Sponsor Message:
Aviation Technical / Operations Forum
My Starred Topics | Profile | New Topic | Forum Index | Help | Search 
Is Mach 0.85 Becoming The Industry Standard?  
User currently offlineA380900 From France, joined Dec 2003, 1116 posts, RR: 1
Posted (1 year 4 months 4 weeks 2 hours ago) and read 11083 times:

757 and 767: Mach 0.8
A330 and A340: Mach 0.82
777: Mach 0.84
747, A380, A350, 787: Mach 0.85

All new airliners seem to be shooting for 0.85. Is there a technical reason for that or is it just a consensus that appeared at this level. Can we expect to see the A320 and 737 replacements to go that fast?

By the way, I made a round trip across the Atlantic not long ago on a 757 and it seemed definitely slower than a 747.

48 replies: All unread, showing first 25:
 
User currently offlineatct From United States of America, joined Mar 2001, 2313 posts, RR: 38
Reply 1, posted (1 year 4 months 4 weeks 1 hour ago) and read 11047 times:

Just because the airliner can go that fast doesn't mean it will. I frequently see 757/767's at .76 etc.


"The way to get started is to quit talking and begin doing." - Walt Disney
User currently offlineA380900 From France, joined Dec 2003, 1116 posts, RR: 1
Reply 2, posted (1 year 4 months 3 weeks 6 days 19 hours ago) and read 10900 times:

Fair enough. They do seem to optimize the planes for one cruise speed and every single last airplane produced seems to be Mach 0.85.

User currently offline26point2 From United States of America, joined Mar 2010, 837 posts, RR: 0
Reply 3, posted (1 year 4 months 3 weeks 6 days 19 hours ago) and read 10901 times:

Most carriers, at least in the US, have been operating at slower speeds the past few years in order to save fuel/cost. We fly the Global Express at M0.85 all day long and I have never seen an airliner match our speed except perhaps 747. 737, 757, A320 are considerably slower. Quite often when 100 miles from destination ATC slows us down to M0.78 for sequencing to follow Southwest (737) or some such thing.

User currently offlinewilco737 From Greenland, joined Jun 2004, 9077 posts, RR: 76
Reply 4, posted (1 year 4 months 3 weeks 6 days 19 hours ago) and read 10901 times:
AIRLINERS.NET CREW
HEAD MODERATOR

An airplane has a design speed where the lift to drag ratio is the best. For the 744 it is M0.855. But due to the high fuel costs we do not cruise that fast anymore. Usually we are at M0.84 or if not heavy even slower. Of course we can still go faster, but we try to avoid it due to the fuel costs.

Problem is still in high demand traffic areas like the NAT OTS system. Many many airplanes on the same route with different speeds. So the 767 and 340s fly slower than M0.80 and we are in a hurry at M0.85... So you don't get your level or your speed...

It would be great to see if all airplanes cruise at the same speed.

wilco737
  



It it's not Boeing, I am not going.
User currently offlinebikerthai From United States of America, joined Apr 2010, 2155 posts, RR: 4
Reply 5, posted (1 year 4 months 3 weeks 6 days 13 hours ago) and read 10736 times:

Technically, doesn't the wing shape effects the efficient speed at which you cruise?
So, if the 737 is being revised to the MAX with a new wing, will they be able to increase that cruise speed?

bt



Intelligent seeks knowledge. Enlightened seeks wisdom.
User currently offlinehivue From United States of America, joined Feb 2013, 1097 posts, RR: 0
Reply 6, posted (1 year 4 months 3 weeks 6 days 7 hours ago) and read 10563 times:

Quoting A380900 (Thread starter):
All new airliners seem to be shooting for 0.85.

The 747 was 0.855 at EIS back in about 1970, right?

Quoting A380900 (Thread starter):
757 and 767: Mach 0.8
A330 and A340: Mach 0.82
777: Mach 0.84
747, A380, A350, 787: Mach 0.85

I think the whole thing is about fuel.The 757/767 were 0.80 for fuel savings. They've been slowly working their way back up to the 747 number as aircraft/engines have become more fuel efficient.


User currently offlineonebadlt123 From United States of America, joined Oct 2004, 51 posts, RR: 0
Reply 7, posted (1 year 4 months 3 weeks 5 days 6 hours ago) and read 10111 times:

At my carrier we plan the following speeds. Note, these are default speeds and we can adjust faster or slower as needed . If there is no operational need to go faster we will leave the flight at default or even slow it down .01. Most of the time in the domestic market, or below FL 300 we will use LRC speeds


A319 - M.78
A320 - M.78

B737-500 - M.76
B737-700 - M.77
B738 - M.78
B739 - M.78
B739ER- M.78/79

B747-400 - M.84

757-200 - M.78
757-300 - M.79

767-200 - M.80
767-300 -M.78
767-400 - M.78

777-200 - M.83

787-8 - M.84

[Edited 2013-06-01 18:22:09]

User currently offline1337Delta764 From United States of America, joined Oct 2005, 6586 posts, RR: 2
Reply 8, posted (1 year 4 months 3 weeks 5 days 2 hours ago) and read 10033 times:

Quoting hivue (Reply 6):
I think the whole thing is about fuel.The 757/767 were 0.80 for fuel savings. They've been slowly working their way back up to the 747 number as aircraft/engines have become more fuel efficient.
Quoting A380900 (Thread starter):
757 and 767: Mach 0.8
A330 and A340: Mach 0.82
777: Mach 0.84
747, A380, A350, 787: Mach 0.85

Don't forget the A300, the world's slowest widebody with a design cruise speed of mach 0.78.

