Aerodynamics thread

[Inst]

We have no evidence of the efficacy of the inlet vents. Once again, we need harder evidence.

The biggest problem remains the drag numbers; to sustain Mach 2.9, the J-20 needs about 311 kN of thrust. This is not achievable with the AL-31s.

There is one other possibility, and this is an even weirder option. Are the WS-15s already installed? We know the WS-15 seems to be in trouble, but getting 180 kN out of WS-15 is an easy way for the J-20 to reach Mach 2.9 (my figures say Mach 3).

Another interesting factor is the Su-57 having reported Mach 2.1 supercruise with izdeliye 30 engines. That implies Mach 2.76 speeds on full afterburner.

This also suggests what the Russians and Chinese are going after with the Su-57 and J-20 as opposed to the F-22. The higher speed is reciprocal with the lesser stealth; stealth coatings won't survive at these super-high speeds, but if you're not that stealthy to begin with, why do you need stealth coatings again?

 

[latenlazy]

Guys it’s probably a bogus number.

 

[Inst]

The thing you're missing is how drag works. The drag force equation has drag increase as the square of velocity; i.e, an engine with 100% greater thrust will only deliver 50% greater speed. A low-drag aircraft can outspeed a better-engined aircraft; note how the SR-71 and MiG-25 had T/W below 1.

The J-20 is a high fineness fighter with a high likelihood of having lower drag than other 5th generation fighters. And that's how supercruise actually works; supercruise is not merely about having the engines and inlets needed for supercruise, but also having an aircraft with low drag profiles up to supercruising speed, and having low drag can compensate significantly for having crappy engines.

Moving up from the AL-31 at 140 kN to WS-15 at 180 kN is only going to bring your speed up by 14%. The main effect of increasing thrust isn't increasing your top speed, but increasing your acceleration and maneuvering ability.

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Put another way, if the J-20 is not capable of supercruise without the WS-15, it won't be capable of supercruise with the WS-15, because drag increases tremendously as you go from Mach .9 to Mach 1.

 

[manqiangrexue]

This also suggests what the Russians and Chinese are going after with the Su-57 and J-20 as opposed to the F-22. The higher speed is reciprocal with the lesser stealth; stealth coatings won't survive at these super-high speeds, but if you're not that stealthy to begin with, why do you need stealth coatings again?

They didn't buy these stealth coatings at Home Depot; each country invents its own and if a breakthrough is made, there could be a stealth coating that could withstand higher mach numbers. Just because the Americans claim that theirs peels off at certain speeds doesn't mean that that's true for everyone else too.

Put another way, if the J-20 is not capable of supercruise without the WS-15, it won't be capable of supercruise with the WS-15, because drag increases tremendously as you go from Mach .9 to Mach 1.

In the gentlest way possible, that statement is stupid as hell. That's like saying if you can't by something with $80, then you can't possibly buy it with $120 either. How about every item in the world that costs between $81 and $120?

To be honest, that's a nice paper you pulled up about the J-20's calculated drag profile, but they're likely to be under-estimating the J-20's design simply because all they can do is look at the shape and do their calculations while the actual J-20 team, on top of being far more capable engineers, have doubtlessly made unseen enhancements to the inside of intakes (and possibly other areas). In other words, with top secret cutting edge designs, there can only be more than what meets the eye, not less, and the paper was written on just what meets the eye.

 

[ZeEa5KPul]

They didn't buy these stealth coatings at Home Depot; each country invents its own and if a breakthrough is made, there could be a stealth coating that could withstand higher mach numbers. Just because the Americans claim that theirs peels off at certain speeds doesn't mean that that's true for everyone else too.

America sets the upper bound on all possible human knowledge and endeavour. Haven't you gotten the memo?

 

[Inst]

@manqiangrexue

Look carefully at the total drag estimations. You're stating it's likely to be an overestimate. Calculate the drag force in kN in the total drag calculations, and study the graph closely. Basically, what it's saying is that the J-20, with present engines, is already capable of Mach 2 supercruise. Also, note the part with the Mach 1 jump, where the drag force jumps from around 5 klb to 20 klb. That's an increase of 300%. A jump of only 20%-35% is not going to be able to push the J-20 to supercruising ability, which is what I mean that if the J-20 can't supercruise now, it can't supercruise with WS-15 either.

