Aerodynamics thread

[Inst]

Frontal area is an imperfect, not necessarily a poor proxy for maximum cross sectional area.

Re: CAD: the problem is that we'd need to get a CAD model up, whereas the model-makers have already done CAD measurements for us.

Re: VTech,

They seem to be including the probe in the dimensions; they seem to have gone off an old Paralay diagram that measured it at approximately 20x13. The final measures are 20.95 x 12.88, implying 4.75% error in the length axis and .93% error in the wingspan axis.

I think one way to consider the supercruise claim would be to scale the AL-31 to the WS-10IPE, which has a claimed maximum thrust of 142kN. That's roughly a 16% increase in thrust. This puts us up to 15.66 kN dry thrust at 10km, assuming linear scaling, which is enough for pseudo-supercruise at Mach 1.1, with afterburner boost. If, however, often factors squeeze in, such as the possibility that the inlets are designed for the potential airflow needs of the higher-powered WS-15, not the AL-31F, it is possible that actual thrust is higher. As I understand it, the DSI geometry has changed from the J-20 2001 prototype to the production models, so it's possible that the inlets are now more optimized for transsonic speeds at altitude, and may be downgraded once the upgraded engines come out.

About the Mach 2.9 speed, if you look around on Chinese sites, they seem to cite 3Mm/h speeds, or Mach 2.9. One possibility is simply going from 87 kN WS-10IPE dry to 180 kN WS-15, afterburner. This clearly puts us at Mach 2.9, but of course, we know the J-20 is not currently capable of Mach 2.9 speeds due to engine weakness.

 

[Inst]

I'll also mention I continue to disagree on airframe weight, it is plausible that the J-20 is heavier than it appears, in the class of 19k tons, but the actual empty weight is not disclosed, and insiders seem to insist that it's roughly 16 tons.

 

[Inst]

Also, there's a very easy explanation for the 15 ton / 16 ton claim. The AL-31s, put together, weigh about 3140 tons. If we assume the empty weight of the J-20 is roughly 19k tons, a J-20 without engines installed easily comes to 16 tons. The astonishing 15 ton claim comes out to about 18 tons, which is believable based off the Su-57 and F-22, aircraft of comparable size. Still doesn't quite float together with the reputed 40% weight savings from 3D printed titanium, however, as well as the claims made that the J-20 is only 20% titanium with 29% composite.

 

[Jura The idiot]

you meant kilos here:

3140 tons

just saying

 

[Inst]

SDF is better when you're not sober.

 

[Jura The idiot]

SDF is better when you're not sober.

huh?

 

[Tirdent]

Frontal area is an imperfect, not necessarily a poor proxy for maximum cross sectional area.

Well, the F-23 (noted for its excellent area ruling) has a higher fuselage frontal area than either the J-20, F-22 or Su-57...

Re: CAD: the problem is that we'd need to get a CAD model up, whereas the model-makers have already done CAD measurements for us.

Decent 3D models have been built of all the aircraft in question for CGI purposes - they'd be no more difficult to get a hold of than a plastic model, and CAD software provides for easy and accurate measurement of cross sectional areas.

Re: VTech,

They seem to be including the probe in the dimensions; they seem to have gone off an old Paralay diagram that measured it at approximately 20x13. The final measures are 20.95 x 12.88, implying 4.75% error in the length axis and .93% error in the wingspan axis.

True, I didn't catch that.

Nonetheless, even 20.1m is not bad for analysis based on what little evidence was available only months after the aircraft first flew. By 2015, AvWeek was estimating 20.4m, and at least some of that additional length will be attributable to the longer tail shelves which had by then appeared on later prototypes:

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VTech were perhaps 0.5m off the actual length of the airframe they were trying to measure - could've been a lot worse. Again, it's the weight estimate which is the one aspect about their presentation that conspicuously misses the mark.

If, however, often factors squeeze in, such as the possibility that the inlets are designed for the potential airflow needs of the higher-powered WS-15, not the AL-31F, it is possible that actual thrust is higher. As I understand it, the DSI geometry has changed from the J-20 2001 prototype to the production models, so it's possible that the inlets are now more optimized for transsonic speeds at altitude, and may be downgraded once the upgraded engines come out.

If the intakes are oversized with the WS-15 in mind (and I agree there's a good chance that they are) it'll be a drag penalty due to increased spillage (or bypass flow, which would be a bit less onerous). At Mach 1.1 to 1.3 they will be functioning largely as normal shock intakes anyway, so the precise configuration doesn't matter a whole lot.

Also, there's a very easy explanation for the 15 ton / 16 ton claim. The AL-31s, put together, weigh about 3140 tons. If we assume the empty weight of the J-20 is roughly 19k tons, a J-20 without engines installed easily comes to 16 tons. The astonishing 15 ton claim comes out to about 18 tons, which is believable based off the Su-57 and F-22, aircraft of comparable size.

Sometimes known as "contractor responsible weight". Equation will go ballistic over the suggestion that such a thing exists though

It would push the claim into more believable territory, but doesn't improve the credibility of the VTech estimate one bit. It's not what they asserted, they specifically included two ~1150kg (RD-33 class dry weight) engines, whatever that was supposed to be.

The Su-57 is a useless benchmark - no hard info on its empty weight is available.

Still doesn't quite float together with the reputed 40% weight savings from 3D printed titanium, however, as well as the claims made that the J-20 is only 20% titanium with 29% composite.

Again, not relevant to the VTech analysis - their method would not have given credit for anything of this kind, such as the realism of that claim is in the first place.

 

[Inst]

What exactly is wrong with the weight estimate? If we assume the 3D-printed titanium claim is real, and it presents 40% weight savings on titanium parts, the J-20 is 11.7% lighter than it would be otherwise. Using the F-22 as a baseline, and subtracting out the engines, we get 16300 kg, multiplied by .883, we get 14392.9 kg, + 3200 for Al-31-type engines, we get 17592.9 kg, still well within the VTech estimates.

 

[Inst]

Actually, scratch that, VTech wants something in the 15700 kg region. One funny possibility is that the Chinese 15 ton / 16 ton claims came out of the VTech estimates, so it's a somewhat circular error.

 

[Air Force Brat]

Control canards can help enhance lift by generating vortices under high AOA conditions.

Canards in and of themselves contribute to total lift production, whereas tailplanes only provide down force to increase the angle of attack of the main wing to the relative wind! they do also increase total lift production through vortices generation, lowering the pressure across the main wing...