[Edited 2013-06-01 22:43:02]


The Pink Delta 767-400ER - The most beautiful aircraft in the sky
User currently offlineMax Q From United States of America, joined May 2001, 4669 posts, RR: 19
Reply 9, posted (1 year 4 months 3 weeks 5 days 1 hour ago) and read 10017 times:

Quoting Reply 7):
At my carrier we plan the following speeds. Note, these are default speeds and we can adjust faster or slower as needed . If there is no operational need to go faster we will leave the flight at default or even slow it down .01. Most of the time in the domestic market, or below FL 300 we will use LRC speeds


A319 - M.78
A320 - M.78

B737-500 - M.76
B737-700 - M.77
B738 - M.78
B739 - M.78
B739ER- M.78/79

B747-400 - M.84

757-200 - M.78
757-300 - M.79

767-200 - M.80
767-300 -M.78
767-400 - M.78

777-200 - M.83

787-8 - M.84

Don't know where you get these numbers from. I work for the same Airline and there are no fixed mach numbers.



Econ is used for planning with a variable cost index depending on the priorities of that particular flight, even if fixed mach numbers were used yours are incorrect (too slow)



The best contribution to safety is a competent Pilot.
User currently offlineKELPkid From United States of America, joined Nov 2005, 6411 posts, RR: 3
Reply 10, posted (1 year 4 months 3 weeks 4 days 10 hours ago) and read 9782 times:

As I understand it, designing an aircraft for high subsonic cruise is almost as difficult a feat as designing it for supersonic cruise...and M0.85 or thereabouts is about the limits for "conventional" subsonic jet design. That said, the 747 has a really high Mmo compared to the others, M0.90 IIRC...a product of a different time   Boeing thought the 2707 had a better future as a passenger aircraft..  


Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineViscount724 From Switzerland, joined Oct 2006, 25659 posts, RR: 22
Reply 11, posted (1 year 4 months 3 weeks 4 days 8 hours ago) and read 9723 times:

Quoting 1337Delta764 (Reply 8):
Don't forget the A300, the world's slowest widebody with a design cruise speed of mach 0.78.

Makes little difference on typical routes that were operated by the A300. If you find old schedules involving routes operated by the A300 and other types, I doubt the published block times are noticeably different.


User currently offlineMax Q From United States of America, joined May 2001, 4669 posts, RR: 19
Reply 12, posted (1 year 4 months 3 weeks 4 days 3 hours ago) and read 9600 times:

Quoting KELPkid (Reply 10):
That said, the 747 has a really high Mmo compared to the others, M0.90

B747 MMO is ,92



The best contribution to safety is a competent Pilot.
User currently offlinebikerthai From United States of America, joined Apr 2010, 2155 posts, RR: 4
Reply 13, posted (1 year 4 months 3 weeks 3 days 18 hours ago) and read 9476 times:

Here's a short homework assignment if anyone is willing to do it.

Please chart the comparison between cruise speed and wing sweep of the different airplane models.
While you are at it, please identified which model have a super-critical wing.

bt



Intelligent seeks knowledge. Enlightened seeks wisdom.
User currently offlineFlighty From United States of America, joined Apr 2007, 8641 posts, RR: 2
Reply 14, posted (1 year 4 months 3 weeks 3 days 15 hours ago) and read 9390 times:

Quoting Reply 7):
787-8 - M.84

It will be interesting to know if the 787 has a "sweet spot" cruise that fast.

Just wanted to add, speed can be important if it permits certain routings or schedules that another aircraft can't do. This isn't only about fuel burn. It can also be about pilot hours (expensive), daily revenue and even aircraft hours. An aircraft moving 5% faster will have 5% fewer hours on it. If speed allows one extra leg per day for a WN 737... wow. You can do the same network with fewer aircraft and pilots.


User currently offlineKlaus From Germany, joined Jul 2001, 21488 posts, RR: 53
Reply 15, posted (1 year 4 months 3 weeks 3 days 10 hours ago) and read 9294 times:

Isn't a relevant part of this the advances in aerodynamics (such as supercritical airfoils) which have reduced the cost penalty of these increments in near-sonic speeds?

If the fuel consumption penalty in going from (for instance) 0.8 to 0.84 mach should be significantly lower now than it was with the older airfoils (and corresponding engines), the revenue opportunity of faster flights would at some point be worth it even with the higher fuel prices nowadays.

If I understand this correctly, the older airfoils should have suffered more from transonic drag than the newer ones do. So with the older designs you have to pay more to get to a higher speed than you do with newer designs.

Other aerodynamic improvements due to newer simulation capabilities should have contributed to this as well (re-shaping fuselage, fairings, stabilizers).


User currently onlineBMI727 From United States of America, joined Feb 2009, 15781 posts, RR: 27
Reply 16, posted (1 year 4 months 3 weeks 3 days 2 hours ago) and read 9150 times:

Quoting Klaus (Reply 15):
Isn't a relevant part of this the advances in aerodynamics (such as supercritical airfoils) which have reduced the cost penalty of these increments in near-sonic speeds?

