neilking
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Airspeed/Groundspeed Transition

Sun Nov 13, 2005 11:42 am

Sorry if the thread title is misleading but this is a thing which has always puzzled me.

An a/c stationary at the start of the runway facing straight in to a headwind of 20kts has an airspeed of 20kts even though its groundspeed is 0kts. And when it rotates at (say) 140kts groundspeed, its airspeed is 160kts.

But once the a/c is airborne (i.e. a cork floating on the stream of the air - or rather a ship steaming in the stream of the tide), then its airspeed must drop back to 140kts (assuming you ignore further acceleration after VR for the purposes of making the point I'm trying to make).

How do you cope with that drop in airspeed at a critical phase of flight?

I'm probably being blindingly stupid here but I've never been able to get my head round this mental puzzle.

Slamclick - you teach this stuff, don't you? Help me out!

Thanks in advance

Neil
Edinburgh, UK
 
Stoicescu
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RE: Airspeed/Groundspeed Transition

Sun Nov 13, 2005 11:54 am

The Stall V1, V2, Vs0, Vne, Vs1 etc. All are calculated in airspeed. So even if it is a 20 kts headwind all the speeds stay the same. Not sure if you ever saw a GA speed indicator it has all those colors on it that indicates Stall in dirty/clean configuration Never Exceed speed, caution range, cruise range etc. if the wind will influence those that means that the airspeed indicator will only be accurate from time to time and it is always accurate. There are speeds like Va that are not on the Airspeed indicator because it changes with weight and balance.

[Edited 2005-11-13 04:17:06]
 
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HAWK21M
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RE: Airspeed/Groundspeed Transition

Sun Nov 13, 2005 1:01 pm

Quoting Stoicescu (Reply 1):
Vs0, Vne, Vs1

What do these three represent.
regds
MEL
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FlyHoss
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RE: Airspeed/Groundspeed Transition

Sun Nov 13, 2005 1:43 pm

Quoting Neilking (Thread starter):
And when it rotates at (say) 140kts groundspeed, its airspeed is 160kts.

Your initial premise was good, but you made a simple error here. The aircraft isn't rotated in reference to groundspeed. Rotate speed (Vr) is an indicated airspeed, so if Vr is 160 kts, the plane is still flying 160 knots through the air. The 20 knot difference (in your example) is already accounted for and the groundspeed remains 140 (assuming no further acceleration).

This fails to examine the instantaneous effects of changing (such as gusting) winds, but that's for another discussion.
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FredT
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RE: Airspeed/Groundspeed Transition

Sun Nov 13, 2005 10:48 pm

Pretty much the only times you care about groundspeed is when you are trying to stop the aircraft on the runway, when navigating and when approaching the limit of how fast the tires are allowed to spin. For basically everything else, airspeed is what you consider. (Even though I am sure there will be a few who find a few other situations.)

Mel,
Vsn, with n a numerical index = Stall speed in a given configuration. Vs0 is landing configuration, I think Vs1 is often used to designate a clean aircraft in the GA world. I've seen Vs,15 for flaps 15, gear down and so on.

Vne - V Never Exceed, the red line on the ASI.
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neilking
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RE: Airspeed/Groundspeed Transition

Mon Nov 14, 2005 12:54 am

Quoting FlyHoss (Reply 3):
The aircraft isn't rotated in reference to groundspeed.



Quoting FredT (Reply 4):
Pretty much the only times you care about groundspeed is when you are trying to stop the aircraft on the runway,

Guys - I totally understand the respective significances of G/S and A/S and that, in my example, the a/c rotated at 160kts (a/s).

The thing I'm still having difficulty getting my head round, though, is this: Imagine I'm walking (in still air) with an "airspeed" of 5kts and I step on to a moving walkway travelling against me at 3kts. If I keep walking at the same speed, my "airspeed" has dropped to 2kts. Is an a/c becoming airborne into a headwind not a bit like it stepping onto a moving walkway travelling against it??

