An “Exciting” Test Flight

Today was a test flight of the F550 and boy did it turn out to be eventful!

A little background.  A while back I had a minor crash when experimenting with batteries to find out why I was only getting 4 minutes of flight time when I should be getting much more.  Since I had to remove the top anyway I figured this was a perfect time to try move all of the electronics inside to give it a much cleaner look.

I got it all in there but the squishly little balls in the anti-vibration plate were heavily compressed (see picture) and I was worried that vibration would cause the APM Flight Controller (FC) to go a little loopy.

To quote the APM wiki.

High vibrations can lead to the copter rapidly climbing as soon as altitude hold is engaged.

I considered flying in the back yard but, since this was practically a full rebuild, I figured I had better give myself some room and there are lots of overhanging trees.  One of the few smart things I did today!

I set up instead at a local field with plenty of room, confirmed I had a good GPS lock, and decided to launch it in “Position Hold” mode, which should use the barometer to automatically hold altitude and the GPS to keep it in the same location.  the plan was to just let it hover until the battery was low.

I armed the motors, pushed up on the throttle and up she went!

She was climbing fast and at a height of about 30ft I backed the throttle off and it stopped right where I needed it…except…that never happened!  Instead she kept climbing like a rocket and, at the same time, started moving away from me.

By now I had the throttle all the way down and she still climbed, topping off at about 45 meters (almost 150′).

Of course, I did what any self respecting drone pilot would do.  I PANICKED!

After having it spin in place for a while I realized that it had sort of stabilized.  It wasn’t climbing anymore and it was staying in place.  But how to get it down?  I had the throttle at minimum and it was just hanging there laughing at me.

Figuring that vibration was messing with the barometer I switched to manual mode (Stabilize).  Of course…since I had the throttle at minimum that immediately turned all the props off and she dropped like a stone!

I quickly gave it throttle again, got it back up in the air, and after a frantic 30 seconds managed to get it back under control.  I had to keep on top of it all the time but it was flying now and responding to my inputs.  Phew!

It was about this time that I realized I never actually hooked up the little beeper to the battery so that it would beep when it was getting low.  No worries though, I had set it to auto land on a low battery warning so I decided to just let it ride.

Unfortunately the jamming in of the FC seemed to have pushed it off kilter and it took constant backward / left pressure on the controls to keep it in place.

All was fine for a while until, suddenly, it took off for the sky at a great rate of knots with no input from me!, topping out this time at around 100′.

Again I put the throttle all the way down to no avail, and then (stupidly) put it in Stabilize mode again.  And, just like before, it dropped like a stone again!

After moving the connections inside

I powered up but this time wasn’t quite fast enough (perhaps because less power was available) and the best I could manage was a crash at a speed low enough to cause pieces to fly off, but not damage anything permanently.

It wasn’t all bad news.  The flight time was better (7 minutes) but still not good enough.  And the gimbal I had mounted to replace the one that went nuts every time the thing took off seems to be working perfectly.

Sadly though it looks like I will be spending a happy evening moving everything back on the top.  Not hard to do…just a PIA.

Building an F450 quadcopter – Removing the Wobble

With plenty of flight modes now available it was time to address the wobble I noticed on the first flight. 

Some Googling pointed to the most likely candidate for wobble and the copter sounding “angry” was that the P-gain was too high.  I don’t know what that means but I figured that this would be easy to take care of.  Take off in Altitude Hold, run Auto Tune and, voilà!  Problem solved.  

Unfortunately it turned out to be more difficult than that.

With my new-found knowledge of how to set up the controller I was able to configure one of the options to be Altitude Hold, and set up a switch to start Auto Tune and I set off to get it done.

It was a somewhat windy day so I chose a local school (it was Saturday) that had a nice square protected on three side by buildings thinking this would give me some protection.

I took off in stabilize, stopped at about 15′ and switched to altitude hold.  Immediately the quad shot up about another 20′.  It was now high enough that it was no longer protected by the low buildings and it started to drift fast!  I immediately pressed down on the throttle and it came down, but much slower than I hoped.  I switched back to stabilize as it got near the ground and it dropped very fast, bounced once and landed up side down.  Bummer!

A quick check and things seemed OK, or so I thought.

I sent her back up again and exactly the same thing happened.  This time I had left myself more room and tried to leave it in alt. hold, but now it was really acting oddly.  Pressing the lever to make it go forwards caused it to climb more each time and, with the wind, it was now high up and drifting over the top of the school buildings.  I was convinced it was going to end up on the roof and, in a last desperate act, pushed forwards all the way.  Finally it responded and shot away from the school.  Bringing it down again was, once again, very slow and with the lever all the way down I switched back to stabilize.  Unfortunately the throttle all the way down in stabilize basically means turn the props off and the quad drop out from the sky like a brick.

