air gap is harmful not usefull

in electrical machines it is the air gap which helps in 
transfer of the energy from stator to rotor or from rotor 
to stator. it also tells about the energy handling capacity 
of the machine.

AIR GAP IS THE PART OF THE MACHINE WHICH STORES THE MAXIMUM
AMOUNT OF STORED ENERGY(REACTIVE POWER WHICH PRODUCES
MAGNETIC FIELD AND GIVEN BACK TO SUPPLY)... IT ALSO REDUCE
THE FRICTION LOSS........ GIVES RELUCTANCE

though it reduces friction still it is not very desirable
because it increases loss,also it produces harmonics which
is responsible for early starting of motor(about at 1/7 of
rated speed)ie crawling.