OyKIE From Norway, joined Jan 2006, 2575 posts, RR: 4 Posted (7 years 3 months 2 weeks 4 hours ago) and read 8907 times:
The 787 has a cruise speed of 0.85 Mach which is according to Boeing similar to today's fastest widebodies.
When the 747 Advanced was promoted they stated a cruise speed of 0.86 Mach. The 747-8 has 0.85. Why is it not as fast as they kept saying before launching the new 747? I know this is a minor detail. You will probably not be gaining more than say 15 minutes on an 10 hour flight. But still curious as to why they decided to make it slower.
Also why isn't the 787 any faster? Why is the 0.85 Mach a "magical" number? The citation X has a 0.90 Mach cruise speed. Could this be achieved in the new planes from Boeing as Max speed? (747-8 and 787)
Dream no small dream; it lacks magic. Dream large, then go make that dream real - Donald Douglas
Btriple7 From United States of America, joined Sep 2005, 1139 posts, RR: 9 Reply 1, posted (7 years 3 months 2 weeks 4 hours ago) and read 8857 times:
There really is no point to going faster. The difference between mach .85 and mach .90 isn't that great. The money spend on extra fuel and faster engines doesn't outweigh the benefits of flying faster. All you gain is an extra half-hour at most. This is partially why the Sonic Cruiser never got off the ground (excuse the pun). It isn't worth going a few tenths of a mach faster if it is to cost a bundles of extra money.
OyKIE From Norway, joined Jan 2006, 2575 posts, RR: 4 Reply 2, posted (7 years 3 months 2 weeks 4 hours ago) and read 8820 times:
Quoting Btriple7 (Reply 1): There really is no point to going faster.
I guess you are right. But still, the 747-8 is according to Boeing supposed to cruise at 0.85 Mach. The 747-400 cruises at 0.855 Mach. And the advanced was going to be at 0.86 Mach. What made Boeing make the 747-8 slower than the Advanced? Is there a good reason for stopping at 0.85 Mach? I would believe that the new technology would make a little increase in speed economically viable.
Dream no small dream; it lacks magic. Dream large, then go make that dream real - Donald Douglas
Btriple7 From United States of America, joined Sep 2005, 1139 posts, RR: 9 Reply 3, posted (7 years 3 months 2 weeks 4 hours ago) and read 8757 times:
Quoting OyKIE (Reply 2): I would believe that the new technology would make a little increase in speed economically viable.
I suppose if you can increase the speed without adding too much money to the plane's operational cost, I would say go for it. The thing is, though, if I was Boeing I wouldn't spend that much time trying to figure out how to make a plane go a bit faster.
Also, we can't be sure what the cruise speed of the 747-8 will be until it is built. When the 777 was designed, the designers said it would cruise at around mach .83, but it ended up with a cruise speed of mach .84 after test flights proved it was more aerodynamic than first thought. We'll just have to wait and see.
FlyDreamliner From United States of America, joined Jan 2006, 2759 posts, RR: 15 Reply 5, posted (7 years 3 months 2 weeks 1 hour ago) and read 8435 times:
The Citation X can cruise at .9, but maximum range is not at .9.
Airbus claims a max cruise of .86 on A340, but in reality it's no higher than .80-.82 for reasons of fuel economy.
Fuel economy comes first. When Dreamliner and 747-8 take to the skies, they'll find out what they really can do.
Boeing refers to the wing on 747-8 as the fastest wing in commerical aviation, what's that about?
"Let the world change you, and you can change the world"
DfwRevolution From , joined Dec 1969, posts, RR: Reply 6, posted (7 years 3 months 2 weeks ago) and read 8373 times:
Quoting OyKIE (Reply 2): But still, the 747-8 is according to Boeing supposed to cruise at 0.85 Mach. The 747-400 cruises at 0.855 Mach. And the advanced was going to be at 0.86 Mach
First, do not take Boeing's commercial website as the gospel truth. There will be discrepencies of some sort because the website is produced by web design and marketing folks, not the engineers. In this case, Boeing's website says the following:
Typical cruise speed (at 35,000 feet): Mach 0.855
Whatever "typical cruise speed" means, it is probably different from max cruise speed, economy cruise speed, and endurance cruise speed, all very different functions.
Quoting Btriple7 (Reply 1): The difference between mach .85 and mach .90 isn't that great. The money spend on extra fuel and faster engines doesn't outweigh the benefits of flying faster. This is partially why the Sonic Cruiser never got off the ground
The Sonic Cruiser concept would have cruised around Mach .95-.98, just barely under super sonic velocity. There would have been a noticable decrease in long-haul trip lenght.
