The Windward Turn Theory

[A Squared]

Originally Posted by Jet_Fan

It is simply HIS groundspeed measured from his frame of reference.

"groundspeed" by definition, is relative to the ground. His velocity in his frame of reference is zero, because it's impossible for him to move relative to himself.

[Jet_Fan]

Originally Posted by A Squared

You're adopting a frame of reference which changes orientation with the person. The only way you can make sense of any of this is with a constant, non-accelerated frame of reference.

the airliner is flying East at 500 knots. the passenger is walking toward the tail at one knot, he is still moving East at 499 knots. He's not moving west merely because he's facing west.

While I obviously think Brecrow's theory is fallacy, I have to agree with him that the analogy the two if you are using here is just as flawed.

He is moving backwards. If I reverse my car at 30mph, what is my car's groundspeed? Clue: the speedo won't be reading 30mph.

[Vessbot]

Originally Posted by A Squared

OK, fine, the airliner is flying west, so west is the positive direction of our frame of reference. The velocity of the westbound airliner is +500 knots. The man is walking walking in the east direction , so that's -1 knot. a positive 500 knots, plus a negative 1 knot equals positive 499 knots, the man is still moving west at 499 knots. it doesn't become negative because of the direction he is facing.

Oh Christ. You're anchoring the +/- to the geographical direction. I was leaving them anchored to the body. Yes I can belabor the point and rework everything to keep to a geographical frame, but the whole purpose of the analogy was to relate it to what we all intuitively understand from flying bug smashers at 100 knots in 10 knot winds (which is where a positive sign on groundspeed always means that ground is moving opposite the direction the nose is pointing). Look at scenario A!

[Jet_Fan]

Originally Posted by A Squared

"groundspeed" by definition, is relative to the ground. His velocity in his frame of reference is zero, because it's impossible for him to move relative to himself.

That's his speed relative to the airframe. When I move backwards relative to the ground, my groundspeed has a minus in front of it.

[A Squared]

Originally Posted by Vessbot

Oh Christ. You're anchoring the +/- to the geographical direction.

Well, yeah, what else are you going to measure ground speed relative to? Again, groundspeed is inherently relative to (in the frame of reference of) the ground.

You can't make any meaningful analysis when the axes of your reference system move whenever a person looks in a different direction.

[Vessbot]

Originally Posted by A Squared

Well, yeah, what else are you going to measure ground speed relative to? Again, groundspeed is inherently relative to (in the frame of reference of) the ground.

You can't make any meaningful analysis when the axes of your reference system move whenever a person looks in a different direction.

Sure you can. If your last sentence is true, then it is not a meaningful analysis that in Scenario A the Cessna went from 90 to 110 groundspeed when turning East to West. Did you look at scenario A? Do you think that's meaningful or meaningless?

[Vessbot]

Hell we do the same thing about airspeed too, and necessarily so. The positive sign is always applied when air is coming from the direction of the nose, and we take that axis with us wherever we point the nose. If we didn't, then an airplane should fly equally well in a tailslide as in normal flight, and I can tell you from personal experience that it certainly doesn't.

[A Squared]

Originally Posted by Vessbot

Sure you can. If your last sentence is true, then it is not a meaningful analysis that in Scenario A the Cessna went from 90 to 110 groundspeed when turning East to West. Did you look at scenario A? Do you think that's meaningful or meaningless?

From a physics standpoint, it is completely meaningless. You are making the identical mistake that Ian was making. In your Scenario A, From the frame of reference of the ground, the airplane went from a velocity +110 knots to a velocity of -90 for a total change in velocity of 200 knots.

[A Squared]

Originally Posted by Vessbot

Hell we do the same thing about airspeed too, and necessarily so. The positive sign is always applied when air is coming from the direction of the nose, and we take that axis with us wherever we point the nose. If we didn't, then an airplane should fly equally well in a tailslide as in normal flight, and I can tell you from personal experience that it certainly doesn't.

Look, it's pretty clear that you simply are unable to grasp the concept of Velocity, how it is different than "speed", why it is imperative that velocity is used in physics and not speed, and why a constant, unchanging frame of reference is necessary to make any kind of a meaningful analysis. I apologize for having wasted your time trying to explain these things and I will stop now.

[Vessbot]

Originally Posted by A Squared

From a physics standpoint, it is completely meaningless. You are making the identical mistake that Ian was making. In your Scenario A, From the frame of reference of the ground, the airplane went from a velocity +110 knots to a velocity of -90 for a total change in velocity of 200 knots.

