Real or Indicated air speed?

[spilok 1,132]

Which should I use...and why is there even a choice?I'd really like to know the reasoning for each choice. Stan

[michal 44]

>Which should I use...and why is there even a choice?>I'd really like to know the reasoning for each choice.> Stan,You should never use *real* (or otherwise called TRUE). This choice doesn't even exist on real aircraft and should not have been included in FS. You use indicated speed because aircraft aerodynamic performance (flaps, gear, etc.) is tied to the indicated speed. In some serious products (say PIC767) having set it to *real* can even lead to harmful side effects.Michael J.http://www.reality-xp.com/community/nr/rsc/rxp-higher.jpg

[ronzie 21]

In flight, IAS (indicated air speed), is used to fly the aircraft because as another poster mentioned, the aircraft dynamics are dependent on air density which varies with altitude, temperature at altitude, and pressure.Real or true airspeed is used in combination with the directional wind component for navigational purposes to calculate ground speed. This is used to estimate time to various waypoints for flight planning including fuel consumption. In marketing literature, the cruise specifications are most often given in true airspeed.Your IAS will decrease for a constant true airspeed as density altitude (altitude compensated for density variation caused by the thinning effects of increasing temperature) increases. As an example, using GPS ground speed or DME, fly at a couple of thousand feet noting your ground speed and IAS. Climb several thousand feet higher, level off, and attempt to reach the same ground speed. Note your lower reading on IAS.Stalls and other aerodynamic specs are always stated in terms of IAS. A single IAS speed represents the lift, thrust, and drag element effects at calibrated air density regardless of actual temperature and altitude.Some speed indicators did have an adjustable second scale that you would have to dial-in a temperature/altitude correction so it would read true air speed. There are also hand-held devices that perform this.Generally ground speed is calculated by timing travel between known points which takes any head or tailwind components into consideration. This will change as wind or direction of flight changes, but it will still help you to estimate your arrival at your next checkpoint if needed and allows to correct estimated times on your flight plan. If you use GPS your ground speed will be more accurate.When using DME, remember that is the distance to the VORTAC. If you are not flying directly to or from the VOR, than it will not be an accurate rendering of ground speed.Now that's a heck of a lot more than you wanted to know, but this question frequently gets asked so I thought to espouse some theory.

[spilok 1,132]

Thanks for the real answers. I'll never use "REAL" again.Unbelievable. I still don't believe there's a choice! Stan

[Guest allblack]

Great response, but I must admit I'm still confused.I posted a message here about a month ago, and people tried to explain but maybe I'm just thick!!I hear repeatedly that you should use IAS and not TAS, but this to me doesn't sound ideal. It seems to me that when I'm simming the most relevant "airpseed" indicator is the ground speed in the GPS.I'll use my same example:Let's say the approach speed for a 737 4 miles out is 132kts IAS. I've got 132kts showing on the airspeed dial, but I have (for arguments sake) a 40kt tailwind. Will I not therefore hit the runway doing 172kts, which is way too fast?Shouldn't my IAS then be 92kts??The opposite would apply for a head-wind, I would have to increase my IAS by 40kts to counter the effects of the head-wind??Therefore it is the GROUNDSPEED that would have to be 132kts, not the IAS.No doubt I'm missing something hugely obvious, but as yet I have no idea what it is.....CheersAllblack

[Guest wathomas777]

Indicated Airspeed is the differential between a static volume of air within your Pitot tube, vs, the dyanamic pressure caused by the motion of the wing through the air. This differential pressure gives you a measure of indicated airspeed. The reason Indicated airspeed is SO important, is it is directly tied into lift. Because, after all, that is what I am primarily concerned with. I have absolutely no interest how fast I am going on the ground. (other than whether I make it back before the FBO's burger grill closes, and I can't grab that $100.00 burger).I am keenly interested, however on how fast the air is moving across my wing. Thus indicated airspeed.Let's use your example. Let's say I have a stall speed of 60Kts (indicated). If I am flying in a headwind of 20 kts, then I only need to maintain a ground speed of 40 kts, to be above stall speed. Consequently, If I am running with a tail wind, I now need to maintain 80 kts ground speed to maitain the same 60Kts across the wind and thus stay aloft.Now, when I am flying, I don't want to have to take my ground speed, then fuss and fiddle with calcualations to determine If I am above stall speed. So, I use indicated. Whether I am travelling 40kts ground speed in a head wind, or 80 kts in a tail wind, that same exact instrument will read 60 kts, and I know if I stay above that, regardless of which way I am pointing, I will still fly.You said "Let's say the approach speed for a 737 4 miles out is 132kts IAS. I've got 132kts showing on the airspeed dial, but I have (for arguments sake) a 40kt tailwind. Will I not therefore hit the runway doing 172kts, which is way too fast?"Absolutely. Which is why you will always be given an approach to a runway that is INTO the wind. In fact the ONLY time you will be directed to land or takeoff in a tailwind is if terrain hazards dictate that the tailwind is less of a hazard than the given approach or departure. (Old Hong Kong Kai Tak was a perfect example).Remember, It's about how much air I am putting over the wing. When I am in Cruise, I like tailwinds because I get to my destination much faster, but when I am in approach, I want a good headwind so my approach speed can be that much slower. But remember, I want a guage that will tell me, irregardless of which way the wind is coming from, where my "stall" point is, and that, is why I use Indicated airspeed. Because my stall point, rotate point, flap extension and retraction points will all be the same value on the guage regardless of which way mother nature happens be blowing.Make sense now? (And my momma said I'd never learn anything from playing computer games.....)(No, I'm not a real pilot, but I play one on the computer.....)Will.

