Aerodynamics thread

[MiG-29]

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

Canards are superior to trailing edge control surfaces for pitch-down moment, precisely because the former do not stall. To bring the nose down from high AoA the canards deflect downward into negative AoA instead of upward. For trailing edge control surfaces, their AoA have to be increased to bring the aircraft's nose down. When the aircraft already has high AoA, this leads to stall of the control surfaces, rendering them ineffective. Here is a statement from Dr. Song's paper:

On page 3 and 4 the article says there are three designs characteristics for the cobra

and it says

Analysis of the Cobra manoeuvre


Three design characteristics are necessary aircraft could execute dynamic entrance into cobra manoeuvre, firstly a high nose pitch up control, secondly lateral stability and control and thirdly a robust pitch down recovery



What does create the high pitch down control? easy tailplanes, why lateral stability well it most not have any yaw nose slip, and third a force to send it pitch down


Now on page 7 it further reads

canard deflection (delta_c) influences the angle of attack in the below-stall range, see Fig-10, but does not influence in the post stall range.......trim angle of attack (alpha) and tailplane (delta_h) in the whole extended range of flight speed are given in Fig 14, Flight speed versus angle of attack for various flight path angles are shown in Fig 15, post stall trim conditions corresponding to these three various path angles and various pitch angles are presented in various are presented in Fig16


It very clear states the tailplane is triming the aircraft at different high AoA during the cobra and you see that in Fig 17-21

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[Engineer]

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

On page 3 and 4 the article says there are three designs characteristics for the cobra

and it says

Analysis of the Cobra manoeuvre


Three design characteristics are necessary aircraft could execute dynamic entrance into cobra manoeuvre, firstly a high nose pitch up control, secondly lateral stability and control and thirdly a robust pitch down recovery

Pitch-down recovery does not equate to using tailplane for pitch-down recovery.

What does create the high pitch down control? easy tailplanes, why lateral stability well it most not have any yaw nose slip, and third a force to send it pitch down

Nope. That is your opinion, and one that is not back up by anything.

The

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explained the mechanism that produces the pitch-down moment:

The recovery from high angles of attack to the classical flight mode in a few seconds only is possible due to moving the center of pressure on main wing back and creating the strong nose-down aerodynamic pitching moment about the center of gravity.

It has nothing to do with deflection of the tailplane. In fact, the the use of the word "only" excludes deflection of tailplane as a contributor.

Now on page 7 it further reads

canard deflection (delta_c) influences the angle of attack in the below-stall range, see Fig-10, but does not influence in the post stall range.......trim angle of attack (alpha) and tailplane (delta_h) in the whole extended range of flight speed are given in Fig 14, Flight speed versus angle of attack for various flight path angles are shown in Fig 15, post stall trim conditions corresponding to these three various path angles and various pitch angles are presented in various are presented in Fig16


It very clear states the tailplane is triming the aircraft at different high AoA during the cobra and you see that in Fig 17-21

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Nope. It very clearly states that tailplane is ineffective at high AoA.

A concentration of characteristic curves Cm for the tailplane setting angle φ[SUB]t[/SUB] being varied at post-critical AoA (i.e. very low sensitivity of pitch moment with respect to the tailplane setting angle) reflects the loss of effectiveness of a horizontal tail at higher AoA.

In Dr. Song's paper, it is clearly stated that trailing edge control surfaces are ineffective at high AoA for pitch-down authority in comparision to canards. The reason is that these trail edge surfaces stall at high AoA. His statement is as follow:

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.

 

[MiG-29]

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

In Dr. Song's paper, it is clearly stated that trailing edge control surfaces are ineffective at high AoA for pitch-down authority in comparision to canards. The reason is that these trail edge surfaces stall at high AoA. His statement is as follow:

The tailplanes are used that is the reason the article has Fig 16 to 21, what you are just trying to do is using a mechanical explanation since the article mentions canards are not used in post stall and do not affect the angle of attack while tailplanes are used at post stall.


You understanding is mechanical at best.


what brings the nose down is hysteresys or delay of vortex burst, this allows to have a pitch down and no nose slice, the tailplanes are still used that is the reason of Fig 14-21, what is bringing the nose down is effectively the positive static stability that has a pitch down force, true but tailplanes are still used, that is the reason the have such figures, the pitch down force prevents the pitch up of a stalled wing, since stalled wings pitch up the nose because the main wing moves the center of pressure yeah definititively the tailplane loses some effectiveness, but not all as you claim, it even says very low sensitivity of pitch moment with respect to the tailplane setting angle it means there is not enough pitch moment to bring the aircraft pitch down under tailplane alone, specially if the aircraft has negative static stability in few words statically unstable longitudinaly, it needs to be stable at high alpha and post stall.


