Question for Viggen experts

So what is the purpose of the automatic fuel regulator on the Viggen?

It seems to seriously reduce military thrust, especially at altitude. In fact, I can't maintain flight above ~7,500 M without turning it off (without afterburner of course).

So what purpose does it serve since it seems to effectively "de-tune" the engine? Does it improve fuel economy? Help with engine preservation (reducing time between rebuilds)? Improve stability during throttle transients?

I haven't seen anything in the manual explaining its purpose and I don't understand why Saab would intentionally reduce engine performance during normal operations. I really prefer flying with it in manual. Is there a benefit to leaving it on?

Thank you for the insight!

-Nick

PS - It reminds me of the E60 BMW M5 where you have to select in iDrive if you want 400 bhp or 500 bhp....of course I want 500! :D

It's just an ECU. That it behaves like it does in the sim at the moment is probably just a bug, but I could imagine it limiting thrust in certain conditions to prevent excessive EGT and such things. Will look up the details in the SFI later.

IRL we never touched that switch. Emergency use only.

Very good question, I can't contribute aything but I've also been wondering if it's really normal/realistic that the Viggen can't stay at high altitude without using the AB. I realize that it was designed primarily for low level flight, but still, it just seems a bit strange that it can't keep up with an airliner without using AB.

Anyone know how realistic this is?

IRL we never touched that switch. Emergency use only.

I feel like there would be...many...forms to fill out if you did.

I feel like there would be...many...forms to fill out if you did.

One could always flip it back upon landing and state "what I don't remember never happened!"

One could always flip it back upon landing and state "what I don't remember never happened!"

Difficult when a signed form before take-off shows parts are OK, and after your flight there is extra wearing or something like that ;)

Difficult when a signed form before take-off shows parts are OK, and after your flight there is extra wearing or something like that ;)

"Where are the fan blades?"

"Unnecessary to my needs, got rid of them."

"Where are the fan blades?"

"Unnecessary to my needs, got rid of them."

:lol:

SFI AJS 37 part 1, chapter 1, section 9, subsection 2.2 says about the fuel regulation system:

2.2. The gas generator's fuel and control system

 

The fuel flow to the gas generator's combustion chamber is regulated by a hydromechanical control system. The pilot inputs a desired fraction of the available thrust by the means of the throttle. The control system ensures that the gas generator starts delivering the requested thrust fraction as quickly as possible, without stalling, flaming out or overheating. The control system also ensures that the gas generator delivers the requested thrust fraction regardless of changes in speed, altitude, air pressure or air temperature. The relationship between throttle position and delivered thrust fraction is linear up to military power.

There's a detailed description on the working principles of the fuel regulator in the SFI but it's like three pages long and I'm not sure if anyone's interested enough to motivate a full translation.

IRL we never touched that switch. Emergency use only.

Hey Hilmerby. Just going by the words you used, were you a Viggen pilot or ground crew?

So it could explain an increase in thrust in certain situations but not in mil power since then probably max thrust without ab would be used in either case.

SFI AJS 37 part 1, chapter 1, section 9, subsection 2.2 says about the fuel regulation system:

That's all it's supposed to do. Disabling automatic fuel control makes the connection between throttle and fuel feed completely mechanical and linear without any interference.

 

There's a detailed description on the working principles of the fuel regulator in the SFI but it's like three pages long and I'm not sure if anyone's interested enough to motivate a full translation.

Thank you renhanxue, that answers the question for me. :)

The difference in DCS seems bigger than what I would expect from the description you posted. Also, when Hummingbird was posting in-game performance test results RagnarDa asked him to ensure that all tests were conducted with the fuel regulator set to manual. Maybe the current implementation gives more accurate performance with the system in manual?

Very good question, I can't contribute aything but I've also been wondering if it's really normal/realistic that the Viggen can't stay at high altitude without using the AB. I realize that it was designed primarily for low level flight, but still, it just seems a bit strange that it can't keep up with an airliner without using AB.

Well the Viggen can comfortably cruise up above 10,000 M in military power and climb in mil with the fuel regulator set to manual. In manual it is much closer to what I expected. In automatic, the max pressure ratio I can attain above 8,000 M is 1.7ish with max engine RPM in the mid 80s%. If I select manual mode the pressure ratio climbs to 2.2 and RPMs climb to about 95%.

Down low the difference is minimal, but above ~6000 M it makes a big difference.

Now I'm even more curious as to why. :)

-Nick

Thank you renhanxue, that answers the question for me. :)

 

The difference in DCS seems bigger than what I would expect from the description you posted. Also, when Hummingbird was posting in-game performance test results RagnarDa asked him to ensure that all tests were conducted with the fuel regulator set to manual. Maybe the current implementation gives more accurate performance with the system in manual?

 

 

 

Well the Viggen can comfortably cruise up above 10,000 M in military power and climb in mil with the fuel regulator set to manual. In manual it is much closer to what I expected. In automatic, the max pressure ratio I can attain above 8,000 M is 1.7ish with max engine RPM in the mid 80s%. If I select manual mode the pressure ratio climbs to 2.2 and RPMs climb to about 95%.

 

Down low the difference is minimal, but above ~6000 M it makes a big difference.

 

Now I'm even more curious as to why. :)

 

-Nick

Haven't had the chance to test it myself yet so - really interesting, thanks! Definitely doesn't seem right the way it's implemented now, but WIP I guess...

...

Well the Viggen can comfortably cruise up above 10,000 M in military power and climb in mil with the fuel regulator set to manual. In manual it is much closer to what I expected. In automatic, the max pressure ratio I can attain above 8,000 M is 1.7ish with max engine RPM in the mid 80s%. If I select manual mode the pressure ratio climbs to 2.2 and RPMs climb to about 95%.

 

Down low the difference is minimal, but above ~6000 M it makes a big difference.

 

Now I'm even more curious as to why. :)

 

-Nick

Great observation :thumbup: (unfortunatelly forum doesn't allow me to add you reputation). I've also noticed that it's not possible to maintain a level flight with Viggen on high altitudes without using AB but I didn't notice that the engine output is reduced in max MP throttle position.

I'll do some more testing today evening but I would assume that this behavior isn't fully correct as manual states the economic speeds up to 10km. I guess if such altitude would be too high for Viggen it wouldn't be listed in the economic speeds table.

Anyway, switching the fuel regulator into manual mode is only mentioned in the emergency procedures section of the flight manual.

I did a short test at 10000 m

- fuel regulator AUTO: RPM 80%, EPR 1,6

- fuel regulator MANUAL: RPM ~92%, EPR, 2,4

As mentioned, in AUTO mode it's not possible to maintain a level flight at that altitude, nor actually get to it without using AB. Really guessing right now but maybe the switch works in reverse way and modes are replaced.

Several times this happened to me:

- Throttle working as usual.

- Engine stall happened because I am an idiot.

- Emergency engine restart procedure - success, RPM back to military idle.

- Engine cannot reach more than 70% RPM, even with throttle at maximum.

- Swap to manual mode.

- Engine works as it should be (possition of throttle represent correct RPM again, AB again possible).

This behaviour is kinda wierd to me. Can someone explain why after engine stall and restart it works so badly in automatic mode?

I'm wondering if this problem isn't related to the issues VEAO is having with the P-40. Maybe the core environmental model isn't passing out the correct air temp/density to the external flight models. It may explain both issues.

AOG