From JAST To J-20

Sometimes the analysis of a new design is one of those areas where you get a whack-on-the-side-of-the-head moment.

This one was induced by the discussion here of the origins of the F-35 design, wherein I suddenly realized what the J-20 reminded me of - Lockheed's immediate pre-JAST/JSF design, tested in the form of a large powered mock-up.

The similarity is quite close in terms of wing/canard relationship, sweep angles, and body shaping, although the Chengdu engineers decided to align the trailing edges of the canards (and rudders) with the trailing edges of the opposite wings, giving them more sweep at the quarter-chord line.

I remember talking this over with Paul Bevilaqua at the 1993 Powered Lift Conference in Palo Alto. If I remember correctly, one reason for the canard delta was that it was good for the cross-sectional area distribution (area ruling) and hence transonic drag.

The challenge was that the shaft-driven lift fan design inevitably had a big cross-section peak well forward, where the inlets wrapped around the fan bay (it needed a large-diameter fan and lots of airflow to work). A canard delta compensated for that by moving the thickest part of the wing as far back as possible.

Somehow I don't think we're going to see a J-20 with a lift fan. However, don't be surprised if the weapons bays turn out to be more capacious (and versatile) than on other designs. It looks like the idea of the canard configuration is to get a large-volume mid-body section through the transonic zone and into supersonic flight with minimal fuss, bother and expenditure of fuel.

Bevilaqua's paper on the origins of the F-35 design cites risk as the reason for the reversion to a quad-tail design for the JSF competition in 1996 - and at the time both Eurofighter and Saab were dealing with unexpected issues in this area.

However, another Lockheed Martin engineer explained that the final JSF planform design was more flexible in terms of being enlarged to meet Navy requirements (given that LO constraints and commonality mandated the same sweep angle for all versions).

That may have been the biggest non-STOVL driver to affect the design, although canards were definitely not popular in the US in the mid-1990s - and I suspect that fitting the canard design on to an LHA elevator might have been a challenge.

Tags: ar99, tacair, j-20, JSF