Aerodynamics thread

[siegecrossbow]

@plawolf

Which is nonsense, because you train how you fight and you fight how you train. If pilots don't regularly push the airframe toward its design limits in peacetime, they will not have experience with how the plane at its design limits work in wartime.

I think you missed his point. I don’t think a heavily inhabited civilian area makes for a good training area. It is one thing to push the envelopes in air shows or in uninhabited areas like Dingxin or Mach Alley and a whole other thing to do it between buildings. In fact I’m surprised that they performed any maneuvers at all in Nantong and that Bay Area in Guangzhou, where J-16/J-10 routinely perform banking maneuvers.

 

[Inst]

I think you missed his point. I don’t think a heavily inhabited civilian area makes for a good training area. It is one thing to push the envelopes in air shows or in uninhabited areas like Dingxin or Mach Alley and a whole other thing to do it between buildings. In fact I’m surprised that they performed any maneuvers at all in Nantong and that Bay Area in Guangzhou, where J-16/J-10 routinely perform banking maneuvers.

I actually had that point in my original post, but I deleted that segment because it's part of the usual tirade attacking the notion of the J-20 as designed for dogfighting.

It's not, the J-20's high supersonic performance implies that what it wants to do is to find a target, go supersonic to extend missile range, fire off missiles, then exploit its supersonic maneuverability to reverse direction and go home.

You've got to remember, if you estimate a 60%-fuel weight of 24000-28000 kg, you're looking at a 1.3-1.53 T/W under the expected 180 kN WS-15. The latter implies the best T/W ratio of any fighter at combat loads, exceeding even the Su-57's 1.42 T/W and well-exceeding the F-22's 1.25 T/W.

The high T/W helps to offset drag and increase the sustained turn rate, true, but where it most contributes is in terms of acceleration to what looks to be an extremely low-drag design.

 

[siegecrossbow]

I actually had that point in my original post, but I deleted that segment because it's part of the usual tirade attacking the notion of the J-20 as designed for dogfighting.

It's not, the J-20's high supersonic performance implies that what it wants to do is to find a target, go supersonic to extend missile range, fire off missiles, then exploit its supersonic maneuverability to reverse direction and go home.

You've got to remember, if you estimate a 60%-fuel weight of 24000-28000 kg, you're looking at a 1.3-1.53 T/W under the expected 180 kN WS-15. The latter implies the best T/W ratio of any fighter at combat loads, exceeding even the Su-57's 1.42 T/W and well-exceeding the F-22's 1.25 T/W.

The high T/W helps to offset drag and increase the sustained turn rate, true, but where it most contributes is in terms of acceleration to what looks to be an extremely low-drag design.

T/W has nothing to do with roll rate. F-104 is optimized for for high supersonic speed, has extremely high wing loading due to small wing size, turns about as well as an air liner, has a lower thrust to weight than many legacy Soviet designs, but has one of the highest roll rates among 2nd generation fighter aircraft. Roll rate depends upon the moment of inertia, which depends on wingspan/number of engines and engine placement. In general, twin engined fighters don’t roll as well as single-engined fighters and widely spaced engines don’t roll as well as closely spaced engines. Case in point, F-15 vs F-16 and F-14 vs F-18.

 

[crash8pilot]

Regardless of the plane's capability or the pilot's skill and experience, it's stupid and downright irresponsible flying maneuvers at the very edge of the jet's performance envelope (I'm talking the full 9Gs a pilot pulls while dogfighting, not a 2 to 3G barrel roll as depicted in the video) that low to the surface. It leaves little to no altitude/room to recover from should something go wrong. Knock on wood but should the pilot pull a little too hard on the stick that low to the ground and loses consciousness - he/she then has no altitude to recover or eject while the plane comes crashing down on a densely populated area.

 

[Inst]

T/W has nothing to do with roll rate. F-104 is optimized for for high supersonic speed, has extremely high wing loading due to small wing size, turns about as well as an air liner, has a lower thrust to weight than many legacy Soviet designs, but has one of the highest roll rates among 2nd generation fighter aircraft. Roll rate depends upon the moment of inertia, which depends on wingspan/number of engines and engine placement. In general, twin engined fighters don’t roll as well as single-engined fighters and widely spaced engines don’t roll as well as closely spaced engines. Case in point, F-15 vs F-16 and F-14 vs F-18.

With the J-20, incidentally, the intended engine for the J-20 is actually supposed to have TVC, so WS-15s with TVC on a twin-engined fighter will improve roll-rate considerably.

The point about T/W is more that the J-20 is closer to a fighter-interceptor; its subsonic maneuverability is at least competitive with 4th generation designs, but its selling point is more its speed, range, and ability to supercruise.

In comparison with the Su-57, the J-20 doesn't trade off stealth for maneuverability, as with the Su-57, but rather trades off stealth for speed and range.

The latter two matter because the strategic / operational factor in an engagement is very important. Speed and range mean that if the enemy does not cluster up, you can exploit a local numerical superiority by concentrating your forces at one point of the enemy line. Even if supersonic flight means that you light up on EOTS/EODAS, you can still supercruise into the combat region before slowing down. Range also means that your bases can be dispersed throughout a massive front, and each base has massive area coverage, and when striking, you can go very deep into enemy territory.