Boeing's logic was that (prior to 9/11) labor cost were a larger cost to airlines than fuel. Eliminate two hours off a 16 hour flight, and you've achieved a 12% labor reduction. Since the Sonic Cruiser was aiming for fuel/seat cost on par with the 767, this would have made sense to some airlines.
In hindsight, it's very fortunate for Boeing that early aprehension and timing ensured the death of the SC. The 787 is a much more economical and versatile product in the wake of 9/11, SARS, and high fuel prices.
Quoting TaromA380 (Reply 4): Maybe because 747-8 has larger engines => more drag ?
Probably not... the 747-8 features aerodynamic improvements that Boeing said would improve cruise speed. Also, the GEnx engines are a reduced-diameter variant from what the 787/A350 will feature. They will be bigger than the current engines, but not much bigger.
Quoting FlyDreamliner (Reply 5): Boeing refers to the wing on 747-8 as the fastest wing in commerical aviation, what's that about?
What's what about? The Citation X isn't a commercial aircraft, and the 747-400 can economically cruise faster than any other commercial aircraft in service. It will still likely edge out the A380.
I suspect what we have with the Boeing website is the confusion of a trivial detail and nothing more.
Gigneil From United States of America, joined Nov 2002, 16215 posts, RR: 88 Reply 7, posted (7 years 3 months 1 week 6 days 23 hours ago) and read 8236 times:
AirlineAV8tr From United States of America, joined Feb 2006, 191 posts, RR: 4 Reply 9, posted (7 years 3 months 1 week 6 days 22 hours ago) and read 8110 times:
Quoting OyKIE (Thread starter): The 787 has a cruise speed of 0.85 Mach which is according to Boeing similar to today's fastest widebodies.
When the 747 Advanced was promoted they stated a cruise speed of 0.86 Mach. The 747-8 has 0.85. Why is it not as fast as they kept saying before launching the new 747? I know this is a minor detail. You will probably not be gaining more than say 15 minutes on an 10 hour flight. But still curious as to why they decided to make it slower.
Also why isn't the 787 any faster? Why is the 0.85 Mach a "magical" number? The citation X has a 0.90 Mach cruise speed. Could this be achieved in the new planes from Boeing as Max speed? (747-8 and 787)
All of today's widebodies cruise between .85, and at or right below .90. While a .01 difference is minimal, over the coarse of a 10-15 hour flight it results in hundreds of pounds of fuel saved- which is Boeing's goal for the near future- to be "Ultra-Efficient". As many of the people responding have said above, cruise speeds have about as many variables as takeoff distances! There is (on my aircraft) a .5 Mach difference between economy, and max cruise speeds. Max cruise, on any aircraft is typically inefficient, and only used on the last leg of trip (to get us home to our families quicker), or in an attempt to make up time so as to make an on-time, or early arrival.
The max cruise on the Citation X is .94 Mach, if I remember correctly. And they actually broke the sound barrier during initial flight testing in Arizona in a shallow dive!
If we went into the funeral business, people would stop dying.-Martin S. (PanAm CEO)
HZ747300 From Hong Kong, joined Mar 2004, 1596 posts, RR: 0 Reply 10, posted (7 years 3 months 1 week 6 days 22 hours ago) and read 8111 times:
Quoting Btriple7 (Reply 1): difference between mach .85 and mach .90 isn't that great. The money spend on extra fuel and faster engines doesn't outweigh the benefits of flying faster.
This is the reason. There might be a few other less substantial reasons, but they are not the primary reason.
CitationJet From United States of America, joined Mar 2003, 2229 posts, RR: 3 Reply 11, posted (7 years 3 months 1 week 6 days 22 hours ago) and read 8074 times:
Quoting FlyDreamliner (Reply 5): The Citation X can cruise at .9, but maximum range is not at .9.
Quoting AirlineAV8tr (Reply 9): The max cruise on the Citation X is .94 Mach, if I remember correctly
Quoting AirlineAV8tr (Reply 9): And they actually broke the sound barrier during initial flight testing
That is true. There is an engineering conference room at Cessna called Mach 1. It has a picture of the Citation X prototype flying in formation with the T-38 chase plane used for the flight flutter testing. The plaque on the photo gives the date that the Citation X first went supersonic.