Thanks
Neil
 
julesmusician
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RE: Airspeed/Groundspeed Transition

Mon Nov 14, 2005 1:14 am

No, your analogy is wrong. The walkway is not in the air, it is on the ground and that is the crucial difference.

Take it step by step:

1) Air blowing at you directly 3 knots will read that airspeed on the IAS of an aircraft.

2) Add going into that at 5 knots and the air will appear to be faster as you are travelling towards it (just like a car travelling towards a car on the opposite side of the road)

3) Your GROUNDSPEED will be 2 knots but you movement through the air will be registered at being 8 knots - that is the AIR is moving at 8 knots towards you regardless of your movement over the ground

4) If you move the ground backwards then you are not applying the same rules as to aircraft vs airspeed. In your instances yes your airspeed would be 2 knots the same as your groundspeed, but runways don't move backwards - you must make the diffference between ground movement and air movement - they are two different physical quantities and cannot be compared - hence the analogy is not applicable to airspeed.

J
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David L
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RE: Airspeed/Groundspeed Transition

Mon Nov 14, 2005 1:16 am

Quoting Neilking (Reply 5):
Is an a/c becoming airborne into a headwind not a bit like it stepping onto a moving walkway travelling against it??

There's no "moving walkway" - that would assume the aircraft had powered wheels.  Smile

Assuming a constant headwind of 10 kts, an aircraft will generate enough thrust to propel itself along the runway with a take-off airspeed of, say, 140 kts, which is a groundspeed of 130 knots. When the aircraft lifts off, it's still travelling at the same speed relative to the air (140 kts) and the air is still travelling at the same speed relative to the ground (10 kts), therefore the aircraft is still travelling at the same speed relative to the ground (130 kts). Ground, air and aircraft continue to travel at the same speeds relative to each other.

If the aircraft were to get its take-off speed through powered wheels alone and then transition to prop or jet thrust alone the moment it lifted off, then you'd get your "moving walkway".
 
Stoicescu
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RE: Airspeed/Groundspeed Transition

Mon Nov 14, 2005 1:22 am

It creates the same lift over the wings even if the groundspeed is less. It doesn't rely matter what the speed in reference with the ground is (groundspeed). An airplane takes off by pulling on the yoke (sidestick) and that puts the elevator up into the wind and when the airflow hits the elevator it makes the nose go up and that's it what it is to it basically.
 
FredT
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RE: Airspeed/Groundspeed Transition

Mon Nov 14, 2005 1:39 am

Quoting Neilking (Reply 5):
The thing I'm still having difficulty getting my head round, though, is this: Imagine I'm walking (in still air) with an "airspeed" of 5kts and I step on to a moving walkway travelling against me at 3kts. If I keep walking at the same speed, my "airspeed" has dropped to 2kts. Is an a/c becoming airborne into a headwind not a bit like it stepping onto a moving walkway travelling against it??

The problem starts already in your view of the moving walkway analogy. If you are walking at 5 kts (brisk walk!) you are travelling at 5 kts, air speed and ground speed.

If you step onto a moving walkway travelling in the other direction, you will still be travelling at 5 kts. Your air speed will not change, nor will your speed relative to the floor change. Your speed relative to the moving walkway will, however, be 8 kts and you will have to break into a run to avoid falling over forward. To put it another way, your moving walkway speed will be 8 kts while your air speed and floor speed will remain 5 kts.

If you walk beside the moving walkway, the situation will be the same. 5 kts airspeed, 5 kts ground speed, 8 kts moving walkway speed.

If you step onto the moving walkway and want to travel at 5 kts moving walkway speed and your aforementioned 2 kts floor/air speed, you would need to decelerate as if you were on flat ground doing 8 kts and wanting to decelerate to 5 kts.

