Thanks to the help and expertise of the veteran fliers at ABC Flying Field in Concord, I was able to precisely balance the weight and the span-wise center of gravity of the blades to within 0.01g and less than a millimeter, and the vibrations are almost completely eliminated! Turns out the small imbalance between the blades in the z-direction had very little to do with the vibrations I was having; the span-wise balance was the key.
When I was first getting into R/C helicopters, I was lucky enough to find a couple of guys who were willing to help me out by teaching me just about everything I needed to know to get airborne. Rusty and Don shared with me details about proper electrical and mechanical setup, radio settings, engine tuning, maintenance, flying tips, and basically anything else I could think to ask them about. I lost contact with them for several years, but when I randomly dropped in on the flying field a few months ago, we picked it up right were we left off. That same day I dropped in on the guys, I met Jim who happened to have an E-Flite Blade 400 for sale. Not wanting to make an impulsive decision — and not having more than $16 cash in my wallet — I made myself sleep on it. Surprise surprise, I still wanted it in the morning. A couple of weeks later this project was born.
So when my frankenstein blades were causing small earthquakes in the downtown Milwaukee area, I turned to the same guys that helped me out the first time I needed to get an R/C helicopter in the air. Back when I started out I pretty much only knew Don and Rusty, but now there were a handful of other heli pilots at the flying field that were happy to offer me their advice. The general consensus seemed to be that my blades’ weight and span-wise center of gravity were most suspect. Don brought to the field a very nice scale and CG balancer. After he was finished with the blades, the vibrations were DRASTICALLY reduced, and everyone there agreed the heli was fit for flying. Unfortunately the wind that day was too strong for me to feel comfortable testing it out, but it was certainly good to see such progress! The batteries were still mounted opposite the circuit boards, so they blocked some of the LEDs. I wanted to mount them on the other side so that all of the LEDs were exposed. Don allowed me to take his equipment home with me so that I could remount the batteries and then re-balance everything.
Don’s CG balancer is called the Koll Rotor Pro. Directions for using it can be found online. Briefly, you use a precise scale to measure the mass of both blades, then cut a piece of balancing material (usually tape) to have a mass equal to the difference in mass of the two blades. Use the Rotor Pro to find the center of gravity of the heavier blade, then mount the lighter blade on the Rotor Pro and place the balancing material on it in the position that causes the balancer to be level. The whole process is very quick and easy, and results in very precisely balanced blades. The tool can also be used to balance the blades chord-wise, but I found that just doing span-wise was good enough for me.
After remounting the batteries and carefully balancing the blades, I found the vibrations to be even less severe than they were at the flying field. I hovered the heli a few feet off the ground for about a minute and then landed to inspect the blades. I was pleasantly surprised to see that nothing had been ejected in midair. However I did notice the board-mounted battery connectors were not in the position in which they started. The centrifugal force from spinning the relatively heavy Deans connectors was enough to bend the board connectors outward by about 25 degrees. Just another tally in the “why these connectors suck” column.
My initial thought was, “Dammit, it’s going to be a pain in the ass to remove those cleanly and solder the leads straight to the board.” Then my buddy who was with me said, “You’ll have to put a little super glue on them or something.” Good call, Rob! So hopefully the super glue will be enough reinforcement to hold the connectors down. It’s definitely an item to inspect after each flight.
The helicopter was considerably heavier feeling in the air. Thanks to the extra weight (and possibly messed up airfoils), I had to give it a lot more throttle to get it airborne. Ultimately I’ll set some new pitch and throttle curves in my radio to use specifically for these blades.
I don’t have a picture or video of the heli flying with the blades yet, so you’ll have to take my word for it.