Any tips regarding RTL please?

I’ve been away for a few months, having previously got my TRex 500 set up to my satisfaction in stabilised flight mode. I’m familiar with Arducopter and its modes with my hexcopter, but I’m very (maybe too) cautious when it comes to my heli. Anyway, today was a very good day weather-wise so after running one battery in stabilised mode I then took the chance on flipping the switch for Position Hold mode with the second battery. It was a complete non-event, and it just sat there for about 3 minutes at about 20ft. height without the need for me to do anything.

So now my next step will be to activate RTL mode on my next outing. My concern is that I don’t want to let the heli completely land itself because of the risk of tipping if it happens to land on a bumpy piece of grass. So I propose to let the heli return to launch point about, say, 8 feet off the ground, and then flip the switch back to Position Hold or Stabilised mode so I can complete the landing manually. Does this sound right to you? Any comments please?

Yes, your plan is good.
Make sure to position your throttle stick around middle when switching. The thing you must avoid, is to have throttle (collective) all the way down, and switch to manual mode, especially on a Heli configured for plenty negative pitch.

Finally: remember that you can reposition while RTL is descending and land precisely on a flat spot without leaving RTL mode.

Hi Allan,
Really the next step would be to see how the aircraft handles in loiter. Position hold does not use the same controller to control the aircraft. Loiter’s controller is pretty similar and gives the best idea of how the aircraft will handle under autonomous control. Remember when exploring new modes, it is best to use a switch to go between the two modes so that if something goes wrong in loiter, you can switch back to stabilize to regain control. In loiter you will want to first see how well it holds position when the sticks are centered. then you can maneuver the aircraft around slowly and then center the sticks and watch how well it comes to a stop and establishes a hover. If the aircraft starts to oscillate and the attitudes start to grow, immediately switch back to stabilize. At that point you’ll have to post a log or try to tune the position controller. You will want to gain confidence in the aircraft flying loiter before moving to RTL. You can even try conducting takeoffs and landings in loiter and that should give more confidence in how the aircraft will perform in RTL. Just remember that on landing in loiter, you have a very detached control of the head because you are actually controlling aircraft position over the ground. Try to minimize inputs once it is close to touching down as it will try to reposition in response to those inputs. once the aircraft touches down, don’t delay in lowering the collective full down. You want the autopilot to declare landed quickly and that depends on the commanded descent rate (your collective position). Once it declares landed then, it stops trying to reposition the aircraft and the head should center itself. If you are ever having trouble landing in loiter and the aircraft looks like it is going to tilt while it is on the ground, it is probably best just to switch to stabilize but be sure your pitch/roll stick is centered.

So after you’ve gotten comfortable with the aircrafts handling in loiter then start testing RTL. What you propose is good. Have it come back to a hover over the Launch point and as it is descending, take over in stabilize. But as @Andre-K says, be sure your collective is centered so you don’t get an abrupt climb or descent. You can check that in loiter, by switching into stabilize mode from loiter and see what the aircraft does. That way you are prepared. Might be better since you have already gained confidence in loiter to switch from RTL to loiter. As you grow more comfortable with RTL. Then let it take the aircraft all the way to landing.

What firmware version are you running? post a param list so I can check your settings.


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Thanks guys.

When I transition from stabilised to position hold I’m always careful not to change the position of the collective, so that when I switch back to stabilised there will be no sudden drop or climb. That worked well in yesterday’s flight even though the stick position for a steady hover is a tad above centre.

But I wasn’t aware that there’s a difference between Position Hold and Loiter – I thought they were just different terms for the same thing! I’ve just read up on it, and I can see how Loiter should give better indication of the FC’s performance, so I’ll re-program my switch to give me Loiter for my next flights, before trying RTL.

Bill, it’s so long since I flew that I’ve forgotten what version I’m running. I’ll let you know, and post a param list this afternoon.

OK, I’m currently using MP 1.3.75 and Arducopter 4.1.2. I see there’s updates available for both of them, so I’ll load them later.

Since I didn’t know the difference between Pos Hold and Loiter, I’ve checked that I’ve actually got Pos Hold programmed on my switch, so I’ll change that to Loiter for my next test flight.

