Erich,
Yup - the Halifax couldn't operate at Lancaster heights, and the Stirling couldn't match either of them.
But they were
NOT designed this way - it was due to deficiencies in design. The idea for the 'heavies' was to fly as high as possible, with the largest bomb-load, for the longest distances. Ironically, the Lancaster seemed to perform the best of the three at low-level ( ie Dams, Augsburg Raids...)
