Aerodynamics thread

[siegecrossbow]

Re: J-20... The New Generation Fighter III

According to starikki in this post the aerodynamic center in a canard aircraft is different from that of a conventional aircraft. In a conventional aircraft the AC overlaps with the center of lift of the main wing. This is not the case for canard aircraft, which have an aerodynamic center that is way closer to the front than the center of lift on the main wing. This is further exaggerated in the case of the J-20 when we take into account body lift.

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[MiG-29]

Re: J-20... The New Generation Fighter III

According to starikki in this post the aerodynamic center in a canard aircraft is different from that of a conventional aircraft. In a conventional aircraft the AC overlaps with the center of lift of the main wing. This is not the case for canard aircraft, which have an aerodynamic center that is way closer to the front than the center of lift on the main wing. This is further exaggerated in the case of the J-20 when we take into account body lift.

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F-22 does have lifting body fuselage traits thanks to wing-fuselage blending, this feature is also seeing on J-20.
что F-22A вобрал в себя наиболее прогрессивные черты истребителей четвертого поколения. Во-первых, это интегральная аэродинамическая компоновка - плавное сопряжение крыла и фюзеляжа
on J-20 i think, the only body lift you can get, is like F-22, with the wing fuselage blending, other possible source at high AoA could be the forebody vortices shed by the nose, you have jets with canards that are stable, in example Mirage III/V/Kfir family or Viggen.

Forebody vortices at high AoA are shed by many aircraft including Mirage 2000, F-15, Rafale, X-31, MiG-23 or MiG-29, but proper body lift is achieved by blending the wing and fuselage like F-16 or cambering the nose like F-16 or MiG-1.44


Forebody shape is driven by
Cockpit visibility requirements which usually govern
forebody camber.
High-AoA handling which influences the length, crosssectional
shape and application of nose strakes
Requirement for radar and laser-ranging installations influece
the nose size and shape
Crew accommodation, including cockpit canopy design,
governs the cross-sectional area
Forward camber: positive camber generates a negative
pitching moment, so reducing the forebody camber will
reduce the horizontal tail download required.

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In recent years many advanced fighters, such as the F-22 and F-35, have incorporated a chine-shaped forebody designed to minimize observables. The chine generally results in stronger forebody vortices being shed compared to traditional fighters with smooth forebodies. For certain flight conditions these vortices interact with the wing leading edge vortices, and improve the maneuvering lift capabilities.1 However, for other flight conditions these vortex interactions are detrimental to the aerodynamic characteristics of the vehicle. This is especially true when the fighter experiences moderate to low angles of sideslip at angles of attack in excess of 25˚. Under these conditions abrupt asymmetric vortex breakdown leads to pronounced pitch-up and significant nonlinearities in lateral stability that could result in roll departure

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[Engineer]

Re: J-20... The New Generation Fighter III

According to starikki in this post the aerodynamic center in a canard aircraft is different from that of a conventional aircraft. In a conventional aircraft the AC overlaps with the center of lift of the main wing. This is not the case for canard aircraft, which have an aerodynamic center that is way closer to the front than the center of lift on the main wing. This is further exaggerated in the case of the J-20 when we take into account body lift.

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Keep in mind that LERX also contributes lift to the aircraft. We can see from the following charts that the early concept of J-20 saw a 17.5% increase in lift (181% vs. 154%) over typical canard configuration through the addition of LERX.

For J-20, it has not one but two sets of LERX: one set in front of the main wings, and a F-22/F-35 style set in front of the canards. Their existence further brings the aerodynamic center forward. Imagine J-10 but with 20~25% of total lift of the aircraft situates in of the aerodynamic center, and this explains why J-20 has its C.G. further forward than J-10 in comparison.

 

[Air Force Brat]

Re: J-20... The New Generation Fighter III

My own view is that turning is like generating lift for the aircraft sideways. (abit with some downward lift to counter gravity.)

In J-20's case the nose area has excessive lift compared to the rest of the aircraft, therefore the canard has to work to 'press the nose down' in line with the rest of the plane, so that the aircraft does not pitch too much and stall.

So it's a balance between how quickly you can change the heading of the plane's nose (which will determine the direction that the enging is pushing the plane forward, as the nose/tail is aligned down the centre of the plane) versus having enough speed in the direction that the nose is pointing to continue to generate the required lift for countering gravity. A nose that moves too fast risk decreasing the velocity of the plane and make it stall.

