Compared with the YF-22, the YF-23 has a longer fuselage and a mid-wing. The front and rear edges of the wing are swept back and forward by 40 degrees respectively, similar to a rhombus. There is no horizontal control surface at the rear of the fuselage, replaced by two vertical control surfaces tilted outward at 50 degrees.
The air inlet is located under the fuselage near the leading edge of the wing. The air inlet and air inlet are fixed structures with no movable parts, which not only reduces weight but also avoids the need for additional frontal radar. Reflection cross-sectional area (RCS). The air inlet is bent upward inside the fuselage and is connected to the engine located on the back of the fuselage. The nozzle is located in the middle of the cambered vertical control surface. The nozzle cannot be directly seen from the rear and below, which reduces the intensity of the infrared signal and also limits the intensity of the infrared signal. Feasibility of installing vector nozzles.
The prototype has only one bomb bay, located in the center of the air inlets on both sides, between the cockpit and the engine. The additional bomb bay planned for the mass-produced version will be located in front of this bomb bay.
The YF-23A shows a completely different design concept from the YF-22A, and also reflects the Northrop/McDonnell Douglas design team's understanding of future air combat requirements.
General layout The overall layout of the YF-23A largely inherits the characteristics of Northrop's conceptual design plan. Its diamond-shaped wing + V-shaped tail layout is between the traditional normal layout and the tailless layout. Single seat, twin engines, mid-single wing, belly air intake.
Like the YF-22A, the YF-23A ultimately did not adopt the once highly requested canard layout. In fact, the American tendency can be seen from the fact that none of the seven companies' plans adopt a duck layout. To a certain extent, this is influenced by General Dynamics' influence at the Big Seven seminar a few years ago - Harry Hill Lake said that "the best place for a canard is on someone else's airplane." The author wrote in "The King of Kings" It has been mentioned in "Wing" that one of the reasons for rejecting the canard layout is the trimming problem. If the canard is designed according to the principle of effective pitch control, then the canard cannot trim the huge nose-down moment generated by the wing lift device. If a trim lift device is required, the canards must be enlarged, and the downwash on the wing will also increase, which in turn weakens the lift effect. And to prevent deep stalls, it may be necessary to add a horizontal tail. On the other hand, from the perspective of transonic area law, it is difficult for large canards to meet the requirements of transonic area law, which increases the difficulty of fuselage design and supersonic drag - this is very important for ATF (especially YF-1) that emphasizes supercruise. 23A), it is particularly difficult to accept.
Another important reason for rejecting duck layout is the invisibility problem. The position, size, and plane shape of the canards are difficult to unify with the stealth requirements. An important principle of stealth design is to minimize (but unavoidable) discontinuities on the body surface (especially in the head-on direction), and canards are difficult to achieve this. If you also want to minimize the number of main beams corresponding to the leading and trailing edges of the wing (that is, the leading and trailing edges are parallel), it will bring greater design difficulties.
Although according to the requirements of the US Air Force, ATF must take into account both stealth and maneuverability, but the design ideas of each company are different, and the emphasis on aircraft performance must also be different. Judging from the fact that the YF-23A finally chose a V-shaped tail instead of the traditional four-tail layout, Northrop's intention to pursue stealth is quite obvious. Their design can greatly reduce the aircraft's side radar reflection cross-section. Due to the reduction of a pair of tail fins, the weight and drag of the aircraft can also be reduced, which is also helpful for improving super patrol capabilities. But what follows is the efficiency of the control surface and the complexity of the flight control system.
Fuselage In order to meet the requirements of "cross-theater range", the ATF must have a large enough fuel capacity and taking into account the stealth requirements (the aircraft cannot carry external auxiliary fuel tanks), all fuel must be carried by the internal fuel tanks. Therefore, whether it is YF-22A or YF-23A, it must provide sufficient internal volume - almost twice that of F-15! From the perspective of body size, the length of YF-23A has increased significantly, but it is still limited. Therefore, the increase in the interior volume must mainly come from the increase in the cross-sectional area of ??the aircraft. If considered from the perspective of trans/supersonic resistance, the increase in the cross-sectional area of ??the aircraft is not conducive to designing the aircraft according to the transonic area law. Properly lengthening the fuselage will help smooth the longitudinal cross-sectional area distribution of the aircraft and reduce trans/supersonic drag. However, lengthening the fuselage will inevitably lead to an increase in the aircraft's longitudinal rotational inertia, which is detrimental to improving the aircraft's agility and precise control capabilities.
The fuselage length of the Su-27 is similar to that of the YF-23A. Pilots who have flown the Su-27 said that the aircraft has a large control inertia and is not that easy to fly.