ABpositive From Australia, joined Nov 2005, 221 posts, RR: 0 Reply 12, posted (7 years 3 months 1 week 6 days 22 hours ago) and read 8040 times:
In my opinion, it could be part of a marketing trick. By having a lower speed they can claim how they have significantly improved the efficiency of the plane.
Dstefanc From United States of America, joined Dec 2001, 63 posts, RR: 4 Reply 13, posted (7 years 3 months 1 week 6 days 21 hours ago) and read 8007 times:
There is a very simple reason why this number is the "magic" number. It has to do with supersonic velocities. The aircraft cruising at or below 0.85 Mach is beneficial, because every portion of the aircraft is traveling at subsonic speeds. What that means is that if You cruise above 0.85 Mach, some surfaces on the airframe will experience supersonic airflow. That is not beneficial, because it reflects on the amount of fuel needed (as pointed out numerous times already). And the largest problem with going partially, and off course fully (fully = every portion of the airframe experiences supersonic flow), supersonic is that the increase in fuel consumption is highly increased. If You are aware of the details that undergo during supersonic flow and shockwaves, You know that very high drag is created due to the very large pressure drop over the shock. Thus, if You have, even local, supersonic flow over the airframe You will be sacrificing a lot of fuel to fly at that speed due to the local shocks. Thus, 0.85 is just about that perfect number that keeps the entire frame of the aircraft subsonic. However, this number is not universal. It off course depends on the design, but since the airframes do not differ from each other that much, the number only fluctuates slightly.
Also, a note on the Citation, I bet they cruise at higher speeds, just because they can afford to. I mean if You have the money to fly around just a few people in a multi-million $$ jet, You can afford the extra fuel cost. Also, time is money when it comes to business, so I bet that is another reason for the increased cruise speed.
Also, just an extra nice little fact somewhat along this subject could be made to props. I don't know if You are aware, but there is a reason that props have to fly much slower than the jets. That's because the rotating blade has a limited RPM speed due to the sonic velocity. Basically, it cannot exceed Mach > or = to 1.0 otherwise it would not make any economical sense to fly the aircraft if the blade would rotate at a supersonic speed, create shocks, and thus would have to overcome an enormous amount of drag.
Hslightnin From United States of America, joined Jun 2005, 36 posts, RR: 0 Reply 14, posted (7 years 3 months 1 week 6 days 21 hours ago) and read 7835 times:
Quoting TaromA380 (Reply 4): Maybe because 747-8 has larger engines => more drag ?
I dont understand your logic here
Quoting AirlineAV8tr (Reply 9): The max cruise on the Citation X is .94 Mach, if I remember correctly. And they actually broke the sound barrier during initial flight testing in Arizona in a shallow dive!
I saw a special on t.v after they roll every X out of the factory during the flight test they bring it up to .99 to check some system, dont remember which.
Sllevin From United States of America, joined Jan 2002, 3376 posts, RR: 7 Reply 15, posted (7 years 3 months 1 week 6 days 21 hours ago) and read 7716 times:
I believe the 747 can cruise at .875 effectively, but fuel costs have caused most operators to slow up a bit.
But as recently as the late 1980s you could see the shadows on the wing caused by the transonic compression...but those days are long gone.
Atmx2000 From United States of America, joined Oct 2004, 4576 posts, RR: 39 Reply 16, posted (7 years 3 months 1 week 6 days 21 hours ago) and read 7695 times:
Quoting Gigneil (Reply 7): What makes you say that? The A380's cruise speed is M0.86.
CaptainStorck From United States of America, joined Sep 2005, 66 posts, RR: 0 Reply 17, posted (7 years 3 months 1 week 6 days 20 hours ago) and read 7494 times:
Cruise Mach # varies with weight. That supersonic airflow mentioned in the above post is due to differential pressure. An airfoil produces lift because of this differential pressure. On a cambered airfoil (which is found on almost all aircraft, with the exception of some aerobatic aircraft), air travels at a higher speed over the top; this means that the air spreads out to an extent, causing lower pressure on the top than on the bottom; boom - lift. Most of you already know this, but just to clarify for everyone.... Anyway, continuing on, early in a long journey when an aircraft and weighs more, it requires a more lift to fly. Extra lift from the same wing is created by having a higher angle of attack (the angle between the wing and the air that it is traveling through). The higher the angle of attack, the faster the air is moving over the top of the wing relative to the air on the bottom. Therefore when an aircraft is heavier, the air on the top of the wing will reach supersonic speed before it would when the airplane is lighter. An airliner that brags of having a cruise Mach # of .86 may begin its journey at .82 or .83 using the economy cruise speeds and accelerate slowly as tens of thousands of pounds of fuel are burned off, only to reach .86 for the last 30-45 minutes of its flight, if at all. If the flight in question is going to be towards the outer limits of the range of aircraft, then the long range cruise speeds will be substituted for the economy speed - LRC is even slower than Econ. The flight may start out at .78 or .79 to conserve fuel and never reach the advertised Mach #. My point? Whether an aircraft manufacturer says that the aircraft can achieve cruise at .85 or .86 is trivial and a non-issue. I apologize if any of the above information is incorrect, though to the best of my knowledge, it is not; feel free to criticize/correct.....
