to figure out which exact conversion process... it's not just the regular "Alodine". There are conversion systems that are specifically for bonding aluminum into honeycomb, aerospace structural bonding, etc. If it is possible, take the time to find/source/acquire these chemicals (or find someplace that does aluminum bonding and have them throw your parts in with their next batch).

In conjunction with the bolts, this chemical process followed by glue will give you the very best, safest, most secure attachment. May be more than is really needed, but may be well worth the extra effort.
Learn something new everyday! Thanks...
Have done a very quick google read up and see the boat builders talk of a similar thing - or the hack approach of clean and degrease the part then rough it up and glue immediately with a glue that is not to exothermic... thanks for the input :)
 

PTAirco

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Sep 20, 2003
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3,670
Location
Corona CA
How to drill the holes straight when installing wing fittings on a spar when the wing is built.

Make a "U" shaped saddle from scrap steel, weld a tube across the opening with the ID of the dia of the hole you will be drilling ( say 1/4". Cut the center of the tube out. C- clamp the fittings to the spar and put the "U" saddle over the spar with the round end of a 1/4" drill bit into the tube and into the hole in your fitting.
Drill from the other side with another 1/4" dia drill bit . That's one drill bit to line up the back side and drill from the other side.

View attachment 119165
What he said!

I once had to build spars from scratch for a Callair cropduster, with no drawings. The spars were massive, almost 2.5 inches thick at the strut attachment points. There must have been 50 holes in each spar and and a drilling jig like Pops shows is the only way to do this accurately. I made mine with various bushings to allow anything from 3/16 to 3/8 bolt holes to be drilled with the same jig.
20160701_152755.jpg
 
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