I have a puzzling question.

[A/SGT.Mav316]

Is there something wrong with the polarities in my flight sim.

If you fly high shouldn't your aircraft fly faster because the air is thinner at high altitudes.

If you are flying low shouldn't your aircraft struggle to acheive high speeds because of the thicker air and turbulance at low levels.

I mean you need full thrust to get your plane off the ground, but at higher altitudes your aircraft struggles to gain higher speeds.

My example is I flew the SST Concorde to an Altitude of 35,000 feet and noticed the fastest I could get the aircraft to go was 756 kias.  :(

But when I flew at 10,000 feet the aircraft was flying at about 1,218 Kias and faster at high speed.  ???

Can someone please explain this to me?

[Saitek]

If I'm correct you are going the same speed, but it appears differently because of the air on the instruments.

[Nav]

'Indicated Air Speed' is just that - the speed indicated by the air pressure in the pitot tube. Because the air is thinner at high altitudes the indicated airspeed will be much lower than the actual airspeed.

So, for example, '275 knots indicated' can be over 550 knots 'true airspeed' at cruising height.

'Mach. speed,' on the other hand, is a computed figure which adjusts the indicated speed for air pressure. Thus it is your true airspeed (but expressed as a percentage of the speed of sound, e.g. 'Mach 0.82'). I think the Concorde used to travel at around 1.5 Mach. or about 1,200 mph.

Best to switch to Mach. speed above say 20,000 feet. Or I think that

[Papa9571]

I know the problem you are facing and until it was explained to me I didn't understand it either.

Mach is an expression of the speed of sound. Don't quote me but at sea level it is around 650 kts and gets progressivly slower the higher you go. ( I know I will get corrected on that )

The Concorde is capable of Mach 2.04, or around 1250 kts and can achieve that at higher alitiudes but not closer to the ground. Why is that?

Well it all has to do with the density of the air you are flying through. The higher you go the less dense the air becomes. That is why mountian climbers need oxygen above a certian level.

The aircraft measures its airspeed by a probe that sticks out into the airflow. The higher you go the less air there is for the probe to measure, and the less air is flowing under and over your wings generating lift. To maintain that lift the aircraft must go faster. So how do you know you are going faster? With the Concorde there are three ways to tell you sre going faster.

First look at the Mach number. If you have your auto throttle set at 300 KIAS and are climbing you will see that number increase. The higher you go the larger that number will become until you reach approxamatly FL500 when it should be around 1.9.  Now using the information on how fast the speed of sound is multiply the mach number by the sea level speed of sound and bingo...you get a number close to 1100. That means that even though you are flying at 300 KIAS you are actually travelling over 1100 kts as measured on the ground.
Second.. look at the speed number on the top right of you airspeed gauge. It should read what your ground speed actually is.
And third... check the GPS and see how fast you are going.

Just a couple of other points. You will notice the orange striped needle in your airspeed gauge move. That needle tells you the fastest you can safely fly the aircraft in KIAS. Anything over that and you will get an overspeed warning. Ignore that warning long enough and nasty things will start to happen. It starts around 350 on the ground and will climb to over 500 as you gain altitude. It will then begin to drop again, setteling back to around 350KIAS at FL 500.

And when you are at your cruising altitude if your aircraft starts rocking you are getting close to a stall. Either increase you airspeed or seek a lower cruising altitude. A drop of 4000 ft should do it.

Sorry to be so long winded but I hope this helps.

[BAW0343]

That my friend was a great explination.  All of what these people have said is correct, I find it easyest just to look at the GPS (if avalable) it will tell you your ground speed is a simple manor  In the bottom left corner you see a set of numbers  it will look like "300 GS" the number of cource will be diffrent most of the time  so you can do the math for more realism or you can look at the gps  ;)

[A/SGT.Mav316]

Thank you for the information guys it was very much useful! From what I understand now the reading KIAS is not to be interpeted as Knots.

you said 300 Kias is approx. 1,100 knots. For me to understand this I refer to 1mb is equal to 1000kb.

Even so when I am flying near the deck the aircraft is flying at 1,250 KIAS which translates to warp speed. But at FL350 i get a 756 KIAS reading which is much slower then the ground travel reading.

So if it is as real as it gets these readings are backwards. I wish Microsoft would just keep the reading in knots instead of Kias. Then it would be much easier to understand

[Craig.]

No, Because as you get higher you travel further quicker. I cant remember the exact equation, someone will know it but for every so many thousand feet up you travel further across the ground, So for every mile you travel in the air your traveling for instance 4 across the ground. Or something like that.
MS have gotten the speed readings exactly correct. You'll notice in most info sheets that a jet fighter for instance will be given a speed of Mach 2 at 35,000 feet. Thats because it couldn't travel that fast at lower altitudes.

