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).