Choplifter HD is a spiritual successor of the original Choplift on Apple II. The mechanics are simiar, but updated. The chopper is in 2D space: it can go left, right, up and down. It can turn left, right, or face the player (out), and this is important as enemies do come from 3 directions (left, right, and out). And you can't hit the enemy if you're facing the wrong way. The game is presented in 3D with parallax and real scrolling to give the impression of 3D but the actual movesments are 2D.
Different helicopters have different handling, such as firepower, speed, carrying capacity, fuel capacity, and so on. Armed helis have missiles and/or chain gun, and all helis need to rescue people (civilians or soldiers or even the occasional news crew). As you play you will unlock better helos by achieving "stars" on the missions.
Enemies have AAA gun sites, soldiers with AK, soldiers with RPG, SAM sites, technical (gun trucks), APCs, tanks, enemy heli, enemy jets, and just general terrain obstructions.
Your objective is basically to rescue as many people that needs to be rescued as possible by flying there, kill enemies in your way, land next to the people (not ON them!) so they get on, then fly back to your base, drop them off, and repeat until either time runs out, or you rescued them all and/or other missions. It can get frantic, when you have enemies on all sides, missile lock on you, and so on.
In many ways this is a twin joystick shooter, as the left stick controls the chopper while the right stick controls the aim (laser sight pointing the chain gun).
Fuel is limited, and any use of "turbo" (speed boost) eats up fuel like crazy. And your helo won't fly without fuel. Rockets are limited in qty and reloaded only at base. Chaingun is infinite but there's a heat limit to prevent it from firing too much. Carrying capacity is limited and the larger choppers don't fly as fast or move as nimbly as the smaller ones. On the other hand, they can often do everything in one trip rather than taking multiple trips.
There are various "campaigns" containing various missions, and each mission usually has a bonus objective as well as news crew to rescue (2 extra people!)
DLC packs adds extra helos to play with (some are very powerful) and more missions.
All in all, it's a fun little game, worth successor to the name.