Aerodynamics thread

[F-15]

LCA is also pretty small and of course is a very different design. I explicitly tied the PAK-FA into my sentence for a reason. I'm not saying LEVCONS are always insufficient on their own, just that Engineer seems to be arguing that on the PAK-FA they are. Anyways, I don't have a stake in this particular argument. Just thought trying to clarify what people were trying to say would help move the discussion along.

LEVCONs control the center of pressure, thus they control the vortex, even the definition of LEVCON says it, by controlling the center pressure they control pitch and lift allowing in LCA for higher elevon effectiveness.
The Naval LCA has higher lift than land based LCAs without the use of canards and it has better vortex control, so his point is moot

 

[latenlazy]

LEVCONs control the center of pressure, thus they control the vortex, even the definition of LEVCON says it, by controlling the center pressure they control pitch and lift allowing in LCA for higher elevon effectiveness.
The Naval LCA has higher lift than land based LCAs without the use of canards and it has better vortex control, so his point is moot

I know how LEVCONs work. As I often say, it's not a question of "what", but "how much".

Also I don't think there's an LCA variant with canards?

 

[Quickie]

After a certain point the difference between the canard deflection angle and angle of attack are too great for the downwash effect to occur. If the canard sustains the difference in angle between itself and the wing it will stall out first, which then disrupts the downwash which then stalls the wing. That or the canard has to deflect downward to prevent over pitching. Both induce a lot of extra drag that isn't overcompensated by extra lift, which hurts the lift drag ratio. LERXes don't have this problem because they generate vortices a different way, but their problem is that they don't begin generating vortices until they've reached a certain angle of attack, and (if I'm not wrong about this point) the inability to terminate that down wash means that an aircraft will have to employ other draggier methods to prevent over pitching. My understanding is the PAK-FA tried to get the best of both worlds by adopting LEVCONS. I suspect the J-20s design may be a different approach to achieve that same purpose.

The PAK-FA still uses the horizontal stabilizers as the main control surface with the associated drag for pitch control despite the LEVCONS being used to complement pitch control in certain flight regimes. This is the same with the LCA which still requires the elevons as the main pitch control surface with its induced drag, which is to be expected for all types of control surfaces. So the question is really the degree of the drag penalty, and how significant it is, and whether there are other solutions to better handle this.

LERXes don't have this problem because they generate vortices a different way, but their problem is that they don't begin generating vortices until they've reached a certain angle of attack, and (if I'm not wrong about this point) the inability to terminate that down wash means that an aircraft will have to employ other draggier methods to prevent over pitching.

Would terminating the downwash be a good idea since it may just kill off the lift immediately, in other words, a premature stalling?

 

[latenlazy]

So the question is really the degree of the drag penalty, and how significant it is, and whether there are other solutions to better handle this.

And that really is the question. It's why I hate it when the discussion turns to "well this feature causes drag" or "that feature causes drag", because all aerodynamic features cause drag.

Would terminating the downwash be a good idea since it may just kill off the lift immediately, in other words, a premature stalling?

Terminating it completely would not be a good idea, but there is still a need to prevent over pitching. In a tailplane design with LERXes the LERXes continue to generate vortices while the tails are deflected upward to push the nose down. In a canard design the wing ends up stalling, but the canards are still aerodynamically loaded so they can push the nose down to recover the aircraft. I suspect that in the J-20's case the canards are mostly responsible for generating lift at high angles of attack up to a point, before they depart and become pitching surfaces, while the LERXes could be optimized for higher angles of attack and take over the vortex generation function.

 

[Engineer]

You simple do not know the LEVCON is a LEX thus it does generate Vortices, it moves the wing center of pressure forward and does generate less drag, this is not said by me but by Sukhoi

So the LEVCON does not need to move forward to reduce drag and keeps the vortex nearer the wing allowing the vortex system to work, moving the canard forward reduces the vortex lift.

While this does not mean a fighter with LEVCON is superior to one with Canards, it only means the LEVCON can allow the designer to reduce drag while maintaining higher lift than if it would had used canards.

