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Afterburner hasn't work if flaps extended because of automatic of SPS system.


Shmal

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SPS system (BLC - boundary layer control, boundary layer blowing) works with these conditions only: 

1. Throttle lever moved between SPS and Maximal marks on the quadrant

2. Flaps are in extended position between 30-45° (25° is take off position)

3. SPS is turned on

 

Additionally automatic sent electric signal to make impossible to light the afterburner! 

 

That is mean the afterburner could not be working while flaps are extended in the landing position!

 

If the pilot while approaching, with the flaps extended in landing position with throttle lever between SPS and Maximal marks, SPS has to be working. But if the SPS is in the On position on the left cockpit wall.

If the pilot decided to move the throttle lever forward, to the afterburner position it won't light. But, the engine will bleed of energy because of changing the nozzle position. That is why it is unrecommended to do so in the flight manual!

 

 

Here is the page conceraning this topic from the Russian manual of MiG-21bis:

МиГ-21 автоматика включения СПС.jpg


Edited by Shmal
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According to the schematics, the afterburner should only be disabled when the SPS is active, which it will not be when the power lever is set to afterburner. Seems like this is only a protection should the afterburner be on despite the power lever position commanding it not to be. Probably a protection should the power lever be missaligned.

 

As on take off you are supposed to use the afterburner and have the flaps extended to 30°, it would not make any sense to disable the afterburner purely regarding to flaps position.

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I have added some additional information in my main post, to clarify the subject!


Edited by Shmal

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Now I stand corrected that the take off flap position is indeed 25° for all MiG-21 versions with the SPS and it does indeed seem to cut out the afterburner when in landing configuration. I wonder why the flight manual does not mention any of this. Could get really nasty in case of a go around.

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I don´t agree, SPS range is 50 - 100 %, SPS AB protection switch is working within this range, flaps more then 30 degrees means out of 25 degree range of flap MIKROswitch, T/O pos. (not a microswitch, then bigger as micro), next notch AB, more than 100%, SPS cuts of, AB protection circuit is not energized, AB is working. What about these pressure switches, any idea ?

 

I need to ask some friends of me, but i can remember some schematics, last time i worked on a 21 was 1992, we had to scrap them all !

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According to technical manual for MiG-21, afterburner should not be functioning with activated SPS switches, and flaps more than 30° down.

Also when tested on the ground in AB regime, when SPS is switched on, assuming flaps are more than 30° down, AB will disengage.

6.I.2-SPS.PNG

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On 1/8/2021 at 9:02 PM, Beta Sokol said:

I don´t agree, SPS range is 50 - 100 %, SPS AB protection switch is working within this range, flaps more then 30 degrees means out of 25 degree range of flap MIKROswitch, T/O pos. (not a microswitch, then bigger as micro), next notch AB, more than 100%, SPS cuts of, AB protection circuit is not energized, AB is working. What about these pressure switches, any idea ?

 

I need to ask some friends of me, but i can remember some schematics, last time i worked on a 21 was 1992, we had to scrap them all !

 

Thats exactly how I understand that diagram, however the text, which I cannot read, seems to say otherwise.

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I dont have manual for R-25 engine, but R-13-300 and R-11F2SK-300 manuals used in older 21s indicate the same thing. When SPS installation is ON (operating, so SPS switch engaged ON and flaps down to landing position), afterburner cannot be engaged regardless of throttle position. 

 

I will forward this bug report.


Edited by Hiromachi
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  • 4 months later...
On 1/12/2021 at 9:58 PM, Hiromachi said:

I dont have manual for R-25 engine, but R-13-300 and R-11F2SK-300 manuals used in older 21s indicate the same thing. When SPS installation is ON (operating, so SPS switch engaged ON and flaps down to landing position), afterburner cannot be engaged regardless of throttle position. 

 

I will forward this bug report.

 

But if the AB is cut-off with full-flaps, why does the RPM go to max 88% now? shouldn't it be 98%? Go-arounds with only 88% RPM seems dangerous? Yeah I know rise your flaps to TO but what if they are stuck?

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It is going to mil thrust. The gauges show it going to mil thrust. It is delivering its mil thrust rated power. If you're seeing 88% you're either using the ctrl+Y telemetry bar, not the gauges in the aircraft, which are the best reference for what the aircraft is actually doing... or if you are reading that off the actual RPM gauge it's because you tripped the AB detent and the nozzle opened without the AB lighting off.

 

If your flaps are stuck fully extended, you either land normally with the boundary layer control system, or if that's INOP you modify your approach. Simple. The 21 at or below mil thrust will be able to land at full flap even with the boundary layer control system disabled, you'll just have to land faster and have less margin for error. I don't know how your flaps would get stuck in the first place, though, as they automatically retract as speed builds and so aren't sensitive to overspeeding like certain other aircraft (in fact there is no limiting speed for flaps listed anywhere in the POH).

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I was using cockpit RPM gauge. Did a bunch of landings & found out the best thing is to turn of AB, so it doesn't get activated (don't have AB detent on my throttle).

 

It's a questionable design not being able to use AB in case of go-around.


Edited by CoBlue

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OK I tried it and the nozzle is scheduling as if the afterburner will run even though it is inhibited. This saps lots of power from the engine and you get a thrust much less than MIL. The RPM decreases to about 89% indicated (80% on the info bar). This is in contrast to just flipping the AB disable switch which prevents nozzle scheduling.

 

Pretty sure this is an oversight. Flaps >30° should be like flipping the AB disable switch so at maximum forward lever you at least get MIL power with normal non-AB nozzle scheduling.

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On 5/31/2021 at 5:03 AM, Frederf said:

OK I tried it and the nozzle is scheduling as if the afterburner will run even though it is inhibited. This saps lots of power from the engine and you get a thrust much less than MIL. The RPM decreases to about 89% indicated (80% on the info bar). This is in contrast to just flipping the AB disable switch which prevents nozzle scheduling.

 

Pretty sure this is an oversight. Flaps >30° should be like flipping the AB disable switch so at maximum forward lever you at least get MIL power with normal non-AB nozzle scheduling.

No oversight, That is correct operation. Manual even says to disable AB braker, if you see nozzle opened indicator with BLC active. Of course you should not be in AB regime with BLC active in a first place.

On 5/31/2021 at 3:35 AM, Frederf said:

What speed are flaps automatically raised by airflow despite full down selection?

I belive its 380-400km/h, manual says BLC will disengage at 360-380km/h, that would indicate to me that flaps will retract close above that speed.  


Edited by Golo
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Not a very good safety design then. If were designing the system the same feature which disabled AB operation would do so by wiring into the same circuit as the AB disable switch. This way a panicked handle forward would result in maximum dry thrust instead of degraded thrust.

 

I wouldn't assume just because manual says proper procedure is to disable AB switch that not setting that switch would result in AB-like nozzle scheduling with all the problems that has. It could be for redundancy or to prevent sudden AB engagement when flaps retract.

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Worth note that the flap auto-retraction happens in stages at a certain speed, but as currently modelled, the flaps are blown back up proportional to airspeed - so that also affects the margin of error a bit.

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15 hours ago, Frederf said:

This way a panicked handle forward would result in maximum dry thrust instead of degraded thrust.

Well its only problem in sim, IRL throttle handle has mill stop detent and you can only get it in AB deliberately. Also that AB cutoff CB is only intendet to be used if there is nozzle malfunction to open position, not when you push throttle past mill stop, that is just bad piloting.

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