Ben Owen
FOAM WEBS . . . A STRUCTURAL MEMBER torch and the glass was left. This showed the cloth was not worked out with straight grain, but was left waived or "kinked". These two errors would reduce torsional rigidity of the wing. Problems included some skin debonding (apparently due to twisting efforts), the foam core was not built to design, there were kinks in the skin ply, and the wing was heavy toward the trailing edge from excessive filler, which aggravated the twist. A complete report on this mishap is available from the EAA Canadian Council (see Canadian Council News, SPORT AVIATION, December 1988). Author's Note: It is, of course, best that solid blocks be utilized where foam
is used as a spar web. However, if you do have to join blocks, the usual procedure on urethane foam is to use either two-part urethane foam as "glue" or epoxy. Vinylester or polyester resin can also be used to bond these. On other types of foam you can use either the epoxy or two-part urethane foam. As with any other procedure of this type, it is best to do sample blocks and test them to be sure they are not decreasing strength in any areas before bonding foam blocks one to the other. Two-part urethane foam used as a joining material makes the foam blocks much easier to shape, whereas epoxy gluing the blocks together makes this area somewhat difficult to sand.
Spar "webs" are part of the structure of an airplane. Foam in the core of wings is a structural member. If foam is not properly used in construction, or maintained in service, structural strength will suffer. There has been an instance of a Q-2 with improperly joined foam that failed in flight. This is the partial history of that aircraft: The airplane was built in 1985 as a Q-2, and later converted to a Q200. The logs revealed a problem with lateral control. 10/24/84 - Noticeable roll right. 12/12/85 - Ran off runway, no damage. 07/29-86 - Patch main wing, left rear, not structural. 07/29-86 - Paint and filler removed to inspect cracks on leading edge. No evidence of damage to glass. 06/12/87 '- Canard remounted at + 0.75 degrees. 06/15-87 - Main wing warped 2 deLeft wing second zero degree ply showing kinking. (Outer 0 degree ply coupon straight grees, painted black, sat in sun, and firmly bonded to the above at time of analysis.) maximum temperature 140 degrees F. During flight the left wing was seen by ground witnesses to fold upwards and separate from the aircraft. The Q-2 wing appears to have failed by "static divergence" - wing lift twists the wing, increasing its angle of attack, which generates more lift, and therefore continues to twist until a failure occurs. This wing obtained its stiffness from two 45
degree plies of unidirectional glass cloth, 45 degrees to the spar and 90 degrees to each other. A very serious construction error is noted in the photographs of the foam. The core is required to be a solid foam block, and this was instead built of three stacked blocks, not fully glued . . . or perhaps they were glued and separated in part while being heated and twisted. The greatest gap is .125" (1/8"), however, no glue is evident in these gaps. Additionally, the skin, which resists twisting, has two layers of glass which should cross each other at 90 degrees but crosses as low as 70 degrees at spots. "Coupons" cut from the skin had the epoxy burned off with a propane 42 JANUARY 1989
Left wing outboard from BL50 showing debonded 3 piece core.
