Hello,

I'm 95% finished with my extra 300S build and am having difficulty with the CG. I built it per the plans and later decided on which engine to use. I went with the biggest motor, OS FS-120S III which is closest to the firewall as it can get without significant modification. The battery is located by the trailing edge of the wing. I need to put 3.25oz's of lead on the tail to balance the CG which to seems like a lot of added weight. Not sure what else I can do. Any ideas. This is with an empty tank.

Welcome to RCU. What kit is it? At 95% complete it's a bit late but yes, 3.5 ounces in the tail is a bit much. If you haven't started to cover yet you can move things around or make changes like installing two elevators and the rudder servo in the aft section of the fuse. Most the time they tend to come out tail heavy. During the build I like to tape all the hardware on the outside of the plane to get an idea of where the gear will need to be to get close to the CG before I hard mount anything. I have a hatch in the bottom of my Extra just behind the canopy where I have my three tail feather servos mounted then mounted all the rest of the gear inside the fuse center section to attain the CG. It's an old Goldberg kit. A heavy tail wheel assembly helps a little but at this stage of the build you may just have to add the lead?

It's a Great Planes Extra 300S kit and is fully covered now. I usually go with the recommended engine but decided to go with the bigger engine this time but didn't realize that the plane would be this nose heavy. The recommended engines for the kit .61 to .91 2 stroke and .91 to 1.20 4 stroke. I'm using the OS FS-120S III 4 stroke. All of the servos are mounted per the build plans. I could move the servos towards the tail but would only gain about an inch or two further from the engine. I don't know if its worth it. I'll take some pic's to provide more detail. I've built planes for many years but had a brain fart on this one. Thanks for the feedback.

Pictures of servos and tail weight needed. Will add engine pic's.

Probably not worth it leaving them above the wing, but if you move them all the way back to the tail, Like the giant scale planes are designed, you'd gain a lot of wiggle room.

good luck

I would start off by moving the rudder servo to the tail. The look to the nose to see what weight can be removed. Any excessive length screws? What spinner and prop are being used? Is the ABS plastic cowl being used and would a fiberglass unit be lighter? Is there a way to make a hatch in the aft fuse to mount the battery farther back? If you post a detailed picture of the engine install with and without the cowl we may have more ideas to lighten the nose.

This old plane was built in 89 but I still do the same thing with my planes today. It's built around a YS 1.20 and was designed for the OS .60 two stroke. It was going to be nose heavy so three servos were located aft but it was all done in the bones. All the gear was located to achieve the CG then hard mounted before covering. Even with the covering done you may still be able to mount the rudder and elevator servos as far back as they will fit. Two elevator servos on the outside around the stab and the rudder servo inside as far back as it will go inside a hatch. The only thing I have in the center section of the fuse is the throttle servo, fuel tank and a battery. The switch is mounted behind the canopy too.
I know it's a pain to do now but that is a lot of lead. Then again I have about a pound of the stuff on the nose of my Swoose but when the plane was designed the guy had to use that much to get the CG on the original too, just a lot of fuse and tail feathers behind the CG. You do what you gota do.

There's not enough structure near the tail to move the servo's to, I would if the structure would support it, I would have to rebuild the tail section to have a solid balsa wood structure. The fuse in not solid balsa wood.
I moved the engine back once, the fuel pump is hitting the motor mount, I could gain about 1/4" if I were to shave the motor mount to allow the fuel pump to fit closer to the firewall.

I see where you guys are going with finding all the weight in the nose, the cowling, prop and spinner weigh 10 5/8 oz. I have pretty beefy screws and nuts to secure the engine to the mount. The prop is a K-series Master Airscrew 15 x 8, can use 15 x 8 to 16 x 6 props with this engine. How do you go about getting a lighter cowling?

I think relocating the battery towards the tail shouldn't be that difficult.

The first pic is the fuel pump/motor mount issue.

To get my OS 120 pumper closer to the fire wall I had to buy the aluminum OS motor mount but at this point that would do almost nothing. There isn't a lot of difference you can make in the nose. Looks like it is what it is.The spinner is the lightest one you can find, mater of fact if I spotted you with that type of spinner I wouldn't pit next to you, I have seen too many of those types shatter so I avoid them. Mentioning props on RCU is opening a can of worms but on my OS 120 in my extra it tested best with the APC 15X8 but mine is a lot heavier plane then the GP Extra. Testing different brands and sizes is really the only way to find the prop you like the best.
Looks like your best bet is to just stick on the lead and forget about it. On your next build you may want to log onto RCU and find an old build thread to see what others have done to there planes and what problems they encountered like the CG. There are a lot of threads on the GP Extra.

Yea, I didn't think it through when I was building it. I knew it would be nose heavy but not this bad. I could always put the recommended engine in it and fly before I do any serious mods.

I've flown a long time and never had this type of nose cone break, I do thorough inspections in between flights though I did have an aileron come loose during one flight. That was loads of fun and I managed to land it.

