There was an attempt to create a 3% Rule, but that proved to be difficult to enforce, (and who wantes to go around with a ruler checking every detail in order to "let somebody have it" anyway). We adopted the "Duck Rule" mentioned earlier--if it looks like a duck, it's a duck. If you want to use a plane for both VR/CS and SPA, I'm positive you will get no arguements from SPA folks, as VR/CS rules are more strict. There is certainly nothing wrong with bringing a plane built to planform to an SPA meet--most planes are original, or very close to the original---it just seems there are lots of mods since that is what everyone wants to talk about in these forums. There will be very few problems/arguements unless the plane has had major mods, and even then you would probably be politely asked to bring a different plane next time.

My "advice" (though I'm sure you don't need it), would be to select a plane close to the 1970, (for VR/CS purposes), cut-off date, with a vertical hinge line for the rudder so "knife edge" won't be a problem. Most late '60s designs were quite advanced, and will fly fine without changes. Most common SPA changes involve small enlargements of the engine compartment to accomodate the 4-stroke engine, (if desired), and a slight lengthening of the fuse.

Bottom line---if someone decides to modify something, make it small, (the smaller the better), so the plane retains it's original look.

My King Altair needed a different rudder for SPA--other than the canopy, that is the only obvious change. Fortunately Vic Husak thought it was a reasonable change.