Aerodynamics thread

[Engineer]

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 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

When you made up a statement for the opposite team then proceed to argue against it, that's call using a

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, which is a logical fallacy.

Instead of your claims of what we have said, what we have actually pointed out is the fact that tailplane is ineffective at high AoA. This is why canards are used on the J-20, and 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.

You then proceed to try to argue how it is not true, and that tailplane is effective at high AoA. But your attempt backfired, and the

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you quoted from states the opposite:

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.

You claim that tailplane generates nose-down pitch moment to support your incorrect position, but the very

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you quoted from has this to say:

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.

Note the use of the word "only", which unequivocally excludes other contributors of recovery. In other words, active deflection of the tailplane is not a contributing factor.

 

[latenlazy]

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

Also mig, you were using hysteresis to claim the Flanker wasn't in stall during the cobra maneuver, when hysteresis has nothing to do with whether the plane is in stall.

 

[MiG-29]

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

Also mig, you were using hysteresis to claim the Flanker wasn't in stall during the cobra maneuver, when hysteresis has nothing to do with whether the plane is in stall.

2.2 Fighter aerodynamics at aigh angles of
attack
In order to realize supermaneuverability in
flight and before training of pilots, it needs to
have mathematical model of aerodynamics at
high angles of attack and solve the problem of
stall and spin (these regimes have to be
excluded). During many years joint Su-ADB
and TsAGI team investigated the aerodynamic
peculiarities at high angle of attack and
elaborated the mathematical model of
aerodynamics with description of some effects
[1][3][4]:
• Nonsymmetrical flow including
nonsymmetrical breakdown of
vortexes at high angles of attack,
and nonsymmetrical yaw and roll
moments as a result; Dynamic lag, static and dynamics
hysteresis in lift, pitch, yaw, roll
moment, side force at high angle of
attack.
On the base of systematical wind tunnel
investigations, flight tests in Su-ADB and Flight
Research Institute (LII), the mathematical
model was developed with using additional
differential equations for parameters, which
determine the scale of phenomena: local on
profile, wing, or global, including whole plane
from nose up to tail.
On the base of this investigations more
detailed studying of stall and spin, requirements
to effectiveness of aerodynamics and thrustvectoring
control, requirements to pitch control
of airplane and requirements to characteristics
of inlets and nozzles were fulfilled.
At that time, Su-ADB continued the flight
tests of first copies of Su-27 fighter and
discovered the possibility to fulfill maneuver
with achievement angles of attack more then
60°.
Joint efforts of specialists and pilots from
Sukhoi ADB, FBW control system deliver
MNPK "Avionika" and TsAGI provided the
modification of control system and
methodology of flight tests. Intensive training of
pilots was carrying out using TsAGI movingbased
simulator. It has opened the "door" for in
flight realization of maneuver, later called
"Pugachev Cobra", Fig. 2.

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Flow Visualization Study of LEX Generated Vortices on a Scale Model
of a F/A-18 Fighter Aircraft at High Angles of Attack
by
Odilon V. Cavazos Jr.
Lieutenant, United States Navy


A water tunnel flow visualization investigation was performed into the high angle of
attach aerodynamics of a 2% scale model of the F/A-18 fighter aircraft. The main focus
of this study was the effect of pitch rate on the development and bursting of vortices
generated fran the leading edge extensions in the high angle of attack range with and without
yaw. Results of this investigation indicate that that the vortex bursting point
(relative to the static case) moves rearward with increasing pitch-up motion and forward
with increasing pitch-down motion. For the same pitch rate, vortex bursting was found to
occur earlier for the pitch-down motion than for the pitch-up motion, implying aerodynamic
hysteresis effects.





During pitch-up motion the LEX vortex appears to be smaller indicating a more stable vortex; this effect leads to a lag in vortex bursting. This indicates that during
pitch-up motion, bursting occurs at a point further downstream than would occur for static
conditions, resulting in a vortex system which is equivalent to a static system at a reduced
angle of attack.

From a careful study of the flow visualization photographs it was determined that
the pitch up motion caused the vortex bursting point to lag the static condition point at
the same AOA.

