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PSA regarding the AJS 37's afterburner


renhanxue

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My only real complaint with the Viggen as an airframe is the core engine. I mean, it's awesome because I can happily fly a 200+ nm flight on the deck but sometimes I just go to full MIL and watch the airspeed ever so slowly climb, only to lose it again in the slightest of turns.

 

The lack of power also makes high altitude cruising interesting (aka impossible above 8-10 km with a clean airframe in my experience)


Edited by Pocket Sized

DCS modules are built up to a spec, not down to a schedule.

 

In order to utilize a system to your advantage, you must know how it works.

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Yeah, the MAX SLÄCKT (military) power is bugged for sure. Well, either drag or thrust.

 

I wasn't saying it's bugged, it's just abysmally low compared to other A/C. It's totally what I was expecting when I saw the afterburner combustion chamber is like 4x the size of the core engine. (And the core engine is a turbofan, which means on its own it will produce less thrust than a turbojet of the same size)

 

The manual mentions that 0.9M is optimal cruise speed at 10km, but I guess that's hypothetical as there is no way you're reaching 0.9M in dry thrust, much less at 10km!

DCS modules are built up to a spec, not down to a schedule.

 

In order to utilize a system to your advantage, you must know how it works.

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Equilibrium airspeed with a clean aircraft is around Mach 0.92 or something like that at 10km. Any external loadout at all (including the drop tank) and you can't maintain level flight at 10km on dry thrust at any airspeed, and forget about climbing there dry. However, with low drag loadouts (basically, anything that doesn't involve the anti-ship missiles, bombs or rocket pods - unsure about the bk 90) you should be able to maintain Mach 0.9 or at the very least 0.85 at altitudes between, say, 4 and 8 km, ish.

 

Best distance economy is achieved with Mach 0.9 at around 7-7.5 km with most loadouts except the draggiest ones (again, rocket pods, bombs, anti-ship missiles). With 16 bombs you need to go down to around 6 km. If you're ferrying a clean aircraft with full fuel though, the manual is correct and 10km is the best cruise altitude.


Edited by renhanxue
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I have to retract my previous statement, after doing some tests. I felt it was way underpowered at MS but after a few quick and dirty tests against the AJ SFI it's definitely ballpark.

 

There are still some high-altitude inconsistencies though --- I should be able to get to 16 km on MAX ZON 2 with a clean aircraft -275 kg, but my afterburner stops working at like 12-13 km.


Edited by Corrigan

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I don't think it should be no but comparing to the acceptable values listed in "Motorvärden vid kontrollflygning" the engine pressure ratio and RPM is below the charts(literally, it's not even included) at higher altitudes when flying in automatic.

 

Using manual puts the values in the acceptable area, I'm leaning towards there being some sort of fuel issue in the engine modeling.

 

Provided I'm reading the charts right.

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Is the Viggen supposed to accelerate stupidly fast at high altitude? Like, I'm pretty sure it beats every other aircraft in the game at 12km accelerating from 0.9 to 2.0.

 

Also, the climb rate in zone 3 doesn't fall off as much as I'd expect at high altitudes, but that could just be a misunderstanding on my part.

DCS modules are built up to a spec, not down to a schedule.

 

In order to utilize a system to your advantage, you must know how it works.

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Not really? There's only one high altitude acceleration diagram in the SFI and it's with a mixed air-to-air loadout (drop tank, two gun pods, one rb 05 and one rb 24), and with that it's supposed to take about four and a half minutes to go from Mach 0.9 to Mach 1.5 at max zone 3.

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  • 2 weeks later...

So I've been thinking and I believe I figured out why the afterburner seems to over-perform so dramatically at high altitude.

 

First off: the core engine fuel control is bugged. It should be aiming for a fixed EGT (or EPR?), leading to a very slight decrease in RPM with altitude. Currently, the fuel control is decreasing fuel flow too much as altitude increases leading to a large drop in RPM, EPR, and thrust.

 

I think the afterburner has the exact opposite issue. Fuel flow isn't being regulated with altitude anywhere near enough. If you climb to 10-12 km (manual or auto fuel control) the afterburner will accelerate you unbelievably fast. Way faster than it would at sea level.

