Clarification on what the "drift angle" on the ground speed/drift angle indicator means?

[jubuttib]

Up until today, and based on what I'd seen in the Hind, I always assumed the drift angle needle is there just to tell you where the helicopter is actually moving vs. where the nose is pointing, i.e. whether you're crabbing and how much.

Reading Chuck's guide on how to use the DISS-15 nav system though suggests otherwise:

This suggests that it's instead about how far off the designed heading you are currently, basically replicating the HSI in a sense.

Can anyone say for certain which it is? I would think it's working the way I originally thought, because regardless of whether the dead reckoning system is turned on or not, and whether you have any source of direction for the HSI or where your heading bug is pointed, the indicator mostly seems to point towards direction of travel. And when navigating I've kept the heading steady with yaw channel/heading hold turned on (confirmed by HSI heading being steady), and adjusted the drift angle shown by the indicator by rolling the helicopter to the left or right a bit.

EDIT: Oh, and the "Drift Angle KM" (or the more accurate "Lateral Deviation km" in the Hind), can anyone confirm how exactly it should be read? Is Chuck's example showing 4 km and Left telling me that the actual track I should be on is 4 km to the left, or that I've deviated 4 km to the left, and should move 4 km to the right?

Edited November 28, 2021 by jubuttib

[admiki]

No. You were correct. If you need to fly certain heading, you head that way and trim out. Then you check Doppler to see if there is any drift. You compensate for that with your new heading.

DISS tells you where YOU are in relation to your planned path and destination.

Edited November 28, 2021 by admiki

[jubuttib]

Cheers, good to have a better idea on what it's telling me.

4 hours ago, admiki said:

DISS tells you where YOU are in relation to your planned path and destination.

OK, so "4km left" means I'm 4km left of the track, and need to correct to the right, correct? FWIW that's how I assumed it was, but I saw a YouTube tutorial that kinda made me rethink...

Timestamp at around 12:58 or so. To be fair, it wouldn't be a bad way to indicate it, telling you to "go this far in this direction".

[admiki]

1 hour ago, jubuttib said:

Cheers, good to have a better idea on what it's telling me.

OK, so "4km left" means I'm 4km left of the track, and need to correct to the right, correct? FWIW that's how I assumed it was, but I saw a YouTube tutorial that kinda made me rethink...

Timestamp at around 12:58 or so. To be fair, it wouldn't be a bad way to indicate it, telling you to "go this far in this direction".

No, he is wrong. I agree it is a bit confusing. Like I said, DISS is telling you where you are in relation to your path and destination. If you look at forward/back window, you will see that it is saying back and counting down, meaning you are "behind" your destination and going towards it. Anyway, easiest way for you is to test it yourself. Set heading to whatever you want and then fly left/right of that and see what DISS will tell you. One thing you need to keep in mind is that when you take heading from the map, you need to convert it to true from magnetic.

[jubuttib]

What you mentioned above was already the conclusion I'd reached from testing, but wanted to still ask around in case I was doing some kind of dumb dumb.