Aerodynamics thread
- Thread starter siegecrossbow
- Start date
There are various weight estimates extant. I'm going with 18500 kg (others feature 17500 kg, and we know the notorious 15000 kg) for empty weight. There's a claimed 11500 kg fuel capacity, so we can do a weight estimate for T/W from there.
As to whether the aircraft is NOT designed to be a dogfighter, the point is that dogfights are increasingly obsolete due to the prevalence of HOBS all-aspect missiles. The USAF has described dogfighting as "knife fighting in a phone booth"; i.e, even if you win, you're likely badly wounded, and for a high-cost heavyweight fighter like the J-20 it's not what you want.
When you consider heavyweight fighters vs lightweight fighters, the primary difference in the 4th generation (although less so in the 5th) is that heavyweight fighters excel BVR due to their large radar size. They're all designed to have "good" dogfighting performance in case an opponent actually get close, but they want to win BVR because they're not trading off cost effectively WVR.
Hell, even the F-22 doesn't want to dogfight. It's simple, in a BVR situation, you have a strong chance of scoring lopsided kill ratios of at least 2:1. In a WVR situation, on the other hand, it gets closer to 1:1 provided both sides have HOBS and comparable pilot skill. The F-22 utilizing its TVC is basically wasting itself in a dogfight because it stands a chance of dying, whereas if it's doing stuff BVR, it can exploit its stealth and sensors and if it doesn't score a decisive victory, it can always choose to run.
The same applies to the J-20. The J-20 is NOT a Su-57, which is designed as a dogfighter. The Su-57, in contrast to the J-20, has highly inferior stealth and desperately wants to get close to try to exploit its agility. Moreover, it uses a DIRCM system to try to foil HOBS missiles, but whether that'd work (or simply spur countermeasures using multi-mode seekers) is in question. On the other hand, it claims to be extremely cheap (likely because they're subsidizing fixed costs), so it can be cost-effective in a dogfight.
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The entire point I'm making, and one point that really riles up fanboys here, is that dogfighting is either dead or suicidal. The J-20 is designed to be capable in a dogfight, but like all serious heavyweight fighters, it wants to settle things at as far a distance as effective, because then it can exploit its supersonic agility advantages without entering a "knife fight in a phone booth".
@siegecrossbow
We're not discussing aerodynamics, we're just discussing specifications of the J-20 (T/W) as well as theoretical factors (why dogfighting is not a decisive advantage for a heavyweight fighter).
That's literally talking about aerodynamics.
Basically every post you've made in the last few pages makes assumptions about aerodynamics that others have contested, and is not suited for this thread. At a certain point, given how many times this discussion has been had over the years, on multiple forums, with the same points reiterated to you and still being misinterpreted to you, I wonder how much of your argument is even in good faith.
But good faith or not, there is a separate thread for this.
F-35 is highly optimized for subsonic maneuverability. F-35 being a maneuverability hog has been debunked since 2017 at several high profile air shows, and it is capable of performing post stall maneuvers without the use of thrust vectoring. Without thrust vectoring engines, the J-20 probably can’t match the F-35 in low speed maneuvers. It probably also can’t internally carry heavy air to ground munitions like the F-35.
What reason is there to believe this beyond the fact that the J-20 hasn't demonstrated these maneuvers? Absence of evidence, evidence of absence and all that.
This I buy, but it can be readily simulated in training with the J-20 "illegally" carrying more payload than it physically can.
There is no evidence that the J-20 could perform post stall maneuvers without TVC. Let’s see what PLAAF pilots have to say:
Zhang Hao: The J-20 is pretty good in the subsonic regime but truly fantastic in the supersonic regime. Once the aircraft goes supersonic, no other aircraft (presumably within PLAAF inventory) could match it.
Li Gang (test pilot): the J-20 has comparable agility to the J-10.
Based on pilot testimonials, the J-20 has standard fourth generation agility/maneuverability, which is impressive considering the fact that it is still encumbered by underpowered engines. Once WS-15 is ready it will be significantly more capable through out its flight envelope.
