Aerodynamics thread

[SamuraiBlue]

It is more appropriate to look at canards vs tail fins, instead of only vortices produced by canards.

Canards generate lift and as such, reduce load on the wings and also drag. Down force on tail fins, increases load on the wings and also drag. It is unclear which configuration is more draggy, until actual calculations are performed on specific configurations.

So there is no particular reason, that canard-delta such as J-20, must be more draggy than tail-delta, such as F-22, or vice versa.

And higher fineness ratios exchange greater skin drag with less wave drag and hence, more optimized for supersonic speeds. The higher fineness ratio of J-20 has an advantage here.

Aughhh, I believe lift and drag are basically the same.
Anything that sticks out from a rocket shape will generate drag.
To my knowledge,canards on most fighters are control canards that does not generate lift.

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In a control-canard design, most of the weight of the aircraft is carried by the wing and the canard is used primarily for pitch control during maneuvering. A pure control-canard operates only as a control surface and is nominally at zero angle of attack and carrying no load in normal flight. Modern combat aircraft of canard configuration typically have a control-canard driven by a computerized flight control system.
Canards with little or no loading (i.e. control-canards) may be used to intentionally destabilize some combat aircraft in order to make them more maneuverable. The electronic flight control system uses the pitch control function of the canard foreplane to create artificial static and dynamic stability.

A benefit obtainable from a control-canard is the correction of pitch-up during a wingtip stall. An all-moving canard capable of a significant nose-down deflection can be used to counteract the pitch-up due to the tip stall. As a result, the aspect ratio and sweep of the wing can be optimized without having to guard against pitch-up.[A highly loaded lifting canard does not have sufficient spare lift capacity to provide this protection.

 

[siegecrossbow]

Aughhh, I believe lift and drag are basically the same.
Anything that sticks out from a rocket shape will generate drag.
To my knowledge,canards on most fighters are control canards that does not generate lift.

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

 

[SDWatcher]

Aughhh, I believe lift and drag are basically the same.
Anything that sticks out from a rocket shape will generate drag.
To my knowledge,canards on most fighters are control canards that does not generate lift.

The diagram illustrates how canards can improve lift, directional stability, and also better turn rates, expecially at high AoA.

 

[SamuraiBlue]

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

Yes but the canard itself does not generate lift, it enhances lift on the main wings.
As for vortices they are drag.

 

[Inst]

Unstable canards subtract lift, stable canards add lift. Depending on flight conditions they could add lift.

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Check this out about vortices. In an ideal case, vortices should be unnecessary, but when the choice is between turbulent flow over the vehicle and laminar flow + vortex drag over the vehicle, the latter case is less draggy.

Also, @Air Force Brat, I was hoping it'd be a purer case, but the 450 km/h is off a modified, turbo-charged, dustbin-fairing Haya. In reality, the Hayabusa gets 202 mph with the limiter off, while the speed record on the S1000RR is 209 mph. Still, this is roughly a 3% difference for a 20% increase in HP/ weight.

 

[Air Force Brat]

Unstable canards subtract lift, stable canards add lift. Depending on flight conditions they could add lift.

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Check this out about vortices. In an ideal case, vortices should be unnecessary, but when the choice is between turbulent flow over the vehicle and laminar flow + vortex drag over the vehicle, the latter case is less draggy.

Also, @Air Force Brat, I was hoping it'd be a purer case, but the 450 km/h is off a modified, turbo-charged, dustbin-fairing Haya. In reality, the Hayabusa gets 202 mph with the limiter off, while the speed record on the S1000RR is 209 mph. Still, this is roughly a 3% difference for a 20% increase in HP/ weight.

That's correct young man, the reality is the J-20 canards contribute very favorably to overall lift production on the J-20, they are all flying, they are distant coupled, not only do they create lift enhancing vortices, but when they are pitched upward smartly, they develop a tremendous"positive pitch rate".

If you were to roll the J-20 inverted, and push the stick forward vigorously, they would again, develop a tremendous "positive pitch rate", as of result of their tremendous lift developing potential...

So in straight and level flight at 50,000 ft, at 500 knts true airspeed, the leading edge of the canards will be pitched upward, and they will be developing a tremendous amount of positive lift!

So in all honesty, anyone with a rudimentary knowledge of aerodynamics can visibly "SEE" lift, in airshows particularly under hard maneuvering, those vapor clouds roll off wingtips like a tornado are always rolling upward when the aircraft is pulling positive G.

So while some might proclaim that the J-20 "trimmed out" for 400 knts would be in equilibrium, in reality those canards are developing lift, and when they are developing lift, they are also creating drag...

off topic: I had a friend loan me his Busa for a whole summer, it needed a battery, he said if you put a battery in it, take it home and pretend its your's, I did, but it looked so nice his neighbor bought it..

I have owned two GSXR-1000 Suzuki's an 04 first, and later an 01, they were both screamers, and I must confess that I loved "pinning the throttle back on the eight fuel injectors",, I only ran the 04 up to the 12,400 RPM redline once or twice, I most often short-shifted around 10,000 RPM,,,

I did have the 01 up to 135, and I admit, I was tempted to pop the 150 MPH barrier?? (they were electronically limited to 186 MPH), I must say that at 135, that 01 was "planted", but alas, I was wearing a short sleeve shirt and not my ballistic jacket, so I backed out of the throttle.

Good Michelin rubber is awesome stuff... but the most important piece of safety gear is between your ears!

