Aerodynamics thread

[MiG-29]

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

Some aircraft are capable of performing Pugachev's Cobra without the aid of features that normally provide post-stall maneuvering such as thrust vectoring. Advanced fourth generation fighters such as the Su-27, MiG-29, and F/A-18 along with their variants have been documented as capable of performing this maneuver using normal, non-thrust vectoring engines. The ability of these aircraft to perform this maneuver is based in inherent instability like that of the F-16; the MiG-29 and Su-27 families of jets are designed for desirable post-stall behavior. Thus, when performing a maneuver like Pugachev's Cobra the aircraft will stall as the nose pitches up and the airflow over the wing becomes separated, but naturally nose down even from a partially inverted position, allowing the pilot to recover complete control.

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Wiki disagrees.

i do not care, in Andrei Fomin`s book Su-27 they say hysteresis is the reason for the cobra, so i do not need your wikipedia explanation, since Fomin book is written with complete support by Sukhoi, in fact Mikahil Simonov the creator of Su-27 is one of the contributors for the book and i know what is hysteresis.



Normal Force Hysteresis
During the grant period, we were made aware of the interesting
experimental results which show significant normal force hysteresis for
pitching delta wings and for a Russian aircraft. Dynamic force coefficients
can exceed static values by 30-50% on the upstroke portion of the pitch,
and can undershoot static values by lesser, but still significant values on
the down stroke portion. Experimental evidence shows this is due to a lag
in the vortex bursting position, thereby maintaining the augmented normal
forces to a higher angle of attack on the upstroke. A lag in the opposite
direction on the down stroke causes the undershoot. A computational
experiment was done to determine if this effect could be predicted by the
adaptive Euler solver. Preliminary results were obtained (Reference 12)
showing the general effect. Both time and resources ran out before more
detailed calculations could be done

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[Engineer]

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

i do not care, in Andrei Fomin`s book Su-27 they say hysteresis is the reason for the cobra, so i do not need your wikipedia explanation, since Fomin book is written with complete support by Sukhoi, in fact Mikahil Simonov the creator of Su-27 is one of the contributors for the book and i know what is hysteresis.

Your argument that you are right just because you are quoting from an authoritative source is known as

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, which is a logical fallacy. The reason being is that when your position is incorrect, it will remain incorrect regardless of whom you are quoting.

Hysteresis refers to a system being dependent on its previous states. In the example of Su-27, and according to the

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, the aircraft has static stability at high AoA, which would prevent the aircraft from gaining any larger AoA. To go past the limit, the aircraft has to have a large initial pitch-up moment, so that inertia takes the aircraft's AoA beyond the limit. During recovery, there is less pitch-rate. This is hysteresis, which has nothing to do with the tailplane.

And since you are unable to answer latenlazy's simple question, I will give you the answer. From

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:

Post stall is the state when the normal flight controls, like ailerons and elevators, are no longer sufficient to maintain control of the aircraft.

This corresponds to the explanation given by

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:

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 also corresponds to this:

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.

Normal Force Hysteresis
During the grant period, we were made aware of the interesting experimental results which show significant normal force hysteresis for pitching delta wings and for a Russian aircraft. Dynamic force coefficients can exceed static values by 30-50% on the upstroke portion of the pitch, and can undershoot static values by lesser, but still significant values on the down stroke portion. Experimental evidence shows this is due to a lag in the vortex bursting position, thereby maintaining the augmented normal forces to a higher angle of attack on the upstroke. A lag in the opposite direction on the down stroke causes the undershoot. A computational experiment was done to determine if this effect could be predicted by the adaptive Euler solver. Preliminary results were obtained (Reference 12) showing the general effect. Both time and resources ran out before more detailed calculations could be done

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And where does it say tailplane being used to generate pitch-down moment? Absolutely no where. Hysteresis has no relevance to the effectiveness of tailplane at high AoA. Your position that tailplane contributes to recovery by producing nose-down pitch moment remains unsubstantiated.

 

[MiG-29]

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

This is a fallacy called

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. You overload your statements with technical terms, but ended up not backing up your point in anyway.

latenlazy asked you a simple question. If there is no stall, why is it call a "post stall" maneuver? It's a simple question to answer.

The hysteresis is caused by a lag in the vortex bursting
position.

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---------- Post added at 05:37 PM ---------- Previous post was at 05:34 PM ----------

Hysteresis refers to a system being dependent on its previous states. In the example of Su-27, and according to the

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, the aircraft has static stability at high AoA, which would prevent the aircraft from gaining any larger AoA. To go past the limit, the aircraft has to have a large initial pitch-up moment, so that inertia takes the aircraft AoA beyond the limit. This is hysteresis, which has nothing to do with effectiveness of the tailplane.

[/URL]:


:

During the grant period the
Principal Investigator became aware of the interesting and important
aerodynamic problem of normal force hysteresis for rapidly pitching wings
and aircraft. The hysteresis is caused by a lag in the vortex bursting
position. Computational studies were undertaken to understand this
problem, even though unsteady flows were not in the original list of
objectives.

