yooyoo
Topic Author
Posts: 5684
Joined: Wed Nov 26, 2003 5:01 am

### Knots Vs. Mach Speed

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?

Andreas
I am so smart, i am so smart... S-M-R-T... i mean S-M-A-R-T

gg190
Posts: 158
Joined: Fri Jan 28, 2005 7:25 am

### RE: Knots Vs. Mach Speed

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

yooyoo
Topic Author
Posts: 5684
Joined: Wed Nov 26, 2003 5:01 am

### RE: Knots Vs. Mach Speed

Why?

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
I am so smart, i am so smart... S-M-R-T... i mean S-M-A-R-T

goinv
Posts: 255
Joined: Wed Jan 05, 2005 11:16 pm

### RE: Knots Vs. Mach Speed

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.

yooyoo
Topic Author
Posts: 5684
Joined: Wed Nov 26, 2003 5:01 am

### RE: Knots Vs. Mach Speed

Makes sense.

Andreas
I am so smart, i am so smart... S-M-R-T... i mean S-M-A-R-T

airfly
Posts: 319
Joined: Wed Mar 09, 2005 5:43 am

### RE: Knots Vs. Mach Speed

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

Mir
Posts: 19383
Joined: Mon Jan 05, 2004 3:55 am

### RE: Knots Vs. Mach Speed

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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