Compare it with a running jumping off a train (or, if you are less adventure minded, something travelling a bit slower). You will be doing maybe 5 kts train speed, but whatever the speed of the train is in ground speed, say 40 kts. The moment you hit the ground, you will notice that you are actually doing 45 kts. Just as when stepping onto a moving walkway.

Now, you are probably equipped to figuring out what is going on when you are taking off with an aircraft. The speed of the aircraft is not changing at liftoff in any way. The ground speed remains the same, the air speed remains the same. The aircraft is essentially running on the moving walkway at 8 kts (air speed) while simultaneously walking on the floor at 5 kts (ground speed) with the speed of the walkway (3 kts) being the headwind.

Cheers,
Fred

[Edited 2005-11-13 17:40:23]
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neilking
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RE: Airspeed/Groundspeed Transition

Mon Nov 14, 2005 8:06 am

I expect the moderators of this board will pull this thread shortly but I'm still not persuaded.

Quoting FredT (Reply 9):
The problem starts already in your view of the moving walkway analogy. If you are walking at 5 kts (brisk walk!) you are travelling at 5 kts, air speed and ground speed.

I agree.

Quoting FredT (Reply 9):
If you step onto a moving walkway travelling in the other direction, you will still be travelling at 5 kts. Your air speed will not change

Disagree - I've stepped onto a moving walkway going in the opposite direction and I haven't quickened my step. Relative to the surrounding floor and still ambient air, my "airspeed" and "groundspeed" has dropped to 2kts (5 - my original walking speed - minus 3 - the speed of the moving walkway back against my direction of travel).

Quoting Julesmusician (Reply 6):
but runways don't move backwards - you must make the diffference between ground movement and air movement - they are two different physical quantities and cannot be compared -

I accept runways don't move backwards - but the air does in a headwind and the air carries an a/c once it's airborne (just like a moving walkway carries me whether I stand still or walk backwards or forwards). Yes, the runway and the air are two different physical quantities but they can and have to be compared because an a/c transits from one to the other on take off - which is my question.

Please pull this thread now - it's doing my head in!!!

Brgds, Neil
 
chksix
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RE: Airspeed/Groundspeed Transition

Mon Nov 14, 2005 8:29 am

Picture yourself on a bike instead.

Pedal against the wind and you'll feel the pressure of a much faster speed than you're travelling.
Turn around and you'll go faster with less pressure on the pedals.

After this, think of how it's possible for a jet to fly to Hongkong in 10 hrs while just doing about 230 knots according to it's airspeed metre. :p
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julesmusician
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RE: Airspeed/Groundspeed Transition

Mon Nov 14, 2005 8:47 am

you are trying to make the effect of the walkway going backwards the equivalent of the effect of the wind. That is where it goes wrong, because going backwards on a walkway does not have the same effect as the wind because it is, a walkway, and hence the ground. The air doesn't move you backwards as you take off into it or as you start the take off roll, whereas the moving back walkway does. What you are doing is trying to say "isn't going backwards on a walkway the equivalent of a 3 knot headwind - nope!"
African Civil Aviation Commission president "You don't want to fly out as a passenger and come back as cargo."
 
chksix
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RE: Airspeed/Groundspeed Transition

Mon Nov 14, 2005 8:55 am

To make the walkway analogy work, you have to stand still on it and count that as zero airspeed. Start walking and you'll see the airspeed meter rise. Your groundspeed will then be compared to the floor outside of the walkway.
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FlyHoss
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RE: Airspeed/Groundspeed Transition

Mon Nov 14, 2005 8:57 am

Quoting Neilking (Reply 10):
Quoting FredT (Reply 9):
If you step onto a moving walkway travelling in the other direction, you will still be travelling at 5 kts. Your air speed will not change

Disagree - I've stepped onto a moving walkway going in the opposite direction and I haven't quickened my step. Relative to the surrounding floor and still ambient air, my "airspeed" and "groundspeed" has dropped to 2kts

NO. Wrong. Your "airspeed" hasn't changed; you're still moving forward at 5 knots. Your "groundspeed" is now 2 knots, so you're correct there.

