KST 325 Servo mounting

[Kenneth Woodring]

Greetings. the KST 325 servo does not come with mounting grommets. I am planning to use a 325 for the Throttle on my Great Planes Big Stik 40 ARF powered by an OS 55AX or Saito 82 4 stroke. I currently have the 55AX mounted on its side with the exhaust going out the bottom. So this requires that I mount a servo low to the "floor" behind the firewall so I can achieve straight linkage to the carb control horn which gives me a minimal "slack" in the throttle control. I have fabricated a servo trey that I can remove if needed (small cap screws and lock nuts through the bottom). I have already laminated 1/64 ply to the bottom of the fuselage.
My question is - do I screw the servo down with no vibration isolation between the trey and servo? I have read on forums of people using pieces of fuel tubing to achieve vibration reduction. I have seen using the plastic triangular pieces, etc. Others have said no isolation is needed. I have tried using traditional grommets, cutting them, etc. UGLY. The pieces of silicone fuel tubing sound practical.
May I please request some input on this topic?
I also have a 14 oz Flat bottom Dubro fuel tank I am installing. I am in the process of converting it to a tail dragger....have moved the main landing gear location, and modified the front wing halves for straight through nylon 1/4x20 (black) hexhead bolts. I am using 8mm aluminum tubing (bolts pass through this) to keep wing secure and not crush it. Bolts go through wing and thread into steel flange nuts held by 1/4" ply. Also reinforced the tail to accept a tail wheel running off the rudder (not a separate servo, but high torque & speed KST).
I had an Ugly Stik 30+ years ago...had a 60 OS on it.... So much fun. The technology now is way beyond those days... Thank you in advance!!
KGw

 

[Konrad]

I don't know how good the silicone tubing is at isolating engine vibration. I use a servo tray that I've mounted the servo on with screws. I then mount this tray with rubber grommets (old Futaba)* and brass eyelets to keep from crushing the grommets to mounts glued on to the airframe This is known as an "Isolation Tray" type mount. This is what I use for AMA 424 racing.

The airframe issue make sure you have a large glue area for the mounting plate to fuselage side. I like to have this plywood plate glued to the front bulkhead. Also make sure that there is filler wood taking up any space between the mounting plate and the wing. You don't want he screws to break the glue joint as they try to bring the mounting plate up to the wing.

Wow 14 oz of fuel. I recall using 12 oz fuel tanks with piped 10cc engines. ( I don't like to trim for fuel loss.)

All the best,
Konrad

* Think Dubro still sells these.

 

[Kenneth Woodring]

I don't know how good the silicone tubing is at isolating engine vibration. I use a servo tray that I've mounted the servo on with screws. I then mount this tray with rubber grommets (old Futaba)* and brass eyelets to keep from crushing the grommets to mounts glued on to the airframe This is known as an "Isolation Tray" type mount. This is what I use for AMA 424 racing.

The airframe issue make sure you have a large glue area for the mounting plate to fuselage side. I like to have this plywood plate glued to the front bulkhead. Also make sure that there is filler wood taking up any space between the mounting plate and the wing. You don't want he screws to break the glue joint as they try to bring the mounting plate up to the wing.

Wow 14 oz of fuel. I recall using 12 oz fuel tanks with piped 10cc engines. ( I don't like to trim for fuel loss.)

All the best,
Konrad

* Think Dubro still sells these.

Konrad: Great point on the fuel tank and trim, it's been so long I did not recall that! I'll give my sizing a second thought. I've been flying electrics to get my skills back..a ways to go there yet. The servo trey mounting idea and point on structure in the fuselage are great as well. I think I had most of those covered..your detail was appreciated. I can't seem to find a pre-mfg trey for the KST 325 except in China on eBay. Do you have a local source? If you are referring to mounting the hand-made trey in this fashion, I think that will work very well too. I have the grommets and brass sleeves.
On the wing front mount, yes understood on the build under the wing. I am actually going to match the fuselage saddle with the screws and nuts being at 90 deg. Again, good bracing as well. I have used rubber bands (in the past) and dowel pins. The stock GP wing rib tabs are not adequate. The black nylon bolts will be best over time. I have built 1/4" ply and bass hard wood into the wing with balsa block to mach the ribs. Thank you for your time and excellent advice.

