Aerodynamics thread

[SamuraiBlue]

Re: Chinese Engine Development

They do generate lift. They stick out in the airstream and it is impossible for them not to generate lift. Not only do they generate lift by exposing to the airstream, they also form votices creating additional lift on the wing.

Not necessarily it depends on the design of the cutting plane. Lift is based on Bernoulli's principle. Basically a wing with a cutting plane that is identical at both upper and lower part will not produce lift as long as the wing is parallel to the air flow.

 

[Inst]

Re: Chinese Engine Development

Ask Blitzo about measurements, amazingly enough. And I got it wrong, it was 20.3x13.4, not 20.6x13.4.

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Here's another picture for J-20 measurements, where someone got about 13.47 and 20.35 length. Even with your .66 ratio, you still get roughly about 13.4 meters wingspan, so you can't seriously talk about 13.5+ wingspan as a minimum.

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Regarding satellite pictures and engineer, you get into the problems either way because even if you're doing a relative measurement, you still have to decide where the shadow starts and where the wing ends for both planes; and even if your margin of error is identical, your error will show up in your final figures because your margin of error won't be the same as a proportion of both measurements as shadow error will be an absolute figure, that is to say, you'll be off by 3 pixels whether you're working with a 150 pixel object or a 15 pixel object, except in the first it's a 2% error and in the second it's a 20% error.

Major cost of an aircraft lies in the engines and avionics. A smaller airframe does not mean cheaper, especially when that smaller airframe uses the same level of engines and avionics. Moreover, there are examples of dog fights that show bigger aircraft being superior in WVR engagements. For example, in Su-27 vs. MiG-29 combats, Su-27 won every time.

Not really, considering that composites and advanced materials in the airframe can mean that the airframe makes up a considerable portion of the total cost. Even then, consider that smaller fighters tend to have both smaller engines and radar than their larger counterparts; the F-35 is designed for about 190kn thrust, while the F-22 has 320kn thrust, meaning that its single engine should be cheaper than its larger counterpart's twin engine with TVC. The F-35 also has a smaller radar than the F-22, with 1200 modules compared to the F-22's 1500 modules, so in that case the F-35 should be cheaper than the F-22 due to having a smaller radar.

With regards to the Su-27 being better than the MiG-29, so? I didn't say that smaller aircraft are intrinsically superior in WVR combat to their larger counterparts, I just said that larger aircraft are better in BVR than their smaller counterparts and that smaller aircraft tend to be more WVR focused to compensate. If MiG can't design their Fulcrums to beat Su-27s in WVR, that's not my problem and it doesn't prove anything.

As a counter example, I could bring up the F-16 trouncing the F-15 in dogfights, because it was designed as a dogfighter and the F-15 was designed more for BVR functions.

That analysis doesn't work. RCS doesn't work in that way because of shaping.

You're basically saying "voodoo!" What I'm saying is that all other factors being equal, the detection range against an aircraft increases as a function of size slower than the effectiveness of a fighter's radar as a function of size. I'm not saying that a F-15 due to its size is stealthier than a F-35 as the F-15, unlike the F-35, is not designed for stealth shaping, but rather than an enlarged F-22, with radar scaling with size, would detect a conventionally sized F-22 first due to the fact that detection range against your aircraft increases slower than the effectiveness of your radar.

 

[latenlazy]

Re: Chinese Engine Development

Ask Blitzo about measurements, amazingly enough. And I got it wrong, it was 20.3x13.4, not 20.6x13.4.

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Here's another picture for J-20 measurements, where someone got about 13.47 and 20.35 length. Even with your .66 ratio, you still get roughly about 13.4 meters wingspan, so you can't seriously talk about 13.5+ wingspan as a minimum.

I did my own measurements with that satellite pic when it came out and got 20.5 meters (.2 meters difference shouldn't be surprising since it reflects maybe a pixel or two of difference). Furthermore, .66 was the lower bound of the range of ratios, so I can seriously talk about a 13.5+ meter wingspan. To that point, I personally felt like that pic above (which I believe Engineer or Deino did when we got that satellite pic) above clipped a pixel or three of the wing and still got 13.47, which is almost 13.5. Anyways, I reviewed the post I made after I did my measurements back in February. I mis-remembered the upper bounds a bit.

"Assuming that the wingspan is only about 13.5 meters, the J-20 should have a wing area of roughly 78-81 sq meters (lower and upper bounds given the range of numbers I used). Assuming a wingspan of 13.8 meters, the J-20 should have a wing area of around 81-84 sq meters. Assuming a wingspan of 14 meters, the J-20 should have a wing area of 83-86 sq meters, all assuming the J-20 is 20.5 meters long."

I'll probably followup with another satellite picture I found where the shadows are less ambiguous to try to reduce error, but I'm pretty certain we're in that ballpark figure. Either way, the point is it's hard to suggest the J-20 is short on wing area. Having around the same or greater wing area as the F-22 is not bad, especially for something that maybe only has 10% more mass. I don't buy that just because the J-20 is longer than the F-22 that it must be significantly heavier. It's not bigger in every dimension, only length, its canards are smaller than the F-22's tailplanes, and its vertical tails are much smaller than the F-22's (I'd be willing to bet that the J-20's vertical tails+ventral fins together take up half the volume of the F-22's vertical tails).