Quoting Dstefanc (Reply 13): Also, a note on the Citation, I bet they cruise at higher speeds, just because they can afford to. I mean if You have the money to fly around just a few people in a multi-million $$ jet, You can afford the extra fuel cost. Also, time is money when it comes to business, so I bet that is another reason for the increased cruise speed.
And as for this, I don't see your logic, if this were so, then why does the Gulfstream 5 or the Global Express cruise at .92? My guess for this is that the wing of the Citation X does not require as great of a pressure differential then other jets in its category... Just a guess though. (May be due to how grossly overpowered the Citation X is; it uses a de-rated version of the engine found in a 717)
Seanp11 From United States of America, joined Jan 2006, 290 posts, RR: 0 Reply 18, posted (7 years 3 months 1 week 6 days 19 hours ago) and read 7147 times:
Quoting CaptainStorck (Reply 17): An airliner that brags of having a cruise Mach # of .86 may begin its journey at .82 or .83 using the economy cruise speeds and accelerate slowly as tens of thousands of pounds of fuel are burned off, only to reach .86 for the last 30-45 minutes of its flight, if at all. If the flight in question is going to be towards the outer limits of the range of aircraft, then the long range cruise speeds will be substituted for the economy speed - LRC is even slower than Econ. The flight may start out at .78 or .79 to conserve fuel and never reach the advertised Mach #. My point? Whether an aircraft manufacturer says that the aircraft can achieve cruise at .85 or .86 is trivial and a non-issue. I apologize if any of the above information is incorrect, though to the best of my knowledge, it is not; feel free to criticize/correct.....
Thats theoretically correct, the way that this is dealt with is through step climbs, instead of increasing the cruising mach #.
The FMS will spit out an optimum altitude, not mach #. The econ cruise, etc. should stay the same number.
MD-90 From United States of America, joined Jan 2000, 8417 posts, RR: 13 Reply 19, posted (7 years 3 months 1 week 6 days 18 hours ago) and read 6825 times:
Quoting CaptainStorck (Reply 17): And as for this, I don't see your logic, if this were so, then why does the Gulfstream 5 or the Global Express cruise at .92?
They don't. High speed cruise on the Global Express is 0.88 Mach, I believe. And best range is still .80, although it only offers a slight increase over 0.85, which is probably the most common cruise speed. The G-V is also .80 up to just below the GLEX.
Sinlock From United States of America, joined Dec 2000, 1532 posts, RR: 3 Reply 20, posted (7 years 3 months 1 week 6 days 13 hours ago) and read 5812 times:
Quoting CaptainStorck (Reply 17): And as for this, I don't see your logic, if this were so, then why does the Gulfstream 5 or the Global Express cruise at .92?
They don't.....
The mmo for G500 and G550 is M.885 with Cruise being M.85
The mmo for Global Express and Global 5000 is M.89 with Cruise being M.85
OyKIE From Norway, joined Jan 2006, 2575 posts, RR: 4 Reply 21, posted (7 years 3 months 1 week 6 days 13 hours ago) and read 5695 times:
Quoting DfwRevolution (Reply 6): Boeing's logic was that (prior to 9/11) Eliminate two hours off a 16 hour flight, and you've achieved a 12% labor reduction. Since the Sonic Cruiser was aiming for fuel/seat cost on par with the 767, this would have made sense to some airlines.
This would still make sense in my eyes. I hope the sonic cruiser is available before I die. If I live till I am 100 years, they have about 75 years to develop it. I keep my fingers crossed.
Quoting DfwRevolution (Reply 8): Airbus was advertising Mach 0.850 just this year. Is this the result of certification testing?
