>>
>
>Much of what you are asking is choice. I would agree with
> And, although wraparound leading edges can be
>used on built up wings, I don't know of anyone who does this
>with foam.
>
>Check with someone who makes foam wings and he can probably
>answer all of your questions.
Ron B.
F4Fguy
I've been doing this since my first foamies in ~1960/65it's easy and fast,but does require a great deal of care to avoid warpage.It builds light,strong and fast.I've used both 1/20"and 1/32" 5-6#balsa and have never had a failure I could ascribe to either the wrapped LE.or the skin thickness.I have had some delam failures using epoxy or aliphatic.
As far as adhesives go,I've used epoxy,several different contact cements,and aliphatic resin.Properly applied I see no difference in the end product.Any of them can get heavy in a heartbeat if over-applied.The one I like best is 3M auto trim cement in an aerosol can.It is a little harder to use but is much stronger than 3M77,and has a longer open time.It also allows straightening MINOR warps while not fully cured.When cured it retains some flexibility to transfer the stresses to the foam.I know most people like epoxy in this application,but IM(NS)HO it's subject to delamination due to the extreme difference in strength and hardness at the bond line,and the fact that if you apply enough to penetrate the crevices of the foam you have applied tooooo much.Aliphatic has much the same problem to a lesser degree.I've tried the "iron on"method with aliphatic,but again, to get a reliable bond,you have to apply too much adhesive.
Lately I've been using"Spider Foam"for cores it's heavier(~2#vs1#)than bead foam,but it cuts more accurately and since it's almost four times as strong in compression,will accept higher bagging pressure.Because of the higher strength it can be cored much thinner so the core weight is comparable to 1# bead foam.
For sheeting I use only 4#-6# A grain.The grain is Very important,especially if you decide to wrap the leading edge,but even if you don't it's still important.Light C grain is too brittle for this application.Since the major stresses are spanwise,A grain is the only choice.
Regardless of which method you choose,I would advise laying out the skins with the leading edge at the angle of the core LE and the trailing edge lined up with the core TE,then piece in the center of the skin running spanwise.This gives a useful increase in torsional strength.
Edge glue the skins on a FLAT surface(top,bottom,& LE if wrapping)and trim to no more than 1/2"oversize re the core.Do all your sanding with the skins flat.If you have a good core with no ripples,this will yield the most uniform skin after installation.This operation is almost mandatory with 1/32"skins to avoid localised thin spots.
Note:For wrapped LE,be sure there no joints within 1"-1 1/2"of the LE center.
The procedure for solid LE has been thoroughly covered by others.I'll try to describe the way I do the wraparound.
(1.)make sure the core is smooth.If not,sand with a sanding bar at least as long as the core until it is.If your'e using bead foam,you'll need fairly coarse paper,80-100 grit.
(2.)Vacuum the core until NO dust remains.
(3.)Mark the centerline of the leading edge skin on both sides of the skin(use a water proof marker)being careful not to score the outside.
(4.)Wet the outside of the LE sheet only,and turn it over(if your work surface is absorbent,lay a strip of wet newspaper under the LE areato keep it damp.
(5.)Apply the adhesive of choice to the entire skin and core.If you're using contact cement,allow it to gas off thoroughly,usually about 5-10 min.When its tacky to the touch,but doesn't pull up from the skin,its ready.Lay waxed paper over all but the LE sheet.
(6.)With the skin flat on the board,line up the center of the core LE at the root with the corresponding centerline of the skin(be very careful to only allow point to point contact).Then line up the tip centers and slowly lower the core to contact the full length.
Note:at this point you only have line contact ,so repositioning is still possible.
(7.)While applying light pressure,slowly rock the core back and forth chordwise.The skin will begin to wrap around a little bit with each rock.Go slowly,to ensure an even wrap.
(8.)When you've almost reached the waxed paper,start moving the waxed paper as you go until its out.
(9.)Trim the skins to the cores, place the assembly back in the shucks and weight as evenly as possible for at least 24 hr.Or place the assembly in a vacuum bag at ~10-12"vacuumfor 12hr.
(10.)Trim and sand the TE and glue on the balsa TE.Applymasking tape to the skin adjacent to the TE and shape the TE remove masking and final sand.
One final note:If you do the wrap with other than contact cement,be watchful for the core trying to"squirt"out of the skin leaving a void at the LE.This is especially a problem if bagging.For other adhesives,use lower vacuum,i.e.5"-7".Also be sure to blot or squeegee off ALL excess adhesive.
Steve