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[latenlazy]

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

The hysteresis is caused by a lag in the vortex bursting
position.

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---------- Post added at 05:37 PM ---------- Previous post was at 05:34 PM ----------



During the grant period the
Principal Investigator became aware of the interesting and important
aerodynamic problem of normal force hysteresis for rapidly pitching wings
and aircraft. The hysteresis is caused by a lag in the vortex bursting
position. Computational studies were undertaken to understand this
problem, even though unsteady flows were not in the original list of
objectives.

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Which says nothing about how effective tailplanes are for pitching at high AoA. All you're saying is that the vortices help the the plane pitch up for the cobra maneuver. It says nothing about tailplane pitch control.

 

[Engineer]

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

The hysteresis is caused by a lag in the vortex bursting
position.

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---------- Post added at 05:37 PM ---------- Previous post was at 05:34 PM ----------



During the grant period the Principal Investigator became aware of the interesting and important aerodynamic problem of normal force hysteresis for rapidly pitching wings and aircraft. The hysteresis is caused by a lag in the vortex bursting position. Computational studies were undertaken to understand this problem, even though unsteady flows were not in the original list of objectives.

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So? Hysteresis has nothing to do with effectiveness of the tailplane. It doesn't alter the fact that tailplane becomes ineffective at high AoA. From your own

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:

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.

The mechanism which does the recovery is passive, and is not related to active deflection in the tailplane. Also from the same 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 tailplane cannot be depend upon for recovery:

For super manoeuvre aircraft the return back from the high AoA to the initial flight condition should be possible independently of the effectiveness of the tail control surfaces.

This is why canard is superior to trailing edge control surfaces at high AoA. The canard doesn't go into a stall, because to produce pitch-down moment, the canard deflects downward into negative 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.

 

[MiG-29]

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

So? Hysteresis has nothing to do with effectiveness of the tailplane. It doesn't alter the fact that tailplane becomes ineffective at high AoA. :

Read Russian?

Область неустойчивости самолёта находится в окрестности углов атаки 30—40°. В этой области может развиться боковое возмущающее движение самолёта и наступить сваливание. Однако его развитие требует определённого времени, и, если выйти из области неустойчивости раньше, сваливания не произойдёт. Для успешного выполнения манёвра «Кобра» самолёт должен развить достаточно высокую угловую скорость по тангажу (в продольном движении), чтобы быстро проскочить участок неустойчивости. Это в какой-то степени аналогично движению человека по узкой переправе без перил: надёжнее преодолеть её бегом, а не медленно и осторожно, пытаясь балансировать.

Кратковременность манёвра спасает ещё от одной неприятности. Дело в том, что на больших углах атаки над крылом, вдоль фюзеляжа самолёта, образуются несимметричные вихри. Они вызывают появление весьма неблагоприятных, так называемых несимметричных возмущающих боковых моментов по крену и рысканью. А при быстром проходе зон образования вихрей они не успевают полностью сформироваться.



Из этого следовал вывод: для выполнения манёвра лётчику необходимо предельно быстро отклонить до максимума горизонтальное оперение на кабрирование. Это предъявляет определённые требования к системе управления самолётом. У Су-27 она содержит отрицательные обратные связи, не позволяющие ему развивать слишком высокую угловую скорость, притормаживает стабилизатор при резком отклонении ручки управления, «смягчает» реакцию самолёта на резкие действия пилота. Поэтому в системе управления необходимо исключить обратные связи и перейти к режиму с «жёсткой» связью ручки управления с поворотным стабилизатором: взяв ручку управления на себя с предельной скоростью, лётчик столь же быстро отклоняет стабилизатор до максимального положения.

here it says hysteresis allows cobra by delay instability and it say says hysteresis avoids yaw assymetries
plus it says the taiplane is pitch up very fast by switching off its artificial deflection limit

 

[latenlazy]

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

plus it says the taiplane is pitch up very fast by switching off its artificial deflection limit

...Which tells us the tailplane is giving good pitching moment before the plane is at high AoA, not during.

 

[Engineer]

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

Read Russian?

Область неустойчивости самолёта находится в окрестности углов атаки 30—40°. В этой области может развиться боковое возмущающее движение самолёта и наступить сваливание. Однако его развитие требует определённого времени, и, если выйти из области неустойчивости раньше, сваливания не произойдёт. Для успешного выполнения манёвра «Кобра» самолёт должен развить достаточно высокую угловую скорость по тангажу (в продольном движении), чтобы быстро проскочить участок неустойчивости. Это в какой-то степени аналогично движению человека по узкой переправе без перил: надёжнее преодолеть её бегом, а не медленно и осторожно, пытаясь балансировать.
here it says hysteresis allows cobra by delay instability

The passage talks about the need of high-pitch rate to initiate the maneuver to keep the duration inside instability region as short as possible. It gives an example of people crossing a stream quickly, rather than slowly balancing their way through. This has nothing to do with the tailplane producing pitch-down moment. It does not alter the fact that tailplane is ineffective at high AoA.

