SAC design and the CAC J-20

Jim Smith had significant technical roles in the development of the UK’s leading military aviation programmes from ASRAAM and Nimrod, to the JSF and Eurofighter Typhoon. He was also Britain’s technical liaison to the British Embassy in Washington, covering several projects including the Advanced Tactical Fighter contest. Here he considers what the J-20’s alleged failed rival reveals about the role and requirement of the Chengdu J-20.

The SAC design features a three-surface layout, with a canard, wing and tailplane. Like the J-20, the canard is relatively ‘long-coupled’, meaning there is a significant distance between the canard and the wing. The wing planforms of the two aircraft differ, with the J-20 having a straked near-delta planform, while that of the SAC design appears somewhat similar to the F-18, albeit with a straight strake leading edge, rather than the curved strake of the F-18.

The Chinese also operate the J-15, a three -surface aircraft derived from the Su-27 series and similar to the Sukhoi Su-33.

Compared to this aircraft, we can see that the tail is somewhat similar, but the wing and canard are more closely coupled. The wing on the SAC design has a larger strake than the J-15, and the planform appears less highly swept than the J-15.

Three-surface Configurations

Let’s discuss three-surface configurations first. There’s quite a good discussion about this on Wikipedia, but here’s my take. I remarked that the SAC design was in some way reminiscent of an F-18, so I’ll start there. 

I can already see the head-scratching – an F-18 isn’t a three-surface configuration. True, but what set the F-18 and the F-16 apart was the use of a fixed forward strake ahead of the wing. This does a number of good things. Firstly, it’s de-stabilising, meaning that (with an appropriate control system) manoeuvre performance can be improved, because pitch stability is reduced. Secondly, at moderate and high incidence, powerful vortices are generated which run over the upper surface of the wing. This generates considerable lift, improving instantaneous turn rate, and also helps maintain lift from the wings at high incidence, allowing flight at very high incidence and low speed (great for airshows, but of questionable utility in combat).

One way of looking at these strakes is to think of them as a highly-swept fixed canard, located in the plane of the wing. That’s how they behave aerodynamically. What about moveable canards, such as those on Typhoon, Rafale and the Su-33? Well, these moveable canards are contributing to the control of the aircraft, as well as providing de-stabilising and additional aerodynamic functions similar to the fixed strakes of the F-16 and F-18.

But having moveable canards and tailplanes provides some additional design freedoms (or opportunities and complexities if you prefer). Let’s assume that all these advanced fighter aircraft are unstable – this offers the rapid manoeuvre response expected of a fighter, but also requires continuous management of the control surfaces to ensure that the aircraft responds as directed by the pilot.

If the aircraft is in the cruise, the pilot will expect the aircraft to be trimmed, and in steady flight. The flight control computers will arrange this by, for example, continuously adjusting the controls in response to gusts and turbulence, and by managing the tendency of the unstable aircraft to diverge from straight and level flight as a result of these disturbances. Choices will be available on how to do this – because either the canards, the tailplanes or other controls, such as thrust-vectoring, can be used to trim the aircraft. 

These choices can allow the control law designer to do some other things as well. For example, thrust vectoring might be used to trim, allowing all control surfaces to remain aligned to minimise head-on signature. Or the canard surfaces might be continuously adjusted to minimise lift-dependent drag. Trimming the aircraft sounds like a minor exercise, but it is important to remember that the aircraft centre of lift will change substantially in supersonic flight, and this will have to be managed, ideally also delivering the minimum signature, maximum manoeuvre margin, and/or minimum lift dependent drag, depending on the decisions taking in designing the aircraft.

If the aircraft is manoeuvring violently, or perhaps flying an approach in turbulent conditions, the canard-tail combination provides a rapid and powerful way of generating pitch commands, and also of trimming out the effects of high-lift devices such as slats and flaps which allow approach speeds to be managed.

I also referred to the SAC design as being relatively ‘long-coupled’, meaning that the canards are further ahead of the wing than is seen in other canard designs such as the Typhoon, Rafale and Su-33. The Chengdu J-20 shares this characteristic.

Why might this be a desirable design feature? It is an interesting choice, because it will, to some extent limit the ability to optimise the flow for minimum drag, because the direct influence of the canard flow on the wing will be somewhat reduced. However, the increased moment arm available will mean greater control power will be available for trimming or for manoeuvre. 

Implications of the configuration

What does this suggest about the aircraft design, and the requirements against which this aircraft and the J-20 were in competition?

To address this, one needs to make some conjecture about the purpose of the aircraft. I suggest the J-20 and SAC-design are in the nature of strategic air defence aircraft. Perhaps, like the MiG-31, their primary purpose could be homeland defence, but it seems likely that this also extends to the defence of China’s immediate area of economic influence, in which I include the South China Sea.

To deter and defeat incursions into this space, the aircraft would require a range of flexible and long-range weapons, clearly including long-range air-to-air weapons, but also possibly including anti-shipping, and in the future, perhaps hypersonic weapon systems. These weapons are all likely to be larger than the AMRAAM-class systems employed by many Western Nations, and in the case of anti-shipping and hypersonic systems may also be heavier.

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This implies the need for the aircraft to have large weapons bays, and the ability to vary loads between different weapons and (probably) auxiliary fuel tanks. In any case, the geography of the air defence region is such that long-range, relatively large aircraft will be needed. High-speed, high-instantaneous-manoeuvrability, long-range and large internal weapons carriage appear to be the principal design drivers for the J-20 and the SAC design.

The Chengdu J-20 and the SAC design have adopted somewhat different solutions to these postulated design requirements. The J-20 is a long-coupled canard delta, appearing in some ways like a growth-version of the European canard fighters, but optimised for the internal carriage of relatively large weapons. The three-surface SAC design does look somewhat like an attempt at the same approach, but starting from a J-15-like three-surface solution, again seeking to provide a configuration with a large internal weapons bay. Both aircraft appear to be designed to accommodate a substantial c.g. range, allowing a wide range of fuel and weapon loads to be managed. The SAC design does also feature a straked wing, which appears to be, relative to the overall dimensions, a little smaller in area than that of the J-20.

Why might the J-20 have been favoured?  An almost impossible question to answer, as cost, industrial policies and other considerations are likely to have been involved. However, it is noticeable that the SAC design has a long forward fuselage, and that the intakes are set relatively far back on the airframe. 

It is possible that this complicates the packaging of the aircraft, as once intakes, engines and main undercarriage have been accommodated, there may be insufficient space for a broad weapons bay like that of the J-20. If the SAC bay has to extend forward into the forward fuselage, then the shape may have been constrained. Carrying two pairs of missiles in tandem (rather than 4-abreast as in the J-20) would be possible, but overall, the shape of such a bay might not offer the range of weapon options available from the J-20.

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