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Knots Vs. Mach Speed  
User currently offlineYooYoo From Canada, joined Nov 2003, 6057 posts, RR: 50
Posted (9 years 2 months 6 days 21 hours ago) and read 7172 times:

Why is there a "switch" on the airspeed indicator to allow a Mach Speed and a speed in Knots?.

When do i switch?...is it necessary?.....or a preference thing?

Somebody, please educate me Big grin


Andreas  spin 


I am so smart, i am so smart... S-M-R-T... i mean S-M-A-R-T
6 replies: All unread, jump to last
 
User currently offlineGg190 From United Kingdom, joined Jan 2005, 160 posts, RR: 0
Reply 1, posted (9 years 2 months 6 days 20 hours ago) and read 7162 times:

Most airspeed gauges display mach and knots (in IAS) at the same time. Above about 25000 you should use mach.

User currently offlineYooYoo From Canada, joined Nov 2003, 6057 posts, RR: 50
Reply 2, posted (9 years 2 months 6 days 20 hours ago) and read 7159 times:

Quoting Gg190 (Reply 1):
Above about 25000 you should use mach.

Why? Big grin

Also, my initial starter is wrong, what i meant to say was on the Auto Pilot "IAS" the speed unit of measure is Knots or Mach.

Andreas  spin 



I am so smart, i am so smart... S-M-R-T... i mean S-M-A-R-T
User currently offlineGoinv From United Kingdom, joined Jan 2005, 264 posts, RR: 2
Reply 3, posted (9 years 2 months 6 days 18 hours ago) and read 7147 times:

This is the way that I would explain it.....

At ground level the air is normal - so when travelling at 100knts there is "normal" air travelling over the wings and you are travelling at 100knts

At 30000 the air is thinner - therefore there are fewer air molecules passing over/under the wings.

So....even though you might be covering the ground at (let's say) 500knts the amount molecules going past the wing is equal to 300knts at ground level.

Therefore your airspeed is bearing little or no relation to how fast you are actually travelling. This is why Mach is used above ( as Gg190 says )about 25000ft.

The Autopilot panel has the switch so you can choose between a sensible indication of airpseed at lower levels and Mach at higher altitudes.

I know my explanation may not be exact according to the laws of physics - but it helps me to understand.



Be who you are, The world was made to measure for your smile. So Smile.
User currently offlineYooYoo From Canada, joined Nov 2003, 6057 posts, RR: 50
Reply 4, posted (9 years 2 months 6 days 17 hours ago) and read 7142 times:

Thank you both for your comments.

Makes sense. Big grin

Andreas  spin 



I am so smart, i am so smart... S-M-R-T... i mean S-M-A-R-T
User currently offlineAirfly From United Kingdom, joined Mar 2005, 322 posts, RR: 1
Reply 5, posted (9 years 2 months 6 days 17 hours ago) and read 7139 times:

Hello there,

Mach number is a quantity that defines how quickly a vehicle travels with respect to the speed of sound. The Mach number (M) is simply the ratio of the vehicle's velocity (V) divided by the speed of sound at that altitude (a).

Therefore: Mach is the division of Velocity and Altitude.

So above 25000ft use Mach as said before...
I usually do that so the velocity precision is better as the air is much thinner...

If you fly with Knots (IAS) your actual speed would be much faster...

Cheers
Lucca M.



L - I - B - E - R - A - L
User currently offlineMir From United States of America, joined Jan 2004, 21865 posts, RR: 55
Reply 6, posted (9 years 2 months 6 days 15 hours ago) and read 7146 times:

I just use my climb speed (300kts) until I get to the mach number I want to cruise at, and then maintain that throughout the rest of the climb (when holding a certain mach number and climbing, IAS will decrease, since the speed of sound decreases with altitude).

-Mir



7 billion, one nation, imagination...it's a beautiful day
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