|Quoting Alien (Reply 216):|
The laws of physics is still in effect. I would suggest you go over to Civ av. There is a thread all about airlines flying with less gas. It makes the take off weigh lower and therefore burns less fuel. The extra cost of carrying around all that extra weight on the KC-30 is only going to cost even more as the price of fuel goes up.
|Quoting Astuteman (Reply 227):|
So as I said - physics does indeed matter. But you need to be considering the right bit of physics. Hence my comment.
Alien, you do seem to have missed a major thread with some excellent contributions, basically about physics, I suppose. It was in tech/ops and can be found at:
There you will find that the laws of physics taken overall, suggest that your boundary conditions were a bit too limited to permit you to obtain valid conclusions.
Yes, for a given aerodynamic configuration lower weight will give you lower fuel consumption. However if extra weight is used to make a more efficient machine, it can well be that the heavier object has a lower fuel consumption - for a given payload that is.
In the particular thread, the difference between the performance of the "heavy" A380 is compared with that of the "light" B747-8 and a number of contributors conclude that the additional weight of the A380 is largely compensated in terms of reduced drag, some from the aerodynamics of the wing but mostly from the extra span. And at least one contributor points out that Airbus might have had an even more efficient wing had is not been restricted to the 80m box/
For example, the posts from that thread include:
OldAeroGuy From United States, joined Dec 2004, 2310 posts, RR
Reply 32, posted Thu Mar 1 2007 (1 year 2 months 6 days 19 hours ago) and read 4111 times:
"Quoting Zeke (Reply 21):
ZEKE: You have basically explained the difference between the 748i and A380, the 380 has a better wing design, allowing it to fly at the same speed with less sweep, thus having less span-wise flow.
OAG: I've only done a partial explanation of the difference in wing designs between the 748I and the A388. The A380 does have a 6.6% advantage in span loading compared to the 748I, but it also has a 19% lower wing loading since its wing is sized to accommodate a stretch to the A389. The higher CD0 caused by this increased wing size reduces the advantage of having a better span loading. Having additional wing area for growth isn't a bad idea, that's what Boeing did on the initial versions of the 747, 767, 777 and 787. Airbus could have reduced the impact of the higher wing area by increasing the span (ie higher aspect ratio) but they were limited by the 80m box. "
and concluding the post from OAG:
"Here is my attempt at a summary:
A388 span loading improves Di/L relative to the 748I.
A388 wing size degrades D0/L relative to the 748I.
The resulting airframe D/L differences, when combined with the TSFC and OEW differences result in virtually the same fuel burn per unit payload, with the 748I being slightly better at missions less than 7000nm and the A388 being slightly better at missions greater than 7000nm. (This point being made by WingedMigrator in Reply 27).
It should explain to SCAT15F in Reply 22 why Boeing didn't want to spend the money for a new 748I wing."