A Smashing Experience with APM2 on an EasyStar!

Yesterday, I had a "smashing" good time with my new APM2 and would like to find out why.  Here's the story.

The EasyStar has run well for several months now with an APM1.  The airplane was upgraded with a larger rudder, ailerons and a 2815/3000kv inrunner.  The extra power is useful when lifting all the electronics and video pod.  The RC radio is a DragonLink on 433mhz, telemetry is an XBee on 900mhz and video is planned for 1.3mhz.

The APM1 worked well and I was in the process of developing more and more complex waypoint plans.  The APM2 arrived recently and it checked out ok on the bench, so I gave it a try in the air.  It ran Manual, Stabilize, RTL and Auto well and I decided to try it with video.  The first video test flight had to be aborted when the EZ started showing "uncommanded" throttle changes.  This was with a 1-watt Lawmate on 1.3ghz, and it seemed like simple RFI.  Video rides on a ReadyMadeRC pan/tilt pod where the antenna is about 4 inches away from the APM.  The EZ did not fly again until a 300 mw Lawmate was installed.  Bench checks were solid, so I took it out to the field.

The first field check was a walking range test.  The RC sticks were tied down and to the side so we would see/hear any jitter or failsafes.  I walked it out to about 1000 feet and there were no problems at all.  Both video and rc control were solid.  The next step was a short flight to capture some initial video.

My preflight procedure is to check correct operation of all controls surfaces in Manual, including a small throttle test.  The next is to check control surface movements in Stabilize.  In this case, the last step was to start AVI capture in Mission Planner.  Takeoff was normal and the EZ quickly climbed to about 400 feet.  In Stabilize, I did some shallow turns with the intent to capture video of the horizon and airfield to evaluate the video system.  At 400 feet altitude and about 1000 feet out, I selected RTL.  This is a standard part of most flights and gives me the ability to check Mission Planner.   The EZ immediately started a descent to the 250 foot default altitude and headed direct to Home.  Passing Home, it turned left to enter its standard counterclockwide holding pattern.  Everything looked fine, so I ducked under the shed to the laptop.

One observer is a retired FAA accident investigator and he immediately pointed out the 45-degree bank angle.  I made a mental note to make it shallower for the next flight.  As the EZ was completing it second complete turn, we heard the motor speed up.  I tried to get the RC back into stabilize but wasn't fast enough.  The EZ was accelerating in a diving right turn and impacted the ground at about a 90-degree right bank.  Mission Planner shows the last groundspeed at 72 mph.  

Everyone on the field ran out to the crash site which was behind the shed.  Fortunately, no one was near and nothing but the ground was hit.  The area was covered with bits of elapor foam and electronics.  The battery had unplugged on impact and was about 20 feet away.  In true form, our retired accident investigator organized everyone to leave the pieces alone and look for structural clues.  That didn't tell us much at all.  The aileron control rods were torn out of the foam and the rudder/elevator servos had separated from the fuselage.  There was no way to judge control surface position.  He did look at the prop hub and saw evidence the motor was supplying power at impact.  So, we picked up the pieces and brought them back for closer exam.  Back at home, the APM2 powered up normally and I downloaded the logs and saved the parameters.

Now, this could be an unfortunate accident.  But, the night before, I read a short post here from another pilot who just lost his airplane in a diving turn into the ground while in Auto mode.  Here's the link to that post - http://diydrones.com/forum/topics/apm-troque-control-problem.  It could be unrelated, but two similar accidents in two days warrants some investigation.

I have attached the Mission Planner telemetry log and the parameters file in the hope that someone familiar with APM's way of handling a level turn under software control can identify the cause.  I'm not flying the APM2 again until I know what happened and I don't want anyone else to have a similar incident.  I'm open to any and all examination of these files, let's just identify the cause.


2012-01-24 09-54-40.tlog

APM2 EasyStar.param

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  • Thanks for all your comments and ideas.  We looked at the APM telemetry data and found that the plane departed from controlled flight just about a half turn from the crash.  At that point, the APM started to correct for a roll into the turn while the plane continued to roll against those outputs.  It used full opposite rudder and increased throttle to try to get back to straight and level.  However, the best guess is that the lower wing (inside the turn) stalled.  The upper wing's lift rolled it over, then the up-elevator pushed the nose down and the increased throttle (plus gravity) drove it towards the ground.  This is a classic approach stall and is usually fatal to any occupants.  My dad, a WW2 Navy pilot, tells the story of watching an F-4U Corsair make precisely this maneuver at Jacksonville NAS.  Fortunately for me, he wasn't that pilot!

    A new FPV EasyStar is already under construction and should be flying in a few days.  Thanks again.

  • Don - Thanks, I started looking at the log yesterday but wasn't organized enough at that point to draw see the trends.  I'm going to try to get all four control channels graphed through the final turn.  Also, check me on this-- If an aileron control rod lets go then the aileron itself would go into trail position.  APM should detect the movement away from the target bank angle towards level and then command more deflection.

    Ric - This airplane has flown extensively and successfully with the aileron mod.  I'm pretty sure it would have shown bad manners before this.  We've even done "upset" tests in Stabilize to check its ability to regain straight-and-level. 

    For anyone looking at the log-- The RTL was set at 84% and the first turn started at 92%.  The accident turn started at 96% on a heading of 105 deg and groundspeed of 59mph. It was already descending at that point. 

  • Just looking at this trying to see if I notice something. Just to be certain, you say it's an easystar, but you mention aileron control rods, so it's been modded to add ailerons, correct?

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