yooyoo
Posts: 5686
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Knots Vs. Mach Speed

Mon Oct 17, 2005 11:40 pm

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 
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gg190
Posts: 158
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RE: Knots Vs. Mach Speed

Tue Oct 18, 2005 12:39 am

Most airspeed gauges display mach and knots (in IAS) at the same time. Above about 25000 you should use mach.
 
yooyoo
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RE: Knots Vs. Mach Speed

Tue Oct 18, 2005 1:03 am

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 
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goinv
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RE: Knots Vs. Mach Speed

Tue Oct 18, 2005 3:07 am

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.
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yooyoo
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RE: Knots Vs. Mach Speed

Tue Oct 18, 2005 3:36 am

Thank you both for your comments.

Makes sense. Big grin

Andreas  spin 
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airfly
Posts: 319
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RE: Knots Vs. Mach Speed

Tue Oct 18, 2005 4:06 am

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
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Mir
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RE: Knots Vs. Mach Speed

Tue Oct 18, 2005 5:30 am

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
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