I have no idea if real attack helicopter would have possibility to tilt helicopter so ground targeting would be possible with fixed guns, while hovering without forward/backward movement.
There could be two key bindings for 'flaps', to increase and decrease angle.
On helicopter this 'increase flaps' could tilt body of copter, with getting nose lower, but without getting forward momentum.
And decreasing should getting nose up.
I did some googling and 'flaps' on planes make sense:
http://www.askcaptainlim.com/flying-the-plane-flying-90/1016-what-is-the-purpose-of-the-flaps-in-an-airplane.html
There was even forum article about fixed gun use on Take On Helicopters:
http://forums.bistudio.com/showthread.php?138776-Mi-24P-Fixed-gun-use
I belive I'm not only one who would like to see this feature.