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+'ve ROC
16th May 2005, 14:31
Could somebody 'in the know' explain the difference between IAS, GS and Mach?

Why does aviation use Kts instead of mph / kmph?

Im I wrong in assuming that Mach x is dependant on air pressure, altitude etc?

Cheers

PIGDOG
16th May 2005, 15:20
IAS is your indicated air speed. This is purely the speed the air is travelling past your aircraft, relatively speaking. (even sitting on the ground, with a head wind of, say, 50 knots, the IAS will be 50.)

GS is you actual speed over the ground. For example, if you're travelling at 60 knots into a headwind of 60 knots, your ground speed will be a big fat 0. You'll be hovering.

Mach speed is very important. This is your speed relative to the speed of sound. At mach 0.5 you are traveling at half the speed of sound. At mach 1 you are travelling exactly at the speed of sound. (although the airflow at certain parts may be travelling faster or slower around you, depending on curves, incipient shock waves and all that.)

Mach speed is dependent on pressure and altitude.
Mach is also varied by temp. Higher temp means higher speed of sound.

Knots are an old nautical term. Not sure why they're still used, but a nautical mile is one minute of longditude. A very clever thing to derive a unit of distance from.

I hope this helps. Maybe others can put it clearer, or more correct. :)

ps I was assuming GS was ground speed, not Glide Slope!!
:D

Rainboe
16th May 2005, 16:03
IAS is the displayed speed as the one you really want- TAS- you don't have an indicator for. A jet may indicate 290kts IAS, but be flying at 480kts TAS which is its 'correct' speed. Knots are frightfully brilliant for navigation purposes- when you are navigating using dividers and chart, you just measure off your distance on the chart (always in nms), and the speed of the aeroplane, and that's that. Hopefully we will convert Europe first to Imperial units (after all Napoleon did lose didn't he?), then stop everyone using those absurd Statute miles. Eventually we can make it a capital offence to do anything in kilometers- surely a totally useless unit- and then we can start working on getting the world converted to pounds and ounces.

+'ve ROC
16th May 2005, 17:50
TAS = True Air Speed, then?

One more thing - Why does one measure high speeds in Mach rather than Kts? When refering to Concorde or a Mil Jet, it always seems to be in Mach?
Surely, as Mach is variable according to altitude, temperature etc, it would be more accurate to use a speed in Kts?

Just curious

Cheers again

Ropey Pilot
16th May 2005, 18:21
As you approach the speed of sound you get all sorts of undesireable effects (shockwaves, mach tuch etc). Airflow is not constant over all parts of the a/c so when you have an indicated mach no. of ,say, 0.8 parts of the ac will be experiencing speeds approaching mach 1 (almost certainly the wings as the shape is designed to speed up airflow).

This will therefore limit the forward speed (without designing it to be supersonic) and you generally cruise at this airframe limiting factor at higher levels.

As you increse alt at a constant mach no your IAS will reduce - ie at FL370 you may find that your mach limit of 0.8 only gives you 250kts, while at FL280 you might manage over 300kts at the same mach number.

This leads to a pay off between fuel burn and speed - the higher you are the less fuel you burn for a given time. However, at lower levels you are travelling faster at a higher burn rate for less time - possibly burning less fuel! So your optimum Flight Level depends on the weather of the day and you will find people requesting different levels to those flight planned (if it is quiet).

The short answer to your last question is that it is accurate to use either it is just of more use to those you mentioned at the time to use mach. (It would also be of little use for a small ac to use mach no as they ARE usually more worried about their speed relative to the ground say for time and distance nav. also at the levels they are flying at IAS will always be more limiting to their airframe than the v. low mach number)

chiglet
16th May 2005, 18:49
"Knots"
In th'old days a "bare chested seaman" [oo-eerr Matron] stood at the edge of the ship/boat/whatever. The Captain/navigatot started his "stopwatch" [eggtimer] and said "NOW". Whereupon our chappie let loose a "LOG" [a piece of wood] attatche to him by a bit of string....However along this bit of string there were tied KNOTS. At the end of the timed run, our hero woud say "Seven and a half knots, zurr" or somesuch. Hence Knots.. [K] Nautical Miles per Hour. :ok:
TAS, True AirSpeed
IAS, Indicated A/S
CAS, Corrected A/S
Mach, a percentage of the "speed of sound" Holds true no matter what Height/Ground/Air Speed you have.
eg M1 [Mach1] at "Sea Level" is [approx] 760 MPH
At 30,00ft is [approx] 630 MPH
Concordes' M2.2 [1440mph] was an "Airspeed" of [approx] 750MPH
I stand to be corrected of course [The USSR etc use KPH and Metres for speed and Altitude]
watp,iktch