Hold [Space] to shoot constantly.
Tips and Tricks:
Submitted by: RM
When the enemy helicopter shoots homing missiles, try to fly in
long horizontal ovals... they overcompensate when, whilst going
to the left of the screen, you quickly turn right, and vice versa.
It may be simpler to just shoot the enemy chopper as quickly as
possible, since by dodging all the time you let the chopper just
send more missiles at you, and you ultimately miss out on shooting
all the planes.