I was lazy today and was almost too embarrassed to post this, but I concluded that my cautionary tale was more important to share than protecting my ego.
Last week, I took the 1/4 Lama out for a maiden since purchasing it from Gonzalo and rebuilding the mechanics and fuel system. On spool-up, the blades were not tracking well so I shut it down and went home. I replaced the head rods with turnbuckles and re-tracked it. I mention this because I could have tried to do these things at the field, but I know better.
Yesterday, I brought the Lama out for a second try. I spooled up and before I even reached mid-stick, she gracefully floated into the air. I actually exclaimed out loud that this machine was the smoothest I had ever flown (it seemed that way). I hovered and performed some initial flight maneuvers and was absolutely blown away by how it flew. I did notice that it had a slight backward drift in a hover and was slightly sensitive so I decided, after 3 minutes, to land and adjust the expo in the radio.
I landed, spooled down and adjusted the expo. I then went into the AXON menu on the transmitter and examined the agility settings; they looked fine for the bank I was flying in, but I could not find a way to trim the elevator from the transmitter. THIS IS WHERE I FAILED MYSELF.
Rather than shut down for the day or pull the deck and connect a cable to the AXON to adjust the cyclic trim, I told the guys around me that I was going to "cheat" and put in 3 beeps of forward cyclic trim. I was well aware of the rule that all trims should be at 0 but I had cheated in the past (likely with a 3X or 401 on the tail) so something in my head told me I could get away with it.
NOT THE CASE. Thanks to input from Gonzalo (full scale pilot), Danny Melnik, Tommy Dean, and Mark Smith, all of whom sent me to the internet to research ground resonance, I now understand why my lazy move was almost guaranteed to cause the mishap I experienced.
HERE IS THE LOWDOWN:
The AXON interprets trim commands as inputs. As such, the AXON sees the radio's trim as a constant, intentional cyclic input. While in the air, the AXON can correct for this. But, on the ground, the skids create a mechanical connection between the machine and the ground. As the rotor nears full speed during spool up, the persistent cyclic input can accentuate the natural frequency of the blades' lead-lag motion. This can cause them to move out of phase, shifting the rotor's center of gravity away from the axis of rotation. As the unbalanced rotor turns, the helicopter rocks on the skids (you can see this in the video). Once the frequency of rocking matches the blade frequency, you get resonance, which is SELF-AMPLIFYING.
THE LESSON: I can fix the damage to the model. I was able to shut down the system fast enough to mitigate the resonance and prevent a rollover. But there was no reason I had to experience this other than LAZINESS. I should have addressed the issue properly but in my desire to fly again, I overlooked an obvious rule of operating these machines.
I am sure many of you already know this rule (as I did). Yet, it didn't stop me from doing a stupid thing. I hope my experience graphically illustrates why -- especially with larger machines -- we should take nothing for granted and ALWAYS take the hard path of patience and deliberation before we fly.
I posted additional insights in the comments. This issue is one experienced by both full scale and model pilots. In the full scale world, instructors will most often NOT induce resonance for the student, but rather ask the student to observe what the instructor does in order to understand how to respond. I can say that having this experience has CEMENTED the learning in my brain.
Michael K.
RCPilotLet This Concept Resonate With YouSeeing Is Believing Watching a model (or full-size) helicopter experience severe ground resonance is a sobering experience. Watch this video and then read about why this occurred and how to prevent or mitigate it in your
Randy C.
Stefan R.
Mark S.
Gonzalo M.
Rene G.
Thomas D.
