Written by Greg Gimlick Helicopters should be like jazz—smooth! As seen in the Winter 2019 issue of Park Pilot
There was an old saying when I was flying Army helicopters: If it had quit shaking or vibrating, it had quit running. That might be partially true, but I don’t completely buy it. Helicopters have a lot of moving parts. Because of that, they’ll never be as smooth as a fixed-wing aircraft, but they don’t have to be bad. Our aircraft depend on us to be diligent about balancing and reducing vibrations. Modern FBL (flybarless) systems also depend on minimal outside interference to confuse their magic. Define the issue: Simply put, if the vibration is low frequency, or there is a visible bounce to the heli, it’s probably from the main rotor system. If it’s buzzing or vibrating at a high rate, it’s probably from the tail drive. Main rotors spin at a much lower rpm than the tail rotor and subsequently cause low-frequency vibrations. Any vibration or bounce can be misinterpreted by the FBL controller as something it needs to correct, and that can cause bizarre actions at times. Main rotor considerations: The most common issues related to the main rotor system are: • Bent main rotor shaft. • Bent spindle. • Blades not balanced. • Blade tracking. • Worn bearings. • Worn blade dampeners. • Bent or worn linkages. If you’ve had a blade strike or crash, chances are that the main shaft is bent. Lay it on a flat surface (glass, etc.) and roll it to see if it’s straight. You can often easily see any damage. You can try to straighten it, but most of the time this is just a stop-gap measure to get you flying quickly. If you think you’ve gotten it “straight enough,” try checking with a dial indicator. Most of the time you’ll be disappointed to see how far off it still is. My recommendation is to replace the shaft.
Lay the main shaft on a flat surface to check to see if it is bent.
This dial indicator shows that the shaft is still off after an attempt to straighten it. It’s better to replace a bent shaft.
Many of us still use flybarred rotor heads. The shaft for the flybar is thin and prone to damage—just replace it. The same goes for the spindle shaft that retains the blade grips. While you’re in there, replace the dampeners. If you’re one of those people who believes that a new blade from a package is perfect and doesn’t require balancing, think again! Most are close, but the smoother the head, the better the heli will fly. Get a quality balancer and learn how to use it!
Buy a good blade balancer and use it!
Tail rotor considerations: • Bent driveshaft. • Worn drive belt. • Bent tail rotor gearbox shaft. • Worn bearings in the shaft drive tail. • Damaged tail rotor blades. These are all self-explanatory. If your machine is buzzing, it’s probably something in the high-speed tail system. Glitches can often be attributed to a drive belt that has stretched with age and use. It can slip a cog on the drive gears and act like a loss of thrust that can confuse any gyro or pilot. If you’re using a direct-drive tail, check to see whether the shaft is still supported by the bearings in the tailboom. They wear and sometimes come so loose that they don’t support the shaft. Balance your tail blades too. Just because they’re small doesn’t mean that they don’t need some love. Spare parts: I keep a box of spares for every helicopter that I own. It doesn’t have to be extensive, but it includes the primary things that become damaged in a mishap. I always stock an extra tail rotor gearbox assembly and a main rotor assembly. Note that I said “assembly.” I don’t want to waste time chasing a problem at the field when I can often swap out an assembly and continue flying. I’ll chase problems in the shop. I also keep extra blades balanced and ready for installation.
This is Greg’s spare parts bin for 450-size helis. Complete assemblies are kept for quick replacements. He also keeps spares for all commonly damaged parts.
True confessions: I don’t try to straighten bent main rotor shafts, spindles, flybar arms, or tail rotor driveshafts. The parts are cheap enough that I replace them instead of spending hours working on something to find out that it’s not quite true enough to make me happy. Your mileage might vary, but unless you own a machine shop, your attempts at straightening things will be difficult. I replace an entire assembly and, when I have time, I rebuild one for the spare parts bin.
Greg keeps a spare tail rotor assembly for quick repairs. A telltale sign of a bent shaft is that the slider is not smooth.
The mystery culprit: Occasionally, I run across someone whose helicopter seems to be well balanced and runs smoothly, but it does random, bizarre things in flight. It usually boils down to something that few of us think about: how the flight controller is mounted. These magical, six-axis gyros look at everything and consider any uncommanded movement to be something it needs to fix. If the double-sided mounting tape is damaged or has been mounted incorrectly, it will move and misinterpret that as something the aircraft is doing and perform a corrective action. Never mount a FBL controller with Velcro! They usually come with extra pads of approved tape, or the company recommends something such as a polyurethane gel pad. It must be secure enough to not move around, yet be protected from minimal vibration. Check the mount to see if your heli occasionally seems to have a mind of its own.
Use the proper mounting material for your flight controller.
Bottom line: Our aircraft are a sophisticated combination of mechanical and electronic components that all need to cooperate with each other. Time spent doing preventative maintenance and building them properly will be rewarded with a smooth flying experience.
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