In your context, with Thurderchief, the J-20 then needs a small LEX, and now it is shaper to increase the vortex system, since prototype 2001 and 2002 had one blunter, the blunter one has more area, so it generates more lift at low Alpha (AoA) but the newer LEX is better for vortex generation, however it is far too small to really being of great impact.
Thus you can see is something less practical of a solution, Eurofighter uses also the same fix of J-20, it has some strakes too.

I can not tell you which fighter is better, only i can tell you the LEVCON can be used for roll and it does control the vortex system shed by the LEVCON (movable LEX) but a Sharper LEX always is better for vortex generation.

F-18E opted for a larger and blunter LEX for more wing lifting area to upset the higher weight of the new Super Hornet, but lighter F-18C has the better LEX design for vortex generation, but also a shaper LEX generates more drag.

So a LEVCON is a better solution than Canard or LEX if you are looking for some conditions where drag and lift are compromised

No one is disputing LEVCON's ability to generate vortex. Canard generates vortex too, but a long-coupled canard is more effective in pitch authority comparing to LEVCON. The reason has to do with length of the moment-arm. While moving the canard forward does reduce vortex lift, the presence of the wing root extension enhances lift. In fact, when used together, the maximum achievable vortex lift is a higher than what LEX or canard can attain independently. This is not said by me, but by the engineer who developed the layout of the J-20. From left to right are lift contribution by LEX, canard, canard with LEX, body-lift along with canard and LEX.

You mentioned that moving center-of-pressure forward reduces drag. You are correct. Essentially, that has to do with making the aircraft more unstable. However, there is a limit as to how unstable the aircraft can made to be, and that limit is governed by the amount of pitch-down authority presence. Canard can simply go into negative angle-of-attack to achieve that, and so far no other aerodynamic methods can compete. Again, this is said by the engineer who developed the layout of the J-20.

LEVCON is not bad. It is less complicate and more affordable, good for poor countries. However, LEVCON is not a replacement for canard.

 

[Engineer]

LEVCONs control the center of pressure, thus they control the vortex, even the definition of LEVCON says it, by controlling the center pressure they control pitch and lift allowing in LCA for higher elevon effectiveness.
The Naval LCA has higher lift than land based LCAs without the use of canards and it has better vortex control, so his point is moot

On the PAKFA, pitch-authority is achieved by tailplane. On top of that, the PAKFA requires thrust-vectoring. That really highlights the ineffectiveness of LEVCON in pitch-authority. Furthermore, by situating at the leading edge of the wing root, LEVCON took up the space for LEX.

Unlike LEVCON, long-coupled canard like that on the J-20 can control vortex lift and pitch authority. The canard also provides space for LEX, so that the use can be used together to achieve greater vortex lift.

 

[F-15]

No one is disputing LEVCON's ability to generate vortex. Canard generates vortex too, but a long-coupled canard is more effective in pitch authority comparing to LEVCON. The reason has to do with length of the moment-arm. While moving the canard forward does reduce vortex lift, the presence of the wing root extension enhances lift. In fact, when used together, the maximum achievable vortex lift is a higher than what LEX or canard can attain independently. This is not said by me, but by the engineer who developed the layout of the J-20. From left to right are lift contribution by LEX, canard, canard with LEX, body-lift along with canard and LEX.


You mentioned that moving center-of-pressure forward reduces drag. You are correct. Essentially, that has to do with making the aircraft more unstable. However, there is a limit as to how unstable the aircraft can made to be, and that limit is governed by the amount of pitch-down authority presence. Canard can simply go into negative angle-of-attack to achieve that, and so far no other aerodynamic methods can compete. Again, this is said by the engineer who developed the layout of the J-20.

LEVCON is not bad. It is less complicate and more affordable, good for poor countries. However, LEVCON is not a replacement for canard.

