Hi guys, I'm building a 80inch aircraft to be powered by a modified 25cc Homelite "weed whacker" gasoline engine. Work is progressing well but I need to consider my throttle servo.

Due to the position and swing of the throttle arm on the Walbro the options for mounting my servo are limited unless I go to fancy bell cranks and the like.

Looking at the engine mounted to the firewall there is plenty of room between both to fit a normal HS-422 servo. However, it's a little tight so I'm looking for a slightly smaller servo like the HS-81.

This would allow me to mount the servo in a semi-protected position behind most of the engine, with just the servo wheel hanging out into the breeze. With a simple ball jointed linkage can then get a fairly straight push/pull on the throttle.

My concerns are this:

1. While my engine runs smoothly it will vibrate. How bad would the vibrations be on the servo?

2. Which type of gear train is better suited to vibrations: nylon or metal?

3. I could use a HS-55 but will that be able to tolerate throttle control and some vibration (also noting it would live pretty much next to the engines heat - but get lots of airflow)?

Oh, the Walbro will have the return spring removed from it so it won't place any undue load on the servo.

In summary, what servo would you use for a gasoline engine and is it such a bad thing to mount it between the engine and fireall (in the open)?

I use a indirect drive on all mine like a standard s148 I am not sure if the hs-81 is indirect drive or not.Indirect drive is where there is a floating part between the servo output and the potentiometer (it wont cause centering problems from vibration like direct drive.)

Darin

I've been mounting mine on the firewall or engine mount box with no problems. All the magazines show the same set up on the gassers as well. Should be fine, but check for radio interference before flying.

AV8TOR

I use the bell crank and golden rod to absorb the vibration.

The reference potentiometer will go first in the servo causing it to hunt and stick close to the intended set point. So most of the time You get a little warning.

I have been using a hobbico mini servo on my cap for three years now with no problems ( knock on wood)

I've seen your setup before and considered it carefully because the layout is near identical to what I will have. The biggest difference is that my carb is mounted the other way around so the needles are accessible for tuning, this means the linkage has to go the other side and I am hesitant to drill holes in the crankcase to suit the bellcrank pivot.

Hmmm, just been carefully looking at my engine and yours. Perhaps there is a way to bellcrank it using a spacer off the lower engine/firewall bolt. I could make a sleeve to sit over that lower bolt, with the sleeve having the pivot point threaded into it. I'll let you know how I go.

the homelite has a lip where the cylinder bolts on, you can drill and tap in this lip and install a bell crank and not drill in the crankcase

Thanks Ken, that's another good idea. What intake manifold are you using there, it has the carb in a totally different orientation than I've seen before (I have a 25cc Homelite with a WA-167)?

Two setups for your review...

Whatever setup you use, try to keep the linkage as tight and slop free as possible. Otherwise you will have problems getting a reliable idle, and also if you set it for full throttle and there is slop in the linkage, engine vacuum can pull the butterfly past full open, closing the throttle partly and losing power. (Don't ask me how I know this...)

Some carb/manifold combos can be "turned over" as you mentioned to put the needles to the rear for easy adjusting, and some can't because the carb pulse hole is then blocked. Check yours because if the pulse hole is blocked, the pump in the carb can't pump fuel. If this is the case, you can drill and tap the pump cover on the carb, and route a hose to a fitting drilled and tapped into the crankcase to make the pump work.

It looks like Ken's setup in the picture above has a "twister" manifold. They cost around $15.00 bucks.

AV8TOR

the carb adapter is a stock item on some homelites certtain models has this manifold , I have several of them . or you can order one but the bell crank system works just fine. I have done this on many of my engines without any problems , the pic was ment to show the lip not the carb mount , I couldn't find a photo of one with the bell crank aded but I will keep looking.

May be I'm old school, but I still prefer to keep at least 8-12" between the gasser ignition and the RX circuits. That means EVERYTHING: switches, batts, servos, etc. This rules out mounting throttle servos near the engine. I know plenty of RCers get away with doing it that way, but I'd rather not tempt fate if I don't have to. Unless you have an extremely tail heavy plane, where you need to cram every last component up front, there is no need to.

I prefer a nice and tight direct connection to the carb with golden rod back to the servo. Having slop in the throttle linkage doesn't shield your servo from vibration; all it does is give engine vibration something to act upon. The airframe transmits just as much vibration, if not more, than the linkage does. I would advise against micro servos like the HS-81 for throttle duty. The gears, motor, and potentiometer are that much smaller and more fragile. The weight saving vs. a standard servo is miniscule.

Av8tor: In you 2 photos, it looks like both engines are mounted with the piston up. Is it just a preference or would the engines run just as good inverted? The engine with the pitts looks like the exhaust must flow over the top of the airplane or is that just a illusion? I inverted it with a couple of clicks to see how it would look inverted!!! Capt,n

Hi Av8tor,
Your first pic is very close to how I now envisage my throttle linkage as both the engine and carb orientation are like mine.

I'm attempting to add pics of my engine so you can see my setup. Hopefully you can see them here and they aren't too big.

What I intend to do is extend the lower standoff by adding a metal sleeve. The sleeve will become the mount/pivot for the bellcrank. Then a conventional setup back into the fuselage.

Thanks Ken, will look for the adapter but I'm starting to think I can get away with a traditional bell crank (see my post with pics - they worked!).

Capn,

Usually if the engine is uncowled, I mount the engine upright as in the picture of the Homelite 30cc on my Ultra Lite Stik 120. If the engine is cowled, like the picture of my GSP Katana with the Kioritz 23.6cc, I mount it inverted. The plane was inverted on a work stand when I took that picture, hence the confusion. I've never noticed any operational difference whether they are mounted upright or inverted.

Actually, it also makes me a bit nervous also to mount the servo next to an ignition engine. The ones I'm running with the servo next to the engine are running on "Gas/Glow", and don't have an ignition system. A lot of people do mount the servos next an engine with an ignition system with no problems however...

AV8TOR

Here's a Homelite 30cc on my Super Decathlon. The carb linkage is a straight shot thanks to the "Twister" mount. Couldn't be simpler. I prefer to use large "semi-rigid" plastic pushrods for throttle linkage.