I think that this problem exists in J-20 and needed to be corrects actually shows that the plane is pretty maneuverable to the extent that it has to be reined in. It is not being corrected because it is a flaw, other plane don't need to correct for this because they didn't have enough nose lift in the first place.

Outstanding No Name, first off you are right, and the terms you have used to describe my climbing around the circle description, are right on, the second thing is that the J-20 is designed to have forward fuselage lift to combat the phenomenon referred to as "mach tuck" the tendency of the aircraft to tuck the nose entering and passed mach one, while going through mach one is no biggie these days, such was not always the case, all airplanes do very bad things when flown past VNE. or VMAX. As Dr. Song so eloquently annunciattes, the J-20 must achieve aerodynamically what other aircraft are able to do on abundant thrust, and hence the reason the J-20 carries a little neg pitch on the canards at high angles of attack subsonically. It is design optimized for VMAX, and to supercruise, without compromising high aoa stability or safety as you manuever to advantage. Another realm that might help you visualize the need for the canards to deflect downward is hydroplane racing Ie Miss Buddweiser and other race boats that have twin sponsons in order to generate lift to get most of the hull out of the water. You will note all the boats have a wing on the rear and some means of lift on the hull. If you watch those races for a couple of heats, invariably somebody gets a little loose and then the front of the boat lifts, the more it lifts the faster it lifts and the next thing you know your boat is in little bitty pieces, and hopefully the cockpit is still intact, having lifted completely off the water before it literally disentigrates. Indy cars since the late sixties have also been designed to generate down force on both ends.

 

[MiG-29]

Re: J-20... The New Generation Fighter III

Outstanding No Name, first off you are right, and the terms you have used to describe my climbing around the circle description, are right on, the second thing is that the J-20 is designed to have forward fuselage lift to combat the phenomenon referred to as "mach tuck" the tendency of the aircraft to tuck the nose entering and passed mach one, while going through mach one is no biggie these days, such was not always the case, all airplanes do very bad things when flown past VNE. or VMAX. As Dr. Song so eloquently annunciattes, the J-20 must achieve aerodynamically what other aircraft are able to do on abundant thrust, and hence the reason the J-20 carries a little neg pitch on the canards at high angles of attack subsonically. It is design optimized for VMAX, and to supercruise, without compromising high aoa stability or safety as you manuever to advantage. Another realm that might help you visualize the need for the canards to deflect downward is hydroplane racing Ie Miss Buddweiser and other race boats that have twin sponsons in order to generate lift to get most of the hull out of the water. You will note all the boats have a wing on the rear and some means of lift on the hull. If you watch those races for a couple of heats, invariably somebody gets a little loose and then the front of the boat lifts, the more it lifts the faster it lifts and the next thing you know your boat is in little bitty pieces, and hopefully the cockpit is still intact, having lifted completely off the water before it literally disentigrates. Indy cars since the late sixties have also been designed to generate down force on both ends.

You know what i can not yet understand, is why people think the J-20 and F-22 by default will have extensive forebody lif, if you see what the concept of fuselage wing blending means is the forebody is almost oval and transitions into the strake and later wing blending the fuselage like the F-16 or MiG-29.
Other jets only have wing-fuselage blending Rafale for example has the wing highly blended upon the fuselage.
Song`s paper talks about wing fuselage blending, which is easy to say the wing blends with the fuselage, and the upper fuselage might have a little camber, on F-22 you see the upper fuselage has the looks more of a mid wing since the upper fuselage is rounded on top and kind of blends too with the wing, something similar you see in J-20, however the forebodies of J-20 and F-22 have diamond cross sections and they are followed by the intakes, on F-16 the nose cross section is semi-oval, it is followed by the transition of the oval fuselage cross section that blends with strakes and blends perfectly with the wing, thus it is called fuselage-wing blended cross section, i do not see that happening on F-22 or J-20, the intakes break the blending.



By the virtue of having diamond cross section, the lift generated by the forebody is null, unless we consider the effects of the forebody vortices at high AoA might have in stabilizing the vortex system and therefore increasing lift at high AoA by reducing assymetric vortex burst.

