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Richard D

Problem with Whiskey compass, DG and the DG on the autopilot

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First of all congratulations on getting this very complex and amazing aircraft released after a long time of waiting.

While performing some turns in your plane, I noticed a design error on the compass and the directional gyros (DG), which also applies to the DG installed on the autopilot.

First of all, let me describe the principles of the involved instruments, so you can understand why the effect I noticed is really a design error and not a design decision. The whiskey compass is a small magnet with a ring around it, which indicates the heading. Since the magnet is always aligned with the lines of the earth's magnetic field, it would not change its direction related to this line. This effect causes the compass change its direction related to the plane when the plane turns. You can visualize this situation by imagining that the plane rotates around the magnet inside the compass. Thus, the numbers on the compass card in all whiskey compasses (at least all I know from real aircraft) increase from right to left. On your compass, the numbers increase from left to right (which is indeed a common display form on some electronic displays used in modern aircraft, e.g. on the GNS430). Though, building a compass with the numbers increasing from left to right as a mechanical device would actually mean that you have to artificially accelerate the ring around the magnet, which nobody would do for such a small instrument (since at least a mechanical construction would also impair the accuracy of the indication and the probability of failure). For further details and some figures, see e.g. the openly available FAA instrument flying handbook, chapter 3, page 11 to 14 (http://www.faa.gov/l...lying_handbook/), where the principles of a whiskey compass are described.

The first generation directional gyros (like the one you included) were built to indicate the heading in the same way as the whiskey compass to allow for an easy comparison of the indications so that the frequent deviations between the instruments, which were due to the effects commonly known as "drift" could be easily noticed and intuitively corrected. See many images of directional gyros from the time where most DC-3 were built on http://images.google...iw=1183&bih=878. These gyros all have the numbers on the card increase from right to left. The famous Sperry autopilots use the same indication as the directional gyros from that time (see http://images.google...iw=1183&bih=878) so this has become some kind of standard.

Actually, the heading indication in your plane is a little confusing for a pilot who is used to the feel of the real instruments. This makes at least me somehow loose the feeling for correcting deviations from the planned heading intuitively, making instrument flight in your aircraft hard. Moreover, for realistic instrument flight, the captain's and FO's DGs are somehow useless when trying to keep your alignment with the approach centerline within your +/- 5 degrees tolerance for errors on a beacon approach without a visible mark (e.g. a small white point or triangle below the compass card) on the point where you can read the exact heading from, so at least I always have to use the DG on the autopilot for that purpose. Though, with some improvements on these issues, performing holdings and some good old beacon approaches with the ADF and a stopwatch could really be fun in your DC-3 ;).

Thank you again for creating this aircraft and for your work on keeping the project up. I hope you can spend the effort to improve the problems with the compasses and DGs, since, at least in my opinion, this would really improve how these instruments feel (at least for pilots that are used to this type of instruments).

Best Regards,


Edited by Richard D

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