J-20... The New Generation Fighter III

plawolf said:

I am also not 100% on the canards angling down when the J20 pitches up. In that picture paintgun posted, the leading edge flaps are also pitch down, indicating that the J20 is indeed diving.

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The AOA of the aircraft is positive (points upward) when climbing and turning. The canards remain close to zero AOA, so they point downward with respects to the plane. You see the samething happening on J-10, Eurofighter, Rafale, Su-33 and other fighter aircraft with canards.

Air Force Brat said:

The J-20 really doesn't seem as stealthy or as polished as Pak Fa, but is an improvement on 1.44

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Air Force Brat said:

...but the J-20 is a relatively new platform, not nearly enough time to fine tune those stealthy characteristics...

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Your second statement doesn't support your first statement at all. In fact, I would call these statements contradictory. PAKFA is a new platform as well, so how did it get "enough time to fine tune those stealthy characteristics" as compared to J-20?

How stealthy a plane is can be gauge from how much effort the designers have put into the details. Take the F-22 for example, its canopy is one piece with no support. Not only this, but the canopy is gold-tinted and the interior of the cockpit has also been designed to minimize radar reflection to the source. Why do the designers need to do these? That's because the rest of the aircraft is very low-observable already and minor details like canopy and cockpit could spoil that low-observability.

Here is the reality; the only other 4th generation fighter (5th generation by Western standard) that has as much attention in details as the F-22 is the J-20. As for the F-35, it has a support for the canopy, but it is placed inside the cockpit so that the canopy remains in one piece. In comparison, PAKFA has to have two supports for its canopy, dividing the canopy into many different pieces. We can see here that PAKFA has less attention in details compared to the other three.

The biggest source of radar return for PAKFA is its inlets. While the other three aircraft has S-ducts, the PAKFA has straight ducts. This is going to ruin the low-observability of PAKFA, so you can see why its designers didn't put as much effort into designing the canopy as the other three aircraft.

Air Force Brat said:

while we don't know exactly what the designers were aiming for, it seems that J-20 is designed for more fuel/weapons for a longer range while still in a stealthy platform, that can fight if necessary.

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If it is a fighter bomber, it would have been called JH-20. J is a designation for fighter.

While we are on the Chinese designation for fighter bombers would the Su-30 have qualified for the JH designation if it were indigenous?

My understanding is that JH is a designation for bombers having some fighter capabilities. Su-30 would be a fighter with bomber capabilities, so not quite the same thing.

My recollection of supersonic aerodynamics is pretty hazy, having followed a course of lectures more than forty years ago and never having needed it. But I remember that the center of force on a wing would move aft when going supersonic. At the same time the fuselage might contribute some more lift. This too is relevant to the contribution needed from the canards under differing circumstances.

Engineer said:

The AOA of the aircraft is positive (points upward) when climbing and turning. The canards remain close to zero AOA, so they point downward with respects to the plane. You see the samething happening on J-10, Eurofighter, Rafale, Su-33 and other fighter aircraft with canards.

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First let me say thank you for attempting to enlighten me, I'm beginning to have an understanding, but seeing the leading edge of the canard angled downward as the J-20 rotates looks a bit wonky, but as they say seeing is believing. I also want to apologize because after reading your comments about my comments, I looked up Su-34 and J-20 on search engine and of course came up with wiki. If I didn't know that I just looked it up on wiki, I would swear that I was just regurgitating wiki on J-20. Wiki does say the J-20 would seem to be stealthier than Pak -Fa or F-35. I looked up su-34 because J-20 is almost as heavy and has similar powerplants, I stick by my casual observations that the J-20 is designed to carry a heavier weapons load, and more internal fuel, and hence that the J-20 will be less agile than its two sisters, just a hunch on my part but the J-20 is a big bird. Again thank you, and with all due respect to all contributers on Sino Defense I really appreciatte the more collegial atmosphere and the genuine friendship, I appreciate your patience with my ramblings, and rest assured I am listening with great interest in your responses. We all seem to have similar interests and come from a love of aviation and engineering, some of our differences I chalk up to blondes, brunettes, or red heads. My wife says I better keep saying that brunettes are the cutest! Thanks again guys and girls if there are any ladies on here.

delft said:

My recollection of supersonic aerodynamics is pretty hazy, having followed a course of lectures more than forty years ago and never having needed it. But I remember that the center of force on a wing would move aft when going supersonic. At the same time the fuselage might contribute some more lift. This too is relevant to the contribution needed from the canards under differing circumstances.