Thanks for the input.

Can't see where or how you have the tank. Have you considered cutting the "motor box" to shorten the nose?
Also, maybe get rid of the universal mount and get a standard beam mount, if you can find some, that would let you move teh engine back a little more.
http://www.gravesrc.com/sigem001-sig...d-eng-mnt.html
http://www.gravesrc.com/han2033-hang...cap-12-15.html

Yes, you'd have to remount and/or shorten the cowl.
Just a couple of ways I would tackle that problem in order to use that engine.
I would rather lose 1/4" or so of the nose instead of adding 3-1/2 ounces of lead.

Ok here is what I see. You can replace the mount with one better suited for the 120. Being a single peice mount it will be a tad lighter. When mounting the engine, drill and tap the mount that would save the large watchers and nuts. 6-32 screws for both the mount and engine are plenty. Try a Vess or Xoar wood 15X8 and the Dubro plastic spinners with aluminum backplate are about the lightest you can get but hold together. The biggest weight loss would be the cowl. Contact Fiberglass specialties and see what a fiberglass peice would weigh. You may be able to drop a few oz there. Being a pumped engine is the fuel tank as far back as you can get it? And lastly, this has little to do with weight but do yourself a favor and get rid of the fuel valve. Sooner or later it's going to be an air leak valve.

Yes, the moving servos would take some work.

I think cutting in like GB has done, then making a hatch for battery access would be very easy, or just patch the covering once you like the battery placement. it would be very unnoticeable on the bottom of the fuse.

good luck

p.s.
I too have had those spinners blow up on big 4 strokes myself, it happens when the prop slips on an engine backfire,, be cautious.

Hi,

Post #4 shows the fuel tank wrapped in foam. It tucks into the motor box.
Are the kit contained nylon bolts OK to use?
Not my fuel valve....... I could at least move it back.
I ordered a new motor mount mentioned by RICKSTUBBZ.
Tap the engine holes directly into the basswood/plywood ?
Should/Can I remove more than a 1/4" from the motor box? I have plenty of lead way on the cowl.

If you can moving the tank closer to the CG will help.Nylon screws for the wing hold down are fine.The O rings in those valves wear out very quickly then it will suck air, cause your engine to go lean. Not a matter of if it's a matter of when.Secure the mount to the firewall with 6-32 screws and blind nuts. Drill the mount and tap 6-32 to secure engine to mount. Tap only about 2/3 through the mount and use a 6-32 screw long enough to just protrude out the back side. This will keep the screw from backing out.Moving the engine back 1/4" will help but 1/2" would be better. I think there is a slightly smaller/lighter muffler available for the engine. if noise is not an issue being a pumped engine the muffler is not required. I realize this is a lot of work but getting the CG right will turn a pig into a good flying airplane. Well worth the effort.

This is an OS mount for the OS 1.20 pumper, it gets the engine back as far as it will go, at this point moving it back doesn't do much for weight transfer. As for the fuel filler, it doesn't mater, the OS pumper has an open vent system anyway, a little more isn't going to hurt it. I did a search and found an old build thread posted in 2011 by Larger Scale88. Two of my old friends were doing the same build so they are both doing the thread. On page 6 it is mentioned that the distance from the fire wall to the thrust washer is 6.25 inches. From page 6 on, the thread is worth reading and there are a lot of photos. Several of them on how they mounted the servos aft and I think Jim ended up with his battery way back there too. Engines are mentioned a lot too but what I found was no one seemed to have a CG problem.
Go ahead and type in Great Planes Extra 300 in the search engine and read the thread from about page 6 on, it should be of help. I don't know how to post the URL or bring up old threads on RCU but it is several pages back on the search when it comes up.
I think it will be a big help to you.

I found the thread and will be reading through it shortly. Right now I'm in the process of moving the battery pack. The bottom fuse of the 300 begins to narrow significantly off the trailing edge of the main wing so I can't go back too far. Building a battery box and support for a hatch. Will be moving the engine back about when the new motor mount arrives about .5" to .75". Possible an inch back, will know more once the new mount arrives. I'm now using a solid aluminum nose cone that I forgot I had purchased, no comment.

Look at the way Tom mounted his elevator servos, they show up well after he covered the plane, he offset them so they fit. That way the rudder servo can be mounted aft too using a pull pull system.

I relocated the battery and need to do some modifications to the new motor mount. I'm sure if I remove some of the firewall I'll be able to move the engine back an inch. After the I've completed both the battery and new motor mount I plan on checking CG.

Great news, she's balanced. I only needed to move the battery back and add 0.5oz of weight on the tail. I can tilt her a little forward and backwards on the balancer and she holds the position. Glad I checked before changing out the motor mount.
Thanks again for all of your feedback!

Glad it worked out, Orange Extras rock!! The Pitts does too but not the picture I wanted LOL

Awesome 300!