What is the concept of duck aerodynamic layout?
For various reasons, the low-speed performance of the tail and tailless aerodynamic layout of ordinary high-speed aircraft is often poor. Duck layout can meet the requirements of high and low speed of fighter. Performance requirements. Because this layout can give good consideration to the slender shape required by high-speed aircraft and the high trim lift coefficient required by aircraft to achieve STOL. This is because: on the one hand, when the slender canard configuration changes from subsonic to supersonic, the stability increment caused by the center of gravity movement is smaller than that caused by the rear tail configuration, which is beneficial to high-speed maneuvering flight. On the other hand, when flying near or at a high angle of attack, it can produce much higher trim lift than tailless and tailless aircraft. This shows that it is also suitable for low-speed flight.
High trim lift
Figure 1 is a schematic diagram of the longitudinal trim mode of tail, tailless and canard aircraft with Jing 'an stability. When the plane flies stably and horizontally in the air, gravity and lift, thrust and resistance are equal, and the moment of the whole plane is also balanced. In order to obtain the balance force, the canard layout scheme of J- 10A was embodied in China's early concept of J-9, but many technical problems involved are that Liu occupies the moment on J-10, and tailless and tailless aircraft need to pay a certain lift price. In flight, the lift Y of the wing and the zero lift moment Mzo of the whole aircraft will produce an arcuate moment on the center of gravity of the aircraft. In order to balance this moment, the tailless aircraft should use the flap to lift and the tailless aircraft should use the elevator to lift and balance, which will reduce the lift of the whole aircraft. Of course, when flying at a small angle of attack, the load of the horizontal tail is not large, and the lift cost it pays is also very small. But when the plane flies at a high angle of attack and measures are taken to increase the lift (such as putting flaps), the situation deteriorates. Because it will bring a lot of extra bow torque when lifting. In order to balance these additional moments, the trailing edge of the horizontal tail must be inclined at a large angle, which will significantly reduce the high-lift effect. If the wing is raised. Sometimes it is even difficult to trim, so we have to take measures to increase the negative lift on the flat tail. There are many examples in this field abroad. Because the boundary layer control technology is adopted on the trailing edge flap of the American F- 14 aircraft, the downward bending moment increases a lot, which leads to the tail approaching stall when balancing, so the flat tail has to be modified to make the leading edge upturn and the airfoil turn into reverse bending. Japan PS- 1 Seaplane blows air on the lower surface of the tail to increase negative lift. Even with the tail layout, it is more difficult for tailless aircraft to balance high lift.
In contrast, the canard layout is superior to the tail and tailless layout, because its pitching moment can be provided by the positive lifting surface (canard) in front of the center of gravity of the aircraft. This really kills two birds with one stone: it not only provides a balancing moment, but also increases the lift. So why did few people use duck layout before? This is because the conventional canard aircraft has three main disadvantages:
(1) There is a strong downwash between the front wing and the main wing, which reduces the lift of the main wing. Although the lift of the front wing is positive, which makes up part of the lift loss, the total lift in peacetime is not necessarily much higher than that of the rear wing.
(2) The problem of duck layout balance is not easy to solve. On the whole. The load of canard wing is larger than that of tail wing, often 3 ~ 4 times that of tail wing. Because the canard is placed in front, the center of gravity of the whole machine moves forward and the center of gravity needs to be adjusted forward. In this way, the duck wing is close to the center of gravity and the arm of force is short, which limits its balance ability. In addition, the main wing washes the front wing, so the front wing tends to stall first at high angle of attack. This is unfavorable for take-off and landing and high angle of attack maneuvering. It was not until the Swedish successfully developed the Saab -37 aircraft in the late 1960s that these shortcomings were overcome to some extent. As a class II aircraft with the number of m, Saab-37 shortened the take-off and landing distance by more than N400 meters without taking complicated lift measures, which met the requirements of short take-off and landing. This achievement has aroused widespread concern in the international community. Saab-37 adopts tightly coupled canard layout, and realizes high lift by using the favorable interference of separation vortex between front and rear wings. (3) The generation, development, rupture and drift of the main wing vortex have great influence on the lift and longitudinal and lateral moment characteristics of the aircraft, which makes the longitudinal moment curve extremely nonlinear and leads to poor handling quality of the aircraft. In order to solve this problem, the conventional canard layout aircraft has to increase the stability of the aircraft in order to obtain a straight longitudinal moment curve. In this way, the trim resistance of the aircraft increases, and the trim ability of the front wing decreases, resulting in poor maneuverability and take-off and landing performance of the aircraft.
One of the solutions is to use fly-by-wire control system to relax static stability.
