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.
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, mplying 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.
This is another one of your fallacies called
, where you are trying to mask your incorrectness with large quantity of text. It is also a fallacy called
, as the text is nothing but a divertion of attention away from answering a simple question "why is the Cobra maneuver called post-stall maneuver if the aircraft doesn't stall".
From this
:
In the post stall regime, lift no longer increases but decreases with the angle of attack.
This is synomsis with the definition of stall, taken from the
where:
The classical stall may be defined as a condition in which the airplane wing is subjected to an angle of attack greater than the angle for maximum lift coefficient.
Further down, the manual gives the following definition:
An aerodynamic stall is defined as a condition in which the wing attains an angle of attack greater than the angle of attack for maximum lift, resulting in a loss of lift and an increase in drag.
So, stall occurs when angle-of-attack is so large that further increase in angle-of-attack causes decrease in lift. This is the case with an aircraft under Cobra maneuver, since without stall the aircraft would fly upward instead of forward with a large AoA angle.
With regards to stall and high angle-of-attack, the manual gives this definition:
Stalls. The stall may be defined by either airflow separation with increasing angle of attack causing loss of lift, control difficulty, or excessive buffet/vibration (see 6.2.2 and 6.2.5)
Flow separation occurs when an aircraft is under Cobra maneuver, as indicated by this picture:
In addition, as pointed out by the above quote, control surfaces (such as the tail plane) lose effectiveness leading to control difficulty.
For post-stall the
, the following is said:
The airframe agility may or not include the aircraft ability to fly and to maneuver at high angles of attack, also described as the post stall flight region, which give rise to new problems to the designer (aerodynamic stall, propulsion ignition,non linear and non stationary behavior, unstable configuration, control of the possible departure).
Quite simply, the aircraft has entered a stall in post-stall maneuver. Your claim otherwise has no basis in realities, and is nothing but pseudo-aerodynamic theories that you made up because you cannot admit that you are wrong.
---------- Post added at 11:35 AM ---------- Previous post was at 11:28 AM ----------
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].
In other words, flow separation is one contributor to stall, which is what the statement "not the sole cause" means. As we see in the following picture of Su-27 in a Cobra maneuver, flow has entirely separated, thus the aircraft is in a stall.
---------- Post added at 11:41 AM ---------- Previous post was at 11:35 AM ----------
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 statement "flow separation not being the
sole cause of stall" is not equivilent to "flow separation not being a cause of stall". We know that flow separation can lead to a stall, as explained by the
:
Stalls. The stall may be defined by either airflow separation with increasing angle of attack causing loss of lift, control difficulty, or excessive buffet/vibration (see 6.2.2 and 6.2.5)
The reason why flow separation is not a sole cause, as explained by this
, is that vortex can re-energize the flow and cause reattachment. Examples of such vortex can be seen while the JF-17 is in a high-G maneuver, as shown by the following picture:
Such vortex does not exist while the Su-27 is under Cobra maneuver.
---------- Post added at 11:54 AM ---------- Previous post was at 11:41 AM ----------
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. .
Hysteresis is not equivalent to no stall occurring, so it doesn't support your claim that the Su-27 is not in a stall when in Cobra maneuver. In analysis of the Cobra maneuver, this
says the following:
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.
Note that "post-critical AoA" is synonymous with stalling, as explained by the
:
An aerodynamic stall is defined as a condition in which the wing attains an angle of attack greater than the angle of attack for maximum lift, resulting in a loss of lift and an increase in drag.
So, in Cobra maneuver, the aircraft has already stalled. When this happens, control surfaces become ineffective, evident by difficulty in control:
Stalls. The stall may be defined by either airflow separation with increasing angle of attack causing loss of lift, control difficulty, or excessive buffet/vibration (see 6.2.2 and 6.2.5)
The only recovery comes from the aerodynamic design of the aircraft, a passive mode of recovery rather than active deflection of control surface. This is explained by the
:
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.