In practical terms, not so much. In practice, the supercritical airfoil is more commonly used to get the same cruise speeds with less sweep which makes the wing less tricky from a structural perspective. Older 747s had 37.5 degrees of sweep, while the 747-400 and -8 make do with 35. The A380 uses 33.5 while the G650 uses 36 degrees but flies faster.

You can use supercritical airfoils to fly that much faster for the same amount of drag, but in practice the speed is usually not worth the cost. Look at the Citation XLS+: it has a straight wing with a supercritical cross section and can do M 0.75 with a ceiling of FL 450 and fly off of a 3,560 ft. runway.



Why do Aerospace Engineering students have to turn things in on time?
User currently offlineKELPkid From United States of America, joined Nov 2005, 6411 posts, RR: 3
Reply 17, posted (1 year 4 months 3 weeks 3 days 2 hours ago) and read 9165 times:

Quoting Klaus (Reply 15):
Isn't a relevant part of this the advances in aerodynamics (such as supercritical airfoils) which have reduced the cost penalty of these increments in near-sonic speeds?

The big trick in moving around near the speed of sound is in controlling shock waves...airfoils and lifting surfaces, by their very nature, accellerate the air moving on the low pressure side of the airfoil. Once that air goes supersonic, a shock wave forms, and the rules governing subsonic flows are out the door in that localized area. This is why Mach 0.92 is tricky (besides the extra drag of moving faster...). And it all has to work well when you're at climb and approach speeds, too  



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineKlaus From Germany, joined Jul 2001, 21488 posts, RR: 53
Reply 18, posted (1 year 4 months 3 weeks 2 days 21 hours ago) and read 9096 times:

Quoting BMI727 (Reply 16):
You can use supercritical airfoils to fly that much faster for the same amount of drag, but in practice the speed is usually not worth the cost.

It still looks like a plausible contributing reason why the design speeds have been going up again even despite rising fuel prices.

Does anybody have the relative fuel consumption numbers at hand for the different kinds of airfoils at different but comparable airspeeds, preferably with comparable engines?


User currently offlinefrancoflier From France, joined Oct 2001, 3790 posts, RR: 11
Reply 19, posted (1 year 4 months 3 weeks 2 days ago) and read 8828 times:

Quoting Max Q (Reply 12):
Quoting KELPkid (Reply 10):
That said, the 747 has a really high Mmo compared to the others, M0.90

B747 MMO is ,92

You're both right.

M 0.90 for JAR certification and 0.92 for FAA certification.
Don't know why the difference though...



Looks like I picked the wrong week to quit posting...
User currently offlineRara From Germany, joined Jan 2007, 2114 posts, RR: 2
Reply 20, posted (1 year 4 months 3 weeks 1 day 8 hours ago) and read 8648 times:

Quoting wilco737 (Reply 4):
An airplane has a design speed where the lift to drag ratio is the best. For the 744 it is M0.855. But due to the high fuel costs we do not cruise that fast anymore.

What's the benefit of a better lift-to-drag ratio if the fuel burn is higher?

In other words, if you burn less fuel at mach 0.84, but the lift-to-drag ratio is better at mach 0.855, then what's the practical benefit of lift-to-drag?

Quoting Flighty (Reply 14):
Just wanted to add, speed can be important if it permits certain routings or schedules that another aircraft can't do. This isn't only about fuel burn. It can also be about pilot hours (expensive), daily revenue and even aircraft hours. An aircraft moving 5% faster will have 5% fewer hours on it. If speed allows one extra leg per day for a WN 737... wow. You can do the same network with fewer aircraft and pilots.

Indeed... theoretically that should be reflected in the cost index, which is a function of fuel cost versus operating cost. In practice, most airlines use the same cost index for all flights because they find it too tedious to calculate.



Samson was a biblical tough guy, but his dad Samsonite was even more of a hard case.
User currently offlineLH707330 From United States of America, joined Jun 2012, 810 posts, RR: 0
Reply 21, posted (1 year 4 months 3 weeks 1 day 3 hours ago) and read 8569 times:

Quoting BMI727 (Reply 16):
Older 747s had 37.5 degrees of sweep, while the 747-400 and -8 make do with 35.

They all have the same basic planform, with the -400 having a small extension and winglet, and the -8 having the same extension and a raked tip (along with a changed cross-section and flap).


User currently offlinelebb757 From United States of America, joined Apr 2013, 9 posts, RR: 0
Reply 22, posted (1 year 4 months 3 weeks 16 hours ago) and read 8473 times:

Quoting Flighty (Reply 14):
Just wanted to add, speed can be important if it permits certain routings or schedules that another aircraft can't do. This isn't only about fuel burn. It can also be about pilot hours (expensive), daily revenue and even aircraft hours. An aircraft moving 5% faster will have 5% fewer hours on it. If speed allows one extra leg per day for a WN 737... wow. You can do the same network with fewer aircraft and pilots.

5% faster does not equal exactly 5% percent fewer hours. However, you are right that speed allows increased profitability in the cases you just mentioned.

Quoting Rara (Reply 20):
Indeed... theoretically that should be reflected in the cost index, which is a function of fuel cost versus operating cost. In practice, most airlines use the same cost index for all flights because they find it too tedious to calculate.