Time for some self-analysis and to find the culprit of the problems with altitude hold.

More Googling revealed that altitude hold issues are often caused by vibration.  Fortunately the APM captures logs every time you fly and I was able to download those into Mission Planner and (after some experimentation) view the vibration logs. This proved to me that this wasn’t the problem. 

It also showed me that this little APM flight controller is super cool. If you have time it is well worth looking into the logs and what they can tell you.  You can even upload these into Goole Earth and get a 3D picture of your flights complete with color coding to show you which flight mode you were in.  Nice!!

Since Auto Tune was off the cards until I could figure out what was wrong with the altitude hold it was time to start asking for help.  Someone on the Quadcopters group provided me with the settings they had that worked and, in comparing those, I noticed that a TRIM_THROTTLE value of 33% seemed much too low compared with his settings.  This is APM’s estimate of the throttle required to maintain a level hover calculated automatically from the pilot’s throttle input while in stabilize mode.  Mine was way too low and actually outside of the 40-60% that is the mid point for altitude hold.

On a hunch that I hadn’t flown in stabilize mode enough for it to get a good reading I went into the back yard and ran through two fully charged batteries doing nothing but trying to keep it in place in stabilize.  Then it was off to a large open area and, voilà!  Alt hold was now working without any crazy behavior!  I was losing altitude on long runs, but for the most part it was well-behaved.  Checking later I found the  TRIM_THROTTLE value of was now a much more sensible 43%.

While working on identifying the Alt hold issue I had also found out that I could assign a knob on the controller to Ch 6, and set that up such that I could adjust the P-Gains while in flight.  I did that, used the knob to back off the Rate Roll and Rate Pitch P gains until it calmed down.  When I got home I found out I had dropped from .15 to .13 so I locked those values in and saved the configuration file to my hard drive as a baseline.

Oh, and for the record, let me just say that testing altitude hold while tying the quad down to a table gives the barometer false readings and doesn’t work, so don’t try this 🙂

But what about Auto Tune?  Now that I had a copter that could stay in the air I went out the next day with a fully charged battery and ran it.  It is very odd watching your quad do it’s dance with no input from you, but it finished without incident.  The results?  Terrible!  It was back to being funky again and very difficult to control.  The good news – remember above when I said I saved my working results as a baseline?  Well, I just restored those.  My old developer habits still keep me honest.

I still have a problem with the craft losing altitude when flying forwards.  I think I might have improved that by stuffing the APM with cotton wool, since I had lost the little piece of foam.  Sadly that was three weeks ago and I have not had a chance to fly since then when it wasn’t raining, snowing or blowing 35mph winds.  I’ll keep you posted on how that goes.

Building an F450 quadcopter – Setting up Flight Modes

After the first flight there were a couple of obvious items that needed to be addressed.  More flight modes and getting rid of the wobble.

First the flight modes.  

I had only figured out how to set up the controller to allow for two different flight modes (stabilze and altitude hold).  With so many modes available on the APM this seemed like a shame.  It took a while but by carefully following the instructions on this page I was finally able to get myself up to six flight modes by using a combination of 2-way and 3-way switches.

Along the way I had an epiphany that the controller sends out frequencies (or something that can be counted) and that the APM looks for values from the different input channels being in ranges (because it isn’t exact).  So a value between 1,231 and 1,360 (for example) can be assigned to a particular flight mode.

Great!  A mystery solved.  Now all those calibration movements make sense.

Not only did I manage to assign the six flight modes, but on the extended tuning I was able to assign Channel 7 to a switch on my controller that would automatically tell the quad to land.  This has been a real life saver because even when it starts going crazy, switching to land will calm it down and bring it down to Earth better than I can do.  

Building an F450 quadcopter – Maiden Flight!

We finally had a break in the weather long enough to get outside and get up in the air!

Before leaving there was some question about which two flight modes I should start with.  Key candidates were Stabilize, Altitude hold and Loiter.  I finally opted for the first two.  Stabilize because it doesn’t rely on anything fancy, and then Alt. hold to make flying a little easier.

The result?  In a word – awesome!  It’s been a while since I was able to fly and it was great to finally get back into the air.   See for yourself.

The F450 has some wobble to it.  Nothing terrible and it should be something we can fix using the Autotune feature of the APM.

S. loved it and wanted to try it out, but was too nervous.  That’s actually good as I would really like things 100% sorted before she does that.  But in the car on the way home she told me she wants a little quadcopter that is all her own 🙂

What a wonderful journey!

I’ve shared my parts list including links to where I bought things in case anyone is interested.  You can see those details here.