Attached is the param file from yesterday’s flight. Any comments will be welcomed.

HK500 after Pos Hold test flight.param (16.6 KB)

I reviewed you parameters and they look pretty good. What is the actual collective blade pitch in degrees for the H_COL_MAX and H_COL_MIN? In order to better protect the heli from in-flight shutdown, I would suggest making H_COL_MID as low as possible. Maybe 5 PWM greater than H_COL_MIN. And I would set H_COL_MIN so it corresponds to -2 deg. Definitely upgrade the firmware to 4.1.5.

Thanks for looking and commenting Bill. I’ll check out the actual pitch angles this afternoon, and upgrade the firmware.

I’ve just seen your posts about mid-air shutdown here, and now have a little better understanding of what’s going on regarding H_COL_MID.

My max and min actual pitch are currently at -2.9 degrees and +10.9 degrees, and H_COL_MIN and _MAX are 1490 and 1706. My H_COL_MID is 1520, so I guess you’re advising that I should tune that down to 1495? Doing that won’t change the stick postion at which it hovers, will it?

I’ve updated MP, but I’ve been unable to update the Arducopter firmware from 4.1.2, so I’ve posted another thread about that. Edit: I’ve now got v4.1.5 installed, using QGC to do the job.

With the current settings, every deg of collective pitch is 15 pwm. So I think you should set H_COL_MID at 1500. That would be just below -2 deg which would be much better protection.

No. That won’t affect the stick position for hover.

Thank you. I’ll follow your recommendations, and then check out Loiter instead of Pos Hold for my next flights.

Regarding the in-flight shutdown situation you mentioned earlier, if that happens, will the vehicle re-start if I switch out of RTL mode before it hits the ground?

I don’t suspect you will have any issues with this after looking at your param file and making this change to the H_COL_MID. The reason I say this is because your hover collective, shown by the H_COL_HOVER, is 55% of your collective range (H_COL_MIN to H_COL_MAX). Which equates to 4.7 deg based on your actual collective pitch values. It’s very unlikely that you’ll ever get to -2 deg in-flight for more than 1 second while descending less than 1 m/s. For user who run very high head speeds and their hover collective is 2 to 3 deg, then I would be more concerned. We can look at your flight in loiter mode when your flying around at some slow forward speed. This is where your collective would be at its lowest for hovering flight. That should give us an idea of how close you are getting to this value.

It will try to restart the engine but the spool up time for the esc is slow and most likely won’t reach flight speed before you hit the ground. So it is best to switch to stabilize and use the best autorotation profile you can set up to land.

Again I think this is very unlikely in your case and also it can only happen in autonomous modes like auto, guided, RTL and land. So it is definitely not a concern for your next flight in loiter.

Thanks for explaining things so well. I’d forgottern about the spool-up time for a ‘cold’ start. Next flight will be with Pos Hold and Loiter programmed onto the ‘mode’ switch so I can compare their behaviour.

I had a couple of flights at the weekend, using Stabilised, Position Hold, and Loiter modes.
I didn’t notice much difference between PH and Loiter, though Loiter was maybe a little less responsive to my inputs – i.e. more firmly under the control of the FC.

But I did have one anomoly while in Position Hold, about 2 minutes into the flight. The heli had been holding position well (though slightly gaining height) for a while when all of a sudden it started a rapid ascent and forward motion. I can see it in the log just after time 16:08:00:000 when I select Baro > Alt, but I don’t know what other parameters I should display so I can figure out why it did it. Switching back to Stabilised mode allowed me to bring the heli back down without any problem.

Below is a link to the .bin file. Can anyone throw any light on what happened, please? And what are the important parameters I should be looking at when reviewing a log file? I notice that vibration increased during that climb, but was that cause or effect? I suspect the latter because it seemed to return to ‘normal’ values when I switched back to Stabilised mode.

Edit: When I try the above link it says I can’t view the file, but have to have a Dropbox account so I can download it. I hope this following one works as intended, though it does look the same to me.

With the vibration problem fixed I today managed three RTLs without any significant issues. Despite a gusty moderate breeze each of the landings was within 2 metres of the launch point.

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