Wind and Groundspeed

[jackyboy]

I can't get an answer to the question below which came about when I was practising for the diversion part of my skills test.

As a rule of thumb, if the wind is 90 degrees to your heading you make no adjustment to your groundspeed.

Having played with my new toy (CX2 flight computer - why do the FAA allow it for exams and the CAA don't??) I find that this is not the case. The higher the wind speed the more the groundspeed changes. Why ??

Anyone know the definitive answer ?

P.S. Passed my skills test last month, so this is just to satisfy my curiosity.

[Flyin'Dutch']

How about the fact that with a 90 X-wind you will have to allow for that when you want to stay on track by 'steering into wind'

Your forward speed vector will be partly used for this. The net result will be that you will go slower forward in your track direction.

FD

Rule of thumbs are just rules of thumb not the precise answer!

[FlyingForFun]

I assume you mean wind at 90 degrees to track, rather than 90 degrees to heading? This would be a far more useful scenario - and it makes a difference to the results.

Wind a constant 90 degrees from track, airspeed constant - as the windspeed increases, the groundspeed decreases.

Wind a constant 90 degrees from heading, airspeed constant - as the windspeed increases, the groundspeed increases.


(Actually, I was typing a reply agreeing with FD, when I noticed this subtle error. FD says, with respect to the wind being 90 degrees from heading, that "The net result will be that you will go slower forward in your track direction". This isn't true - but it's subtle enough that I didn't catch it until I actually tried to draw it so that I could describe how to prove it!)

FFF
-------------

[drauk]

It all becomes pretty clear when you draw the vectors on a bit of paper. Your groundspeed along your track will not be changed by a 90deg cross-wind, but your groundspeed to the point that you would reach if there was no wind (and thus presumably where you want to actually go) will be changed - the amount dependent on your speed and the wind speed.

[Mike Cross]

The difference is between Track and Desired Track.

Track is where you actually go, Desired Track is where you intended to go.

This is easily forgotton by people who use GoTo's on their GPS. Put in a GoTo with a crosswind and your track will not resemble a straight line in the slightest, nor will your time en-route be as calculated.

Any sailors who have been "sucked down the Swinge" will know exactly what I mean (and no it's not illegal or immoral:D )

Mike

[bar shaker]

A
------ W
.
..
...
....
.....
......
.......
T__TMG

I'm not sure if this will work when its formatted but it should explain in simple terms.

The wind strength is W to A and your intended course is A to T. To counter the drift, you must fly the distance of A to TMG and as this distance is longer than A-T, your ground speed will (A to TMG)/IAS not (A to T)/IAS.

Not a dig at you (and I am a big fan of GPS), but Nav exams should be computer free.

[2Donkeys]

I guess that everybody is agreeing, although I have never heard anybody use the term "desired track" in aviation conversations before.

In zero wind, you point the aircraft at its destination and the heading you steer is the same as the track across the ground.

Once you introduce a wind across the ground track, you will need to vary the heading you steer in order to avoid being blown off your ground track. Part of your thrust is thus deployed countering the effect of the wind, and the speed at which you progress towards your destination is reduced.

No more to it than that.

2D


PS: Lets not worry too much about great circles :D

[bookworm]

This is easily forgotton by people who use GoTo's on their GPS. Put in a GoTo with a crosswind and your track will not resemble a straight line in the slightest, nor will your time en-route be as calculated.

Not quite sure what you're driving at here, Mike. Why shouldn't I or wouldn't I fly a straight line track?

[CSX001]

I think Mike is becoming confused with boating from one bank of a river to another ;)

If you hold off the drift induced by the wind, there is no reason at all why your ground track will not be a straight line.

Charlie.

[Mike Cross]

bookworm

A GoTo in your GPS gives bearing, distance and a track line from your current position to the waypoint. Your current position is moving, therefore this track line will also move around your destination as a result of you deviating from the desired track.

Imagine the wind is from the South and you stick a GoTo in your GPS to a waypoint 20 nm East of your current position. You set off heading 270. As you fly along the wind drifts you North. Your GPS will still be giving you bearing and distance but the bearing will have changed to something less than 270 as your destination is now further South relative to where you now are.

The GPS will be showing the track from your current position to your destination, not the track from where you started, as it would if you had set a Route rather than a GoTo. Your resulting track over the ground will be a curve, drifting to the North of a straight line between your start point and destination with your heading gradually turning more South to correct for the drift.

