Saab J 29 Tunnan and JAS 39 Gripen compared: Part 2 – Time twins

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The JAS 39 Gripen entered service with the Swedish Air Force in June 1996 and is now the sole combat type in the Flygvapnet. Paul Stoddart compares this fourth generation aircraft with its ancestor, the portly yet effective, J 29 Tunnan which entered service 46 years earlier. 

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(Part 1 can be read here

The J 29 concept started with a straight-wing, but this did not last. In November 1945 Saab obtained a windfall of German research data. The future was clearly swept. By February 1946, a 25-degree swept wing design had been selected. Automatic leading edge slats were fitted to prevent the airflow over the wings from separating in high angle of attack (AOA) manoeuvres.

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At the transonic speeds achieved by post-war aircraft, shockwaves forming on the tailplane would render conventional (ie trailing edge mounted) elevators, downstream of the shock, ineffective. Those speeds also moved the mainplane centre of pressure rearwards resulting in pitch-down that the ineffective elevators could not correct. The solution was the flying tail (pioneered by the Bell X-1 seen here- and Miles M.52 concept) in which the entire horizontal tailplane could move in pitch. Shockwaves still formed on a flying tail but its area ahead of the shock front would remain an effective control surface.

The Tunnan was the first Western European jet fighter to have an all-moving tail – something which has since became standard for transonic and supersonic aircraft of conventional tail layout.

Fighter agility depends, among other things, on a rapid roll rate, something the J 29 prototype had in spades. Fitted as it was with full-span ailerons, the prototype had a pilot nauseating rate of roll of 180 degrees per second – this was excessive. The ailerons were reduced to around 65% of the span, with the remaining inboard section replaced by flaps, and the aircraft was tamed. 

Roll out the barrel

Following four prototypes, 224 J 29As were produced. From March 1953, the J 29B became the standard Tunnan version. This had internal wing tanks that added 154 Imperial gallons (700 litres) of fuel taking the internal total to 462 Imperial gallons (2,100 litres), a 50% increase. Twin 99 Imperial gallon (450 litre) drop tanks could also be carried so offering a total load of 660 Imperial gallons (3,000 litres). The tanks were fitted at roughly mid-span on the outer of the two main pylons with the inboard hardpoints retained for weapons. Gripen uses a 242 Imperial gallon (1,100 litres) drop tank that can be fitted on the centreline and inboard underwing pylons. With three tanks in place, total fuel carried is 1,386 Imp gallons (6,300 litres); the internal load being 660 Imp gallons (3,000 litres).

The Tunnan’s wing was shoulder mounted, meeting the fuselage somewhat above the centreline. The wing was a one-piece structure and ran straight through the capacious fuselage passing just behind the cockpit rear pressure bulkhead and above the intake duct. Gripen also has a shoulder-mounted wing, set roughly at the centreline of the slim fuselage. This is slightly below the level of the canards (that in turn are mounted just below the upper surface of the intakes). Fuel tanks are fitted in the upper part of the fuselage middle section with the intake duct(s) and main undercarriage bays placed below.

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A one-piece wing could have been mounted level with the fuselage upper or lower surfaces as per the Jaguar and Phantom respectively.

A low mounted wing on the diminutive Gripen would have offered insufficient ground clearance for loading under-wing stores without a longer and heavier undercarriage. To achieve favourable interaction, the canards have to be set above the level of the mainplanes. Lack of suitable alternative mounting points for the canards would rule out the high wing location option. Furthermore, in order to reduce drag, it is best to avoid forming acute angles at wing-fuselage fairings. A mid wing configuration arguably offers the best overall solution in this and the other respects; it is therefore an entirely reasonable design choice.

The Tunnan was of monocoque structure built from aluminium alloy. High strength and stiffness were required to withstand the loadings imposed by transonic flight and a very fine standard of surface finish was also achieved in order to reduce skin drag. In structural terms, the Gripen marks a major change for Saab with composite materials (carbon fibre, glass fibre and Aramid) accounting for 20% of the structure by weight. Fatigue life consumption is reduced by a gust alleviation system. Aircraft disturbance is sensed by the flight control system, which prompts control surface reaction to alleviate the loads imposed.

Insane in the mainplane

The most obvious difference between these classics is in their lifting and control surfaces. Although radical at its inception for its swept wing and flying tail, the J 29 was standard in being longitudinally stable with a conventionally sited tailplane. Such tailplanes apply a download to balance the mainplane’s lift (the main plane centre of pressure being ahead of the centre of gravity). In turn, the mainplane must generate additional lift to counter the tail’s down force and as a result lift-induced drag is increased. The Tunnan’s primary flying controls were the tailplane for pitch and the ailerons for roll. By contrast, the Gripen controls pitch by the canard while the inboard and outboard elevons on the delta wing act in both pitch and roll. The canard applies a lifting force to balance the mainplane and this co-operative interaction reduces the overall lift-induced drag.

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During pre-Gripen studies a conventional layout was considered. 

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Saab originally reversed the traditional arrangement with the Viggen and adopted a tail-first or canard design although it retained natural longitudinal stability. With the JAS 39, the full potential of the canard was realised. Full time, full authority, digital, fly by wire flight control systems (FCS) allowed the adoption of artificial stability in pitch with attendant gains in agility and aerodynamic efficiency. At supersonic speeds the centre of lift on all wings moves aft promoting a nose down moment. A conventional aircraft trims this by increasing the tailplane download whereas the opposite applies with the canard, a more efficient solution. An unstable canard design offers more lift during take-off and landing, better supersonic turning performance and lower supersonic drag. The FCS keeps the Gripen’s instability in check and allows the full envelope to be exploited without the risk of overstress or departure from controlled flight. This carefree handling facility enables the pilot to concentrate on the mission while the FCS controls the load factor, AOA, angle of sideslip and roll rate. Another function unavailable to the Tunnan is CG control. The fuel control system not only monitors the fuel remaining but also balances the amounts drawn from the various tanks to keep the CG within limits.

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This article is Paul’s personal view of the development of the Tunnan in comparison with the Gripen A.  It contains no implication of Ministry of Defence policy nor should any be inferred.

If this article interests you, support Hush-Kit.net with a donation (buttons above and below). If this goes well we’ll be able to give you much more! Recommended donation £15. Many thanks for helping to keep us impartial and independent. 

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