Lots of interesting input. This was obviously a question that was lurking and just waiting to be asked!
I'm sure that a wise aeronautical engineer back in the day realizes that there is no plausible reason to deliberately enter negative G with the flaps extended, nor be pulling more than 2G. That said, the airframe will withstand more, it is just not needed for certification.
Thanks Pilot DAR (and GTE for the follow up). There is an analogous situation with demonstrated crosswind velocity in gliders. The regulations require demonstration of satisfactory landing characteristics up to 0.2 Vso. Given the generally low stall-speeds of gliders and the unwillingness of manufactures to spend more money than is necessary during certification, we end up with ludicrously low demonstrated crosswind values in the POHs. For example, the Flight Manual of the DG1000S that I fly has this in the Limitations section:
2.12 Crosswinds
The demonstrated crosswind velocity is 15 km/h (8 kts.) according to the airworthiness requirements.
However, later on in the Normal Procedures section, we have this informative but at the same time useless wording:
4.5.1 Launch
... Take-off with strong crosswind is possible.
PDR1 wrote:
Not really a surprise, just the transient vs the steady-state case. Flaps mostly (nearly always) produce a steady-state nose-down trim change, ...
The C182 that I used to fly required a strong push and lots of nose-down trimming when going from zero to 40º flap. Are you saying that if I didn't push, it would have eventually stabilized in a nose-down attitude? I can't do a test, because it was written off last year.