Random Engine shut downs

[_JMax]

I am having a great time learning the Mig 21. So far just SP,making my own missions etc. What is the cause of the constant engine spool downs/shut downs?? Every time I fly I get this random and unpredictable problem. I almost always get an in flight re-start,but not always. I must be missing something. Thanks!

[Rudel_chw]

No track file, hard to pinpoint a cause without it, maybe you have negative G for too long?  Check this video:

 

 

[Volator]

What Rudel says. The 21 is very susceptible to near 0 or negative  G's, especially when in afterburner.

[_JMax]

On 4/18/2025 at 10:44 PM, Volator said:

What Rudel says. The 21 is very susceptible to near 0 or negative  G's, especially when in afterburner.

Yes that seems to be it. Ham-fisted pilot is the cause. ...although the Mig 21 is a little bit sensitive on this. But as long as I was more careful with the negative G's,I dont seem to get the shut downs so far. Thanks Volator and thanks Rudel. 

[Art-J]

Negative Gs are somewhat manageable, the fuel system is designed to handle them for a few seconds, which should be plenty for going into a dive. It's staying at about zero G that shuts the engine MUCH faster. Afterburner exacerbates the issue even more.

Time limits are mentioned somewhere in the module manual (and one of tutorial missions?), but I don't remember them off top of my head - haven't flown the -21 for quite a while.

[Rudel_chw]

5 minutes ago, Art-J said:

Time limits are mentioned somewhere in the module manual (and one of tutorial missions?), but I don't remember them off top of my head - haven't flown the -21 for quite a while.

 

15 seconds when flying at MIL power.

5 seconds when using the Afterburner.

3 seconds when the emergency afterburner is used.

 

After a negative G maneuver the pilot should fly 30 seconds at positive G, because of the time needed to refill the Inverted flight Fuel tank.

[_JMax]

On 4/22/2025 at 5:21 AM, Rudel_chw said:

 

15 seconds when flying at MIL power.

5 seconds when using the Afterburner.

3 seconds when the emergency afterburner is used.

 

After a negative G maneuver the pilot should fly 30 seconds at positive G, because of the time needed to refill the Inverted flight Fuel tank.

Thanks Rudel....I am now able to mitigate the shut-downs with this knowledge,but I sure need to be careful.And shutdowns occur usually,of course,in the middle of a dogfight...

[zerO_crash]

Let me help you a little:

 

What you cannot fail to recognize in the MiG-21Bis (as one of the modules yielding this tendency), is the trim position at different airspeeds. The faster you go in the MiG-21Bis, the more the aircraft wants to pitch up, thus - the faster you go, the more nose-heavy you trim. If you think about it for a second, that means, conversely, that the slower you go, the more the nose will want to droop (unless paid close attention to).

 

If you consider how a BFM fight typically starts; you retain more energy at the start of the fight, than you have towards the end. In other words, whether kinetic, or potential, your net falls over time. Why does this matter? Because it tells you that you typically will go in fast with heavy forward trim, and finish slower and in need of readjusting trim back. In particular, if you end up dogfighting in the vertical, you will lose speed, yet typically maintain the same pressure on the stick. At that point your G will lower, more than you'd expect. I am mentioning this, because a pilot is esp. susceptible of entering the 0.5G - 0G regime when going vertical (following or escaping from an opponent). If you are not aware of your trim at all times, you are done for (engine flameout due to insufficient fuel). Remember this; if you go vertical, either actively retrim, or keep pulling the stick harder back. Also, one eye on G-meter if doing below 1G maneuvers. Above that, you have warning lights on both sides of the cockpit front glass. 

 

To add to the times Rudel mentioned; the culprit of flameouts in the region sub 1G, is very simply that the engine gets starved of fuel. The fuel tanks in the MiG-21Bis have generally a very clean layout, where you have individual fuel tanks in different sections of the plane connected in line to a 9L cannister (reservoir), and finally to the engine. That 9L tank is there, because at sub 1Gs, the main fuel pumps are incapable of delivering fuel fast enough to the main engine (hence why the time you can fly in these different regimes lowers, the higher thrust you demand from the engine). The technical solution, then, was to engineer in a small tank (it was not there on the early MiG-21 variants - hence a couple accidents), with a fuel pump sufficient to operate even in the extreme and deliver fuel to that Thumansky R25-300. In those extreme G-conditions, the tank will deplete faster than it is refilled by fuel tanks from the feed side. 

 

Your takeaway from this should be to keep in mind that you have a 9L bottle back there, which gives you options in sub 1G envelope, but keeping in mind that it isn't big, you don't want to hang out there for too long. Don't forget the nose-heavy trim either, when losing airspeed. You'll be good!

Edited April 23 by zerO_crash

[_JMax]

Awesome post,zero_Crash.....very helpful. It is easy to get sloppy with the trim and you have identified my issue I think. Generally I do ok with the Mig21,but now recognize that  being out of trim sneaks up on me. And being mindful of that 9 litre tank(only 9 litres?!!...You would think they could have put a bigger tank in,lol),is good advice. Thanks!