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Bruce
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Ground Speed VS Air Speed

Sat Dec 20, 2003 3:03 am

I'm a little confused about ground speed versus air speed. Which one tells you how fast the airframe is physically moving thru the air?

Ground speed includes the added tailwind (or less the headwind), right? Why would that not be the airspeed? and then there is "indicated" and "actual" airspeed! so many terms for speed and differences in the numbers, its confusing!  Confused  Confused

bruce
Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens
 
flyf15
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RE: Ground Speed VS Air Speed

Sat Dec 20, 2003 3:20 am

The simpliest way to say it is that groundspeed is true airspeed corrected for wind. Say you're flying along at 150kts into a 10kt headwind. You'll be moving through the air at 150kts, but only moving along the ground at 140kts.

Now for some of the various airspeeds:

Indicated airspeed (IAS): What the pilot reads off the airspeed indicator
Calibrated airspeed (CAS): Indicated airspeed corrected for instrument error
Equivalent airspeed (EAS): Calibrated airspeed corrected for the compressibility of air
True airspeed (TAS): Airspeed at which the aircraft is moving through the air. Will be different than EAS unless you are at sea level with a standard aptmosphere due to changes in air density affecting instrument readouts.

Most of the time, at least in general aviation, all we care about is indicated airspeed (for performance related issues such as climb speed and such), true airspeed (which can be calculated knowing pressure altitude and temperature) to know how fast we're actually moving through the air, and groundspeed (which can be found from various instruments such as a GPS or DME) to know how fast we're actually making our way to our destination.
 
Mr Spaceman
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RE: Ground Speed VS Air Speed

Sat Dec 20, 2003 3:25 am

Hello Bruce.

This topic has been discussed before many times. If you do a search, you'll find some very good info & explanations about this.  Big thumbs up

Here's some good info.....

http://www.airliners.net/discussions/tech_ops/read.main/45537/

I hope this helps.


Chris  Smile
"Just a minute while I re-invent myself"
 
SkyGuy11
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RE: Ground Speed VS Air Speed

Sat Dec 20, 2003 3:37 am

If there was no wind, airspeed = groundspeed

If it's windy out, your airspeed is the same (still moving the same speed through the air), but the wind will change your speed over the ground.

Wind is air in motion, when you're in the air you move with it.
.
 
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Bruce
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RE: Ground Speed VS Air Speed

Sat Dec 20, 2003 3:41 am

But why.... would your airspeed (speed thru the air) not change? If you have a tailwind pushing you along doesn't that make the plane go faster?

If I have a 150kt airspeed plus 50kt tailwind from behind me then why wouldn't my speed thru the air be 200kt?


bruce
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RE: Ground Speed VS Air Speed

Sat Dec 20, 2003 3:43 am

Hey,

>If I have a 150kt airspeed plus 50kt tailwind from behind me then why wouldn't my speed thru the air be 200kt?

If you have a 150kt airspeed, you are traveling 150kt with respect to the air - the windspeed is already factored in.

Having a 50kt tailwind would make you look faster *from an observer on the ground.* If I stand on the ground and watch a plane above go 50kt (airspeed) with a 50kt tailwind, I have to add the velocity vectors of the wind and the plane together. If the plane has a 50kt airspeed, but a 50kt headwind, I will see the plane staying in the same place  Smile

Google "relative velocity" if you want some other ways to look at it.

[Edited 2003-12-19 19:45:48]
 
SkyGuy11
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RE: Ground Speed VS Air Speed

Sat Dec 20, 2003 3:51 am

Because all motion is relative.
-Air is a soup (fluid)
-Wind is air in motion
-You're IN the air

When you fly with wind (air in motion), you're drifitng with the wind. This happens to ALL planes regardless of size / weight. Your speed relative to the air is the same as always: 150kts in your example.

But because your medium is in motion also, your speed relative to the ground is now 200kts in your example.

Take a fish tank and slide it across a table. The table is the ground, water is the air, fish is the plane. You can slide it across the table as fast as you want and the fish will be in exactly the same place relative to the water.