 

[Konrad]

I make my tray with 1.5mm (1/16) plywood. I used a coping saw to cut out the servo hole. In vibration environments I like to add a bit of flexible glue (Canopy glue) on the top of the screws. I can peal this off should I need to, prior to removal of the screw.

Wow going from electric to slimer! Most go the other way. In today's hobby environment what if anything do you use as a fuel proof paint/ sealant. When I flew glow I lost more airframes to oil rot than crashes!

All the best,
Konrad

 

[Konrad]

Something I try to do with my Slimer servos is to "harden" them against vibration. Most servo's fail the feed back pot. There is little I can do to stop that other than what we have been talking about. The other failure is wire fatigue. To address that problem I often open up my servos and try to support as much of the internal wires as is practical with some electrically safe silicone. The Idea isn't to pot the wires but to offer some support, other than the solder joint, to keep the wires from moving (vibrating) in the case. https://alofthobbies.com/electronic-safe-silicone-adhesive.html

I honestly don't do this for my sport ships, but on my high rpm models I do try to do this. I'm also trying to move over to Hall Effect servos in my racers. I don't have enough experience (time) to tell if they are any better than the pot feed back servos when it comes to vibration tolerance.

All the best,
Konrad

 

[Kenneth Woodring]

I make my tray with 1.5mm (1/16) plywood. I used a coping saw to cut out the servo hole. In vibration environments I like to add a bit of flexible glue (Canopy glue) on the top of the screws. I can peal this off should I need to, prior to removal of the screw.

Wow going from electric to slimmer! Most go the other way. In today's hobby environment what if anything do you use as a fuel proof paint/ sealant. When I flew glow I lost more airframes to oil rot than crashes!

All the best,
Konrad

Konrad:
Great info! I found epoxy paint (Klass Coate Satin clear - have to mix it - pint cans- A&B). First time to use it, but saw it in one of my many searches. I will use the Klass Coate on the inside of the fuselage. I will use Sig Dope on the firewall (for color only) over roughened epoxy rather than Coverite or similar and then Klass Coate over that. . I too found you cannot seal these areas in this environment with plastic iron-ons. I am also clamping all fuel tubing joints and running fill tube and vent tubing to exterior fill points...trying to keep splashes and spills from the interior. I have created a hatch for behind the firewall and to access the throttle servo. If I shrink the fuel tank I may put the Frsky receiver in foam there. I purchased a Frsky Horus X10S transmitter. Dealing with setting it up, etc. Trying to find "FrOS For Dummies" Manual.
Your point about the servo wiring is so cool!! I just re-wired and lighted my utility trailer and did the same thing with the wires coming from the lamps to keep down vibration. I have a Liquid Tape it's called. Super stuff. I may try opening the Throttle servo and see if there is anything I could do. Great suggestion. It may be too small...we'll see.
To be honest the mini-servo is an experiment. I am using the standard high torque high speed servos on the Rudder, Elevator and Ailerons. The KST's run $45-70 per servo. I wanted the direct linkage (and short) to the throttle. Used flexible cable in the past...did not like it. Did not like bending wire control rods to the Throttle either or to anything for that matter. So liking to engineer these planes, I thought I'd try the mini-throttle on the servo. Will be first time as well!
Konrad...thanks so much for sharing. You have a lot of experience and super ideas!!
Regards,
Ken Woodring, Huntsville, AL
BTW - Andrew Jesky (Pattern Champion) flies at our Club field... absolutely amazing what he does with pattern ships and helicopters. So fun to watch.

 

[Konrad]

Having a world class hobbyist/flier is a great asset to a club.