But if you look at YF-23, you see the chines blend right away with the wings and the upper fuselage and a flat bottom fuselage.
If the J-20 has better lift characteristics than convetional 4th generation canard delta fighter how come it does deflect more the canards, it does not make sense from my point of view, specially since some of these jets have fuselage-wing blending, lerxes or strakes too.

It does not make sense since the turn it is executing is relatively not very tight and not very quick, so it means to do it quicker it requieres even more deflection, that is not logic.

In my personal opinion, J-20 has wing-fuselage blending, but has been optimized for stealth with a set of contradictory requierements (like any stealth fighter) leaving some compromises that can only be fixed with TVC nozzles and are up to a level fix by the carriage of internal weapons

 

[Quickie]

Re: J-20... The New Generation Fighter III

Do you think the J-20 will deflect the canards further to turn tighter?
In my opinion J-20 will improve not becuase as you think its aerodynamics are uncompromised, but because once it adds TVC nozzles it will increase high AoA agility

... as you think its aerodynamics are uncompromised,...

Where did that come from? Never claimed that.

Do you think the J-20 will deflect the canards further to turn tighter?

It's quite ridiculous to just base your analysis on just the degree of canard deflections.
The J-20 rely entirely on its canards and tail control surfaces for pitch control and so we are bound to see more action in these control surfaces. The rafale and gripen, because of the way it's designed, also uses its elevons, probably more than its canards, for pitch control. (The LCA Tejas depends entirely on its elevons since it doesn't have canards)

In fact, you can see how ineffective the rafale canards is in pitch control during landing and in a slow speed maneuovre in one of the videos, i.e. even a >45 degrees canard deflection doesn't seem to affect the aircraft pitching much. Would you then say the rafale is a very stable aircraft just based on these instances?

 

[delft]

Re: J-20... The New Generation Fighter III

The leading edge flaps increase camber, they increase lift without the need to pitch the nose up, they do however increase drag, and in order to maintain airspeed, you must pitch the nose down or increase thrust.

Not quite. The leading edge flaps allow you to go to a higher angle of attack without stalling and so reach a higher lift coefficient.

 

[MiG-29]

Re: J-20... The New Generation Fighter III

Where did that come from? Never claimed that.




It's quite ridiculous to just base your analysis on just the degree of canard deflections.
The J-20 rely entirely on its canards and tail control surfaces for pitch control and so we are bound to see more action in these control surfaces. The rafale and gripen, because of the way it's designed, also uses its elevons, probably more than its canards, for pitch control. (The LCA Tejas depends entirely on its elevons since it doesn't have canards)

In fact, you can see how ineffective the rafale canards is in pitch control during landing and in a slow speed maneuovre in one of the videos, i.e. even a >45 degrees canard deflection doesn't seem to affect the aircraft pitching much. Would you then say the rafale is a very stable aircraft just based on these instances?

It is not my analysis, it is airfleet magazine`s and it corresponds with some facts about canards.

Long coupled canards have a longer arm, thus a very good respose and lower need for deflection when the canard is near the nose of the aircraft like Tu-144, thus Eurofighter moved forward the canard from its original position on the EAP, the eurofighter technology demostrator, originally the canard on EAP was closer to the wing, the Eurofighter moved the canard for one single reason lower drag and better pitching moment, so moving the canard forward from the wing in a long couple canard position will reduce drag and canard deflection.

On a close coupled canard a higher longitudinal instability means a quicker pitch moment too, thus lower deflection at high AoA but still higher drag at cruise flight.

The reason the J-20 is not moving the elevon too much is simply because elevons are flaps and have a nose down pitch movement, thus canards are usually the pitch control force as you can see while they take off.

Tailess delta fighters like LCA use a high relaxed stability reducing elevon deflection, with relaxed stability the tailess aircraft will pitch up easily.

Fighters with canards use a near neutral longitudinal position when they relax the stability, otherwise the jet will need to deflect the canards excesivly to create a pitch down force and keep the nose straight at level flight, so for supercruising excesive longitudinal instability will mean higher trim drag, Rafale can supercruise, and the same applies for Eurofighter.


J-20 deflects a lot the canard, according to the analisys simply because it has a short lever arm on a long coupled canard, the reason is the canard`s position is very close to the center of gravity, thus during turns it requieres higher canard deflection than other similar canard delta fighters.

