How good was the JA-37 Viggen?

I always thought that the performance of JA 37 has always been somehow underrated. To some extent it can be attributed to the lack of export sales and the Sweden’s neutrality – there were simply not enough information, rumors from exercises etc., to generate some sort of the consensus on the Viggen’s performance. Also, despite the originality of the design, there are practically no books dedicated to the development of Viggen family, what makes extremely difficult to trace all the in-service upgrades and modifications, which, as usually in Sweden, can be expected to be rather extensive.

The fighter variant JA-37 came to live as a deep modification of the Viggen’s first generation planes. Its first flight took place in 1974, while it entered squadron service in 1980 (for reference, the AJ version prototype flew in 1967). As a result, the aircraft is commonly disregarded, as people say that it offered phantom-like capabilities while being produced in the same period as the teen fighters. That however, is not entirely true.

The JA-37 had a number of modification in comparison to AJ-37. The main differences were:

– Uprated RM8B engine, with the thrust increased from 115.6 to 125 kN, was used. The responsiveness of the engine was also greatly improved (and the smoking was reduced).
– An Oerlikon KCA cannon, famous for its performance (range in excess of 2 km), was added.
– The onboard computer, air data computer, inertial navigation system etc. were updated (most of these systems was actually borrowed from the F-14…)
– A pulse-Doppler PS-46/A radar was introduced. It sported a lookdown/shootdown capability as well as ability to track up to 2 targets while scanning. I’ve came across some information that it benefited from solutions used in F-15 AN/APG-63, but no specifics were given. The cited range is almost always given as either 48 or 75 km, what looks awfully like the case, where everybody is citing an old brochure, with no clue of the real performance.
– The initial armament consisted of 2 SkyFlash missiles, combined with 2-4 Rb. 24 (AIM-9J). Later, it was upgraded to Rb.72 (AIM-9L) and Rb.99 (AIM-120B). It is worth noting that after the AMRAAM upgrade it was common to see a Viggen with 4 such missiles.
– Cockpit was fitted with 3 CRT screens. The HOTAS was introduced at some stage, but I’m not sure if it was there from the beginning.

The JA 37 went through numerous upgrades, often benefiting from the Gripen’s technologies. The most important were:

1985 – introduction of the data link (encrypted communication between up to 4 fighters and ground control), which allowed passive firings about a decade before any other country sported such system
1987 – flares, ground controlled secondary information via data link, Rb. 74
1990 – radar upgrade, improved resistance to jamming and multi target tracking (I don’t know the specifics)
1992 – automatic gun aiming (from what I’ve heard, extremely effective)
1996 -1998 – upgrade to the JA-37D standard – Rb. 99, cockpit upgrades, MIL-STD-1553B databus introduction, U95 jammer pod, flare and chaffs dispensers BOY 401, ECM and ECCM suites enhancements, TARAS terminal (improvement to data link).

These are the main upgrades I am aware of, but there were probably multiple more.

Another poorly known aspect of Viggen’s capabilities is the airframe performance. Recently, a performance manual of JA 37 Viggen became available thanks to this guy: https://www.reddit.com/r/hoggit/comments/5swfsd/ja_37_air_superiority_viggen_turn_performance/
Below are presented some of the E-M diagrams, given for different armament configurations (examples in the table).

I must say that I have some problems with understanding how the instantaneous turn rates are to be determined. The problem here is with the Nz G limit lines. They are changing from 7G at 0 meters, to 4G at 8 and 12 km. I have never seen such behavior on E-M diagram, so I don’t know what to make of it, as usually the G limit is not changing with the height. If anyone have any suggestions how to read it, I would be thankful. Nevertheless, even if we assume that at 4 km the airframe is limited to 6G, and at 8 km just to 4G, the turning performance of Viggen is still quite good. Its sustained turn rate at 0 km, with A-A configuration is about 13.3 deg/s, so in the Phantom’s ~14 deg/s ballpark. However, its instantaneous performance is rather spectacular, with 24.5 deg/s, so similar or even better than some of the teen fighters. The performance suffers rather badly with the increase of height (again, the G-limit question), but the instantaneous turn rate at 4 km (18.5 deg/s) is still great (for comparison, at 4570 meters the clean F-16 block 50 has 18.8 and F-14D in A-A configuration has 18.1 ). The acceleration of Viggen is a bit less spectacular, as at sea level it takes about 30 s to accelerate from 0.55M to 1.05M, so similar to F-4, and much slower than F-16 (about 20 s).

It seems to me, that the case of Viggen is similar to Tornado ADV – a very good weapon system, inside a rather mediocre airframe (in comparison to teen fighters and MiG-29/Su-27). Still, I would say that it had a clear edge over the MiG-23/F-4/Mirage F-1 generation (excluding F-4F, which is debatable) and thanks to the upgrades it remained a very competitive aircraft up to its retirement. However, there are still many aspects, which remain unknown, at least to me:
– what was the performance of the PS-46 radar? How greatly it was improved during service?
– what is going on with the G-limit – height dependence?
– how did it perform in actual training exercises, are there any rumors?
– and finally, why this marvelous aircraft seems to be forgotten by all aviation authors?