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Old Jan 31, 2007, 11:05 PM
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Build Log
MoJo .60 Build Log

I just started a new build of a MoJo 60, one of those famous slab-sided profile monsters, and thought it might be fun to document it here for anyone who might be interested. The plane spans 55 inches, has 1,000 square inches of wing area and a projected weight of 5.5 to 6.0 pounds. A popular engine choice for this plane is the Saito 1.00, and thatís what Iíve chosen to use. This is my first Saito and Iím very excited to try it out. Itís almost too pretty to use.



Anyway, if anyone sees anything worthy of comment, donít be shy. If you see me make some glaring mistake or omission, please chime in and help steer me in the right direction. Iíve built a few kits over the years, but this plane is pretty cutting edge and has some techniques that I have never used before. It features a 1/2-inch epoxy-glass tube that runs the from engine mount all the way to the tail feathers. This reportedly gives it unsurpassed durability and a much longer than average life span for a down-on-the-deck type of plane. There is also tons of CNC machined parts, and Iím anxious to experience something better than the die-smashing of years gone by. Here is a pic of the contents of the box. I tried to lay it all out neatly, but Iím not so sure I succeeded.



The construction starts with the motor mount. Two identical plywood mounts are laminated together. The manual specifies epoxy or wood glue for this task, and since I always manage to make a mess with epoxy, I broke out my trusty bottle of Elmers white glue. Here is everything ready to proceed.



I bought that darn Woodpecker years ago and have probably only used it half a dozen times, so I decided to dust it off for this plane. I ran it back and forth over the mating sides of the mounts and then smeared a liberal quantity of glue all over both halves.



I then put them together and twisted them back and forth a couple of time to distribute the glue. I then pulled them apart, added a bit more glue and put them back together for the last time. As you can imagine, there was a mess of glue oozing out all over.



But since white glue cleans up so easily, in no time at all, it was looking good.



I then placed the assembly between two pieces of wax paper-lined scrap shelf material and topped it off with a gallon of fuel for a little added pressure.



When dry, it was time to fit the Ďglass tube into the mount. I gathered the mount, some epoxy and a piece of 60-grit sandpaper, which was used to rough up the tube.



After a few swipes with the paper, I cleaned off all the dust and any remaining contaminants with a bit of denatured alcohol. I also took a minute to line one side of the mount with several strips of painterís tape in an attempt to prevent the epoxy from oozing all over the place.




After mixing the epoxy, I slathered it around the tube and inside the slot in the mount and slid the two pieces together. I put a few more pieces of tape around the tube itself to prevent the epoxy from flowing too far. I mixed up a second batch of epoxy and added a little microballoons to it and used it to make a nice fillet between the tube and the mount. After this is dry, Iíll flip it over, remove the tape and do the same on the other side.



While the mount/tube assembly was curing, I turned my attention to the wing. The first step is to assemble the two main spars. These are each made up of two 5/16-inch square balsa sticks, a 12 x 5/16-inch strip of carbon fiber fabric, and a 12-inch strip of 5/16-inch by 1/8-inch basswood spar doubler. The first step is to lay out the portion of the plans that show the spar on your workbench.



Next, measure the spar length on the plans, which is 27.5 inches.



Then cut the 5/16-inch square stock to length and butt-glue them together over the spar layout section.




This pic shows where the basswood doubler goes, but before gluing it in place, I had to glue the CF fabric in place.



I used medium CA for this and it worked very well. I originally tried thin because I thought it would penetrate better. However, I found that it wicked into the CF so fast that it never made it through to the woodÖor at least thatís what I concluded. I laid down a bead of medium CA, placed the CF on top of it, and smoothed it out with my finger wrapped in scrap plastic. It looks kind of ratty in this pic, but I intend to clean it up after the ďsandwichĒ is complete and the CA is fully cured.



I applied a bead of medium CA to the basswood doubler and placed it on top of the CA. I clamped it in place and set it aside to cure overnight. I know CA doesnít take that long to cure, but it was quitting time and spars are a component that we rely on pretty heavily so why not let it really cure?



Thatís all Iíve done so far, but I hope to get back to it very soon. I snapped a quick pic of my building table just to show that itís pretty much maxed out with this plane. The table is 6 feet x 2 feet and is crammed into a corner. For those who say they donít have room to build (I said that for years!), youíd be surprised what you can accomplish if you want something bad enough.



The wing plans are all taped down and covered with wax paper, which is convenient because thatís the next step in the manual.
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Old Jan 31, 2007, 11:38 PM
And You're Not
Timbuktu, Mali (Happy?)
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Just make sure you make the spars fit the plans, not the manual (which is a Mojo 40 manual with a 60 addenda). The bottom fuse stick doesn't fit the plan just right, be sure you put it the right way. And be sure to use epoxy to glue the ply motor mount pieces together...uh, nevermind.
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Old Feb 01, 2007, 08:17 AM
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Thanks Chuck, I'll make sure the spars go in right. As far as the epoxy for the mount, the manual said wood glue was OK, so that's why I used it. Do you really think the difference is significant?
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Old Feb 01, 2007, 06:26 PM
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just finished my 60 .went togather great. whish i would have gottem th 2 pc wing
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Old Feb 01, 2007, 11:36 PM
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3dnut, what engine did you use and what was your final weight? I'm trying to ballpark what mine will weigh.

Thanks,

Erik
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Old Feb 02, 2007, 10:01 AM
And You're Not
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It'll be fine, I imagine that's what is holding the ply itself together, anyway. The more critical joint is gluing the mount/tube into the wing. I'm a firm believer in Polyurethane glue for that.
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Old Feb 02, 2007, 10:32 AM
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Quote:
Originally Posted by Jeffery
It'll be fine, I imagine that's what is holding the ply itself together, anyway. The more critical joint is gluing the mount/tube into the wing. I'm a firm believer in Polyurethane glue for that.
Understood. I won't let you down this time.
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Old Feb 04, 2007, 11:25 PM
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I was finally able to get a little more time in the workshop and made a bit of progress on the wing. The first order of business was to build the center rib assembly. You first lay down one of the two identical center ribs and retrieve the motor mount/stick assembly.



Lay the mount/stick on top of the center rib and align the spar notches, as indicated in the photo.



Also shown in the photo are two 1/2-inch thick half-ribs. One is placed on either side of the stick and glued to the center rib, making sure the spar notches line up. Note that you must not get any glue on the stick or mount. The goal here is to use the mount/stick as tool to get the proper alignment.



After the half-ribs are in place, you glue the second center rib to the top of the half ribs ONLY.



Finally, glue the four leading edge half-ribs to the front half of the enter ribs, again aligning the spar notches as before. The result is shown below.





This is a very strong assembly and is the foundation for the rest of the wing assembly. To continue, fit one of the spars into the center rib and line it up over the plans.



Make sure it is square in all respects. The instructions say it is not necessary to pin the spar to the work bench because the spar notches set the proper taper. However, one of my spars had a pretty significant bow and to ensure the wing went together straight, I did end up pinning down one side of the bottom spar. As a result of this, the center rib also needed to be shimmed because the bowed spar was now pushing it out of alignment. This is not a big deal and is to be expected with balsa. No matter how careful kit cutters are, they cannot control what happens to balsa after itís been cut.

After getting the spar lined up, I pinned and glued the two tip ribs in place.




After that, things started moving really fast. The remaining ribs were added to the bottom spar, followed by the top spar.



After double checking all the ribs for proper placement and alignment, each was lightly glued to the spars with a bit of thin CA. Iíll go back over all the joints later after the wing is off the board.

I took a long sanding block and trued up the training edges of all the ribs. There was very little sanding necessary to achieve a nice straight edge. Here is a long aluminum scrap that I use to check for alignment. It looks close enough for me.



And here is one of the TE pieces glued in place.