What variables does the FMC manage?? I'm pretty sure about fuel cost, maintenance and crew salaries. It would be interesting to know if it also takes into account aircraft utilisation and as Flighty mentioned the possibility of an extra leg.


User currently offlinebikerthai From United States of America, joined Apr 2010, 2155 posts, RR: 4
Reply 23, posted (1 year 4 months 3 weeks 12 hours ago) and read 8408 times:

Quoting Rara (Reply 20):
then what's the practical benefit of lift-to-drag?

From a simplistic stand point, at a given payload, the higher the lift-drag ratio, the less thrust you need to maintain speed, which directly impacts the efficiency of you're engines.

bt



Intelligent seeks knowledge. Enlightened seeks wisdom.
User currently offlineonebadlt123 From United States of America, joined Oct 2004, 51 posts, RR: 0
Reply 24, posted (1 year 4 months 2 weeks 2 days 14 hours ago) and read 7984 times:

Quoting Max Q (Reply 9):
Don't know where you get these numbers from. I work for the same Airline and there are no fixed mach numbers.



Econ is used for planning with a variable cost index depending on the priorities of that particular flight, even if fixed mach numbers were used yours are incorrect (too slow)

I never said these were fixed in stone mach numbers. Now I may not be correct on the Sabre system, but old school wise it generally picked one of the above mentioned mach numbers 9 times out of 10 as it was the most efficient speed it liked to chose based on our preset variables. I cant tell you how many flight plans I run daily and see those numbers along with LRC, but LRC is different

They aren't too slow to be chosen, they are chosen because they are cheaper speeds to run given that time-frame. South side planning does not use CI planning.


User currently offlineRara From Germany, joined Jan 2007, 2114 posts, RR: 2
Reply 25, posted (1 year 4 months 1 week 3 days 8 hours ago) and read 7704 times:

Quoting bikerthai (Reply 23):
From a simplistic stand point, at a given payload, the higher the lift-drag ratio, the less thrust you need to maintain speed, which directly impacts the efficiency of you're engines.

So you're saying that the better the lift-drag ratio is, the more efficient the engines work - meaning you burn less fuel?

Why then will you burn less fuel at mach 0.84 in a B744, if the lift-drag ration is best a 0.855?



Samson was a biblical tough guy, but his dad Samsonite was even more of a hard case.
User currently offlineprebennorholm From Denmark, joined Mar 2000, 6486 posts, RR: 54
Reply 26, posted (1 year 4 months 1 week 3 days 5 hours ago) and read 7664 times:

Quoting Rara (Reply 25):
Why then will you burn less fuel at mach 0.84 in a B744, if the lift-drag ration is best a 0.855?

You are really splitting hairs here. No plane has best L/D ratio at one single Mach number. It varies greatly with actual weight and altitude.

A heavy B744 at 5000 feet altitude will have a best L/D ratio at around M0.40, and it changes all way up to operational ceiling.

If you want to fix best L/D to one single value, then it is best to look at wing angle of attack.

Even at one certain weight and one certain altitude the same plane does not have best L/D at one certain Mach number, but at one certain IAS. Lift changes with speed (IAS), but a Mach number is a variable depending on air temperature. The same IAS at typical cruising altitude can vary Mach 0.02 depending on actual air temperature.

If you are heavy and high enough, then best L/D hits MMO. If you go any higher, then you have to accept a lower L/D ratio.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlinebikerthai From United States of America, joined Apr 2010, 2155 posts, RR: 4
Reply 27, posted (1 year 4 months 1 week 2 days 18 hours ago) and read 7740 times:

Quoting Rara (Reply 25):
Why then will you burn less fuel at mach 0.84 in a B744, if the lift-drag ration is best a 0.855?

When we talk about L/D ratio, most of us are just talking about the wing L/D. There is also parasite drag (includes the fuselage) that increases with increasing speed no matter how good the wing L/D ratio is.

As I stated, my explanation is in simplistic terms. Things get complicated at cruise when you weigh is constantly changing (with fuel burn). You'll have to ask a pilot to get a more thorough answer.

bt



Intelligent seeks knowledge. Enlightened seeks wisdom.
User currently offlinePihero From France, joined Jan 2005, 4597 posts, RR: 77
Reply 28, posted (1 year 4 months 1 week 10 hours ago) and read 7499 times:
Support Airliners.net - become a First Class Member!

Quoting lebb757 (Reply 22):
What variables does the FMC manage?? I'm pretty sure about fuel cost, maintenance and crew salaries. It would be interesting to know if it also takes into account aircraft utilisation

We are talking *Cost Index* here. It's just the ratio of hourly costs /fuelcosts . These include crew salaries, hourly maintenace costs / amortisation or leasing costs... the fixed costs are not taken into account.

Quoting bikerthai (Reply 23):
From a simplistic stand point, at a given payload, the higher the lift-drag ratio, the less thrust you need to maintain speed, which directly impacts the efficiency of you're engines.

