Why were Rate 4 Turn and balance indicators phased out in favour of Turn coordinators
Pre-war and early post war aeroplanes usually were equipped with Turn and Balance Indicators with the Turn needle calibrated with instrument markings to Rate 4 turns. In common parlance then, an instructor would ask his student for a Rate 4 Turn - not just a steep turn. The "skid" or "balance" needle part of the instrument was also calibrated to show the number of degrees the aircraft was out of balance. Tiger Moth instrumentation for example. When the pilot flew on instruments it was called "Primary panel" flying or sometimes "Limited Panel" flying. Now for modern airliners it is called flight on standby electrical power.
These early T&B indicators were gradually phased out to be replaced with Bat and Ball turn indicators for which the turn needle (the bat) was only calibrated to one marking and that represented Rate One. The ball was just that and sometimes called the skid ball. Finally the ubiquitous Turn Coordinator appeared in early Cessnas and nothing else has replaced them so far.
Would appreciate any comments as to why the old Turn and Balance instrument which was very good for instrument flying skills was phased out over the years for the now largely current Turn Coordinator in light aircraft?