Gyro drift

[VulcanB2]

Hi,

Is it possible for you to add gyro drift to the gyro compass? It seems I have to use the FSX in-built gyro drift which is wholly inaccurate (drift rate is far too high, so consequently I leave it disabled).

Best regards,
Robin.

[some1 - A2A]

What drift rate are you expecting?

For average USA/European latitude, the drift rate would be about 10 degrees per hour when sitting still on a runway. I just loaded an aircraft in the FSX and the drift rate seems to be quite accurate.

You can find formulas for drift rate here:
http://en.wikipedia.org/wiki/Heading_indicator

[AKar]

Hi Robin,

As the previous post indicated, the drift of a simple heading gyro is actually related to the Earth rotating, plus or minus the error caused by imperfect mechanical function of the gyroscope and gimbal. The amount such a gyro drifts can be derived mathematically as in the link. It is in practice a whole different matter than a drift of an IRU position in an airliner for example, as the IRU knows (well.. has an idea about) it's position and, in principle, the effects of Earth rotation are included in it's position calculation**. I often hear people mixing those up, though very understandably.

(**During the alignment phase, the IRU actually determines it's latitude based on the Earth's rotation. The error compared to latitude input given helps the unit to determine if the alignment was successful and if the unit can be used for navigation.)

-Esa

[VulcanB2]

Hi,

I'm aware of the above - I write simulations of them. It detects North by sensing East.

The problem is the gyro drift in FS is too much, and it would be great if A2A could add this to the simulation instead, so it is the correct rate, and takes into account the direction of flight (which also affects the drift rate).

Best regards,
Robin.

[some1 - A2A]

Robin, maybe you have missed my previous answer in this thread, but I cannot fix it if I don't know what value of gyro drift you are expecting.

I have not tested if FSX simulates gyro drift component from moving aircraft, but for an aircraft that goes at C172 cruise speed, this error is about 3 degrees per hour, 4 degrees if we are in northern Alaska. That is less than 30% of the error caused by earth rotation and only in the worst case scenario when the aircraft is flying east or west. So in most situations in the simulator it is negligible.

[VulcanB2]

Robin, maybe you have missed my previous answer in this thread, but I cannot fix it if I don't know what value of gyro drift you are expecting.

I have not tested if FSX simulates gyro drift component from moving aircraft, but for an aircraft that goes at C172 cruise speed, this error is about 3 degrees per hour, 4 degrees if we are in northern Alaska. That is less than 30% of the error caused by earth rotation and only in the worst case scenario when the aircraft is flying east or west. So in most situations in the simulator it is negligible.

On a 2+ hour flight, uncorrected, that is at least 6 degrees of error then (I often fly ~2 hours in the 172).

Here is the FSX gyro drift after 42 minutes. I set the heading bug so the line was exactly down the middle. This must be nearly 10 deg/hr drift rate!!! I think we both agree this is completely wrong. I'm currently sat at Heathrow, RW27R.

Microsoft seem to be modelling a free gyro, rather than one that is corrected for latitude.

I calculate the earths rotation to be ~15.041 deg/hr. I'm currently at N51° 28.66', so for a free gyro this works out at cos(51.47766667) * 15.041 = 9.36 deg/hr drift rate for an uncorrected gyro.



Best regards,
Robin.

[VulcanB2]

FSX gyro drift after 1 hour.

It would be great if you could model the gyro drift yourself, and make an option to set the latitude nut. AFAIK it is set to the latitude the aircraft will most likely operate in. Given we can fly world-wide with ours, a user setting for latitude (whole degree resolution would be sufficient) would be great.



Best regards,
Robin.

[some1 - A2A]

Thanks, now I understand what you mean. We will check if dg's installed in those planes have latitude nuts and how realiable they are on average.

I'm currently at N51° 28.66', so for a free gyro this works out at cos(51.47766667) * 15.041 = 9.36 deg/hr drift rate for an uncorrected gyro.

It's sin(latittude), not cos(latitude). At the poles you get sin(90) = 1, which gives full 15 deg/hour drift. At your latitude the drift is 11.65 deg/hour, so it matches what you see in FSX for uncorrected gyro without latitude nut.

[AKar]

Microsoft seem to be modelling a free gyro, rather than one that is corrected for latitude.

We will check if dg's installed in those planes have latitude nuts and how realiable they are on average.