It's kinda like that.

Or you can just picture a train moving on tracks. You can walk to the front of the train just fine at the same speed as usual, even though you may be moving 75mph relative to the ground.
.
 
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RE: Ground Speed VS Air Speed

Sat Dec 20, 2003 4:30 am

A comparison that I like to make is that it's like swimming in a river. If you're swimming upstream your "groundspeed" will be your swimming speed minus the speed of the water current. Let's say you're a strong swimmer and can make 4 mph. If you swim into a 3 mph current you will only make 1 mph along the riverbank. If you turn around and swim with the current your speed along the bank will be your swimming speed of 4 mph plus the speed of the current (3 mph) for a total speed of 7 mph. The speed at which you can swim has not changed, the speed along the riverbank has. The exact same principle applies with airspeed & groundspeed. Well, almost... When you're talking airspeed there are several other "types" - indicated, calibrated, true, etc. I'm running out of time so if you're still confused, do a search - there's been a lot of threads on this subject in the past.

Jetguy
 
Mr Spaceman
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RE: Ground Speed VS Air Speed

Sat Dec 20, 2003 6:31 am

Jetguy.

I just wanted to tell you that I think your description of how the comparison between airspeed & groundspeed is like swimming in a river is an "excellent" way for someone to get their head around the idea of different speeds and lose that mental block.

I'll always remember your comparison, incase someone asks about this subject again.

Good work!  Big thumbs up


Chris  Smile

"Just a minute while I re-invent myself"
 
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Bruce
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RE: Ground Speed VS Air Speed

Sat Dec 20, 2003 7:16 am

It makes a little more sense now when i think of "relative" velocity. And that air and wind are different. AND, your airspeed will vary depending on the pressure and density of the air?

So, the only use for groundspeed is for your ETA to destination, and flight duration planning?

also....about "indicated" airspeed, if what the pilot reads is "indicated" from the instrument, then why have the others. The instrument itself should automatically calibrate its input for error and air density, especially on today's modern jets with computer-controlled instruments.

bruce
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RE: Ground Speed VS Air Speed

Sat Dec 20, 2003 7:25 am

And, except for taking written exams you can pretty well dump the "calibrated" and "equivalent" airspeed terms. They are mostly used by the engineers who contribute to the performance section of the airplane manual.

Unless the plane has an FMC or the like to do it for you, you will have to understand TAS as the startpoint for flight planning.

IAS will be what the pilot looks at. Whizwheel or pocket calculator can solve from that to TAS adjusting for the thinner air as you climbed to your cruise altitude.

With TAS and the wind information you can accurately predict a groundspeed and that is what your flight time will depend on. It should be mentioned though, that wind correction is a full 360 degree concept. It is not just (in fact hardly ever) plus or minus the wind speed. It is a geometric vector based on the angle between your heading and the direction the wind is coming from. For example, if the wind is from straight off your left wing, you will have to aim into the wind a little bit to make good the desired course over the ground. This will have the effect of giving you a slightly lower groundspeed, but certainly not lower by the amount of the wind speed.

One last rule of thumb. In any round-trip situation the wind is against you. That is because a headwind will act against you for a longer period of time than the headwind will act for you.

Happiness is not seeing another trite Ste. Maarten photo all week long.
 
liamksa
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RE: Ground Speed VS Air Speed

Sat Dec 20, 2003 9:16 am

Indicated airspeed (IAS) is a measure of the aerodynamic performance of the aircraft. If the pitot tube believes the aircraft is travelling through the air at 200kts so does the wing. So basically lift is generated according to IAS (or actually calibrated airspeed) and is therefore the deciding factor in how to fly the aircraft.
 
Mr.BA
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RE: Ground Speed VS Air Speed

Sat Dec 20, 2003 10:23 am

How is TAS measured? IAS is measured by the pilot tube near the nose of the aircraft, what about TAS?

For example you're cruising FL370 Mach.85 with an OAT is -20 degrees celcious (for example) and you cruise in the same conditions same speed but your OAT is -70 degrees celcious, your GS would be slower. I can't figure out this logically, anyone help?

