Throttle servo challenge
 

I'm not a beginner but this is a new one for me and I know that many experienced modellers frequent this forum.

Here's the problem. The SC52 four stroke throttle arm is 4cm below the servo arm and 1.5 too far in towards the centre of the fuse.

I'd like to do a good job with the linkage but there isn't an obviously simple, clean and tidy way to do it.

If I drilled a hole in the firewall right behind the throttle arm it would be at or just below the tank tray which can be seen at the left in the fuse picture. The linkage would need to run along below the tank tray and then right up to the servo arm. The angle would need to be extreme and then how would you connect it to the servo arm ?

What I have in mind is to move the throttle servo (maybe replacing with a 9g sub-micro) from where it is (the top one in the photo) and mount it on blocks glued to the forward landing gear mount. I could then have a short straight linkage with no binding. This risk there would seem to be that if the landing gear plate gets ripped out then so does the servo and maybe the RX as well. There doesn't seem to be anything else that I could fix it to in that area though.

And to make things even more interesting, having got the mount on the firewall and drilled for the engine, I discovered that I'd not left enough room to have both a bolt for the mount and one for the carb in the same vicinity. You can see the problem in the third photo. I suspect that everyone would say that four bolts are mandatory so I have the option of either fetching and drilling another mount with the engine further forward this time or maybe drilling another hole a little higher up in the mount and firewall. The latter seems quicker, easier and cheaper.

Any advice about these two issues from more experienced modellers please ?

Thanks

RE: Throttle servo challenge
 

That's easy. Try to make it as straight as possible. I'd leave the servo where it currently is.

http://www3.towerhobbies.com/cgi-bin...&I=LXK055&P=ML

RE: Throttle servo challenge
 

Thanks for the quick reply. The clevice wouldn't be at right angles with the servo arm though. It would be hanging down from it and more than likely binding ?

RE: Throttle servo challenge
 

I just reread your post.

Looks like your engine is scooted all the way back on the mount. Is that the correct distance the manual calls for? If it is move the engine forward on the mount a bit (away from the firewall) just enought to clear the fourth bolt I'm sure the extra few millimeters won't be that big a deal and won't have a negative effect. Also, why are you inverting the engine? How about a sideways install? Would that make for an easier throttle linkage installation? Yes, I'd use all four engine mount bolts.

RE: Throttle servo challenge
 

Which plane is that? I know it's a J3 Cub,,but which one? I have the Great Planes J3 Cub ARF and have my .70FS engine mounted sideways.

RE: Throttle servo challenge
 

While the model seems well built, the manual is poor to say the least. There is no mention of the distance between the back of the prop plate and the firewall so I have had to work it out the best that I can looking at the cowl. I e-mailed the makers (Green Model USA) on Wednesday and had no reply from them. I had considered a sideways mounting but the cylinder head would stick too far out of the cowl for my liking. The other problem is that the tank and foam around it take up the majority of the area behind the firewall so no matter where the throttle arm is, the tank will always be in the way. Any drilling to the sides of the tank would need to be through the reinforcing triangular stock. I've looked at the dubro four stroke extenders but there would still be the vertical distance issue.

RE: Throttle servo challenge
 

It's this one [link]http://www.maxfordusa.com/gm-piper-j3-cub.aspx[/link]

RE: Throttle servo challenge
 

Well, the only way to fit that fourth bolt on the motor mount to the firewall would be to slide the engine forward on the mount. Sounds like a pain by looking on the pics you have already drilled the mount, but that's the only solution I could see moving it forward. Again, it'll only be a few millimeters and I doubt it would hurt anything. I do like your idea of installing the cowling and looking at the spinner backplate, you probably want about 1/3 or 1/4 inch clearance for the prop. I you would like I could measure mine on my Cub.

