Compass offset

I was doing ground reference flights yesterday around Kutaisi and kept feeling that the Yak's HSI is slightly off from the L-39. So I created a mission spawning both in the same spot and found a delta of ~5 degrees. Is this a bug or intentional? If intentional, does anyone know the reason?

Here are the screenshots of both cockpits to demonstrate the difference.

L-39's HSI and magnetic compass are in agreement.

Yak-52s HSI and magnetic compass are in agreement.

L-39C Pattern Kutaisi.miz

Yak-52 Pattern Kutaisi.miz

L-39 tends to operate on true (magnetic corrected for variation) by way of a special knob that inputs the local variation. The instruments are slaved to magnetic input. The dial biases the magnetic sensor in the wing by the set amount. If that knob is set at 0 then the L-39 operates without variation correction and everything's in magnetic.

Looks like the Yak is set up for the heading indicator to read true ~140 while the magnetic compass reads magnetic ~135. That's unusual to me but if that's what they do then that's what they do. A heading indicator can be set with any intentional offset you like (not like it's automatically slaved or anything) so if you want magnetic operations then you can just do that. I'm guessing if you checked the external GUI unit orientation it would be ~140.

The L-39 appears to be ~125 magnetic and ~135 true which is the right direction but a little large in magnitude.

The Caucasus has 5-6 degrees east variation, more or less depending on where and when. East variation means that when you're facing directly north the magnetic pole is to your east by that amount. The result is your magnetic heading is always that much less than your true.

Yak52 has the same system The correction unit is located behind the rear seat.

Got it, by adjusting the PU-26 panel on the right side of the forward cockpit and setting it to ~42 degrees N lattitude I got a reading that more closely follows the L-39. I should have thought of checking this.

Now that we have more map options will it be more common for it to be @ zero by default?

Latitude? Latitude entry is to combat Coriolis drift due to the Earth turning. It would be part of a heading and attitude reference system. ARS can be so good as to maintain reference to an attitude in space which is maintained despite the rotation of the Earth. Latitude setting allows compensation for on what axis this rotation is.

But latitude has nothing on magnetic variation.

Usually modules which have a magnetic variation adjustment knob will set it automatically to the variation at the spawn point no matter which terrain or location or date. This value is available to module developers in all cases so even in NTTR or Persian Gulf it should work properly.

Frederf,

Thanks for the clarification. I mentioned latitude because that's the unit the PU-26 accepts as input. After reading the Mi-8 manual on the PU-26 unit (hint ED, please make the Yak manual more complete), it is clear that it corrects for drift due to rotation rather than magnetic declination. Until your reply I was not aware that was a factor. Thanks! Always more to learn. In either case, correcting for drift brings the Yak inline with the L-39.