Yes Streak, the Yak-9U has very good numbers, for a 1 v 1 situation.
Speedwise the Yak is only slightly slower than the P-51D, and thats if the P-51 is using WEP. Once the WEP is gone, the Yak is nearly equal from 20k on down. There is also a narrow band between 5k-9k where the Yak is the same speed as the P-51, even if the Pony is using WEP. By the way, the Yak doesn't have WEP, so its always at its max power/performance.
Most importantly though, is the fact that the Yak accelerates much faster than the P-51, and retains alot more energy in vertical manuevers as well. So if the Pony decides to run, he better not wait until he has bleed alot of speed off, or the Yak is very close to translating onto his six. Because the Yak will catch him before he can get his P-51D up to speed.
Flat turn wise, they are about equal to the left, but in right hand turns, the Yak is again noticeably superior.
Rollrates, both are excellent aircraft, with neither having a real advantage.
The P-51D is superior in range, and gunset/ammo.
So in a 1 v 1 with equal pilots, the P-51 has got to hope for a snapshot (he can afford to take spraying snapshots where the Yak can't) or play the old fuel game. Hit and run, hit and run. Even then, thats a risky proposition. Of course the Pony can disengage at will, as long as he does it early enough in the fight so that acceleration doesn't play a role, and he still has WEP.
The P-51 is probably the better Arena plane, where the Yak is probably the better dueling plane.
I use to fly the P-51 almost exclusively when I cared about my K/D rankings, but now I tend to fly the Yak almost exclusively, because I find it more fun

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Vermillion
**MOL**, Men of Leisure