 

[Engineer]

Re: Chinese Engine Development

Regarding satellite pictures and engineer, you get into the problems either way because even if you're doing a relative measurement, you still have to decide where the shadow starts and where the wing ends for both planes; and even if your margin of error is identical, your error will show up in your final figures because your margin of error won't be the same as a proportion of both measurements as shadow error will be an absolute figure, that is to say, you'll be off by 3 pixels whether you're working with a 150 pixel object or a 15 pixel object, except in the first it's a 2% error and in the second it's a 20% error.

Again, that problem has to do with absolute measurements, since it has to do with finding the exact measurement of both planes before comparisons. A satellite picture with J-20 and a Flanker side-by-side can show which one is bigger, and we have such picture already.

Not really, considering that composites and advanced materials in the airframe can mean that the airframe makes up a considerable portion of the total cost.

Not at all. To get an idea, let's consider two aircraft of the same size, each with a pair of engines of the same size, and similar suites of avionics. A 314-seat 777-200 costs around 296 millions, whereas a 323-seat 787-10 with much higher use of composite costs 288.7 millions. There isn't much difference. Engine and avionics take up most of the cost, that's a given.

Even then, consider that smaller fighters tend to have both smaller engines and radar than their larger counterparts; the F-35 is designed for about 190kn thrust, while the F-22 has 320kn thrust, meaning that its single engine should be cheaper than its larger counterpart's twin engine with TVC. The F-35 also has a smaller radar than the F-22, with 1200 modules compared to the F-22's 1500 modules, so in that case the F-35 should be cheaper than the F-22 due to having a smaller radar.

Smaller avionics and engines usually take sacrifices in performance. F-35 is designed with one engine in mind, but the result is that the aircraft cannot supercruise. Smaller airframe means less fuel is carried resulting in less range. Sensor performance is correlated with aperture size, so less number of elements in a radar imply less performance. To get the same performance in a small package needs higher cost. There is no free lunch here.

With regards to the Su-27 being better than the MiG-29, so? I didn't say that smaller aircraft are intrinsically superior in WVR combat to their larger counterparts, I just said that larger aircraft are better in BVR than their smaller counterparts and that smaller aircraft tend to be more WVR focused to compensate. If MiG can't design their Fulcrums to beat Su-27s in WVR, that's not my problem and it doesn't prove anything.

As a counter example, I could bring up the F-16 trouncing the F-15 in dogfights, because it was designed as a dogfighter and the F-15 was designed more for BVR functions.

The Su-27 vs. MiG-29 example shows smaller aircraft does not equate to being more WVR focused. It doesn't really need to prove anything other than putting a big hole in the argument about smaller being better.

You're basically saying "voodoo!" What I'm saying is that all other factors being equal, the detection range against an aircraft increases as a function of size slower than the effectiveness of a fighter's radar as a function of size. I'm not saying that a F-15 due to its size is stealthier than a F-35 as the F-15, unlike the F-35, is not designed for stealth shaping, but rather than an enlarged F-22, with radar scaling with size, would detect a conventionally sized F-22 first due to the fact that detection range against your aircraft increases slower than the effectiveness of your radar.

That does not change the way RCS works. F-35 has larger RCS compare to F-22, even though F-35 is smaller. An increase of 10% in length, width and height doesn't translate to 21% increase of RCS.

 

[Engineer]

Re: Chinese Engine Development

Not necessarily it depends on the design of the cutting plane. Lift is based on Bernoulli's principle. Basically a wing with a cutting plane that is identical at both upper and lower part will not produce lift as long as the wing is parallel to the air flow.

So, when the airfoil is not parallel to the air flow, lift is created. Therefore, canard on J-20 contribute to lift. I rest my case.

 

[SamuraiBlue]

Re: Chinese Engine Development

So, when the airfoil is not parallel to the air flow, lift is created. Therefore, canard on J-20 contribute to lift. I rest my case.

I suggest you do a little more studying in how aeromechanics works since if a cut plane symmetric canard is pointed downwards it has a complete opposite effect creating down force.

 

[Engineer]

Re: Chinese Engine Development

I suggest you do a little more studying in how aeromechanics works since if a cut plane symmetric canard is pointed downwards it has a complete opposite effect creating down force.

So, when canard is pointed downward, it creates negative lift as opposed to no lift. And when the canard is pointed upward, positive lift is created. I rest my case.

 

[thunderchief]

Re: Chinese Engine Development

They do generate lift. They stick out in the airstream and it is impossible for them not to generate lift. Not only do they generate lift by exposing to the airstream, they also form votices creating additional lift on the wing.

One of the rules of aerodynamics is : everything that increases lift increases drag

Considering canards - in normal flight you would want to keep them in a position that least interfere with air stream (minimum lift and minimum drag) . Of course, this is not entirely possible. As I sad, unstable fighters like J-20 need to move them frequently and rapidly to keep steady.

