It's still a pretty imprecise term. What would your answer to the "sideways"case be?

Yep

No, they both get to the destination, unless one it on the wrong flight.

Same as a ball that's facing sideways.

Next question.

Not in the frame where the air is still.

If you move sideways, are you moving forwards or backwards?

Thats the point-neither.

so do you not HAVE a groundspeed?

The groundspeed is still there, it didn't go anywhere. A chopper can have a - groundspeed and a + airspeed at the same time, while a ball just has groundspeed.

Yes, but can you have a groundspeed which is neither poitive nor negative?

A good starting point would be a definition of speed- I always just assumed it was the magnitude of velocity. You seem to have a different take on it.

It looks like you're moving backwards. It's what happens when you fly at 50 knots airspeed into a 60 knot headwind. Groundspeed is -10 knots.

Fortunately for the guy with the negative groundspeed, the destination is behind him.

Speed in relation to what?

Yes, good one. :)

Well, velocity is in relation to a frame of reference. If we are talking groundspeed, then relative to the ground.

The magnitude of velocity is the rate of change of position-m/s, km/hr, furlongs/ fortnight- it is how much your positionn has changed in the reference frame and how long the change took. I don't see how that can have a negative value.

How do you quote on an Android??

Vesbot, you didn't deal with the guy standung sideways- is his groundspeed positive or negative?

It's embarrassing that the people on the correct side are chasing their tails on definitions and reference frames. Brercrow must be (justifiably) having a good laugh.

Yes, if you consider the aircraft a point object (like a ball) without a heading, then groundspeedvelocity can only have a direction and positive magnitude. But it's slightly more complicated than that, in that an aircraft has a set of body axes and everything is normally calculated relative to them as the direction component of the reference frame.

Again, before anyone got lost in the weeds of this thread, everyone would agree without a moment's hesitation of the trivial truth that in my scenario A, groundspeed is 90 initially and then 110. Not negative 110.

And again, imagine a dead calm day with no wind. Then, everything that is true about airspeed is true about groundspeed. If groundspeed is only positive, then airspeed is always positive, and therefore airplane should fly just the same in a tailslide as they do going forward. True or untrue?

It's zero in relation to the body axes, but that definition starts being useless then. You can say it's 500 sideways, which is kinda clunky and not useful for much, but it is at least useful in figuring out how soon he'll get to the destination.

Now it's your turn again to deal with the Cessna flying at 50 knots into a 60 knot headwind. What's his groundspeed?

Right, helicopter flying into a headwind such that it has a groundspeed of -2kts but at the same time an airspeed of 5kts . Pilot of helicopter uses yaw pedal to change heading by 180. What's the helicopter's groundspeed when it's on its new heading?

Vesbot,

Yes we are just talking semantics.

The magnitude of the ground velocity of the Cessna is 10 knots, its direction being 180 degrees to its heading.

Its position over the grond is changing by 10 nautical miles every hour, and there is no such thing as a negative nautical mile.

I would maintain speed is a scalar quantity, and therefore has no direction. By invoking a positive or negative you are now actually defining a reference frame, and we are talking velocity, not speed.

I think our passengers actually demonstrate this- how can you change speed without accelerating?

Jetfan

7kts.

I don't understand your point.

It's flipped around and gone from -2kts groundspeed to +7kts GS. Or are you saying GS is never negative in relation to the motion of an aircraft?

I understood what you meant, sure.

lets say he pedals throgh 90 degrees- now whats jis groundspeed?

Point being, I would say his ground speed has gone from 2kts to 7 knots. His ground velocity frim 2cknots North to 7 knots South (or whatever).

A cessna flying into a 60kt headwind at 50kts, what's its GS?

The reference to the Earth is purely one of convenience.

Because we measure wind velocity relative to the Earth!

e.g. if you are heading 270 and you fly through a gradient from 270/10 kts to 270/30 kts then we have a +20 knot effect.

The same as if you went from 090/5 to 270/15.

It really doesn't matter what the absolute values are relative to the Earth, only what the change is.

