Difference in fuel density between main and slipper fuel tanks

[Lau]

While making a fuel and payload calculator, I noticed a difference in fuel density used by ED between the internal and external fuel tanks. Any reason for this, perhaps a variation in octane that could provide and extra boost when switching tanks ?

Also, both drop tanks types carry the exact amount of fuel, with one being just heavier, what’s the point, better air penetration ?

ED Lbs per Gal int		6,4
ED Lbs per Gal Ext		6
		GAL	LBS
Top Tank		48	308
Bottom Tank		37	237
Total internal		85	545
			Lbs
45 Gal Torpedo drop Tank empty weight			49
45 Gal of Fuel			270
45 Gal Slipper drop Tank empty weight			35
45 Gal of Fuel			270

 

Best,

Lau

[Dawson]

I hadn't noticed this before but it looks DCS is mistakenly using US gallons which reduces fuel load by 20%. It should be 85 imperial gallons (102 US gallons).

[Skewgear]

Standard weight of avgas is 6lbs per US gallon.

For imperial gallons that figure is 7.2lbs per gallon.

https://aviation.govt.nz/assets/publications/products/fuel-conversion-factors-sticker-avgas.pdf

I don't have access to my rig at rhe moment but if someone could please check what DCS shows the full internal fuel weight as and post a screenshot, that would be helpful.

I don't know where Lau's weights come from. AP1565J Vol 1, the Spitfire IX descriptive handbook, gives the correct empty and full weights plus moments to add or subtract from the CG. It also gives the 45 gallon torpedo tank as a 50 gallon item, interestingly. See below, and for blister tank read slipper tank.

You can see by some elementary sums that the RAF weight for petrol is 6.2lbs/gallon.

[saburo_cz]

In DCS Spitfire.

Internal fuel weight is 247kgs, it is 2,905kgs per gallon.

If i add external fuel tank, no matter which one, fuel weight is 370 kgs, then it is 2,85 kgs per gallon.

Yes, when i add external fuel, weight per gallon goes down. It shoud not...

Edited March 23, 2024 by saburo_cz

[Skewgear]

247kg is 544lbs. This equates to a fuel weight of 6.4lbs/gal. Which is neither the RAF WW2 100/130 octane petrol weight nor the modern avgas weight.

I will raise this.

[Lau]

@Skewgear

Thanks for taking the time to have a look at this, by the way the figures I used are from the ME. I have created a topic on the bugs & problems, please flag it as reported when possible.

Link to the problem report

Your findings on the cylindrical external tank capacity are interesting, not sure how the Spit will cope with that extra weight since it has a very limited payload for fuel, especially with bombs mounted.

Best,

Lau

[Skewgear]

As belated closure, this was fixed quite a while ago. Now 85 gallons means 85 gallons!

It should also have been fixed for all other DCS warbirds, as the fuel weight/density figure was previously set on a per-module basis but has since been tied to a single global figure.