In fact, we can see the difference in design ideas between YF-23A and YF-22A just from the characteristics of the fuselage design. Judging from the fuel capacity inside the aircraft, the YF-23A carries 10.9 tons of fuel, and the YF-22A carries 11.35 tons of fuel. Considering that the bomb bays in the aircraft are designed to carry the same amount of bombs (the reason why I say design is because of the combat bombs of the YF-23A (the cabin is still on the drawing), then the internal volume of YF-23A will not be larger than that of YF-22A. The fuselage length of the YF-23A is significantly longer than that of the YF-22A (the latter has an actual fuselage length of more than 18 meters due to the tail brace and flat tail), which means that even when the maximum cross-sectional area of ??the aircraft is the same, , YF-23A can also obtain a smoother cross-sectional area distribution (that is, smaller trans/supersonic resistance), and of course it also obtains a larger longitudinal moment of inertia. It is not difficult to see that in order to solve the resistance problem caused by the increase in cross-sectional area, YF-23A and YF-22A made completely opposite choices. The former chose speed performance at the expense of agility and precise control capabilities. This also reflects to a certain extent the positioning of future fighter jets by the two major groups. In appearance, the fuselage of the YF-23A is quite similar to the Lockheed SR-71 Blackbird. It looks like the front fuselage and two separate engine nacelles are directly embedded into an integral wing. The front fuselage is mainly equipped with a radar cabin, cockpit, front landing gear bay, avionics bay and missile bay. The cross-section of the front section of the front fuselage is approximately a rounded hexagon that is symmetrical up and down, and then gradually transitions to a circular cross-section, and finally fully integrates with the wing at the middle section of the fuselage. The cross-sections of the rear air inlets and engine nacelles are still trapezoidal and transition in a very smooth curve to the wings or the "beaver tail" of the rear fuselage, which helps reduce mutual interference drag. As mentioned earlier, the Air Force canceled the requirement to use thrust reversers, but Northrop did not modify the design, forming a very obvious "groove" in the rear fuselage, bringing unnecessary increase in drag.
The side-strip wing layout has greater advantages in lift characteristics than the canard layout at high angles of attack - this is one of the factors that influenced Northrop's choice of the overall layout of the YF-23A. As far as traditional side strips are concerned, the increase in their length (and area) has obvious benefits in increasing the lift at high angles of attack. However, the greater the span, the greater the pitching moment generated at high angles of attack; this becomes a factor restricting the size of the side strips. But obviously the side strips of the YF-23A are different from the traditional side strips on the third-generation aircraft. Its three-section straight-line narrow side strip design is quite distinctive, extending forward from the leading edge of the wing to the top of the radome. This kind of side strip is quite similar to the YF-22A side strip.
The edge strips of YF-23A have the following functions: generate edge strip vortices, induce vortex lift on the wing, and improve the lift characteristics of the wing; use edge strip vortices to form the upper surface of the wing The boundary layer replenishes energy and delays the wing stall; it acts as an aerodynamic "wing knife" to prevent the boundary layer from accumulating toward the wing tip and delay the wing tip airflow separation (in fact, due to the large root-to-tip ratio of the YF-23A wing, high speed or There may be an obvious trend of wingtip separation at high angles of attack); the separation of the nose vortex at high angles of attack provides better pitch and directional stability - until the third generation supersonic fighter, the nose of the aircraft at high angles of attack The problem of asymmetric vortex separation remains unresolved and is an important factor limiting aircraft from entering the stall realm.
But from a traditional point of view, the side strips of YF-23A are too small, and it is still doubtful whether they can generate strong enough eddy currents to play their due role. If it is indeed possible, then one possibility is that the working principle of the aircraft's edge strips is different from traditional edge strips. Another possibility is that there are other auxiliary measures to help improve the lift characteristics of the wing. Some information mentions that "the vortex generated by the nose and inner wing has no impact on the tail." This may mean that there may be some measure on the inner side of the YF-23A wing to generate vortex, which acts similar to the edge vortex. role. There are two control panels on the top of the YF-22A's air inlet, which are used to control the vortex on the upper surface of the wing. The YF-23A may have a similar design - there is a deflation slit in the boundary layer of the air inlet on the inside of the wing. We do not rule out the possibility that the boundary layer airflow is accelerated and discharged through it, thereby improving the airflow condition on the upper surface of the wing. .
The huge diamond-shaped wing can be regarded as one of the most prominent appearance features of the YF-23A.
The leading edge of the wing is swept back 40 degrees, the trailing edge is swept forward 40 degrees, the dihedral angle is 2 degrees, the wing area is 88.26 square meters, the aspect ratio is 2.0, and the root-tip ratio is as high as 12.2. The most important influencing factor why Northrop chose such an awkward wing plane shape was stealth. The YF-23A's stealth technology is inherited from the B-2, and the two have similarities - one of which is the X-shaped four-lobe reflection characteristics. To achieve four-lobe reflection, the leading and trailing edges of the wing must be parallel in the horizontal plane. As a result, Northrop had no more choices: either adopt a swept-back design to form a swept trapezoidal wing, basically similar to the B-2 wing; or adopt a forward-swept trailing edge design to form a symmetrical diamond-shaped wing.
