I am building my first model with a 35 cc gas engine and I have been advised by a fellow modeller to be extra carefull of the vibration issues as opposed to glow engines. I was even advised to use thread lock on the screws holding down the servo arms !!! Can any one please advise.???

Yes this is correct... thread lock all servos if they are metal gear servos (and they should be) Also DO NOT use Karbonite gear servos. Balance your prop and make sure everything is tight. Loctite the engine bolts as well.

Also make sure you have no metal to metal contact anywhere... ie: linkages. Throttle linkage should have plastic ends.

Also range check it when it's done without the engine on... then with the engine on... you should get atleast 80% of the range you got with the engine off.

The advice Sinergy gives is spot-on. Do what you can to avoid RFI or EMI from metal-to-metal contact.

Also note the airframe vibration isn't much of an issue with a well-balanced gas engine. Big 4-stroke engines shake far more than any of my gas engines, especially through the mid-range.

Please remember that there are a number of different thread lockers available and you definitely don't want to use red Loctite (which is the one not designed for removing) or similar on those servo arms.

Loctite comes in colors to designate the recommended use, i.e. blue is the next color down and is considered removeable, but I like green. It's the next step down and is considered a penatrating variety so you must make sure you don't use a lot of it, but that is true of all thread lockers.

The other piece of advice is to replace your stock servo arm screws with the socket head type for Allen wrenches, that makes removing the screw a much easier experience as opposed to messing up the head of a Phillips type screw. Also, if you choose to not replace the stock screws, be aware that a lot of those screws use a JIS (Japanese Industrial Standard) type slot, not a true Phillips so it's easier to strip those screws out using the wrong type screwdriver.

I like to replace all my servo arm screws, but I keep a set of JIS screwdrivers around as well...

For engine mounting, I prefer to use the nylon insert type locknuts as opposed to a thread locker.

What's wrong with Karbonite gears?

TIA,

CR

Zeeb, do you have the source handy for those socket head servo arm screws? I see something along that line at Servo City, but they come in three lengths, apparently all the same diameter & pitch, and no apparent distinction between Futaba, JR & Hitec. I checked TBM, who have a nice selection of servo accessories, but socket head servo arm screws don't seem to be among them. I definitely agree the Philips head screws are not the easiest to deal with.

Whilst all the advice given is very good, let it be noted that gassers in fact vibrate less than glow engines, due to the controlled ignition if the mixture. In the heyday of gassers, these engines were much larger than modellers were used to, and those large chain saw converts vibrated more than a .60 of course. Of nowadays practice, if a 1.60 glow is compared with a 1.60 gas, the gasser vibrates less.

Re Loctite...There are many myths concerning strengths and colors...The different grades are different colors, but green 680 is the strongest..If you put green 680 on the brass screw that holds the throttle plate on the shaft and try to remove it, the screw will twist off before it unscrews...
The best way to find out about Loctite is THEIR website, certainly not here...

Thank you gents for all the valuable info. No one mentioned the servo arms! Am I to assume the standard issue that comes with the servo is good enough ???

This is where I purchase mine:
http://rtlfasteners.com/RC/d.html

Walt...

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Nothing if all is done properly.


Ed Cregger

Walt, I appreciate the URL you provide, but that relates to servo mounting screws (which I do already use) and not for the screw that holds the servo arm to the servo hub, which is what zeeb is referring to. I will say that I've never had a problem with the servo arms coming loose from the hubs, but I still don't like the philips head, don't like the little star type lockwashers, and would prefer to use a socket head version, if I could get the right diameter & thread pitch.

Look in the inventory of screws in a large model race carhobby shop -I find they have -usually- a very good assortment of small screws. both machine and sheet metal types .

Balance the prop. You'd be surprised the crap that people will ship you (Menz 22x8 from DA for example...WAY off!!!). Always slap it on a balancer before it goes on your engine...or watch the plane shake apart.

Check with the servo manufacturer to find the correct size of the servo screws used. Then go to one of the bulk hardware companies like RTL or MicroFastener and order 100 of them for 5 or 6 bucks in the head type you desire. All manufacturers DO NOT use the same size screws.

visit http://www.mcmaster.com/ for fasteners as well.

Hitec do not recommend them for gassers, they suggest metal geared servos. Karbonite is strong but they can't handle the vibration. I assume the vibration hammers the gear teeth hard enough that they wear faster.

The screws in the servo hubs can come loose. Had it happen on my first gasser recently. Luckily, my pre-flight check picked it up (not to mention a ball link nut coming off while idling).

I use Tamiya blue locktite, which seems to be popular with the local R/C car guys. It stays somewhat gummy and therefore won't just 'snap' loose. It's also easier to remove parts when you need to. It only goes rock hard (but still removable) after about 12+ months of exposure to the rigors of R/C planes.

Just for the record, all major brand metal servo hubs seem to use metric 3x.50 screws. Servo City has them in three lengths, the shortest being 8 mm. The stock screw on my JR metal hubs is 6 mm, but I tried an 8, and it fit OK. This size also fit my Hitec metal gear servos as well, a few chosen at random. The McMaster-Carr part no. is 91239A113 for M3x.50x8 button style socket heads, less than $6 for 100. The resin gear screws are metric 2.5x1.0 pitch, and that appears to be common for at least Hitec & Futaba; did not have a resin gear JR to check, but I bet they're the same.

Yes, mikenlapaz, McMaster-Carr is great for hardware shopping, best website I've seen anywhere for quick finding of what you're looking for, good prices, vast selection and fast delivery. They are my hardware store of first and last resort.

I find a dab of silicon glue on the head of the servo screw is best . It holds firm and is easy to remove.

Ditto Ditto.

Hitech is 2.6x8mm. In my JR 8611 3x8mm works great.

Silversurfer has the right idea as not all screw sizes are the same, some are just different lengths, others have different diameters and/or thread pitch...

Find the size you need from the manufacturer and order them on-line. I've not had much luck finding them in any LHS or hardware stores, and if you do find them at the LHS they tend to be about 10 times as expensive as the folks Silversurfer mentioned.

Ralph is right concerning the Loctite, and I have to confess making a mistake when I said Loctite as I use Permatex brand where the color coding is a little more straight forward than the Loctite. In fact, there are so many Loctite compounds it can be difficult to determine which is best for a specific application.

And as for not using thread lockers on Karbonite gears, not only does Hitec not recommend them for gassers, but not for models over twelve pounds as well as the fact that if any of the threadlocker happens to get onto the gears it will deteriorate the material.

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Thanks, Raven. I wasn't aware that Hitec did not recommend them. I'll go to their website and give a gander.

I used to fly 35cc and a bit larger engines with "stock" servos that came with cheap radios. Never had an issue, but that was long before the engines were producing today's power levels and the models were mostly scale or semi-scale with little control surface deflection.


Ed Cregger