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lumpy
24th Feb 2002, 21:01
According to my JAR Instrumentation course notes the sign convention for an aircraft flying east would be negative and the opposite for west. That is, the gyro would drift by e.g. -7.94 degrees per hour for an aircraft flying East at 400 kts along 45N. Now, with a different set of notes the sign convention has changed and I'm struggling to visualise the mechanics of Transport Wander.

Any gyro boffins out there?

Ta.

EGCC4284
24th Feb 2002, 22:32
Is it not -6.66 per hour.

I thought it was Ground speed X Tan 45n divided by 60 = answer.

East in northern hems = -6.66.

Please someone correct me if Iam wrong.

Checkboard
26th Feb 2002, 06:58
Transport wander is negative for East in the Northern Hemisphere, and positive for the Southern Hemishpere.

Capt Pit Bull
27th Feb 2002, 05:46
You can learn signs by rote if you wish. Alternatively:

Most people know where the sun rises and sets. This tells you direction of Earth Rotation.

That is all you need to figure out sign of earth Rotation. If you can't figure it out, with a sketch if necessary, then there are all sorts of compass / instrument / navigation things that are going to be hard work.

(If stuck, I can tell you how to visualise this in a simple and effective way).

Once you've sussed out Earth Rotation for one hemisphere, the other hemisphere will be opposite.

As far as transport wander is concerned, the basic mechanism is the same as Earth Rotation. We have changed the angle between the gyro datum and the compass datum. How this happened is irrelevant. I.E. the effect is the same whether the earth rotated us around through space, or whether we flew round the Earth, or a bit of both.

I.E.:

Any time you fly the same direction as the Earth rotates, Transport Drift gives you more of the same (so same sign as Earth Rotation in that hemisphere).

Any time you fly in the opposite direction as the Earth rotates, Transport Drift causes the reverse effect, so opposite sign to Earth Rotation for that hemisphere.

Take that on board and you'll never need to memorise another table of plus and minus signs again! And you'll be able to recreate it from scratch should your brain dump in the middle of an exam.

CPB

Keith.Williams.
28th Feb 2002, 03:35
Lumpy,

The responses above deal with the magnitude of transport wander but probably don't do much to help you get your head around the basic concept. . .I am unable to draw diagrams in this forum, so I will describe how to draw one on paper. This has the advantage that you can do it in a couple of seconds, so if your mind suddenly goes blank in the exam, this might help you to recover. For the purposes of simplicity, I will ignore the distinction between true north and magnetic north.

Take a plain sheet of paper and draw an N in the centre. Let this represent the North Pole. Using this as the centre, draw a 1 inch diameter circle to represent a line of latitude. Now using the same centre, draw a circle of 4 inches diameter to represent the equator. We can now imagine that we are looking down on the earth from a point in space directly above the North Pole.

On this scale the 1 inch circle represents a latitude of about 75.5 degrees north, so the surface of the earth within it is almost parallel to the surface of the paper. Though not entirely accurate, this statement is close enough for government work (as Stephen Byers would probably deny ever having said).

We would therefore be looking almost straight down on an aircraft parked on the 1 inch circle. So a gyro aligned on the North Pole would appear to point straight at the N at the centre of the circle. At the bottom of the 1 inch circle, draw a ¼ inch arrow pointing up the page towards the N. Let this represent the spin axis of a gyro aligned on the North Pole. Now draw a slightly thicker ¼ long horizontal line across the centre of the arrow, to represent the rotor of the gyro. Mark this gyro position 1.

To represent flight on a westerly heading, draw a small arrow pointing in a clockwise direction on the circle next to gyro position 1. Now move around the circle 90 degrees in a westerly direction and draw another gyro. Because the gyro is rigid relative to space, which is represented by the page, it must still point straight up the page. Draw another clockwise arrow to represent the westerly direction of flight, and mark this position 2.

Go a further 90 degrees around the 1 inch circle and draw a third gyro and clockwise westerly flight direction arrow, directly above the N. Mark this position 3. Once again the gyro must point straight up the page. Now move a further 90 degrees around and repeat the process for position 4.

Throughout this experiment the aircraft was always heading west, but look at what has happened to the gyro. It was pointing north in position 1, but in position 2 it is pointing in the direction of flight, which is west. But the gyro will be telling the compass that this direction is north, so the compass will indicate that the aircraft is flying north on heading 0. At position 3 the gyro is pointing south (away from the N), but still telling the compass that this is north. So the compass will indicate easterly flight on a 90 degree heading. At position 4, the gyro points in the opposite direction to flight. So the gyro is pointing east, but again telling the compass that this is north. So the compass now indicates that the aircraft is heading south on heading 180. When the aircraft returns to point 1, the gyro will again point north and the compass will indicate the real heading of 270. (All of this will probably sound dreadfully confusing, if you have not bothered to draw the diagram!!!!)

Although the flight was always on heading 270, the compass indicated 0 at position 2, 90 at position 3, 180 at position 4, and 270 upon returning to position 1. This increase in compass reading from 270 to 0 to 90 to 180 to 270, represents positive wander. So flying west in the northern hemisphere causes positive transport wander. Flying east would have the opposite effect, causing the compass to wander in a negative sense.

To see what happens in the southern hemisphere, simply hold the paper up to the light and look through it from the reverse side. This reverses the effect, so when flying west in the southern hemisphere, transport wander is negative.

Now let's look at what happens at the equator. We now need to put the paper face up on the desk and draw a gyro at the bottom of the 4 inch circle. But at the equator the surface of the earth is at right angles to the paper, pointing not up and down the sheet, but into and out of it. So to point north our gyro must point straight out of the paper. To represent this draw a small circle with a dot at its centre. The dot represents the point of the spin axis. Now draw similar gyros at positions 2, 3, and 4, noting that in each case the spin axis must point straight out of the page. We can now see that the gyro has not drifted, but continues to point straight up towards the north. So transport wander at the equator is zero.

For the purposes of the exam you do not need to derive the equation for how transport wander varies with latitude, but you do need to know what it is and how to use it:

Drift due to transport wander in degrees per hour is

= (E/W component of groundspeed in Kts x Tan of latitude) / 60

Using negative values of E/W flight to represents W/E flight in this equation will give an answer of the correct sign.

For example flying west at 400 kts at 45 degrees north gives. .Drift due to transport wander = (400 x Tan 45)/60 = 6.67 degrees per hour.

And flying east at 400 Kts at 45 degrees north gives. .Drift due to transport wander = (-400 x Tan 45)/60 = -6.67 degrees per hour.

For the southern hemisphere just represent southern latitudes as minus values of equivalent northern ones. So for example, 45 south becomes - 45 north. This reverses the sign of the Tan and hence gives the correct sign in the answer.

. .At the end of the day the problem is caused by the fact that gyros are rigid with respect to space, rather than with respect to the earth.

[ 28 February 2002: Message edited by: Keith Williams. ]</p>

skymonkee
28th Feb 2002, 04:52
Well thats cleared that up then.I would advise getting thru the exam by hook or by crook and if yu are a pilot ,land at the nearest airport if all yer nav stuff packs up and go to a hotel as soon as posible till its fixed.

The only transport you should need to "wander" about is "wanderng" where the hotel shuttle bus is.. .Hope this helps.