Кратковременность манёвра спасает ещё от одной неприятности. Дело в том, что на больших углах атаки над крылом, вдоль фюзеляжа самолёта, образуются несимметричные вихри. Они вызывают появление весьма неблагоприятных, так называемых несимметричных возмущающих боковых моментов по крену и рысканью. А при быстром проходе зон образования вихрей они не успевают полностью сформироваться.
here it say hysteresis avoids assymetric yaw assymetries

This simply says that the maneuver is kept short so the adverse effects do not affect the aircraft. Again, it says nothing about tailplane being effective at high AoA, thus your claim remains unsubstantiated.

Из этого следовал вывод: для выполнения манёвра лётчику необходимо предельно быстро отклонить до максимума горизонтальное оперение на кабрирование. Это предъявляет определённые требования к системе управления самолётом. У Су-27 она содержит отрицательные обратные связи, не позволяющие ему развивать слишком высокую угловую скорость, притормаживает стабилизатор при резком отклонении ручки управления, «смягчает» реакцию самолёта на резкие действия пилота. Поэтому в системе управления необходимо исключить обратные связи и перейти к режиму с «жёсткой» связью ручки управления с поворотным стабилизатором: взяв ручку управления на себя с предельной скоростью, лётчик столь же быстро отклоняет стабилизатор до максимального положения.

here it says the taiplane is pitch up very fast by switching off its artificial deflection limit

This has absolutely nothing to do in using the tailplane to generate pitch-down moment during recovery. Pitching up does not equate to pitching down.


Recovery is achieved passively by aerodynamics, and has nothing to do with deflection of the tailplane. From the

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:

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 authors' use of the word "only" excludes other means of recovery, such as usage of the tailplane. Regarding the effectiveness of tailplane, the

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has this conclusion:

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.

Read "loss of effectiveness of a horizontal tail at higher AoA". Yet, despite it is coming from the same paper you quoted from, you still try to argue for your incorrect position. Clearly, discussing aerodynamics is not your objective.

Recovery should not be dependent on the tailplane, as explained in the same

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:

For super manoeuvre aircraft the return back from the high AoA to the initial flight condition should be possible independently of the effectiveness of the tail control surfaces.

This is why canard is superior to trailing edge control surfaces at high AoA. The canard doesn't go into a stall, because to produce pitch-down moment, the canard deflects downward into negative 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.

 

[MiG-29]

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

...Which tells us the tailplane is giving good pitching moment before the plane is at high AoA, not during.

haha why do not you do this, send to sukhoi a letter and contac Viacheslav Averanov who is a Sukhoi test pilot, and flies Su-30MKIs recomendation of not pulling off the control stick, just continue pulling it up and do not pull of the control stick since at post stall according to you and engineer tailplane deflection does not affect angle of attack at post stall and he does not need to bring the tailplane to neutral position since inertia will bring it down perhaps Pogyiosan will ask his engineers to change the tailplanes control system haha.

You can tell Viacheslav you saw him flying a Su-30MKI during MAKS 2005 and he was pulling off to quickly his control stick at minute 0:40 to 1:40 hahaha

you can also send to Eugeny Frolov a letter too he fies a Su-30 at minute 1:40 specially at minute 2:08 when he does the cobra haha

[video=youtube;9zxb0Q6hZgA]http://www.youtube.com/watch?feature=player_embedded&hl=en&v=9zxb0Q6hZgA[/video]

 

[latenlazy]

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

haha why do not you do this, send to sukhoi a letter and contac Viacheslav Averanov who is a Sukhoi test pilot, and flies Su-30MKIs recomendation of not pulling off the control stick, just continue pulling it up and do not pull of the control stick since at post stall according to you and engineer tailplane deflection do not affect angle of attack at post stall and he does not need to bring the tailplane to neutral position since inertia will bring it down perhaps Pogyiosan will ask his engineers to change the tailplanes control system haha.

You can tell Viacheslav you saw him flying a Su-30MKI during MAKS 2005 and he was pulling off to quickly his control stick in at minute 0:40 to 1:40 hahaha

[video=youtube;9zxb0Q6hZgA]http://www.youtube.com/watch?feature=player_embedded&hl=en&v=9zxb0Q6hZgA[/video]

Well...1) I do not know Russian. 2) There IS a good reason for deflecting the tailplane in the cobra maneuver. It's just not doing what you're saying it does. The tailplane is not pitching the nose down at high AoA. After the AC shifts behind the CG, it's pushing the plane from behind its CG instead of in front of it, so the nose swings back downward as force is applied to the tail instead of the nose. That is what's pitching the nose back down in the cobra, and it has nothing to do with the tailplanes. The tailplanes probably come in to assist the plane back into level flight, after the plane is already in recovery from stall and its nose is pitching back down.