Chengdu J-10 Multirole Fighter Air Craft News & Discussions

[enroger]

So the plane will always try to pitch down? It sounds like a stable design as the center of gravity is in front of the lift center of the main wing. A disturb increases AOA will generate more lift on the main wings and push the nose down as the main wings always generate more lift than the canards. Am I right? I remember I saw a video of Rafael on level flight, the canards were clearly moving up and down(sure it was negative AoA).
Also, it seems the wingtip of J10's canards twist down a lot and it seems they are trying to reduce the downwash?
as seen in last post by LKJ86.

No, it is the exact opposite; when in positive AoA the plane naturally wants to pitch up, the canards have to actively deflect downward to keep the nose down, the design is unstable.

My point is that in those instance the canards does not produce negative lift to hold the nose downward, it merely has to produce less positive lift because lift was uniformly distributed between main wind and canard.

I can't say I noticed any canard twist? If there is twist it is very subtle...

 

[TOPGUN]

J-10C
View attachment 636117 View attachment 636118 View attachment 636119
Via CCTV 7 and @沉默的山羊 from Weibo

I am in love with J-10C

 

[laser911]

No, it is the exact opposite; when in positive AoA the plane naturally wants to pitch up, the canards have to actively deflect downward to keep the nose down, the design is unstable.

My point is that in those instance the canards does not produce negative lift to hold the nose downward, it merely has to produce less positive lift because lift was uniformly distributed between main wind and canard.

I can't say I noticed any canard twist? If there is twist it is very subtle...

I mean when talking about stability of aircraft, people normally use this assumption: when the center of lift is behind the center of gravity, the aircraft is stable as if a small disturb increases the AOA, the lift increase and try to push the nose down to decrease the AOA. Therefore stabilise the aircraft. If the center of gravity in these canard fighters is in front of the center of lift of the main wing, it looks like a stable plane for me.
Found a few pages talking about this tho. Not saying they are 100% correct but make sense to me.
https://aviation.stackexchange.com/...t-dont-involve-a-flight-surface-that-provides
https://www.secretprojects.co.uk/th...ontrol-in-usa-and-other-countries.1624/page-3

For the twist, if you look at the image in ljk86 posted and my post, the wingtip front lead is facing down while rear edge seems level or slightly upward.

 

[enroger]

I mean when talking about stability of aircraft, people normally use this assumption: when the center of lift is behind the center of gravity, the aircraft is stable as if a small disturb increases the AOA, the lift increase and try to push the nose down to decrease the AOA. Therefore stabilise the aircraft. If the center of gravity in these canard fighters is in front of the center of lift of the main wing, it looks like a stable plane for me.
Found a few pages talking about this tho. Not saying they are 100% correct but make sense to me.
https://aviation.stackexchange.com/...t-dont-involve-a-flight-surface-that-provides
https://www.secretprojects.co.uk/th...ontrol-in-usa-and-other-countries.1624/page-3

For the twist, if you look at the image in ljk86 posted and my post, the wingtip front lead is facing down while rear edge seems level or slightly upward.

If you replace "center of lift" with "aerodynamic center" in your sentence than you would be right, those two are very different things
https://en.wikipedia.org/wiki/Aerodynamic_center

In any case you can't possibly eye-ball a plane and tell where their CoM, aerodynamic center are. However you can tell whether a canard delta is stable or unstable by looking at how their canards behave. I've said already J-10's canards exhibit typical unstable behavior (pitch down when positive AoA to keep nose down just like rafale/typhoon/gripen...etc). If the plane does not have fbw it would easily flip over due to inherit instability...

As for the twist, you may be right it's there, but honestly its too subtle... I don't know the purpose of it. However J-10 main wing does have twist, usually it is for the purpose to prevent wingtip stalling before wing root (since the twist made incidence AoA smaller for wing tip), since wingtip stalling is disastrous for roll authority. I don't know what the twist does for canards though.

 

[laser911]

If you replace "center of lift" with "aerodynamic center" in your sentence than you would be right, those two are very different things
https://en.wikipedia.org/wiki/Aerodynamic_center

In any case you can't possibly eye-ball a plane and tell where their CoM, aerodynamic center are. However you can tell whether a canard delta is stable or unstable by looking at how their canards behave. I've said already J-10's canards exhibit typical unstable behavior (pitch down when positive AoA to keep nose down just like rafale/typhoon/gripen...etc). If the plane does not have fbw it would easily flip over due to inherit instability...

As for the twist, you may be right it's there, but honestly its too subtle... I don't know the purpose of it. However J-10 main wing does have twist, usually it is for the purpose to prevent wingtip stalling before wing root (since the twist made incidence AoA smaller for wing tip), since wingtip stalling is disastrous for roll authority. I don't know what the twist does for canards though.

Sorry, was using the wrong word. It is difficult to tell the actual AoA from pic, but from I think the AoA of canards are still positive (although it pitchs down but just smaller AoA than main wing) when the aircraft at high AoA. At least in some situations. As some pics show white steam above the canards.
EF2000: cant find any
Rafael: difficult but found one

https://in.reuters.com/article/braz...nch-fighter-jet-sources-idINDEE81C00X20120213
Jas39: found a few

J10: found a few

 

[enroger]

Sorry, was using the wrong word. It is difficult to tell the actual AoA from pic, but from I think the AoA of canards are still positive (although it pitchs down but just smaller AoA than main wing) when the aircraft at high AoA. At least in some situations. As some pics show white steam above the canards.
EF2000: cant find any
Rafael: difficult but found one

https://in.reuters.com/article/braz...nch-fighter-jet-sources-idINDEE81C00X20120213
Jas39: found a few

J10: found a few

Yes, this is exactly what I'm talking about a few posts back, lemme quote it again:

The canards must be positive AoA with respect to airstream in those pic you see, they are just negative deflect with respect to the plane. This is expected for a relaxed stability delta canard plane, the canards are still producing lift in those instances, the more AoA the plane is pulling the less lift from canards to keep the nose down.

