The Goonies and Groonies are all "short, squat" designs that somehow get "aerodynamic exemptions" from the one caliber rule. I have heard of people changing the engine mount in the old Goonies from 13mm to 18mm, not adding nose weight and having stable flights. Rocksim shows that to be impossible.

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Carl McLawhorn
NAR#4717 L2
semroc.com