Airspeeds are relative to the air.

 

Ground speeds are  relative to the ground.

 

If there is a wind speed then there must be a differences between them.

Think i understand it a little bit more guys. So if you have a TW at 30kts your acft will be moving over the ground 20kts faster but in the AIR your doing lets say TAS 500kts thats not affected but the ground speed is as the air is moving you faster?

By definition, "speed" (velocity) has to be measured in relation to something else. Speed is expressed in terms of distance traveled vs. time

 

The airspeed of an aircraft is measured in relation to the airmass through which the aircraft is flying. GROUNDspeed however is measured in relation to the fixed surface of the earth OVER which the aircraft is flying. They are two entirely different things.

 

The "wind speed" of the airmass is measured in relation to the ground. A 100 knot westerly wind means that if you select a "parcel" of air molecules at a given moment in time, after 1 hour passes, that group of air molecules will have moved 100 nautical miles to the east of where they started

 

Think of a hot air balloon, which is not powered in the sense that it cannot move forward through the air (only up and down). If that balloon is suspended in an airmass moving at 50 knots, then it will have a "ground speed" of 50 knots... But its AIRSPEED will be zero. Passengers in the balloon's basket will feel no "wind" at all - the air around the balloon will appear to be dead calm - even though the people on the ground below (who are "attached" to the surface of the earth) will experience all the effects of a full gale windstorm - because they are stationary on the ground, while the air moves past them at 50 knots.

 

The balloon will have an airspeed (relative to the airmass) of zero, but a ground speed (relative to the earth) of 50 knots.

 

Once an aircraft leaves the ground and becomes suspended in the air - it moves with the airmass, and feels no "wind" - because the "wind speed" of the air mass is defined ONLY in relation to the ground.

 

You are undoubtedly familiar with the moving walkways found in most large airport terminals. If a given walkway is moving at 2 MPH, and you step on to it then stand still, your speed (relative to the walkway) will be zero - but your speed relative to the other persons in the terminal NOT on the walkway will be 2 MPH. If you choose to walk in the same direction the walkway is traveling at a "walking pace" of 3 MPH, you will be moving at 3 MPH in relation to the walkway but zipping by your fixed surroundings at 5 MPH.

 

If you choose to walk AGAINST the direction the walkway is moving, (similar to being on a treadmill) you will still be moving at 3 MPH in relation to the walkway but only progressing at 1 MPH relative to your fixed (non walkway) surroundings.

Think i understand it a little bit more guys. So if you have a TW at 30kts your acft will be moving over the ground 20kts faster but in the AIR your doing lets say TAS 500kts thats not affected but the ground speed is as the air is moving you faster?

 

Yes, and its all relative to the `fixed` point you choose. In this instance everything is relative to the ground.

 

But the Earth is rotating so the ground is actually travelling at 1,040 mph as it spins on its axis... 

 

Then the planet is also travelling through space in an orbit, at around 18.5 miles per second... 

 

That fixed ground is no such thing. Now you see why they choose a fixed value for TAS - things get very confusing unless you start with an assumed Constant.

Jim what can i say? That was amazing piece of work there thanks alot to the rest who have been helping me understand this more.

 

I get the adjust of it now, but only one thing that remains - How come the DEST air distance decreases with a TW and ground distance increases? Because if A to B is 500nm how can that change? Only thing that can change like we've all been talking about is TTG? With a TW your time to the DEST will be less

Distance is distance right?

Hey Vernon,

 

just a question, why do you want to know all that stuff. I mean, by all do respect, these things have no or little effect in the sim and it almost have no effect on the fight at least you will edit all the atmospheric variables for each leg between waypoints? Or do you want to start with your ATP as these are question that ATP students ask ? Dont get me wrong its cool though someone asking such questions as it is a kind of refresher for us real world jet drivers. 

I was wondering the same thing. It's a dispatchers problem, not mine

Jim what can i say? That was amazing piece of work there thanks alot to the rest who have been helping me understand this more.

 

I get the adjust of it now, but only one thing that remains - How come the DEST air distance decreases with a TW and ground distance increases? Because if A to B is 500nm how can that change? Only thing that can change like we've all been talking about is TTG? With a TW your time to the DEST will be less

Distance is distance right?

 

No distance is the relative travelling required between two points: Again, the air mass in which you travel in your plane is itself moving. If it's a tailwind then it is actually shortening the apparent distance.

You leave your house and get in your friends car

You travel in the car on the motorway to your other friends house 50 miles away

You're not walking while you are in the car but still you are closing the distance to the house

When you get there, the car is parked on the street and the pair of you walk the last few yards to his front door

You've only `walked` those few yards but you have travelled 50 miles to be there

On another day you both do the same trip, but there's no space outside his home, so you have to park round the corner. And walk back. Distance travelled appears further.

 

More worryingly. You have only two friends. Learn from this 

OK, now let's talk about `slant range`...

 

No distance is the relative travelling required between two points: Again, the air mass in which you travel in your plane is itself moving. If it's a tailwind then it is actually shortening the apparent distance.

You leave your house and get in your friends car

You travel in the car on the motorway to your other friends house 50 miles away

You're not walking while you are in the car but still you are closing the distance to the house

When you get there, the car is parked on the street and the pair of you walk the last few yards to his front door

You've only `walked` those few yards but you have travelled 50 miles to be there

On another day you both do the same trip, but there's no space outside his home, so you have to park round the corner. And walk back. Distance travelled appears further.

 

More worryingly. You have only two friends. Learn from this 

OK, now let's talk about `slant range`...

 

 

Hilarious, Loius you made my day man Now lets talk about time speed and space...

 

 

Distance is distance right?

Varying distances can only be explained by the space time continuum where time (the fourth dimension) can be stretched by large masses resulting in longer "felt" distances.

B) 

But I think this is not the case here with your PFPX.

As I understand: the problem is that your flightplanner shortens the air distance/time in tailwind situations (which would be correct) while it extends the ground distance/time.

 

That would probably suggest the use of a different flightplanner.

Or wait: in tailwind situations from A to B it would be normal to use an opposite SID (head wind take off) from A and in addition a downwind STAR into B also prolonging the ground distance.

So you will be faster at B with tail wind and longer ground distance...

Would make "sense"

As an extreme example, if you were trying to fly 100 NM (ground distance) directly into a 100 knot headwind in a Cessna 172 at a TAS of 110 knots, your ground speed would be 10 knots, the trip would take 10 hours, and your air distance would be 1000 NM!

 

Its just really strange how 100nm can change to 1000nm in your extreme example above but you're still only doing 100nm ground distance lol

I've never actually understood the differences and so i decided to start it off by posting it on here! Thanks for all the support

These are my results from PFPX.

 

EPGD / EGKK. I modified the winds for the same route, to a TW and HW.

 

HWC 27

 

GND DIST 763

AIR DIST 810

 

TAS 454

GS 430

 

TWC 22

 

GND DIST 763

AIR DIST 725

 

TAS 441

GS 473

Can anyone explain why my TAS changes i thought this never changed?

Those airspeed and groundspeed values are averages for the route that were backed into after establishing the time enroute. When fighting a headwind, the longer time enroute means you will have spent more time at cruise altitude at cruise speed than if being helped by a tailwind. And since the time spent in climb and descent, which is done at slower speeds, is the same for both hw or tw, the average tas will work out higher if fighting a headwind and lower if being helped by a tailwind.