Using a Mk-19 grenade launcher on a parked AH-9 at point-blank range requires far too many rounds to cause it to explode. Not sure whether this is the result of weak grenades, a super-strong helicopter, or both.
Normally, in games and sims it is possible to destroy a helicopter with one 40mm grenade, because the grenade would tear open the surfaces of the aircraft and the flow of air across the surface would widen the hole and change the helicopter's flight characteristics, making it hard to control. I would say that a helicopter should not be able to survive more than three 40mm grenade direct hits. Maybe two for the Littlebirds and Loaches.