The underlying configuration control issues are discussed in the other Nimrod thread, but I think someone said that just enough of the MR2 was retained to make it a modification, not new build.
Yes, Tuc, I think that it was I who wrote that. Even though the wings are new, they are, to all intents and purposes, the same shape and size of the ones on the MR2. By keeping to the the original Nimrod wingplan design, with only a few minor changes, the MRA4 is legally a modification that does not require the degree of certification that a new design would need.
No-one has told me what causes the porpoising, so I can only guess. The MR2's CG envelope (to maintain straight and level by trimming the elevators) is huge. The 2 aircraft are the same shape and size and the kit inside the MRA4 cannot be so heavy or displaced (they would have moved it, if it is the cause) to reduce the CG envelope to what it is now. So, I reckon its the engines: they are bigger and heavier and perhaps slightly aft of the Speys. Who knows, but, as someone has already said, this is a schoolboy error.
Regards
Ed