Why does the PA-28 C.G. envelope have the top left corner cut off rather than allowing higher weights at a forward C.G. like it does at the aft C.G.?
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With "top left corner" you mean the combination of a heavy airplane and a forward cg location.
This is to limit maximum stick forces. Manoeuvring (= flight at a specified load factor ≠ 1) needs force proportional to the distance between the cg and the neutral point ("static margin") and the mass of the airplane. There are several situations in which certification requires not to exceed specific maximum forces. For example, FAR Part 23.145 says:
(b) Unless otherwise required, it must be possible to carry out the following maneuvers without requiring the application of single-handed control forces exceeding those specified in §23.143(c). The trimming controls must not be adjusted during the maneuvers:
(c) At speeds above VMO/MMO, and up to the maximum speed shown under §23.251, a maneuvering capability of 1.5 g must be demonstrated to provide a margin to recover from upset or inadvertent speed increase.
Another would be out of trim situations as those in Part 23.255:
(f) In the out-of-trim condition specified in paragraph (a) of this section, it must be possible from an overspeed condition at VDF/MDF to produce at least 1.5 g for recovery by applying not more than 125 pounds of longitudinal control force using either the primary longitudinal control alone or the primary longitudinal control and the longitudinal trim system.