Shit, Warcry and I are laughing so fucking hard. I just highjacked 21 helicopters without landing. Here's how I did it.
1. Cause shitloads of chaos so at least 3 helicopters and a strike team comes after you.
2. Get to the highest building you can find.
3. Use the whipfist grab attack to pull yourself to a heli and hijack.
4. Mash X to ascend then get out when a few heli's are below you.
5. Hold Left trigger(target) then use the whipfist grab attack to get onto helicopters and hijack.
Repeat steps 3 - 5 until you've beaten my record :3
When you're falling down and cannot whipfist them from there, use X+B (on Xbox) to use the cannonball attack(If you've got it) to bring you closer to it then whipfist the bastard. At #19 I cannonballed at a Heli from about 3/4 up the Empire state then landed ON the helicopter, I then hijacked without even using the whipfist :D