Say there's no wind (it's one of those perfect mornings) and you're flying around. You fly East for a while at 100 knots airspeed, and turn around and fly West. By that same premise, your airspeed is now -100 knots too, for a total change of 200 knots. Hmmmmm?

(if there's no wind then the airmass frame and the ground frame are the same, so you should just be able to do a find-and-replace of "ground" with "air" to your post and everything still hold true.)

No, it's perfectly valid, for certain contexts, to rotate the axes as needed.

[Brercrow]

Originally Posted by Vessbot

Yes, there will be, for the reason explained by the math I posted.

What's the groundspeed before the turn? -499
What's the groundspeed after the turn? 501

The difference is 1000.

I read your post again, and it still rearranges the terms in error. Look at scenario A, and see that obviously everything is right. This is common sense to a private pilot. And then look at how every element translates to scenario B. Have I made any mistakes in the translations?

The difference between these two examples is that in one the windspeed is less than the airspeed and in the other the windspeed is greater than the airspeed. In both cases airspeed is positive. In the first case the groundspeed reverses direction and in the second case the groundspeed is in the same direction. That means that in the first case you are entitled to take the difference between the groundspeeds but in the second case you have to take the sum. In other words along with common sense there is a minus sign missing.
I don't really mind where you stick it.

I should add that this is because calculation of groundspeed is a triangle of velocities problem envolving the cosine of the wind-angle. Less than 90 degrees is positive Greater than 90 degrees is negative

(1+500)+(1-500)
=501 +(-499)
=2

[Vessbot]

Originally Posted by A Squared

Look, it's pretty clear that you simply are unable to grasp the concept of Velocity, how it is different than "speed", why it is imperative that velocity is used in physics and not speed, and why a constant, unchanging frame of reference is necessary to make any kind of a meaningful analysis. I apologize for having wasted your time trying to explain these things and I will stop now.

Quit being a martyr. You know it's perfectly reasonable, given the standard set of assumptions, to say that your groundspeed is 90 knots one way, then 110 the other way. In fact, before this thread, you would not have even blinked at saying exactly that. You know it.

[Jet_Fan]

Originally Posted by A Squared

From a physics standpoint, it is completely meaningless. You are making the identical mistake that Ian was making. In your Scenario A, From the frame of reference of the ground, the airplane went from a velocity +110 knots to a velocity of -90 for a total change in velocity of 200 knots.

It's not even a mistake, let alone the same mistake. The whole point is to explain to people like Ian, that what he saw was different to what the plane actually experienced.

Now, a ball can't travel backwards, but people and planes can so + and - is a way of expressing that relative movement.

[Wizofoz]

Originally Posted by Jet_Fan

no, it's -499 because he is facing the tail. If he stops it goes back to -500. If he turns to face the cockpit while stationary it goes from -500 to +500 almost instantly.

No-it doesn't matter which way you are facing, it matters which way you have nominated as positive and negative in the reference frame you are using.

Acceleration is absolute- if his velocity changed by 1000kts in a fraction of a second, he'd be crushed by the Gs.

[Jet_Fan]

Originally Posted by Wizofoz

No-it doesn't matter which way you are facing, it matters which way you have nominated as positive and negative in the reference frame you are using.

Acceleration is absolute- if his velocity changed by 1000kts in a fraction of a second, he'd be crushed by the Gs.

We are looking at is groundspeed not his velocity. His groundspeed can be + or -.

[Wizofoz]

Originally Posted by Jet_Fan

He is moving backwards. If I reverse my car at 30mph, what is my car's groundspeed? Clue: the speedo won't be reading 30mph.

The magnitude of your ground VELOCITY is 30 MPH

[Wizofoz]

Originally Posted by Jet_Fan

We are looking at is groundspeed not his velocity. His groundspeed can be + or -.

What does a negative ground speed look like?

[Wizofoz]

Originally Posted by Jet_Fan

We are looking at is groundspeed not his velocity. His groundspeed can be + or -.

So if two people are standing in the isle facing each other, one is going positive and one negative? Does one therefore not get to the destination?

What is the ground speed of someone facing sideways?

[Jet_Fan]

Originally Posted by Wizofoz

What does a negative ground speed look like?

Land a parachute with a negative groundspeed and have someone video you. Whether groundspeed it + or - does matter sometimes.

[Wizofoz]

Originally Posted by Jet_Fan

Land a parachute with a negative groundspeed and have someone video you. Whether groundspeed it + or - does matter sometimes.

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