Helicopters - Greg Gimlick

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.



RC helicopters are just as susceptible to degraded performance because of the lack of maintenance and minor damage as their big, full sized brothers. I keep a well stocked inventory of spare parts to replace any damaged or worn sub systems on the helicopter if needed.

Greg, when were you in, what did you fly and in what units did you serve ? Me, '68-71, OH-23, OH-13, UH-1D and AH-1G, C/7/17 Air Cav and 1st Armored Div.

Hi Rob, thanks for your service.

I spent most of my life in the 82nd Abn and 18th Abn Corps. I retired in '90 after flying OH-13, UH-1H, Oh-58A & C, AH-1G, OH-6, and got to fly an Apache, but wasn't rated in it. Flew some civilian stuff like A-Star 350 and Long Rangers while at PHI.

This article was devoid of any 'real' help that would aid a modern heli pilot.
"Many of us still use flybarred rotor heads." I've been flying helis for many years, and haven't seen a fly-barred heli since 2013. No one at my two RC clubs (one of which is SOLELY helis) has flown a flaybarred heli since 2012.
"Never mount a FBL controller with Velcro!" Who in the world still thinks this is even an option when mounting a FBL unit?

I'm not certain how much time the author has building, setting up, flying, or repairing helis, but he would be best served to travel back to 2009, and submit the article then.

With they dying popularity of helis in general, this article most certainly doesn't do justice to what is quite possibly the most technical and detailed portion of the RC hobby.

Thanks for taking the time to read and comment Timothy. I'm sorry if the gist of the article offended you, but I can assure you I get mail all the time from people who have bought a used or heavily discounted helicopter that still uses a flybar. While they may not be the norm by any stretch, there are a lot of folks first trying helis out who have them before moving up to the latest gear. There is no shortage of flybar helis online.

You mentioned my reference to velcro for mounting and once again, you underestimate the people out in the middle of nowhere without help nearby who do just that. I've also gotten emails with photos of wires connected to each other using standard household wire-nuts. Yup, it happens and I've stopped being surprised by things I hear and see. I just try to help by educating them, not harassing them.

I agree with the substance of the article. But I object to the lead in line equating jazz with smoothness. It *can* be smooth (ish), but that's not the norm. Ever heard of Thelonious Monk? Rahsaan Roland Kirk? I'll take my helis smooth, but my jazz edgy. :-)

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