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Min/Max Mach Numbers Of Aircraft  
User currently offlineCuriousOne From Bahrain, joined Sep 2009, 3 posts, RR: 0
Posted (5 years 3 months 3 weeks 4 days 2 hours ago) and read 9578 times:

For a small school project about aviation, I wanted to research the Mach number margins of the following aircraft:
- B744
- B763
- B774
- A320
- A346
- A380
I tried to google them, but I didn't find all answers. Some sites mention the "typical cruise mach number", others mention a "maximum mach number" but not "maximum cruise mach number" which can be different as I understand it. And I have not found a single note about any "minimum cruise mach number".
Anyone out there who can help me out?
Thanks a lot!!!

5 replies: All unread, jump to last
 
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 1, posted (5 years 3 months 3 weeks 4 days 1 hour ago) and read 9566 times:



Quoting CuriousOne (Thread starter):
For a small school project about aviation, I wanted to research the Mach number margins of the following aircraft:

"Margin" usually means the difference between two numbers. Which two are you looking for?

There are, typically, three mach numbers of interest:
Cruise Mach (can't remember the right symbol for this one)...this can vary a lot with economics
Mmo = maximum operating Mach
Md = dive Mach

Cruise Mach is usually listed by the OEM, or various websites, but there's no really good definition so the numbers will vary. It could be maximum cruise, economy cruise, or some mix.

Mmo will be listed in the aircraft flight manual and, I think, on the Type Certificate Data Sheet. The former is often available on www.smartcockpit.com and the latter is always available on the FAA website.

Md is occasionally listed on the TCDS or AFM, but not always. You can figure out what the minimum Md is by working it backwards from Mmo and the relevant regulations, but the actual Md may be higher than that.

Quoting CuriousOne (Thread starter):
I tried to google them, but I didn't find all answers. Some sites mention the "typical cruise mach number", others mention a "maximum mach number" but not "maximum cruise mach number" which can be different as I understand it. And I have not found a single note about any "minimum cruise mach number".

You're right that those can all be different numbers. There isn't a minimum cruise mach number because that's not parameter anybody uses for anything. Minimum speed is going to require low altitude (high air density), which means you'll be speed limited rather than mach limited anyway.

Tom.


User currently offlineCuriousOne From Bahrain, joined Sep 2009, 3 posts, RR: 0
Reply 2, posted (5 years 3 months 3 weeks 4 days ago) and read 9546 times:

Thanks for your answer, Tdscanuck.
To be more precise about what I was looking for, that was the "maximum Cruise Mach number" and the "maximum Mach number" as they can be quite different, as I understand it.
I searched for the data in Wikipedia, and I found some of the following:
B744: cruise & max
B763: cruise & max cruise
B773: cruise & max cruise
A320: cruise & max
A346: cruise
A380: max cruise & max
Especially the big difference on the A380 between max cruise Mach number (0.89) and max Mach number (0.96) leads me to believe that I cannot just assume the max Mach numbers of other aircraft to be their max cruise Mach numbers as well. There seems to be a huge difference.
And in respect to the "minimum cruise Mach number" I am looking for data for a scenario when air traffic control has to sequence at high levels with the Mach number technique. I am looking for (the at least theoretical) scenario if it was possible by aircraft's layout and thus Mach number range that e.g. an A380 could be set up with same speeds behind an A320, for example.
Oh, and thanks for the link for SmartCockpit. I will browse through there and have a look if I can find some information I need.
I'm sorry to be so little educated in this field, but any help will be highly appreciated.


User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6544 posts, RR: 54
Reply 3, posted (5 years 3 months 3 weeks 3 days 16 hours ago) and read 9498 times:



Quoting CuriousOne (Reply 2):
Especially the big difference on the A380 between max cruise Mach number (0.89) and max Mach number (0.96)

The A380 and M=0.96 was one test flight before certification, and it should never be repeated.

The purpose was to demonstrate that there was a sufficient safety margin well above max cruise Mach number, which could be reached in case of some incidense without destructive flutter.

I remember that the captain on that test flight told that it was a very delicate thing to do. First of all it had to be done in a rather steep dive since the engines are not designed to work very well at such high speed. And as he said: "We want to make sure that we do not get anywhere near M=0.97".

Various "maximum Mach numbers" do not tell anything about the quality of the plane. It is only something which the designers agree upon after having evaluated thousands of other compromises.

Minimum Mach number is totally irrelevant since it is totally dependent upon altitude density and weight. And of course actual high lift device configuration. Depending on the circumstances it can on the same plane in principle be anything from around M=0.20 and up to max cruise Mach number. In the real world, however, things like max certified takeoff weight, max certified altitude, or simply inability to reach max altitude because of heavy weight, will limit the latter to a somewhat lower value.

There are also somewhat complicated operating rules which dictate that the pilots observe a margin - airliners shall always keep a certain margin to "coffin corner". As opposed to the Lockheed U-2 spy plane which was designed to fly as high as possible where min and max Mach number matched, and the slightest disturbance by turbulence would make the plane fall out of the sky.

Various minimum speeds are always expressed as IAS (indicated air speed) most often expressed in knots (KIAS). IAS (as opposed to TAS - true air speed) is the figure at any altitude where the air reacts on the plane the same way as it does at the same figure of TAS at sea level. If you are going 250 knots IAS at 35,000 feet, then you are actually going more like 500 TAS. Mach numbers are even more different since speed of sound expressed as TAS decreases with decreased temperature at altitude.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 4, posted (5 years 3 months 3 weeks 3 days 12 hours ago) and read 9474 times:



Quoting CuriousOne (Reply 2):
Especially the big difference on the A380 between max cruise Mach number (0.89) and max Mach number (0.96) leads me to believe that I cannot just assume the max Mach numbers of other aircraft to be their max cruise Mach numbers as well.

They're definitely not the same. Some aircraft can't reach their max Mach number in level flight (i.e. cruise), and while many aircraft may operate at max cruise Mach on a routine basis, few if any will ever reach max Mach except in flight test.

Quoting CuriousOne (Reply 2):
And in respect to the "minimum cruise Mach number" I am looking for data for a scenario when air traffic control has to sequence at high levels with the Mach number technique. I am looking for (the at least theoretical) scenario if it was possible by aircraft's layout and thus Mach number range that e.g. an A380 could be set up with same speeds behind an A320, for example.

Definitely possible. Although the optimum Mach may be different, there's very significant overlap between all types so pretty much any commercial jet can speed match any other at the lower end of the range. At the high end you may get separation...a 737 can't catch a 747 going max cruise Mach, for example.

Tom.


User currently offlineCuriousOne From Bahrain, joined Sep 2009, 3 posts, RR: 0
Reply 5, posted (5 years 3 months 3 weeks 2 days 20 hours ago) and read 9416 times:

Thanks a lot, guys!
Those explanations help indeed a lot!
Thanks for your time answering to such a "dumb-struck thing" like me...  Smile
Appreciate it great deal!


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