Inlets absolutely generate a large proportion of thrust at high speed. 'Lazy and naive' is to imagine that its simply a case of making the inlet bigger or longer to get more thrust out of the engine. Available thrust depends on lots of interrelated factors, only one of which is air density. Correlation does not equal causation.
J-20 Inlet Discussion
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Inlets absolutely generate a large proportion of thrust at high speed. 'Lazy and naive' is to imagine that its simply a case of making the inlet bigger or longer to get more thrust out of the engine. Available thrust depends on lots of interrelated factors, only one of which is air density. Correlation does not equal causation.
You're looking at things the wrong way.
Turbofan engines have a maximum output in a bench test. This output is not going to be exceeded by (most) inlet designs under (most) conditions.
Under other flight regimes, however, the turbofan will be less effective because the aircraft is moving faster relative to thrust, because the feed air is insufficient, and so on.
What the inlet can do is to keep the engine as close as possible to the bench test situation, by changing the pressure, by assuring greater airflow at altitude, etc.
The J58 is a radical case, true, as it's a hybrid turbofan / ramjet, but you can see how the engine / inlet combination works to modify the air volume to the turbofan at different flight conditions.
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As far as inlet design goes, the biggest benefit of increased inlet length is better control of supersonic shockwaves, allowing the engine to operate more effectively at higher speeds. With stealth aircraft, it also gives more space for curvature, reducing RCS.
The exceptions you've mentioned are typically on the SR-71 class; i.e, conical inlets with diffusers, which we know the J-20 doesn't use.
The drawback of increased inlet length, as others have mentioned, is more drag, reducing engine performance at lower speeds.
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Generally speaking, however, I've remeasured the J-20 vs the Su-27 inlets. Both seem to be in approximately the .67 m^2 range. This implies that the J-20 is not drawing more air than the Su-27 at sea-level and at low speeds. However, the J-20 inlets may be more efficient than the Su-27's at higher speeds due to inlet length controlling for shockwaves.
On the other hand, the Su-27 features a variable inlet:
The J-20, at least on the front, is a fixed DSI inlet. There are rumors of hybrid DSI, i.e, with the longer span, there's substantial space for diverters to exist behind the stealth-optimized DSI.
We also know that the J-20 had its DSI modified between the prototype stage (2001) and its LRIP / production variant (2011, later versions). The DSI became larger (implying reduced airflow but increased compression at the first stage).
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As others have said, I don't think anyone here is an expert or even an educated layman on engine inlets here. This seems to be even more of a black art than engines, since people will talk about OPRs, compressor / turbine stages, bypass ratios, etc. This is why this stuff is worth discussion and investigation.
The open source NASA information is fairly good, but I've already posted it.
The most relevant portions are:
Basically, at supersonic speeds, variable geometry creates a bump within the inlet to generate compression shocks.to slow down the incoming air. DSI is basically a fixed bump at such speeds, implying less efficient airflow at low speeds.
This is the other part, and much more relevant to what secretprojects is saying. An oversized inlet will create spillage drag at low speeds, reducing its low-speed performance, which is why intakes are typically matched to some performance value for the engine.
The numbers we're looking for regarding the inlet is "total pressure recovery" and "pressure recovery", and how these vary based on inlet length.
The most relevant portions are:
Basically, at supersonic speeds, variable geometry creates a bump within the inlet to generate compression shocks.to slow down the incoming air. DSI is basically a fixed bump at such speeds, implying less efficient airflow at low speeds.
This is the other part, and much more relevant to what secretprojects is saying. An oversized inlet will create spillage drag at low speeds, reducing its low-speed performance, which is why intakes are typically matched to some performance value for the engine.
The numbers we're looking for regarding the inlet is "total pressure recovery" and "pressure recovery", and how these vary based on inlet length.
Also, for reference purposes, the F-35 seems to have a total inlet area of 1.06m^2 considering it's single-engined. Put another way, the J-20's airflow is crap for the WS-15, suggesting that AFB is right that the inlets need to be redesigned once the WS-15 comes into flight. The MiG-31 has an inlet area of 1.1m^2 per engine. IIRC (I'm not in the mood to redo the calculation), the F-22 has an inlet area of .62m^2.
