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idude
Topic Author
Posts: 8
Joined: Thu Jan 18, 2018 5:06 pm

Ground Speed Question

Thu May 31, 2018 12:36 pm

Why is it that when an airplane going west flies at a slower ground speed than going east when the IAS is the same?

For example, using a simulator, if I'm flying at 16000 feet with an IAS of 122 KTS, going east, I get a ground speed of 179kts whereas going west I get 132 KTS, a difference of 47 KTS. Does it
have to do with earth's rotation?
 
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Francoflier
Posts: 5917
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Re: Ground Speed Question

Thu May 31, 2018 2:27 pm

Winds.
 
amishfarmer
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Joined: Wed May 30, 2018 2:37 pm

Re: Ground Speed Question

Thu May 31, 2018 3:07 pm

Franco is correct. Look up High altitude winds effects on aircraft.
 
Virtual737
Posts: 1212
Joined: Tue Jul 19, 2016 6:16 am

Re: Ground Speed Question

Thu May 31, 2018 3:57 pm

Imagine yourself on a windy day. The wind is blowing at 20 knots from the west. Standing still and facing west, your ground speed is 0 knots and your indicated airspeed is 20 knots.

Now face east. To get an indicated airspeed of 20 knots, you would have to travel east over the ground at 40 knots. 20 knots to overcome the 20 knot tailwind (to make your indicated airspeed 0) and another 20 knots to get to the target of 20 knots.

Wings generate lift based on airflow. That airflow is moving independently of the ground. With zero wind, indicated airspeed and ground speed would be the same regardless of direction of travel, assuming level flight of course.

None of this takes into account the difference between IAS (indicated airspeed) and true airspeed (which is velocity in relation to the air mass itself) or pressure differential at altitude.
 
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AirKevin
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Re: Ground Speed Question

Fri Jun 01, 2018 12:03 pm

Try this with your simulator. Put your plane on a runway of an airport of your choice, then give yourself a direct headwind of 150 knots. See what happens.
 
KingOrGod
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Re: Ground Speed Question

Fri Jun 01, 2018 10:29 pm

idude wrote:
Does it have to do with earth's rotation?


Yes. It's largely because the earth is flat, and this effect is more pronounced the closer you get to the edges. It's the little known "Flat Earth Effect". :geek:
I once worked at a station very far south, and it was so fast coming in, but took forever to get out again. :crazy:
 
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Moose135
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Re: Ground Speed Question

Sat Jun 02, 2018 4:25 am

idude wrote:
Does it have to do with earth's rotation?

No, that only effects you if you are taking off on a treadmill.
 
idude
Topic Author
Posts: 8
Joined: Thu Jan 18, 2018 5:06 pm

Re: Ground Speed Question

Sat Jun 02, 2018 12:50 pm

Francoflier wrote:
Winds.
So the winds are always blowing from west to east?
 
rotation18L
Posts: 41
Joined: Sat Jun 23, 2001 11:30 am

Re: Ground Speed Question

Sat Jun 02, 2018 1:56 pm

I think winds blowing west to east is generally true; i.e., the jet stream.
 
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glen
Posts: 364
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Re: Ground Speed Question

Sat Jun 02, 2018 2:05 pm

idude wrote:
Francoflier wrote:
Winds.
So the winds are always blowing from west to east?


Not generally, but on an average latitude (between 30° and 60°) that's indeed the most common winddirection in the upper atmosphere. This is caused by the rotation of the earth, disturbed only by the position of High and Lowpressure systems and the position of frontal weather systems. So you were somehow right, although only as an indirect cause, with your assumption about the earth's rotation.

You seem to have made your observation about the groundspeed on a simulator, I assume a PC-Simulator.
Go into the weather settings, where you can probably change wind values, and set wind speed to zero and you will see a constat groundspeed in all directions. Or change the wind direction to another value and you will see a change in groundspeed in the other directions.

But you will see an additional effect. You wrote you are flying at 16000 feet. At this altitude you will also see a difference in Indicated Airspeed and Groundspeed, even with zero wind. This is caused by the thinner air at this altitude, thus the Air Speed Indicator measures a lower speed than the effective speed of the aircraft through the air. This effective speed is called True Airspeed and equals the Ground Speed with zero wind.
 
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Spacepope
Posts: 5478
Joined: Tue Dec 28, 1999 11:10 am

Re: Ground Speed Question

Sat Jun 02, 2018 2:41 pm

Moose135 wrote:
idude wrote:
Does it have to do with earth's rotation?

No, that only effects you if you are taking off on a treadmill.


...but what if your cargo is a load of pigeons who are flying around the cabin all at the same time?
 
Virtual737
Posts: 1212
Joined: Tue Jul 19, 2016 6:16 am

Re: Ground Speed Question

Sat Jun 02, 2018 5:45 pm

Spacepope wrote:
Moose135 wrote:
idude wrote:
Does it have to do with earth's rotation?

No, that only effects you if you are taking off on a treadmill.


...but what if your cargo is a load of pigeons who are flying around the cabin all at the same time?