Ground ONLY equals (true) airspeed in zero wind conditions.

Using a moving walkway is somewhat specious, though. Try thinking of swimming in a river instead. If the river is moving at 5 knots, and you're swimming upstream at 5 knots, you're still moving through the water (your "airspeed") at 5 knots, but you're "groundspeed" is 0. If that doesn't make sense, consider the opposite path; swimming downstream at 5 will result in "groundspeed" of 10 knots.
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David L
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RE: Airspeed/Groundspeed Transition

Mon Nov 14, 2005 9:17 am

Quoting Neilking (Reply 10):
I've stepped onto a moving walkway going in the opposite direction and I haven't quickened my step. Relative to the surrounding floor and still ambient air,

The moving walkway in the analogy represents the moving air in the real take-off example. You can't include the effect of the air in the walkway analogy - you'd be adding an airspeed factor twice.  

[Edited 2005-11-14 01:19:14]

[Edited 2005-11-14 01:20:22]
 
timz
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RE: Maintaining Airspeed

Mon Nov 14, 2005 10:44 am

Quoting Neilking (Thread starter):
But once the a/c is airborne ... then its airspeed must drop back to 140kts

That's the part that's baffling all of us. Why must it?

Say the aircraft's airspeed for takeoff is 60 knots, and the headwind is 59 knots. The aircraft faces into the wind, accelerates a bit, and lifts off at an airspeed of 60 knots, which it can then maintain (or increase). Why do you imagine its airspeed must drop instantly to 1 knot?
 
Mr.BA
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RE: Airspeed/Groundspeed Transition

Tue Nov 15, 2005 3:28 pm

To add to Timz's post, maybe breaking it into smaller parts might help.

For example, an aircraft lines up at the threshold and is stationary waiting for the takeoff clearance. It experiences a headwind of 20 knots. The airspeed of that aircraft is 20 knots, while groundspeed is 0 knots at that point of time. As the aircraft accelerates it gains groundspeed, as well as airspeed. Airspeed at any point in time would simply be

current groundspeed + 20 knots (windspeed)

Assuming the headwind is a steady of 20 knots, in this case, assuming Vr is 160 knots, the aircraft would need an airspeed of 160 knots in order to lift off into the air. As mentioned above, Vr is calculated based on airspeed, ie, rate of airflow over the wings. This doesn't change when the aircraft lifts off from the ground. The ground is stationary, and groundspeed would be at 140 knots at the point of rotation. In other words, the speed relative to the ground is 140 knots while with the wind is 160 knots because the wind is blowing against your direction of travel already in the first place.

Seems amateur but I hope it does help!
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comorin
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RE: Airspeed/Groundspeed Transition

Fri Nov 18, 2005 1:51 pm

Quoting Neilking (Thread starter):



Quoting Timz (Reply 16):

If an aircraft is flying with a tailwind of 1 knot at 150 knots groundspeed, and the tailwind drops to zero, the aircraft will eventually slow down to 149 knots, all other factors being constant (Airspeed = Groundspeed-Tailwind). This change is not instantaneous, but depends on the momentum and drag of the aircraft. The apparent 'headwind' of 1 knot will slow the aircraft down until steady state is reached, and this 'headwind' becomes zero. Similarly, the opposite occurs for headwinds.

In your example, as a plane takes off the runway, it's motion is propelled by reaction with air, and not the runway. So the plane's forward motion is stirctly determined by airspeed, and there is no groundspeed to airspeed transition - the difference equals headwind or tailwind. In the case where there is a crosswind component, then you would have to add this vector once the plane unsticks from the runway to get correct groundspeed.

Hope this helps, correct me if I'm wrong.

Sub-question: What happens when there is an abrupt transition in subsonic flight from tail to headwind? Could a plane overspeed during this transition?

Thanks

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