Su-27 Engine Failure / ReStart & Stall

[ekg]

When I try to climb high (altitude > 10K) with afterburner and I stall something ends up causing the engines to shut down. The plane also inverts upside-down and falls vertically. I usually can't recover. Sometimes after falling for a while they will restart sometimes they won't.

 

Any ideas why that happens? I find that I can't handle this bird as easily as in Flanker 2.5 :) (not complaining this is a good thing). Does anyone have any ideas off the top off their head? I don't have track at the moment but I can provide one if necessary.

[sobek]

If you subject the plane to negative g for a period longer than a few seconds, the fuel lines will run dry and the engines will starve.

[ekg]

If you subject the plane to negative g for a period longer than a few seconds, the fuel lines will run dry and the engines will starve.

 

That actually kinda makes sense. When I climb and start stalling I usually push on the stick so that would result in some negative G's (not enough for a red-out though). Is the correct manoeuvre then to first rotate upside-down and pull back?

[Svend_Dellepude]

Is the correct manoeuvre then to first rotate upside-down and pull back?

 

That would probably make more sense. Pilots avoid negative G's IRL because they are more difficult to handle.

[DarkFire]

When I try to climb high (altitude > 10K) with afterburner and I stall something ends up causing the engines to shut down. The plane also inverts upside-down and falls vertically. I usually can't recover. Sometimes after falling for a while they will restart sometimes they won't.

 

Any ideas why that happens? I find that I can't handle this bird as easily as in Flanker 2.5 :) (not complaining this is a good thing). Does anyone have any ideas off the top off their head? I don't have track at the moment but I can provide one if necessary.

 

What you're experiencing is a combination of a couple of things:

 

1. The cache tank on the Su-27 is small. The cache tank will start to empty at any G level <0 or during level inverted flight. You can fly inverted but to continue in this condition you must have a G load of >0 or the engines will be starved of fuel and will flame out. Fortunately at high altitude (above 10,000m) you have plenty of time to recover positive airspeed and re-start the engines. It's worth bearing in mind that at low level at full afterburner the contents of the cache tank will be used in less than 4 seconds.

 

2. When you say you're climbing to altitude at full afterburner, are you trying to zoom climb? That's possible for the sake of a zoom climb but as a technique to reach a cruise altitude it's ineffective and wasteful of fuel. The flanker climbs nicely under military power with ~30-35 m/s climb rate up to @6,000m altitude and after than ~20m/s climb rate to whatever cruise altitude you want. No need to use the afterburners unless you want to reach very high altitude, but if that is what you want to do I'd suggest reaching ~12,500m then levelling off to gain speed before climbing again once you reach ~1,100Km/h TAS. 90-95% engine RPM will easily get you that sort of cruise speed at that sort of altitude. If you're trying for an altitude record then I'd recommend levelling off at 12,500 metres then accelerating in full afterburner to 2.60M before pulling up in a 3-4G climb to a pitch angle of 60-70 degrees. This will get you to 26-27 Km altitude (as indicated on the HUD) but you will stall eventually.

 

3. The Flanker behaves really, really badly under negative AOA conditions. It's this which causes the infamous inverted stalled departure which is so difficult to recover from. If you're stable in level flight at almost any altitude the preferred method of diving is very much to roll inverted before pulling down in to a dive. If you watch videos of real VVS Flanker drivers this is how they do it.

 

4. Stall recover from a positive-AOA stall condition is fairly easy as the nose will drop automatically. The aircraft can roll a bit but this is easily corrected when you start to gain airspeed. Stall recovery procedure is to set throttle to low or idle, wait for the airspeed to build to ~300 IAS, wings level, gradually increase engine RPM and pull up to the horizon. Recovery from a negative AOA condition is much, much harder and I've never been able to do it with less than ~5,000m altitude. If you do end up in this condition the most effective method I've found is to enter direct control mode and try to 'rock' the aircraft out of the stall using alternate positive and negative pitch inputs. Even then it doesn't always work.

Edited April 1, 2016 by DarkFire

[Stuge]

As far as I know if you get into a stable, non-oscillating inverted stall and manage to flame out the engines, then that's it. You can't recover.

 

As long as the engines are running idle, recovery is possible.

[Esac_mirmidon]

From the Su-27SK manual.

 

Maximum service ceiling profile: No payload, or 2 R-27 + 2 R-73.

