kentb

kentb

Well Known Member
I have noticed as I play around with the Seattle Avionics Voyager flight planner that the world is not flat.:eek:
I entered a flight plan from KPDX (PORTLAND, OR) to KMLS (FRANK WILEY FIELD, MT) and scale to map so that I can see the entire route. When you grab the map and slide it sideways you can detect that the earth is not flat.
It is also noticeable that the path is not a straight line.
I know that the shortest distance between two point is often a curved line when plotted on the earth surface, but I usually don't fly far between landings and so don't notice this often.

Now the point of my post, could one of you high powered CFIs remind me (I am sure that I at one point I knew this), the NavLog notes that the Meg Hdg is 56 degrees, if I plug KHLN into my GPS (430), will it show me 56 degrees all the way to KHLN? Assuming that my AP is flying and keeping dead on course.

Kent
 
I've never flown a 430, but every other GPS and Loran I've ever flown will update the course to be flown as you proceed along the route. Even on relatively short (400-500nm) east-west trips there can be quite a few degrees of course change. It gets more pronounced at higher latitudes. (Interesting bit of trivia: in a no-wind situation in a perfectly rigged airplane (yeah, right!) the airplane doesn't have to accomplish a turn to effect the course change on a GC route.)

BTW if you want a quick and dirty look at a great circle route, just stretch a string between departure and destination points on a globe.;)
 
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on a recent trip

from Las Vegas to Columbia, SC I saw 15^ or more heading change. Don't remember the exact numbers, but it was a non-stop flight in a C-550. Even on much shorter flights we see a few degrees change. :eek:
 
I won't swear to it, but I think that the particular mapping method used in Sectionals or WACs is such that a straight line drawn on them is close to a great circle route.
 
Magnetic headings

from KPDX (PORTLAND, OR) to KMLS (FRANK WILEY FIELD, MT) my Jepp flighstar shows a no wind MH of 062 degrees for 26 NM during climb, 073 degrees for 615 nm in cruise and 078 in the descent for 56nm. These are magnetic headings so the earth is definitely round and the compass variation also changes quite a few degrees as you travel east. (true course + or - vatiation is Magnetic course and MH with no wind). Variation 17 deg. east at Portland and KMLS is 12 deg. E. So 5 degrees change in variation there too.
I turn to my initial heading push direct to then enter on my 430 and 496!
I also write down the time, my heading, time to destination and GS while in cruise in case the goodies decide to quit working...and I have to get there the old fashion way.
Hope this helps more than confuses.:)
 
Kent,

The way I was taught about great circle routes, if you were to put two sticks into a globe that went through your departure and arrival points and met at the core of the globe (for accuracy) then ran a string between them on the globe, that's your GC route. If you flatten out the globe to a map projection, it shows a bow to the north in the northern hemisphere, and a bow to the south in the southern hemisphere. If you fly between two points on the equator, its a straight line on a flat map too.

Your mag course will increase when on a great circle route west to east, and it will decrease when on a GC route east to west.

That flip-flops south of the equator...but the water in a sink drain goes round and round the other way down there too! Be fun to take a trip in the RV to prove that, wouldn't it! (Rosie...?!?!)

Scary stuff this flat earth! Peering from the edge is fun though! ;)

lookoutbelow.jpg


Cheers,
Bob
 
rvmills said:
If you flatten out the globe to a map projection, it shows a bow to the north in the northern hemisphere, and a bow to the south in the southern hemisphere. If you fly between two points on the equator, its a straight line on a flat map too.
Click to expand...

This sort of depends on the map projection used...somewhere around here I have an interesting (ok, well, interesting to a mathematician) book called (duh) Map Projections: A Reference Manual . Projections can be defined that have all sorts of properties...equal area, parallels as line, parallels as circles, local invariances, etc. Fascinating stuff, actually.

There's no way to get a map projection that gives you great circles as straight lines over the entirety of the globe (or even hemisphere), but there are some projections that give you that property locally, IIRC. (ETA: Look up gnomonic projections for maps on which great circles are straight lines).
 
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My experience with a Garmin GNS 430 is that if I plug in my destination airport using the GOTO button, all I have to do is follow the magenta line. Actually, I just engage the autopilot and it follows that line. I am always going straight...or so it seems.

So take the Wizard of Oz saying and change it to "Follow the magenta line, follow the magenta line."
 
