Main Rotor

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.

Find ideas of things to check if your heli doesn't stay super tight on horizontal axis when hovering, typically circular moving around axis. Can happen since day one or slow degradation.

1) Check the shaft of all swashplate servos: excessive play can result to lack of precision. Compare the servos, they must be identical. Check also when new.

2) No excessive play of ball links, consider experiencing another brand, some are more tight while still smooth, thanks to high grade polymer.

3) Consider replacing bearings if lot of hours
of run. Some can feel difference after major overhaul. See all exchanged parts in picture, more than strictly necessary but since on it..

Hello,
In the heli courses videos
In your pitch curve video. To get the pitch curve -100/0/+100 with a value of -100 to -2 degrees. Where do you enter this value so as not to go below -2 because in the axon the settings are -11/+11
I have tried various parameters but I cannot reproduce your pitch curve.
I would like to understand. If you could help me .....
I changed Gyro and radio so I am starting with the axon and the Jeti DC16-II and I would not like to make a mistake.
Very cordially.
Gilles

Autorotation pitch.

For our scale helicopters how much is needed? -2,-3,-6, more??

All my scale helis use similar pitch curves: -3, hover pitch, and +10.

I practice Autos on all types on a sim but only with my 700 pod and boom in real life. The exception being on a couple of occasions due to real life failures: once with my 40lb BK117 which was mostly successful with just bent rear struts and some cracked fiberglass. And a second time with my 700 size 14lb MD500 somewhat less successful due to a delay in actuating the Auto.

It has been suggested that -3 degrees isn't enough...at least initially, and the Auto curve should have something more than that as a starting point.

Thoughts?

From Heli-Tec web site:

Dear customers,
First of all, we wish everyone a happy new year!
The year 2024 will also begin with changes for us:
After more than 20 years, we have decided with a heavy heart to stop producing rotor blades at the end of the year. We are already looking for a successor who would like to continue rotor blade production. The successor would take over all rotor blade shapes (several 100 pieces), original models, raw materials, and stocks of finished and salable goods from us. We also disclose a list of our purchasing sources upon acquisition. After the takeover, we are happy to call for a limited period of time. to be available for advice. If a successor can be found who already has experience in building rotor blades, that would be great. At least it should be someone who is no stranger to dealing with fiber composites.
If you are interested, please send us an email to info@derblattschmied.com.
In the coming days we will reopen our shop so that you can order all sets again - initially only in light gray.

Hi Michael , My name is Stefan from Belgium and I am verry new to the rc heli world. I've seen your course video about the pitch settings but I don't understand 1 thing. Your curve is 2 degrees negative to about 12 degrees positive. When setting an FBL ( Spirit FBL) you must therefore have a linear curve. ( 12 degrees negative - 0 degrees midstick - 12 degrees positive ) Not many people fly in our club Helis and they fly 3D , not scale . How do I change that 12 degrees negative to about 2 or 3 degrees negative without bending my curve significantly? I have a Jeti dc 16 transmitter. Sorry if this is a stupid question but I want to learn how to set it up properly. You will find very few videos for tuning scale helis and their transmitter settings. Thank you in advance Regards Stefan