Aerodynamics thread

[MiG-29]

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

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

This video show a real Flanker pilot pulling up and off the control stick, not like you claim pilot control is needed in post stall and that means tailplane control
[video=youtube;9zxb0Q6hZgA]http://www.youtube.com/watch?v=9zxb0Q6hZgA[/video]
hahah i read english and a bit of Russian and both papers say tailplanes trimming at post stall FiG-14-21 of course you did not read that neither

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

 

[Engineer]

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

This video show a real Flanker pilot pulling up and off the control stick, not like you claim pilot control is needed in post stall and that means tailplane control

Pulling up has nothing to do with using the tailplane to generate the pitch-down moment for recovery. Your claim that tailplane is effective at high AoA remains unsubstantiated. Furthermore, aircraft possessing stability at high AoA means pilot input is not necessary. This is also addressed by the

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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.

hahah i read english and a bit of Russian and both papers say tailplanes trimming at post stall FiG-14-21 of course you did not read that neither

Fig. 14-21 shows trimming conditions that the paper then use to calculate the velocity of the aircraft. They have nothing to do with effectiveness of the control surfaces at high AoA. As far as the effectiveness of the tailplane is concerned, the same

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is pretty clear:

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.

What does "loss of effectiveness of a horizontal tail at higher AoA" means? Since you have problem with English, let me explain it to you. It means they are useless at high AoA in bringing the nose back down.

The same paper explained the mechanism behind the pitch-down moment toward the end of the Cobra maneuver. It has nothing to do with active deflection of the tailplane:

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.

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

No where in this quote do they ever mention tailplane being effective at post-stall condition in producing pitch-down moment. Once again, you are putting words into people's mouth and drawing conclusions that are not there.

 

[latenlazy]

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

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

Well, to quote you...

"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"

Note that inertia has already begun pulling the jet back down and the nose has already begun to pitch down before the tailplanes are used, which means the plane is already recovering from stall. This doesn't show the tailplanes can be used to control pitch when the wings have stalled. Rather the Flanker depends on the shifting of the AC behind the CG at post stall to pitch the nose back down, and once the wings are no longer stalled the tailplanes can be used again to help control pitching. The key take away here is that the tailplanes can't function to control for pitch while the wings are stalled, but there are other ways to pitch the plane to recover from stall other than using the tailplanes.

 

[no_name]

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

This discussion thread is good. I've learned alot of new things.

 

[MiG-29]

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

Well, to quote you...

"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"
Note that inertia has already begun pulling the jet back down and the nose has already begun to pitch down before the tailplanes are used, which means the plane is already recovering from stall. This doesn't show the tailplanes can be used to control pitch when the wings have stalled. Rather the Flanker depends on the shifting of the AC behind the CG at post stall to pitch the nose back down, and once the wings are no longer stalled the tailplanes can be used again to help control pitching. The key take away here is that the tailplanes can't function to control for pitch while the wings are stalled, but there are other ways to pitch the plane to recover from stall other than using the tailplanes.

Su-27 is not stalled, if it was stalled, it will spin, at 60 degrees of AoA the Su-27 is stalled if the pitch up is not brief, Cobra is done in seconds.

Hysteresis means the vortex system is still there, suction is still there, what happens is the resultant force of vectors of the aerodynamic forces move the pressure center behind the center of Gravity, thus the Su-27 goes back to horizontal flight.


The tailplanes are still trimming and they have to stop pitching up, therefore the pilots pulls off the control stick and the tailplanes go back to neutral position, this allows the jet to pitch down since the resultant of the aerodynamic forces and inertia bring the nose down.

The original paper says, the supermanoeuvrable fighter is unstable at low AoA and stable at post stall, by having the center of lift behind the center of gravity it is stable.

The pape says the tailplane loss effectiveness, but still have a pitch component, true the inertia and static positive stability brings the nose down, but it does not mean the tailplanes are not used or do not affect post stall, otherwise the pilot would not pull off the control stick.

the key of the cobra is duration, if it is brief means returning to horizontal flight but if the pitch up lags means Su-27 will spin and its departure of controlled flight.

why it is brief? because the tailplanes allow for a high pitch up rate allowing for hysteresis and inertia to work and bring the nose down.

if the jet has a low pitch up rate will depart, that is the reason F-14 gets up to 70 deg of AoA because the vortex will burst and lateral unstability will take place in the form of yaw instability and nose slice

 

[latenlazy]

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

Su-27 is not stalled, if it was stalled, it will spin, at 60 degrees of AoA the Su-27 is stalled if the pitch up is not brief, Cobra is done in seconds.

Hysteresis means the vortex system is still there, suction is still there, what happens is the resultant force of vectors of the aerodynamic forces move the pressure center behind the center of Gravity, thus the Su-27 goes back to horizontal flight.