I remember reading that Airbus as a reply to Boeings 747 Advanced being faster, that they where really impressed by flight test data so they said maybe the cruise speed would be increased.
Quoting CaptainStorck (Reply 17): I apologize if any of the above information is incorrect, though to the best of my knowledge, it is not; feel free to criticize/correct.....
CaptainStorck. That sounds very plausible
Dream no small dream; it lacks magic. Dream large, then go make that dream real - Donald Douglas
DarkBlue From United States of America, joined Sep 2003, 233 posts, RR: 11 Reply 22, posted (7 years 3 months 1 week 6 days 12 hours ago) and read 5296 times:
Quoting Hslightnin (Reply 14): Quoting TaromA380 (Reply 4):
Maybe because 747-8 has larger engines => more drag ?
I dont understand your logic here
GEnx engines have a much larger bypass ratio (~8) comared to the current CF6 engines (~5). When TaromA380 said that the 747-8 has larger engines he was meaning that they are physically larger (larger fan diameter => larger nacelle). A larger nacelle will lead to increased drag in two ways. Even with improved aerodynamics, it will still have a much larger wetted surface area than the current CF6 engines. Second, a larger nacelle will lead to a heavier engine weight. More weight means that you need more lift. More lift leads to more drag.
ChiGB1973 From United States of America, joined Mar 2004, 1605 posts, RR: 1 Reply 23, posted (7 years 3 months 1 week 6 days 11 hours ago) and read 5068 times:
Quoting DfwRevolution (Reply 8): Airbus was advertising Mach 0.850 just this year. Is this the result of certification testing?
I am sure any substantial amounts of speed would require a different amount of certification. I wonder if this is the breaking point between FAA certification points? Does it put the aircraft in a different category that would require more testing or a different certification?
AeroWeanie From United States of America, joined Dec 2004, 1600 posts, RR: 52 Reply 24, posted (7 years 3 months 1 week 6 days 11 hours ago) and read 4995 times:
Quoting Hslightnin (Reply 14): I saw a special on t.v after they roll every X out of the factory during the flight test they bring it up to .99 to check some system, dont remember which.
While Mmo might be .92, certification requires that the airplane be OK out to a dive speed of .07 Mach higher (this used to be .05). Thus, the aircraft must be capable of a M=.99 dive.
Quoting Sllevin (Reply 15): But as recently as the late 1980s you could see the shadows on the wing caused by the transonic compression...but those days are long gone.
They are still there - you just have to be in the right position relative to the sun to see them. This was taken on a 757 - note the line on the wing.
26 ZSOFN: Back in June 2003 I was on BA049 LHR-SEA on one of their 744s and the transonic compression wave was quite clearly visible - much more so than the ex
28 DarkBlue: Yes 9.5 sounds about right, but only for the 787/A350 applications (GEnx-1B & -1A). The 747-8 version (GEnx-2B) has a smaller fan so BPR won't be abo
29 Max Q: In terms of fuel economy and economy of operation at high gross weight one would be more likely to climb at a high indicated airspeed to (especially i
30 PhilSquares: On the 744/744F, we use a CI of 130 and SOP dictates you cruise at Econ Cruise. But, sometimes the flight plans are planned and filed at higher speeds
31 N1120A: What is today's widebodies? The A345/346 does M0.825-0.83 and the 777LR aircraft are at .84. Now, I am not a pilot which is why I am asking you this
32 KELPkid: There is some tricky engineering on the Citation X to allow it to cruise that fast: the fillet between the wing and the fuselage had to be engineered
33 BuckFifty: The speeds decrease as the weights lighten due to the fact that the lift required is less. So if we remained at constant altitude as the weights decre
34 PhilSquares: Hadn't heard that one before. I agree on short flights the FMS gives you a very unrealistic altitude profile, but on longer flights it's very accurat
35 L-188: Well somebody correct me if I am wrong but doesn't the 747-4 and the Citation X share the same 45 degree wing sweep? Which BTW is the same sweep used
36 AeroWeanie: The "tanks" are "Kuchemann Carrots", designed to smooth out the area rule distribution. The mods applied to the AA aircraft are in addition to these.
37 N1120A: Oh, I know about it being an alternate issue in that a 744A really doesn't take restrictions at all when an alternate is not required. I was just not
38 CitationJet: That is true. The wing/fuselage fairing changed during development flight testing. The sweep of the Citation X wing is 37 degrees at the c/4. The HT
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