@bozon can it be that in your series of how to kill yak, you shot 7mm rounds instead of 20mm?
No definitely only 20 mm.
I believe that the Yak 3 is not designed by HTC to be this durable - it is an artifact of the damage model itself. The "all or nothing" model favors parts of small area. The yak wings are so small that a typical volly of hits registers on different parts of the wing - wing tip, root, ailerons, flaps, gears... and thus spreads the damage between them. Since it is "all or nothing" a volley that will register all the hits on a spit's wingroot is spread all over the wing components of the yak and the result is nil actual damage to any of them. It is funny to unload a volley of hispanos into a yak 3 wing and the only damage is the gear falling off.
Other small planea tend to show a similar effect. The Brew is also quite durable for a plane that was stripped of most of its armor by the Finns.
The post the I made of "how to kill the yak 3" is of course an extreme fluke. But the general feeling is that the yak is harder to bring down than most other planea due to the above mentioned effects. I hope HT plans some upgrades to the damage modeling down the road. I can see why this would not be his #1 priority, but I hope it is on his list.