The problem is that we don't really fly at that schedule, which is *best endurance*, only used for holding. Our speed schedules are between a Maxi Range and an *Econ* Mach in the vicinity of the Long Range Mach. As a set of ballpark figures,the Maxi Range is obtained at the maximum of Sqrt CL / Cd . By definiotion, >Long Range is at a schedule that gives 1% worse specific range than the MR. ( The Mach is sensibly greater, though )

Quoting prebennorholm (Reply 26):
No plane has best L/D ratio at one single Mach number. It varies greatly with actual weight and altitude.

Plus the fact that a modern airfoil has a very flat polar : there is in reality a range of best L/D speeds.

Quoting bikerthai (Reply 27):
Things get complicated at cruise when you weigh is constantly changing (with fuel burn). You'll have to ask a pilot to get a more thorough answer.

It's not really that complex.
You have first to introduce the notion of *specific range*, basically how far you can fly on a given mass of fuel.
At a given Mach, the *Optimal Altitude* is the one at which you have the best specific range, and we demonstrate that it is a function of the aircraft mass and the static pressure : Weight / Ps is then a constant ---> one can see that with diminishing weights, you must reduce Ps, hence you have to climb in order to keep that ratio constant. We'' ve just explained the *Step Climlb* necessity : the lighter you are, the higher you should go... By a similar reasoning, with diminishing weights, to remain at the same Mach number means you reduce your AoA, therefore going farther from the ideal SqrtCL / Cd angle ---> you have to reduce your Mach number.
That are the reasons why airliners end up higher than their firdst cruising level and at a slower Mach number
All these computations are now continuously performed by the FMS... They used to be done "by hand" using graphs by the flight deck crews.
The best illustration is given by the - now almost forgotten - cruise-climb : We were cleared to a *block altitude from -say FL 310 - to FL 370. Last time I used it was on a 744 from LAX to PPT. With an FMS, 'twas a doodle as we climbed some 100 ft every 5 minutes. Skies are a lot busier nowadays   



Contrail designer
User currently offlinehivue From United States of America, joined Feb 2013, 1097 posts, RR: 0
Reply 29, posted (1 year 4 months 6 days 16 hours ago) and read 7333 times:

Quoting Pihero (Reply 28):
Skies are a lot busier nowadays

How much (if any) effect does this fact have on airlines being able to actually fly the best specific range? Could routes/airways be arranged so that cruise-climb could still be used on a regular basis if it's actually an advantage?


User currently offlinePihero From France, joined Jan 2005, 4597 posts, RR: 77
Reply 30, posted (1 year 4 months 6 days 14 hours ago) and read 7308 times:
Support Airliners.net - become a First Class Member!

Quoting hivue (Reply 29):
Could routes/airways be arranged so that cruise-climb could still be used on a regular basis if it's actually an advantage?

The block altitude clearance now belongs to military traffic - especially tankers refuelling other jets in a hippodrome.
There are still some die-hard pilots and controllers doing it - I heard - in the Oakland oceanic FIR... but that must be an exception.
On the LAX-PPT sector I was referring to in my previous post, the economy was around 5 to 7 hundred kg of fuel.



Contrail designer
User currently offlinebikerthai From United States of America, joined Apr 2010, 2155 posts, RR: 4
Reply 31, posted (1 year 4 months 6 days 14 hours ago) and read 7299 times:

Quoting Pihero (Reply 28):
It's not really that complex.

From a hardware engineer standpoint . . . what you just explained was somewhat complex
 Wow!
bt



Intelligent seeks knowledge. Enlightened seeks wisdom.
User currently offlineSandroZRH From , joined Dec 1969, posts, RR:
Reply 32, posted (1 year 3 months 3 weeks 6 days ago) and read 6721 times:

Quoting wilco737 (Reply 4):

The A340 is optimally flown at M.81, the A330 at M.80-M.81.


User currently offlinewilco737 From Greenland, joined Jun 2004, 9077 posts, RR: 76
Reply 33, posted (1 year 3 months 3 weeks 5 days 22 hours ago) and read 6693 times:
AIRLINERS.NET CREW
HEAD MODERATOR

Quoting SandroZRH (Reply 32):
The A340 is optimally flown at M.81, the A330 at M.80-M.81.

Optimally yes, but many friends of me are 330/ 340 Pilots and they fly slower than M0.80 at times.
I know you fly them as well, but at LH the cost index was reduced a lot that let the cruise speed drop even more.

Optimally the 747 has a design speed of M0.855, but we do not fly this speed very often anymore. Usually it is at .84 or .85.

wilco737
  



It it's not Boeing, I am not going.
User currently offlineplanewasted From Sweden, joined Jan 2008, 533 posts, RR: 0
Reply 34, posted (1 year 3 months 3 weeks 5 days 17 hours ago) and read 6570 times:

I remember reading that Emirates says the A380 is more economical at the higher cruise speeds.

User currently offlineStarlionblue From Greenland, joined Feb 2004, 17069 posts, RR: 66
Reply 35, posted (1 year 3 months 3 weeks 5 days 16 hours ago) and read 6558 times:

Quoting planewasted (Reply 34):
I remember reading that Emirates says the A380 is more economical at the higher cruise speeds.

This is unfortunately rather a vague statement. Higher relative to what?

[Edited 2013-07-01 08:04:47]


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineseabosdca From United States of America, joined Sep 2007, 5620 posts, RR: 6
Reply 36, posted (1 year 3 months 3 weeks 5 days 16 hours ago) and read 6560 times:

Quoting A380900 (Thread starter):
Can we expect to see the A320 and 737 replacements to go that fast?

