J-20 5th Generation Fighter VII


voyager1

Captain
Registered Member
Thanks for the reply. I understand when it comes to wings. But I'm not too sure about the canards. Perhaps we should expect it to behave much like the wings, perhaps we shouldn't, I don't know. That's why I'm asking as my knowledge on this point is rudimentary at best.
Obviously such small surfaces wont flex as much, the wing example was something I quickly found on YouTube so I could give you a better idea on how it looked in real life

This is more of a physics and material science question so AFAIK the general rule is that small surfaces flex much less than bigger ones. With these high speeds, and sudden changes in the aircraft direction the canards must flex otherwise they will get destroyed.

I am sure the Chinese did their own homework before investing billions into this project.
 

crash8pilot

Junior Member
Registered Member
Thanks for the reply. I understand when it comes to wings. But I'm not too sure about the canards. Perhaps we should expect it to behave much like the wings, perhaps we shouldn't, I don't know. That's why I'm asking as my knowledge on this point is rudimentary at best.
Principles of Flight was my worst and most hated exam I had to write when I was qualifying as a commercial pilot, so you'll have to excuse my piss poor explanation lol. That said wings, ailerons, rudders, elevators, canards, even fan blades on a turbofan engine, are all aerofoils. They'll all share the same aerodynamic and aeroelastic properties.

While flutter is seen as dynamically unstable, it can create a statically stable effect to the overall airframe. Just think of the springs in your car suspension - the more bounce the springs absorb, the smoother the car ride. The same analogy can be drawn to the video voyager posted of the 737's wing fluttering and what I mean by dynamic instability (wing flapping about) vs static stability (stable flight path).

Now how all this applies to the J-20's canards, I'm not entirely sure... I'd imagine they're made of stronger material than the wing of an airliner considering they'd be designed for maximum performance in combat rather than straight-and-level cruise flight.
 

Ndla2

New Member
Registered Member
Control surfaces by the very nature of composite usage will flex and deform when under load, as do the wings of the aircraft. It should be fine as long as flutter isn’t introduced.

Obviously such small surfaces wont flex as much, the wing example was something I quickly found on YouTube so I could give you a better idea on how it looked in real life

This is more of a physics and material science question so AFAIK the general rule is that small surfaces flex much less than bigger ones. With these high speeds, and sudden changes in the aircraft direction the canards must flex otherwise they will get destroyed.

I am sure the Chinese did their own homework before investing billions into this project.

Principles of Flight was my worst and most hated exam I had to write when I was qualifying as a commercial pilot, so you'll have to excuse my piss poor explanation lol. That said wings, ailerons, rudders, elevators, canards, even fan blades on a turbofan engine, are all aerofoils. They'll all share the same aerodynamic and aeroelastic properties.

While flutter is seen as dynamically unstable, it can create a statically stable effect to the overall airframe. Just think of the springs in your car suspension - the more bounce the springs absorb, the smoother the car ride. The same analogy can be drawn to the video voyager posted of the 737's wing fluttering and what I mean by dynamic instability (wing flapping about) vs static stability (stable flight path).

Now how all this applies to the J-20's canards, I'm not entirely sure... I'd imagine they're made of stronger material than the wing of an airliner considering they'd be designed for maximum performance in combat rather than straight-and-level cruise flight.
Thank you guys for the reply. I appreciate it.
 

Richard Santos

Senior Member
Registered Member
Does it means that the canards cant take that much of a g-loading? Not good at all considering J-20 is supposed to be the most advanced fighter China has to offer :(


Aero-elastic behavior is unavoidable. Whether it is good or not depends on how well it is taken into account, or even leverage for desirable behavior, in the design of the component. Typically the fact that airplane parts will flex under load is perfectly understood. What is harder to understand and predict is how exactly each component flexes in detail when under true flight load, particularly if the load is rapidly changing because the aircraft is maneuvering.

In very bad scenarios, imperfect understanding of aeroelastic behavior can lead to designs that allow different components on the same aircraft to collide as they flex through their range of deformation. One extreme case had helicopter rotor blade strike the helicopter’s own canopy.

In more common case, and pertinent here, is the fact that things like canards are not perfect rectangles, but sweeps back. This means lift forces that tend to bend it upwards would also tend to twist it. The tip of rear swept airfoil will tend to twist in a way that increase the angle of attack. If this kind of twisting is not considered and counteracted in the design of the airfoil, then what will happen is as lift load on the airfoil increases, the tip’s angle of attack will also gradually increase as the air foil twist. So eventually the airfoil tip will stall even if in theory, the whole airfoil has not reached stall angle of attack.