Of course you could try flying so that the bearing to your destination remains constant as CSX001 suggests but this requires you to establish a suitable offset to your heading, which usually involves deviating from the direct track while you work out how much offset is necessary.

It's similar to the problem trying to home on an NDB by simply pointing at it.

2D

Not an aviation GPS but the map display on my Garmin 12XL can be set to
DTK Up (Desired Track Up)
Track Up
North Up

My Magellan 315 offers
Course Up (equivalent to Desired Track Up)
Track Up
North Up

I was using "Desired Track" to indicate the track you wished to fly and "Track" to indicate the track you actually flew. For some unaccountable reason these always appear different on my GPS:ugh:

Mike

PS
I am assuming here that we are talking about a straightforward GPS without a heading input and therefore not capable of calculating wind drift and presenting a course to steer.

[CSX001]

Mike

I don't know what kind of aeronautical GPS you use, but your description does not match the way mine works.

So there I am flying along, and I decide I want to route direct to DETling.

I hit GOTO DET and a large pink line pops up on the screen from my current position to DETling.

I could hold my current heading and not turn towards DETling, and the line will not follow me. Rather, the line will stay where it was, and I will deviate from it. This is common functionality amongst the Garmins; less so amongst the cheaper more generic GPSes.

If I wish to fly a constant track towards DETling, I just follow the original line. I normally have "track up" on the map, making this very easy. If the track line is perfectly vertical and the little aeroplane is on the track, I know that I am holding off exactly the correct drift.

Tell me where I am wrong...

Charlie.

It occurs to me that I should have added.

Most fixed GPS installations in aircraft are also connected to the HSI or VOR CDI Indicator.

When you hit GOTO, the needle of the HSI/CDI is initially centred. If you fail to track correctly towards the target you leave the Track line, and your HSI/CDI shows a deviation.

If the installation were to work the way you describe it, the HSI would always be centred.

Charlie.

[Brooklands]

Imagine the wind is from the South and you stick a GoTo in your GPS to a waypoint 20 nm East of your current position. You set off heading 270.

I take it this is the scenic route then! :D

[Mike Cross]

No wonder I never end up where I intended to go!

I of course meant West from your current position:ooh:

My apologies to all, I was wrong.:uhoh:
I have just carried out a test on my Garmin 12XL and it does not work in theway I had assumed. The manual is not at all definitive so I've been walking around a bit.

If you set up a GoTo it draws the line from your current position to the waypoint. This line does NOT move as your current position changes, so you can see where you are in relation to your desired track.

There are probably now some puzzled people out on Southsea Common saying "I wander what that nutter was doing walking round in circles?"

Mike

[bookworm]

No, I think I see what you mean, Mike. The problem you highlight is the tracking vs homing issue, which is, as you suggest, similar to problems encountered if you just "follow the needle" on the way to an NDB.

If you make your heading equal to the bearing (BRG) to the waypoint indicated by the GPS, you'll fall into exactly the same trap and curve your way to the waypoint. The important difference is that with a GPS you have an output of your actual track (TRK) over the ground. If you make the TRK equal to the BRG to the waypoint, you will fly in a straight line to the waypoint.

The GPS makes drift correction much easier by telling you your TRK, but you still have to remember that your heading needs to be offset for drift.

I'm always amazed at the number of people who don't have the TRK displayed as a data field. The only argument I can think of for not having it there is that it spoils the fun of getting lost. :)

[Timothy]

Oooo...you lot do think a lot don't you? :-)

Just two buttons do it for me...D on the GPS and NAV on the autopilot and you get there in a straight line at exactly the time predicted by the GPS, every time.

W

[bookworm]

Depends on your autopilot, I guess.

If I do that with our KFC 200, the GNS430 moves the needle on the HSI as appropriate, and the autopilot waggles us from side to side keeping the beambar kind of somewhere around the middle, because it only knows how we're doing by reference to the beambar. I can usually do much better in HDG mode by matching the track with the required bearing. The beambar stays centred.

With an autopilot with GPSS, the GNS430 tells the autopilot to turn until the track is the same as the required bearing. And then that pretty little display autoslews so you don't have to put down your coffee...

[Timothy]

...ah well, there you go. My S-TEC is rock solid in NAV and wobbles everywhere in HDG. What can you do?:}

W