You'll have a big mess to clean up when you eventually land. Wind probably wont affect that though.
 
idude
Topic Author
Posts: 8
Joined: Thu Jan 18, 2018 5:06 pm

Re: Ground Speed Question

Sat Jun 02, 2018 7:51 pm

glen wrote:
idude wrote:
Francoflier wrote:
Winds.
So the winds are always blowing from west to east?


Not generally, but on an average latitude (between 30° and 60°) that's indeed the most common winddirection in the upper atmosphere. This is caused by the rotation of the earth, disturbed only by the position of High and Lowpressure systems and the position of frontal weather systems. So you were somehow right, although only as an indirect cause, with your assumption about the earth's rotation.

You seem to have made your observation about the groundspeed on a simulator, I assume a PC-Simulator.
Go into the weather settings, where you can probably change wind values, and set wind speed to zero and you will see a constat groundspeed in all directions. Or change the wind direction to another value and you will see a change in groundspeed in the other directions.

But you will see an additional effect. You wrote you are flying at 16000 feet. At this altitude you will also see a difference in Indicated Airspeed and Groundspeed, even with zero wind. This is caused by the thinner air at this altitude, thus the Air Speed Indicator measures a lower speed than the effective speed of the aircraft through the air. This effective speed is called True Airspeed and equals the Ground Speed with zero wind.


Thanks for clearing that up for me!
 
idude
Topic Author
Posts: 8
Joined: Thu Jan 18, 2018 5:06 pm

Re: Ground Speed Question

Sat Jun 02, 2018 7:52 pm

Spacepope wrote:
Moose135 wrote:
idude wrote:
Does it have to do with earth's rotation?

No, that only effects you if you are taking off on a treadmill.


...but what if your cargo is a load of pigeons who are flying around the cabin all at the same time?


Stop wasting bandwidth with needless jokes!
 
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Spacepope
Posts: 5478
Joined: Tue Dec 28, 1999 11:10 am

Re: Ground Speed Question

Sat Jun 02, 2018 11:27 pm

idude wrote:
Spacepope wrote:
Moose135 wrote:
No, that only effects you if you are taking off on a treadmill.


...but what if your cargo is a load of pigeons who are flying around the cabin all at the same time?


Stop wasting bandwidth with needless jokes!

False premise. All jokes are needed!
 
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Moose135
Posts: 3249
Joined: Mon Oct 04, 2004 11:27 pm

Re: Ground Speed Question

Sun Jun 03, 2018 1:47 am

idude wrote:
Stop wasting bandwidth with needless jokes!

You're new here, you'll learn there are no needless jokes on the A.net forums!
:rotfl:
 
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Starlionblue
Posts: 20814
Joined: Fri Feb 27, 2004 9:54 pm

Re: Ground Speed Question

Mon Jun 04, 2018 3:30 am

Spacepope wrote:
idude wrote:
Spacepope wrote:

...but what if your cargo is a load of pigeons who are flying around the cabin all at the same time?


Stop wasting bandwidth with needless jokes!

False premise. All jokes are needed!


Do we dare link to the infamous treadmill thread? Or would that open a rift in the space-time continuum...?
 
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BartSimpson
Posts: 634
Joined: Sat Mar 05, 2016 5:01 pm

Re: Ground Speed Question

Mon Jun 04, 2018 9:39 am

Spacepope wrote:
Moose135 wrote:
idude wrote:
Does it have to do with earth's rotation?

No, that only effects you if you are taking off on a treadmill.


...but what if your cargo is a load of pigeons who are flying around the cabin all at the same time?


You can do that but take care that there is no fish in the cargo bay of the FI B757!
 
stratclub
Posts: 1382
Joined: Fri Jan 05, 2018 10:38 pm

Re: Ground Speed Question

Mon Jun 04, 2018 11:10 pm

Moose135 wrote:
idude wrote:
Does it have to do with earth's rotation?

No, that only effects you if you are taking off on a treadmill.

The absolutely lamest Myth Busters episode I ever saw address the completely ridiculous non myth subject of taking off from a conveyor belt.
https://www.youtube.com/watch?v=YORCk1BN7QY
 
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Starlionblue
Posts: 20814
Joined: Fri Feb 27, 2004 9:54 pm

Re: Ground Speed Question

Tue Jun 05, 2018 3:19 am

stratclub wrote:
Moose135 wrote:
idude wrote:
Does it have to do with earth's rotation?

No, that only effects you if you are taking off on a treadmill.

The absolutely lamest Myth Busters episode I ever saw address the completely ridiculous non myth subject of taking off from a conveyor belt.
https://www.youtube.com/watch?v=YORCk1BN7QY


IMHO it was a legitimate myth since many people, including a not insignificant fraction of forum members, believed it.

The way it was done was a bit lame but they don't have an infinite budget.
 
Chemist
Posts: 946
Joined: Tue Oct 20, 2015 4:46 am

Re: Ground Speed Question

Tue Jun 05, 2018 3:49 am

But can you do the impossible turn better going east or west?
 
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Starlionblue
Posts: 20814
Joined: Fri Feb 27, 2004 9:54 pm

Re: Ground Speed Question

Tue Jun 05, 2018 3:51 am

Chemist wrote:
But can you do the impossible turn better going east or west?


:D

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