===

Besides the usual fanboy (supersonic maneuvering matters a lot in a dogfight!) vs cynic arguments, does anyone have any idea whether the J-20 can carry PL-XX/21s on its fuel pylons? It'd sacrifice stealth while doing so, but it'd provide an alternative counter-AEW&C option to the J-16 / J-11s, and what's more, the J-20s can supercruise. It might be able to obtain a Mach 1.4 supercruise capability (sufficient dry thrust to break the Mach barrier) if it has such excellent T/W, which given its high-speed design, would make it well-suited for interception (and this is a role of air superiority fighters, air superiority fighters can't strike, interceptors can't dogfight, strike fighters can't intercept).

 

[siegecrossbow]

With the J-20, incidentally, the intended engine for the J-20 is actually supposed to have TVC, so WS-15s with TVC on a twin-engined fighter will improve roll-rate considerably.

The point about T/W is more that the J-20 is closer to a fighter-interceptor; its subsonic maneuverability is at least competitive with 4th generation designs, but its selling point is more its speed, range, and ability to supercruise.

In comparison with the Su-57, the J-20 doesn't trade off stealth for maneuverability, as with the Su-57, but rather trades off stealth for speed and range.

The latter two matter because the strategic / operational factor in an engagement is very important. Speed and range mean that if the enemy does not cluster up, you can exploit a local numerical superiority by concentrating your forces at one point of the enemy line. Even if supersonic flight means that you light up on EOTS/EODAS, you can still supercruise into the combat region before slowing down. Range also means that your bases can be dispersed throughout a massive front, and each base has massive area coverage, and when striking, you can go very deep into enemy territory.

===

Besides the usual fanboy (supersonic maneuvering matters a lot in a dogfight!) vs cynic arguments, does anyone have any idea whether the J-20 can carry PL-XX/21s on its fuel pylons? It'd sacrifice stealth while doing so, but it'd provide an alternative counter-AEW&C option to the J-16 / J-11s, and what's more, the J-20s can supercruise. It might be able to obtain a Mach 1.4 supercruise capability (sufficient dry thrust to break the Mach barrier) if it has such excellent T/W, which given its high-speed design, would make it well-suited for interception (and this is a role of air superiority fighters, air superiority fighters can't strike, interceptors can't dogfight, strike fighters can't intercept).

You haven’t addressed my points. I’m not debating maneuverability here, which I concede that the Raptor and Su-57 are superior in currently. I’m saying that roll rate has nothing to do with thrust to weight. Do you agree?

 

[Inst]

You haven’t addressed my points. I’m not debating maneuverability here, which I concede that the Raptor and Su-57 are superior in currently. I’m saying that roll rate has nothing to do with thrust to weight. Do you agree?

They're semi-related; elevators induce roll by elevating the aircraft in different directions on different parts of the aircraft, using air-speed and induced drag to rotate the airframe.

The point regarding T/W is more that the J-20 is not designed to be a superlative dogfighter like the Su-57 is. It's closer to the F-35; maneuverability (at realistic fuel percentages) is good enough, but the J-20 wants to exploit its sensor advantage (purportedly the largest AESA of any 5th gen, EODAS, EOTS), its speed, and its range. The T/W helps with its maneuverability, true, especially since the J-20's planned TVC will bleed off energy like crazy and the T/W countering the energy loss, but what the T/W helps with more is the raw acceleration.

 

[ZeEa5KPul]

@siegecrossbow There's an aerodynamics thread where we put all of @Inst's "contributions", which are OT in this thread.

Aerodynamics thread

I created this thread at Blitzo's request so we can stop cluttering threads about specific fighters with aerodynamic explanations, questions, and debates.

www.sinodefenceforum.com

 

[Xizor]

They're semi-related; elevators induce roll by elevating the aircraft in different directions on different parts of the aircraft, using air-speed and induced drag to rotate the airframe.

The point regarding T/W is more that the J-20 is not designed to be a superlative dogfighter like the Su-57 is. It's closer to the F-35; maneuverability (at realistic fuel percentages) is good enough, but the J-20 wants to exploit its sensor advantage (purportedly the largest AESA of any 5th gen, EODAS, EOTS), its speed, and its range. The T/W helps with its maneuverability, true, especially since the J-20's planned TVC will bleed off energy like crazy and the T/W countering the energy loss, but what the T/W helps with more is the raw acceleration.

It indeed doesn't have guns ( for now) or the slated engines ( for now). But how does that make the fighter not designed to be a dogfighter? I would reserve all comments about the fighter considering that it is a development in progress ( batches).

And in order to argue about T/W , do you know the weight of J-20? The Tsinghua paper that Wiki uses categorizes its weight ( air combat, max takeoff etc as a good 2-4 tons less than F22)
Do compare with the openly available numbers for both the aircraft.

 

[latenlazy]

T/W has nothing to do with roll rate. F-104 is optimized for for high supersonic speed, has extremely high wing loading due to small wing size, turns about as well as an air liner, has a lower thrust to weight than many legacy Soviet designs, but has one of the highest roll rates among 2nd generation fighter aircraft. Roll rate depends upon the moment of inertia, which depends on wingspan/number of engines and engine placement. In general, twin engined fighters don’t roll as well as single-engined fighters and widely spaced engines don’t roll as well as closely spaced engines. Case in point, F-15 vs F-16 and F-14 vs F-18.

And high aspect ratio wings tend not to roll as well as low aspect ratio wings. But a central factor beyond the distribution of features is how concentrated or distributed your center of mass is.