Should I consider Obechi for my HLG wing?

I'm building an HLG with white foam cores, The instructions say to use 1/32 balsa sheeting w/ epoxy. I'd like to use 1/64 obechi, any weight issues or performance issue's I should consider.

From : Joe Hahn

Bob,
We experimented with obechi early in the Monarch development program in '92. We found it to be great structurally, but too heavy. A handlaunch wing sheeted with obechi would come out about 2 ounces heavier than an identical wing built with 7 lb/cu.ft. balsa(light density). 6 ounces as opposed to 4. BIG difference in performance and handling. In our opinion, not the way to go if you want a ship that responds to lift well and has good handling characteristics.

I'm not sure if the obechi we used was 1/64" thick, however. We used whatever was "standard" material at the time. I think it was a little thicker than 1/64", but only slightly. If your obechi is actually 1/64" thick, or preferably LESS, it may work fairly well. If you try it, let us know how it goes!!

Joe Hahn
DJ Aerotech

From Don Stackhouse : 

Bob, just a few additional comments. The obechi we used was .024" thick, which as Joe said was what was available at the time. As I remember, it's finished weight "on the wing" was equivalent in weight to about 11 or 12 lb./cu.ft. 1/32" balsa. 1/64" (.016" thick) obechi should be about equivalent to about 7 to 8 lb. 1/32" balsa. This ignores that the amount of epoxy absorbed by the obechi should not be significantly different from that absorbed by the thicker obechi we used, so in actual practice you won't save quite as much weight as the thickness change would suggest. It's a guess at best, but I'd estimate that your assembled weight would be more equivalent to about 9 lb. 1/32" balsa, about equal to a little heavier than good quality balsa like we used to use on the older Monarchs. The bottom line is that your wing's weight might not be much heavier than a really good balsa skinned wing, but it probably won't be any lighter.

Check to make sure your obechi really is .016" thick. I've seen a lot of fractional sized wood sheet from various suppliers that was thicker than it said on the label. It's not unusual to find what's claimed to be 1/32" (.031") balsa that's actually more like .040" to .045". If your obechi is more than .016", then all the possible weight savings I discussed above just evaporated. By the way, the 1/32" balsa we used really was .031", and our obechi really was .024" thick.

Where you really have to watch weight on an HLG is in the wingtips. Any extra weight here has very significant detrimental effects on roll and yaw control response and yaw stability. This is especially true on 2-channel rudder+elevator models. This is why we use white foam instead of blue for the tip panels of the Monarch 'D' series.

Don Stackhouse @ DJ Aerotech
djarotec@bright.net