AW139 lost tail taxying DOH

[blakmax]

clarification on vacuum

Hi 500e. Just to clarify about applying vacuum to remove air etc. That is a waise move and should be continued, but with some modification. It is important to remove trapped air.

The problem is that epoxies absorb humidity in the cured and especially the uncured state, and many small voids in composites and adhesive bonds are actually caused by release of that moisture as heat is applied. As the resin or adhesive becomes fluid, the bubble will expand because of the low pressure outside and the increasing pressure in the bubble. That forces the adhesive out. Very high vacuum and you end up with an aero-bar (a bubbly confectionery sold down under).
The trick is to make the voids smaller before the adhesive gels. Try this: Apply full vacuum during heat up so that any trapped air can be drawn out. Now once the adhesive has started to flow, reduce the vacuum to about 10 inches Hg (about 5 psi) for the rest of the cure cycle. There is enough pressure to hold the shape, but the voids reduce significantly because of the increase in pressure outside the joint.

Regards

blakmax