As far as the cylinder sticking out,,,that's what the full scale plane looks like. :D Might solve your throttle cable/push rod problem. ;)

RE: Throttle servo challenge
 

I see. Good looking plane. Nice price too. Brother, move the engine forward on the mount, get that fourth bolt in there and flip the engine sideways. That's what the Cub should look like anyway, with the head coming out the side of the cowl. It'll be easier on you when installing the glow ignitor unless you go the remote route. It also might be just a tad easier on the engine (sideways) as the whole carb/fuel tank height issues sometimes come up and might be easier getting to the high speed needle. But that's just me.

RE: Throttle servo challenge
 

Okay, okay, hold on. Here's another thought. If you mount the engine sideways you'll need to reverse the mounting of the carb. That would put your highspeed needle on the top (where it shoud be) the glow plug easy to get to and a straighter line between the throttle servo arm and the engine throttle arm??? Maybe?

RE: Throttle servo challenge
 

Two comments:
1. You can redrill the mount for a bolt hole in a more convenient place, and you can cut away the part of the mount that interferes with the carburetor bolt.

2. You can drill your throttle link hole wherever it works, outboard of the mount. Then you can bend the wire around like a paper clip, with the clevis in front (or at an angle, or whatever's necessary. The bends won't hurt anything.

This assumes you can balance the plane with the engine at that location and the cowl fits where you want it.

Good luck,
Dave Olson

RE: Throttle servo challenge
 

I put one of these together for someone who purchased it off e-bay and had no modeling experience. Move the engine all the way out to the end of the mount. Have you tried fitting the cowl with engine where it is now? I think we put a couple of 90 degree bends in the throttle wire and all worked out. Not a bad plane, takes a little trial and error to get everthing to fit. Don't be afraid to make changes, add a tank floor, move a throttle servo or whatever. I build em so they are serviceable later, easy to get tank, battery, receiver in and out etc. Then balance it! Good luck, it will make a nice plane with a little work.

RE: Throttle servo challenge
 

Yes, despite not wanting a cylinder head sticking out the side (yes, like the full scale ones :D), that seems to be the cleanest way to hook up the throttle. The arm sticks out to the outer edge of the firewall so it should be fairly easy to sort out. I've turned round the carb and sat the mount on the firewall and here are some pics. I've not had a four stroke before. Is it practical to have the muffler located like in the pics ? Got to have it tidy one way or another ;)

I'm resigned to the fact that I'll have to get another mount. Another 5mm forward and it should be ok. I can use one firewall hole but I'll need to make three new ones. Can I get away with not buying dowel to fill the other three holes ?

RE: Throttle servo challenge
 

Hmm. Just spotted a flaw with the plan. The end of the carb nipple will be flush with the cowl so the fuel pipe to it would be outside the cowl !! Remind me why I've focussed more on nitro again [:@]

RE: Throttle servo challenge
 

Just redrill the mount closer to the engine mount leg, and then set up the throtle linkage as shown.

Cheers

RE: Throttle servo challenge
 

For the missing bolt, I use the following method. Use an allen head bolt, drill a larger hole in the mount part way so the head of the allen bolt is counter-sunk into the mount.

For the throttle- use the ball link for the throttle arm. I use a piano wire often for throttle. Drill a hole for the wire to exit firewall and miss tank and mount. Have the end go past the carb a ways. Now bend a u shape into the wire bringing the ball end back toward the throttle.

Hope that made sense.

RE: Throttle servo challenge
 

I ran into sort of the same thing as you. I am mounting a Magnum .91 FS in the nose of a Model Tech Formula 3D. It is very narrow and short. To get a straight shot with a push rod, I would have to drill a hole through the center of the fuel tank. That is not really an option so I went with a strange setup but it works. I saw a pic of it some where and just did what I thought I saw. I used a scrap piece of aluminum, nylon screw, some quick links. It may look ugly but works pretty well. It all fits inside the cowl so you never see it any way.


You can see I sanded down the mount ends to clear the cowl.

Dru.

RE: Throttle servo challenge
 

I had a plane with a OS 46FX and I changed it to a OS 52 four stroke. I used the same throttle guide tube but made a new rod. Seems to work fine
http://image2-6.rcuniverse.com/e1/ga.../lg-123515.jpg