Foreplanes
These top views of the J-20 and Eurofighter, show that the J-20 has its main wing at a farther aft position than the Eurofighter`s main wing and its canards are closer to the center of gravity, than the ones of Eurofighter Typhoon; in terms of effectiveness, the Eurofighter design is more effective since it can reduce the size of the canard with respect its wing and its lever arm is more effective.
The Typhoon also has a canard with lower swept than its wing to understand it let us see:
Minimum induced drag is obtained for the 25-degree canard at low lift coefficients for both research models regardless of canard position. The 60-degree canard had the highest value of induced drag at low lift coefficients and the lowest value at lift coefficients near (C L) max lift coefficients. the Intermediate canards C0 and L2 have values between the 25- and 60-degree canards. Thus, for good low lift performance characteristics, i.e., range and endurance, the low sweep canard Is best. When maneuvering capability is the dominant design factor, the highly swept canard generates the best performance. The Intermediate canards are good compromises having lower Induced drag than the 60-degree delta shaped canard at low lift coefficients and slightly higher values of induced drag near maximum lift.
AERODYNAMIC CHARACTERISTICS OF THE CLOSE-COUPLED CANARD AS APPLIED TO LOW-TO.MODERATE SWEPT WINGS VOLUME 2: SUBSONIC SPEED REGIME by David W Lacay
LifT to drag/ ratio In still optimized at positions forward of the canard wing overlap. As with, (L/D))max, lowering the canard reduced the lift to drag ratio.
Position has a minimal effect of C but moving the canard forward and downward increased minimum drag.
AERODYNAMIC CHARACTERISTICS OF THE CLOSE-COUPLED CANARD AS APPLIED TO LOW-TO.MODERATE SWEPT WINGS VOLUME 2: SUBSONIC SPEED REGIME by David W Lacay
It is pretty obvious the Eurofighter`s canard is for lower drag thus trying t increase range at very low AoA and level cruise flight, J-20 is further limited in the sense stealth forces the canard to have the same swept angle than its wing
The J-20 copied the canard solution of the F-15 ACTIVE, which has canards with dihedral and two canards mounted on the upper inlet area forward of the wing, this foreplane configuration was also studied in the HiMat aerodynamic testbed UAV by NASA
Basically this plane represents a crude attempt to make a Jet fighter plane taking certain ideas from various American and Russian projects, take for example the forebody of the plane, basically it is a plane that takes the american patented air intakes design named Divertless supersonic inlet that does not require a boundary air layer diverter, which in the United States was implemented for the first time and used in a testbed F-16 and subsequently in the US F-35 fighter.
The plane is quite large almost the size of the Russian Su-27 plane, about 21 meters in length, and the fuselage is quite long in fact it can be argued that only its fuselage without taking into account the wings or the vertical and horizontal stabilizers is probably around 3 meters longer than the F-22`s fuselage , this is if we consider only the distance from the tip of its radome to the end of the nozzles of the jet engines, this suggests two things, a need for more internal volume, and that it has internal air intake ducts longer than the ones seen in the F-22, this can only be explained if you consider an operational combat radius that requires more fuel than the F-22.
The Al-31 is known to be a less effective jet engine than the one used in the F-22, the F119-PW-100, this indicates that the plane would be used in long-range flights if it had efficient turbines, in future variants. And for the time being it would give enough combat range with less efficient turbines due to a greater fuel capacity.
Another reason why the plane is quite large is the contradictory design of its wings and horizontal stabilizers, on the one hand it uses its main delta wings and canards (the foreplane horizontal stabilizers)in high-mounted positions like the canards of the F-15 ACTIVE or the X-36, this prevented to put the canard above the wing as in fighters like the Rafale or the Gripen and that also increased its length by moving the wing backwards.
The centre of lift lies aft on a Delta wing. This means that the horizontal tail can only be effective if it lies even further aft, this also forced a big canard size on the J-20 because its wing is farther aft than aircraft like Eurofighter or Rafale.
It is known that the centre of lift of Sweptback wings and Trapezoidal wings lie ahead of that of the center of lift of Delta wings.
The lowest values of drag due to lift are those of canards above the wing cord configuration as those fitted to aircraft like Rafale, canards that are at the same level of the wing chord like X-29 have lower lift increase. In order to increase its lift the J-20 in fact copies the solution given to an F-15 variant with canards called F-15 ACTIVE since its canards have an angle in relation to its wing root chord in English called dihedral to improve the generation of vortices of low pressure but which is a less effective position than the usual position of the canard with a chord root above the wing root chord of the main delta wing like in the case of the French Rafale, that has a high-mounted canard with a mid-mounted wing; the coplanar root chords of the J-20`s canards and wings moved the main wing much further back from the center of gravity and generated the need for a canard of larger size, however this position is not the best because it moves the main wing lift aft from the center of gravity of the aircraft unlike the Eurofighter that has its wings further forward and its canards very close to the nose and forward of the cockpit thus reducing the canard`s size and the turbulence generated on the wing and improving the lever arm that the canard has; nor is it like the Rafale with a smaller canard and near the wing to generate vortices more efficiently.
