The bad news

I’ll start off with the bad news. I’ll go over all of our crashes since the last post. There haven’t been many and most were fairly minor, so it isn’t all that bad.

Our first crash was with the DAO when we were attempting an auto-takeoff. We had some false starts and then, with a running start, the plane took to the air. Instead of following a straight-out heading, the plane immediately banked right and made 2 climbing circles at full throttle before nosing down from a relatively low altitude and hitting the deck. I have not dissected the cause of this crash but we are pretty certain that we needed to command APM to restart the mission after the aborted takeoffs before the successful one. We think the plane was attempting to loiter at home when something caused it to nose down. From this we learned not to use a front-line workhorse aircraft for testing. Fortunately, damage was pretty minor and some glue got us back in the air.

The second crash was when we went to Belize. This was all pilot error. I was coming in for a landing on a very small beach under FPV control. The beach was only about 15m long and 5-10m wide, and I had to make a low approach over mangroves and a small lagoon to keep the plane oriented into the wind. The mangrove canopy about 50m short of the beach necessitated a fairly steep glide-slope. I was on my second approach to the beach (we had already flown a portion of the island earlier) and from my view with FPV it looked like I was approaching too low and was in danger of landing short in the small lagoon. According to the spotter on the beach I was not, in fact, too low but I was flying remotely so that was not relayed to me in real time. I pulled up and added throttle to clear, which put me too high and in danger of landing long and hitting palm trees at the edge of the beach. To avoid that, I nosed back down hard, digging the nose into the sand and flipping the plane onto its back. Damage was minor and repaired within an hour, allowing for a third flight and a successful landing on the same beach. From this crash, we learned that sometimes there is no replacement for an LOS landing.

Making repairs on the DAO after our hard landing on the beach

Our next crash came a little over a week ago. Marcus and I were working on ironing out issues with the DAO which had persisted since the Belize trip and crash. Repairs on the tail had left the geometry of it a few degrees out of alignment and something (probably exposure to heat during transit) had caused the wings to warp (a detail we didn't discover until after this crash). Due to these 2 problems, the plane had flown really poorly since returning from Belize. In the process of doing test flights we simply got too low got the black plane below the dark tree background, and lost orientation. The plane went down causing minor but not insignificant damage. From this crash we learned that the foam airframes are disposable and are easier to replace than repair. New airframes after significant crashes are just a part of doing business. As a result we retired the original DAO.

Our fourth and final recent crash was just a few days ago. We had gotten in our new airframes (which are awesome, but I'll talk about that in more detail in an upcoming post). We put the new wings on our new primary airframe to evaluate their performance. The new wings have a few significant differences from the old, including much larger ailerons (about twice the size as the old DAO wings). We aren't entirely sure what caused the crash during our test flight, as we shut down the laptop shortly before it happened (we were winding down for the day), but I am pretty confident it was a tip stall. We had already done about 15 minutes of flawless test flying and Tim and I were both very, very impressed with the new wings. We entered a slow left bank in FBW-A at about 40m AGL, with roll limited at 30 degrees. In the bank the plane suddenly rolled past the 30 degree limit into a dive and was unrecoverable. My belief, based on review of the onboard camera footage and the dataflash log of the RC I/O data, is that in the bank there was a gust of wind coming from the right of the aircraft, which caused the APM to drop the left aileron to fight the roll caused by the wind gust. This drop of the aileron on the slow wing (the wing to the inside of the turn) put the angle of attack too high, leading to a tip stall in the left wing. This caused the plane to roll over and dive. At this point the plane should have been easily recoverable, but I believe one or both of the batteries slid to the nose of the aircraft, moving the CG too far forward to be able to pull out of the dive. Fortunately damage was minimal and limited only to some deformity of the plane's nose. From this we learned that tip stalls are real and we need to keep airspeed up to avoid them. We also need to secure batteries better so an event like this doesn't lead to an unrecoverable dive.

Skip ahead to 10 minutes to see the crash.

So, that is all I have for crashes. It has been a pretty tame few months without too much excitement. Most of our flying has been uneventful and some it has produced downright astounding results... but more on that later.