Aerodynamics thread

[Inst]

@SamuraiBlue

I think the deal is that you can achieve supercruise or high speed both through drag and engines. The Concorde, for instance, could supercruise long before the advent of current-generation engines, through drag control.

The thing is, though, engine technology is hard. As far as I understand, IFI's engines aren't really that good, and the main players in the engine game are either the Americans or the Europeans, including the British. If you can't improve your engines to enable supercruise, you work instead on the drag design, perhaps sacrificing maneuverability as in the case of the J-20.

 

[plawolf]

It seems a little unreasonable to insist on people needing to produce advanced calculations few here would even be able to understand to point out some obvious flaws in methodology.

From a general aerodynamic modelling POV, there is precious difference between a Mig25 and Mig31, yet the supersonic performance between the 2 are vast.

In terms of supersonic flight characteristics, it’s not just aerodynamic shaping that is key, how key design features like the inlets are executed are also of fundamental import.

Without the kinds of detailed tech specs even national intelligence agencies would not dream of being able to obtain on the J20, it would be impossible to definitively judge if it’s unusual design optimisation was intended to achieve extremely high speed, or to attain high speed with modest thrust.

Often designs intended for one purpose also end up having characteristics that were not explicitly intended, but which proved beneficial nonetheless.

The SR71 blackbird is a good example in point. It was designed for extreme speed, but the design also had unexpected low observably characteristics that lead to the stealth age.

The J20 has underpowered engines, as its designers knew it would. There are many design characteristics to compensate for that, so it stands to reason that being able to achieve high speeds or even super cruise with lower than optimal engine thurst would be been a serious design consideration.

The unintended beneficial side effect could be that they J20 would also be capable of very high speeds once it does finally get the WS15.

However, it must be remembered that just because the basic aerodynamic shape of a plane allows for something, it does not necessarily mean the rest of the plane was designed or optimised to do that.

To go to M3 would require very specialised materials and design choices that the J20 is unlikely to have been designed or built with. Not least because such features would clash with its primary LO design goal.

Canards and the raised bubble canopy are the most obvious design features one would not have gone with if high speed was of paramount import.

As such, I think the VTech paper is being taken out of context, since they are just looking at the basic aerodynamic shaping without considering any of the actual design choices of the J20.

I think the reason there is so much emotion invested in this study is because it is effectively rehashing the same underlying assumptions and motivations of the now infamous 23m debates, with one side being seen, rightly or wrongly, as wishing to paint the J20 as a one-trick-pony speedster with questionable stealth and agility, while the other side is just as fiercely opposed to that theory.

 

[latenlazy]

You have no understanding of how the model works; i.e, the software they're working with may not need a detailed three-dimensional image.

The other thing is, the F-16 isn't actually that agile. It's supposed to have a max STR of 18 degrees, whereas the Rafale is reputed to have a 22-24 peak STR. The only advantage of the F-16 is that its peak cornering speed occurs around Mach .9, whereas Rafale and other fighters peak out earlier at around 700-800 km/h.

1) Whereas you understand what they did perfectly of course. Any model that tries to calculate characteristics of a real object using a simplified shape is making simplified assumption. What the VTech paper literally did was it took some rudimentary two dimensional measurements using Paraly’s rather early imprecise three view, took a blind stab at how much they thought the J-20 weighs with some very rough guestimations, gave it a specific aerofoil without any actual measurements, and used those to try to derive lift and drag coefficients. If what they did were sufficient to give you realistic outputs then actual plane designers are stupid for going through the trouble of making real to scale physical models for their wind tunnel tests and computer simulations, and there wouldn't be a point to the continued push towards ever more powerful computational models.

2) The Rafale does 22-24 degrees/sec STR at what altitude? At what speed? At what payload and fuel fraction? Do you have actual flight envelope charts for the Rafale or are you just cherry picking hearsay? Do you even know for sure that the person who decided to assign 22-24 degrees/sec as the Rafale's turn radius even knew whether it was still bleeding energy? A bare F-16 peaks at 26 degrees/min at sea level at over 300 knots. There's a reason why I keep pointing out that we can't make hard comparisons without flight envelop charts. STR varies constantly at different flight regimes. We don't know, meaningfully, how any of these planes compare to one another without knowing what the STR is for each plane at each point of their flight regimes.

 

[latenlazy]

Also, for that matter, I tihnk we actually have a few studies for the J-12 / J-XX project that showed thin-slice models not different from the ones used in the VTech papers.

Shocking. If you employ the similar assumptions you will get similar results. That still doesn't mean those results are representative of the real thing.

Yes. For aerodynamic contribution to drag at supersonic speeds.

I don't need calculations to tell you that a flat plate will have very different drag profile than a full bodied three dimensional object.

 

[latenlazy]

I think the reason there is so much emotion invested in this study is because it is effectively rehashing the same underlying assumptions and motivations of the now infamous 23m debates, with one side being seen, rightly or wrongly, as wishing to paint the J20 as a one-trick-pony speedster with questionable stealth and agility, while the other side is just as fiercely opposed to that theory.