That's actually an interesting point. I don't know if 172's gyro has drift setting for latitude, but I've assumed that it hasn't - at least a field settable one. Per operational check instructions it may be allowed to drift surprisingly much in real life: that is less than 4 degrees in a 10-minute period. That's of course 24 degrees in an hour, which would easily account for the maximum perceptible natural drift of about 15 degrees and some mechanical inaccuracy (as a wrong latitude setting?) above that too.

I'm aware of the above - I write simulations of them.

Sounds interesting! For PC flight simulations or for other purposes, if you don't mind me asking? I'm rather interested in avionics (both mechanical and solid state / mems) and and their modeling, though never had a chance yet to get into flight simulation specific issues.


(I actually would like the gyro instruments to be added into custom simulation too, though primarily to make them less surgical in precision (the mechanical AIs for example may indicate some bank angles after longer turns), and to allow a single instrument failure which happens sometimes. Same goes with radio navigation instruments. But of course I understand there are limits of what's practical and even worth of modeling!)

-Esa

[VulcanB2]

It's sin(latittude), not cos(latitude).

Ooops yes.

I don't know if 172's gyro has drift setting for latitude

Yes they do, generally. They are set to the average latitude they are likely to be operating in by maintenance. For a C172 based in the UK for example, this could be 53 degrees North for example. Note that it will NOT correct for transport rate drift, i.e. the error present due to the aircraft moving (maximum in the east/west direction, and dependent upon ground speed).

Sounds interesting! For PC flight simulations or for other purposes, if you don't mind me asking? I'm rather interested in avionics (both mechanical and solid state / mems) and and their modeling, though never had a chance yet to get into flight simulation specific issues.

Flight simulation for real-world training purposes.

Best regards,
Robin.

[Alec246]

The C152 from my Flight School has a Gyro that will Drift WAY more than those formulas suggests. We have to every 10 minutes correct the Gyro with the Compass. But it's probably some issue with the instrument that has not been addressed.

[Great Ozzie]

I have been trying to find a maintenance reference to directional gyro drift limits... best I have come up so far:

Kershner in his "Instrument Flight Manual" (third edition) states a maximum drift error of 3° per 15 minutes.

I did come across this RC ALLEN 15 SERIES DIRECTIONAL INDICATOR (28V) at Aircraft Spruce & Specialty stating calibrated to an accuracy of 3° maximum drift error after 10 minutes of ±1.5° scorsby motion.

fwiw
-Rob

[AKar]

They are set to the average latitude they are likely to be operating in by maintenance. For a C172 based in the UK for example, this could be 53 degrees North for example.

Okay, thanks! Is the knob field settable, or do you need to open the instrument for that? I've never actually been handling the DG from C172R/S, and did not find any real reference using part catalog's P/N.

(Also, I did not find any reference for adjustment in C172R/S AMM; under EASA rules in Europe, at least when I last had anything to do with the Parts a few years back, the adjustment would thence be performed as per instrument's maintenance instructions, which in turn would make the adjustment to be component maintenance, requiring a component-specific license from the maintenance organization. If that sounds difficult enough.., in principle, it needs to be done by instrument shop anyway just because the C172 AMM has no instructions for it. Though there could be a bailout somewhere.)

Note that it will NOT correct for transport rate drift, i.e. the error present due to the aircraft moving (maximum in the east/west direction, and dependent upon ground speed)

Yep, of course.

I have been trying to find a maintenance reference to directional gyro drift limits...

Best I could find so far is the one I wrote earlier from C172 AMM, which gives you the limit of 4 degrees in 10 minutes at constant heading. Of course there probably are instrument-specific bench test limits in component maintenance manuals of the instruments themselves, but I got none at hand.

Edit: The link from your post referred to TSO-C5 which in turn refers to SAE AS 8021 which would probably give the minimum requirements for a new instrument at least.


-Esa

[Great Ozzie]

Edit: The link from your post referred to TSO-C5 which in turn refers to SAE AS 8021 which would probably give the minimum requirements for a new instrument at least.

Yeah I had looked thru TSO-C5f... nothing there. The AS is where I guessed it would be but that is $70.00 to download.

Btw... it would seem then Robin's observations are still "ok" as far as expected drift is concerned.

[VulcanB2]

calibrated to an accuracy of 3° maximum drift error after 10 minutes of ±1.5° scorsby motion.

...to me that implies that 10 minutes of straight and level flight should produce less than 3 deg/10 mins error.

Interesting topic!

Best regards,
Robin.