Hangar Flying => Hangar Flying => Topic started by: Alrata on January 27, 2010, 05:12:56 PM
Title: Voids under wing skin
Post by: Alrata on January 27, 2010, 05:12:56 PM
Bought a Long Ez project that has some voids between the blocks of foam and several places under the skin next to the spar. Does anyone have any experence fixing these? Need advice
Title: Re: Voids under wing skin
Post by: go ez on February 01, 2010, 03:41:44 AM
Depends on whether you have voids (how do you know?) or delaminations where the skin has separated from the core.
If the latter, drill a couple of holes in the skin and inject microslurry (mixed epoxy & microballoons to a thin cream consistency) under the skin then apply a release film (cling film / saran wrap) over it then weight down (some use vacuum techniques) sufficient to get the skin to contact the foam. It is worth removing the weight / release film after the initial application to ensure that inevitable ( and preferable) excess slurry is extruded from the holes and then wiped away from the skin -then re-apply the release film & weight. Leave to cure for 48hrs depending on temeperature.
If voids are truely present, you need to cut away the skin and insert a foam plug with micro, re-shape and re-skin with adequate overlap onto surrounding bare glass to ensure strength & rigidity is retained.
Some areas on the airframe are much more critical if damaged than others so you also need to be aware of the implications of a repair.
Simple if you know how of course - if you don't, get help - ideally by joining CSA and getting a local buddy to help/advise.
Regards Steve
Title: Re: Voids under wing skin
Post by: Spodman on February 01, 2010, 10:59:35 AM
Small delaminations, or bubbles up to 2-inch diameter, may be filled with epoxy by drilling a small hole into the bubble and filling the void with epoxy. - Quote from the Cozy plans, what do yours say?
Not terribly keen on the micro idea, epoxy is required.
Also if these deficiencies are in a critical area - like over the spar - maybe the wing is no good. Expert advice and eyeballs needed on your machine I think
Title: Re: Voids under wing skin
Post by: go ez on February 02, 2010, 07:00:51 AM
I agree that epoxy would be right if the delamination was glass on glass ie say wing skin over spar cap but in that situation I think it would be essential to have a very close look at the cause.
However since all layups on foam are preceded by a micro / epoxy slurry to ensure good bonding, it doesnt seem unreasonable to use the same for re-establishing that bond.
It was suggested by Rutan for repairing the landing brake after it was found that the urethane near the reinforcing wood hard point was breaking up due to movement/working.
ie CP43 p4
"If you have already built your landing brake or are already flying, an appropriate repair/reinforcement is as follows: drill several 1/16" or 1/8" diameter holes through the skin below (or aft) of LB19 plywood insert. Inject pure epoxy or a very liquid slurry into each hole, forcing the slurry/epoxy into the damaged urethane foam. When this cures it will greatly stiffen this area of damaged foam. Remove the LB18 brackets and layup 4 plies of UND over the plywood insert and down the inside skin of the landing brake as shown."
I agree that pure epoxy would do the job too though, if a little heavier and I agree the plans & CP notes call out pure epoxy. Thanks for pointing that out. :)