1. Junior Member
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## IAS/MACH

Can some one explain the differance between IAS and MACH and there use and how you calculate one to the other
Tony:-wave

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## RE: IAS/MACH

I'll explain what I can. I'm sure some corrections will follow because I'm not expert.

IAS is indicated airspeed. The pressure of air going into the pitot tube is compared to the "static" pressure measured somewhere inside the wall of the airframe. The difference is used to calculate how fast the airplane/wing is hitting the air calculated as airspeed in knots (nautical miles per hour I think).

MACH is the speed of the airplane relative to the speed of sound which is MACH 1. I don't know how this is calculated.

With the ability to climb to high alititude in jets the measure of airspeed soon becomes unreliable because of thinner air. As a jet climbs at a stable indicated airspeed the Mach speed will still increase because the plane has to keep picking up ground (actual) speed to maintain airspeed in thinning air. Around 25000ft a jet can begin to base their speed on a Mach number like M.74 (74% speed of sound) and then the indicated airspeed will start to drop (air thinning until cruise altitude).

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## RE: IAS/MACH

The speed of sound is the rate at which sound travels through the air and varies according to the temperature of the air. At sea level with standard atmospheric conditions (+15'C), the speed of sound is 660 knots. In the stratosphere at about -60'C, the speed of sound drops to 575 knots. In Artic regions where the temperature is -60'C at sea level the speed of sound is 575 knots. It is not a function of altitude.
Mach number is calculated by dividing the airspeed of the airplane by the speed of sound in the atmospheric temperature conditions at the time of flight.
As the message above states, it is more reliable at higher altitudes than IAS.

4. ## RE: IAS/MACH

Would not density altitude have an effect on sound propagation, that is altitude pressure variations as well as OAT?

Rephrasing, wopuld not sound propagation be a function only of air density as affected by altitude and temp?

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## RE: IAS/MACH

>Would not density altitude have an effect on sound
>propagation, that is altitude pressure variations as well as
>OAT?
>
Yes, you are correct, density does have an effect, however, since temperature and density are very much related, temperature is the controlling factor. Cold air is denser because the molecules are moving slowly and are packed tightly together. Warm air, of course, is lighter and the molecules are moving rapidly. Therefore, temperature controls the density which controls the speed of sound travelling through it.
Hope this helps.

Cheers
Glenn

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## RE: IAS/MACH

Hi Ronzie,

The speed of sound is governed purely by air density. The more dense the air (or anything else) the faster sound travels through it. This is because in dense air the molecules are closer together and therefore pass on the sound "shockwave" more rapidly to the next molecule as there is less distance for it to travel.

Various factors affect air density including :

1. Altitude - The air is thinner up high as gravity has less of an affect on it. Down at ground level gravity, and therefore the weight of air above it, squishes it down to make it more dense. At sea level it is at it's most dense as this is the lowest point (generally speaking - You can't fly an aircraft down a pot-hole :) ).

2. Temperature. Cold air is more dense. Hot air is less dense due to the more "energetic" rapidly moving molecules making their bonds to the next atom floppier. This is why you get more power from a turbo-charged engine when you put a large intercooler on it to cool down the air going into the engine. More dense air in the cylinders is like more air in the cylinders which makes more Wooosh :)

3. Humidity. Wet air is more dense than dry air. Having water in the air means it is occupying the same space making the air more dense. This is also why putting water injection on an engine gives it more power. It cools down the air too giving a double-wammy effect which effectively increases the compression ratio in an engine which gives more power.

The speed of sound also varies between gasses, and other substances for the same reasons. The speed of sound through metal will, for example, be much faster than in air or something less dense (than metal) like rubber

Here endeth your A Level Physics lesson :)

7. Ian
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## RE: IAS/MACH

This is so much bollocks. The speed of sound only depends on the air temperature. Please don't try to teach A-level Physics without a Physics degree.

http://www.lerc.nasa.gov/WWW/K-12/airplane/sound.html

Ian

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## RE: IAS/MACH

>This is so much bollocks.
>The speed of sound only depends on
>the air temperature. Please don't
>try to teach A-level
>Physics without a Physics degree.
>
>
>http://www.lerc.nasa.gov/WWW/K-12/airplane/sound.html

>Ian

Hmmm...you may want to retract your statement before the wolves chew you up ;-)

Density ( pressure ) is a very big part of the equation....it says so right in your link.

I'm no physics expert, but I can tell you that sound travels faster through 70o water than it does through 70o air....why would that be if the medium through which it travels has no bearing, rather only temperature ? You can view air at 14.7 PSI and air at 7 PSI two separate mediums. Sound will certainly travel faster through 70o air at 14.7 PSI than it will at 7 PSI at the same temperature....yes ?

Regards,

Bob

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## RE: IAS/MACH

Actually Ian this is not bollocks. It is fact.

If you had bothered to read that entire document before flaming me you would have noticed that air pressure (my explanation of being squished by gravity) and density were also a huge part of the equasion!

Let me know when you get your foot out of your mouth :) :)

10. Ian
Member
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Mar 2005
Posts
110

## RE: IAS/MACH

I will certainly let you know.

This is a FlightSim board and the sim only flies through air. In FS the molecular weight is a constant only letting the air temperature have an effect on the speed of sound. Yes the speed of sound in water is about 1500 m/sec and is also effected by temperature, but it is not a gas and it not relevent to flight simulation. Have you read the link?

I will withdraw the word bollocks but you need to read some books on aerodynamics.

"The speed of sound depends on the state of the gas--more specifically, the square root of the temperature of the gas. The speed of sound (a) is equal to the square root of the ratio of specific heats (g) times the gas constant (R) times the absolute temperature (T). "

a = sqrt(g * R * T)
g and R are both constants as this equation.

If YOU read it YOU would see g is NOT 32.174 ft/sec2 but Cp/Cv = gamma which is 1.4 in air.

Ian

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