9U is awesome plane, arguably the most underrated in the game. (Including the 109G2, the P-47 family and the 190A5) One month I decided to fly it simply to make myself a better shooter in other rides. With so few bullets to spit you become very conscious of shot
setup vice the shooting itself. It is a survivalist assassin among fighters not a meat grinder like the 47.
I flew it intermittently over the course of a month. When I checked my stats toward the end of the month the other rides were all decent 3 to 5-1 K/D but the Yak was sitting pretty at 53-0! I was shocked! Checking the stats jinxed me though, couple sorties later I collided with the severed wing of my B-24 victim...
Vs. American rides play the angles game, the Yak will turn inside any U.S. fighter (Wilcat excluded) before flaps become a factor. When the flaps comes out, you should have kept a little E in reserve and take the fight vertical. This is especially important against the Corsair family since they have an uber tight turn with flaps out. If you bleed the U.S. fighters of their E and get their flaps popped out they will be wallowing under you asking for a nice easy close range shot in short order. Running down lone jugs or hogs is great fun. Even a jinking pony can be overtaken. Usually some kind of scissors is the defense- just chop throttle, throw in rudder, lag them a little and wait for a shot. Once slowed only the 38 will have an advantage out of the blocks and even then the advantage will be minimal and temporary. Pick one wing or the other and keep giving it little squirts as the American iron flops around. (Cockpit works great too but is harder)
Vs Spits, BnZ tactics are best. Come in level behind with a minimum of deflection and a good bit of overtake (I set my convergence at 200), I normally aim at the left wing root. The reasoning here is that the wing root is weak (same for jugs, B-24s) and if you miss they will probably jerk away from your tracers. Collision is a concern in this plane- don't press between the cheeks. After they jerk away and acquire you, take your slight offset to the left and gently pull into the vertical . If you're lucky the spit will make a 45 deg turn of greater to the left after you and attempt to climb up your tail. You should see 400-600 and a plus sign at this point with tracers. Continue your gentle left handed spiral pulling just enough to keep their solution off. If they are getting a solution and you are running out of E, put your wingtip on him to minimize your profile and pull a little more. When the spit teeters out 600 below you, if your wingtip is on him, throw in some left rudder...
190s deserve special mention. Don't bother shooting straight at the 6 of a 190 that knows you're there. They are too slippery, too small in profile and too tough. Your bullets will go much farther on spits.
Start fights around 10k if you can and keep your escape heading in mind. This is important in the Yak since in many situations you will have more enemies than bullets. As the fight drops lower and or you run out of bullets/find yourself very much defensive, consider kicking out the bottom of your defensive spiral at ~6k pointing home. Why 6k? Look at the Yak's speed performance chart. You are maximizing your speed advantage over anything that can turn inside you at that altitude. Treat the 6k like it's ground level and you will cut your deaths by 10% right there. Even if the enemy is still overtaking (as might be the case with the K-4 or La-7) you are still maximizing your speed and hence shortening the time to get towards friendlies with bullets.
In closing: Know the performance of the entire planeset. The Yak has some notable advantage over 98% of opponents. This plane generally out-turns anything it can't outrun. (Light 4-Hog possible exception) The Yak can engage anything in the mid-range of performance with confidence but the extremes are still troublesome IE the Brew and K4. The Brew is only a problem if you slow down, so don't if there's one in the area. The K4 can run you down at any alt and the tater is deadly when you get into a scissors. (Which they are usually better at than Ponies) Once slowed, he has as much of an E advantage over you as you did vs the American rides. You are more stable than him in a tight turn but it all depends- be sure to turn right! Past my bedtime...good luck and congrats on finding a gem!