Fuel flow rate wrong

[Boagord]

@crazydunc  and I have developed an aircraft Drag Index and weight calculation system independently.

A .exe and an XLS respectively.

 Then we have collaborated on the data extraction about the fuel flow, as noted in Appendix B (-229 engines) of the F-15E-Flight-Manual (15-April-1993)

Currently, there is a significant discrepancy between the document (which applies well to the engines 229) and what is observed in DCS .

Is this issue known? If not, we can provide specific figures or exemples.

Edited February 5, 2024 by Boagord

[draconus]

19 hours ago, Boagord said:

Currently, there is a significant discrepancy between the document (which applies well to the engines 229) and what is observed in DCS .

...we can provide specific figures or exemples.

You should.

[Boagord]

So, here is the @crazyduncwork : 
https://github.com/crazydunc/F-15E-Strike-Eagle-Performance-Calculator/releases/tag/1.0.0.9

In the attached file there is my XLS. 

My xls is less precise than his .exe because I am far less experimented,  I haven't yet integrated the pylons, and some conditional DIs like those of the 610 gal fuel tank but perhaps more transparent in understanding how it operates.

The source of the work is a file F-15E-Flight-Manual-15-April-1993.pdf that can be found on Google (I'm not sure if I'm allowed to share it here). The fuel flow tables were taken from page 704 and the subsequent pages of this PDF.

The DIs table was made from pages 625 and the following ones.

As an exemple this loadout give a weight of ~80 000 lbs and a DI of 100:

Spoiler

*****************************************
**          F-15E Strike Eagle         **
**         loadout exported from       **
**          F15E Perf Calculator       **
*****************************************
 
30/01/2024 03:00:39
 
Basic Airframe Weight (Aircraft + 2 x CFTs + Crew): 39416 Lbs
Basic Airframe Drag Index:                          21.3
Payload Weight:                                     8034 Lbs
Payload Drag Index:                                 79.1
Total Drag Index:                                   100.4
Internal Fuel:                                      21585  Lbs
External Fuel:                                      11895  Lbs
Lateral Imbalance:                                  -101  Lbs
Total Fuel:                                         33480 Lbs
Gross Weight:                                       80930 Lbs
*******************************
Internal Gun Ammo: 289 Lbs
*******************************
STA2A
Store: AIM-120C-5 with a LAU-128/A
Category: AirToAirMissiles
Quantity: 1
Drag Index: 2.8
Weight: 467 lbs
*******************************
STA2
Store: 610 Gal Fuel Tank
Category: FuelTanks
Quantity: 1
Drag Index: 8.2
Weight: 320 lbs
Fuel: 3965 lbs
*******************************
STA2B
Store: AIM-9M with a LAU-114/A
Category: AirToAirMissiles
Quantity: 1
Drag Index: 3.3
Weight: 274 lbs
*******************************
LCFT
Store: GBU-10C/B
Category: GuidedWeapons
Quantity: 1
Drag Index: 10.7
Weight: 2081 lbs
*******************************
LTP
Store: LANTIRN TGP (AN/AAQ-14)
Category: Pods
Quantity: 1
Drag Index: 7.4
Weight: 621 lbs
*******************************
STA5
Store: 610 Gal Fuel Tank
Category: FuelTanks
Quantity: 1
Drag Index: 12.2
Weight: 320 lbs
Fuel: 3965 lbs
*******************************
LNP
Store: LANTIRN NVP (AN/AAQ-13)
Category: Pods
Quantity: 1
Drag Index: 9.5
Weight: 520 lbs
*******************************
RCFT
Store: GBU-10C/B
Category: GuidedWeapons
Quantity: 1
Drag Index: 10.7
Weight: 2081 lbs
*******************************
STA8B
Store: AIM-9M with a LAU-114/A
Category: AirToAirMissiles
Quantity: 1
Drag Index: 3.3
Weight: 274 lbs
*******************************
STA8
Store: 610 Gal Fuel Tank
Category: FuelTanks
Quantity: 1
Drag Index: 8.2
Weight: 320 lbs
Fuel: 3965 lbs
*******************************
STA8A
Store: AIM-120C-5 with a LAU-128/A
Category: AirToAirMissiles
Quantity: 1
Drag Index: 2.8
Weight: 467 lbs
... (NaN Ko restants)

Since the gross weight is 80,000, pages 722-723 should be used, and on this page, the column corresponding to a DI of 100.

This give : 

Do you observe the same flow rates? Is the documentary method correct?

I hope my explanations are clear.

fuel_consumption.xlsx

Edited February 7, 2024 by Boagord

[draconus]

Do you ignore temperature effect? I know this alone doesn't explain such big differences.

It can also be DI calculations but also not that much.

Did you test some simple case like clean aircraft w/CFTs?

[Boagord]

Temperature is calculated for 15° at SL, and standard as indicated in the PDF.  I don't remember my setting in the ME.

DI calculation is inside the spoiler, but As you said there is a huge difference.

I did not yet test with a clean aircraft, I lack of time.

If you want to try

[crazydunc]

9 minutes ago, draconus said:

Do you ignore temperature effect? I know this alone doesn't explain such big differences.

It can also be DI calculations but also not that much.

Did you test some simple case like clean aircraft w/CFTs?

Morning! 

For Testing the Fuel flow comparison, I included the temperature effect using a standard rate of change.  For SEPC, I don't include the temperature model as it's such a small difference, and there is already a 20% margin in the figured vs the DCS fuel burn. 

I have spot-checked from DI 30 to DI 120 and found there to be a 12-25% difference throughout. The DI100 figures @Boagord have used above was just the example i had to hand when we were discussing this. 

 

Thanks

Duncan 

[Boagord]

After some tests, the fuel flow  of a clean aircraft is spot on, but it seems that the drag caused by the stores is a tiny fraction of what it should be:

 

In the documentation, the relationship between the DI (Drag Index) and the fuel flow appears to be of the form ax + B

For example, for 50,000 lbs, 520 KTAS, and 10,000 ft, the relationship appears to be close to: 10518 + DI x 73.875

A clean F15-E has a 21,3 DI and the FF is very close to the documented one but the coefficient of the variable linking the FF and the DI appears to be close to 74 in the documentation and 13 in the measurements taken in DCS.

null

Although I cannot validate all of this as I don't have enough time, I am certain there is an issue somewhere because in no configuration can I achieve a fuel flow requirement exceeding 15,000 lbs/hour for 60,000 lbs, 520 KTAS, and 10,000 ft, whereas I am supposed to have this result between 60 and 80 on the DI.

Edited February 8, 2024 by Boagord