I think it is almost impossible to do a very 'crisp' hammerhead in Aces High. At least, if it is possible, I can't do it.
What you need to do instead is actually 'flop' the plane back over onto its back, so that you can be headed back down the right way once the bandit runs out of speed and stalls. It takes a lot of practice to get a feel for when you should start coming back down, but if you can see the enemy plane's nose start to wiggle, you know they are about to stall and you need to stop climbing so you can get them while they are hanging there. In Aces High, this is known as a "rope-a-dope", cute, huh?
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In a 109G10 at least, if you have your pick between trying to turn in lag pursuit, and just going straight vertical- go vertical. That way, you can keep an eye on what the enemy plane is doing, and you can easily point your plane in the right direction once you come down off your climb. You CAN turn with several planes (as a general rule, I don't try to turn with N1K2's, Spits, or Zeros, but most anything else is fair game)as long as you don't turn so much that you lose your initial advantage. If you are having trouble with snapshots, I recommend loading the gondolas on the plane, and going with the 20mm nose cannon. That will give you a lot more ammo to play with, and the gondolas only marginally effect the turn and roll rate (the climb rate is affected as well, but it is outstanding even with gondolas).
In general, you don't want to 'saddle up' on someone unless you are absolutely, positively sure that none of his friends are around to jump in once you get low and slow. It is better to stay high, take snapshots when you get them, and in general wear down your opponent.
If you want to fly a 109 that performs similar to the G10 (it is slower), try out the G2 with gondolas. It can't turn with N1K2's or Zekes or Spits, but it turns a damn sight better than the G10 does.
And I just realized I answered pretty much none of your questions :-).
As far as 'rolling in the vertical' goes, picture this. You come down on a Spit's 6, (both heading 000), and you are about 500 yards away when he breaks hard left and settles into a heading of 270. Since there is no way you are going to turn to 270 behind him, you pull up on the stick until you are going just about straight up, then you roll to the left (or right, whichever floats your boat), until a line drawn from your butt to the top of your head is pointed at 270. You then pull back on the stick again, and you will be behind the spit again, above him again, but inverted. Just roll 180 degrees to get the top of your head pointed back at the sky, and you will be in exactly the same position you were to begin with. You will probably have lost some altitude and speed (i.e. energy), but the Spitfire lost altitude and speed (i.e. energy) as well. So you still have an overall E advantage.
[ 09-30-2001: Message edited by: Urchin ]