The J-20 has canards that carry some of the weight of the J-20 since the internal weapons bays are positioned forward of the center of gravity.
Another problem is that the canard is not parallel to the inclination of the vertical stabilizer nor is it aligned and in a position parallel to the wing like other stealth technology stealth planes, besides by being without a control mechanism to pitch the plane in flight such as thrust vector control nozzles, it will simply increase the deflections that the canard will need for trimming and it will increase its radar signature footprint and it will make it easier to detect.
Studies have shown that for angles of attack greater than 16 degrees, an increase in the canard sweep results in an increase
in lift developed by the canard when the canard is above or in the wing chord plane. This increased lift results in a lift increase for the total configuration for the canard above the wing chord plane.
For the canard in the wing chord plane, the increased canard lift is partially lost by increased interference on the wing.
For the configurations with the canard in the wing chord plane, increasing the canard dihedral angle from -18.60 to 18.60 increased the maximum lift coefficient of the configuration. For the configurations
with the canard above the wing chord plane, the highest maximum lift coefficient was
developed when the canard had no dihedral.
"THE EFFECT OF CANARD LEADING-EDGE SWEEP December 1974" this can explain why the J-20 has canard with dihedral
From the front view, the J-20`s canard are less effective than the F-22`s aft positioned horizontal stabilizers in terms of stealth, first because the J-20 lacks thrust vectoring nozzles for longitudinal trim and its canards are ahead of the main wing and not aligned with both the wing, the engine nacelles fuselage lateral walls, intakes lateral walls and vertical stabilizers.
This series of pictures show as the canards are deflected to pitch up the aircraft, the vortex system appears at high AoA, once the aircraft starts climbing the vortex system disappears.
The aircraft has very well developed wing LERX, wing and canard vortices.
In order to understand the complexities of canard equipped aircraft, like the J-20, we will see the aerodynamics of Su-30MKI/MS.
From the analysis of the lift force coefficient, at low attack angle, the Su-30MS`s canard deflection (δ) causes a negative lift which causes the lift coefficient to be smaller than the standard canard condition. Hence, the use of standard canard configuration δ = 0° gives the best CL results whereas, at high AoA, the deflection gives the ability to sustain the vortex center, delay the vortex breakdown location and be able to retain the CL. This occurrence shows that deflection angle (δ) significantly affect the ability of the aircraft to regulate lift and delay the stall.
Therefore, the employment of deflection angles of the canard is suggested for high AoA to delay the stall needed for maneuvering movements. The experiment shows that at canard deflection angles of 30°–40°, it is still possible to form a vortex center that still produces a lift force up to AoA 80°, as seen In that occurrence, the The Su-30MS can maintain its highest CL and delay the stall up to AoA 80°
J-10 has performed the Pugachev`s Cobra, albeit with Thrust Vectoring nozzles, no J-10 has ever performed the Pugachev`s cobra on aerodynamics alone, at least publicly.
J-20 has not performed any post stall maneuver at least publicly, it is interesting to speculate if it can perform it, in order to do it its all moving v vertical tail and ventral fins and the whole aircraft will need high stability at high AoA in yaw and roll movements and its V vertical tail and wing elevons and flaps will be needed to bring it to level flight, but at least up to know it is very likely it will need thrust vectoring control nozzles to do it
These picture shows several images of a Su-30MS model in a water tunnel, being pitch up to different (AoA) Angles of attack, the Vortex Breakdown location(VBL) moves from the trailing edge to a leading edge position as the AoA increases.
By increasing its AoA, the Su-30 also increases its pitch up force, the original Su-27 can recover thanks to Hysteresis, or dynamic lag of vortex breakdown, it needs to have a mathematical model of aerodynamics at high angles of attack and solve the problem of stall and spin aerodynamics with description of some effects such as Nonsymmetrical flow including nonsymmetrical breakdown of vortexes at high angles of attack, and nonsymmetrical yaw and roll moments; the Pugachev`s cobra is the result of Dynamic lag, static and dynamics hysteresis in lift, pitch, yaw, roll moment, side force at high angle of attack.
To put it in simpler terms the aircraft has to be pitched up very quickly in order to delay vortex break down and have enough pitch down force to bring the aircraft to level flight and recover the aircraft from stall, this is popularly known as the Pugachev`s cobra, where no nonsymetrical flow at yaw or roll moments occur and its horizontal aft stabilizer is capable to pitch down the Su-27 .
The aerodynamics of the Su-27 allows it to use "lift-hysteresis" and perform manoeuvres like the Pugachev`s Cobra, which is essentially rapid deceleration using a sudden high angle-of-attack manoeuvre,
