I am wondering which type of radiator is commonly used in helicopter powertrain? is it similar to that of airplane or car? And usually where are they located?
The heat exchanger you will find in most rotorcraft drivetrains is for gearbox lube oil. These are liquid/air heat exchangers, and they commonly employ a fan to ensure adequate airflow over the core during hover. The cores are usually furnace brazed plate-fin type construction since that tends to be more durable.
Rotorcraft transmissions are effectively oil cooled, and the heat rejected through the lube oil system can easily be 3% or 4% of the engine input power. So the lube oil heat exchangers on some high-power rotorcraft can get quite large.
Turboshaft engines also have their own lube oil heat exchanger. These are sometimes liquid/liquid designs that use fuel to cool the engine oil. But the amount of heat rejected through the engine oil system is far less than that of the main rotor transmission.
Here's a link to a manufacturer of rotorcraft oil heat exchangers: