novice
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Mach Number Characteristics

Fri Oct 12, 2012 5:50 pm

"Mach number is used as a speed reference at high altitudes, usually above 26000 ft, because the MN becomes the aircraft's limiting speed in preference to IAS. That is, up to approximately 26000 ft, an aircraft will climb at a constant IAS against an increasing MN, where the MN speed reaches the aircraft's MN limiting speed. Then above 26000 ft the aircraft is flown at a constant MN with a decreasing IAS for an increase in altitude."

From the above i take it that the mach number limiting speed is the mach number speed that you reach when you change from IAS to MN?

I would i also to right in saying when you are climbing at a constant IAS to 26000 ft your TAS would always have to be increasing due to ram air affects and also the density becoming less meaning you would have to fly faster to keep the indicated airspeed constant?

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DashTrash
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RE: Mach Number Characteristics

Fri Oct 12, 2012 6:55 pm

Quoting Novice (Thread starter):
From the above i take it that the mach number limiting speed is the mach number speed that you reach when you change from IAS to MN?

Not exactly sure how to answer that, but in practice you climb at an IAS until you reach your mach number specified in the profile.

Standard for us in my last airplane was 300 KTS above 10K, transition to M.80. If you flew it in VNAV, the aircraft would pitch for 300 until 300 was .8, then it would hold .8 all the way up until level off where it would accelerate closer to Mmo.

Quoting Novice (Thread starter):
I would i also to right in saying when you are climbing at a constant IAS to 26000 ft your TAS would always have to be increasing due to ram air affects and also the density becoming less meaning you would have to fly faster to keep the indicated airspeed constant?

Yep. TAS increases as you climb at a constant indicated airspeed.
 
vikkyvik
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RE: Mach Number Characteristics

Sat Oct 13, 2012 1:00 am

Quoting Novice (Thread starter):
From the above i take it that the mach number limiting speed is the mach number speed that you reach when you change from IAS to MN?

Whatever Mach number you end up climbing at likely won't be a limiting speed (that is, it wont' be Mmo or Md or anything). It might be your best climb Mach number, for efficiency, or climb rate, or whatever.

Quoting Novice (Thread starter):
I would i also to right in saying when you are climbing at a constant IAS to 26000 ft your TAS would always have to be increasing due to ram air affects and also the density becoming less meaning you would have to fly faster to keep the indicated airspeed constant?

Yes, but there is also less drag the higher you go, so it takes less thrust to fly higher. Point being, you don't necessarily have to keep increasing thrust to fly faster as you get higher.
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BMI727
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RE: Mach Number Characteristics

Sat Oct 13, 2012 1:05 am

Quoting vikkyvik (Reply 2):
Yes, but there is also less drag the higher you go, so it takes less thrust to fly higher. Point being, you don't necessarily have to keep increasing thrust to fly faster as you get higher.

Thrust lapse often eats up the drag reduction.
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Mir
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RE: Mach Number Characteristics

Sat Oct 13, 2012 2:26 am

Quoting Novice (Thread starter):
I would i also to right in saying when you are climbing at a constant IAS to 26000 ft your TAS would always have to be increasing due to ram air affects and also the density becoming less meaning you would have to fly faster to keep the indicated airspeed constant?

It's not really a matter of having to fly faster - you just maintain the same IAS (just like you normally would) and your TAS ends up being faster as you climb all on its own.

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vikkyvik
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RE: Mach Number Characteristics

Sat Oct 13, 2012 9:18 am

Quoting BMI727 (Reply 3):
Thrust lapse often eats up the drag reduction.

I was referring to TAS in my post, since that's what he was asking about - so you don't necessarily need to increase thrust to attain higher TAS as you climb.
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Jetlagged
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RE: Mach Number Characteristics

Sun Oct 14, 2012 9:56 pm

Quoting Novice (Thread starter):
From the above i take it that the mach number limiting speed is the mach number speed that you reach when you change from IAS to MN?

The words you quoted in the first post refer to limiting speeds and Mach (Vmo, Mmo) not climb speeds and Mach. As Vikkyvik pointed out, climb speeds aren't limiting speeds, but the changeover from using IAS to Mach is similar, though may happen at a different altitude.