As an example of how inlets can affect engine performance, the F110s on the F-14 actually increase in total thrust at speed and altitude vs sea-level thrust because the fixed inlets limit airflow so much.
As an example of how inlets can affect engine performance, the F110s on the F-14 actually increase in total thrust at speed and altitude vs sea-level thrust because the fixed inlets limit airflow so much.
Just a hypothetical of what the J-20 inlet design might look like. Please note that there's weapons bays blocking the lower section of the intake diffuser in front of the engines.
The upper-mounted DSIs in the J-20 mean that air is primarily routed to the bottom, before hitting the lower-mounted weapons bays and being routed to the top, before diffusing.
On the intake dimensions, would @Totoro like to take a double check?
I'm getting stuff that varies from .97m^2 to .67m^2 depending on how I measure and which photograph I use.
The problem is, the WS-15 is going to be a much more air-hungry engine than the WS-10 / AL-31 if it's low-bypass. Putting simply a WS-10 intake on a J-20B isn't going to work because the quantity of air required is going to increase.
I'm getting stuff that varies from .97m^2 to .67m^2 depending on how I measure and which photograph I use.
The problem is, the WS-15 is going to be a much more air-hungry engine than the WS-10 / AL-31 if it's low-bypass. Putting simply a WS-10 intake on a J-20B isn't going to work because the quantity of air required is going to increase.
This comes up via Deino. In the inlet thread, he accused me of trying to argue that the J-20 is an interceptor again. I think we need a dedicated thread to explain the nomenclature.
First, did I ever claim the J-20 was an interceptor? The closest I think I've come to that is saying that the J-20 is closer to an interceptor than an air superiority fighter. My preferred nomenclature is currently fighter-interceptor, because I think it best describes the J-20. Which gets us to point two, what's in a name?
A name, properly, is a referent to an object or a concept. It is not the same as the object or a concept, which is what it describes.
For instance, let's say the Chinese decide to re-designate the J-20 the H-19, or B-19 in Chinese nomenclature.
Has this changed what the J-20 is? Is the J-20 no longer capable of air-to-air combat? Has the J-20 suddenly picked up a massive weapons bay and fuel reserves to make it a regional bomber?
No, it's just a change in nomenclature. And it's not like this hasn't happened before.
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This is the "F"-117. The aircraft actually has the role of an attack aircraft; it has no radar to maximize its stealth, and is uniquely unsuited for air-to-air concept. But it's designated F-117, not A-117, because the USAF, in order to find pilots to fly a highly-expensive and crucial stealth attack craft, needed to pretend it was a fighter aircraft in order to attract the best of the best.
Which gets us to the second part of names.
Names present preconceptions of what an aircraft is. For instance, if a semi-modern or modern bomber were put up against a semi-modern or modern fighter in an air-to-air engagement, the fighter would, at worse, miss its kill, and would be expected to shoot down the bomber.
This is irrelevant of details, which makes it extremely important to laymen, who can't possibly be expected to know things like sustained turn rates at altitudes, drag force at speed and altitude, and so on.
If I say, for instance, the air superiority fighter the F-15 is facing off against a strike version of the F-16, the layman will assume that somehow the F-15 will shoot down the F-16, even if they might not be aware that this is due to the F-15's superiority in BVR characteristics and is more likely to see the F-16 first and loose BVR missiles at the F-16 first before either killing the F- 16 at range or killing a kinematically depleted F-16 in WVR.
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Now, we return to the J-20. There are some people who seem to emphasize the notion that the J-20 is an air superiority aircraft, and blush at the notion that the J-20 might be more similar to interceptors than they'd like to bear. They seize upon any evidence that the J-20 can not merely dogfight, but is good at it. These people do so because they want to counteract the narrative in the West that the J-20 is an interceptor or a striker, and that the primitive Chinese could never make something that could match the F-22 or F-35. In doing so, they've made gross errors.
First, let me point out, that most Western media coverage of the J-20 is propaganda.
Sources like The National Interest and Popular Mechanics fall into standard Western media bias, i.e, they tell their readers what they want to hear (that our planes are better and there's no threat), as well as propaganda lines that favor established elites (don't worry, American military supremacy will endure, and you should continue to suck up to the American world system / Tianxia).
