Making a func_physbox Float on the Z-Axis?

Im making a helicopter that works with thrusters but when i move the heli upwards, it falls straight back down
but i want it to stay floating in the air.
Is there anyway to achieve this? thankyou

http://www.facepunch.com/threads/1143212

http://www.facepunch.com/threads/1143216

http://www.facepunch.com/threads/1143330