D10A - External compass calibration

[john88588]

This may be a dumb question, but here goes anyway.

From the NOAA site, the inclination value for my location in North Carolina is 64.6 degrees.

When I enter my inclination (dip) angle as the initial step of my compass calibration , should I enter + 64.6 or - 64.6?

Thanks, John

 

[dynonsupport]

It's positive. The web site will show a "-" if it's needed.

It's + in the northern hemisphere and "-" in the southern as a general rule of thumb.

 

[Canadian_JOY]

Followup Q?Re D10A - External compass calibration

I have a follow up question concerning compass inclination - actually a few questions. These are asked in hopes of improving my understanding, not in response to any problems with my unit.

I know the inclination is used in calibration of the EFIS and EDC. What I'm wondering is whether or not this calibration would need to be re-executed if one changed latitudes significantly, eg if one flew from Canada to South America? Is the calibration performed with a single inclination value sufficient to allow accurate performance across a wide range of latitudes?

Also, I was left scratching my head about this inclination value and its relationship to EDC mounting angle. If the EDC needs to be mounted such that it's base is perpendicular to the instrument panel, in many aircraft this would mean that it's base is horizontal in level flight. But when on the ground doing a compass calibration in a taildragger that same EDC might be several degrees off horizontal. I guess I'm trying to figure out how an inclination value relates to accurate calibration when there may be several degrees difference between local geographic inclination and the inclination which the EDC actually experiences during calibration.

Please enlighten those of us who have difficulty with this concept (I'm only guessing that I may not be the only one who's pondered this question, and I'm reasonably certain the engineers at Dynon have pondered it and come up with the right answers!).

 

[dynonsupport]

It's pretty easy to understand actually. The compass is a 3D device. We can look for the gravity vector any direction we want because we have sensors in each axis.

We also have this handy attitude reference- the EFIS. This means that we know which way is down, as long as the EFIS and compass are at the same angle relative to one another.

Thus, when on the ground, the EFIS is telling the compass "look down 13 degrees for your gravity vector, since the nose of the plane is up 13 degrees". We do exactly the same thing in flight, throughout all attitudes.

As for declination, we only need that during calibration. This pre-tells us what way the gravity vector will be, which is needed to calibrate all three axes.

Once calibrated, we only need to know the direction of the magnetic vector in 2D, so we just look through the pre-calibrated compass based on the angle of the EFIS and see which direction is strongest. At this point dip angle only effects magnetic strength, not direction.

Ok, maybe it isn't so easy

 

[Canadian_JOY]

Hmmm, so my head scratching can end now. Thanks for the reply. As I read through it I had one of those moments when you go "Doh! I shoulda figured that out!"

I'm going to throw one more silly question out in hopes you can do your usual magic and get sense through this concrete between my ears...

Given the accelerometers etc which comprise the attitude sensing portion of the EFIS, can the EFIS determine relative heading without compass input? I guess what I'm trying to ask is if there's sufficient equipment on board the box to allow it to know which way the airplane has rotated about its vertical axis and by what magnitude, without having to reference magnetic north?

As always, your thoughtful replies are greatly appreciated. What goes on inside the EFIS is magical stuff...

 

[dynonsupport]

Unfortunately it isn't really possible to do heading without magnetometer information. Until you start using very expensive rate sensors of a much higher grade and type (we're talking Boeing-type stuff), the rate sensors and accelerometers alone aren't good enough to produce heading without drift drifting over time (kind of like a conventional DG). Our heading does in fact use the whole ADAHRS platform in the short term so that turn initiations are picked up immediately and crisply. Over the long term, though, you need the calibrated magnetometers in the loop to determine ultimate heading truth.

 

[Canadian_JOY]

Thank you for the reply. I guess my next train of thought would go toward using instantaneous heading indications compared to GPS track info to provide a method of auto-correcting for ADAHRS drift. This is a project near and dear to me as I've done a fair amount of flying in the region of compass unreliability and the inability of the Dynon to provide a grid heading reference is, in my opinion, quite a serious operational limitation. Methinks there have to be some smart ways to get around this limitation...

 

[dynonsupport]

GPS = ground track, magnetometer = where then nose is pointed.

The difference is wind. We use the difference between GPS and the heading to calculate wind. You can't correct your heading with GPS unless there is zero wind, and there's no way to know that without a good, calibrated magnetometer.

As far as I can see, you need a true gyro to do grid heading, and the Dynon EFIS is not that, and none of the sub $1M EFIS units are. As far as I know, nobody, including the G1000, does grid heading. It sounds like your best bet is an old, mechanical gyro.