Canards improve ITR but STR, not so much. Any how we are doing aerodynamics discussions again. Let’s get back to topic.
Canards actually *can* improve STR. As I understand it you need to optimize the vortex shedding from the canard to minimize drag while maximizing the lift enhancement over the main wing over a sustained angle of attack when exercising a sustained turn. Alternatively, as ZeEa5KPul suggests, you can use the canard as a positive pitch device to maintain the optimal angle of attack for your LERXes to maintain the lowest drag to lift ratio in order to maximize sustained lift of the wing at a particular speed envelope. What’s interesting about high angle of attack scenarios with all moving canards is that in order for the canard to maintain lift and not stall out, there are some high angles of attack where it should also able to be positioned to shed vortices over the main wing in conjunction with the LERXes. This may be beneficial or harmful to overall lift to drag ratio though, since the interactions between the two vortex systems may be very complex (though if we watch the vortex patterns at high AoA maneuvers like during the 2018 Zhuhai show we might be able to discern a bit more about how these complex interactions work). The optimal control laws for these scenarios I imagine can get pretty tricky, given the number of complex interactions involved.
Now, sustained turn rates aren’t just about how aerodynamically optimal your air flow is at the angle of attack of your turn, but also a function of how much force you can generate to push against net drag in a turn. So having a higher thrust to weight ratio is beneficial toward a better sustained turn rate. Assuming good transient thrust (how quickly the engine can ramp up thrust output), it’s actually also highly beneficial for employing ITR maneuvers because you can recover energy much faster after expending energy in a tight maneuver, which allows you to be much more nimble (and survivable) after an instantaneous vector change.
*If* the J-20 is actually around 17500-18500 kg empty the way it’s been reported in the rumor mill (I believe this figure came from gongke), and assuming its current engines are around 135-145 kN, it actually should have a slightly better thrust to weight ratio than the F-35, which means its STR should be comparable if not better, assuming the J-20 isn’t aerodynamically limited. If on the other hand the J-20 is actually around 20000-21000 kg, it should currently have slightly worse T:W ratio than the F-35, and thus worse STRs, unless it is more aerodynamically optimized than the F-35 (and when we get into the topic of optimization it’s not enough to specify the sustained turn rate, but the sustained turned rate at what altitude and air speed, since the max sustained turn can vary for each different plane at those different conditions).
As for the F-35’s maneuverability, my understanding is, based on what I can piece together from testimonials, that it may not in fact have better sustained turn rates than the F-16. Instead, it may have more advanced aerodynamic control laws and/or a more refined/complex vortex management, which gives it a better ITR, a much wider maneuver envelope, and perhaps more nimble ability to change flight vectors (agility), the combination of which gives it a range of attack vector options that can in combination with the right tactics outclass a straight energy advantage and traditional tactics by the F-16. This I think would be actually pretty consistent with what we see in flight shows, since doing a max STR turn is a pretty visually boring maneuver.
As for why we haven’t seen the J-20 execute the kinds of maneuvers we’ve seen the F-35 execute...one possibility is simply that the pilots haven’t been authorized to show what the plane can do at flight shows. After all we know Flankers are fully capable of some pretty jaw dropping maneuvers from flight show performances, but we have never seen any of those crazy maneuvers executed by any Chinese Flanker. Another possibility is that the J-20’s flight control system hasn’t been developed and optimized for those maneuvers yet. Perhaps all the efforts placed into developing flight control laws have been put into realistic and usable combat maneuvers rather than dazzling air show tricks, so they simply haven’t relaxed the maneuver range for the plane. This is, presumably, what happened with the F-35, and may explain the discrepancy between earlier and later performance evaluations. Another possibility is that the J-20’s design simply isn’t capable of executing those maneuvers safely because it’s too aerodynamically complex to control safely in corner case airflow conditions, and TVC is necessary not only to further enhance the plane’s maneuver range but to provide a fail safe in rare corner case airflow conditions. Why risk crashing a plane that costs maybe 100 million USD just to show off to a crowd? Another possibility could be that the plane is engine limited, perhaps not in terms of overall thrust, but in terms of transient thrust performance, or perhaps in effective thrust at very challenging parts of the flight envelope reached in some extreme maneuvers (in other words, there are flight conditions where the engine stalls out so the plane can’t safely complete the maneuvers without risk).
I think simply put, we just don’t know how to properly evaluate what the J-20’s performance is next to the F-35. We don’t even know if the F-35’s airshow maneuvers demonstrate anything that might present a meaningful advantage against a J-20, or if they’re simply impressive visual displays. I certainly wouldn’t make assumptions about STR or ITR for either plane, in comparison with each other or with legacy 4th gen planes. Maybe those are simply not the relevant maneuverability parameters where either design maintains innovation and advantage over their predecessors, or each other. I’ve come to believe those number comparisons tell us no more about meaningful performance in combat than how well a car design can handle on a race track from some max turn rate numbers that we read off a piece of paper, or how superior one boxer might be against another based on body fat percentage, punching power, or arm length.