I doubt it. Widebodies' speed has crept back up as their range has grown. There's less benefit to optimizing for higher speed if you're not flying as far. I'd expect the A320 and 737 replacements to be optimized for speeds similar to the current A320 and 737.


User currently offlineRara From Germany, joined Jan 2007, 2114 posts, RR: 2
Reply 37, posted (1 year 3 months 3 weeks 5 days 8 hours ago) and read 6437 times:

Quoting SandroZRH (Reply 32):
The A340 is optimally flown at M.81, the A330 at M.80-M.81.
Quoting wilco737 (Reply 33):
Optimally yes, but many friends of me are 330/ 340 Pilots and they fly slower than M0.80 at times.

What exactly is "optimal" at M0.80 if fuel burn is lower at lower speeds? Or why else do they fly slower?



Samson was a biblical tough guy, but his dad Samsonite was even more of a hard case.
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17069 posts, RR: 66
Reply 38, posted (1 year 3 months 3 weeks 5 days 7 hours ago) and read 6423 times:

Quoting Rara (Reply 37):
What exactly is "optimal" at M0.80 if fuel burn is lower at lower speeds? Or why else do they fly slower?

Lower fuel burn can mean lower endurance, but not necessarily longer range, especially in a headwind. Highest endurance speed is lower than highest range speed.

As for optimal, turbofans tend to have a sweet spot where cruise speed is a bit higher than longest range speed, but this increase gives several percent shorter trip time with only slightly increased fuel burn.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinePihero From France, joined Jan 2005, 4597 posts, RR: 77
Reply 39, posted (1 year 3 months 3 weeks 5 days 6 hours ago) and read 6418 times:
Support Airliners.net - become a First Class Member!

Quoting Starlionblue (Reply 38):
Lower fuel burn can mean lower endurance,

Higher, surely ?

Quoting Rara (Reply 37):
What exactly is "optimal" at M0.80 if fuel burn is lower at lower speeds? Or why else do they fly slower?

The associated costs of a given flight are of two orders :
1/- the fixed costs : mainly related to overflight and landing fees, flight ground assistance, line maintenance, insurance... etc...
2/- the variable costs, including crew salaries, consumables (fuel and oïl ) ownership / leasing costs on a flight hour basis... etc...
You take them all, put them in a box, shake it well... et voilà ! you come out with a cruise Mach number at which the trip cost is minimal, i.e. optimum.
The computations were done by hand and were the basis of the min trip cost we'd used for years.
It's now done by the FMS when you input the *cost index* ( see the concept by searching the best document called "Getting to grips with the cost index" somewhere on the .net.
Starlionblue's above post gives a few more clues on what happens.



Contrail designer
User currently offlineSmittyOne From , joined Dec 1969, posts, RR:
Reply 40, posted (1 year 3 months 3 weeks 3 days 18 hours ago) and read 6168 times:

Quoting Pihero (Reply 39):
The associated costs of a given flight are of two orders :
1/- the fixed costs : mainly related to overflight and landing fees, flight ground assistance, line maintenance, insurance... etc...
2/- the variable costs, including crew salaries, consumables (fuel and oïl ) ownership / leasing costs on a flight hour basis... etc...
You take them all, put them in a box, shake it well... et voilà ! you come out with a cruise Mach number at which the trip cost is minimal, i.e. optimum.
The computations were done by hand and were the basis of the min trip cost we'd used for years.
It's now done by the FMS when you input the *cost index* ( see the concept by searching the best document called "Getting to grips with the cost index" somewhere on the .net.
Starlionblue's above post gives a few more clues on what happens.

Am I right in saying that all of this can and does go right out the window if/when ATC has you following a slower aircraft?!


User currently offlinePihero From France, joined Jan 2005, 4597 posts, RR: 77
Reply 41, posted (1 year 3 months 3 weeks 3 days 16 hours ago) and read 6139 times:
Support Airliners.net - become a First Class Member!

Quoting SmittyOne (Reply 40):
Am I right in saying that all of this can and does go right out the window if/when ATC has you following a slower aircraft?!

It would be a very rare occurrence to be blocked for very long by a slower airplane ahead. The simplest way is to accept a lower level, at your optimum Mach, then overtake the traffic and resume your own cruise schedule... or get a higher step level even if initially it won't be optimal. Time will arrange that.



Contrail designer
User currently offlineSmittyOne From , joined Dec 1969, posts, RR:
Reply 42, posted (1 year 3 months 3 weeks 3 days 15 hours ago) and read 6144 times:

Quoting Pihero (Reply 41):
It would be a very rare occurrence to be blocked for very long by a slower airplane ahead.

Good to know!

On a lighter note, I can almost picture ATC waving this:



User currently offlineairmagnac From Germany, joined Apr 2012, 316 posts, RR: 52
Reply 43, posted (1 year 3 months 3 weeks 3 days 12 hours ago) and read 6086 times:

Quoting Rara (Reply 37):

What exactly is "optimal" at M0.80 if fuel burn is lower at lower speeds?

Total cost
Made of energy cost and time-related cost

Quoting Rara (Reply 37):

Or why else do they fly slower?

