Improvements for compass navigation and wind correction

[benjaprud 0]

Hello Little Navmap,

First let me stress how much I appreciate this little piece of software. It's rare nowadays to come across some tasteful, beautifully built and efficient software like this, I absolutely love it !

Lately I've been exploring flying long flights with low and slow planes at high latitudes in areas where navaids are scarce. That prompted me to raise my game when it comes to considering things like magnetic deviation and wind angle correction, which is actually a nightmare with the current sim flight planning tools I commonly use. It turns out that Little Navmap is the only sim related software I've found capable of giving me the average wind speed over the course of my flight, not that I would have expected anything else.

However, the only way to get and display winds aloft data right now seems to be by sourcing the wind data from NOAA, which certainly does the job perfectly well enough. Active Sky does provide an API that can be accessed from HTTP GET requests that seems perfectly capable of providing winds aloft data across the globe. Using X-Plane's wind data would result on the average wind speed prediction to report "no wind". Given that LNM already has some form of Active Sky integration, would you consider expanding that to include this data as well ? I'd expect the Active Sky wind data to be somewhat different from the NOAA one given all the interpolation work that goes under the hood, and more accurate for Active Sky users, although wheter the actual benefits are worth the effort remains to be proven. It would also make LNM aware and capable of predicting the winds while using Active Sky's historical and custom weather modes (although to be honest I personally don't use these myself).

One thing i realise is that covering the whole globe grid would probably need some thousands of HTTP requests to the API every weather refresh, in the API's current state (something around 64800 for a 1° resolution if I'm not mistaken), which may not fit well into the beautiful workflow of LNM, but I'm no programmer enough to judge on that front. There may however be some way to limit the feature to provide wind information to the current flight plan only.

I'm just rambling thoughts at this point and should probably wrap this up. I'm curious to hear about your considerations on the matter.

Thank you.

Edited December 19, 2020 by albar965
Title changed

[albar965 1,040]

Glad you like LNM.🙂

ActiveSky has a file containing the winds aloft for the flight plan: https://github.com/albar965/littlenavmap/issues/419

Not sure about the HTTP API. This is probably not usable for showing world wide winds but probably good enough for flight plan winds.

Keep in mind that the winds are just predictions updated four times a day. I wouldn't be surprised if AS use the same source. X-Plane does.

Alex

 

[benjaprud 0]

Hi Alex, thanks for you answer.

It came to my mind just after posting that ActiveSky probably uses NOAA as a source, thus the benefits would probably not be worth the effort, apart from allowing the use of other than live weather modes.

I'm still poking around to find a reliable method to estimate my magnetic heading. Thus far I was using SkyVector's PDF nav log feature which does calculate and display magnetic deviation and wind correction angles. But after having checked their results against my own calculus, I've found that SkyVector's data and calculus is wildly off. It seems that the only way forward for me is to do the calculations myself, which I'm fine with since it adds a fun layer to the flight simulation. However for that to be possible I'd need to know the predicted average wind values on each leg of my flight plan.

In LNM I've found two different ways of displaying the average wind. One is the average wind over the entire flight in "Fuel Report", the other is the "Wind °M/kts" column in the flight plan. However it seems that I can't get these two values to agree. Trying today with a single leg flight from BIHN to BIKF at 10000 feet, the fuel report displays 94°T, 34kts while the flight plan reports 75°M, 15kts. MD is 9,5°W at departure and 13,1°W at destination. From my few tests I can't understand the relevance of the wind values in the flight plan. Having that column display the average wind along the leg like the fuel report does along the entire flight could be the ideal way for me to gather the information I need to do my heading calculations.

[albar965 1,040]

A few hints if you deal with magnetic declination: Declination is shown in the statusbar for the cursor position. Here is also a chapter which explains how LNM deals with declination in flight plans: https://www.littlenavmap.org/manuals/littlenavmap/release/2.6/en/FLIGHTPLAN.html#magnetic-declination

You can also customize the labels along the flight plan lines in options on page "Map Display 2" and choose to show true and magnetic course. Same for measurement lines.

The "Wind °M/kts" column in the flight plan shows the wind at the waypoint. This is the same as the yellowish wind icon on a flight plan waypoint. I could actually change the column to show average wind since this is already calculated internally. Makes sense IMO.

Alex

[benjaprud 0]

Thanks for the tips, over the last days I've become very wary of double-checking the reference against which each value is expressed, and turned on every option that allows for displaying true and magnetic headings, which LNM does well. I hadn't paid much attention to the status bar yet.

I've finally managed my first LNM planned precision flight using only the compass and gyro ! I made it to the destination within 1,6 nm of the intended track after 361 nm of flying into moderate crosswind, which is less than a degree of error. To get to that point I had to learn quite a bit and ended up reinventing the wheel, the exact procedure pilots use for flight planning.