What you are claiming is canards are as as ineffective as tailplanes, however that is not the case because it says

canard deflection (delta_c) influences the angle of attack in the below-stall range, see Fig-10, but does not influence in the post stall range.......trim angle of attack (alpha) and tailplane (delta_h) in the whole extended range of flight speed are given in Fig 14, Flight speed versus angle of attack for various flight path angles are shown in Fig 15, post stall trim conditions corresponding to these three various path angles and various pitch angles are presented in various are presented in Fig16

and they show you the degree the tailplane is effective, yeah true tailplanes by themselves can not bring the jet down the need hysteresys to do it, that is the reason only Su-27 does it and not F-18E, and to prove see any cobra and you see the tailplane is used all the time and in the F-22 is the same, the tailplanes are used all the time because they are aided by the hysteresys of the wing-LEX that helps the jet bring the nose down, but it does not mean tailplanes still do not trim or are not used as you claim, they are still trimming the SU-27.

However the key is pitch up duration since a brief pitch up will allow for hysteresys and inertia to pull down the nose, but if the duration of the cobra is extended, the aircraft will get into a spin and will slip into uncontrolled flight.

In fact what the pilot is doing is pulling up the control stick to pull up the nose, later it needs to stop pulling up and the pilot pulls off the control stick to stop the pitch up before it goes beyond 110 degrees, later when the nose pitch down force gets the nose at 10-20 deg of AoA it pulls up again to stop going into negative AoA or diving and later it will stop pulling down the nose up once horizontal flight has been achieved, this is achieved by deflecting the tailplanes and you see it in Fig17
Here you see the picture of the tailplane defelction during cobra

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[Engineer]

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

The tailplanes are used that is the reason the article has Fig 16 to 21, what you are just trying to do is using a mechanical explanation since the article mentions canards are not used in post stall and do not affect the angle of attack while tailplanes are used at post stall.


You understanding is mechanical at best.

False. What you are trying to do is misinterpret the paper for things that are not there. Fig. 16 to 21. show trim conditions, and from these the paper calculate velocity of the aircraft so that the Cobra maneuver can be analyzed. The figures say nothing about "tail planes are used at post stall" which is entirely your own opinion.

Aren't you good at quoting things? If the paper actually says that tail planes are effective at high AoA, you would have gave a direct quote already. So why isn't that the case? The answer is simple: the paper does not share your incorrect position, so now you are relying on your creativity with the figures.

The paper is pretty clear on tail planes' effectiveness:

A concentration of characteristic curves Cm for the tailplane setting angle φ[sub]t[/sub] being varied at post-critical AoA (i.e. very low sensitivity of pitch moment with respect to the tailplane setting angle) reflects the loss of effectiveness of a horizontal tail at higher AoA.

Trailing edge control surfaces lost effectiveness at higher AoA. No matter how hard the tail planes wiggle, the above fact isn't going to be altered. It is amusing to see you so desperately distorting facts from that paper like a condom over an elephant's trunk to argue against the conclusion of the exact same paper.

what brings the nose down is hysteresys or delay of vortex burst, this allows to have a pitch down and no nose slice, the tailplanes are still used that is the reason of Fig 14-21, what is bringing the nose down is effectively the positive static stability that has a pitch down force, true but tailplanes are still used, that is the reason the have such figures, the pitch down force prevents the pitch up of a stalled wing, since stalled wings pitch up the nose because the main wing moves the center of pressure yeah definititively the tailplane loses some effectiveness, but not all as you claim, it even says very low sensitivity of pitch moment with respect to the tailplane setting angle it means there is not enough pitch moment to bring the aircraft pitch down under tailplane alone, specially if the aircraft has negative static stability in few words statically unstable longitudinaly, it needs to be stable at high alpha and post stall.

