Flight Test Log 21
Attempt at flying between two waypoints. Not successful, flight ended in crash.
Had pitch trim problems.
Date: 2006/07/06 (Thu)
Scale: 55 %
Mass: 962 g (2.12 lbm)
Location: PCC West Campus
Elevation: 730 m (2400 ft)
This was an attempt at flying between a pair of GPS waypoints 350 m apart.
The objective was to see if the airplane would do a racetrack or figure-8
pattern about the waypoints.
Prior to switching to autonomous flight, the pilot had trouble trimming the
airplane. There didn't seem to be enough nose-down trim. When we switched to
autonomous mode, the airplane didn't seem to be tracking waypoints very well.
After awhile the pilot had trouble controlling the airplane and it crashed.
You can hear the thump on the video. We walked over to the crash site and
expected to see the airplane reduced to a pile of confetti, but it turned out
damage wasn't all that bad. Damage consisted mostly of a separated wingtip,
elevon hinge damage, detached motor mount and broken propeller (see below):
Damage to port outboard elevon hinge
(note exposed Robart hinge point):
The airplane had crashed into a small bush, which might partially explain the
lack of serious damage.
The internal equipment held up well against crash loads. The components are
attached with velcro, long strips of which line the floor of the avionics bay.
The only problem was with the 6 VDC voltage regulator, which was partially
pulled up from the velcro (see below).
We're not sure what is causing the pitch trim problem, which had shown up in
previous flights over the past few days. The center of mass is OK and is
actually slightly ahead of its usual position. We suspect the problem is
caused by changes in shape of the airplane, perhaps due to accumulated
(Added 2006/07/09) A new theory is that the pitch trim problems were caused
by a thrust line misalignment. Prior to the most recent set of flight tests,
the motor mount had broken off and had been repaired. It's possible the
repair could have resulted in a misalignment.
We had never actually measured the thrust angle after that repair, and
unfortunately it's difficult to measure now because the motor mount broke
off again, in the crash of 6 July.
(Added 2006/07/18) More on the thrust line misalignment theory -- we
recently discovered a bent motor bracket. See news page entry of 18 July 2006.
Last updated 2006/07/18 FLM