Wednesday, April 9, 2008

RC Heli Rebuild

In the last article, I mentioned how I was about to rebuild my helicopter. The original heli had some serious design flaws, and I had started to grow out of the instability problems at high speed, the inability to fly in anything other than absolute dead calm, and the fragility of the rotor blades from withstanding any sort of impact whatsoever.

I go through rotor blades very regularly on this heli. Here is a picture of the original construction, with some battle-damage showing. Note the busted landing gear, the broken rotor blade. There are portions of the plastic fuselage that also broke off.

I was sick on Saturday, so I stayed in bed all day long. Near the end of the day, I was feeling better, but felt MUCH better when Stacy announced that my package of replacement pieces arrived.

I tore into the packaging like a kid at Christmas time. I managed to take a few pictures just in case I forgot how to reassemble the parts that I took off. The slideshow below shows the reconstruction steps.

This collection of new pieces did not come with any instructions. So for the most part, I had to figure out how the old helicopter worked well enough, and figure out where the new pieces fit together in the new approximate positions of where the old obsolete components were. There were a few instructions on how to assemble the rotor shaft, which had key information about where specific washers were, I suppose those washers were quite important.

I did make many mistakes in the reconstruction process.

The first mistake was where I missed a collar that attaches to the rotor shaft. This piece keeps the rotor shaft from sliding up and down, and keeps the two big white gears from grinding against the wrong pinions. I realized this mistake after getting everything assembled, and the only way to install this collar was to back out the whole rotor shaft. This cost about 45 minutes of small screws.

The second mistake I hoped to avoid by taking good pictures of everything on the old helicopter as I disassembled it. The electrical hookup of the motors I got mixed up. The motor for the top blade was hooked in backwards, so when I powered up the engines, both rotor blades spun the same way. And with so much torque coming in the same direction, the fuselage of the helicopter spun very madly in the opposite direction.

The third mistake I made simply is indefensible. It took a while to diagnose the problem though. The main symptom was the wild spinning of the fuselage, either full left or full right. I could have the heli on the ground with no engine torque; power up the engines slowly, and the fuselage does not spin. Increase the throttle a little tiny bit. Still OK. Not enough power to lift off. More. Heli spins to the right madly. Give it a little tiny tiny bit of left "rudder" and the heli spins madly to the left.

It took me a while to figure out that I had hooked up the motor for the left engine into the electrical hookup for where the right engine should go. I guess the onboard electronics got confused and the incorrect hookup caused a divergent negative dynamic stability problem. (Those are some 5 dollar words there).

Once I got all of these components hooked up, I did another hour worth of trimming and lengthening pushrods here, tweaking the "proportional" and the "gain" riostats. I finally got it flying pretty well. The first thing I noticed is that the controls were much more sensitive. I also discovered, by means of a few crashes, that the rotor blades are not indestructible, as I broke one of my new blades right at the blade root.

The evening of flying ended when I crashed it and the new rotor hub for the bottom blade broke. I suppose one of the disadvantages of super strong rotor blades is that the damage just moves to the rotor hub. I had no replacement rotor hub parts, so the flying was done until some more replacement pieces came in.

A very long week of waiting around and not being able to fly my heli dragged on. After a full week, a new box of pieces came in from Canada, I replaced the rotor hub for the lower blades, and the new refurbished heli flies a lot better than ever before.

After all the fixing, adjusting and correcting, finally I got this thing flying smoothly. After all the fixes, this thing flies like a dream. I wonder how long it will be for me to crash it and need new parts again. I started all this RC flying because the soaring season here in Switzerland is short compared to flying in Virginia. I have to start getting back to the airfield more often, especially now that Spring is here.



Dan Ponjican said...

Wow very cool post. I just picked up a Syma heli and am loving these little things!

fpv flying said...

This is one of the coolest hobbies ever! I liked it so much! Flying and assembling them. Nice!