 

[Inst]

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Re Vortex generation and how they can reduce drag. There are actually two ways the canards can reduce drag, though. First would be through vortex generation, we've covered that.

The other way, though, is through surface area distribution.

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Of course, the J-20, as a canard delta, is not going to have the perfect taper of a Sears-Haack body, but it's far closer. By having canards in the front, you can start the area taper earlier and more moderately than you would if you had a wing + tailplanes. With tailplanes, the worse problem becomes that tailplanes are not forward swept, and as such, you have more difficulty tapering off the cross section, although it can be done. And moreover, with no need for a tail in the rear, you can stick arbitrary tailbooms to control rear taper, as the J-20 does.

 

[Tirdent]

I suspect most will have noticed, but I got the text green/red text highlighting the wrong way round compared to the sea level static markings in the AL-31F diagram (the green text refers to the red cross and vice versa).

I can't agree here; according to pilot reports as well as the information in the documentary, the J-20 is faster than you think. The J-20 apparently has enough power to reach Mach 1 without reheat, and according to the CCTV claim, the J-20 can reach 1500 km/h without reheat.

Arguably close enough (<10% off) for an advanced AL-31F version or WS-10X, given the margin of error in drag estimates based on outdated geometrical information. I'm really sceptical about an AL-31F or WS-10 powered J-20 being able to break Mach 1 in military thrust, though.

AFB: Fineness ratio. The J-20 is 1.62 vs the 1.4 of the F-22 and 1.44 of the Su-57.

Fineness ratio is NOT length/span for an aircraft. In subsonic aircraft, it is only relevant for optimizing fuselage length/*diameter* ratio for minimum parasitic drag (span is driven by lift induced drag and hence biased toward very high aspect ratio, i.e. large span for a given area). For a supersonic vehicle where wave drag dominates, it would be more appropriate to think of fineness ratio as the ratio of length to the maximum diameter of a Sears-Haack body having the same longitudinal area distribution as the aircraft in question.

It's cross-sectional area that matters, not width - and that means you have to account for variations in fuselage height and vertical tail integration too. In other words, the Su-57 with its flat fuselage and very small fins is probably a lot better than the length/span ratio suggests, J-10 vs. Gripen (underslung as opposed to lateral intake configuration) is another case where length/span is misleading.

 

[Inst]

Well, the outdated geometrical information is possibly an underestimate; in your experience, how wrong would it be to assume drag of a flat body, instead of a bulging smooth body? I would actually not be surprised if actual drag on the J-20 weren't significantly higher as the VTech paper assumes a flat body.

About AL-31F; I'm not sure. From the number of exhaust panes we've seen, we know the J-20 is running some kind of AL-31F derivative, but it's not clear which exact AL-31F is working. The Su-35S, for instance, can reach Mach 1.1 supercruise (i.e, AB past Mach barrier, then coast at Mach 1.1), so perhaps the 117S engines are supercruise capable. But the notion of a J-20 as less draggy than a Su-35 is something hard for me to believe.

As for cross-sectional area,

Roughly comes out to peak body width of 4.27 meters, peak body height of 1.84 meters. In comparison, the F-22 comes out to peak body width of 4.17 meters, with a peak body height of 1.91 meters. Su-57 has a peak body width of 6.13 meters, but a peak body height of 1.51 meters.

Ultimately, I think the best way to do any of this would be to build or buy accurate subscale physical models, then take a knife to them, to create subsections for volume measurement.

 

[Tirdent]

Well, the outdated geometrical information is possibly an underestimate; in your experience, how wrong would it be to assume drag of a flat body, instead of a bulging smooth body? I would actually not be surprised if actual drag on the J-20 weren't significantly higher as the VTech paper assumes a flat body.

As noted in the VTech presentation, the flat panel model was only used to calculate lift induced and trim drag contributions, for which purpose it's adequate. While the outdated geometry will probably create a certain error, the method is sound - the result should be in the right ball park.

And credit where it's due - at 21.5m length, 13.0m span and 77m² wing area the geometry assumptions are markedly closer than some others which were circulating at the time (23m and up). My biggest beef is in fact with the weight estimates, and then ironically it's primarily the method (which unfortunately isn't elaborated) rather than the relatively minor discrepancy in the dimensions.

Run the F-22 through that algorithm and I guarantee it will produce a huge error - most likely the method derives from (and really is only valid for) previous generation designs. Even if you add 1000kg (based on actual AL-31F dry weight scaled with the same multiplier of 1.3) to account for the inexplicably low estimate for installed engine weight, it's still unrealistically low.

Although the weight error has a certain impact on total drag via lift induced drag, that contribution is relatively less important in the supersonic regime. It does, due to far too favourable a value for fuel fraction (fuel per aircraft gross weight, an important term in the Breguet range equation), explain the implausibly good range results.

As for cross-sectional area,

Roughly comes out to peak body width of 4.27 meters, peak body height of 1.84 meters. In comparison, the F-22 comes out to peak body width of 4.17 meters, with a peak body height of 1.91 meters. Su-57 has a peak body width of 6.13 meters, but a peak body height of 1.51 meters.

Frontal area is also a poor proxy for maximum cross sectional area.

Ultimately, I think the best way to do any of this would be to build or buy accurate subscale physical models, then take a knife to them, to create subsections for volume measurement.

A CAD model would be *considerably* more efficient.

At any rate, I'd say we can consider these Mach 2.9 speeds debunked at this point.