Although I forgot to take pictures, the next two steps were very quick and painless. I glued in the 1/4-inch x 1/4-inch sub spars, which are located midway between the leading edge and the main spar. They slipped easily into place without the slightest problem. After that, I cut and glued the shear webs in first three rib bays out from the center section.

The next step is to partially sheet the leading edge. The instructions say to use a piece of paper to carefully measure the necessary dimensions and then transfer this to the supplied 1/16-inch x 4 inch balsa sheets. I wasnít real sure how to do this accurately, so I just kind of stumbled through. I wonít know how well I did until we flip the wing and finish up the sheeting. I first cut a long piece of paper to the approximate size I would need.



I then carefully taped one edge of the paper in place over the center of the sub spar. I wrapped the paper forward around the leading edge and back under the wing, where I secured it in place in several pieces of tape.



You can see in the above pic that I placed a small reference dot at each of the sub spar notches on the other side of the wing. I then removed the paper, lined up a straight edge on the dots and trimmed the paper to shape.



Here is the pattern being used to cut one of the 1/16-inch sheets of balsa to the proper shape. Note that the sheet is not quite large enough for the pattern.



I trimmed the sheet according to the pattern.



And used the scrap portion to make up the difference.



After edge gluing the pieces together and truing up the edges with a long sanding block, I had two pieces of leading edge sheeting ready to be glued into place.



I lined them up on the sub spar and taped them into place. Note that the sheeting starts in the middle of the sub spar. This is to allow room for the cap strips to be added later.





A tiny bit of thin CA was used along the joint between the sub spar and the sheeting. When the wing is removed and turned over, Iíll be able to fully glue the sheeting to both sides of the wing.

The last thing I did was to sheet the center section of the wing. This required edge gluing 4 pieces of 1/16-inch balsa sheet together to form one large sheet big enough to cover the whole center section. I used masking tape to first tape the edges together.



Then I opened up the joints, applied some white glue, flattened that sheets and scraped away the excess glue with a scrap of balsa. The result was a nice big sheet of balsa with nearly invisible joints.



This was then pinned and glued in place on the center section. I used lots of pins and tape because there is a slight compound curve and the balsa didnít want to lay down properly.



Next up is to flip the wing and finish sheeting the leading edge.
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Old Feb 04, 2007, 11:35 PM
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Dang, I'm just at the "build spar" step...
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Old Feb 05, 2007, 08:30 AM
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Dang, I'm just at the "build spar" step...

Yeah, but it's on probably your 10th MoJo!
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Old Feb 11, 2007, 02:38 PM
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wow - good work guys wish i had a shed and workspace like yours - all the gear and equipment
did you cut the parts ? or is it a build yourself kit?
well done so far
love to see it finished and flying
lovely engine too!
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Old Feb 11, 2007, 07:45 PM
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Nice work man. I hope everything works out.

I have a noob question though. How do you fold the LE sheeting so that the balsa doesn't break; Do you use water or something;

Keep it up.
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Old Feb 11, 2007, 10:31 PM
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Quote:
Originally Posted by kipouras
Nice work man. I hope everything works out.

I have a noob question though. How do you fold the LE sheeting so that the balsa doesn't break; Do you use water or something;

Keep it up.
mist water on the outside (non glue surface) and let it set till it's flexible.

Works great.
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Old Feb 12, 2007, 09:29 AM
And You're Not
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He better get cracking or post an update, I have passed him by
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Old Feb 12, 2007, 09:40 AM
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Originally Posted by Jeffery
He better get cracking or post an update, I have passed him by

Easy Killer, I've been cracking, but have been lax in updating my progress. I've got some pics, but they need to be resized and cropped and all that garbage.

The wing and fuse are done. I'll hopefully be starting the ailerons, stab and elevator tonight. That will finish up the framing, which means I REALLY need to decide on a covering scheme. I guess I've put that off long enough. So far, all I know is that the bottom will be white with a big fat PRO BRO on it. It's the top that's got me worried.
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