What you are claiming is canards are as as ineffective as tailplanes, however that is not the case because it says

canard deflection (delta_c) influences the angle of attack in the below-stall range, see Fig-10, but does not influence in the post stall range.......trim angle of attack (alpha) and tailplane (delta_h) in the whole extended range of flight speed are given in Fig 14, Flight speed versus angle of attack for various flight path angles are shown in Fig 15, post stall trim conditions corresponding to these three various path angles and various pitch angles are presented in various are presented in Fig16

and they show you the degree the tailplane is effective, yeah true tailplanes by themselves can not bring the jet down the need hysteresys to do it, that is the reason only Su-27 does it and not F-18E, and to prove see any cobra and you see the tailplane is used all the time and in the F-22 is the same, the tailplanes are used all the time because they are aided by the hysteresys of the wing-LEX that helps the jet bring the nose down, but it does not mean tailplanes still do not trim or are not used as you claim, they are still trimming the SU-27.

However the key is pitch up duration since a brief pitch up will allow for hysteresys and inertia to pull down the nose, but if the duration of the cobra is extended, the aircraft will get into a spin and will slip into uncontrolled flight.

In fact what the pilot is doing is pulling up the control stick to pull up the nose, later it needs to stop pulling up and the pilot pulls off the control stick to stop the pitch up before it goes beyond 110 degrees, later when the nose pitch down force gets the nose at 10-20 deg of AoA it pulls up again to stop going into negative AoA or diving and later it will stop pulling down the nose up once horizontal flight has been achieved, this is achieved by deflecting the tailplanes and you see it in Fig17

You are employing what's known as

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, where you use a bunch of technical sounding words trying to mask the fact that your own position has been proven as incorrect. At the end of the day, deflection tail planes is still ineffective in bringing down the nose.

The paper explains the actual mechanics for bringing down the nose:

The recovery from high angles of attack to the classical flight mode in a few seconds only is possible due to moving the center of pressure on main wing back and creating the strong nose-down aerodynamic pitching moment about the center of gravity.

No matter how you try to spin it to shove the words "tail planes" in, their use of the word "only" unequivocally excludes other contributors such as active deflection of tail planes. Furthermore, the paper says:

A concentration of characteristic curves Cm for the tailplane setting angle φt being varied at post-critical AoA (i.e. very low sensitivity of pitch moment with respect to the tailplane setting angle) reflects the loss of effectiveness of a horizontal tail at higher AoA.

Simply put, for the tail planes to be able to generate pitch-down moment, they must increase their own AoA. This will lead to stall and ineffectiveness of the control surfaces when the aircraft already has high AoA to begin with. Canards on the other hand reduce AoA into negative region to generate pitch-down moment, thus does not have the problem of stalling. As a result, canards are superior to tail planes in generating pitch-down moment. This is explained in Dr. Song's paper in the following statements:

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.

Here you see the picture of the tailplane defelction during cobra

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Flapping your arms wildly does not indicate it is effective in making you fly.

 

[MiG-29]

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

False. What you are trying to do is misinterpret the paper for things that are not there. Fig. 16 to 21. show trim conditions, and from these the paper calculate velocity of the aircraft so that the Cobra maneuver can be analyzed. :

haha at post stall angles and pitch angles, look i will put you in few words what are you doing you have absolute no understanding of Cobra.


Russian explanations and the book by Andrei Fomin, "Su-27" say what is really cobra.

To start, tailplane deflection is not by a computer, is what the pilot does, what the pilot does during cobra?
first thing he will switch off the artificial stability and this will allow the tailplane to achieve a very high pitch up rate, allowing the Su-27 to pitch up the nose very fast in fact something in the range of 70 deg/s, so what the pilot does is pull up the nose how? yeah first the pilot will pull up the control stick really fast, this will pitch up the nose, by deflecting the tailplanes negatively, in Fig17 this is when the tailplane has a -12 degrees of deflection.

Second what does he do?

he will pull off the control stick to stop pitching up and returning the tailplane to neutral position, in fig 17 is -2 degrees deflection.

Now what does happen? the jet by having a high pitch up rate does not stall, so it retains vortex lift and dynamic lift thanks to Hysteresis so the pendulum motion starts, and sum of aerodynamic forces and inertia moment will move the center of pressure behind the center of gravity making the Su-27 a stable jet in post stall or in other words a pitch down force will bring the nose down.

The jet passes for a state of equilibrium, at 70 degrees of AoA when the center of gravity and center of pressure are in the same place after but at 110 degrees the center of pressure is well behind the center of gravity so it becomes stable.