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You are of course quite right deflt, not only does the center of lift change, but depending on fuel tankage and weapons stores, center of gravity changes as well. I used to fly right seat with a friend who flew the Mitsubishi MU-2 light turbo prop. The mains were tip tanks and invariably fuel burn and such, one tank would be heavier than the other, and as the Mitsi had nearly full span fowler flap, the roll control was by spoilers. I swear that thing would fly sideways unless you used the small trim tab, or prefferably crossfed the tanks to balance the fuel load. As a result, if you were out of trim, it was highly obnoxious to try to fly it on the guages, however once trimmed out, it was almost on rails. As you so insightfully point out, the center of lift is dependant on airspeed and requires corresponding pitch trim changes as airspeed or load changes.

Air Force Brat said:

Wiki does say the J-20 would seem to be stealthier than Pak -Fa or F-35. I looked up su-34 because J-20 is almost as heavy and has similar powerplants, I stick by my casual observations that the J-20 is designed to carry a heavier weapons load, and more internal fuel, and hence that the J-20 will be less agile than its two sisters, just a hunch on my part but the J-20 is a big bird.

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Estimate of J-20's length is between 20m to 21m. Compare to Su-30's length of 21.94m and Su-34's length of 23.35m, the J-20 is clearly shorter. In addition, J-20's span-to-length ratio is smaller than Su-30. So while J-20 is big, it is not as big as Su-30 or Su-34.



It makes little sense that J-20 being smaller than Su-34 would actually be as heavy. In addition to weight, agility is also dependent on engines, wing loading, and stability of the aircraft.

The more powerful the engines the better, and production of J-20 will employ WS-15s. These will be F-119 class powerplants, not the Al-31F class on the Su-34 which you claimed.

The lower the wing loading the more agile, although now days fuselage produces some lift so we would have to weight in fuselage area some how. In any case, while we have no numbers on the wing loading of J-20 or PAKFA, we do know that F-35 is criticized to have a very high wing loading, and on top of this the aircraft is also overweighted. Based on these information, F-35 would actually be the least maneuverable of all 4th generation fighters.

As for stability, unstable aircraft is more agile than a stable aircraft. Stability measures how easy a system can be disturbed. For example, a normal pendulum is stable because it always go back to its original position, whereas an upside down pendulum is unstable because a slight nudge would cause the pendulum to deviate from its upright position. For aircraft, unstable means a small control input translates to large attitude change. Back in the days when F-15 and Su-27 were designed, flight control system is not sophisticated enough to have unstable aircraft, whereas modern fighter designs are unstable. On top of this, J-20's canard configuration is also less stable than traditional configuration. Su-34 wouldn't come close as far as being unstable is concerned.

So you see, there is little to support the claims that J-20 is not agile and is a fighter-bomber. I will also mention again that J is a designation for fighter aircraft, which means J-20 is not a fighter-bomber.

Engineer said:

Estimate of J-20's length is between 20m to 21m. Compare to Su-30's length of 21.94m and Su-34's length of 23.35m, the J-20 is clearly shorter. In addition, J-20's span-to-length ratio is smaller than Su-30. So while J-20 is big, it is not as big as Su-30 or Su-34.

It makes little sense that J-20 being smaller than Su-34 would actually be as heavy. In addition to weight, agility is also dependent on engines, wing loading, and stability of the aircraft.

The more powerful the engines the better, and production of J-20 will employ WS-15s. These will be F-119 class powerplants, not the Al-31F class on the Su-34 which you claimed.

The lower the wing loading the more agile, although now days fuselage produces some lift so we would have to weight in fuselage area some how. In any case, while we have no numbers on the wing loading of J-20 or PAKFA, we do know that F-35 is criticized to have a very high wing loading, and on top of this the aircraft is also overweighted. Based on these information, F-35 would actually be the least maneuverable of all 4th generation fighters.

As for stability, unstable aircraft is more agile than a stable aircraft. Stability measures how easy a system can be disturbed. For example, a normal pendulum is stable because it always go back to its original position, whereas an upside down pendulum is unstable because a slight nudge would cause the pendulum to deviate from its upright position. For aircraft, unstable means a small control input translates to large attitude change. Back in the days when F-15 and Su-27 were designed, flight control system is not sophisticated enough to have unstable aircraft, whereas modern fighter designs are unstable. On top of this, J-20's canard configuration is also less stable than traditional configuration. Su-34 wouldn't come close as far as being unstable is concerned.

So you see, there is little to support the claims that J-20 is not agile and is a fighter-bomber. I will also mention again that J is a designation for fighter aircraft, which means J-20 is not a fighter-bomber.

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A very liberal use of transitive property there XD

Regarding the change in center of pressure with speed thing, I'm quite positive that the eurofighter, for example, becomes aerodynamicly stable when going supersonic because of the center of pressure moving aft. Also the cabards are locked in place above that speed, so maneuvering is purely done by wing mounted controll surfaces. I don't know about the other canard deltas, though.