Engine stalls

Last weekend was spent instructing people flying the Viggen simulator at the air show at the local airbase Säve. Though it was hard work it was also much fun and revitalised my commitment to this project.

We've been discussing for some time how to simulate engine stall, something that was quite common in the AJ37. After some comments on the lack of this feature by some previous Viggen pilots I've now seriously started to dig into this.

From my sources I've found that there were three major types of engine stalls. Reversing the engine with too much thrust at too low speed. Increasing throttle before the engine RPM has decreased enough after throttle decrease or decreasing the throttle to rapidly at high altitude and high alpha. Finally, you can get engine stalls just by flying with high alpha at high altitudes. Engine stalls are perceived as loud "bangs" in the fuselage or when at low altitude as a growl. You will also see a small flutter in the needle of the engine pressure gauge. If the condition that triggered the stall is maintained, the engine will eventually stop.

Engine stall when reversing on ground has a very simple condition. Under 130km/h you risk getting engine stall with high thrust. However I still need to d some more research before I'm able to have a complete condition for this.

I've been told that for increasing or decreasing the throttle at high altitudes and high alpha. A realistic condition would be to start stalls at alpha>20 degrees at 5000 meters and at alpha > 15 at 6000 meters. Below 1000 meters it would be virtually impossible to get engine stalls this way so I chose to set the condition to alpha>30 at 1000 meters. In the end I got a nice curve that could be described with a simple second degree equation. I'll calculate a factor from the change of throttle and let the sensitivity for this value be linear to the alpha and the altitude. And hopefully I will get a nice behaviour for this kind of engine stalls.

Engine stall from just high alpha at high altitude should be quite simple but I only have one value for this so I need to do more research before I start implementing this. What I do know is that at 6000 meters it's reasonable to assume that you will get engine stalls at 18 degrees alpha.

Implementing engine stalls will be an important step towards our goal of realism. Another important thing I wanted to add is inducing engine fire when using afterburner when reversing. However that is not possible in FS2004 so this is a feature that will have to wait for us to switch to FSX.