Simviation Forum - AC Speed

Aug 31^st, 2004 at 10:12pm

richardd43 Offline
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The Learjet and a few other aircraft show a max speed of 330 KIAS or .81 Mach which are not the same.

The Learning Center info for the Learjet gives it a cruise speed of 464 KIAS.

I changed the aircraft CFG file to allow the higher speeds.

I guess my question is why are the aircraft configured to go slower than their actual speeds?

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Reply #1 - Aug 31^st, 2004 at 10:18pm

alrot Offline
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Richadd i thing you got a mistaken information from that learning center i even saw with my own eyes the top of the speed as b737 is the same as the real one there been a lot of disscusion about this issue but belive me
both speed mach lear fs9 lear real life i sure of that perhaps it just another model?
cheers

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Reply #2 - Aug 31^st, 2004 at 10:34pm

Skittles Offline
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Also, Your MACH speed at low altitudes will yield different KIAS than at high altitude. I cannot reach MACH .81 without the overspeed warning until I get over FL20.

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Reply #3 - Aug 31^st, 2004 at 10:44pm

Nexus Offline
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How they compare speeds are a mystery sometimes I agree.

464 KIAS at cruise altitude is virtuallly IMPOSSIBLE.
So I assume that number is TAS, and not KIAS.
But under which conditions and altitude?...beats me. That's why you should always take those numbers with a grain of salt.
You also have to remember that the speed of sound (mach) decreases as the air gets tinner. So the speed of sound is much slower at 40.000ft than at ground level, for example.

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Reply #4 - Aug 31^st, 2004 at 11:36pm

Skittles Offline
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Quote:

How they compare speeds are a mystery sometimes I agree.

464 KIAS at cruise altitude is virtuallly IMPOSSIBLE.
So I assume that number is TAS, and not KIAS.
But under which conditions and altitude?...beats me. That's why you should always take those numbers with a grain of salt.
You also have to remember that the speed of sound (mach) decreases as the air gets tinner. So the speed of sound is much slower at 40.000ft than at ground level, for example.

I find I can push 500-520 at FL32. That's from looking at GPS which show Speed over Ground or close to TAS. IAS still reads about 300-330. It all does get confusing.

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They both will work perfectly, until you open windows.

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Reply #5 - Sep 1^st, 2004 at 8:48am

Mr.Ray_Ban Offline
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but it´s logical, or not? At FL300 the air is so thin that if you fly 500 Kias OVER GROUND there´s less air to stream through you pitot, and you get different speeds shown on your display. At FL350 you can sometimes get a Overspeed warning at 250 knots, but over ground your speed is 500 knots anyway....that depends on the actual winds and air pressure und such stuff, too.

that was a very informative post/ answer by me, Mr. Ray_Ban, and i send greetz with it, of course!!!

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Reply #6 - Sep 1^st, 2004 at 9:39am

Chris_F Offline
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Speed over ground is a poor measure of aircraft flight performance as it can be significantly influenced by wind. Aircraft max speeds (and min speeds) are always given in air speed, and it's important to always monitor the plane's airspeed and not its speed over ground (there's a switch for this in the Realism settings).

Mach number does indeed change with pressure (and thus altitude) and temperature. So Mach will not always line up with any specific number.

Flight envelope (max and min airspeeds) will always change with air pressure (and thus altitude). More pressure (more air) will mean lower max speeds and higher min speeds. This is because the air exerts more force on the aircraft. This force translates to lift, drag, and structural stress significantly chaning the aircraft's performance.

Pitot tubes measure velocity by the difference in air pressure between the ambient pressure and the stagnation pressure. They should read accurately at all altitudes. Mach indicators should also self correct for air pressure and temperature.

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