I should hope the J-20 does better than a run-of-the-mill 4th gen in maneuverability, even with WS-10 engines. While I don't believe the design choices like canards and ventral strakes significantly damage RCS, their effect isn't negligible, so they had better deliver some kinematic payoff. However, as I don't have evidence to the contrary aside from the early design documents and a layman's understanding of aerodynamic layout, this isn't a hill I want to die on.
Canards improve ITR but STR, not so much. Any how we are doing aerodynamics discussions again. Let’s get back to topic.
I think it's time to discuss some actual aerodynamics in this thread.
ITR is a function of the initial torque when the aircraft executes a maneuver and transitions from low to high AoA, while STR is a function of the torque at a high AoA. The punchline here is that since the canard can move, it can always have zero AoA by orienting it along the relative wind, no matter the AoA of the fuselage. Perhaps the high ITR/low STR holds for static canards, but they needn't hold properly controlled dynamic canards.*
Now, it's certainly possible that during a sustained turn, the canards can't be oriented to maintain favourable coupling to the main wing, but that's fine in the J-20's case. Even if the coupling is lost entirely, the canards are still large (high lift) and far from the center of mass (long moment arm), which means high torque, which means high STR. The brilliance of the J-20's design shines through when we consider that even if the canards are at low AoA, the LERX is at high AoA and can function to generate more lift along the main wing.
I conclude from this admittedly handwavy analysis that the J-20 is capable of post-stall maneuvers at least as well as the F-35, even with WS-10 engines. The pilot testimony is just that - testimony of their experience. None of them claimed to have pushed the airframe to its absolute limits, they just reported what they experienced within the limits of their flight instructions and the settings of the FCS.
Incidentally, I don't believe there's a TVC WS-10 being installed on the J-20. The J-20 rumoured to be pulling cobras is just that... a yellow J-20 with standard WS-10Cs doing a cobra.
*Important caveat: The component of the lift parallel to the moment arm does not apply torque (remember vector cross products). In the extreme case where the canards are perpendicular to the fuselage, the torque is zero. While this is a fly in the ointment of the analysis, I believe the conclusion still holds.
I think we should be careful with these hard and fast "rules." I don't think this is a good rule in general, and whatever merit it has I don't think it applies to the J-20's layout. Canards - like every control surface on an aircraft, work by applying a torque along the moment arm to the center of mass of the aircraft. In addition, coupled canards generate "excess" lift by reducing the pressure above the main wing to which they're coupled.
ITR is a function of the initial torque when the aircraft executes a maneuver and transitions from low to high AoA, while STR is a function of the torque at a high AoA. The punchline here is that since the canard can move, it can always have zero AoA by orienting it along the relative wind, no matter the AoA of the fuselage. Perhaps the high ITR/low STR holds for static canards, but they needn't hold properly controlled dynamic canards.*
Now, it's certainly possible that during a sustained turn, the canards can't be oriented to maintain favourable coupling to the main wing, but that's fine in the J-20's case. Even if the coupling is lost entirely, the canards are still large (high lift) and far from the center of mass (long moment arm), which means high torque, which means high STR. The brilliance of the J-20's design shines through when we consider that even if the canards are at low AoA, the LERX is at high AoA and can function to generate more lift along the main wing.
I conclude from this admittedly handwavy analysis that the J-20 is capable of post-stall maneuvers at least as well as the F-35, even with WS-10 engines. The pilot testimony is just that - testimony of their experience. None of them claimed to have pushed the airframe to its absolute limits, they just reported what they experienced within the limits of their flight instructions and the settings of the FCS.
Incidentally, I don't believe there's a TVC WS-10 being installed on the J-20. The J-20 rumoured to be pulling cobras is just that... a yellow J-20 with standard WS-10Cs doing a cobra.
*Important caveat: The component of the lift parallel to the moment arm does not apply torque (remember vector cross products). In the extreme case where the canards are perpendicular to the fuselage, the torque is zero. While this is a fly in the ointment of the analysis, I believe the conclusion still holds.