[BUG] Engine performance

[MYSE1234]

I've been working on a mission planning tool, and have therefore been gathering data about fuel consumption for Viggen at different speeds and configurations. In the data I could see some strange things happening.

 

One of those things were fuel consumption dropping with altitude, as expected. However when you get to 5km alt. it starts going back up again, to almost the same consumption as on sea level. Which it shouldn't be doing.

Another thing is the mach number at mil power. The mach number is the same until you get to 5000m, where it starts increasing smoothly, very close to the real values. Only 5000m offset.

At sea level with 92% NHT, you should have a mach number of around 0,67. In DCS it's 0,82.

 

The real data is from "SFI AJ37, del 3. Kap IV", page 11 and 16.

As with all data it's not going to be 100% accurate, so my measured values might not be 100% accurate, but quite close.

 

Feel free to have a look yourself.

Edited September 22, 2021 by IronMike

[TOViper]

Erm ... if you could write all labels in English please? :tomato:

I recently noticed the high fuel consumption in high altitude compared to earlier stages of the module as well, but wasn't enough to write things down.

Edited June 20, 2020 by TOViper

[MYSE1234]

Erm ... if you could write all labels in English please? :tomato:

I recently noticed the high fuel consumption in high altitude compared to earlier stages of the module as well, but wasn't enough to write things down.

 

I could do that, but that would require about the same amount of effort as using google translate on the few words that are needed to understand. :music_whistling:

It wasn't meant for other people to see, and I just can't be bothered translating it. The few who need to see it know what it all says, the rest is a bonus.

Edited June 20, 2020 by MYSE1234

[Brown Floyd]

It wasn't meant for other people to see, and I just can't be bothered translating it.

 

Edit: Thanks, man!

Edited June 23, 2020 by Brown Floyd

[MYSE1234]

Brown Floyd said:

Edit: Thanks, man!

 

I thought the graphs would be pretty self explanatory, and would be fine as they are. I guess I was wrong.. 

 

Edit: Alright, it should now all be translated to english so that every one can understand it.

Edited September 22, 2021 by MYSE1234

[TOViper]

I thought the graphs would be pretty self explanatory, and would be fine as they are. I guess I was wrong.. :noexpression:

Edit: Alright, it should now all be translated to english so that every one can understand it.

 

 

Cool, thank you!

If wonder if you could translate the whole SFI as well ... I mean ... its not that long ... is it? :P

 

 

The obvoius deviation M0.12 at 0m using 92% of RPM of the HT is amazing/stunning to see ... while the "bump" at 4000m is unexplainable to me at the moment.

Edited June 23, 2020 by TOViper

[MYSE1234]

I did some more calculations and the current fuel flow at sea level is equivelent to ISA +35, which is 50°c.

 

Cool, thank you!

If wonder if you could translate the whole SFI as well ... I mean ... its not that long ... is it? :P

 

 

The obvoius deviation M0.12 at 0m using 92% of RPM of the HT is amazing/stunning to see ... while the "bump" at 4000m is unexplainable to me at the moment.

 

The bump is there because I needed to use zone 1 on the A/B to keep 92%. It shouldn't be needed, but that's what it's like.

[RagnarDa]

Thanks for the very detailed report! I've addressed the biggest issues noted here, hopefully it won't break something else...

[MYSE1234]

Thanks for the very detailed report! I've addressed the biggest issues noted here, hopefully it won't break something else...

 

Love to hear it, thank you very much. :thumbup:

Might I ask exactly what's been changed?

[Airhunter]

Thanks for the very detailed report! I've addressed the biggest issues noted here, hopefully it won't break something else...

 

You're the man Ragnar! Seriously appreciate your constant work on our beloved Viggen and listening to feedback and reports.:thumbup:

[Rover]

Latest update:"Tweaked engine behavior at medium altitude.".... thanks for being so specific Heatblur :D

[QuiGon]

Latest update:"Tweaked engine behavior at medium altitude.".... thanks for being so specific Heatblur :D

Indeed.... :music_whistling:

 

So what has changed? Speed? Acceleration? Fuel consumption?

[MYSE1234]

The spreadsheet is updated.

 

And the answer to what has changed is, not much. The only notable change is that you don't need afterburner to keep 92% at 4000m. The fuelflow at 92% does seem to keep to the correct "curve" (albeit a bit too high, 0,14%/min), but that goes wrong at 5000m, as with the other things.

Edited July 16, 2020 by MYSE1234

[IronMike]

On 7/16/2020 at 3:23 PM, MYSE1234 said:

The spreadsheet is updated.

 

And the answer to what has changed is, not much. The only notable change is that you don't need afterburner to keep 92% at 4000m. The fuelflow at 92% does seem to keep to the correct "curve" (albeit a bit too high, 0,14%/min), but that goes wrong at 5000m, as with the other things.

 

 

How up to date is your chart? Thanks again for putting it together. It doesnt look fixed to me and imo needs a lot more attention. Thread redesignated as [BUG].

Thanks again for this.

[MYSE1234]

4 hours ago, IronMike said:

 

How up to date is your chart? Thanks again for putting it together. It doesnt look fixed to me and imo needs a lot more attention. Thread redesignated as [BUG].

Thanks again for this.

Haven't updated it since last year when it was "fixed", but I am using all of the data in a fuel planner and that is still very accurate. So I'm certain it's still bugged, and the charts should still be accurate.

[IronMike]

2 hours ago, MYSE1234 said:

Haven't updated it since last year when it was "fixed", but I am using all of the data in a fuel planner and that is still very accurate. So I'm certain it's still bugged, and the charts should still be accurate.

 

Thank you!