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[latenlazy]

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

2.2 Fighter aerodynamics at aigh angles of
attack
In order to realize supermaneuverability in
flight and before training of pilots, it needs to
have mathematical model of aerodynamics at
high angles of attack and solve the problem of
stall and spin (these regimes have to be
excluded). During many years joint Su-ADB
and TsAGI team investigated the aerodynamic
peculiarities at high angle of attack and
elaborated the mathematical model of
aerodynamics with description of some effects
[1][3][4]:
• Nonsymmetrical flow including
nonsymmetrical breakdown of
vortexes at high angles of attack,
and nonsymmetrical yaw and roll
moments as a result; Dynamic lag, static and dynamics
hysteresis in lift, pitch, yaw, roll
moment, side force at high angle of
attack.
On the base of systematical wind tunnel
investigations, flight tests in Su-ADB and Flight
Research Institute (LII), the mathematical
model was developed with using additional
differential equations for parameters, which
determine the scale of phenomena: local on
profile, wing, or global, including whole plane
from nose up to tail.
On the base of this investigations more
detailed studying of stall and spin, requirements
to effectiveness of aerodynamics and thrustvectoring
control, requirements to pitch control
of airplane and requirements to characteristics
of inlets and nozzles were fulfilled.
At that time, Su-ADB continued the flight
tests of first copies of Su-27 fighter and
discovered the possibility to fulfill maneuver
with achievement angles of attack more then
60°.
Joint efforts of specialists and pilots from
Sukhoi ADB, FBW control system deliver
MNPK "Avionika" and TsAGI provided the
modification of control system and
methodology of flight tests. Intensive training of
pilots was carrying out using TsAGI movingbased
simulator. It has opened the "door" for in
flight realization of maneuver, later called
"Pugachev Cobra", Fig. 2.

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Flow Visualization Study of LEX Generated Vortices on a Scale Model
of a F/A-18 Fighter Aircraft at High Angles of Attack
by
Odilon V. Cavazos Jr.
Lieutenant, United States Navy


A water tunnel flow visualization investigation was performed into the high angle of
attach aerodynamics of a 2% scale model of the F/A-18 fighter aircraft. The main focus
of this study was the effect of pitch rate on the development and bursting of vortices
generated fran the leading edge extensions in the high angle of attack range with and without
yaw. Results of this investigation indicate that that the vortex bursting point
(relative to the static case) moves rearward with increasing pitch-up motion and forward
with increasing pitch-down motion. For the same pitch rate, vortex bursting was found to
occur earlier for the pitch-down motion than for the pitch-up motion, implying aerodynamic
hysteresis effects.





During pitch-up motion the LEX vortex appears to be smaller indicating a more stable vortex; this effect leads to a lag in vortex bursting. This indicates that during
pitch-up motion, bursting occurs at a point further downstream than would occur for static
conditions, resulting in a vortex system which is equivalent to a static system at a reduced
angle of attack.

From a careful study of the flow visualization photographs it was determined that
the pitch up motion caused the vortex bursting point to lag the static condition point at
the same AOA.

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...Which doesn't say anything about stall on the wings. Just because the flanker frame can reach a high angle of attack and recover control, does not mean the wings haven't stalled past a certain point.

 

[MiG-29]

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

...Which doesn't say anything about stall on the wings. Just because the flanker frame can reach a high angle of attack and recover control, does not mean the wings haven't stalled past a certain point.

you can live in blindness, vortex burst means wing stall and pitch up if you chose to live in darkness up to you

" The advantage of the hybrid planform over the
conventional wing is due to the LEX induced vortex flow which increases in strength with
increasing angle of attack. The stable vortex flow creates an area of high negative
pressure on the wing upper surface which increases lift and delays separation of laminar
flow in the basic planform"


Prediction of separation characteristics over wings and airfoils in steady flow at high
AOA has been the subject matter of various researchers for the past several years. As the
angle of attack is increased, lift also increases; but soon the upper surface flow starts to
separate near the trailing edge. Further increase in the angle of attack causes the
separation point to move forward toward the leading edge, eventually resulting in stall.
The lift producing mechanism of a hybrid planform wing at lower angles of attack is
similar to a conventional wing but is accompanied by flow separation from the LEX and
the formation of counter rotating vortices called LEX vortices [Ref. 8]. External flow is
drawn over these vortices and is accelerated downward causing the flow to reattach
resulting in additional lift, commonly called the voi. lift. At high angles of attack the
flow separates, but flow separation is not the sole cause of stall. Instead, lift is lost due
to a breakdown (bursting) of the vortices. This vortex breakdown on stationary wings has
been investigated extensively by Wedemeyer [Ref. 9].

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[latenlazy]

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

you can live in blindness, vortex burst means wing stall and pitch up if you chose to live in darkness up to you

Um. I'm sure the vortex helps the wing achieve a higher AoA without stalling. However, that does not mean that the wing remains unstalled throughout the entire maneuver. If it did, then the flanker would not need a passive way of recovering back to level flight.

 

[MiG-29]

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

Um. I'm sure the vortex helps the wing achieve a higher AoA without stalling. However, that does not mean that the wing remains unstalled throughout the entire maneuver. If it did, then the flanker would not need a passive way of recovering back to level flight.