You know nothing, the movable LEX has been applied to LCA, the canard configuration was studied, but rejected, for one simple reason, size and drag, LCA needs to remain small, so no canard was the best, but how to achieve lift in that condition? well something you have not even understood yet, the LEVCON combines two different qualities, the ability to re-attache the vortex and the ability to create drag, the PAKFA can roll with LEVCONS like the LCA, it does it by generating different lift coefficient on each wing, in fact the LEVCON is no more than a canard without gap.

You have a fixation thinking the longer arm of the canard and the pitch control of a canard means the canard is superior to the elevon on LCA or aft tail of PAKFA, something which is for starters not true, an airplane chooses a configuration because it has some specifications and a mission to fill.

The limit of the agility for a fighter is the total lift it generates and the drag it opposing it, if the lift is higher and the thrust good you get a world beater.

It is not a factor determined by this or that characteristic, but the set of features and elements it has.

You think i am attacking J-20, i do not care about the jet, how good it is will depend in the whole aerodynamic configuration and thrust it has plus the avionics and tactics.

If it has canards and Lex it does not make it superior to LCA or Gripen, since what most aircraft like birds fight is weight and drag.

The Canard is just a fix a patch to the fact it uses a delta wing, only that, same the LEVCON on LCA or PAKFA they are fixes, on PAKFA because the fighter designers did not want to use canards due to the drag they generate and the challenges canards have for stealth.

On LCA they did not use canards because the jet needs to be small, so first there is no space to attach a canard on its small fuselage.

There also many ways to approach a problem, one is thrust vectoring other is advanced mounted sights and missiles.

LCA uses elevons and the LEVCON is doing all what the canard does, PAKFA skipped the triplane configuration and uses conventional aft tail, do you think having aft tail makes it inferior? well it does not if you do not believe it see the flight test of the fighter tumbling in the air with absolute control basically spinning like a leaf in fall falling

[video=youtube;HXfc3ft6zUE]https://www.youtube.com/watch?v=HXfc3ft6zUE[/video]

 

[Quickie]

And that really is the question. It's why I hate it when the discussion turns to "well this feature causes drag" or "that feature causes drag", because all aerodynamic features cause drag.



Terminating it completely would not be a good idea, but there is still a need to prevent over pitching. In a tailplane design with LERXes the LERXes continue to generate vortices while the tails are deflected upward to push the nose down. In a canard design the wing ends up stalling, but the canards are still aerodynamically loaded so they can push the nose down to recover the aircraft. I suspect that in the J-20's case the canards are mostly responsible for generating lift at high angles of attack up to a point, before they depart and become pitching surfaces, while the LERXes could be optimized for higher angles of attack and take over the vortex generation function.

Terminating it completely would not be a good idea, but there is still a need to prevent over pitching

.
Reducing the effectiveness of the vortex generation would in effect put the aircraft nearer to overpitching condition, which means that in the end the aircraft is better to rely on its main pitch control surfaces to prevent overpitching.

In a tailplane design with LERXes the LERXes continue to generate vortices while the tails are deflected upward to push the nose down. In a canard design the wing ends up stalling, but the canards are still aerodynamically loaded so they can push the nose down to recover the aircraft.

When the aircraft is stalling or near stalling, the tail horizontal stabilizers lose its effectiveness. On the other hand, the canards have the advantage during stalling since they still retain their effectiveness in controlling pitch. This was also discussed in a Chinese research paper posted here awhile ago.

 

[Engineer]

You know nothing, the movable LEX has been applied to LCA, the canard configuration was studied, but rejected, for one simple reason, size and drag, LCA needs to remain small, so no canard was the best, but how to achieve lift in that condition? well something you have not even understood yet, the LEVCON combines two different qualities, the ability to re-attache the vortex and the ability to create drag, the PAKFA can roll with LEVCONS like the LCA, it does it by generating different lift coefficient on each wing, in fact the LEVCON is no more than a canard without gap.

You have a fixation thinking the longer arm of the canard and the pitch control of a canard means the canard is superior to the elevon on LCA or aft tail of PAKFA, something which is for starters not true, an airplane chooses a configuration because it has some specifications and a mission to fill.

