Fuel Consumption on the B-17G

[Serenity]

(Yay! I get to make a thread that actually looks intelligent!)

Alright, now, in real life, as you increase altitude, you make your mixture leaner, so as to maintain the fuel:air ratio. This means that you would burn less fuel per hour, correct?

Well, being the geek I am, I wanted to find the best cruise altitude for the B-17G. To do this, I went into LW Blue, and flew one of my Strategic Bombing Raids. I opened my E6B, and recorded my fuel burn rate at the deck, 5k, 10k, 12k, 15k, 18k, 22k, and 26k. Here is where my question comes in. Is mixture modelled? And at what intervals? Because I got the following burn rates:

Deck: 1120 GPH
5,000: 1120 GPH
10,000: 1120 GPH
12,000: 1120 GPH
15,000: 1120 GPH
18,000: 1120 GPH
22,000: 1120 GPH
26,000: 1120 GPH

Now, granted, I only took it to 26,000, but im planning a follow up, taking it to 35,600. However, I would think that in 26,000 feet the mixture would become at least a LITTLE leaner. Am I misunderstanding the way mixture works? Or is this really some kind of bug?

[frank3]

Well, at higher alts, you will fly faster at the same throttle-settings, so relatively your fuelburn is less

[Serenity]

Originally posted by frank3
Well, at higher alts, you will fly faster at the same throttle-settings, so relatively your fuelburn is less

Er... my question though was, at the same throttle setting, wouldnt the punds of fuel burned per hour be reduced though?

[Brooke]

It seems to depend on aircraft.  Turbocharged aircraft in AH seem to hold the same fuel consumption up to a critical alt, then it starts to fall off.  The criticial alt for the P-47 is about 30k -- probably the same for the B-17 and B-24, which are at least over 25k in my testing offline.

For supercharged aircraft (mechanical, staged superchargers), they can have fuel consumption stay the same -- or even go up, as in the case of, say, the Spit V -- as they get to a critical alt, then it drops off; and the critical alt seems to be typically much lower than 30k (like 20k or 25k).  It depends on the staging of the supercharger, which is mechanically tied to the RPM of the engine.

[hitech]

Serenity: For turbo charged engine, you are consuming a constant amount of air until critical alt. I.E. your manifold pressure is staying constant hence your fuel remains constant. I havn't looked at the specific numbers for the b17 but my guess would be fuel consumption might raises slightly with altitude do to lower air temperature and hence more power and air at altitude. But it is also possible that as the Turbo RPM increases so does the air temp in the manifold, and hence fuel consumption would remain constant.

Now do not confuse the more power with more brake HP, because some of the excess power is used by the turbo charger.

[Serenity]

Thank you verry much for your prompt and helpful reply sir!

[bj229r]

AR234's actually use FAR less fuel at higher alts...slightly different engine tho:rolleyes:

[Anyone]

Originally posted by bj229r
AR234's actually use FAR less fuel at higher alts...slightly different engine tho:rolleyes:

always laugh at people whining about the ar234 range..... theres no problem with range at 19k