Inertia will pull down the jet back, then the nose starts diving, so what the pilot does? he will again pull up the nose to stop going into negative AoA at 15-20 degrees of AoA so again the tailplanes are deflected negatively in FiG17 -5 degrees, but once the nose is flying horizontally he will go pull of the control stick and set the tailplane back to -2 in FiG17
You can see in this picture the pilot trims the jet with tailplanes

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and here you see the shift of the center of pressure going backwards as in fig 1 and 2 of the paper; passing by a state of equilibrium

if you want to see the whole secuence watch the video at minute 4

[video=youtube;emb6er8LOtk]http://www.youtube.com/watch?v=emb6er8LOtk[/video]

 

[latenlazy]

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

haha at post stall angles and pitch angles, look i will put you in few words what are you doing you have absolute no understanding of Cobra.


Russian explanations and the book by Andrei Fomin, "Su-27" say what is really cobra.

To start, tailplane deflection is not by a computer, is what the pilot does, what the pilot does during cobra?
first thing he will switch off the artificial stability and this will allow the tailplane to achieve a very high pitch up rate, in fact something in the range of 70 deg/s, so what the pilot does is pull up the nose how? yeah first the pilot will pull up the control stick really fast, this will pitch up the nose, by deflecting the tailplanes negatively, in Fig17 this is when the tailplane has a -12 degrees of deflection.

Second what does he do?

he will pull off the control stick to stop pitching up and returning the tailplane to neutral position, in fig 17 is -2 degrees deflection.

Now what does happen? the jet by having a high pitch up rate does not stall, so it retains vortex lift and dynamic lift thanks to Hysteresis so the pendulum motion starts, and sum of aerodynamic forces and inertia moment will move the center of pressure behind the center of gravity making the Su-27 a stable jet in post stall or in other words a pitch down force will bring the nose down.

The jet passes for a state of equilibrium, at 70 degrees of AoA when the center of gravity and center of pressure are in the same place after but at 110 degrees the center of pressure is well behind the center of gravity so it becomes stable.

Inertia will pull down the jet back, then the nose starts diving, so what the pilot does? he will again pull up the nose to stop going into negative AoA at 15-20 degrees of AoA so again the tailplanes are deflected negatively in FiG17 -5 degrees, but once the nose is flying horizontally he will go pull of the control stick and set the tailplane back to -2 in FiG17
You can see in this picture the pilot trims the jet with tailplanes

In other words, the tailplanes don't do anything to pitch the plane down after a cobra. Inertia is what's pitching the nose down.

 

[MiG-29]

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

In other words, the tailplanes don't do anything to pitch the plane down after a cobra. Inertia is what's pitching the nose down.

The pilot stops pitching up with the tailplane going back to -2 after have been in -12, he pulls off the control stick why? remebers this is done in seconds
the sequence is in seconds in Fig17 they show you after pulling off is just 2 seconds and he will pull up again to -5, a but i know you can not even read Russian, Russian papers say tailplane deflection is excellent in fact they say control is achieved during the whole manoeuvre that is the reason the control stick goes up off and again up off and the tailplanes are trimming

 

[Engineer]

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

View attachment 6307haha at post stall angles and pitch angles, look i will put you in few words what are you doing you have absolute no understanding of Cobra.


Russian explanations and the book by Andrei Fomin, "Su-27" say what is really cobra.

To start, tailplane deflection is not by a computer, is what the pilot does, what the pilot does during cobra?
first thing he will switch off the artificial stability and this will allow the tailplane to achieve a very high pitch up rate, allowing the Su-27 to pitch up the nose very fast in fact something in the range of 70 deg/s, so what the pilot does is pull up the nose how? yeah first the pilot will pull up the control stick really fast, this will pitch up the nose, by deflecting the tailplanes negatively, in Fig17 this is when the tailplane has a -12 degrees of deflection.

Second what does he do?

he will pull off the control stick to stop pitching up and returning the tailplane to neutral position, in fig 17 is -2 degrees deflection.

Now what does happen? the jet by having a high pitch up rate does not stall, so it retains vortex lift and dynamic lift thanks to Hysteresis so the pendulum motion starts, and sum of aerodynamic forces and inertia moment will move the center of pressure behind the center of gravity making the Su-27 a stable jet in post stall or in other words a pitch down force will bring the nose down.

You go on and on with technical sounding words, but none of it support your idea that tail planes is effective in high AoA. It is a fallacy called

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, because you are trying to mask your incorrect position with long winding texts. It is also

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as you are trying to divert attention from the point of contention.

