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Afterburner inhibition on final. Why is that even thing?


DmitriKozlowsky
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Why is afterburner and engine power so inhibited by something in latest Mig-21bis version. On final, my speed bled off, and I attempted a goaround. But for some reason even after gear is retracted, AB would no light, and engine seemed to loose power, and KIAS bled off . I found myself in impossible situation. Full mil power was insufficient to complete approach. Could not light afterburner with gear down and Nav mode to LANDING, flaps LANDING. I initiated GOAROUND retracted gear, and still burner would not light, and mil power was insufficient. I could not lower nose, to gain speed, as I had no altitude to trade. I thought my approach was OK until burner failed.

Personal thought. This feature is kind of BS! to me. It makes aircraft unsafe on final .  I mimicked, inadvertently the infamous F-100 crash on final. Pilot error , on my part, was there, but no way to correct it. Aircraft was properly configured for landing. Fuel below 700 liters. AG ordinance expended, ext tank empty, 2 X R-60 on rails, aircraft speed about 350, gear down, flaps full down, until I realized I was sinking while simultaneously noting lack of engine power and no burner light. So I initiated Go Around. Gear UP, engine to full AB. But got no burner light. Raising nose to get a bit more time in air. Did not work, so it was time to leave the office.

Track attached.

Mig_21_bis_failedGOAROUND.trk

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It depends on flaps settings. And it's a real thing. 

 

Also, if you do a search, you will find already a few topics about it. 

 

 

 

And now that you know what happens, you can adjust your approach to a more safe and secure approach together with a safer procedure. 

 

 


Edited by razo+r
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OK, I read previous post. Thank you. But what a what??!!!! None of this helps. Trained approach , with fuel below 700 liters, no AG, and just 2 X R-60, is 400 KIAS while keeping runway threshold right at the edge of visible nose, flaps full down, gear down. 10 km final with start AGL at 500 meters. Hold 400 KIAS until after inner marker, begin to raise noise and adjust throttle to have 300 KIAS at threshold, and T/D with 10-12 AOA with 280-250 KIAS. Pop drouge.

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Allow me to elaborate.

Since MiG-21PFM variant, they did away with fowler flaps and used what's called blown flaps, or flaps with "boundary layer control" - SPS. Aircraft takes pressure from your engine's compressor and blows highly pressurised air over the top of your flaps to keep the air energised (and thus increasing lift, by some 900 kg). (Cutting throttle obviously results in no more air energisation, and you lose your lift.)

Using SPS in turn decreases pressure in the engnie, so if you try lighting the afterburner - it won't. But the nozzle scheduler will still open the nozzle. This is why there's a dramatic decrease in pressure and thrust. In fact, if this happened to you at higher speeds, somewhere in stratosphere, your engine might as well implode from the pressure drop (it happened to a fair bit of our MiG-19's), it's just how jet engines work.

In my opinion however, the 21 is still one of the easiest to land jets in DCS. I fail to understand how you ran out of energy on approach...


Edited by Koty
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I think you mean Km/H, not KIAS. I would never, ever, lower full flaps so far away as 10Km away. It's obvious the aircraft doesn't hold itself with that kind of drag while on final. I don't know why anybody would do that. All training missions and manual say you don't lower full flaps until final, that means really close to the runway. Depending on the situation sometimes I wouldn't lower full flaps until really short final. Also, if you aren't coming with the right AoA and glide path you're probably incoming too low and in the reverse command region, which is just fatal in this aircraft. If you don't know what that is look for it and read about it, it's the region (in a power available/drag chart) in which you need more power to fly slower… that may pop up something in your head.

 

S!


Edited by Ala13_ManOWar

"I went into the British Army believing that if you want peace you must prepare for war. I believe now that if you prepare for war, you get war."

-- Major-General Frederick B. Maurice

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That's correct if you come in 4º glide slope as per PRMG. Any other situation, and I know what happens with the average virtual aviator 🙄, you shouldn't lower full flaps that far away (meaning 10 or 10+ Km away) as the OP said. 4-5 is nice, and pretty close to the runway as I said, it's perfect indeed, but only if you came all the way also in almost perfect approach, but in the virtual World that's not always the case 😅, hence I can't recommend anything else than what I said, despite the manual, to the bulk of the pilots trying to learn the beast. Real numbers are only for super geeky virtual pilots matching every of those numbers, the rest needs a compromise 😁.

S!

"I went into the British Army believing that if you want peace you must prepare for war. I believe now that if you prepare for war, you get war."

-- Major-General Frederick B. Maurice

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DCS PRMG is too darn steep. Real PRMG is adjustable but 2°40' (two and two thirds degree) is more common.

MiG-21 approach is a sort of constant deceleration process without really stabilizing too long at any particular speed. With a light fuel load and stores expended or jettisoned you're looking at 360 km/h medium final slowing to 330 km/h close final at 80-90% N1.

Anyway, SPS is blowing engine bleed air through tiny holes in a hollow shaft at the root of the trailing edge flaps. I always thought afterburning gets the bleed air temps too high or something. But the explanation of bleed air pressure loss causing AB failure to ignite makes more sense. These older airplanes run closed loop logic running the engine without actually checking its state. The nozzle is scheduling like AB is running but it ain't which is not optimum thrust. What's interesting if you switch SPS off at the cockpit switch and select AB with the lever it will inhibit AB in a way that doesn't schedule as if the AB is gunna AB so you don't have the same loss of thrust.

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  • 4 weeks later...

I find if you fly most of the approach using take off flaps, and only then go over to landing flaps at maybe the last 100 metres, and cross the threshold at about 320kmh, you are probably going to grease without running out of power. Any faster and you are going to bounce, any slower and you are starting to get too much high angle of attack.

 I agree the glideslope with the mig21 seems far too steep. Did I read somewhere the devs used their own glideslope code instead of the DCS one?

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MiG-21 devs used so much stuff they came out with instead of ED stuff, not just glide slope. On their benefit we have to remember it was the very first third party module and probably SDK tools weren't available the same as now they are for sure. Also, since it was modelled out of a "third party air force" (Croatian IIRC) either, not Russian air force, probably some procedures and information from them wasn't very Russian standard or even real stuff either (no matter the model is aged, it's or was their front line fighter, why disclose every detail) so maybe they actually did or do use that kind of glide slope in their AF for the MiG-21 or they just told so to the devs even if it's not real. We now know so many things still undisclosed not so many years ago.

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"I went into the British Army believing that if you want peace you must prepare for war. I believe now that if you prepare for war, you get war."

-- Major-General Frederick B. Maurice

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