Wind correction for GC steering
-
OK - my 2 cents (and at current exchange rate 2 pence as well). Are you sure it is a bug? Just thinking about this for about 10 seconds, the tadpole should never be aligned precisely - the reason being that the tadpole follows a great circle, and the steerpoint diamond is line of sight… Now whether that is the same as your bug I am not sure…
-
The STPT diamond is line of sight. Great circles are also line of sight. No issue.
-
The STPT diamond is line of sight. Great circles are also line of sight. No issue.
Not quite true. A straight line between two points on a sphere is a great circle, where a line of sight is not viz: https://www.flightradar24.com/blog/flight-paths-and-great-circles-or-why-you-flew-over-greenland/
It might not be the issue, nor might it be how the STPT is mechanised (however I believe it is, otherwise why have both a tadpole and a stpt?), however the straight line in RL is not Lat2-Lat1^2 + Long2-Long1^2 on the surface of a sphere.
-
Rhumb vs. GC line should be super tiny at that dozen miles.
I’m curious if GCSC isn’t aligned as expected from any direction and what happens if you actually follow the GCSC exactly. Do you miss the waypoint? Do you fly a curved path?
-
Rhumb vs. GC line should be super tiny at that dozen miles.
I’m curious if GCSC isn’t aligned as expected from any direction and what happens if you actually follow the GCSC exactly. Do you miss the waypoint? Do you fly a curved path?
Rhumb lines cross all lines of longtitude at the same angle. It is an approximation of a great circle (and has a great advantage that it can be drawn in a mercator projection chart as a straight line).
As a great circle (equivalent to a straight line on a plane) is the shortest distance over a sphere between two points, you will not miss the steerpoint, as the great circle will be between your current position, and the steer point if you steer a great circle.
-
Great circles happen to be straight lines on a sphere. Line of sight also happens to be a straight line on a sphere.
Also, for BMS, a rhumb line and a great circle are identical, because there is no curved earth navigation code. Geometrically? It is flat. On a flat plane, line of sight, great circles, and rhumb lines are simply straight lines.
If you follow the tadpole, you fly a curved path (not a great circle) which does overfly the waypoint. The degree of curvature is proportional to the amount of wind perpendicular to your flight path. As a result, your heading changes over time. In short you are blown off course by the wind.
-
This post is deleted! -
Ok … we neeeeed … someone …
-
I thought this was what the DRIFT C/O switch was for…if it is OFF (HUD “caged”, in my lingo) you’d fly the arcing path to/over the steerpoint, and if it is in NORM that the system will show you the wind corrected steering to fly?
…which is a reason I prefer to land with it in OFF. For VFR approaches, anyways.
-
mmm … sorry but, IMO, everything is fine in 4.33.2
NORM
DRIFT C/O
Looks alright to me! (?)
-
Clearly merits investigation. Ill have to see about replicating my results from yesterday.
-
Holy sh*t… 6 pages on steering. The steerpoint diamond is based off LOS. Herm the Sperm or the GCSC or the tadpole is all based off, surprise surprise, great circle steering.
The shortest route is by placing the FPM over the tadpole. This is wind corrected as long as you’re not in DCO. If you are in DCO then you will just home to the station with a constantly updated heading.
At long distances the tadpole and FPM will not match the steerpoint diamond in azimuth because because you’re not flying a straight line to the steerpoint but an arcing path with the curvature of the Earth. As the distance decreases then the difference (or error as the -34 calls it) will become less and less. This is because your heading is actually changing when you fly a Great Circle route. Think of it this way, when you’re looking at the steerpoint diamond you are looking at some point in space by looking through the Earth (at far distances). That’s all it is. Additionally, if you could just put the FPM on the steerpoint diamond there would be no reason for Herm the Sperm.
-
Er… got a page number for that?
A constant line of sight doesnt have a constant heading anyway. That cannot be the source of this error.
The tip about drift cutout is very useful - I dont think BMS reflects that presently.
-
nooooo its back
Nothing had changed in this area
Please fly 30kt crosswind to show if there is a problem
Here I just see what could be a rounding issue
-
The one thing that is clear from the above is that I shall not call it the tadpole or GCSC anymore. Herm the Sperm it will be for now on.
-
Tadpole placement represents GC steering error by position relative to FPM. There’s nothing wind corrected about it. When GCSC is on FPM that’s zero GC steering error by construction. Since GCSC-FPM relative position is how the steering error is expressed then putting the thing on the thing works the same, drift c/o or not. The GCSC tail however might not be perfectly vertical at thing-on-thing alignment. At no steering error you might have a relative bearing to steerpoint other than 12 o’clock in a crosswind.
I have read however that the steerpoint diamond isn’t rendered on the HUD with the same care as the GCSC and can be wrong at long distances. Falcon isn’t a spherical navigation environment and I seriously doubt the code is copy-pasted from the real jet as we’d have a lot more noticeable discrepancies. For fun real world Kunsan-Pusan is calculated constant azimuth and GC:
291.64° initial
290.29° final
290.97° constant directionThat’s a 124NM path and up to about ~1° difference from sitting in the middle which would be noticeable on the HUD as a diamond misalignment. That shouldn’t happen in a flat Falcon Earth of course as GC and constant az solutions are one and the same.
-
Seems to be working now. Perhaps it was just a small amount of roll, or a heading misalignment, or something. Pendulum effect perhaps.
Although according to FoxThreeTwoShip above, the current DRIFT C/O mechanism is incorrect.
-
Great circle route (shortest route) vs constant bearing route (loxodrome or Rhumb line).
On the two pictures below, the shortest route is the red route. The constant bearing is the blue route:
To get something more visual go there: http://www.sciences.univ-nantes.fr/sites/genevieve_tulloue/Meca/RefTerre/Orthodromie1.php
… Clic Orthodromie (9729Km) and then Loxodromie (11354Km).
-
Pretty sure no one is confused between great circles and rhumb lines, but that is quite visual enough.
-
Pretty sure no one is confused between great circles and rhumb lines, but that is quite visual enough.
… I’ve just seen the opportunity to give some info for ppl who are not familiar with this. Not everybody here are professionals.