The Hobbit Hole III - Journey to the Cross-roads!

[HairOfTheDog]

Welcome to The Hobbit Hole!

Journey to the Cross-roads
The Two Towers Edition

Come on! Come in! -if you would like to have some seedcakes and a pint and relax a while. (If it is a special occasion, we still have a few bottles of the old wineyards left!)

Our first thread ( New Zealander builds Hobbit hole ) reached 4,100 posts, and we thought that was big. Our second thread (The New Hobbit Hole ) held us for over 48,000 posts, and we loved it dearly. We talked about moving to a new thread for the last 38,000 posts, but we are really slow to muster! Finally, the time has come. Tomorrow (at 12:01 am, to be precise!) The Two Towers comes out, and we start a new chapter.

[Sam Cree]

Yes, I think current is analogous to a boat for wind to a plane.

The Gulf Stream is the main one to worry about in this area. It's speed is known, so it is possible to figure for it ahead of time with some accuracy.

Wind doesn't have that much effect on power boats, it's more the wave action it produces that affects them. However, wind has a major effect on sailing vessels, causing "leeway," which routinely throughs their courses off by as much as 15 degrees.

What about compass deviation on airplanes?

[Sam Cree]

throughs = throws

[Sam Cree]

"The effect of wind on a boat would depend on the sail area of the boat and would be much harder to develop a rule about."

One method I have heard of is to just look astern and measure the angle of your wake vs. your heading.

[HairOfTheDog]

Compass deviation meaning instrument error, is tested and known for each plane... it is negligible, really.... a 2-3 degree error in a compass that is jumping and rolling 20 degrees constantly in flight is not much of a concern.... at calm moments, the compass is checked against the Heading indicator and adjusted.

Most navigation is not time-distance where heading is your most important thing to know. Navigation with use of aids, where we are tracking a set course toward or away from a beacon on the ground, gives us a course line, and the pilot will make adjustments to heading as the nav instrument indicates variations in course. We watch the movement of that line on the nav instrument I put up earlier.... if it is moving, then we are either approaching or leaving course, if it is centered and stationary, we make small heading corrections if it begins to move.

[HairOfTheDog]

Pretty good test, that!

[HairOfTheDog]

Here is a better perspective perhaps on the effect of wind on a plane, and adjustuments to heading...

In a boat traveling at 10 knots, you are much more a victim of a 10 knot current or a 10 knot wind. And the wind and current would be much more apparent to you because at slow speeds, you remain in the same weather or move through it slowly.

In an aircraft, traveling at 180 knots, a 10 knot wind will cause me to make a few degree correction to crab into it. But it won't remain the same.... over the course of 180 miles traveled in an hour, the wind will have varied quiet a bit over the distance, or even changed direction completely, but more than likely not remained the same.... the wind is a trend we consider, that it is either strong or mild and a general direction, but we are constantly adjusting for real life based on the navigation instruments, or in dead reckoning, testing our predicted heading, against what we see happening on the ground. Dead reckoning is a tough gamble out at sea or in poor visibility with no real landmarks, whether in a boat or in a plane.

[osagebowman]

Hair; you and Sam are quite the navigators. Curious to know if you reset the compass declination for a long trip.

A friend at work is quite the sailing enthusiast, bare-boating in the carrib etc. Finally broke down and got a gps and loves it. A lot of work is being done with gps now, the 3 meter accuracy is plenty good for most uses.

[Sam Cree]

Yeah, the current varies accross the Gulf Stream too, I just figure for average. The actuality is that I go in an S curve only crossing the rhumb line twice, but usually ending up close to the target.

Some boats have quite alot of compass deviation, and it's normally different on each heading. Nice that planes don't have that headache.

Boats do the same, check the DR against reality, then adjust.

Thank God there are not that many 10 kt currents to deal with!

Nowadays, of course, people navigate mostly by GPS, point to point, same as airplanes I guess, the thing takes care of drift, variation, does everything for you.

However it is still considered correct to make the waypoints in terms of lat/lon, only now, down to one / one thousandth of a nautical mile (minute). It's cool. That means within about 12 feet!

I still keep my dead reckoning skills up to speed, though. Going accross the Stream, or around the Bahamas, I always keep a DR plot going, even if just in my head.

Hair, most of the time when I fly, the airplane is not bucking even as much as a boat might be expected to, especially offshore in any kind of a sea. 'Course if you are flying at 10,000 or so feet, it can get pretty rough. But then try crossing the stream in 10 foot seas aboard a 30 footer on a compass course, which used to be done pretty routinely.

Yes, that aids to navigation thing makes great sense for a plane, are there many beacons and signals to follow? Plus, I guess you can often look down.

[JenB]

Hi all,

I'm back safely from my drive and discovered a new used bookstore on my way home, and found two books I'd been wanting there for a ridiculous bargain. That makes up for five hours traveling!

[HairOfTheDog]

Yes, there is an extensive network of VOR beacons as a backbone to air navigation, and often an instrument landing aid for poor weather even at small fields. And that is with the old time technology, nowadays, even higher-end small planes will also have RNAV or GPS.

[HairOfTheDog]

Yes and no..... Sam would, because he converts to true for navigating in a boat.

I would not, because I use magnetic, and true north is only 'interesting', not relevant...

[HairOfTheDog]

I shouldn't have said that it isn't relevant.... because it is.... In a dead reckoning situation with no nav-aids, we measure the direction on the chart in true against the Lat and Long.... and convert it to magnetic to give us the course heading.... over long flights, it will change a couple degrees.

[Sam Cree]

Not sure that there is such a thing as compass declination, but, no, I check the *deviation* formally rarely, since it doesn't usually change from year to year. I keep an eye on it anyway, so usually have confidence in it.

I am not usually correcting magnetic variation to true, but laying out a course according to true bearings, then uncorrecting the heading so that my compass will take me where I expect it to! Since the thing reads magnetic.

[Sam Cree]

Cool that that guy has another book. How long is this one? Long, I bet!

[HairOfTheDog]

over long flights, it will change a couple degrees.

Long flights being long in the world of little planes traveling 100 knots where I only go a couple hundred miles.... of course in an airliner, it will change many degrees over a couple-thousand mile flight.

[Sam Cree]

Sorry, yes, I uncorrect for variation everytime I lay out a course. My boat doesn't have deviation, so no problem there.

[HairOfTheDog]

Declination and variation the same thing? - That is the assumption I was making.

Compass, Declination, Mapping

[Sam Cree]

ack! every time

[Sam Cree]

Yes, it's the same according to that link. I have never heard it called that by a boat person. In celestial nav declination refers to the height of a star, etc. I think, above the horizon. Gotta check if I'm right on that.

Maybe in map reading it's declination, that could be.

[HairOfTheDog]

There is another error that exists related to air travel that I think is called magnetic tilt error, meaning an apparent error at high altitudes in upper latitudes because the compass is pointing down toward the earth as well as north....

This is a cool website I just found that shows graphically and explains the turning errors that happen in aircraft that make the Mag compass so useless during a turn.

The Whiskey Compass