Dalton computer
 

Hello all,

I am planning to fly to a boat and they do not have relative wind indicators. If I know the boats course/speed and the wind how do I calculate the relative wind over the deck using a Dalton computer?

Many thanks.

Just take a clean sheet of paper, draw the speed vector ( arrow ) of the ship, put the wind vector at the speed vector ( tip to tip ). Connect both ends of the vectors and you get the relative wind vector....

skadi

The easiest way using the Dalton I can see, without getting unduly complex, would go like this:

1. Put windface centre grommet on any convenient round number on the TAS centreline scale, eg 100 kt. Plot the boat's course and speed as a wind cross, eg if the boat's true course is 270 and it's doing 20 kt, you'll plot a cross at 270/20 from the centre point.

2. Say wind is 180 at 30 kt, rotate the ring until 180's at the top, and, starting from the first cross you made, go straight down 30 kts worth of distance (using the original 20 kts worth of wind as a distance reference). Make another cross where you end up.

3. Rotate the ring again until the second cross is at the top and read off the wind (I get about 325 at 35 kt, give or take).

This is just like adding the vectors on a bit of paper, but a bit easier because you can do it on the computer face. The downside is that it's only as accurate as your distance estimate for plotting the second cross, but it should be pretty bloody close.

Now you know the sum of the real wind plus wind due to boat movement, you can easily work out how it will affect the deck using the clock code:

eg, in this example, boat nose pointing 270-ish, sum of the 'winds' 325, that's about 60 degrees off the nose, so I would apply all the wind as crosswind and 1/2 of it as headwind, call it a 17 kt headwind and 35 kt crosswind from the right, assuming I'm approaching up the centreline of the boat from astern.

If you're coming from another direction, just work it out the same way, or if you just wanted it in naval terms, you wouldn't bother with the clock code and say the wind is green 055 at 35 or whatever they say.

Whoops - it goes to show I shouldn't work on flight planning problems and drink beer at the same time!

Sorry, I stuffed up in my method above by not being consistent with which way I applied the wind. On reflection, it should have gone like this:

1. Get boat's course and speed. From the centre grommet on the wind face, plot out in that direction the appropriate number of knots. (eg for boat going 270 at 20 kt, put 270 at the top and plot 20 kts straight up on the TAS scale. Make a mark.

2. Get the wind direction and speed. Make a mental line from the centre grommet in that direction (ie TOWARDS where the wind's coming from), and then parallel that from the first mark a distance equating to however many knots of wind there are. Make a second mark where you end up. (eg for the wind 180 at 30kt, go from the first mark 30 kts worth of distance (estimating from the TAS scale) in the same direction as 180 is from the centre point.)

3. Rotate the outer ring until the second mark is at the top. You can now read off the wind.

What I've done there is just add vectors, really. We're looking from the boat's frame of reference, and there's one relative wind velocity caused by the boat's speed, to which we must add the actual wind. If you 'step' into wind both times, you'll end up with the right answer. For the example I used above, it should have been about 220 at 37 kt.

Just working through another example - boat 330 at 25 kt, wind 210/15, I get the resultant 285/22.

Takes about 10 seconds.

Excuse my ignorance but why would you want to try to calculate it...?
If the wind is blowing check the water for the wind direction, and land accordingly. If there's not enough wind to pick up a wave then it is neglegible.
Trying flying parrallel next to the boat, keeping same speed and heading and check what you ASI gives you as this is what you'll be landing in.

If you're dead bent on getting results, I suggest using this APR E6-B2WHL Pilot Flight Computer - Marv Golden Pilot Supplies
I bought one for my JAA exams and the wind arm worked a treat and is quick and accurate... and you won't have to take your hands off the controls to fuddle with pens and rulers :ooh:

Another thing is, that almost every boat has at least one relativ wind direction indicator: The national flag!

skadi

Really......why does everyone need to make this difficult...

Skadi got it right.....but if in doubt, why not ask the boat to stop in the water and look at the flag again.......

Ship Operations
 

Be Careful.





Charlie

Oh yes.

Depending upon the deck size and location....having the ship/boat steer a course that puts the relative wind at the best direction for the deck makes sense as well....just as adjusting the ships course to minimize deck movement is an issue.

Relative Wind direction alone is not the whole answer.

It is very useful to be able to talk with the ship's captain, or whoever's in charge on the bridge. If you don't have marine VHF with a pre-arranged channel to talk to them, then they should have at very least an aeronautical hand-held set which can be kept charged up. Generally hand-helds are okay up to 5 miles away.

Most vessels make at least a little smoke which should give you a good idea of the relative wind. As someone has already stated, the vessel's flag/s can also be good indicators.

Trying to pre-calculate relative wind isn't much use, because the vessel could easily change course before you get there.

