Aerodynamics thread

[SDWatcher]

It maybe a matter of semantics, of whether to call the results academic or else. A senior or masters-level project is usually considered academic. As I have mentioned earlier, it canbe right or canbe wrong. Even published journal papers canbe wrong, nothing anymore sacred.

But what we have seen here so far, are criticisms based on unfounded accusations or status assassinations, instead of genuine rebuttals of the contents. Would such approaches be even more questionable, and frankly, more dishonorable, than a senior or masters-level project?

I would say it again, the results canbe right or canbe wrong, as with any other pieces of information. But it is certainly wrong, to dismiss the contents on the grounds of unsupported speculations or merely a matter of personal status of the authors.

As far as observing Chinese military is concerned, it should be obvious that we are trying to extract some meaningful insight from whatever information are available, expected to be incomplete and prepared to be skeptical.

 

[Jura The idiot]

"Student presentations" hmmm... So Its' credibility is highly questionable, since It's not an 'article' and I can hardly call it 'academic'... Still, if there is an actual publication, I'd very much like to read. But if there isn't one, we should do go on with J-20 rather than some student studies.

I guess only
SDWatcher
knows why s/he has been fighting for
#2902

Only if estimated with the oversimplified equation, F= ma+ bv**2, where,
F= force, m= mass, a= acceleration, b= drag coefficient, and v= velocity.
For both supercruise or maximum speed, a= 0.

But according to the calculations of the paper from Virginia Tech, the Total Drag canbe modeled as (measured from the graph),
D= 15+ 30* (M- 1.2) in klb, where,
D= total drag, M= Mach number.

So with AL-31, J-20 may supercruise at about Mach 2.0 with a maximum speed of about Mach 2.8 (graph only extends to Mach 2.5).
With WS-15, J-20 may supercruise at about Mach 2.3 with a maximum speed of about Mach 3.4 (graph only extends to Mach 2.5).

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since Monday at 6:25 AM

 

[ougoah]

Looks like a hurried study for the purpose of academic discussion or student training? Seems like lot's of assumptions, broadly defined details, though based off what I imagine is decent aerodynamics foundations. Wouldn't really draw serious conclusions from that paper. Accurate drag coefficients in different flight envelopes are probably VERY detail oriented.

 

[azretonov]

It maybe a matter of semantics, of whether to call the results academic or else. A senior or masters-level project is usually considered academic. As I have mentioned earlier, it canbe right or canbe wrong. Even published journal papers canbe wrong, nothing anymore sacred.

Master's-level project? Hardly... Even a Bachelor's-level project would (& should) likely contain more data and information related to the study -- whether the results are right or wrong is something completely different, since one can still learn a thing or two from a study with mistakes and wrong results. That's why journals keep those studies on their data bases -- learning from others' mistakes. Citing them as a sole source would be laughable at best.

 

[Inst]

@Deino, well, we're discussing the J-20, aren't we?

@Others, do we have any listing of Western academic papers for the J-20? There's definitely the Taiwanese study for the J-20's RCS, what we still need is a study for the J-20's drag level.

From what we know, the drag level of the J-20 at Mach 1.35 at 10,000 meters does not exceed 174 kN, since to reach Mach 1 from Mach 0.9 requires more power than to reach Mach 1.35.

 

[Inst]

As I've mentioned in the main thread, there's purportedly a retraction amending it to Mach 1.4 supercruise. With that said, however,

using a linear speed increase from 87 kn per engine to 180 kn per engine (WS-15 with AF), you should be able to get Mach 2.9 out of it.

 

[Jura The idiot]

in
J-20 5th Gen Fighter Thread VI Today at 4:33 PM
:

There's an addendum on SpaceBattles where people are claiming that there was an error in the numbers given; it should have been 二十五 instead of 五十二. It's possible that this retraction is due to opsec, but if it's 25 km per minute, it maps to 1500 km/h or around Mach 1.4 supercruise at 10 km altitude. This roughly maps the "enough power to reach the speed of sound" claim.

Anyways, the drag conversation needs to be in the aerodynamic thread.

indeed

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(LOL)

 

[Tirdent]

Oh dear.

Quite apart from the outdated geometry assumptions and data used in the VTech paper (also apparent in their weight estimates, as discussed here a couple of months ago), you are completely neglecting engine thrust degradation with altitude due to the drop in air density.

AL-31F dry thrust (at sea level, zero speed, ISA conditions) is not ~39klb for two engines, but only about 34klb (2x76kN) - and then only on the test stand: installed in the aircraft, it's not even 33klb.



According to the drag graph, Mach 1.1 @ 35000 feet requires >14klb of thrust - but in those conditions a pair of AL-31Fs (presumably with Su-27 inlets, though at such low supersonic Mach the design hardly matters) delivers just 60kN/13.5klb. If the various errors in the VTech analysis turn out favourably, this is close enough that Mach 1.1 might be doable on dry thrust - but only if the J-20 punches through the sound barrier in reheat.



Of course, for all we know the engine might be an advanced Salut-developed derivative with capabilities more like the 117S (which would in fact hit 39klb ISA SLS), however that needless to say is subject to similar thrust degradation. It might push speed out to Mach 1.3, but almost certainly no more than that (and it'd likely still be incapable of accelerating up to that point without afterburner).

 

[SamuraiBlue]

@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.

Don't really understand what you mean by drag control.
With a certain surface size there is a certain amount of drag. You can't really go below that threshold no matter how you design the airframe.
That threshold increases exponentially with velocity since drag increases exponentially with velocity.
Without an engine having enough dry thrust to overcome that threshold there is no way a plane can super-cruise, it's as simple as that.
Not arguing whether J-20 can super-cruise or not since I don't have an answer for that just laying down a simple fact that is all.

 

[Quickie]

Oh dear.

Quite apart from the outdated geometry assumptions and data used in the VTech paper (also apparent in their weight estimates, as discussed here a couple of months ago), you are completely neglecting engine thrust degradation with altitude due to the drop in air density.

AL-31F dry thrust (at sea level, zero speed, ISA conditions) is not ~39klb for two engines, but only about 34klb (2x76kN) - and then only on the test stand: installed in the aircraft, it's not even 33klb.

View attachment 48266

According to the drag graph, Mach 1.1 @ 35000 feet requires >14klb of thrust - but in those conditions a pair of AL-31Fs (presumably with Su-27 inlets, though at such low supersonic Mach the design hardly matters) delivers just 60kN/13.5klb. If the various errors in the VTech analysis turn out favourably, this is close enough that Mach 1.1 might be doable on dry thrust - but only if the J-20 punches through the sound barrier in reheat.

View attachment 48267

Of course, for all we know the engine might be an advanced Salut-developed derivative with capabilities more like the 117S (which would in fact hit 39klb ISA SLS), however that needless to say is subject to similar thrust degradation. It might push speed out to Mach 1.3, but almost certainly no more than that (and it'd likely still be incapable of accelerating up to that point without afterburner).

I can't read Russian but I think the graph says the engine can push up to 42 kN (84 kN for 2 engines) at Mach 1.40 at about 9 km altitude. Depending on the drag curve, the J-20 may be able to super-cruise faster than Mach 1.1 at a lower altitude than 35000 feet.