 

[laser911]

Yes, this is exactly what I'm talking about a few posts back, lemme quote it again:

But it seems eurofighter doesnt. Is it because it has a longer lever or it actually doesnt provide much lift?

I also check some more pic of J10, it seems the canards sometimes go totally negative AoA at almost level fly.

 

[enroger]

But it seems eurofighter doesnt. Is it because it has a longer lever or it actually doesnt provide much lift?

I also check some more pic of J10, it seems the canards sometimes go totally negative AoA at almost level fly.
View attachment 636758

eurofighter doesn't what?

Those photos can be deceptive, you can't tell the true flight vector from this picture; what is the true horizon? is the plane in leveled flight and not descending/ascending/maneuvering?...etc

Edit: actually true horizon can be obtained from the picture, you notice those towers in the background? Assuming those are vertical then horizon is tilted a few degrees.

One of the key advantages canard delta has over conventional layout is the reduction of trim drag, since canard produce positive lift to maintain trim. For canard to actually produce negative lift in leveled flight(thus increase trim drag) would imply staggeringly incompetent design....

 

[laser911]

eurofighter doesn't what?

Those photos can be deceptive, you can't tell the true flight vector from this picture; what is the true horizon? is the plane in leveled flight and not descending/ascending/maneuvering?...etc

Edit: actually true horizon can be obtained from the picture, you notice those towers in the background? Assuming those are vertical then horizon is tilted a few degrees.

One of the key advantages canard delta has over conventional layout is the reduction of trim drag, since canard produce positive lift to maintain trim. For canard to actually produce negative lift in leveled flight(thus increase trim drag) would imply staggeringly incompetent design....

It seems eurofighter doesn't need much lift from canards.

Could J10's canards be negative AoA but producing 0 lift or very little lift as it is not using symmetric airfoil?

 

[enroger]

It seems eurofighter doesn't need much lift from canards.

Could J10's canards be negative AoA but producing 0 lift or very little lift as it is not using symmetric airfoil?

yes, Ef2000's canard is pure control device, no lift, not even vortex generation.

also yes, I maybe a little hasty in saying negative AoA->negative lift, not true for asymmetric foil.

However that picture does not show what you think it shows. As I said pay attention to those towers in the background to correct for true horizon (the picture itself is tilted), then you'll see even if you assume the plane is in leveled flight, the canards are at about zero degree with respect to horizon.

 

[LKJ86]

"J10C0272"

Via CCTV and @沉默的山羊 from Weibo

"J10C0274"

Via CCTV 7 and @沉默的山羊 from Weibo

 

[laser911]

yes, Ef2000's canard is pure control device, no lift, not even vortex generation.

also yes, I maybe a little hasty in saying negative AoA->negative lift, not true for asymmetric foil.

However that picture does not show what you think it shows. As I said pay attention to those towers in the background to correct for true horizon (the picture itself is tilted), then you'll see even if you assume the plane is in leveled flight, the canards are at about zero degree with respect to horizon.

Thanks a lot. One more question from me. The aerodynamic center, known as the point where the moment is not affected by lift, but for canard planes like J10, the lift and pressure distribution varies a lot. How the aerodynamic center not moving backward/forward when canards are moving?

 

[enroger]

Thanks a lot. One more question from me. The aerodynamic center, known as the point where the moment is not affected by lift, but for canard planes like J10, the lift and pressure distribution varies a lot. How the aerodynamic center not moving backward/forward when canards are moving?

That's a very in depth question that is a little outside my knowledge, you'll really have to study an aircraft design textbook...

In general, not only aero-center would shift due to control surface movements, all the flight coefficients will shift as well. This does not only apply to J-10 but to all aircraft as well.

Simplest way to deal with this is to start with a default model, then treat all control surfaces variation as linear perturbation as if each control surfaces change independently from each other. Of course if you do this you'll miss all the control surfaces coupling especially the canards have such large impact on main wing flow field.

Modern plane development exhaustively test different control surface config with CFD and wind tunnel tests then followed by years of test flight to correct the flight model, it gets very complicated...

 

[laser911]

That's a very in depth question that is a little outside my knowledge, you'll really have to study an aircraft design textbook...

In general, not only aero-center would shift due to control surface movements, all the flight coefficients will shift as well. This does not only apply to J-10 but to all aircraft as well.

Simplest way to deal with this is to start with a default model, then treat all control surfaces variation as linear perturbation as if each control surfaces change independently from each other. Of course if you do this you'll miss all the control surfaces coupling especially the canards have such large impact on main wing flow field.

Modern plane development exhaustively test different control surface config with CFD and wind tunnel tests then followed by years of test flight to correct the flight model, it gets very complicated...

Indeed. I also found the leading edge flaps of J10 move downward a bit most of the time, but this one upward a bit when in air. It seems the FCS is far more complicated than I thought.

 

[LKJ86]

J-10C

Via 解放军报