I've heard arguments on both sides. Would like a consensus. What is your reasoning for using, or not using, the rubber grommets to mount your flight control servos in your turbine model? What is the common practice among experienced turbine pilots?

well, there is little if any vibration in a turbine model, so that being said you could probably get away without using grommets. flight induced vibration (harmonics) may be another thing to consider. no real data to confirm how high those levels are yet. also finding a suitable fastener to take the slop out the now oversized servo mounting hole (without a grommet) will also be something to consider. you definetely dont want any chance of servo movement. i would use them.(grommets)

I don't use them. I don't want the servo to be able to move whatsoever. I use the servo mounting screws from Micro-fasteners.

i checked out those servo mounting screws from micro fastener, it does not appear that they have a bushing or collar to take up the extra slop in the servo ear. is the threaded portion of the screw the same diameter as the hole in the servo? if not seems to me that you could have the potential for servo movement. the grommet, bushing method may allow slight, and i do mean slight movement but, will most likely return the servo to its original position. if your servo moves due to smaller screw diameter, you better find your flight trims quickly.

I don't use grommets I cut small peices of tygon tubing and fit the original screws and steel washers.

M

A screw to fill the servo mounting hole would be over 3/16 of an inch thick and impractical. The servo screw holes are drilled into the mounting plate so that the screw flanges (or washer, if I'm using one) snug up to the servo case and the shaft of the screw is snug to the inner surface of the hole. This is so that if, for some reason, the screws loosen the slop will be minimal. It is not the screw filling the mounting hole in the case that makes the servo mounting solid, it is the pressure that the four mounting screws apply to the servo mounting ear that secure it to the mounting plate.

Do a test, mount servos up both ways and work them back and forth to simulate usage forces and see what you think. Go with the method that makes you more comfortable.

If lack of vibration is the main reason you are considering not using grommets, then don't forget to consider that the engine is not the only source of vibration. Simply taxying on a rough surface causes enormous amounts of vibration too.

Gordon

I've been known to epoxy, polyurethane glue and other such methods to make sure my servos never move. Once I put a servo in a jet, it's never coming out unless there is a problem with the servo and then it's a huge project, but I have yet to have a servo come loose in any way. If I lose an airplane, I take the carnage, send it to horizon and ask them to put a new case on it/and or fix repair the servo. It comes back to me in new condition and I glue the little *******s into the next project.

I'm not willing to take any chances on this issue. Servo movement does not sound like something I'd be willing to accept FWIW.

We always used grommets here and never had any issue with them... (as far as I know).


Regards, Enrique

I have never seen or heard of an issue that installing grommets caused a problem. The servo cases are designed for the grommets and I will always install gromments on a servo unless the mount itself (such as the Robbe thin wing servo mount) precludes the use of grommets.

Never had any problems using the grommets even when one of the screws came loose because of vibration the other screws and grommets held the servo in place and tight, so I will keep using them.
I think it all comes to where you are screwing the screws and how solid is the surface.

CARS II

Use the grommets. Gordon has it right.

Just a question? Wouldn't control surface movement (minor flutter, back pressure) over time cause the lip to weaken perhaps even crack under some extreme manuevers? I thought the grommets not only aided in vibration damping, but also assisted in side and front load support for heavy torquing on the servo case. Has anyone not using grommets experienced any failures in this regard?

Interersting topic!!!!!!!!!!!

Thanks for the input gentlemen, I truly appreciate it. I made the choice to go without grommets a year ago when I was rigging my first turbine model. My reasoning at the time was that turbine models are typically very fast, and fast models are prone to flutter. There are lots of "tricks" to prevent flutter, but the best way (barring counterbalancing) is to eliminate slop and springiness in the linkage. To me, servos properly mounted with grommets and eyelets seem fairly easy to wiggle back and forth a tiny amount - a perfect foothold for flutter. There's no engine vibration, so it seemed like a good idea to eliminate the grommets and firm up the control linkages. So far, everthing's working great - even the Hitec digitals

I do see the argument about the bumps of taxiing and landing. However, it's a different kind of "jolt" than the millions of tiny vibrations that a servo would see from a typical glow model. Do the grommets absorb the jolt of taxiing across a crack in the pavement? I really don't know for sure, but they certainly wouldn't hurt.

Even though I went without grommets, I'm not entirely comfortable with it and don't know that I would recommend it to others. That's why I wanted to find out if the jet community had settled on a common practice.

I have run without grommets for years for the same reasons, namely fear of flutter. In addition I counterbalanced all of the flight surfaces in my last bandit. Probabaly not as big a deal in the 200 mph speed limit days we live in unless you operate without AMA membership.

A lot of low frequency vibration comes from out of round tires, so changing them frequently helps. I still wrap the Rx in foam however

It's not really the one or two bumps from a crack that concern me, so much as the much more substantial vibration that you get from many of the "paved" runways I've flown from. Surface quality varies greatly: runways designed just for model aircraft tend to be fairly smooth, but many of the full-scale runways have a much coarser tarmac grade that subjects the model to pretty harsh abuse. I can literally see the wire-gear on my Bobcat continually vibrating back & forth as I taxi on some of these runways, and that tells me a lot about the force being transmitted to the aircraft. So does the fact that on one occasion on my Bandit, I forgot to put Loctite on the screws when I put one of the main gear retracts back in the plane, and it took only two landings for the screws to back out completely.

Gordon