Posted on

Almost there

The new FDE is now “almost finished” – which really means that we are now in the kingdom of Pareto and Murphy. The final stage of any project is inevitably where most of the work is being done, and where everything inevitably goes wrong. That said, I am actually optimistic about releasing 4.1b2 within the next few weeks.

The big difference between the previous (current) version of the Draken FDE and this attempt is that I am now starting with a blank sheet and some good books on aerodynamics, instead of just tweaking a stock FDE until it squeaks. The 4.0 FDE performs OK within, say, 75% of the envelope. The new version will hopefully be able to “touch the edges”.

I started by remodeling the engine, which now produces net dry thrust within 5% of the real thing all the way from the runway to the “coffin corner”. It also has the correct thrust-specific fuel consumption and a more linear throttle response. All based on the sparse documentation from Rolls-Royce, Volvo and Saab that I managed to find.

The next step was to rework the lift and drag parameters to achieve the correct airspeed, angle-of-attack, pitching moment, etc. during climb and cruise. A great help in this was that I finally found some actual airfoil data for Draken. At the moment I am working on stability and control derivatives, which turned out to be the trickiest part. I don’t have the real values, so it is basically a lot of maths, guesswork, and test flights.

Putting the final touches on the derivatives today, then adding the afterburner and starting on the supersonic parts.