Wind "lanes" on the sea surface are at right-angles to the prevailing wind, so you only need to figure out which of two directions the wind comes from. As you approach the deck at say 50 knots (or before your ASI becomes unreliable at low speed) compare it with the groundspeed showing on the GPS. Groundspeed must be lower than or equal to airspeed.

Generally the limits of vessel movements for offshore passenger flights by day are pitch 3 degrees either side of vertical, roll 3 degrees either side of vertical and heave 3 metres total. This is quite a lot and night limits are about half of these.

Have chocks ready for the wheels after landing, unless your machine has skids. Beware if your aircraft has an autopilot and a low-ish main rotor e.g. S76. The autopilot will try to drive the rotor disc tilt to maintain level and this could cut heads off if the vessel is pitching/rolling much. Rather disengage the autopilot if there's no likelihood of needing to lift off quickly and avoid letting pax on/off with rotors running.

Be even more aware of the tail rotor in relation to vessel's aerials, etc which could be moving around. Get some dual training in offshore landings and don't land if you have doubts while attempting to do so.

No, wind lanes are in the direction of the wind, at right angles to the chop, like this:

Google Image Result for http://upload.wikimedia.org/wikipedia/commons/4/42/Loch_Bealach_Culaidh_-_geograph.org.uk_-_727673.jpg

Obviously this picture's on a lake, but the same thing happens on the sea under the right conditions.

O-P. might be an idea to tell us if you`ve ever landed on a ship before, what ship/size deck,what aircraft,and deck landing limits,so better answers can be offered...

Steady on there, sycamore, you're bringing logic into the mix!

Mmmmm ...

Comedyjock ... you make me very nervous for you ....

I assume from your question you have very little experience with shipboard/maritime operations ... if wrong I apologise ..

I would REALLY suggest you should before embarking on this flight seek out an Ex Naval (Heli) or 'Offshore' aviator and pick his brains (whatever might be left ?? :O) and get some in depth advice as to how to go about your proposed task.

ALL deck landings should be considered as 'confined areas' and treated with utmost respect ... the opportunities for massive 'F**k Ups' are endless .. :eek:

We really need to know what size ship and what a/c you propose to use ... in order to offer better advice (as per sycamore's advice) ...

Be vewy vewy careful ... relative wind might be the least of the issues you encounter !!!

Arm out of the window With respect and after 30 years of flying to offshore destinations including boats and barges, wind lanes almost never appear in neat straight white lines as depicted in the link. Mostly the sea state doesn't allow for such straight lines to form and the only clearly visible clue is the line formed by the oncoming waves at right angles to the wind.

I have mostly looked carefully without success to see these straight white lines, but almost invariably was obliged to resort to the waves from which such lines or lanes might arise.

In the North Sea the wind strength needs to blow from a constant direction for many hours and get above about 30 knots before such lanes become visible. I guess that the person asking about Dalton computers wouldn't be operating in such strong winds, so the wave lines would be an easier clue to observe.

I accept that wave lines aren't actual wind lanes, but like the lanes they are a consequence of the wind and seem more reliable as an indication. Hopefully he will see this distinction for himself.

Fair cop.

In the area where I've done most of my overwater winching and shipboard ops, around the big breakwater which is the Great Barrier Reef, the sea state can often be such that wind lanes form with a decent breeze over a relatively calm sea.

However, the waves being at right angles to the wind is also a bit of a furph, I must say. Depending on the swell, coastal and sea floor shape, prevailing wind conditions and no doubt a lot of other variables, the waves don't necessarily show you what the current wind's doing as you would certainly be aware, I'm sure.

I guess the bottom line for comedyjock is what spinwing said. May I also add for comedyjock, make sure the helideck is rated to take the weight of your machine! Don't land on it unless you know you won't go through it.

Since comedyjock has been posting on PPRuNe for about ten years and was a FAA Lynx pilot with a lot of experience, I suspect that he may know a little about deck landings, wind lanes and relative wind over the deck ;)

Colibri49, I would not agree with your assertion re wind lanes. They certainly do not form a right angle to waves! An early lesson taught to all ASW pilots is to recognise wind lanes since life is much easier if you approach the dip into wind, and the only conditions where wind indications/lanes are negligible or difficult to see are very light (<5 knots) in some sea states. I never recall having problems with conditions you describe:

Wind lanes do not require hours of steady wind nor be above 30 knots, but when the conditions get a bit rough in the North Sea you will be able to get wind direction from the spume/spray of the top of the wave crests instead. There are many natural ways of getting wind direction, they should be a basic part of any aviator's early education: be it overland or overwater.

comedyjock, re your Dalton query I'd suggest that it is a basic wind/course calculation: pencil at the ready?

Pity comedyjock didn't give a bit of background to save us all going off on tangents, not that we wouldn't've anyway...

Looks like he's posted and p****d off!:)