Just speaking for myself, I'm making a debate out of this because I'm tired of seeing the same trashy amateur hour guess work being peddled as sound analysis over and over again.

 

[Inst]

@plawolf
You guys are way too attached to the J-20 being subsonic agile. Remember, WVR combat is currently somewhere between a knife fight in a phone booth and rugby with suicide bombs. Most major fighting forces have HOBS missiles that can quickly turn any WVR engagement into a telefrag.

Put another way, if you put an F-4 vs an F-16, and both are using HOBS, both aircraft die, even though the F-16 is way more sophisticated and agile than the F-4. So who really cares? In the US's case, they ended up going with the F-35, which has questionable agility subsonically and relies on its sensors, weapons, and subsystems to kill before WVR is achieved. In the Chinese case, it appears that they went with the high-altitude high-speed J-20, aiming to do a slashing attack on slower aircraft with kinematically superior IR missiles.

The idea of the J-20 as a high-speed interceptor doesn't mean that it's not an air superiority aircraft. As I mention a thousand times, the last time someone scored an air-to-air kill against an American fighter aircraft was in a MiG-25 doing a slashing attack vs an F-18.

I mean, the US is going to field 2400 F-35s and around 190 F-22s. The idea of symmetrically competing with the USAF is insane, especially if you're going to counter medium-weight mass-produced fighters with heavy-weight boutique fighters. Even in rumors, the Chinese are only going to produce 600 or so J-20s. They are not going to be fielding 2400 J-20s to out attrition the F-35s, the J-31s are better suited for that, and the J-20s won't win an IR stealth war against the F-35s simply because the J-20s have higher thrust and thus higher heat.

 

[latenlazy]

@plawolf
You guys are way too attached to the J-20 being subsonic agile. Remember, WVR combat is currently somewhere between a knife fight in a phone booth and rugby with suicide bombs. Most major fighting forces have HOBS missiles that can quickly turn any WVR engagement into a telefrag.

Put another way, if you put an F-4 vs an F-16, and both are using HOBS, both aircraft die, even though the F-16 is way more sophisticated and agile than the F-4. So who really cares? In the US's case, they ended up going with the F-35, which has questionable agility subsonically and relies on its sensors, weapons, and subsystems to kill before WVR is achieved. In the Chinese case, it appears that they went with the high-altitude high-speed J-20, aiming to do a slashing attack on slower aircraft with kinematically superior IR missiles.

You're way to attached to your faux analysis.

 

[latenlazy]

Okay, and you smell bad.

Better a bad virtual smell than a real inept mind.

 

[Inst]

pot, kettle, etc.

Let's put it another way. You literally assume that the PLAAF is run by simpletons and idiots who want to simply build a better aircraft than the United States, when the Chinese military tradition has always been about asymmetrical warfare. You NEVER meet the enemy's strength with strength. Do you really think that somehow the J-20 is going to be engaging in daring dogfights with the F-22, guns blazing, when we haven't even been able to confirm a gun port on the J-20 and HOBS on both sides are simply going to turn it into an attritional battle?

I am not saying that the Chinese are not playing the air superiority game; for them to set the J-20 up as a pure interceptor, the J-31 would have to pick up the slack and the J-31 is a strike fighter, not an air superiority craft. I am saying that the Chinese are playing the air superiority game THEIR way, not the same way the Americans or Russians are aiming to do, with highly-stealthy and subsonic maneuverable aircraft. The goal instead is to have a fast aircraft that has strong maneuverability at high speeds. Look at the J-20's aerodynamic design. It is an aircraft with relatively low drag, even if it's not as low as the VTech model (we have confirmation of Mach 1.4 supercruise with AL-31 / WS-10 engines), as well as long-coupled canards and TVC for exceptional supersonic maneuverability.

If it has to sacrifice subsonic maneuverability to do so, so be it. With F-16 level STR, it's roughly around the same level as the F-35 and behind the Eurocanards. ITR shouldn't be that bad, given the presence of both canards and TVC, but it just means that the J-20 is like the F-22, it only goes to WVR battle when it has to and would rather keep the fight BVR whenever possible.

 

[Inst]

Put another way, the advantage of the Chinese military tradition is that it's never self-glorifying; the Confucians have always seen it as crass. Chinese troops are willing to do things that are ignoble and undignified by other military traditions, whether it's rolling on the ground like children (rattan shield soldiers) or frequent ambushes. The important thing is, it works.

Remember WW2; the Japanese came into the war with a super-agile dogfighter that could rock any American fighter in a low-speed turning battle. How did the Americans respond? Not by building a more agile dogfighter, but by adopting tactics like the Thach weave, diving to escape, and boom-and-zoom tactics that rendered the Mitsubishi Zero's agility irrelevant.

Likewise, the high-speed J-20 might not be "manly" in getting into a WVR dogfight and winning through skill and elan, but against slower fighters, it simply works; you run in, you fire missiles, and get out. The enemy can't fire back because you're stealth, and you're too fast for IR missiles to do much damage.