J-20... The New Generation Fighter II

[siegecrossbow]

未来战斗机除了具备近代先进战斗机的中低空机动性外，还要具备超声速巡航能力，以及过失速等非常规机动能力；同时，隐身特性也是一个需要重点考虑的因素 ⋯。因此，未来战斗机的气动布局，要在满足外形隐身约束条件的前提下，尽可能降低其超声速阻力，改善最大升力特性和大迎角下的稳定性、控制性，同时兼顾亚跨声速升阻特性。如此多的设计点，对气动力设计提出了新的挑战。必须采用新的气动力概念和措施，以及配套的总体与控制措施，在众多的设计点之间权衡折衷取舍，才能获得可行的解决方案。

This is the abstract of Dr. Song Weng Cong's article. Let me briefly summarize:

The "future fighter" will not only have good manoeuvrability at medium and low heights but have the ability to supercruise. The future fighter mentioned in the article is, in essence, a stealth fighter whose design will reduce drag as well as maximize lift.

I just find it funny that Dr. Song's work, presumably the result of 10s of thousands of hours of windtunnel testing, could be so easily dismissed by visual inspections of a few photographs.

 

[johnqh]

Some other interesting points in Dr Song's paper.

Dual all-moving vertical stabilizers, relative size of 10% to 13% (normally 20% to 25%).
Use of movable LEX to control under high AoA. (I guess it is possible to use canards for the same purpose)
Some charts shows testing result with AoA as high as 60%.

 

[MiG-29]

In China they refer to two types of LERX. One of them they call "bian tiao" and the other they call "bian tiao yi". I don't know how to properly translate the two.

LEX or LERX stands for leading edge root extention, in the english language it does not matter the size, however the size and shape will affect some aerodynamic qualities of the aircraft, long thin LERXes increase pitch up tendencies, LERXes have the function of increase lift at high AoA but they add bit of drag too

 

[MiG-29]

You call those LERX?
Size does matter, you know

Those are LERXes you can not deny it, LERX stand for leading edge root extention, therefore the Rafale has LERXes, size and shape do matter that is true, longer they are increase pitch up tendencies, however LERX are called sometimes strakes, shape also matters higher angles of sweep increase the vorticity strength, that is the reason you have very sharp LERXes on the Su-27, LERXes also move the main wing center of lift forward, so they add a degree of relaxed stability to the whole design.

Since LERXes were used and studied in the West first specifically F-5, F-16 and later on MiG-29, the technical terminology is Western in origin regadless in China they use another one.

 

[siegecrossbow]

Those are LERXes you can not deny it, LERX stand for leading edge root extention, therefore the Rafale has LERXes, size and shape do matter that is true, longer they are increase pitch up tendencies, however LERX are called sometimes strakes, shape also matters higher angles of sweep increase the vorticity strength, that is the reason you have very sharp LERXes on the Su-27, LERXes also move the main wing center of lift forward, so they add a degree of relaxed stability to the whole design.

Since LERXes were used and studied in the West first specifically F-5, F-16 and later on MiG-29, the technical terminology is Western in origin regadless in China they use another one.

never mind

 

[MiG-29]

Some other interesting points in Dr Song's paper.

Dual all-moving vertical stabilizers, relative size of 10% to 13% (normally 20% to 25%).
Use of movable LEX to control under high AoA. (I guess it is possible to use canards for the same purpose)
Some charts shows testing result with AoA as high as 60%.

The only moveable LERXes in existance are find on the T-50 and Naval LCA and these are called LEVCON
LEVCON


LEVCON ( Leading Edge Vortex CONtroller ) is a deflectable aerodynamic device in wing apex region LEVCON surface is deflected +20 (down) to 30 (up) from its neutral position.Downward deflection of LEVCON is used for reducing aproach speed for carrier landing


And effectively canards do the same thing LEVCONs do however the LEVCON does not increase the drag as canards do since they do not generate downwash

 

[MiG-29]

You know it's no coincidence that real-life examples of high mach aircrafts like the Blackbird, Concorde, Space Shuttle (which is applicable during reentry to the atmosphere) all have relatively small aft wings and a longer body compared to other slower supersonic aircrafts.

Jet engine power aside, I can't imagine something in the shape of the F-22 flying at more than 3 mach without breaking up.



Are you now alluding to the point that being longer and narrower is better?



I think you still miss the point. A body that's slightly wider and longer than another body, can still be more streamlined than that other body. i.e. The shape of the 2 bodies also have to be taken into account. Furthermore, an aircraft fuselage is just one of the many components contributing to drag, with the wings contributing a large part of it.

The drag equation states that drag (D)is equal to a drag coefficient (Cd) times the density of the air (r) times half of the square of the velocity (V) times the wing area (A).
D = .5 * Cd * r * V^2 * A

pay attention Drag is equal to times the wing area (A).
area is importat since it is multiplying a bigger area makes for a bigger result, thus the J-20 having a longer fuselage will have higher drag, of course shape matters but since both designs have similar cross sections they won`t have a big difference in shape you can not expect the J-20 to have lesser drag just a relatively higher but not by much in the best case

 

[Schumacher]

......

I just find it funny that Dr. Song's work, presumably the result of 10s of thousands of hours of windtunnel testing, could be so easily dismissed by visual inspections of a few photographs.

Now you know. Dr Song and his J20 have ruffled some serious feathers.
Hell hath no fury like a fanboy scorned. LOL

 

[Quickie]

The drag equation states that drag (D)is equal to a drag coefficient (Cd) times the density of the air (r) times half of the square of the velocity (V) times the wing area (A).
D = .5 * Cd * r * V^2 * A

pay attention Drag is equal to times the wing area (A).
area is importat since it is multiplying a bigger area makes for a bigger result, thus the J-20 having a longer fuselage will have higher drag, of course shape matters but since both designs have similar cross sections they won`t have a big difference in shape you can not expect the J-20 to have lesser drag just a relatively higher but not by much in the best case

The equation doesn't tell you anything without the required data. The Cd is different for different aircraft wings. This is in addition to different Cds for different aircraft bodies.

 

[Quickie]

The only moveable LERXes in existance are find on the T-50 and Naval LCA and these are called LEVCON
LEVCON


LEVCON ( Leading Edge Vortex CONtroller ) is a deflectable aerodynamic device in wing apex region LEVCON surface is deflected +20 (down) to 30 (up) from its neutral position.Downward deflection of LEVCON is used for reducing aproach speed for carrier landing


And effectively canards do the same thing LEVCONs do however the LEVCON does not increase the drag as canards do since they do not generate downwash

All vortex generating surface increases drag, including the LEVCON, although the vortex generated may in the end reduce the overall drag. The canard or tail elevator both generate downwash/upwash for pitch control reason.