1 - Get the course of your flight/leg (the track from departure to destination expressed relative to true north). That's the true course (TC).

2 - Calculate and apply the wind correction angle to that value since winds are expressed relative to true north. I used this calculus form to do that, real pilots may use their E6B or any other kind of flight computer. Using that form you'd have to input your true airspeed at the intended altitude and the reciprocal of the reported average wind value (found in the fuel report in LNM) since winds are expressed in the direction which they come from, not the one they're heading towards. That gets you the true heading (TH).

3 - Apply the magnetic deviation at the starting position, since that's where you'll be lining up the gyro against the compass ("East is least, West is best"). That gets you the magnetic heading (MH). If you have to realign the gyro while in-flight (which is hardly accurate with all the compass wobbling), you'd have to estimate what your current position is and adjust your heading based on the magnetic deviation at this position.

4 - Apply the compass deviation which is indicated on the compass before lining up the gyro. That gets you the compass heading (CH).

Now it all makes sense looking at a pen and paper flight planning sheet, which drives you into following all these steps in the correct order.

In LNM, the current "Wind °M/kts" value certainly makes sense in the way the flight plan is currently laid out, but is not really helpful for enroute navigation. I guess the staggered layout that's used on pen and paper flight plans makes more sense for compass navigation. That's one reason why I really liked using SkyVector's PDF flight plans before learning about the existence of LNM, and also before discovering that their WCA calculus and MD data are wrong. As much as I'd like to suggest you to introduce this kind of layout to LNM, I realise that's a quite involved change, and one that may have other downsides I haven't considered. It's still an interesting discussion to have. Another easier way could be to just add a "Wind °T/kts (leg)" to the flight plan.

Thanks for reading my rants, I've modified the title of the thread to let it reflect the way it's heading.

[benjaprud 0]

Oh well, I can't edit much since I'm a new user. The current thread title is there to stay.

[albar965 1,040]

@benjaprud  Keep in mind that the course given in LNM (flight plan table and map) is always the start of a great circle route. It will be different along the line for long distances. Only the measurement lines show the start and end course for a GC line. I think there is also a bug in this display since it uses the declination of the start point only. So magnetic arrival might be wrong.

I'll simply change the wind columns to show average wind at the leg for the next release. This makes sense IMO.
What new thread title do you suggest?

Alex

[benjaprud 0]

22 hours ago, albar965 said:

It will be different along the line for long distances.

@albar965 Well... 😱 Thanks for pointing that out...

I had to read about what's a great circle route and stare at the map for a good two hours to wrap my head around the pieces I was missing. I was afraid that it would make my previous flight once again a lucky shot. I should have known these things after playing so much KSP.

If I get it correctly, assuming you have a perfect directional gyro with no drift and perfectly compensated against the earth's rotation at your latitude, following the same heading on the gyro would guide you along a great circle course (aka straight line). That's the "cheat" I've been exploiting since my virtual plane has very little gyro drift, at least in clear air. Today's attempt was a lot bumpier all along the way and didn't go so well.

On the other hand, if you fly referencing the compass which you'd probably better do in a real aircraft, it makes navigating these high latitude long legs a nightmare. I'm not sure how one would navigate these apart from establishing a timetable that tells you which heading to follow based on your estimated position along the way. I guess at that point you'd probably want to cross-reference your position against other means of navigation, like a sextant or some visual reference when available (or use modern navigation systems but that's not the point here).

20 hours ago, albar965 said:

What new thread title do you suggest?

I was thinking about "[Feature suggestion] Improvements for compass navigation and wind correction". Something that might bring in other people interested in the subject.

One other thing I've noticed is that the status bar gives feedback about weather data updates. However it seems that if I leave LNM opened overnight, the message "Winds aloft downloaded from NOAA, forecast from <date/time> to <date/time>" always indicates the same period around which LNM was started, leaving me unsure if the winds aloft data is effectively updated over time.

[albar965 1,040]

@benjaprudTitle changed. Had to truncate it since it was a bit too long.

About GC: That's the confusing thing: If you like to use the shortest route you have to change course from time to time. Depends on route, obviously.

Winds aloft are downloaded by LNM every 30 minutes but are updated only four times a day.

Alex

 

[benjaprud 0]

This is what I was refering to (I'm UTC+1) :

https://i.postimg.cc/fLjQ1V5D/Capture-d-cran-2020-12-20-211134.jpg

[albar965 1,040]

Thanks.

I already noticed that the forecasts arrive a bit late on the server but there might be something wrong when downloading.Will check.

 

Edit: Just updated the code. If it downloads older reports in the next LNM version it means that nothing newer is available.

Edited December 23, 2020 by albar965