Connect and get the inside scoop on Airline Companies

Welcome to Airline Pilot Forums - Connect and get the inside scoop on Airline Companies

If this is your first visit, be sure to check out the FAQ. Join our community today and start interacting with existing members. Registration is fast, simple and absolutely free.


User Tag List

Post Reply
 
Thread Tools Search this Thread
Old 05-30-2020, 01:55 PM   #1  
Gets Weekends Off
Thread Starter
 
Bahamasflyer's Avatar
 
Joined APC: Sep 2017
Posts: 348
Default What altitude does mach# become meaningless?

Just watched the launch of SpaceX and it got me wondering? What altitude does one have to reach in order for mach # to become meaningless? Is it a sudden thing, or gradual?
Bahamasflyer is offline  
Old 05-30-2020, 02:14 PM   #2  
Gets Weekends Off
 
Myfingershurt's Avatar
 
Joined APC: Jul 2015
Position: MD-88 FO
Posts: 316
Default

Quote:
Originally Posted by Bahamasflyer View Post
Just watched the launch of SpaceX and it got me wondering? What altitude does one have to reach in order for mach # to become meaningless? Is it a sudden thing, or gradual?
I’d say once you leave the atmosphere completely. Sound doesn’t travel in the vacuum of space so there would be no Mach number.
Myfingershurt is offline  
Old 05-30-2020, 02:58 PM   #3  
Gets Weekends Off
 
Excargodog's Avatar
 
Joined APC: Jan 2018
Posts: 4,016
Default

Meaningless in what respect? In terms of using external control surfaces? More a function of air density than actual Mach number. The higher you go the more irrelevant Mach is.

The NF-104 had rocket thrusters because it went high enough the rudder and ailerons ceased to work.

https://youtu.be/TwpJ7fIDmCI

It was used when you went over 75,000 feet IIRC. Without them it was kind of like running out of speed in a rolling uphill scissors only more so. You could move the controls but the aircraft was basically in ballistic flight because the density of the air flowing over the control surfaces was inadequate to provide any meaningful control.

The RCS starts about 45 seconds in to the video.
Excargodog is offline  
Old 05-30-2020, 04:15 PM   #4  
Prime Minister/Moderator
 
rickair7777's Avatar
 
Joined APC: Jan 2006
Position: Engines Turn Or People Swim
Posts: 26,019
Default

Quote:
Originally Posted by Bahamasflyer View Post
Just watched the launch of SpaceX and it got me wondering? What altitude does one have to reach in order for mach # to become meaningless? Is it a sudden thing, or gradual?
Gradual, and it becomes meaningless when the atmosphere becomes so thin as to have negligible affect on vehicle performance.

On re-entry, the shuttle started measuring mach at M 25. Nope, didn't forget the decimal.
rickair7777 is offline  
Old 05-30-2020, 07:45 PM   #5  
Gets Weekends Off
Thread Starter
 
Bahamasflyer's Avatar
 
Joined APC: Sep 2017
Posts: 348
Default

Quote:
Originally Posted by rickair7777 View Post
Gradual, and it becomes meaningless when the atmosphere becomes so thin as to have negligible affect on vehicle performance.

On re-entry, the shuttle started measuring mach at M 25. Nope, didn't forget the decimal.
So does this not happen until reaching the Karman line (300K I think)? Or is it where the practical end is, perhaps at around 100K?

I'd love to know what IAS M25 is during reentry!
Bahamasflyer is offline  
Old 05-30-2020, 08:01 PM   #6  
Gets Weekends Off
 
Excargodog's Avatar
 
Joined APC: Jan 2018
Posts: 4,016
Default

Quote:
Originally Posted by Bahamasflyer View Post
So does this not happen until reaching the Karman line (300K I think)? Or is it where the practical end is, perhaps at around 100K?

I'd love to know what IAS M25 is during reentry!
About 17,500 mph typically for low earth orbit.
Excargodog is offline  
Old 05-31-2020, 02:37 PM   #7  
Gets Weekends Off
Thread Starter
 
Bahamasflyer's Avatar
 
Joined APC: Sep 2017
Posts: 348
Default

Quote:
Originally Posted by Excargodog View Post
About 17,500 mph typically for low earth orbit.

I meant indicated airspeed, not true.
Bahamasflyer is offline  
Old 05-31-2020, 02:55 PM   #8  
Gets Weekends Off
 
Excargodog's Avatar
 
Joined APC: Jan 2018
Posts: 4,016
Default

Quote:
Originally Posted by Bahamasflyer View Post
I meant indicated airspeed, not true.
Calibrated airspeed is about 8500kts at 100,000 feet. That was the closest I could come. Any higher was out of range for the calculator. Barometric pressure at that altitude is down to 0.329 inHg. Indicated airspeed outside the atmosphere would be zero. Nothing coming in the pitot tube.

Excargodog is offline  
Old 05-31-2020, 08:03 PM   #9  
Line Holder
 
Barnstormer's Avatar
 
Joined APC: Aug 2008
Position: MD-11 CA
Posts: 57
Default

Quote:
Originally Posted by Bahamasflyer View Post
Just watched the launch of SpaceX and it got me wondering? What altitude does one have to reach in order for mach # to become meaningless? Is it a sudden thing, or gradual?
Max Q is probably the most import factor.
The max q condition is the point when an aerospace vehicle's atmospheric flight reaches maximum dynamic pressure. This is a significant factor in the design of such vehicles because the aerodynamic structural load on them is proportional to dynamic pressure. This may impose limits on the vehicle's flight envelope.
Barnstormer is offline  
Old 06-03-2020, 02:33 PM   #10  
Gets Weekends Off
 
John Carr's Avatar
 
Joined APC: Jul 2013
Posts: 3,504
Default

Pretty sure once you’ve gone plaid, no more need for a Mach meter.
John Carr is offline  
 
 
 

 
Post Reply
 



Thread Tools Search this Thread
Search this Thread:

Advanced Search


Related Topics
Thread Thread Starter Forum Replies Last Post
Any "Latest & Greatest" about Delta? Guard Dude Delta 199225 07-03-2020 04:08 PM
Eights on Pylon Mistek89 Flight Schools and Training 38 09-23-2014 06:51 AM
Critical Mach Number RJcursed4life Technical 12 10-05-2013 10:33 AM
Crossing Restriction Question cantwin Technical 6 04-28-2012 02:04 AM
Vmo indicated airspeed decrease with altitude r1830 Technical 14 02-22-2010 04:21 PM


All times are GMT -8. The time now is 03:51 PM.