According to the text, contrary to Eurofighter that has its canards near the nose and farther from the center of gravity; J-20`s designers wanted the canard behind the supersonic shock and they positioned the canard on the intake like gripen followed by the wing; setting the wing behind after the canard and not below the canard like Rafale, thus the center of gravity is well ahead of the MAC of the wing

 

[Quickie]

Re: J-20... The New Generation Fighter III

It is not my analysis, it is airfleet magazine`s and it corresponds with some facts about canards.

Long coupled canards have a longer arm, thus a very good respose and lower need for deflection when the canard is near the nose of the aircraft like Tu-144, thus Eurofighter moved forward the canard from its original position on the EAP, the eurofighter technology demostrator, originally the canard on EAP was closer to the wing, the Eurofighter moved the canard for one single reason lower drag and better pitching moment, so moving the canard forward from the wing in a long couple canard position will reduce drag and canard deflection.

On a close coupled canard a higher longitudinal instability means a quicker pitch moment too, thus lower deflection at high AoA but still higher drag at cruise flight.

The reason the J-20 is not moving the elevon too much is simply because elevons are flaps and have a nose down pitch movement, thus canards are usually the pitch control force as you can see while they take off.

Tailess delta fighters like LCA use a high relaxed stability reducing elevon deflection, with relaxed stability the tailess aircraft will pitch up easily.

Fighters with canards use a near neutral longitudinal position when they relax the stability, otherwise the jet will need to deflect the canards excesivly to create a pitch down force and keep the nose straight at level flight, so for supercruising excesive longitudinal instability will mean higher trim drag, Rafale can supercruise, and the same applies for Eurofighter.


J-20 deflects a lot the canard, according to the analisys simply because it has a short lever arm on a long coupled canard, the reason is the canard`s position is very close to the center of gravity, thus during turns it requieres higher canard deflection than other similar canard delta fighters.

According to the text, contrary to Eurofighter that has its canards near the nose and farther from the center of gravity; J-20`s designers wanted the canard behind the supersonic shock and they positioned the canard on the intake like gripen followed by the wing; setting the wing behind after the canard and not below the canard like Rafale, thus the center of gravity is well ahead of the MAC of the wing

Repeating all those aerodynamic principles here, with some errors here and there, don't really get into the point of the discussion.

Fighters with canards use a near neutral longitudinal position when they relax the stability, otherwise the jet will need to deflect the canards excesivly to create a pitch down force and keep the nose straight at level flight, so for supercruising excesive longitudinal instability will mean higher trim drag, Rafale can supercruise, and the same applies for Eurofighter.

By this principle, the J-20 must be very longitudinal unstable since a few of the members here are puzzling over the seemingly pitching down of the canards in most of the maneouvres as seen in the pictures and videos.

Any which way, the article is still wrong in the way it determines the J-20's longitudinal stability by just considering the position of the wing's MAC with respect to CG, and by committing the error of neglecting the overall lift of the aircraft, escpecially the forebody lift enhanced by the few vortex generators at the inlet and LERX as describeed in Dr Song's paper.

 

[no_name]

Re: J-20... The New Generation Fighter III

It does not make sense since the turn it is executing is relatively not very tight and not very quick, so it means to do it quicker it requieres even more deflection, that is not logic.

I think the fact that canards on J-20 has to pitch down more to hold the nose down makes it potentially more maneuverable. If you want tighter turns all you need is to do is to relax the amount of canard deflection downwards.

From what I saw in the video the J-20 is doing a whole lot of deflection could be due to:

-Turning at low speeds, therefore the rate of pitch of the nose needs to be limited, otherwise you are exposing more of the plane cross section to incomming air flow, increasing drag, makes the plane slow down more and eventually stall.

-Engine not at full power, therefore the aircraft is not accelerating enough and this will limit how fast it can turn. This is interrelated with the previous point.

-J-20 will due to have more powerful engine than it currently has, so with it the canard downward deflection could be relaxed, giving it faster response.

-As far as I see the video of J-20 shows it flying and doing maneuver at slower speeds than the F-22 videos.

-Using low speed performance to deduce supersonic performance may not be on track as the centre of lift shifts with different airspeed


Though I think I'll just wait for high performance revealing test flights like what we've seen with the F-22 demos to convince people