What happens in air combat ? You could deflect your canards fully to rapidly change direction. Lots of lift (positive or negative) but unfortunately lots of drag and large loss of energy (i.e. speed).

Now we come to wing loading part. In a low speed, large wing creates less drag for same amount of lift, then smaller wing. You would notice that low speed aircraft usually have straight, larger (and ticker) wings then aircraft optimized for high speed (example A-10 vs F-16) . If a fighter like J-20 slows down, because of relatively small wings optimized for speed (delta-configuration) it will become relatively unmaneuverable because of poor lift. It will have to regain speed and during that time it will be vulnerable.

So, decision for J-20 pilot would be something like this : I have a lot of speed. Should I make sharp turn to get into opponents tail and loose precious energy, or should I keep my energy and my distance . I would say that any smart J-20 pilot would choose option number two, especially considering that china has other planes for dogfight and J-20 will be armed with array of BVR weapons.

 

[Engineer]

Re: Chinese Engine Development

One of the rules of aerodynamics is : everything that increases lift increases drag

Considering canards - in normal flight you would want to keep them in a position that least interfere with air stream (minimum lift and minimum drag) . Of course, this is not entirely possible. As I sad, unstable fighters like J-20 need to move them frequently and rapidly to keep steady.

What happens in air combat ? You could deflect your canards fully to rapidly change direction. Lots of lift (positive or negative) but unfortunately lots of drag and large loss of energy (i.e. speed).

Now we come to wing loading part. In a low speed, large wing creates less drag for same amount of lift, then smaller wing. You would notice that low speed aircraft usually have straight, larger (and ticker) wings then aircraft optimized for high speed (example A-10 vs F-16) . If a fighter like J-20 slows down, because of relatively small wings optimized for speed (delta-configuration) it will become relatively unmaneuverable because of poor lift. It will have to regain speed and during that time it will be vulnerable.

So, decision for J-20 pilot would be something like this : I have a lot of speed. Should I make sharp turn to get into opponents tail and loose precious energy, or should I keep my energy and my distance . I would say that any smart J-20 pilot would choose option number two, especially considering that china has other planes for dogfight and J-20 will be armed with array of BVR weapons.

See my reply here.

 

[Engineer]

One of the rules of aerodynamics is : everything that increases lift increases drag

Considering canards - in normal flight you would want to keep them in a position that least interfere with air stream (minimum lift and minimum drag) . Of course, this is not entirely possible. As I sad, unstable fighters like J-20 need to move them frequently and rapidly to keep steady.

What happens in air combat ? You could deflect your canards fully to rapidly change direction. Lots of lift (positive or negative) but unfortunately lots of drag and large loss of energy (i.e. speed).

Now we come to wing loading part. In a low speed, large wing creates less drag for same amount of lift, then smaller wing. You would notice that low speed aircraft usually have straight, larger (and ticker) wings then aircraft optimized for high speed (example A-10 vs F-16) . If a fighter like J-20 slows down, because of relatively small wings optimized for speed (delta-configuration) it will become relatively unmaneuverable because of poor lift. It will have to regain speed and during that time it will be vulnerable.

So, decision for J-20 pilot would be something like this : I have a lot of speed. Should I make sharp turn to get into opponents tail and loose precious energy, or should I keep my energy and my distance . I would say that any smart J-20 pilot would choose option number two, especially considering that china has other planes for dogfight and J-20 will be armed with array of BVR weapons.

This is what I call a self-contradictory argument. You argued not having lift is bad due to lack of maneuverability, at the same time that you are arguing having lift is bad due to drag.

At cruise, the best location to keep a canard is not a position at minimum lift and minimum drag, but at a position that maximizes the lift and minimizes the drag for the entire aircraft. That is also known as optimizing the lift/drag ratio. There is no requirement that canard has to create zero lift.

As to the canard having to move frequently, it is not unique on the J-20 as fighters with tailplane also face the same problem.

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. After all, the whole point about instability is to enable tiny control surface movement to cause large change in the aircraft's motion.

When it comes to wing loading, the metric is only useful to aircraft that purely rely on wing for lift such as airliners and military transports. The metric is nearly useless in gauging performance of modern fighters, starting from the F-16. The reason being is that modern fighters also employ the effects of vortex for lift. This is done by extending the root of the wing forward, resulting in a surface that slices the air at an angle, causing the air to swirl and creating more suction above the wing. The vortex effect increases as the angle-of-attack of the aircraft increases, in essence allowing the aircraft to get more lift when it needed the most.

In addition to the vortex effect I described above, the J-20 also generate another vortex system through the canard. This is achieved whenever the canard is in a non-zero angle-of-attack position. Unlike the vortex system earlier, the system from the canard is also tunable, resulting in many aerodynamic wonders that is not doable with a traditional configuration. The flat belly of J-20 also acts as a lifting surface, turning most of the fuselage into a wing, which creates more lift.

So, on J-20 there are three methods of generating lift in total. At cruise, most of the lift comes from the delta wing, minimizing drag. In a turn, the total amount of lift can be ramped up to 1.8 times of what the wing is capable of producing. So in your scenario, a J-20 pilot would have no problem making sharp turns to get behind an opponent's tail.