Gta 4 question? please?

Question

okay i chose to take the money not kill dimitri now i have to kill him every time im supposed to get under the helicopter jocob keep going too fast how to make under the helicopter any special way. hurry please i cant stop playing until i beat this mission

Answer 1

no matter where jacob is, as long as you go up the ramp at a reasonable speed, it will go into a cut-scene where niko climbs into the chopper.

Answer 2

just go off of the ramp at the end of the beach. If you go on the jump when it's highlighted, you'll catch the chopper.  

Answer 3

The other two guys talking about the ramp (jump) are the other alternate ending (bike ramp jump onto heli to chase Pegorino who is in a boat) not the one you're on which is you're in the boat trying to get under Jacob's heli so that you can chase Dmitri who is getting away in a heli.

It's not really a trick, but the thing you need to do is from the very beginning of driving the boat, try to follow the shortest distance around the rocks and through the channel, and unless your boat is bouncing too much at the beginning from waves before you really get into the channel, always keep the acceleration maxed out so you're always going full speed.  As soon as you see Jacob coming in the heli, you need to continue to keep max speed and don't turn too much, just head straight for Jacob, and you will be able to hit the marker to get in.

Good luck!