Aerodynamics thread

[latenlazy]

Not very likely you going to get supercrusing performance with these engines. You would need at least something in the rank of 117S . That doesn't mean J-20 could not start a fight from superior position (higher speed and height) . Relatively low T/W would matter later in the combat when you need to regain energy quickly.

Also, if you get into prolonged air combat where STR matters, you are already subsonic 99% of the time. Simply saying, J-20 would start with supersonic speed, use excellent ITR to get first shoot. But in the process it would bleed lot of energy . If it misses, things would get ugly because it would get in a low-speed dogfight were delta-fighters with mediocre T/W don't have much chance.

As for J-20 low-speed handling, I admit it is enigma. We could only make conclusions based on what we saw, and what we know about similar aircraft in similar circumstances . If PLAAF records a demo of J-20 with superb low-speed handling (like Americans did with F-22, and Russians with PAK-FA) I would be willing to eat my words

Not the current engines clearly. The J-20's a dud without F119 class engines. I don't think we should assume that it will continue to be underpowered though. If the WS-15 fails (and I doubt it will), it's unlikely that China wouldn't find another solution, even if it has to be foreign.

Most air combat is not prolonged, and with improving missiles and sensors the expectations is that they will only become shorter engagements. Furthermore, as you mentioned, the whole point of having F119 class engines is to provide the acceleration necessary to overcome energy bleed in prolonged air combat. We're not having the same debate if one of us is assuming the J-20 won't be powered with adequate engines and the other is.

As for J-20 air shows...you'll be waiting for a long time. China doesn't like to showboat, especially in its development phase. Russia has a very strong incentive to do so that China doesn't share, at least not with the J-20. The F-22 itself wasn't showboated until its production run, and despite all claims that the YF-23 was aerodynamically superior, we of course never saw it showing off.

 

[delft]

2. Its likely the vortex never completely detaches in the first place, but simply remains within the boundary layer over the top of the wing reducing pressure due to increased velocity caused by the turbulence, all the talk in the world likely won't help you visualize this phenomena, but a little blue smoke in a wind tunnel will show you the whole truth. In flight, yarn tufts have been used to visualize these effects, as with the F-18, when they were getting a lot of transonic buffeting on the vertical stabs????

A vortex within the boundary layer lacks the energy to produce the effects you need. The old formula is that the vortex energizes the boundary layer.

 

[Quickie]

Mirage-2000 is a pure delta and of an old design of late 70s. A better comparison would be the more modern delta canard design like the Rafale, Typhoon and even the J-10. But even between these designs, there're significant differences.

As for low speed handling of the earlier J-20 prototype. (We haven't yet seen any example of the new prototype.)

[video=youtube;u9nOMfa40FY]http://www.youtube.com/watch?v=u9nOMfa40FY[/video]

 

[F-15]

Great Pictures of the PAKFA F-15, thanks for the good questions and thanks for these great pix, you can see the vortices in the vapor cloud and that is very helpful to "illustrate" their function, good post young man. brat

There are many pictures showing its vortex system, perhaps you need just find them

Basicly the movable LEX is a vortex flap

 

[Engineer]

Oh, yea Tell me again what is fastest military aircraft ever, what is fastest fighter ever, and when were they designed ? And, while we are at it, why are 5th gen fighters generally slower then their 4th gen counterparts ?

You have answered your own question. Being the fastest is not equivalent to having more advanced flight dynamics, which is why fighter designs ceased striving for maximum speed.

You sad maximum lift and minimal drag. Mutually exclusive. You could have either on of them , or compromise between the two.

No, I never said that. I said "maximizes lift and minimizes drag... known as optimizing the lift/drag ratio".

Only thing that is fundamentally changed since 60's and 70's is use of computers. Basically, today you could work with inherently unstable elements like cranked delta, you could move center of the mass aft etc. But basic laws of aerodynamics have not changed and you still cannot generate lift if you don't have wings (please don't attach pictures of rockets to prove me wrong ) . That is why even very modern fighters have large wings if their designers want superb low-speed handling (PAK-FA , F-22) .

Laws of physic does not change, but how much of that physics can be discovered and utilized is correlated with advancement in technologies. You pointed out one example of this when you bring up the issue of working with unstable designs. Utilization of vortex lift also correlates with advancement in technologies, especially with computer power. Computers enable the effects of vortex to be better studied, predicted and utilized. Modern fighters owe their superb handling to vortex lift, and do not rely just on wing area. That is why wing loading is a bad metric for gauging performance of modern fighters.

Any aircraft loses energy in turn, it is just the matter how much . Delta fighters are known to have good ITR and bad STR. Classical example are Mirage-2000 and F-16 in Hellenic Air Force. They had both aircraft. Mirage has better ITR so it would get a chance for a first shot . But if Mirage pilot failed to score a kill, F-16 would beat him with superior STR and power-to-weight ratio. Same thing could happen to careless J-20 pilot,and I don't think China would risk their new birds in such encounter (at least not until they get sufficient number of them to cover loses ) .

The J-20 is a canard-delta, not a delta fighter. The flaw in your analysis is that you try to isolate the canard completely out of the discussion, rendering your description entirely irrelevant to the J-20.

 

[Engineer]

Not very likely you going to get supercrusing performance with these engines. You would need at least something in the rank of 117S . That doesn't mean J-20 could not start a fight from superior position (higher speed and height) . Relatively low T/W would matter later in the combat when you need to regain energy quickly.

Also, if you get into prolonged air combat where STR matters, you are already subsonic 99% of the time. Simply saying, J-20 would start with supersonic speed, use excellent ITR to get first shoot. But in the process it would bleed lot of energy . If it misses, things would get ugly because it would get in a low-speed dogfight were delta-fighters with mediocre T/W don't have much chance.

