Stilton

Correct me if I'm wrong, but the B763 manual I got
gives these optimum cruise # for 420.000 lbs

FL290
M.802
FF/ENG 6990

vs B787 (also 420.000 lbs optimum cruise)

FL390
M.848
FF/ENG 5346

Thats a huge difference, a 23% save in aerodynamic & engine efficiency alone!!

XPM

On a NM/1000# Fuel, it is better than that due to the higher TAS associated with 0.046 increase in Mach number.

GF

Why is the 763 optimal cruise at such a low level? I would have expected
FL330 or FL350

Greetings,
Between 290 up to 330 you have the highest TAS for the same Mach :ok:

XPM,


I'm not sure how you obtained cruise fuel flow numbers for the 767-300 at 420,000 pounds considering the highest maximum take off weight available is 412,000 !




Further more your cruise data numbers are 10000 feet apart !



Heavily favouring the 787 in an unrealistic comparison. The 767 is a very good high altitude Aircraft and unless stuck down at FL290 by ATC would normally cruise at FL350 or higher depending on weight and other factors.



11000 pounds an hour total is a much better average fuel flow figure for the 763, in comparison, if your numbers are correct the 787's not that great..

We can compare any another weight if you like..:rolleyes:

http://www.xplanefreeware.net/morten...b767cruise.jpg

Everything I have seen from Boeing indicates that the 787-800 will have an OEW of at least 20,000 lb higher than the 767-300ER.

I suggest looking at identical weights at the same cruise altitude to make your comparison more relevant.

Or even .... Identical payloads.....:8

Stilton,

Thats pointless! Modern acf are designed to cruise at optimum altitudes to stay in the low drag bucket of the airfoil.
Thats why the 787 is equipped with cruise flap so it can cruise more efficient slightly above or below it's optimum altitude.

SR,

True, unfortunately the B787 OEW is still unknown and Boeing is
probably still shaving off it's weight. The FCOM
indicates minimum landing weights around 220.000 lbs. So as mentioned
by Zeke, a figure around 20.000 lbs heavier than the B763
is probably in the ballpark.

In that case it will STILL be WAY superior to the B763.

A same payload example.

B763, weight 320.000 lbs
FL 350 (optimum level)
FF 5286
M.802

B787, weight 340.000 lbs (added 20.000 OEW)
FL 410 (optimum level)
FF 4570
M.855

In fact, you can add another 80.000 lbs to 420.000 lbs to the 787,
it would STILL be more efficient than a 320.000 lbs B763.

And thats not counting the speed advantage OR need to plan less trip fuel
advantage which will give a weight reduction

The higher the grossweight, the bigger the advantage of the 787 vs 76X
will be.

XPM

Not pointless at all XpM But you don't seem to get the point. :ugh:



If your numbers just posted are remotely accurate, however then maybe it will do what they say on the 'brochure'

Perhaps a more valid comparison would be a 767-XXX to a 787 over the same route with the same payload.

Then would you please enlighten me what a comparison
same weight AT same altitude would tell you with
regard to acf efficiency?

At the same weight/payload the 787 would cruise MUCH higher than the 763.
So at whatever altitude you would pick in between their optimum levels
BOTH acf would be inefficient ==> Pointless <==

XPM

Naturally the 787 will cruise higher than the 767 and faster for that matter.



However when you compare it's cruise efficiency at 390 while only looking at FL290 numbers for the 76 you are simply not being realistic.

Why do you care how much the aircraft weighs, look at the PAYLOAD CAPABILITIES, start with 150 pax, then 200 pax.

I don't care how much the aircraft weighs nor what altitude it flies at, but I do care can it carry my required payload and at what cost.

Unfortunately we cancelled our 787 order, so I wont get to play with this toy.

Mutt

I hate to bring the news to you Stilton, but this is very much reality.
In a few months time 787's will pass 5-10.000 feet above your 767
on the same sector, getting there before you at a much lower cost with
more satisfied passengers and more payload. Better get used to the idea..;)

LR Cruise (optimum)

Wgt B787 B763

300k FL430 FL370
340k FL410 FL340
380k FL400 FL320
420k FL390 FL290
460k FL370 N/A
500k FL350 N/A

I relinquish all hope of getting my point across, Xpm.



Happy landings..

It's the 'side by side' flight that would show the difference in fuel burn. Compare NYC to Europe. JFK-CDG is is approx 3100 n.m.

767-300 EOW 205,000
787 EOW 255,000 (target was 108K kg/238,000 lbs, reports that it's 8% overweight)

TOC for a 767-300 is approx. 90 nm to FL300. Estimate a 787 would take 130 n.m. to TOC. TOD to destination is approx. 130 n..m. Assume a 787 fuel burn to TOC is 8,000 lbs.

Assume a 60 kt tailwind. Total 787 cruise distance would be 2850 n.m.

Now add 200 pax (40,000 lbs) plus cargo (20,000) = ZFW of 315,000 and figure out the cruise fuel burn of a 787 at it's optimum altitude and speed. A 767-300 would burn approx. 75,000 lbs with 1+50 arrival fuel(alt+rsv).

Anyone with the 787 cruise data that can figure this out?

787 cruise altitude
 

Reading about the cruising altitude of the 7 late 7 & understanding that the cabin altitude is going to kept at 6000 feet, what is the max dif of the going to be?

Most aircraft have a max dif of 8psi but presumably the Dreamliner is going to be much more.

tristar 500

Looks like a press diff of 9.18 (searching the internet).

Found this site that has lots of info and found the answer to my question. Apparently a 787-8 carrying 200 pax would burn about 60,000 lbs on a 3,000 nm flight, or approx. 15,000 lbs less than a 767-300 (20%). At current fuel prices that's about $4800 less fuel.

Boeing 787 Dreamliner : Analysis