The tailplanes are still trimming and they have to stop pitching up, therefore the pilots pulls off the control stick and the tailplanes go back to neutral position, this allows the jet to pitch down since the resultant of the aerodynamic forces and inertia bring the nose down.

The original paper says, the supermanoeuvrable fighter is unstable at low AoA and stable at post stall, by having the center of lift behind the center of gravity it is stable.

The pape says the tailplane loss effectiveness, but still have a pitch component, true the inertia and static positive stability brings the nose down, but it does not mean the tailplanes are not used or do not affect post stall, otherwise the pilot would not pull off the control stick.

the key of the cobra is duration, if it is brief means returning to horizontal flight but if the pitch up lags means Su-27 will spin and its departure of controlled flight.

why it is brief? because the tailplanes allow for a high pitch up rate allowing for hysteresis and inertia to work and bring the nose down.

if the jet has a low pitch up rate will depart, that is the reason F-14 gets up to 70 deg of AoA because the vortex will burst and lateral unstability will take place in the form of yaw instability and nose slice

...So if it's not stalled, then why is it call "post stall" maneuver.

 

[Engineer]

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

Su-27 is not stalled, if it was stalled, it will spin, at 60 degrees of AoA the Su-27 is stalled if the pitch up is not brief, Cobra is done in seconds.

This is incorrect. Stalling refers to the situation where the critical angle-of-attack of the airfoil is exceeded. From the following images, we see this being the case. Also, we see flow has long separated from the wings.

In fact, the Cobra maneuver is called post-stall maneuver for a reason.

Hysteresis means the vortex system is still there, suction is still there, what happens is the resultant force of vectors of the aerodynamic forces move the pressure center behind the center of Gravity, thus the Su-27 goes back to horizontal flight.


The tailplanes are still trimming and they have to stop pitching up, therefore the pilots pulls off the control stick and the tailplanes go back to neutral position, this allows the jet to pitch down since the resultant of the aerodynamic forces and inertia bring the nose down.

The original paper says, the supermanoeuvrable fighter is unstable at low AoA and stable at post stall, by having the center of lift behind the center of gravity it is stable.

You are still trying to massage your own opinion into a

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when the authors have concluded the exact opposite. As stated by your paper:

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 aerodynamic center moving behind center of gravity to produce nose-down moment has absolutely nothing to do with deflection of the tailplane.

The pape says the tailplane loss effectiveness, but still have a pitch component, true the inertia and static positive stability brings the nose down, but i does not mean the tailplanes are not used or do not affect post stall, otherwise the pilot would not pull off the control stick.

the key of the cobra is duration, if it is brief means returning to horizontal flight but if the pitch up lags means Su-27 will spin and its departure of controlled flight.

why it is brief? because the tailplanes allow for a high pitch up rate allowing for hysteresis and inertia to work and bring the nose down.

The

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made a different conclusion:

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.

This means without the shift in aerodynamic center, recovery is not possible. Due to stall, the tailplane loses effectiveness and unable to generate pitch-down moment. This is why canard is superior at high AoA, as 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.

 

[MiG-29]

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

...So if it's not stalled, then why is it call "post stall" maneuver.

simple if the pitch up lags the jet will crash because hysteresis means delay of vortex burst, it means the vortex still is there, but at 110 degrees of AoA will burst, but because it is delayed the jet can be brought back to horizontal flight,
that delay is what keeps lateral stability, once one of the vorteces burst, yaw assymetries can happen.
These do not happen because the vortex is delayed, it is know the vortex does not burst right away, it delays and lags behind the actual pitch up of the aircraft, for that reason the Cobra is done is seconds and the SU-27 has a AoA limit of 35 deg AoA

 

[latenlazy]

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

simple if the pitch up lags the jet will crash because hysteresis means delay of vortex burst, it means the vortex still is there, but at 110 degrees of AoA will burst, but because it is delayed the jet can be brought back to horizontal flight,
that delay i what keeps lateral stability, once one of the vortex burst, yaw assymetries can happen.
These do not happen because the vortex is delayed, it is know the vortex does not burst right away, it delays and lag behind the actual pitch up of the aircraft, for that reason the Cobra is done is seconds and the SU-27 has a AoA limit of 35 deg AoA

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.

 

[Engineer]

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

simple if the pitch up lags the jet will crash because hysteresis means delay of vortex burst, it means the vortex still is there, but at 110 degrees of AoA will burst, but because it is delayed the jet can be brought back to horizontal flight,
that delay is what keeps lateral stability, once one of the vorteces burst, yaw assymetries can happen.
These do not happen because the vortex is delayed, it is know the vortex does not burst right away, it delays and lags behind the actual pitch up of the aircraft, for that reason the Cobra is done is seconds and the SU-27 has a AoA limit of 35 deg AoA

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.