Mach vs. Speed hold switch (my final post)

This is my final question for now...

Display your speed in Mach or IAS, it matters not. It is like MPH vs Kts. Velocity is velocity no matter how you measure it. As an aircraft moves through the air, the air molecules near the aircraft are disturbed and move around the aircraft. If the aircraft passes at a low speed, typically less than 250 mph, the density of the air remains constant. But for higher speeds, some of the energy of the aircraft goes into compressing the air and locally changing the density of the air. This compressibility effect alters the amount of resulting force on the aircraft. The effect becomes more important as speed increases. Near and beyond the speed of sound, about 330 m/s or 760 mph, small disturbances in the flow are transmitted to other locations isentropically or with constant entropy. But a sharp disturbance generates a shock wave that affects both the lift and drag of an aircraft. The ratio of the speed of the aircraft to the speed of sound in the gas determines the magnitude of many of the compressibility effects. Because of the importance of this speed ratio, aerodynamicists have designated it with a special parameter called the Mach number in honor of Ernst Mach, a late 19th century physicist who studied gas dynamics. The Mach number M allows us to define flight regimes in which compressibility effects vary.

You will need a climatologist to tell you why there are areas of differing pressure on this planet…but here are. The inches of pressure vs Millibars of pressure is just another unit of measurement thing. SAE vs Metric.

A Mach number is an expression of the ratio of your aircraft's speed to the speed of sound. The speed of sound varies with altitude.
If the speed of sound at a given altitude is 700 miles per hour (1,126.5 kilometers per hour or 312.9 meters per second) and your speed is 700 miles per hour you are traveling at Mach 1. If your speed is 1.5 times the speed of sound (1,050 miles per hour for this example) you are traveling at Mach 1.5 or, if your speed is 3/4 of the speed of sound (525 miles per hour for this example) your Mach number is 0.75.

Air pressure (barometric pressure or atmospheric pressure) is defined as the force per unit area exerted against a surface by the weight of the air molecules above that surface. Atmospheric pressure varies with weather. The average pressure at sea level is 29.92 inches of mercury or 1013.25 millibars. Air pressure also varies with altitude, becoming less as you fly higher. Weather forecasts and pilots express atmospheric pressure in inches of mercury, meteorologists speak in terms of millibars. They are different ways of expressing the same thing just I measure in inches and feet but a European or Canadian measures in millimeters and meters (a much better way, btw.).
On a bright, sunny day the air pressure will tend to be high or greater than 29.92 inches of mercury (1,013.25 millibars) as shown on a barometer.
On a dull, rainy or stormy day the air pressure will tend to be low or less than 29.92 inches of mercury.
Air pressure also varies with altitude. A barometer must be calibrated so that it shows the sea level pressure equivalent for it's altitude above sea level. An altimeter measures *only* air pressure and, if you know the local barometric pressure it will accurately tell you your altitude. Conversely, and some pilots use it this way, if you know your altitude, as on the surface of a given airport, and set your altimeter to that altitude, it will show you the ambient barometric pressure. In flight, however, you are usually traveling from an area with one barometric pressure into an area with a different pressure. Air Traffic Control (ATC) will tell you what the pressure is for the area in which you are flying - or for the airport where you are about to land - and that will allow you to set your altimeter to accurately indicate your elevation above sea level.

Hg is the chemical symbol for mercury. "Inches of mercury" is how high a column of the liquid metal mercury will rise into a vacuum, as in a mercury barometer, when "pushed" by the ambient air pressure.

I hope this helps. 😀
How come it's your last post?

Pete

Good replies already.

The main reason that you have the option of IAS or Mach is the speed you are flying. For slow speeds, such as those below 250 miles per hour for under 10,000 feet or for landing, it's more important to know the airspeed, because it's a real number, such as where the aircraft stalls. However, for high speeds, the Mach number is much more important, because Mach is how you can expect your aircraft to fly through the air. As you approach the speed of sound, parts of your aircraft start to develope shockwaves because the air over them is actually exceeding the speed of sound, even if the vast majority of the aircraft is not, on some aircraft, this can be quite low. I think the 737 is around .74 Mach. Whether you are going 300 miles per hour in a super dense atmosphere or 800 miles per hour at 100,000 feet(or it may be vice verse, I always have trouble with sound dynamics) the aircraft will still start to buffet at .74 mach. So that way, your air data computers will determine the atmospheric conditions and maintain your mach speed instead of you trying to feel out how fast the aircraft is going and if your passengers are getting a smooth ride.

And lastly, Mach is a ratio of the speed of sound through atmosphere, while a knot is one second of latitude at the equator, a little more than a mile. 1.15 miles to be exact.

OOPS! Careful now, Worries. A knot is one nautical mile per hour. And, just to be sure of this point, one is NEVER supposed to say "Knots per hour"! That would be acceleration but it's beyond the scope of this discussion. A knot is one nautical mile per hour.

A *nautical mile* is one second of lattitude and it doesn't matter if it's at the equator or not. It's the same regardless of where on the earth it is as long as it is *latitude*. Degrees of *longitude* vary, grow smaller in span, as one leaves the equator going either North or South and they shrink to zero as one reaches either pole.

A nautical mile is 1.1515 times longer than a statute mile. A nautical mile is 6080 feet (by international treaty). A statute mile is 5280 feet.

Just do a Google on this stuff.

Respectfully, Pete

Well written Pete 😀

Gee - there's so many intelligent people out there.

Wish I was intelligent

Thanks guys