Wiggling of the tailplane is not an indication that it is effective and a contributor to pitch-down moment. This is the same idea that flapping your arms rapidly does not make you capable of flying. The paper is pretty clear on the tail's effectiveness at post-critical AoA situations:

A concentration of characteristic curves Cm for the tailplane setting angle φ[sub]t[/sub] being varied at post-critical AoA (i.e. very low sensitivity of pitch moment with respect to the tailplane setting angle) reflects the loss of effectiveness of a horizontal tail at higher AoA.

The jet passes for a state of equilibrium, at 70 degrees of AoA when the center of gravity and center of pressure are in the same place after but at 110 degrees the center of pressure is well behind the center of gravity so it becomes stable.

Inertia will pull down the jet back, then the nose starts diving, so what the pilot does? he will again pull up the nose to stop going into negative AoA at 15-20 degrees of AoA so again the tailplanes are deflected negatively in FiG17 -5 degrees, but once the nose is flying horizontally he will go pull of the control stick and set the tailplane back to -2 in FiG17

Inertia producing pitch-down moment has nothing to do with tailplane deflection. Aerodynamic center moving behind the center-of-gravity resulting in pitch-down moment also has nothing to do with tailplane deflection. In other words, pitch-down moment is not caused by the active deflection of the tailplane. The actual mechanics for bringing down the nose is explained in the paper:

The recovery from high angles of attack to the classical flight mode in a few seconds only is possible due to moving the center of pressure on main wing back and creating the strong nose-down aerodynamic pitching moment about the center of gravity.

The use of the word "only" quixotically excludes other contributors such as deflection of the tailplane.

You can see in this picture the pilot trims the jet with tailplanes

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Actually, from the smoke you can see all the aerodynamic surfaces have stalled, indicating all the aerodynamic surfaces become ineffective. The paper is pretty clear on the effectiveness of the tailplane in such a condition:

A concentration of characteristic curves Cm for the tailplane setting angle φ[sub]t[/sub] being varied at post-critical AoA (i.e. very low sensitivity of pitch moment with respect to the tailplane setting angle) reflects the loss of effectiveness of a horizontal tail at higher AoA.

Canard is superior to tailplane because the former goes from zero into negative AoA to dump lift in order to produce pitch-down moment. Thus, canard does not run into the stall issue. This is explained in Dr. Song's paper:

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.

---------- Post added at 04:34 PM ---------- Previous post was at 04:29 PM ----------

The pilot stops pitching up with the tailplane going back to -2 after have been in -12, he pulls off the control stick why? remebers this is done in seconds
the sequence is in seconds in Fig17 they show you after pulling off is just 2 seconds and he will pull up again to -5, a but i know you can not even read Russian, Russian papers say tailplane deflection is excellent in fact they say control is achieved during the whole manoeuvre that is the reason the control stick goes up off and again up off and the tailplanes are trimming

This is irrelevant. Tailplane is not what produced the pitch-down moment necessary to bring the nose down from a post-stall condition. There is only one thing that generates the pitch-down moment, as explained by the paper:

The recovery from high angles of attack to the classical flight mode in a few seconds only is possible due to moving the center of pressure on main wing back and creating the strong nose-down aerodynamic pitching moment about the center of gravity.

 

[MiG-29]

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

You go on and on with technical sounding words, but none of it support your idea that tail planes is effective in high AoA. :

. Основными факторами, которые определили успешное выполнение им манёвра «Кобра», стали высокая эффективность его поворотного стабилизатора и малый запас статической устойчивости.



Read Russian? it says very clearly the tailplanes are working perfectly but of course you did not googled Russian papers about cobra to even know and understand the first paper i gave you

 

[Engineer]

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

. Основными факторами, которые определили успешное выполнение им манёвра «Кобра», стали высокая эффективность его поворотного стабилизатора и малый запас статической устойчивости.



Read Russian? it says very clearly the tailplanes are working perfectly but of course you did not googled Russian papers about cobra to even know and understand the first paper i gave you

Read English? It says very clearly that tailplane is ineffective at high AoA. This is from your own

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:

A concentration of characteristic curves Cm for the tailplane setting angle φ[sub]t[/sub] being varied at post-critical AoA (i.e. very low sensitivity of pitch moment with respect to the tailplane setting angle) reflects the loss of effectiveness of a horizontal tail at higher AoA.

From Dr. Song's paper:

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.