Finally did my first source vehicle mission. Used a Cargobob to grab the black and white "M4K3B4NK" X80 Proto. The bastard AI chopper nearly killed me in the Cargobob. Thankfully, I managed to put the X80 down safely and only got a $696 damage fee, due to me pulling a hard maneuver while landing. The engines nearly died on me because of those AIs. I'm definitely going to need a door gunner in future missions. :|
Door gunner won't help as the AI choppers will continuously spawn. You can even land, grab your rocket launcher, and the choppers just keep on coming. You'll be out of rockets and they'll keep spawning every time you take one out. How you avoid them; as soon as you hear the text come through and see the chopper on the HUD map, immediately begin forward progress. The AI chopper is faster so it will catch you if you don't have that huge cushion of altitude. As well, because it gains altitude faster than the cargobob it will catch you in vertical motion as well. What it struggles to do, however, is gain altitude while making forward progress. It can move faster than the cargobob vertically and making way forward, but it cannot gain altitude as fast WHILE making forward progress. So, if you immediately begin forward movement toward your warehouse when the chopper spawns below(and usually a little behind) you, by the time the chopper gains enough altitude for you to be in range you are already on close approach to the warehouse. Once you are close enough to the warehouse the chopper disengages.
Using this method, the AI chopper rarely even gets a shot off at me. It is also important to note that there is a 'sweet spot' in forward angle relative to progress efficiency. You'll notice that after you push forward on the stick tilting the cargobob, it initially begins forward progress, but then as the nose drops further it just loses altitude without as much forward power. By modulating the forward motion of the stick you can make more efficient progress while not losing as much altitude. You end up with a somewhat phugoid motion as you progress forward. It can take some time to get the proper feel for it, but after a few runs or some practice you'll become more efficient at it.
Edit: I've mentioned this before, but for those running MTU800 or less that can easily snag a solo public session; don't be afraid to select 'find new session' when you get dealt a tedious pickup mission. I'd rather take the extra minute or two to load up a new room and begin a new pickup mission that will take me 3 minutes to complete, than spend 12 minutes battling with people on a barge, potentially losing the cargobob and having to fly halfway round the map for another one. If you have your spawn set at last location or your office, it's most convenient as you'll drop back right there and quickly be able to start a new pickup.
This process also works for annoying supply pickup missions, but be sure you're set on last location so that when you respawn into the new room you are next to your business. If you spawn a block or more away you may face an instant raid, so make sure you won't spawn in your apartment, office, or yacht instead. Interaction menu selects this, for those unaware. Once you spawn back in next to your business, go back inside and start a new resupply mission. Dragging 25 miles round the map for 10 minutes to take out the bikers for 1 bar of supplies? No thanks, I'll take the 'drive to Crenshaw and steal this van after killing 12 baddies' for 2+ bars instead.
It's all about efficiency. Everything you do in this game is engineered by R* with a focus on $/hr potential. That is why as they've refined(read:nerfed) things most people find that no matter what you do for your grind there is a max potential earning per hour, which you can often achieve in various ways but always works out to be
about the same. This isn't unintentional, so making sure not to fall into the time sink traps is beneficial to your cash making progress.