Treat a wirble as to separate things, the turret and a chassis, one shot from a tank or shots from a fighter will take out the turret, but repeted shots to the turret from a tank wont kill it or "pop the top off" next shots should be at the chassis to kill it, (long shots at one youll most likly hit the chasis anyway)
I agree they take alot of rounds from planes and ive been in that spot waiting to be killed and get tired of it and tower out,
BTW SKYROCK field and town acks can turret a WW as easy as anything else, but i know what your saying there, one night LTARogue and i were taking down a town and an ack started firing and hitting me but when i stopped the rounds were missing the turret that should have takin me out as previous times when i got to close to an ack.
a really odd thing i seen one night, an enemy WW sitting beside a town ack and the ack was firing at every thing but him, seen it from my tank and got him but there must be a dead zone or something that an ack cant see.