Autopilots and reported distance from Take off.....

[karlak]

So, having now completed a couple of sessions FPV using a Bix 3 and a Cyclops Storm autopilot, we were out yesterday and were pushing the boundaries a "bit" further which has led to a question.

The Storm reports back on distance from where you took off along with the altitude (as well as all the other things), but what is noticeable is that the distance from you seems to be affected by altitude ?

What I mean is, if flying at at 200 metres and am 500 metres downwind I would have thought that the OSD would report both figures as being that.  But, when I come back and am immediately overhead, I would then expect alt to be still 200 metre and distance to home to be 0.  But, this doesn't seem to be happening.  The DTH was still showing as being a fair distance away.

Is this just how autopilots are ?  Do they not work out that you are actually back at your take off point, all be it at altitude ?  Just found it a bit confusing and ofcourse something that would be made worse the higher you were to fly.

Hope that kind of makes sense what I am saying :)

Thought the Storm worked really well, but did have a couple of RTH issues, where it didn't want to hold the set altitude for some reason.  Oh and I now very much see the advantage of a FC with true airpseed after a RTH test where it turned from a tail wind into a head wind to come home, luckily I was quick enough to knock it back to manual and altitude to save a walk into a farmers field :) . Really must get a 60% throttle set on the RTH switch.

[Coyote]

Some autopilots work differently but a lot use 3 separate readings.

Altitude
Distance from home
Exact distance

Altitude is a given.

Distance from home ignores altitude and just shows distance from launch point.

Distance from launch point takes into account altitude and distance with a bit of πr squared giving the exact distance from you to the plane.

[CurryKitten]

That's not how the Storm should work (unless they've done some big firmware changes lately), distance to home shouldn't take alt into account at all.  The only way I could see anything like that happening in normal circumstances is taking off with a low number of sats, and then getting more later which will improve the accuracy of your GPS fix, but ultimately may mean that your home position is some way away from where it first thought.  It's not normally far though, I think the worst I've seen is 20m

[jimbo19]

We definately had good sat lock on the storm before we took off each time - strange

[karlak]

Thanks for the replies.

It was certainly not showing the distance down to tens of metres when approx overhead.  When I go out next time will try and take a bit more notice and perhaps try and record it to review. 

It had good Sat lock before take off and when back and landed it did show home distance as correct, so did seem linked to flying at altitude.


Have to say, this FPV stuff is very addictive.  I still am amazed at the tech and what it can do, pretty amazing really.

[karlak]

We definately had good sat lock on the storm before we took off each time - strange

I wonder if there is a problem somewhere, remember it did a couple of funnies when testing RTH and wouldn't go to the set altitude.  Need to get the latest firmware loaded before next time I think

[CurryKitten]

We definately had good sat lock on the storm before we took off each time - strange

Really odd then - what firmware are you on... wondering if they introduced a new menu giving an option to take alt into account.  If not, be interested to know if it's reproducable on another day.  Only reason I mention it was I've had GPS 'spaz-outs' on occasion (although not on the Storm) where it tells me it's locked and has 8 sats, but suddenly it thinks I'm a mile away and thus i must be doing 300kpm to get there.  Which is very odd indeed

EDIT: Just seen the replies.  Yep be really useful to get a recording of it if you can

[FPVSteve]

Did you fly from the same location as last time, and did you reset home before you took off?

[karlak]

Yes, same location and reset before the next flight to zero things.

F/W is currently 2.1, but Jimbo19  "can't be arsed" to remove the FC to upgrade to 2.2 ;)

[jimbo19]

Yes, same location and reset before the next flight to zero things.

F/W is currently 2.1, but Jimbo19  "can't be arsed" to remove the FC to upgrade to 2.2 ;)

LOL - if it was a major change in the FW from 2.1 to 2.2 then yes I could understand

[helimadness]

GPS is not perfect and I have seen fairly big temporary errors in the past. I do a lot of ocean racing on sailboats so use GPS for a lot of things other than navigation so it plays a bit part nowadays. It can be very good but not 100% reliable 100% of the time.

As for losing height during RTH. I have had Storms in the past and recently setting up a Tornado on a FX61. I had trouble with it over correcting and also losing a lot of height in turns. Problem was max roll angle was too much. I had to reduce it to 20 which sorted it out. I noticed that during a turn by the flight controller the pitch was maxing out and still not enough to maintain height. Also if flying too slow ran out of pitch control.

The Bixler flys fairly slow but. I had one of them with a storm. I think default settings worked fairly well from memory. Try checking your pitch angle when in say PA mode at different air speeds. I found with the FX61 under say 40 kph the flight controller was getting to the max 10 deg limit I had set just to keep it level. So if I went RTH and it wanted to turn was going to lose height for sure. At 50 Kph the pitch angle in level flight in PA mode was down to maybe 2 deg. So had 8 deg up it sleeve to work with. Either going faster or reducing bank angle in the turns made it better.

Hope that helps. Still got a a bit of work to get mine perfect but starting to sus it out.

As for the Auto throttle Im thinking of disabling it. Still dont really have my head around it. The fix throttle failsafe position worked for me very well on my Storm. I flew my Bixler to the radio limit all the time while testing different aerials etc. Always came back. Once back in radio range but then the normal throttle stick works with RTH on from memory.

Cheers

[BlueFlyer]

Pi r squared has nothing to do with it... that's all about circles.

when it comes to distance from launch point (hitherto referred to as "home") it's an ACTUAL distance from one GPS position in 3 dimensions (home), to another GPS point in 3 dimensions (the aircraft in the sky)

you see, many people think GPS co-ordinates are simple X and Y, like would be useful for a flat map on a table for instance, or something like google maps.

but really, a GPS co-ordinate also includes a third dimension.... distance from the satellites, and it is this third dimension that gives us the "altitude" number on the OSD. it uses the distance from each satellite to literally triangulate your device in 3 dimensional space, not just as a plot on a flat surface.

this third value is usually hidden and quoted GPS co-ordinates on screen just give you a position on the surface of the earth, and "could" be at any altitude... but the third value IS in the raw GPS data and that's how it measures the distance from home.

In a simpler way, think back to school and Pythagoras theorem



You are at point A
the aircraft is at point B
the line Y represents the distance from the ground to the aircraft B directly above it.

Some expect the "distance from home" value on a OSD to be the distance from A to C (line X)... when in reality a simple OSD will just use a straight line measurement from one GPS point in 3 dimensions to the other... a straight line.

therefore the distance displayed as "from home" will be the line Z which you can clearly see is a longer line than line X

only the more complex OSDs can tell you the ground distance separate from the straight line distance... I know with the vector you can choose to display distance from the pilot (straight line) and ground distance as separate values on screen.

[BlueFlyer]

makes you wonder now how many people have actually reached the goals they say they have lol ;)

[karlak]

So just a case of some FC's will take altitude into account and some wont.

I guess the test is to fly directly overhead at 100 metre altitude and see what dist to home says.

[Coyote]

Lmao pi r squared. My bad, where did I get that from, I was referring to a square plus b squared not pi r squared.