Mach confusion!

n57flyguy · March 2nd, 2006, 10:56

I was trying to look up a reasonable definition of mach, the speed. Ok I know it defers with barometric presure like any V speed but how fast is mach in general? How is it determaned? sorry if this is confusing but If you guys could help me out, I'll be more specific. Thanks.

Chris_Ford · March 2nd, 2006, 11:11

Mach is your speed relative to the speed of sound given your current conditions, at standard temp/pressure/sea level, it's 761.2 MPH (I think it's around 660 knots, but I'm not 100%). It differs with temperature and pressure changes (and altitude, but that just changes those two variables).

For example, Mach .78 might be cruising speed for an airliner (mach .92 if you're NJA Capt

) at altitude... This is 78/100s of the speed of sound at the given conditions. This means that the airspeed indicator might be indicating 250 kts, but really at such heights, indicated airspeed is kinda sorta worthless, which is why they use the mach speed.

Hope that made a little more sense, although I will point out that I have never flown anything (except a sim) that uses Mach at altitude, so I could be off on the operational aspects of this post...

Rugby51 · March 2nd, 2006, 11:57

The speed of sound depends solely on temperature. As temperature increases, the speed of sound increases.

As Chris mentioned, Mach is then the ratio of aircraft speed to the speed of sound.

B767Driver · March 2nd, 2006, 13:28

I think this is probably a Commercial or ATP written test question.

What happens if while climbing at a constant IAS, the mach number remains the same? A temperature inversion exists. With a standard lapse rate the mach number should increase while climbing at a constant IAS.

zerouniform · March 2nd, 2006, 13:39

Quote:

Originally Posted by B767Driver

I think this is probably a Commercial or ATP written test question.

What happens if while climbing at a constant IAS, the mach number remains the same? A temperature inversion exists. With a standard lapse rate the mach number should increase while climbing at a constant IAS.

MN = TAS/LSS (Local Speed of Sound)

Local Speed of Sound depends on temperature as stated above, so as you increase altitude (normally decreasing temperature) the LSS will decrease as well.

If climbing at a constant IAS, the MN increases because the TAS will increase (air density is less) and your LSS is decreasing (temperature decreases). At higher altitudes (usually above FL 260) jets will consider MN limiting due to the aforementioned and structural Mach limitations.

B767Driver · March 2nd, 2006, 16:20

Ah yes...I should've said climbing at a constant TAS...good catch.

n57flyguy · March 2nd, 2006, 17:16

I see but still confused. Say Im in a Gv going .88 mach. So that is 88% of the speed of sound? How fast is the speed of sound in average conditions above FL260? Maybe Im over complecating things but Im confused.

B767Driver · March 2nd, 2006, 17:34

Quote:

Originally Posted by n57flyguy

I see but still confused. Say Im in a Gv going .88 mach. So that is 88% of the speed of sound? How fast is the speed of sound in average conditions above FL260? Maybe Im over complecating things but Im confused.

google speed of sound vs temperature....I'm sure you will find a calculator to illustrate the variances.

TonyC · March 2nd, 2006, 17:58

Quote:

Originally Posted by n57flyguy

I see but still confused. Say Im in a Gv going .88 mach. So that is 88% of the speed of sound?

Yes.

Quote:

Originally Posted by n57flyguy

How fast is the speed of sound in average conditions above FL260? Maybe Im over complecating things but Im confused.

There is no average condition above FL260. Yes, you are over-complicating things. The speed of sound varies with Temperature. Google "speed of sound" and click on the first link, and you'll find that the speed of sound in air on earth is approximately (331.4 + .06 T) meters per second, where T is the Temperature in celsius. A rudimentary comprehension of math and a casual observation of the formula will lead one to understand that warmer temperatures will produce higher speeds, and cooler temperatures will provide slower speeds.

Furthermore, a rudimentary understanding of the atmosphere, specifically a knowledge that the temperature at higher altitudes is lower than at lower altitudes, will lead to the understanding that the speed of sound generally decreases as altitude increases.

Keep in mind, the speeds discussed so far have no relationship, and are therefore not dependent upon, airplanes or their methods of measuring speeds. In other words, these are true speeds, not indicated or equivalent or... or anything. They're "True Airpseeds."

Feel free to plug a temperature into the "calculator" you'll find on that link. For example, try -37C as a temperature (typical for FL260, perhaps?) and you'll see that the Speed of Sound at that temperature is 315.539999 m/s or 706.809599 mph. (Type in a temperature and press TAB) Compare that with +15C, where the speed is 762.832 mph.

.

n57flyguy · March 3rd, 2006, 06:40

Okay, I knew I was thinking to hard! I'll look it up and post anymore questions I have, Thanks again guys.

March 2nd, 2006, 10:56	#1
n57flyguy Old Skool Join Date: Jan 2006 Location: Pennsylvania Posts: 3,769	Mach confusion! I was trying to look up a reasonable definition of mach, the speed. Ok I know it defers with barometric presure like any V speed but how fast is mach in general? How is it determaned? sorry if this is confusing but If you guys could help me out, I'll be more specific. Thanks. __________________ -Paul It ain't always 65 and sunny

March 2nd, 2006, 11:57	#3
Rugby51 Junior Member Join Date: Sep 2003 Location: The fix formerly known as CHECO Posts: 67	The speed of sound depends solely on temperature. As temperature increases, the speed of sound increases. As Chris mentioned, Mach is then the ratio of aircraft speed to the speed of sound. __________________ CFI-MEI-CFII

March 2nd, 2006, 17:16	#7
n57flyguy Old Skool Join Date: Jan 2006 Location: Pennsylvania Posts: 3,769	I see but still confused. Say Im in a Gv going .88 mach. So that is 88% of the speed of sound? How fast is the speed of sound in average conditions above FL260? Maybe Im over complecating things but Im confused. __________________ -Paul It ain't always 65 and sunny

March 3rd, 2006, 06:40	#10
n57flyguy Old Skool Join Date: Jan 2006 Location: Pennsylvania Posts: 3,769	Okay, I knew I was thinking to hard! I'll look it up and post anymore questions I have, Thanks again guys. __________________ -Paul It ain't always 65 and sunny

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March 2nd, 2006, 11:11	#2
Chris_Ford Banned Join Date: Jul 2004 Location: Phoenix, AZ Posts: 7,329	Mach is your speed relative to the speed of sound given your current conditions, at standard temp/pressure/sea level, it's 761.2 MPH (I think it's around 660 knots, but I'm not 100%). It differs with temperature and pressure changes (and altitude, but that just changes those two variables). For example, Mach .78 might be cruising speed for an airliner (mach .92 if you're NJA Capt ) at altitude... This is 78/100s of the speed of sound at the given conditions. This means that the airspeed indicator might be indicating 250 kts, but really at such heights, indicated airspeed is kinda sorta worthless, which is why they use the mach speed. Hope that made a little more sense, although I will point out that I have never flown anything (except a sim) that uses Mach at altitude, so I could be off on the operational aspects of this post...

March 2nd, 2006, 13:28	#4
B767Driver Old Skool Join Date: Sep 2003 Posts: 1,883	I think this is probably a Commercial or ATP written test question. What happens if while climbing at a constant IAS, the mach number remains the same? A temperature inversion exists. With a standard lapse rate the mach number should increase while climbing at a constant IAS.

March 2nd, 2006, 16:20	#6
B767Driver Old Skool Join Date: Sep 2003 Posts: 1,883	Ah yes...I should've said climbing at a constant TAS...good catch.