What is up with the J-31?

[Versus]

When you are having an unlimited supply of fuel, you don't need to be efficient. True statement that eastern engines are crude and primitive, but they fit into the tactics of large numbers to counter sophisticated opponent. Engine is at least five times more harder to construct than the aircraft itself. They are super complicated devices.

[Skywatcher]

The J31 is an example of trying to copy the F35 and failing because you can't make the engine. The impact on combat load and internal fuel must be severe. The Chinese are probably developing it as a cheap stealthish option for local air defense and light attack. It's a defensive weapon not an offensive one.

Also, it's in a much earlier state of development than the J20. Notice the absence of apertures for anything useful.

The sole flying J-31 is the production version what the X-35 and YF-22 are to the Lightning II and Raptor (I don't even know how functional the avionics are)

Well, the Chinese want something that's more optimized for A2A, so that makes it lighter than the F-35, since the fuselage isn't so voluminous. Though I expect that its normal takeoff weight and MOTW to drastically expand once mission creep/future upgrades come up.

The R-33s are a temporary measure (apparently the production WS-13Es have 10+ tons of thrust each).

[SteveDaPirate]

When you are having an unlimited supply of fuel, you don't need to be efficient. True statement that eastern engines are crude and primitive, but they fit into the tactics of large numbers to counter sophisticated opponent.

When I think of unlimited fuel I tend to think of Western doctrine more than Eastern. Unlimited fuel requires aerial refueling assets, and the East just doesn't seem interested in buying very many. The Chinese have ~20 Xian H-6Us, and the Russians operate only ~20 Ilyushin Il-78s along with some buddy tanking with fighters.

Meanwhile, the USAF alone has north of 500 KC-135s, and a few KC-10s and still contracts with private companies to provide refueling services as well. The USMC has ~80 KC-130s, and both the USMC and Navy use fighter buddy tanking, and refueling kits with V-22s.

If anything, I would expect Eastern engine design to focus on efficiency even over performance, since they seem to like to operate their aircraft on internal fuel rather than gassing them up en-route to a target.