Many a early fighter jets of 1st and 2nd generation crashed mysteriously because their swept wings twisted and stalled in hard maneuvers even when the nominal angle of attack of the wing was still well below stalling. This is why the 3rd and 4th generation of fighters such as F-16, F-15, and Su-27’s wings are naturally twisted. Look carefully at their wings. Their wings are twisted when the aircraft is sitting on the ground so the tips have lower angle of attack than the roots. This is to enable the tip to twist aero-elastically through a number of degrees without causing the angle of attack of the tip to exceed that of the root, so wing tip would not suddenly stall during hard maneuvers.

For fighters like j-20, with composite wings and canards, such tip twist is no longer necessary. The composite material can be designed to be non-isotopic, so even if made into swept back shape, they don’t twist as they deform under load. You can see from the j-20 cockpit footage that the tips of canard might deflect upwards, but it does not twist as it deflects.
 

siegecrossbow

Lieutenant General
Staff member
Super Moderator
Aero-elastic behavior is unavoidable. Whether it is good or not depends on how well it is taken into account, or even leverage for desirable behavior, in the design of the component. Typically the fact that airplane parts will flex under load is perfectly understood. What is harder to understand and predict is how exactly each component flexes in detail when under true flight load, particularly if the load is rapidly changing because the aircraft is maneuvering.

In very bad scenarios, imperfect understanding of aeroelastic behavior can lead to designs that allow different components on the same aircraft to collide as they flex through their range of deformation. One extreme case had helicopter rotor blade strike the helicopter’s own canopy.

In more common case, and pertinent here, is the fact that things like canards are not perfect rectangles, but sweeps back. This means lift forces that tend to bend it upwards would also tend to twist it. The tip of rear swept airfoil will tend to twist in a way that increase the angle of attack. If this kind of twisting is not considered and counteracted in the design of the airfoil, then what will happen is as lift load on the airfoil increases, the tip’s angle of attack will also gradually increase as the air foil twist. So eventually the airfoil tip will stall even if in theory, the whole airfoil has not reached stall angle of attack.

Many a early fighter jets of 1st and 2nd generation crashed mysteriously because their swept wings twisted and stalled in hard maneuvers even when the nominal angle of attack of the wing was still well below stalling. This is why the 3rd and 4th generation of fighters such as F-16, F-15, and Su-27’s wings are naturally twisted. Look carefully at their wings. Their wings are twisted when the aircraft is sitting on the ground so the tips have lower angle of attack than the roots. This is to enable the tip to twist aero-elastically through a number of degrees without causing the angle of attack of the tip to exceed that of the root, so wing tip would not suddenly stall during hard maneuvers.

For fighters like j-20, with composite wings and canards, such tip twist is no longer necessary. The composite material can be designed to be non-isotopic, so even if made into swept back shape, they don’t twist as they deform under load. You can see from the j-20 cockpit footage that the tips of canard might deflect upwards, but it does not twist as it deflects.

The main wings of J-20 still retains the twist however, as does that of the F-22 but it is less apparent due to the relative thickness of the wings.
 

Richard Santos

Senior Member
Registered Member
The wings of F-22 and J-20 are statically twisted for a different reason. These wings do not twist dynamically to any appreciable degree under either increasing lift, or shift in center of lift. The slight permanent static twist from root to tip is designed to ensure if the aircraft stalls in extreme maneuvers, the wing tip is the last to stall and remains least stalled. This ensures aileron control is maintained for as long as possible to facilitate recovery in the case the aircraft stalls.
 

Richard Santos

Senior Member
Registered Member
They were done with afterburners. It is just that the flames aren’t visible due to the lighting.

In actual service, air combat maneuvers like these would be done only in relatively close combat, and then it would be done mostly without afterburners to avoid giving marginal IR shots an increased chance of hitting it. You would only light up afterburners if you lost a lot of energy and must gain the energy back quickly regardless of consequences.

Afterburners should be thought of as mostly an emergency device in combat, not something you light up whenever you feel the need for speed.
 

siegecrossbow

Lieutenant General
Staff member
Super Moderator
A resident in Anshan claims to have spotted a J-20 this afternoon (presence confirmed by satellite image earlier this year). The next piece of big news would be either high def image or multiple J-20s flying in formation in the region.
 

Volpler11

New Member
Registered Member
From a mechanical engineering point of view, it is possible to design the wing to be stiff so it wont flex. But it would be heavier and more bulky. If you can get away with some flex, it would reduce the weight of the aircraft, which is important for a combat aircraft. Also modern wings, especially when made with composites are designed to flex in flight for better aerodynamic performance. Just google Boeing 787 wing flex. It is meant to mimic how bird wings looks.
 

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