That argument about thrust-to-weight ratio has been around since the first photo of J-20 appears. It is predicated on the assumption that J-20 is of similar size to F-111, which we now know to be false.

As for J-20 low-speed handling, I admit it is enigma. We could only make conclusions based on what we saw, and what we know about similar aircraft in similar circumstances . If PLAAF records a demo of J-20 with superb low-speed handling (like Americans did with F-22, and Russians with PAK-FA) I would be willing to eat my words

That argument about J-20 having poor low-speed (subsonic) handling is based primary on the use of a delta wing. As such, we only need to find another aircraft that also features a delta wing but with good low-speed performance, and that would be sufficed to make your argument falls apart. Do we have the video you spoke off?

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.

 

[thunderchief]

You have answered your own question. Being the fastest is not equivalent to having more advanced flight dynamics, which is why fighter designs ceased striving for maximum speed.

No, aircraft designer decided to make compromise and to decrease speed in order to gain maneuverability and stealth. They would not need to do that if aerodynamics has made such significant steeps forward in the last 30-40 years. They would simply increase both speed and maneuverability.

Laws of physic does not change, but how much of that physics can be discovered and utilized is correlated with advancement in technologies. You pointed out one example of this when you bring up the issue of working with unstable designs. Utilization of vortex lift also correlates with advancement in technologies, especially with computer power. Computers enable the effects of vortex to be better studied, predicted and utilized. Modern fighters owe their superb handling to vortex lift, and do not rely just on wing area. That is why wing loading is a bad metric for gauging performance of modern fighters.

No amount of computer power would eliminate need for lifting surface i.e. wing . And a rule of thumb is that aircraft with lower wing loading is usually more maneuverable at low speed , everything else being equal . Even vortex lift needs surface to act on .

The J-20 is a canard-delta, not a delta fighter. The flaw in your analysis is that you try to isolate the canard completely out of the discussion, rendering your description entirely irrelevant to the J-20.

Flaw of your analysis is that you think that canards carry weight of the plane. They do not except in special circumstances . And when they do, they bleed energy. Canards are not replacement for the wing, they are replacement for the tail .

 

[thunderchief]

That argument about thrust-to-weight ratio has been around since the first photo of J-20 appears. It is predicated on the assumption that J-20 is of similar size to F-111, which we now know to be false.

Argument is based on ability/inability of Su-27 to supercruise, because it has similar size and same engines as J-20. And we had that argument before, so I'm not going to repeat it

That argument about J-20 having poor low-speed (subsonic) handling is based primary on the use of a delta wing. As such, we only need to find another aircraft that also features a delta wing but with good low-speed performance, and that would be sufficed to make your argument falls apart. Do we have the video you spoke off?

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.

First of all, that video is not a proof of maneuverability (ability to rapidly change direction in flight) , but ability to fly with high AoA . J-10 is good at that, but again it is not anything special . Go to 2:38 and watch :

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Demonstration of maneuverability is something like this from 3:45 . PAK-FA demonstrates its ability to turn tightly at low speeds. Nothing special but reasonably well :

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[Engineer]

No, aircraft designer decided to make compromise and to decrease speed in order to gain maneuverability and stealth. They would not need to do that if aerodynamics has made such significant steeps forward in the last 30-40 years. They would simply increase both speed and maneuverability.

Fighters do increase in speed and maneuverability, with J-20 being one such example.

No amount of computer power would eliminate need for lifting surface i.e. wing . And a rule of thumb is that aircraft with lower wing loading is usually more maneuverable at low speed , everything else being equal . Even vortex lift needs surface to act on.

The amount of lift that can be generated is no longer proportional to the wing area, which is why that rule of thumb with the wing loading goes out of the window. While vortex lift does need a surface to act on, that surface does not have to be the entire wing. Vortex is typically generated close to the fuselage, consequently acts on the fuselage as well as inner portion of the wing the most. Counter intuitively, reducing wing area can in some cases improve lift-to-weight ratio, through removal of unneeded structural weight.

Flaw of your analysis is that you think that canards carry weight of the plane. They do not except in special circumstances.

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. The flaw of your analysis is that you are trying to exclude the canard completely from a discussion, in which case you end up with something that is wholly irrelevant to the J-20.

And when they do, they bleed energy. Canards are not replacement for the wing, they are replacement for the tail.

Nice try with the strawman, but no one here claimed canard is to replace the wing entirely. Canard generates vortex which contributes to lift, and such lift is not factored in the metric of wing loading. You are right in that canard does bleed energy, specifically to form the vortex. However, the aircraft as a whole doesn't lose much energy since the energy from the vortex is used in improving lift.

 

[Engineer]

Argument is based on ability/inability of Su-27 to supercruise, because it has similar size and same engines as J-20. And we had that argument before, so I'm not going to repeat it

I am glad you realize that the J-20 is of similar size and has same engines to the Su-27. These similarities mean the notion of J-20 having horrendous thrust-to-weight ratio doesn't have much to stand on.

First of all, that video is not a proof of maneuverability (ability to rapidly change direction in flight) , but ability to fly with high AoA . J-10 is good at that, but again it is not anything special . Go to 2:38 and watch :

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Demonstration of maneuverability is something like this from 3:45 . PAK-FA demonstrates its ability to turn tightly at low speeds. Nothing special but reasonably well :

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A turn is nothing but an aircraft flying at a sustained high angle-of-attack, so the video I have shown you demonstrates exactly what you are looking for. The video also does not need to show something special. In fact, not showing anything special is the entire point, since it shows an aircraft featuring a delta wing does not have impaired handling characteristics.

If you still have issue with the video, then feel free to

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. Unlike your claim, an aircraft featuring a delta wing can still have great turn performance. This is because your analysis of a delta configuration is not applicable to a canard-delta configuration.