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Selective reading, again. The linked document actually states this: (2) For stick controls, W/140 (where W is the maximum weight) or 15 pounds, whichever is greater, except that it need not be greater than 35 pounds.
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To be
airworthy......
That is not the ideal by any means nor did the Spitfire have unacceptable stick force gradients.
It had a low stick force gradient and that served to aggravated the neutral longitudinal stability issue.
Although steepening the stick for per G gradient was the fix used to increase the pilots ability to safely control the aircraft, the stick force gradient was
not the issue with the Spitfire.
The issue was the longitudinal stability was neutral and not positive. That makes for a twitchy airplane that is easy to stall, hard to precisely maintain a load factor in a turn, and easy to overstress the airframe.
3/4 of an inch from cruise to stall is not safe and would not be considered acceptable.
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FAA formula for chimp-proof civil aircraft.
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That is for
all aircraft seeking certification in the United States and since most of us are all on the same standard now, much of the world.