tphuang said:let's put it this way, in an internal plaaf military exercise between the mkk and J-10. People who say this claimed that J-10 was faster and moved much better than mkk despite carrying much heavier payload.
If you read the article, it basically mentions that the engine and plane structure allows for really good manuverability at high speed. I don't know the exact physics behind this, but think about it this way:
f=ma, and F = thrust - drag. Normally, drag increases when speed increases, but decreases when you go higher up (less resistance).
Since J-10 has good aerodynamic configuration, therefore the drag doesn't increase that much at higher speed, so acceleration is still possible, so the speed can still go up.
On the other hand, J-8's configuraiton is nowhere near as good, so even with slightly larger thrust, the force becomes 0 when speed is really fast. Then, acceleration is no longer possible. Even if it's frame might be able to stand for greater speed, the drag will stop it.
If the aircraft is thrust-limited, it will reach it's maximum speed will be determined by how fast it can go before thrust=drag. As you said, drag increases with speed and decreases with altitude. However, drag is not the only variable. The thrust also decreases as speed increases, and it decreases more for turbofans than for turbojets. There are also some other facters that affect how well the engine will perform at high speeds, like the type of inlet used.
It is true that the J-10 has good aerodynamics, but I'm not convinced that the J-8's aerodynamics aren't good for a plane that wants to go fast. The J-8 was designed as an intercepter, so flying fast is what it is designed to do. It has a high ceiling of over 20,000m which also helps to reduce drag. If you compare it with similar aircraft, the fact that it can only go up to mach 2.2 is actually somewhat surprising. This is why I think that it is probably materials-limited rather than thrust-limited especially when you consider that even older versions with significatly weaker engines can also acheive mach 2.2.
tphuang said:As for J-11, it needs a greater difference between thrust and drag, because mass is greater, so acceleration is smaller if thrust and drag difference do not change. J-11 is also larger, so the drag will be much larger than that of J-8/J-10.
If J-11 has the same net force acting on it, then yes it will accelerate slower. However, that really has no effect on top speed.
Being larger tends to work in the favor of the J-11. Drag tends to be roughly proportional to surface area, while thrust is more proportional to volume. That is why an old civillian passenger liner like Concorde can easily fly at mach 2 while some modern small fighters (such as FC-1 and LCA) can't. The Flanker's top speed is Mach 2.35, which is a pretty high target for a single engine fighter. If the J-10 really can fly at mach 2.4, then that is very impressive.