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Old 10-06-2017, 04:20 PM
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BEC BEC is offline
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Join Date: Feb 2009
Location: Auburn, Washington
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....except that the SP-25 shape was NOT "definitively dimensioned in Estes Tech. pub. TR-11". There was a long tedious thread on that exact subject over on that other forum, which ended abruptly when I showed Bill Simon, the designer of the Alpha, TR-11 and he basically told me "never heard of it". When I asked him what the definition was, he told me it was SP-25.

As most know, that was a printed pattern you had to transfer to the balsa and then cut for yourself was the official definition of the original Alpha fin shape. The dimensions in Dr. Gregorek's TR-11 are clearly taken from built examples and do not match the original pattern.

Subsequent to that whole discussion, I have had the opportunity to compare fin shapes among several different printings of SP-25, the die cut fins that followed, and the current laser-cut fins. The fin shape changed with the laser cut version, with a reduced span and slight change in the angle of the tip with respect to the root (they were not quite parallel initially).

So yes, I guess I'm a purist, at least from a lineage of the design point of view.

Note that the shape in the yellow handbook pages in some catalogs is not the same.

As soon as I saw that language in the description for those 3D printed parts I wondered if the vendor wasn't the architect (I think that what he said he was) with whom I was arguing in that TRF thread and who simply couldn't wrap his mind around a part being graphically defined or the idea that the bill of materials for knowing the cost of the kit would be, for the fins, a sheet of BFS-30 (which is what was in those kits, of course) rather than explicitly costing a dimensioned fin part. *sigh*
Bernard Cawley
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