Paul Mac Cready and Weight savings

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Paul Mac Cready and Weight savings

Post by RTH » Sun Sep 28, 2008 2:37 pm

Paul MacCready was the driving force behind the human powered Gossamer Albatross that crossed the English Channel. Readers might be interested in the extent to which MacCready’s engineers went to shave weight off the vehicle so it could make the crossing using the pilot’s leg power.

I was working at Douglas Aircraft Company in 1978 doing early work in applying composites to commercial aircraft. One of Mac’s part-time engineers, Jim…, also worked at Douglas and came to me asking whether we could shave weight off the aluminum pedal hub and sprocket unit that transmitted leg power to the prop. He asked if the hub could be replaced with a carbon/epoxy version with a targeted weight savings of 1.5 oz. I told him that kind of weight reduction was ridiculous and to go away until he had bigger parts to make. Jim insisted so I gave it some thought and came up with a scheme.

The sprocket had teeth on it, made for a normal bike chain, that engaged a Kevlar thread with crimped beads on it. The beads were spaced to fit between the sprocket teeth. Composites would not make for a good toothed-sprocket but if the aluminum sprocket were machined away from the aluminum hub as a toothed ring, the hub could be made of flat composite stock and perhaps the aluminum ring could be heat-shrunk onto the hub. In fact if we did it right, a series of toothed rings with different pitches could be supplied and changed by throwing the ring into an oven at 250F, allowing it to expand, then dropping the ring over the hub edge and letting it contract. No one had ever tried this heat-shrink fitting of composites before and we didn’t know if the composite would delaminate or somehow break down or if the whole ring would just slip on the hub and be worthless.

I designed the hub to be a 1mil/inch dia. interference fit with the ring at room temperature and to be a loose fit at 250F. Parts were fabricated and machined and the heat-shrinking was performed very easily. But then we needed to prove the torque carrying capacity of this thing. So we bolted the toothed ring down to a workbench, adapted a fitting to the hub and found a big torque wrench. When the strongest shop workers applied 175 ft-lbs and nothing moved, slipped or broke, we declared the task a structural success!

The weight reduction was actually 2.25 oz over the all-aluminum version. I still thought it was dividing hairs but the Albatross bunch loved it and it was the hub assembly that pedaled across the Channel. In the 42 years I spent in aerospace, I never saw anyone like MacCready’s guys so sensitive to weight savings.

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