The advantage of using a swept trapezoidal wing is that the sweep angle selection limit is smaller and can be optimized as needed; but compared with the triangular wing, the disadvantages are also obvious: lower structural efficiency; smaller internal volume , which has a particularly large impact on ATFs that require cross-theater range; the problem of aeroelastic divergence is obvious; the choice of relative thickness of the wing is limited, which is not conducive to choosing a smaller relative thickness to reduce supersonic drag. If you choose the trailing edge forward sweep design, when the wing leading edge sweep angle (trailing edge forward sweep angle) is small, this kind of wing is closer to Northrop's customary thin wing with small sweep angle (typically such as F-5, YF-17), faces the same problem as the swept trapezoidal wing - extraordinary endurance and excellent supersonic performance are huge contradictions that are difficult to solve for this kind of wing. The use of symmetrical diamond-shaped wings with large sweep angles is advantageous for stealth purposes - the F-117 uses a sweep angle of up to 66.7 degrees in order to deflect radar waves significantly - but aerodynamic restrictions have rejected this. One possibility: the aspect ratio is too small and the aerodynamic efficiency is extremely low. Whether such an aircraft can fly is a question. Moreover, if the trailing edge forward sweep angle is too large, the efficiency of the lift/control device on the trailing edge of the wing will be drastically reduced until it is unacceptable.
Under comprehensive trade-offs, only a symmetrical diamond-shaped wing with a medium sweep angle can achieve a more satisfactory balance point in stealth, endurance, aerodynamics and other aspects. As for why the sweep angle of 40 degrees was chosen exactly, the author believes that when other conditions are basically met, the beneficial interference of optimizing the edge strip vortices should be one of the influencing factors. However, even so, the trailing edge forward sweep angle of 40 degrees also seriously affects the efficiency of the wing trailing edge aerodynamic device: YF-23A must use a larger flap downward angle to ensure the lift effect, but this also increases The separation trend of the boundary layer on the upper surface of the wing not only increases the difficulty of controlling the boundary layer, but also reduces the lift effect. On the other hand, the aileron efficiency of the YF-23A is also poor, resulting in an inefficient roll rate. meet the requirements, which ultimately affects the outcome of competitive test flights.
As far as the characteristics of the wing are concerned, Northrop's priority is stealth first, followed by supersonic speed and endurance, and finally maneuverability and agility.
In order to improve the lift characteristics of the wing, the YF-23A adopts a leading edge maneuverable flap design, which accounts for about 2/3 of the wingspan. There is information that the aircraft uses a slat design, but the characteristics of the slats cannot be seen in the YF-23A test flight photos. And from a stealth perspective, when the slats are extended, the slit formed will become a good reflector of electromagnetic waves, which is absolutely unacceptable to Northrop.
In fact, leading edge flaps still have a negative impact on the aircraft's stealth characteristics. The best solution is the mission-adaptive wing technology proven on AFTI/F-111, which can avoid discontinuities and gaps on the wing surface. Unfortunately, this technology has not been put into practice until today. In this regard, the YF-22A adopts the diamond-shaped groove design inherited from the F-117, which makes the area become a low radar reflection area when the flaps are deflected. The YF-23A, which strongly pursues stealth, does not consider this detail. The only explanation is that in the aircraft's typical combat state (super patrol), the wings are symmetrical airfoils and do not need to deflect flaps.
The design of the aerodynamic control surface located on the trailing edge of the YF-23A wing is quite unique and can be regarded as the highlight of the YF-23A. Some information states that the inner side of the wing is a flap and the outer side is aileron, but the actual situation is far from that simple. The simple distinction of flaps and ailerons is not in line with Northrop's "one thing, multiple uses" design concept embodied in the YF-23A. Judging from the test flight photos of the YF-23A, both the inner and outer control surfaces are involved in lift and roll control. Therefore, the author positions it as a "multi-purpose flaperon".
The reason why it is said to be "multi-purpose" is that in addition to the functions of traditional flaperons, these two pairs of control surfaces also serve as speed brakes and drag rudders. When the inner flaperons are deflected downward at the same time, the outer flaperons are raised at the same time. Deflection, while ensuring that the wing does not generate additional lift increment, generates symmetrical aerodynamic drag and acts as a speed brake; when only one side of the flaperon adopts upward/downward deflection, small symmetrical drag is generated and acts as drag The role of the rudder - this must have been inherited and developed from the design of the B-2. This design is quite novel and effectively reduces weight, but the complexity and development risks of the flight control system inevitably increase.