[Guest SoarPics]

HI Allback,The answer is in your own example.If you flew the sim in TAS, and you were on final with that kind of tailwind (which would be a deadly mistake), the aircraft would indeed be flying thru the air at 92 kts. The sim is telling you 132kts TAS, but it's not taking into account that 40 kts of that speed is tailwind. So you and your 747 would very soon become a smoking hole in the ground because the airplane is only seeing 92 kts of air passing over it's wings... not enough to keep it airborne.The airplane (specifically the wings) only cares about the speed of the air it's flying thru. If you were flying 132kts IAS into a 40 kt headwind, your GPS would show your ground speed as 92 kts. Make that a 40 kt tailwind, and the GPS will show a ground speed of 172 kts. But the airplane is still moving thru the air at 132 kts IAS. And that's what you as a pilot cares about most... the speed of the air passing over the wings. IAS tells you that, TAS won't.For flight purposes the pilot only wants to know IAS because it tells him how fast the air is moving over the wings. More moving air means happier wings. For navigation purposes, the pilot will refer to the GPS (or DME if he's flying a VOR radial) to tell him when he can expect to arrive at a waypoint or destination.Hope this helps,Edit: Well, it seems wathomas has explained it in much the same way... but a few minutes sooner. LOL

[gosta 0]

Hi there,>Let's say the approach speed for a 737 4 miles out is 132kts IAS. I've got 132kts showing on the airspeed dial, but I have (for arguments sake) a 40kt tailwind. Will I not therefore hit the runway doing 172kts, which is way too fast?Shouldn't my IAS then be 92kts??http://hifi.avsim.net/activesky/images/wxrebeta.jpg

[Guest SoarPics]

LOL Too many teachers and not enough students. :-)

[Guest wathomas777]

Sorry, but I go hogwild over someone on this forum that actually wants to learn something, not just troll or complain.

[gosta 0]

Impressive - three simultaneous answers, and all of them saying more or less the same thing... :).Just goes to show - Avsim is the best place to go for information about FS.Cheers,Gosta.http://hifi.avsim.net/activesky/images/wxrebeta.jpg

[Guest Larry S]

I fly radio control (rc) planes. Everything works the same except scaled down; size, weight, cruise speed, stall speed.. all smaller.Since we stand stationary in one spot, and watch the plane, we see everything in effect as true or ground speed. I've seen rc planes hover over the runway. If the headwind is faster then the stall speed,and with some of the really light planes it doesn't take much wind, the ground speed can be zero, but the IAS is still above stall. I lost a plane in the scenario you gave. Lost the engine shortly after takeoff and had to deadstick. No altitude yet, so not much room to glide, and the shortest route back to the runway was where I just came from. A quick right turn, then a quick left, and I'm lined back up on the runway. I kept what looked like normal landing speed (ground speed), but forgot about the tailwind. The left wing stalled first, the plane rolled hard onto it's left side and fell like rock. If I had been using IAS, I would have come in too fast, as you said, and rolled into grass, but I would have survived.

[Guest allblack]

To you all - thanks VERY MUCH for your comments, and taking the time to explain.I will study this info. at my leisure (probably at work tomorrow!!) and get back one way or another.Thanks again.Allblack

[ronzie 21]

If you are serious about simming and want to know about the why's behind flight technique, consider browsing at a bookstore for the FAA handbook publications which usually are less expensive than commercial publications. You can also check on line at Amazon. I got the Instrument handbook on the shelf at Borders.Baicly relaxing not in front of your computer, you can read at your leisure the ground school info.

[Guest]

>Great response, but I must admit I'm still confused.>>I posted a message here about a month ago, and people tried to>explain but maybe I'm just thick!!>>I hear repeatedly that you should use IAS and not TAS, but>this to me doesn't sound ideal. It seems to me that when I'm>simming the most relevant "airpseed" indicator is the ground>speed in the GPS.>>I'll use my same example:>>Let's say the approach speed for a 737 4 miles out is 132kts>IAS. I've got 132kts showing on the airspeed dial, but I have>(for arguments sake) a 40kt tailwind. Will I not therefore hit>the runway doing 172kts, which is way too fast?>>Shouldn't my IAS then be 92kts??Yikes! Heck no! First of all you wouldn't be landing with a 40 knot tailwind EVER! ;-) To use your example for this though, assume the planes stall speed is 100 knots. If you slowed the plane down to keep your GS at 134 you would stall the plane. Indicated is the ONLY speed you should use! GS is nice to know and that is what determines how fast you get from point A to Point B but it's not used for aircraft reference speeds such as V1, VR, Stall etc... as indicated is exactly what it says "Indicated." if flying into a headwind your indicated will be higher than GS most likely. With that if the aircrafts stall speed is 100 knots, you could probably be flying over the ground at 80 knots if you have a 20 knot headwind or so and not stall the plane because the headwind adds 20 knots to the aircrafts "Indicated" speed.ah well, that probably confused you more, but just realize the most important thing is the speed of the wind going over your wings to keep you in the air and that is "INDICATED." ;-)