Using separated vortex to obtain high lift.
It is found through experiments that when the angle of attack is very small, the airflow is separated from the leading edge and rolled into a detached vortex for a thin wing with a large sweep angle above 45 degrees. The vortex center pressure of this separation vortex is very low, and the lift of the wing surface is improved due to the difference between the upper and lower pressures. We know that the total lift of delta wing is equal to the sum of potential lift and vortex lift.
Potential flow lift is the lift calculated according to potential flow theory. In Figure 2, the dotted line represents the total lift, the dotted line represents the potential lift (the circle is the experimental point), and the difference between the two lines is the theoretical vortex lift. It can be seen that the lift line slope and maximum lift coefficient of delta wing are greatly improved because of vortex lift. If the canard is closely coupled with the main wing with large sweep angle, it will produce favorable interference, and the efficiency of vortex shedding is higher and the vortex lift is greater. When the canard is placed in front of the main wing, the separated vortex of the front wing enters the low pressure area on the upper surface of the main wing, which is beneficial to the stability of the vortex center, delays the breakdown of the vortex and improves the stall angle of attack of the front wing.
In addition, the separation vortex of the front wing not only induces a vortex lift on the front wing, but also induces a vortex lift on the main wing when it sweeps the upper surface of the main wing. The existence of front wing vortex also helps to control the leading edge vortex formed on the main wing and delay the stall of the main wing. Because the main wing is washed down by the front wing (inner wing section) on the one hand and washed up by the front wing (outer wing section) on the other hand, the total amount of washing down is reduced. Because of these favorable disturbances, the lift and stall angle of attack of the close-coupled canard aircraft are higher at high angle of attack (up to more than 30 degrees, while the stall angle of attack of the ordinary tail aircraft is only a dozen degrees). This is of great significance for expanding the maneuvering flight range of aircraft and improving the take-off and landing performance of high-speed aircraft.
In the interference between the front wing and the rear wing, except the downwash of the front wing to the main wing, other interferences are beneficial, which makes the lift of the close-coupled canard aircraft much larger than that of the ordinary canard aircraft with the same wing area. In the take-off state, the lift coefficient of tightly coupled canard aircraft can reach twice that of tailless delta-wing aircraft!
Of course, due to the large amount of downwash interference, the favorable interference is not enough to offset the unfavorable interference at a small angle of attack. Even so, at small angles of attack, the maximum lift-drag ratio of tightly coupled canard aircraft is equivalent to that of the same class of rear-wing aircraft. With the increase of attack angle, the favorable interference is gradually greater than the unfavorable interference. When the angle of attack reaches about 16 degrees, the favorable interference of tightly coupled canard aircraft exceeds the unfavorable interference, and the lift coefficient of the whole aircraft is higher than the sum of the lift coefficients of a single front wing and a single main wing, which is beyond the reach of ordinary rear-tailed aircraft. Because for a tail plane. There is also the downwash problem of the main wing to the tail wing, which increases with the increase of attack angle. Even if the tail generates positive lift, its total lift coefficient is always lower than the sum of the lift coefficients of two independent wings.
Large scraping angle
There is another advantage of the canard layout aircraft: because the main wing is at the rear and the tail of the fuselage is short, the ground friction angle (the angle at which the tail touches the ground, which is determined by the angle between the connecting line between the main wheel and the tail nozzle and the ground horizontal line) can be designed to be relatively large, which is beneficial for the aircraft to take off and land at a higher angle of attack (14 ~ 18). However, the rear fuselage of ordinary tail-wing aircraft is long, and the friction angle with the ground is often only 8 degrees and 9 degrees.
The close-coupled canard-wing aircraft also has shortcomings: the contradiction of difficult balance has not been fundamentally solved, which greatly limits its application scope and performance. In order to overcome this contradiction, domestic and foreign aircraft design departments have adopted a series of technologies. For example, spanwise blowing or chordwise blowing is used to improve the trim ability of the front wing; Or we can use fly-by-wire control system and active technology to relax the static margin of the aircraft, so that the front wing can be freed from the heavy burden, and the direct lift and direct side force control can be realized by the coordinated action of the front wing and the main wing moving wing surface. Gust, typhoon, JAS 39 and other new generation fighters with canard aerodynamic layout are all equipped with fly-by-wire control systems, which can realize active control, so they are one step ahead of Saab -37 and their aerodynamic performance is greatly improved.
New types of canards appeared in the 1990s, and they still have great potential in aerodynamics. It can be predicted that with the application of new technologies such as two-dimensional nozzle, composite material, forward sweep, dynamic lift and active control, the performance of canard aircraft will be greatly improved.