But, if you wanted to calculate it based on reported wind values,we measure wind relative to a stationary point on the ground. This might give the impression that the relationship to the ground is fundamentally important rather than being a book keeping convenience.

This, and everything else resting upon it, is total bollocks.

Groundspeed is a Scalar. An objects ground vector is comprised of its ground speed and its track.

When an aircraft is heading directly into a wind that exceeds the TAS then it's track is the reciprocal of the heading.

If you try and call that a negative ground speed you are taking account of the same thing twice.

This is true. But why does it matter?

This is true too.

Also true...

Hold the phone! Why does that justify taking the sum? That makes no sense. The whole premise is that we are comparing the difference between the two groundspeeds (i.., before and after) which, by definition of the mathematical operation, is performed by subtraction. In no universe is addition meaningful to any of this. There is a difference in one of the initial conditions (the airmass is so fast compared to the airspeed that the groundspeed starts out negative) but this does not justify changing the manner in which we compare the initial and final condition. Groundspeed is airspeed minus headwind (equivalent to airspeed plus tailwind) always.

Well, AOA will be cracking on for 180 degrees. That might have a small part to play.....

+ve airspeed and Alpha(plus Beta for 3d) gives us a system that covers all the bases for where the air is coming from.

No it's not, if you simply define groundspeed as the component of velocity of the ground relative to the airplane's X axis, signed as positive moving nose-to-tail.

And don't act as if this is esoteric or meaningless, as we, conventionally, do exactly that with airspeed as I've pointed out a few times by now.

Or you can say that the airspeed is negative. I think communication is goddamn impossible sometimes. One time my boss was trying to tell me there's no such thing as negative lift. Definitions and accounting is like 99.9% of most problems.

...is just another way of saying its GS has become negative.

Jet Fan, I think it's time me and you resign from this part of the project, and think up a new and different counterexample to demonstrate that initial and final ground velocity is irrelevant to accelerations that are measured and enacted only by reference to the uniform airmass... :ugh::{

If you walk down to the tail of an airliner your 'track is [now] the reciprocal of the [your] heading' though. ;)

His ground speed is 10 knots. His ground velocity is 10 knots in the direction of the wind. Speed and velocity are two different things. A good definition of velocity is from Wikipedia, which I use for its clarity:

Please note that velocity must have a frame of reference and that's what's leading to all this BS. Define your frame of reference then do all the vector sums from that frame. You can't mix both the 'air' reference and the 'ground' reference. Use one or the other. There is no energy generated when a plane turns constant circles in a constantly moving air stream. The velocity of the reference air stream over the ground doesn't make one whit of difference to the aircraft.

I'm a dumb winemaker and even I understand this.

As you love wiki so much, let's see what it has to say about groundspeed:

What is 50 - 60?

Apparently groundspeed = | airspeed - headwind |

Ever since the 3rd grade I always wondered when in the world you'd ever use the | | symbol to strip out a negative, and I've finally found it!

This might help some in here:

Thanks Vessbot, you said it for me.

Race Fan, I didn't say I loved Wiki, I said their definition was clear.

NO. It really isn't. It might look like it, but if you just choose to invert the sign of something on a whim, expect difficulties if you apply that thought process to a navigation or reference system.

It's not a whim, it's perfectly in accordance with the simple principle that groundspeed is airspeed minus headwind component.

Said what for you?

We are all on the same page, but just someth8ng for the poitive_negative guys to mull- you are defining positive ground speed as being in the direction the aircaft is ponting- well, if there is any cross wind, the aircraft isn' t tracking that way- so is a teack 30 degrees off the nose " moslty positove "

answering the question on a 50 knot airspeed in a 60 knot headwind- how far will the aircaft be from its starting position after one hour?

Can I propose a gedanken (thought experiment)?

A Cessna is flying into a 10 kt headwind and has ground speed 90 kts. Instantaneously the headwind increases to 20 kts. How long does it take for the Cessna's ground speed to attain 80 kts?

I've always wanted to know.