[Papa9571]

No is is not backwards. Knots is a nautical term that avaiation borrowed. It is a measure of speed.

KIAS is the measurement of how fast, in knots, you are moving through the air,  not how fast you are moving on the ground. If you are traveling at 300 KIAS ( Knots   Indicated Air Speed ) and are close to the ground you are actually moving at 300 knots. That would be your ground speed. In your case you said you flew over the carrier at  1250 KIAS. At sea level KIAS and ground speed are close to the same if you have no winds.

Try this simple test. Turn on your GPS and look at the number in the lower left hand corner. It is labeled GS and is a measure, in knots, of how fast you are travelling over the ground. It gets its speed measurements fron the GPS satellites and should read close to what you airspeed is if you are at sea level.  If you are flying at 1250 KIAS that number should be close to 1250. Now begin a slow climb and watch that number and your airspeed. If you are flying with full afterburners the gps number should stay close to the same and you airspeed number should start to fall. When you reach 35,000 feet your airspeed will have fallen to 756 KIAS but your ground speed should have stayed close to the same.

Remember that air probe I talked about and how it measures the speed of the aircraft through the air?  At 35,000 feet the air is thinner than it is at sea level so there is less air to measure your speed.  That means even though you are still going as fast as you were when you were close to the ground there is less air to measure your speed. That causes your airspeed reading to fall even though you are still flying as fast as you were when you were close to the ground.

I hope this helps. It took me a while before I got it and several people were patient and kept at it until I finally got it.

Dont feel stupid by asking the question. It is the smart ones that ask questions. That shows thay are still willing to learn.  

Dave

[Jakemaster]

Ground speed more truly reflects your actual speed, even at a fairly low altitude.  This morning in my DC-3 I was flying at 6500ft with a KIAS of 140.  My GS was around 160

[Craig.]

or just try true airspeed

[Brett_Henderson]

So if it is as real as it gets these readings are backwards. I wish Microsoft would just keep the reading in knots instead of Kias. Then it would be much easier to understand

There are quite a few thing that ARE as real as it gets about the sim. The relationship between indicated and true airspeed at different altitudes is one of the MOST realistic things here.  SURE.. it would be easier to understand your aispeed indicator if it always read true airspeed, but that's NOT what happens in real life.. real planes.

There is a setting under realism to have the airspeed indicated true, but I can't imagine why anybody would wanna "cheat" like that.. It defeats the whole point in learning real pilot stuff..

[elite marksman]

I think that true airspeed would actually make it harder to fly, not that I use it. Say your flying a 773 at FL390 at 300 KIAS you should be around .8Mach. Now if you set it to true airspeed, it would be reading much higher, probably between 500 and 600kts. If you lose an engine but dont decend for some reason, your airspeed will drop considerably, probably close to or below stall speed for a clean 777. If I'm in KIAS, and I see my airspeed indicator get near 200 with no flaps I'm going to do something to avoid a stall. If I'm using true airspeed though, there is no easy way to tell how fast the air is actually moving over my wings, and therefore I won't know that I just stalled out until I start flying like a brick.

[A/SGT.Mav316]

Ok I fully understand now. KIAS is just a measure of airspeed and not to be taken in term as knots which is ground speed. I will take the concorde up again and fly to 35,000 feet again and look at the GPS to see if the ground speed remained close to 1250 KIAS.

Thanks for pointing this out guys.

[A/SGT.Mav316]

Guys you have been 100% correct. I Flew the aircraft at 10,000 feet got a reading of 1,287 KIAS and 1,134 knots from the GPS. Then I took the concorde up to 40,000 feet and it registered 626 KIAS and 1,234 knots.

I understand how the guage works now. The hud display is reading true airspeed not ground speed. Thank for pointing that out guys I was confused there! It is as real as it gets there is nothing wrong with the readings!

Thanks again, I learn something new everyday!  ;D

[Nav]

I flew the SST Concorde to an Altitude of 35,000 feet and noticed the fastest I could get the aircraft to go was 756 kias.  But when I flew at 10,000 feet the aircraft was flying at about 1,218 Kias and faster at high speed.

Oh dear, A/SGT.  :)

'KIAS' just stands for 'Knots Indicated Air Speed'. A 'knot' is a nautical mile, 2,000 yards.

But the airspeed indicator simply reads LOWER in the thinner air high up.  So 1,218 knots ('indicated') at 35,000 and 756 knots ('indicated') at 10,000 are probably the SAME speed; it's just that the 'clock' reads lower as the air pressure drops at height.

The shortest way to put it is that mere indicated airspeeds in knots are useless at height. Best if you did what everyone else does and switch to (computed) Mach. speed above 20,000 - it only takes a couple of minutes to get 'acclimatised' and start thinking in mach numbers instead of knots. Coming back down, switch back to knots as you descend through 20,000.