Flying Scotsman said:
This sort of depends on the map projection used...somewhere around here I have an interesting (ok, well, interesting to a mathematician) book called (duh) Map Projections: A Reference Manual . Projections can be defined that have all sorts of properties...equal area, parallels as line, parallels as circles, local invariances, etc. Fascinating stuff, actually.

There's no way to get a map projection that gives you great circles as straight lines over the entirety of the globe (or even hemisphere), but there are some projections that give you that property locally, IIRC. (ETA: Look up gnomonic projections for maps on which great circles are straight lines).
Click to expand...

Granted...good call. I'll add the caveat of "maps or charts one might normally use in the cockpit or in the planning room". But there are some pretty cool map projections out there, some with some pretty skewed land masses...didn't know one was a great circle=straight line projection. Are the continents recognizable, or are they all stretched and scrunched up? Cool stuff, and point well made!

Ron Lee said:
My experience with a Garmin GNS 430 is that if I plug in my destination airport using the GOTO button, all I have to do is follow the magenta line. Actually, I just engage the autopilot and it follows that line. I am always going straight...or so it seems.

So take the Wizard of Oz saying and change it to "Follow the magenta line, follow the magenta line."
Click to expand...

True Ron, but as you follow the magenta brick road ("pay no attention to the man behind the curtain!"):p, your course and heading magically change...mysterious stuff, and good for an occasional bar bet win perhaps.

Cheers,
Bob
 
rvmills said:
Granted...good call. I'll add the caveat of "maps or charts one might normally use in the cockpit or in the planning room". But there are some pretty cool map projections out there, some with some pretty skewed land masses...didn't know one was a great circle=straight line projection. Are the continents recognizable, or are they all stretched and scrunched up? Cool stuff, and point well made!
Click to expand...

Gnomonic projections aren't horrible in terms of distortion.

Aren't Sectionals and WACs Lambert Conformal Conic projections? (Yes, I'm lazy tonight and don't want to go upstairs to get my flight bag out LOL!)
 
Flying Scotsman said:
..........Aren't Sectionals and WACs Lambert Conformal Conic projections? (Yes, I'm lazy tonight and don't want to go upstairs to get my flight bag out LOL!)
Click to expand...

Yes, they are...and furthermore, on a long cross country or any other flight in which you maintain a constant altitude, you are actually diving the whole way, in order to follow the curvature of the earth.

Regards,
 
Thanks for the education...

What prompted me to question this, was the post about not using you glass and doing nav the old fashion way.
So if I printed out my plan from Voyager and was flying along with the GPS/AP taking to my next airport and the GPS went out. What would I do. I can just enter the heading from the plan into the AP and continue!
I will need to break out the map and start figuring out heading based on what I see down below or if available start using VOR (what is that?) :D

Kent
 
pierre smith said:
Yes, they are...and furthermore, on a long cross country or any other flight in which you maintain a constant altitude, you are actually diving the whole way, in order to follow the curvature of the earth.

Regards,
Click to expand...

:) Except that the surface of a sphere is locally isomorphic to a flat plane...
 
Cool....

pierre smith said:
Yes, they are...and furthermore, on a long cross country or any other flight in which you maintain a constant altitude, you are actually diving the whole way, in order to follow the curvature of the earth.

Regards,
Click to expand...

I should get better fuel burn, sense I am going down hill.:rolleyes:

Kent
 
Don't forget changes in magnetic variation. On some parts of our tiny blue planet (not the nicest bits-think Hudson's Bay) a 500 mile trip will give you a magnetic heading change of 180 degrees.
 
pierre smith said:
Yes, they are...and furthermore, on a long cross country or any other flight in which you maintain a constant altitude, you are actually diving the whole way, in order to follow the curvature of the earth.

Regards,
Click to expand...

kentb said:
I should get better fuel burn, sense I am going down hill.:rolleyes:

Kent
Click to expand...

And I should be going faster (don't tell Bob Axsom this...or the other racers!)

Flying Scotsman said:
:) Except that the surface of a sphere is locally isomorphic to a flat plane...
Click to expand...

I knew that! ;) (ahem)

Ron Lee said:
Actually it may be more like a satellite in LVLH mode if I remember correctly.
Click to expand...

I knew that too! ;) (ahem again)

(You guys been goin' to that "Elippse" school of big words and acronyms?! :p)

Good stuff!

Cheers,
Bob
 
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