|Nov 24, 2013, 11:03 PM|
|Nov 28, 2013, 01:34 PM|
The XC-142 has crashed - Again
I was flying Thanksgiving morning and the plane was performing beautifully. Hover and slow forward flight were uneventful. I had already burned off one battery and was nearing the end of a second. Each flight was a takeoff to hover, transition to slow forward flight, climb 2 mistakes high, and then transition to “almost” Full Forward Flight. I was past 45 degrees wing angle and moving forward, carefully, 4.5 degrees per flight.
The plane was working as perfectly as I could have hoped for. My dutch roll problems of the previous flight had been solved by eliminating the flaps and mostly by programming some P stability feedback in yaw into the main motors. The transitions from hover to slow forward flight were near perfect, and the transitions to “almost” fast forward flight were showing no significant trim changes. I wasn’t doing anything radical, but the plane was easy to fly in all flight modes with no bad habits that I could find.
On the last flight I had programmed the wing tilt to +60 on a scale from –200 to +200, so the wing was about 18 degrees from horizontal. It was almost certainly flying almost entirely on the wing at that point. I was 2 mistakes high and made the transition to “almost” fast forward flight without incident and was just flying around. Everything was responding normally, and in trim, so I decided to bring it down for another 4.5 degrees of wing tilt. I pulled the throttle back a bit to descend and then noticed that it was still climbing slightly. I pulled the throttle back a bit more to descend and then noticed that it was getting very slow, so I gave it a little forward stick to push the nose down.
The nose dropped, and it went divergent, as if entering an outside loop. From there it went into a random tumble and I could not say exactly what happened. Based on the recovery from the previous tumble I flipped the switch into slow forward flight mode, or so I thought, I had actually flipped into full hover mode. The gyros began to resolve the tumble and brought it level in roll, but it was still oscillating wildly in pitch. It did several 90 degree excursions and did at least one 360 degree flip. I was pretty sure it would stabilize before hitting the ground, but it never did. It hit nose down, but not at a high rate of speed.
In hind sight I should have increased the throttle to both increase control authority and give it more lift. I also would have been better off in slow forward flight mode where a little forward airspeed would have provided some much needed aero damping.
The real problem was letting it get slow in “almost” fast forward flight mode in the first place. I would not normally be flying in this configuration except briefly when passing through to fast forward flight. Letting it get slow in fast forward flight mode is of course not allowed, but I would expect that with a decrease in throttle it would drop the nose like any other plane. It may well do that once I get fully into fast forward flight mode, I don’t know yet.
The problem remains that I have a dangerous region that needs to be avoided or preferably mitigated so you can’t get into it in the first place, or if you do, you can recover with normal flight controls. I have some ideas about adjusting the tail rotor throttle curve that will hopefully solve this problem, but first I have to put the plane back together.
At this point it is a jigsaw puzzle from the wing forward. It all should be fairly simple to repair, but of course the scars are accumulating.
Bob O. has some video of the earlier flights and the latter part of the tumble and final crash. I don’t know if he got the actions leading up to the crash or not. He has a brand new I-phone that he is just getting used to, so hopefully he will figure out how to put it all on You-tube and share it.
|Nov 28, 2013, 04:11 PM|
If bob o. Download the app: CAPTURE,
He can post his videos on his YouTube .
It take a little while before I found out.
|Dec 01, 2013, 05:08 PM|
100% Transition Complete
This morning, 12/1/13, with the new enlarged horizontal stabilizer, the XC-142 completed the last steps at 70%, 80%, 90%, and of course 100% wing tilt out of a scale of –100% to +100%. I did not explore the flight envelope much at these partial transition steps.
The first flight with a full transition was uneventful with a smooth clean transition to SFF and a 2nd clean transition at altitude to FFF. Trim in FFF is good, with a possible tendency to climb slightly, but this is easily adjusted. Flight characteristics in the air are very normal, but with a bit of adverse yaw present at times. This is probably related to the high P value on yaw that was used to overcome dutch roll. Coordinated rudder on the turns solves this problem. I can also mix some ailerons to yaw if I want to make this automatic.
The reverse transition from FFF to SFF can be a little over dramatic if you make the transition with too much speed. The plane drops the nose and continues to fly on the wing, before slowing and leveling to the normal SFF attitude. I will need to experiment with more delay in the TX on the flap switch to make this smoother. I have the delay set to a short 4 seconds so I could get back to my SFF safe mode ASAP in a tumble situation.
A 2nd flight with a transition to FFF was performed with a new battery. The flight envelope was explored, including modest push overs, high banked turns pulling nearly full up elevator, several rolls, and one loop. Aside from the occasional adverse yaw, everything was completely normal and predictable. A pushover and release to neutral stick resulted in an immediate pitch positive recovery. One of the rolls involved a touch of down elevator when inverted and the result was as expected. Inverted flight has not been attempted yet. Slow flight characteristics have not been explored when in FFF mode except that throttle was reduced from about 80% in level cruise to about 40% and the nose dropped automatically to maintain acceptable airspeed.
Unfortunately, I played too long, because at 20 ft altitude, on final, with less than 20 seconds to touchdown, in SFF mode, the tail rotor quit, it flipped on its back and crashed upside down. I later determined that the battery was down to 4%. For future reference, I need to remember that high power settings, like 80% throttle, as are needed in FFF, will flatten the battery as fast or faster than hover mode. The low pitch fixed pitch props are forced to churn a lot of air to maintain adequate airspeed and are therefore inefficient in FFF mode. The damage should be repaired this afternoon but further flight testing will probably have to wait until next weekend, weather permitting.
All of this testing was done with the battery 1” ahead of its ideal hovering location to insure adequate pitch stability in FFF. This has compromised hover stability to some degree, especially in a descent. I will want to shift the battery back in increments and determine if my FFF pitch stability is still adequate. It certainly feels fine in the air, with nice trim and control harmonization in all flight modes.
Increasing the horizontal stabilizer area by 33% seems to have solved the pitch stability problem in FFF, though it is unclear if it ever had one. Hopefully it solved the pitch divergence and tumble problem during a partial transition between SFF and FFF. I do not, however plan to explore high power pushovers in that region any further. I have decided that it makes no sense to be doing hard maneuvering during mid transition. Now that I know that FFF is a “safe mode” I won’t be spending more than a second or so passing through that region anyway.
I will try to post some video of the flight, but much it was at "safe altitude" and the I-phone camera doesn't have adequate zoom to make it a very good video.
I must give full credit to the OpenAero 2 VTOL firmware provided by HappySundays without which this project would not be possible. The various transition related features in this firmware have provided the flexibility to handle the extreme trim changes throughout transition, turn off the tail rotor in FFF, and mitigate various stability problems what would have prevented this project from succeeding. With this firmware, a pilot friendly aircraft is possible.
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