The tuning of the new Viggen is divided into different challenges:

- Take off preformance
- Behaviour from sub-to supersonic
- Acceleration & Top speed
- Max G to supersonic
- Landing performance
- Stall behaviour

Above includes parameters such as

- Speed
- Alpha
- Weight
- Inertia (radius of gyration)
- Cm-alpha over entire aircraft
- Drag
- Thrust

To achieve best solution requires knowledge of absolute values to reflect the Viggen, production of test cases, and know how about the relation between parameters. It is easy to just build a good plane, but very hard to create a specific fighter. Current work focus on the hard parts, such as super stall and smooth transition to supersonic.

To create the Viggen we have defined known limits within the flight envelope. These are:

• Top speed M1.1 at MSL
• Top speed M2 at high altitude
• Normal landing speed 127 kt at 15 deg alpha, landing weight.
• Max Roll speed is 270 degrees/s
• Time to max speed MSL: 100 sek
• Time to 30000 feet: 100 sek.
• Minimum speed with flap: 100 kt. (actually 97 kt).
• Minumum speed to gain max G: 400 kt
• Max allowable G to get throught the sound barrier: 4G.

Other aircraft data that we have absolute control over is the outline of the Wings, because we have used blueprints provided by FMV (Defence Material administration) which clearly defines every angle in degrees.

For the engine we have used standard catalogue data, but I know from rumour that it is even stronger if you pull the plug to get it in "war mode". We use normal data.

One issue is the inertia, where I only use estimates based on my experience of seeing the Viggen fly.

Current state in data collection is that I have requested more precise wing data as well as the flight manual from the authorities. The manual was made in english for austrian pilots who flew the Viggen in Sweden (the rented flight hours). I was their teacher in RADAR and weapon systems.

The cm-alpha (momentum of aircraft in different alpha) for the entire aircraft is not optimized right now. This is important for high pitch rate induction as well as super stall behaviour. In super stall the aircraft is actually stable but the alpha pivots around a much higher alpha and it is very hard to "flip back" to the normal pivot point (0 degrees up to 23 degrees). It flies "normal" in above 23 degrees but with very low lift. The momentum (Cm) or tendency to flip backwards is stronger than the force gained from elevators also depending on the very low lift that is created. The "stability class" of the Viggen is "near instable", but not instable. (The new JAS39 Gripen is super instable).

Recent winnings is accurate painting instructions for the splinter version, so don't be surprised if we give you a perfect splinter JA soon. We will make sure that you "tailspotters" out there will recognize the exact Viggen individual. It will probably be the 323 which was used to demo the Viggen in airshows in the early 80:s.

The pain is that we do not have accurate wing profile data, but by using known envelope data we close in on the real thing. The other issue is the constructive interference between the canard and main wing which is a design feature of the Viggen to actually be able to fly at those low speeds. x-plane does not mimic that well, except for the downwash forn canard onto main wing.

To solve this I use a combination of profile data, as well as flap data. It is hard to optimize because you don't want to change pitch or other things when flap is extended (between 250 and 150 kt). You also don't want to drop the elevators more because it differs to much from real Viggen. In normal landing the elevators shall pivot around +/- 5 degrees.

//Erik Norberg