On the auxiliary panel, rear-right, you'll find this bad boy:
It's the ballistics data settings selector. It has different modes that you need to set, depending on which type of unguided weapon you are using. If you're only carrying 1 type of rocket or gunpod, and it is mounted at the start of the mission (the helo "spawns" with it), it should be in the right postion.
If you change load-out during the mission though, or have different weapons on the inner and outer hardpoint (S-8 + gunpods, for example) it will need to be set manually:
0 - S-8 KOM / OM
1 - S-8 TsM
2 - S-13
3 -
4 - S-8 OFP2
5 - UPK-23
6 -
7 - FAB 250
8 - FAB 500
9 - KGMU AO
10 - KGMU AT
Failing to set it correctly will result in the targeting reticle not displaying in the correct place on the HUD.