Another thing to note is how the drag drops between Mach 1 and Mach 1.3. Note the claim going around about the F-35's pseudo-supercruise, that it's capable of supercruising at Mach 1.3 after afterburners push it to Mach 1.3 Apply the insight from the graph. Now, if the F-35 can supercruise at Mach 1.3, but not Mach 1.4, it implies that the F-35 can only achieve a pseudo-supercruise and would need an additional 33% thrust to be able to supercruise without engaging afterburners, and what's more, that'd push it to Mach 1.5 supercruise. That is to say, even with a substantial engine upgrade to 220 kN or even 240 kN, the F-35 would still be unable to supercruise due to its drag profile.

That is to say, the most important aspect in being able to supercruise is not engine power, but your drag profile. If the J-20 is designed to be supercruise capable, the engines don't matter that much, it either supercruises or doesn't supercruise.

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Basically, the big secret of supercruising is that the trick is not mainly the engine, but your drag. Note the commonalities between all supercruise-capable aircraft. The true supercruise capable aircraft all have internal bays and no external storage. Even the Concorde does not place engines out in pods, as other commercial airliners do, but integrated into the body.

 

[Richard Santos]

Polycarbonate Bubble canopy is generally thought to be unable to survive aerodynamic heating that occurs much beyond Mach 2.5. At Mach 2,5 the fighter will already glow brightly in IR From friction heating and might as well fire off flares to attract the attention of any IR sensors. In addition, it takes a different engine design to operate with reasonable efficiency at Mach 2.8 than one optimized to operate at Mach 1.8. Normal Mach 1.8 engine would lose enormous amount of thrust simply slowing intake air before the Fan/compressor if operating at Mach 2.8. The notion that stealthy j-20 could, much less be designed to, operate at substantially faster than Mach 2.5 is simply jingoistic idiocy.

 

[Inst]

Stated max speed of the F-15 is about 3000 km/h.

The combat scenario I'm looking at is one where the range of radar-guided missiles is significantly reduced due to stealth + jamming. The only accurate missiles, then, are IR missiles. IR missiles tend to be short-ranged compared to radar missiles, due to the limitations of the seeker as well as their basic design, so high speeds are used to improve their terminal kinematics.

 

[Richard Santos]

Stated max speed of the F-15 is about 3000 km/h.

The combat scenario I'm looking at is one where the range of radar-guided missiles is significantly reduced due to stealth + jamming. The only accurate missiles, then, are IR missiles. IR missiles tend to be short-ranged compared to radar missiles, due to the limitations of the seeker as well as their basic design, so high speeds are used to improve their terminal kinematics.

Max speed of F-15 is Mach 2.5. F-15’s canopy and radome will last less than a minute at Mach 2.5 before suffering heat damage.. That speed is generally prohibited in normal operation. F-15 is only given that marginally ability to reach Mach 2.5 at altitude because of the hysterical MiG-25 scare of late 1960s. It is now generally acknowledged the Mach 2.5 requirement was a mistake and imposed design compromises that reduced F-15’s overall effectiveness as air superiority fighter.

The program that led to the F-15 grown out of the tail end of the faster is better era in fighter design. So the design specification knitially called for top speed substantially faster than the F-4, So specification initially called for at least Mach 2.7. But experience in vietnam and various NATO exercises suggested flying faster than Mach 2 is more often a liability, not an asset, in air combat. In addition the design compromises needed to achieve Mach 2.7 is incompatible with maneuverability and situational awarenesss requirements shown to be more important than speed by combat experience. So the top speed was reduced to Mach 2.5. It required would have been reduced even more, to Mach 2.3, but for the mig-25 scare.

 

[SDWatcher]

........The drag force equation has drag increase as the square of velocity; i.e, an engine with 100% greater thrust will only deliver 50% greater speed........

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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