One of our Canberras on the OCU (early 70's) came back from a major service and during air test she ran out of nose down down trim at 110 kts (should have been 450ish) - we filed as much as we could off the elevator t/e 'spoilers' but to no avail - after a lengthy investigation it transpired that there were no ballast weights fitted under the cockpit floor
The CofG must have been just a little aft !!