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


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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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Posted (edited)

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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