How owuld pressure changes affect any of these speeds?

Thanks a lot appreciate it!
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SkyGuy11
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RE: Ground Speed VS Air Speed

Sat Dec 20, 2003 10:40 am

BA; this isn't really a technical explanation but here goes:

You're airspeed indicator measures speed by pretty much counting the number of air molecules hitting it per second.

Anytime the air is less dense, you have to go farther in the same amount of time to hit the same # of air molecules. This means that your airspeed indicator might say 100, but on a hot day you'll be going faster than on a cold day.

Things that make the air less dense include heat, humidity, and altitude.
.
 
liamksa
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RE: Ground Speed VS Air Speed

Sat Dec 20, 2003 10:57 am

For example you're cruising FL370 Mach.85 with an OAT is -20 degrees celcious (for example) and you cruise in the same conditions same speed but your OAT is -70 degrees celcious, your GS would be slower. I can't figure out this logically, anyone help?

The Mach no. of an aircraft is defined as the speed of the aircraft (TAS) divided by the local speed of sound. eg: Aircraft speed 400kts, local speed of sound 500kts, Mach no. = 400 / 500 = M0.8

Speed of sound is related to the temperature, such that as temperature increases, the speed of sound increases. ie: the speed of sound is faster at sea level (warmer) than at altitude.

So in your example, at a constant M0.85 but at a colder temperature, the speed of sound will be less, and your speed (TAS) will be 85% of this lesser value. (slower)

Rob.
 
SlamClick
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RE: Ground Speed VS Air Speed

Sat Dec 20, 2003 11:35 am

Here goes.
  • IAS is read right off the dial.


  • CAS is IAS corrected for installation error and nothing in the plane shows you CAS. The manufacturer knows how to figure it.


  • EAS is CAS corrected for compressibility and nothing in the plane shows you EAS


  • TAS is EAS corrected for (reduced pressure at) altitude


  • All you need (in the real world) is IAS because that is what you can see and fly by, and TAS because that is what you use, plus or minus wind factor to find your GS. GS over a fixed distance will tell you how long. GS and the time flown will give you distance flown.

    Any whizwheel (aviation circular sliderule) or flight-specific pocket calculators will give you TAS to expect at cruise as long as you know the "recovery factor" specific to the airplane. So will the aircraft performance charts. So will your previous experience in the airplane.

    To give you some idea of the effect of altitude on airspeed, in the plane I fly now these are typical numbers. At about 35000 feet, I will be indicating maybe 260 knots. My TAS, as shown by the magic boxes is about 450 knots.


    Happiness is not seeing another trite Ste. Maarten photo all week long.
     
    Kay
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    RE: Ground Speed VS Air Speed

    Wed Dec 24, 2003 6:51 pm

    Slamclick,
    is that the 2% for every 1000 feet that pilots add to get an approximate figure of true air speed at altitude?
    35,000ft that makes:

    2% * 35 = 0.70
    1.7 * 260 Knots = 442 Knots

    also, 576 is the standard speed of sound for 35,000ft, so
    442/576 = 76.7,

    so, in standard atmosphere, 260knots at 35,000ft makes mach .77?

    kay
     
    saintsman
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    RE: Ground Speed VS Air Speed

    Wed Dec 24, 2003 7:49 pm

    Bruce,

    A couple of other things to help you visualise airspeed / groundspeed.

    If you are flying at 100 kts (and you can see it on your airspeed indicator to prove it) and are flying into a head wind that is also 100 Kts (hypothetically of course), your groundspeed will be zero. This is a simple mathematical equation 100 - 100 = 0. You know that your aircraft is flying because you are in it and if you look down at a spot the ground you will not be moving away from it.

    Hopefully such big differences in the sums will help you see the difference.


    TAS is achieved by adjusting the airspeed with a temperature factor. As the aircraft goes higher, and the air becomes less dense, the temperature falls approx 1.8 dergrees per thousand feet. There are other things to do with air desity I believe, but temperature is the major factor and the easiest to measure and compensate for.

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