 

I'm not sure how the afterburner fuel flow is regulated IRL, as the burner is far too hot for a temperature sensor. Perhaps it's based on tailpipe pressure? Or maybe it follows a predetermined profile based on altitude/Mach? I'll see if I can get in touch with somebody who knows.


Edited by Pocket Sized

DCS modules are built up to a spec, not down to a schedule.

 

In order to utilize a system to your advantage, you must know how it works.

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First off: the core engine fuel control is bugged. It should be aiming for a fixed EGT (or EPR?), leading to a very slight decrease in RPM with altitude. Currently, the fuel control is decreasing fuel flow too much as altitude increases leading to a large drop in RPM, EPR, and thrust.

 

Do you base this statement on the bug thread about climbing performance? (https://forums.eagle.ru/showthread.php?t=181927) Or do you have further evidence? Interesting, if so!

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Do you base this statement on the bug thread about climbing performance? (https://forums.eagle.ru/showthread.php?t=181927) Or do you have further evidence? Interesting, if so!

 

Yes. It seems that the actual engine performance is more or less bang on (when manual fuel is used) but the fuel control is throttling the engine back much more than it should.

 

I connected the dots and realized the afterburner likely has a similar fuel regulation issue. The acceleration and thrust is accurate at low altitude but at high altitude the afterburner goes into hyperdrive. 0.9M climb performance actually INCREASES with altitude due to less drag, almost as if the thrust isn't falling off at all.


Edited by Pocket Sized

DCS modules are built up to a spec, not down to a schedule.

 

In order to utilize a system to your advantage, you must know how it works.

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  • 11 months later...

Anyone knows why the afterburner stops working at high altitude (>12-14 km or so)? It doesn't restart until you land and restart your engine.

 

Is that how it is IRL or some kind of issue with the module?

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Anyone knows why the afterburner stops working at high altitude (>12-14 km or so)? It doesn't restart until you land and restart your engine.

Is that how it is IRL or some kind of issue with the module?

 

A question I was asking myself since months, but was to lazy to post for some reason ... thanks frosen for asking this question.

 

I experience burner going off somewhere between 13 - 14 kms depending on ISA conditions, and in speed regimes from approx M0.8 to 1.85.

 

Also, I am still not able to fly faster than M1.85 no matter what I do.

But this issue is handled in another thread which can be found here: https://forums.eagle.ru/showthread.php?t=194415

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  • 3 months later...

Why did I miss this thread... very cool discussion.

 

I have a PDF here about the Jaktviggen and looks like the RM8B produces around 203kN (45,740 lbf) at Mach 1.1. This is ridiculous! Just like the FF, 115,265 lb/h.

 

The Viggen intake system works really well at transonic speeds, especially at .9 and 1.1. Very impressive how the airframe is able to do that. Ram recovery is certainly very well used here.

 

About the F110 Fuel flow, at sea level, standard day conditions, the F110-GE-400 (based on -100) producing a thrust of 26,950 (static thrust), it has an SFC of 2.104 lbs/lbf/h. Which gives me a fuel flow of 56,702 lb/h.

 

The RM8A at Mach 1.1, sea level, shouldn't it make around 160 kN instead?

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Why did I miss this thread... very cool discussion.

 

I have a PDF here about the Jaktviggen and looks like the RM8B produces around 203kN (45,740 lbf) at Mach 1.1. This is ridiculous! Just like the FF, 115,265 lb/h.

That sounds way too high. There's really not much difference between RM8A and RM8B when it comes to thrust, and what difference there is is mostly at dry thrust. Both of them make around 165 kN at Mach 1.1 at sea level at max zone 3. There's a thrust diagram for the RM8A in the OP, and for the RM8B, see SFI JA 37 del 4, page 79.

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Please check the PDF out in the attachments.

 

Take a look at Page 19, table 4, Mach 1.1.

Oh, that one. It's an assessment from first principles by someone with no access to any actual aerodynamic data. I dunno why you'd trust that over the actual declassified flight manuals for the real aircraft.

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