At high angles of attack the
flow separates, but flow separation is not the sole cause of stall. Instead, lift is lost due
to a breakdown (bursting) of the vortices. This vortex breakdown on stationary wings has
been investigated extensively by Wedemeyer [Ref.

please conect the dots

Results of this investigation indicate that that the vortex bursting point
(relative to the static case) moves rearward with increasing pitch-up motion and forward
with increasing pitch-down motion. For the same pitch rate, vortex bursting was found to
occur earlier for the pitch-down motion than for the pitch-up motion, implying aerodynamic
hysteresis effects

 

[latenlazy]

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

At high angles of attack the
flow separates, but flow separation is not the sole cause of stall. Instead, lift is lost due
to a breakdown (bursting) of the vortices. This vortex breakdown on stationary wings has
been investigated extensively by Wedemeyer [Ref.

please conect the dots

Results of this investigation indicate that that the vortex bursting point
(relative to the static case) moves rearward with increasing pitch-up motion and forward
with increasing pitch-down motion. For the same pitch rate, vortex bursting was found to
occur earlier for the pitch-down motion than for the pitch-up motion, implying aerodynamic
hysteresis effects

The vortices of the flanker don't sustain themselves throughout the entire maneuver either. They DO burst before the maneuver is completed, as Engineer showed earlier. However, the lifting force generated continues to pitch the nose of the plane via inertia before the AC shifts behind the CG and drag takes over. You're the one who needs to connect the dots. If the wings were not stalled during the maneuver then the Flanker would not need a passive way to recover pitch control.

 

[MiG-29]

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

The vortices of the flanker don't sustain themselves throughout the entire maneuver either. They DO burst before the maneuver is completed, as Engineer showed earlier. However, the lifting force generated continues to pitch the nose of the plane via inertia before the AC shifts behind the CG and drag takes over. You're the one who needs to connect the dots. If the wings were not stalled during the maneuver then the Flanker would not need a passive way to recover pitch control.

okay chose to live in darkness i will believe Andrei Fomin`s book Su-27, hystersis is the reason and why the Su-27 does not enter into spin at 70 deg of AoA, after 70 deg the static dynamic stability has changed signs and the lift pushes the nose down along the moment of inertia.



Live in darkness no problem a stalled wing pushes the nose up and creates yaw assymetries this would make the Flanker uncontrollabe like an F-14 at 110 deg of AoA.

So live in darkness is your choice.


When this happens the tip stalls, and since the tip is swept to the rear, the net lift moves forward.

Further increases in angle of attack would cause an inward progression of the stall. A loss in load near the wingtip may, depending on the sweep angle, taper ratio, and aspect ratio, cause a forward shift in the wing aerodynamic center.

This causes the plane to pitch up, leading to more of the wing stalling, leading to more pitch up, and so on. .

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[latenlazy]

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

okay chose to live in darkness i will believe Andrei Fomin`s book Su-27, hystersis is the reason and why the Su-27 does not enter into spin at 70 deg of AoA, after 70 deg the static dynamic stability has changed signs and the lift pushes the nose down along the moment of inertia.



Live in darkness no problem a stalled wing pushes the nose up and creates yaw assymetries this would make the Flanker uncontrollabe like an F-14 at 110 deg of AoA.

So live in darkness is your choice.

The reason the plane doesn't spin once the wings have stalled and the vortices have burst in the cobra maneuver is because the nose gets pushed up so fast the airframe reaches a positive static margin. It has nothing to do with vortices preventing stalling of the wings through the entire maneuver. You should read your own sources.

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"The pitch motion is highly dynamic (with the acquired kinetic energy of pitch) allowing the aircraft to overshoot its trim angle of attack at post stall region. The provision of high lateral stability, especially at angles of attack about 30 degrees, is sometimes referred to as a black art, and the only way to overcome this instability is to cross it in a reduced time before passing into the fully separate flow at higher angles of attack."

So yes, you are right that a hysteretic process is what prevents the Su-27 from entering into a spin, but that hysteretic process has little to do with vortices and the angle of attack which the wings stall. Those only matter in so much in that they help enable the nose attain a rate of pitch into post stall. The real hysteretic phenomenon is the pendulum motion created by the shifting of the AC from in front to behind the CG through the maneuver, which then pushes the plane back into level flight.

When this happens the tip stalls, and since the tip is swept to the rear, the net lift moves forward.

Further increases in angle of attack would cause an inward progression of the stall. A loss in load near the wingtip may, depending on the sweep angle, taper ratio, and aspect ratio, cause a forward shift in the wing aerodynamic center.

This causes the plane to pitch up, leading to more of the wing stalling, leading to more pitch up, and so on. .

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This doesn't help your case, as the whole point of the cobra maneuver is to encourage the nose to keep pitching until the air frame has reached positive static margin in post stall.