The ultimate purpose of transport aircraft is to create movement from point A to point B. As there are costs related to the time it takes to cover the distance between the two, you want to minimize this time, ie maximize the ground speed.

However, this speed comes at a cost : you have to spend energy to produce it.
With gross simplifications, the process to do so is to take chemical energy (stored in the fuel) and transform this into kinetic energy. This is done by the propulsion system, with an efficiency reflected in the specific consumption.
Part of this kinetic energy is then bled off to be converted into potential energy (lift) by the wing, to compensate for the weight of the aircraft. But this also comes at a price, in the form of drag (made of heat, and various vortex movements in the local airflow). The overall "efficiency factor" of the wing is reflected in the L/D. So the lower the weight, or the higher the L/D, the less energy you need to bleed off. Taking it to the extreme, if weight was zero or if L/D was infinite, you wouldn’t have to bleed any energy at all
What’s left of the kinetic energy is the airspeed, which is then combined with natural energy (wind) to produce the ground speed.

You therefore also want to minimize the energy costs, ie reduce weight, increase L/D, reduce specific consumption, and find favorable winds. However, all these elements depend on some common parameters : pressure, density, temperature, altitude, speed, AOA. So you have to balance out all these things.

However, the important item is total cost, ie fuel cost + time cost, both of which are to some extent contradictory. So you also have to find the proper balance between energy and time. If you use slaves to pilot and maintain the aircraft, but fuel is expensive, then your time-related costs are negligible, and so reducing speed will not have much of an impact compared to what you gain in energy costs. If fuel is virtually free, then you can spend lots of it to go fast and reduce your time costs.

To summarize, the problem is made of L/D, weight, temperature/density/pressure, altitude, AOA, airspeed, wind, and price of fuel and price of time (more precisely, the ratio between the two, ie cost index). You might have some imposed waypoints too. And I left out the energy used to power the various systems.
To make matters worse, all those parameters change all the time. And many of them are inter-dependant to some extent.

The FMS basically stores a very detailed model of the links between all these parameters, and any limits on them. You feed it with the most up-to-date data available regarding their current values, and it calculates the best speed/mach/altitude/vertical speed/heading to maintain over the next few minutes in order to obtain the optimal total cost.
Or as Pihero beautifully put it :

Quoting Pihero (Reply 39):
You take them all, put them in a box, shake it well... et voilà !




More details here :
http://www.google.de/url?sa=t&rct=j&...vyyNvmRiLpcw&bvm=bv.48705608,d.ZWU

Quoting Pihero (Reply 39):
the best document called "Getting to grips with the cost index" somewhere on the .net.



One "oh shit" can erase a thousand "attaboys".
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17069 posts, RR: 66
Reply 44, posted (1 year 3 months 3 weeks 3 days 6 hours ago) and read 6018 times:

Quoting Pihero (Reply 39):
Quoting Starlionblue (Reply 38):
Lower fuel burn can mean lower endurance,

Higher, surely ?

Yes!

Oops damnit...



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineSandroZRH From , joined Dec 1969, posts, RR:
Reply 45, posted (1 year 3 months 2 weeks 5 days 8 hours ago) and read 5619 times:

Quoting wilco737 (Reply 33):

I hear you, they are starting to introduce CI0 standard at my company aswell. It's not going to work though as connections at ZRH are so tight that we usually will have to fly faster anyway  


User currently offlineRara From Germany, joined Jan 2007, 2114 posts, RR: 2
Reply 46, posted (1 year 3 months 2 weeks 2 days 16 hours ago) and read 5300 times:

Quoting airmagnac (Reply 43):

Thanks for the explanation. I take it, then, that the B744's L/D ration isn't best at M0.855, but at some speed lower than that, but that M0.855 is the speed at which the different cost factors usually nicely balance out?

What would be a typical max endurance speed for the B744, and would that be the speed at which the L/D ratio is optimal?

Does a cost index of 0 command max endurance speed?



Samson was a biblical tough guy, but his dad Samsonite was even more of a hard case.
User currently offlineairmagnac From Germany, joined Apr 2012, 316 posts, RR: 52
Reply 47, posted (1 year 3 months 1 week 4 days 20 hours ago) and read 4898 times:

Quoting Rara (Reply 46):
I take it, then, that the B744's L/D ration isn't best at M0.855, but at some speed lower than that, but that M0.855 is the speed at which the different cost factors usually nicely balance out?

We're dealing here with a what could be called a "multi-layered" optimization problem. Ultimately, the goal is to optimize the airline operating costs, themselves dependent on a multitude of trip costs, themselves dependent on fuel costs and time costs for each trip, themselves dependent on engine performance (Cs), airframe performance (L/D, W), wind, themselves dependant on [...]

Not particularly easy to solve ! And to make matters worse, our human brains are rather lazy devices that like to take the easy path. Our intuition is that the "optimum of the sum is the sum of all optimums". It is a very common misconception*. So our first (and often, last) answer is that if you have the best L/D value, the lowest specific consumption and the highest tail winds, then voilà ! you’ll get the best trip costs. Or in the same vein, if each trip operated by an airline is done at the lowest trip cost, then the airline total operating costs will be minimized.
That is true if all the elements of the problem are independent of one another.
But in real life, and particularly in the case of trip costs, the elements are linked through altitude, Mach and so on. For global airline costs, you can’t consider each trip separately, because as SandroZRH mentioned, the trips are interrelated through airline schedules for connecting flights.
Optimising each element seperatly will usually give you a rather good result, but it will rarely give you the best result.


