Vertical AoA causes engine shut off?

[nullsys]

Hey all,

I was just wondering if anyone knew the exact reason the engines cut off when going over certain vertical climbs (especially with the afterburners on) ?

 

Nothing worse than being in a fight and suddenly having to do an emergency restart.

Thanks!

[Kuky]

Negative G's

[Buzzles]

Negative G's

 

Or zero G's, which the '21 likes even less.

[DaveRindner]

I can try. At and above critical AOA, the aircraft is near stall, and air flow around lift surfaces, and ESPECIALLY INTAKE(s) is interrupted. The disturbed air ingested into engine compressor, does not flow around axial blades, and the volume of air is insufficient. The engine suffocates. In humans we call it hyperventilating. Man and engine are trying to breath, but there is too little air (oxygen at atmospheric concentration) ingested, for breathing. The low pressure compressor blades stall, and engine flames out. Oh, yeah, the man grabs a brown bag and breathes normally, reestablishing nice flow into his lungs. The engine has not shutdown cold, it just cannot burn the fuel mist, and generate hot gas for exhaust. For American designers inlets that use a spike to control volume of air for given conditions, the stall condition of interrupted air intake, is called UNSTART. In RL, crew can anticipate an unstart about 5-10 seconds into the future. First there is rumble at inlet, followed by a loud bang, followed by engine flameout with compressor still spinning due to windmilling. SR-71 was infamous for unstarts, and dealing with them compromised a large chunk of SR-71 aircrew training. Luckily for both -21 and -71 unstarted engine can be restarted in flight.

From high-med. high altitude, place throttle at MIN, point nose down 10-15 to keep AC IAS at 600 kph or more. Keep wings level. Switch APU to ON, switch EMERGENCY AIRSTART to ON (UP) position. The engine should relight after 5-10 sec. If its not happening press (and perhaps hold) START button. Assuming you have fuel, and no damage to fuel system, the engine will relight. Once you go below 1000 AGL with no relight, time to contemplate ejection or dead stick onto runway or road, if possible. If engine relights, place throttle as needed to maintain level flight, level off, and don't forget to turn APU OFF.

In MIG-21 it is an emergency, but really only dangerous at low altitude.

Aircraft's system automatically moves inlet spike, based on a schedule of IAS, air pressure, altitude. Obviously the system can be damaged or malfunction. In DCS: MIG-21, if you stall the aircraft, expect a flame out. If aircraft goes into a spin, usually following stall, expect a flameout.

 

Some 4th generation fighters, F-15, F-14,SU-27, MIG-29, and 4++ gen. F/A-18E/F/G deal with inlet and compressor unstarts by angling the intake leading edge. So those aircraft can fight at high AOA, and not stall the compressor. At high speeds angled intakes use bypass doors or equivalent to prevent inlet stalls and unstarts. USAF's fighter/bomber F-100 Super Sabre was notorious for inlet stalls, especcially on final. Which is worst time. You are low, slow, flaps down, gear down&locked, so AC is draggy, and has flight characteristic of a washing machine.

The ATR-72 tragedy in Taiwan yesterday appears to be a stall following engine failure. Plus, on CNN and BBC, there is speculation that crew shut down their remaining good engine by mistake. ATR stalled on port wing, did uncommanded roll to port, and into the river they went. May God bless and keep the souls of those who perished!

 

For more information: Google or YT SR-71, UNSTART, Col. Richard Graham, compressor stall.

Edited February 7, 2015 by DaveRindner

[Einherjer]

Vertical AoA causes engine shut off?

 

Thats right so far, but hyperventilating is when you breath too much - you loose to much co2 and your muscles in the lips and fingers begins to spasm. By breathing into a bag you re-inhale you're co2 and the pH number comes back to normal. The oxygen saturation is at the time near the normal value.

The engine stall here is more a kind of breathlessness or dyspnea

End of medical discussions ... We don't want to treat a crying teenager here - back to topic

[effte]

Going by the OP, high alpha isn't the issue as much as - as previously pointed out by Kuky and Buzzles - unloading (low/negative G). The fuel system in the MiG can't cope, so the engine flames out from fuel starvation. Just roll inverted and pull positive Gs. Tends to be less of an issue in real life, as you'd probably do that anyway just to save yourself the discomfort.

 

DaveRindner,

very nice explanation buty you're mixing compressor stalls and unstarts in the same description. It gets kind of muddy to follow.

 

A compressor stall is when we ask too much from the compressor, making the blades stall. It can also be caused by disturbed airflow into the compressor face from e g high alpha or beta conditions. The stalled compressor allows the high pressure behind the compressor to go forward and out the inlet, with a loud bang and often flames. While it can cause a flameout, it typically doesn't. I can tell you from first-hand experience that it is something which will get your attention - and quickly. There you are, all cosy up in cruise, catching up on the paper work and - BANG BANG BANG BANG! :D

 

An unstart is when we're trying to shove too much air down the inlet, more than the compressor can ingest. This causes the shockwave system inside the inlet, set up to deliver subsonic flow to the compressor face, to become all messed up. The normal shock (a shock perpendicular to the airflow, with supersonic flow on the upstream side and subsonic flow on the downstream side) moves forward and possibly even out of the inlet where what we really want is an oblique shockwave off the tip of the spike, with supersonic flow both up- and downstream of the shockwave.

 

One can lead to another so you can have both at the same time, but they are different phenomena.

 

I'd drop the analogy with breathing, and introducing stalled wings along with compressor stalls has been found to cause immense confusion. :)

 

Cheers,

/Fred

[DaveRindner]

Thank you. I though they are a variation of the same problem. Which is why modern fighters no longer use nose intakes. I went through Graham's SR-71 book, which explain UNSTARTs in detail, and my understanding of UNSTART and compressor stalls is that its a variation of same condition , stall, which is a perturbed air flow around airfoil or intake.

But thank you. It makes sense. So then the spike in SR-71 and in MIG-21 have similar functions. Limit the air into compressor. I thought that spike slows the airflow to subsonic, to allow combustion at supersonic speeds.

[Luzifer]

Thank you. I though they are a variation of the same problem. Which is why modern fighters no longer use nose intakes.

 

Probably more because a nose intake limits what kind of radar you can stuff into it.