These sources have no interest in the truth, and if the USAF or its related entities (consulting firms, aerospace manufacturers) tell them something, they'll gobble up the sources based on standard Western journalistic practices (consult an expert, don't doubt the expert, etc).
These aren't windmills worth tilting at, because the sociological forces underlining their bias will be enduring. People will continue to give them pageviews or pay for paper copies, because they're providing a product for which there is demand (confirmation bias).
Second, when we name the J-20, we want to give a concept that is closest to the truth without necessitating details.
With this in view, what is the underlying reality of the J-20?
-The J-20 has an aspect ratio (1.58) that's closer to the MiG-31 (1.68) than the F-35 (1.48) or the F-22 (1.39). The high length to width of the J-20 reduces its drag coefficient, even if elements like canards and ventral fins add to it.
-The Song Wencong design documents showed that the canard / lerx / delta / body lift configuration was chosen in order to improve the fineness ratio of the wings, as well as reduce the aspect ratio, with the aim of meeting the requirements of stealth, supermaneuverability (an ITR concept, not an STR concept, as it refers to post-stall maneuvering where STR is impossible)), range, and supercruise.
-The J-20 design is a long-coupled canard, not a close-coupled canard, when close-coupled canards are the norm for maneuvering capability. Long-coupled canards are more optimized for high-speed maneuvering as they provide more control authority at high speed.
-Videos of the J-20 in aerodynamic maneuver have been underwhelming. We've definitely seen a 22.5-30 degree / sec instantaneous turn (
-Leaks currently imply that the J-20 lacks a gun, which is considered necessary for dogfighting.
-Other leaks claim that the J-20 has good maneuverability subsonically, but excellent maneuverability supersonically, and this is in comparison to J-11s and J-10s.
-Modern air combat does NOT want to enter a dogfight, because the prevalence of off-boresight missiles turn dogfights into what pilots describe as "a knife fight in a phone booth". Dogfights are now attritional engagements where the question is how many missiles you've brought to the fight, how good are they, and how many airframes do you have to absorb enemy missiles. The sole exception would be the Su-57, whose manufacturers claim is in the 35-50mn range per unit and has laser dazzlers to offset HOBS missiles. I'll also point out that some Western observers claim that it's closer to 4+++, i.e, better than a Eurofighter, but worse than a F-35. Which plays into its cost structure.
-As further evidence of the above, the F-35 doesn't want to enter a dogfight either; its STR is abysmal, but its manufacturers don't care. They claim its sensor fusion and advanced sensors (combination of EOTS, cutting-edge radar, and EODAS) mean that the F-35 can settle any fight at BVR.
-There are leaks claiming that the J-20 bested PLAAF airspeed records in testing, further suggesting its high-speed orientation.
-Interception is extremely crucial in modern air warfare, as stealth warfare involves the use of AEW&C to provide targeting information for stealth fighter AESA. Likewise, strike is also extremely crucial, as there will be ground and sea-based radar platforms.
-As a counterpoint, certain posters have pointed out that the J-20 is advertised as being able to fulfill air superiority roles, interception, and escort.
I think from the evidence shown, it's clear that the J-20 is designed to have exceptional high-speed performance, in both acceleration and maneuverability. This characteristic befitts an interceptor. On the other hand, we have evidence that the J-20 is intended for air superiority roles via combat claims.
What we can definitely agree upon is that 5th generation air combat is not going to be similar to 4th generation air combat. F-22s, for instance, rely more on their stealth than their STR / ITR (TVC) to achieve combat effectiveness. Against 4th generation aircraft, the incredible BVR superiority of F-22s mean that it never has to close in to shoot down enemy aircraft.
Likewise, HOBS-IR-UV means that once aircraft enter into dogfights, 5th gens might as well be 4th gens because any STR superiority is never going to come into play.
The conventional concept of air superiority (strong BVR capabilities, good dogfighting abilities in WVR) is now invalid because if you're dogfighting, something has gone horribly wrong or you're finishing off a weak opponent.
So what we're basically stuck with are two terms, "fighter-interceptor" and "high-speed air superiority fighter".