Looking at only the trip costs, and leaving aside the airline costs :
For a set of conditions on those parameters, if you take the best L/D, you may get such a high engine burn and long trip time that the overall costs are high. And reciprocally, best trip time may not be a good choice either because it uses way too much fuel. The better choice may be to sacrifice a bit on L/D, engine consumption, airspeed and winds ; it may not be optimal setting for any of these items, but it may be the optimal setting in terms of trip costs.
So rigorously speaking, for each mission, there will be a different optimal Mach, optimal altitude…depending on the payload, atmospheric conditions, ATC constraints...

All that is a bit mathematical though, and will be of interest mainly for the engineers writing the ops manuals. In practical terms, it may be possible (even likely) that you will always end up with the same values (within a few %). It is easier for everyone to just remember one ball-park value which will be good enough for almost all cases. Which is likely the case of the M0.855 number provided by Wilco for the 744. For more detailed stuff, just stick the full cost model and the current numbers into a computer

So to answer your question more clearly, M0.855 is not THE Mach for which L/D is better, but it is probably close enough to the real number (which may be higher or lower). To “balance out all the cost factors”, you then have to mix in the airspeed, winds, trip time and engine aspects. That calculation will give you a combination of Mach, altitude etc… to get the best trip cost.


* And a major issue in airplane design. The best airplane is NOT the airplane with the best flight controls, the best air conditioning, the best fuselage structure, the best nacelle aerodynamics…it is the airplane with the best overall integration of these various elements. Of course you want each element to be very good on its own, but they do not necessarily have to be the best-of-the-best. But this principle is not always reflected in the organisations in charge of designing aircraft...
Other simpler example : you want to minimize the time to travel from Paris to Lyon in France. Train is 2h (TGV), plane is 1h. Plane is twice as fast, so you take the plane, right ? But that’s just looking at one element. If you add in time to go to the airport from downtown (45min), time from LYS to downtown (a good 30min), time margin for lines at check-in and security and for boarding, time for de-boarding, you end up with a total trip time by plane of maybe 3 ½ hours. Compared to maybe 21/2 hours by TGV if you are going downtown to downtown. So the optimum trip time is obtained by sacrificing a bit on the actual travel time


Quoting Rara (Reply 45):
What would be a typical max endurance speed for the B744, and would that be the speed at which the L/D ratio is optimal?

Different optimization problem here : the objective is not to minimize trip costs, but to maximize flight time.
However, an advantage in this case is that you can obtain the relatively simple Breguet equation by adding some simplifications to the problem. This will show that max endurance is indeed linked to best L/D, as well as weight and specific consumption.
Which is logical, because coming back to the energy study, in this case you just want to transform fuel into kinetic energy and then into lift to counter the weight. Period. So the goal is to optimize those 2 transformations, which means first minimizing engine specific consumption and second, minimizing weight (if possible) and maximizing L/D
Again, L/D, weight and Cs are not completely independant, so the rigourous solution is not quite so trivial, but Breguet is good enough as a first approximation.



One "oh shit" can erase a thousand "attaboys".
User currently offlineB747400ERF From United Kingdom, joined Jun 2013, 477 posts, RR: 1
Reply 48, posted (1 year 3 months 4 days 9 hours ago) and read 4150 times:

Optimum econ cruise speed for the 744 is around .84

Top Of Page
Forum Index

Reply To This Topic Is Mach 0.85 Becoming The Industry Standard?
Username:
No username? Sign up now!
Password: 


Forgot Password? Be reminded.
Remember me on this computer (uses cookies)
  • Tech/Ops related posts only!
  • Not Tech/Ops related? Use the other forums
  • No adverts of any kind. This includes web pages.
  • No hostile language or criticizing of others.
  • Do not post copyright protected material.
  • Use relevant and describing topics.
  • Check if your post already been discussed.
  • Check your spelling!
  • DETAILED RULES
Add Images Add SmiliesPosting Help

Please check your spelling (press "Check Spelling" above)


Similar topics:More similar topics...
How Important Is An Internship In The Industry? posted Thu May 6 2010 09:59:35 by KLM672
Can The 777 Cruise At Mach 0.85? posted Mon Feb 25 2008 17:10:44 by Iloveboeing
What Is That Scope On The 787 ZA002 Tail? posted Sun Oct 23 2011 11:52:18 by ferpe
What Is This Hump On The 77W? posted Fri Oct 8 2010 15:00:07 by c5load
What Is This Device In The Cockpit For? posted Sat Jan 16 2010 16:00:46 by JAGflyer
What is this part on the tail? posted Sun Jun 29 2008 16:53:07 by Imiakhtar
What Is This Flap On The DC10 For? posted Sat Jul 28 2007 01:52:21 by NEMA
What Is This Panel On The A-380? posted Thu Jul 5 2007 00:37:43 by SLCPilot
What Is This Lump On The 737? posted Sun Oct 1 2006 13:18:37 by Monteycarlos
Why Is Fuel Coming Out The Wing? posted Sun Oct 23 2005 13:49:51 by Wrighbrothers

Sponsor Message:
Printer friendly format