 

1.- Acceleration at sea level to 650-670 IAS at full afterburner and climb at Mach 0.8-0.85.

 

2.- Climb to 11.000 - 12.000 meters at Mach 0.8-0.85.

 

3.- At 11.000 - 12.000 meters, level flight and accelerate to Mach 1.6 in afterburner.

 

4.- At Mach 1.6 climb keeping a vertical speed of 3-5 m/sc until service ceiling.

 

Service ceiling under afterburner ( 50% internal fuel without payloads ) is 18.500 meters.

[ekg]

As far as I know if you get into a stable, non-oscillating inverted stall and manage to flame out the engines, then that's it. You can't recover.

 

As long as the engines are running idle, recovery is possible.

 

The thing is that the engines themselves are actually not flamed out. I did a little bit of playing around with this. Basically what I did is just fly inverted for a while and the engines will shut off. If you can then flip back they will come back on. It seems to be pretty consistent. The likely cause is what DarkFire described with the fuel cache tank being drained when upside-down. Now if it's a stable inverted stall and the plane is falling like a rock then there's no way to recover even though the engine could function if inverted back.

 

One thing I would like to confirm though is that this is a feature (in other words a design flaw in the Su-27 itself) rather than a bug in the modelling.

[pr1malr8ge]

The thing is that the engines themselves are actually not flamed out. I did a little bit of playing around with this. Basically what I did is just fly inverted for a while and the engines will shut off. If you can then flip back they will come back on. It seems to be pretty consistent. The likely cause is what DarkFire described with the fuel cache tank being drained when upside-down. Now if it's a stable inverted stall and the plane is falling like a rock then there's no way to recover even though the engine could function if inverted back.

 

One thing I would like to confirm though is that this is a feature (in other words a design flaw in the Su-27 itself) rather than a bug in the modelling.

 

 

Umm, Ok let me just make sure you under stand something here.. Flame out means exactly what it says. No flame in the hot section of the engine. This can be the result of fuel starvation, a compressor stall, FOD damage, oxygen deficiency, and a few other reasons.

Regardless of the fact the engine is producing NO THRUST and spooling down to a speed where only impact/ram air through the inlet duct is keeping it spinning.

So yes they are flamed out.. In your account where you flip back over the engines start up again is a result of a few things. A. the engine has enough residual heat still in the hot section and the compressor is still turning fast enough that when fuel is re-introduced it will self ignite.

or B. the engine has enough speed still and with a combinition of fuel and the igniters the engine can be restarted.

 

Other wise to restart the apu bleed air or a steep dive to windmill is needed to spin the compressor then fuel + ignigters are needed for a restart.

[DarkFire]

As far as I know if you get into a stable, non-oscillating inverted stall and manage to flame out the engines, then that's it. You can't recover.

 

As long as the engines are running idle, recovery is possible.

 

Yep, under these conditions the aircraft doesn't have sufficient airspeed to provide control authority. If you have an oscillating stall usually the engines won't flame out which can give you enough forward airspeed to rock the aircraft out of the stall. If you have enough altitude to play with. The lowest altitude I've seen anyone recover from was a smidgen under 3,000m though personally I've never been able to recover from anything under ~5,000m.

[Stuge]

Now if it's a stable inverted stall and the plane is falling like a rock then there's no way to recover even though the engine could function if inverted back.

 

Yes there is :)

 

Here's my recovery technique:

 

1) Throttle idle, to prevent flameout and also idle thrust gives enough control authority to recover.

 

2) Assume direct control (hit the S key!)

 

3) Fire away all missiles. Press alt-W to activate launch override. (Jettison won't work while

inverted)

 

3) Trim neutral (Ctrl-T)

 

4) Start rocking the nose. Stick fully aft to induce nose movement towards the sky, then release stick to neutral to induce nose movement towards the ground. Repeat in sync to increase oscillation.

 

5) Once the nose is pointed sufficiently downwards (perhaps 45 degrees or more) and you move the stick from neutral to full aft, the plane will recover.

 

This technique requires minimum altitude of about 3000m.

Edited September 30, 2015 by Stuge

[Eristoff]

DarkFire,

 

I also needed this info, half a year later. Your reply was very useful, thanks a lot!

[DarkFire]

DarkFire,

 

I also needed this info, half a year later. Your reply was very useful, thanks a lot!

 

Glad it helped, and welcome to the forum :)