The limit of the agility for a fighter is the total lift it generates and the drag it opposing it, if the lift is higher and the thrust good you get a world beater.

It is not a factor determined by this or that characteristic, but the set of features and elements it has.

You think i am attacking J-20, i do not care about the jet, how good it is will depend in the whole aerodynamic configuration and thrust it has plus the avionics and tactics.

If it has canards and Lex it does not make it superior to LCA or Gripen, since what most aircraft like birds fight is weight and drag.

The Canard is just a fix a patch to the fact it uses a delta wing, only that, same the LEVCON on LCA or PAKFA they are fixes, on PAKFA because the fighter designers did not want to use canards due to the drag they generate and the challenges canards have for stealth.

On LCA they did not use canards because the jet needs to be small, so first there is no space to attach a canard on its small fuselage.

There also many ways to approach a problem, one is thrust vectoring other is advanced mounted sights and missiles.

LCA uses elevons and the LEVCON is doing all what the canard does, PAKFA skipped the triplane configuration and uses conventional aft tail, do you think having aft tail makes it inferior? well it does not if you do not believe it see the flight test of the fighter tumbling in the air with absolute control basically spinning like a leaf in fall falling

LEVCON can be used to enhance lift and reduce drag, no one disputes that. What you do not understand is that canard not only do the same, but do so better. With longer moment arm, a long-coupled canard provides better pitch authority, which is simple law of physics. This pitch authority is a necessity especially for an unstable aircraft at high angle-of-attack. LEVCON cannot provide such pitch authority alone, and must rely on tailplane as well as thrust-vectoring-control. In other words, LEVCON offers some advantages as canard, but cannot serve as a replacement for canard. Furthermore, to design a canard layout properly is challenging, risky, costly and not everyone can do it. So, the next best alternative after canard is to control vortex through LEVCON.

 

[Engineer]

In a tailplane design with LERXes the LERXes continue to generate vortices while the tails are deflected upward to push the nose down. In a canard design the wing ends up stalling, but the canards are still aerodynamically loaded so they can push the nose down to recover the aircraft. I suspect that in the J-20's case the canards are mostly responsible for generating lift at high angles of attack up to a point, before they depart and become pitching surfaces, while the LERXes could be optimized for higher angles of attack and take over the vortex generation function.

If you remember, Dr. Song, the engineer responsible for the layout of the J-20, wrote specifically about the use of canard for pitch authority:

4.3 Aerodynamic control mechanisms

The requirement for high AOA pitch down control capability is closely related to the longitudinal static instability requirement. The greater the longitudinal static instability, the higher the demands for pitch down control capabilities. As described in chapter 3, the future fighter will hopefully increase its longitudinal static instability to around 10% its average aerodynamic chord length to enhance the trim's lift to drag and lift characteristics. As a result there should be a corresponding improvement in the pitch down control capability. We can categorize two types of control surfaces based on the relative position of the pitch control surfaces with respect to the aircraft's center of mass: positive load pitch down control surface and negative pitch down control surface. Control surfaces placed behind the center of mass, including the vertical stabilizers and trailing edge flaps, generate pitch down control torque by increasing lift. They are considered positive load control surfaces. Control surfaces placed in front of the center of mass, like the canards, are negative load control surfaces. Since the main wing's ability to generate lift tends to saturate under high AOA conditions, the positive load control surfaces' pitch down control capabilities tend to saturate under high AOA as well. Therefore it will be wise to employ negative load control surfaces for pitch down control under high AOA conditions. Figure 7 compares the pitch down control capabilities of the canards and horizontal stabilizers. From the high AOA pitch down control stand point, it will be wise to use canards on the future fighter. Canards on close coupled canard configuration aircraft have relative short lever arms. Employing the LERX canard configuration can increase the canards’ lever arms while retaining the benefits of positive canard coupling. Considering the overall lift enhancement effect and pitch down control capabilities, we can set the canards’ maximum relative area to around 15% and the maximum canard deflection to 90 degrees.