In subsonic aircraft, at lower altitudes the limiting factor is dynamic pressure so IAS is the value that is used. The airframe also has a Mach number limit, but at low levels you reach the limiting airspeed first. As you climb the the speed of sound reduces so the Mach number for a given TAS increases. Eventually you get to an altitude where Mmo and Vmo coincide. Above that altitude Mmo becomes the limit.

Climb speed and Mach number are related in a similar way and change over at the altitude where the climb IAS is the same as the climb Mach.

BTW, Mach number is a ratio, not a speed. So you would never say "Mach Number speed".
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WingedMigrator
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RE: Mach Number Characteristics

Mon Oct 15, 2012 4:42 am

Quoting Jetlagged (Reply 6):
As you climb the the speed of sound reduces so the Mach number for a given TAS increases.

   This is the entire crux of the matter. The speed of sound goes as the square root of temperature, dropping as you climb simply because it gets colder.
 
novice
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RE: Mach Number Characteristics

Tue Oct 16, 2012 5:25 pm

When climbing at a constant mach number why does the IAS decrease? Is it because the air is less dense i.e less air molecules going through the pitot tube?
 
tdscanuck
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RE: Mach Number Characteristics

Tue Oct 16, 2012 6:15 pm

Quoting Novice (Reply 8):

When climbing at a constant mach number why does the IAS decrease?

Speed of sound is a function of only of temperature (at least in normal air). As you climb, the air is getting colder so the speed of sound is dropping. Thus, to fly at constant Mach, you need to slow down (lower TAS). At the same time, you're climbing so the density is dropping so IAS (even at constant TAS) is also dropping. So you've got a double whammy...your TAS is falling and your IAS is dropping relative to TAS.

Tom.
 
novice
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RE: Mach Number Characteristics

Tue Oct 16, 2012 7:51 pm

Quoting tdscanuck (Reply 9):
Thus, to fly at constant Mach, you need to slow down (lower TAS).

How do you lower your TAS when flying at a constant Mach?

and from jet

Quoting Jetlagged (Reply 6):
In subsonic aircraft, at lower altitudes the limiting factor is dynamic pressure so IAS is the value that is used. The airframe also has a Mach number limit,

Is the mach number limited to do with the adverse affects of flying close to the speed of sound?
 
Fabo
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RE: Mach Number Characteristics

Tue Oct 16, 2012 8:15 pm

Quoting Novice (Reply 10):
How do you lower your TAS when flying at a constant Mach?

Physics does it for you. You dont do it yourself, you have a mach indicator and you hold that in place.

Quoting Novice (Reply 10):
Is the mach number limited to do with the adverse affects of flying close to the speed of sound?

You could say that.
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vikkyvik
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RE: Mach Number Characteristics

Tue Oct 16, 2012 10:14 pm

Quoting Novice (Reply 10):
How do you lower your TAS when flying at a constant Mach?

Climb. That's what Tom was talking about. Climb at a constant Mach, and your TAS will decrease.
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georgiaame
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RE: Mach Number Characteristics

Tue Oct 16, 2012 10:38 pm

I love this section, even though as a non engineer and a mathematically challenged individual, I barely have a clue as to what you are talking about. OK, in colloquial English, IAS is indicated air speed. TAS??? Next, my question. All of this stuff is well and good. This past weekend, a total nut job jumped out of a tiny pressurized capsule at 24 miles up trying not to become one with the earth. He went supersonic and lived to tell about it. Not only supersonic, but 839.9MPH for a brief period of time. Can anyone discuss how this topic would apply to this lunatic? In advance, thanks!
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vikkyvik
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RE: Mach Number Characteristics

Tue Oct 16, 2012 10:57 pm

Quoting georgiaame (Reply 13):
TAS???

True AirSpeed. The actual speed of an object through the air.

Quoting georgiaame (Reply 13):
Can anyone discuss how this topic would apply to this lunatic?

Well, he stands a much higher chance of breaking the sound barrier at a higher altitude, since the temperature and therefore the speed of sound are much lower. Also, the air is much thinner, so less drag.

As he descended, his terminal velocity would slow as drag increased, and the speed of sound would increase as temp increases, until he was below the speed of sound.

Now seeing a photo of him with shockwaves emanating from his spacesuit would be awesome!
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