chemie

So I decided to document my Mojo 40 build here in RCU as there are none (that I can find). There is a very good post over at the ProBro site which details this build. I have noticed that many of the Bros hold RCU in great contempt but I enjoy both sites and thought it would be nice to cover the build here too. All comments and suggestions welcome.

This is my first profile build and will be my first attempt at 3D flying. There are only 1-2 folks at my field who 3D so I will likely have to self-teach. I am expecting lots of crashes so I am building this one "on the cheap".

To start, here is my equipment:

1. Mojo 40
2. ST-51 engine. I know this is not the best for 3D but it is what I have. It was in my SSE and I did practise harriers this summer and did not have any throttle issues. It seems to throttle smoothly and never cuts out so I will give it a try. Maybe I will upgrade if this 3D works out.
2. I am trying out some PowerHD servos from [link=http://www.valuehobby.com/products.php?category_id=21]Value Hobbies[/link]. I am trying out the 6001HB. I thought about some MG but they are heavy and I know weight is an issue for good 3D builds. I have my doubts on these chinese servos but I figure I can test out 5 for the price of a single 3010 so it is worth a try. If they are no good, I will put 3010 in the tail and use 3004 from the SSE for the ailerons.
3. Going with 4-40 hardware with Dubro 2-56/4-40 ball links (I might just use 2-56 for the ailerons...will decide later).
4. Will try out my new slot machine with GP nylon hinges.
5. APC 12x4 to start.
6. I also have some Orange Tx on the way from [link=http://www.hobbycity.com/hobbyking/store/uh_viewItem.asp?idProduct=14300&Product_Name=OrangeRx_Futaba_FASST_Compatible_8Ch_ 2.4Ghz_Receiver_%28PREORDER%29]HK[/link] for my Futaba 2.4 system. The $30 (versus $90) is just too hard to pass up. I have ordered 2 to try these out but I will start with a Futaba Rx initially so I can not blame my dumb thumbs on the Rx. These Rx have been getting good reviews and appear to be equal to the Futaba at 1/3 the price (of the 7 channel; about 1/4 price of the 8-channel it is meant to replace). Will let everyone know what I think once they arrive.
7. Dubro [link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXAZF4]diamond lites wheels[/link].
8. Dubro [link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXD850]tail wheel[/link].
9. Throttle cable is GP [link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXK056]flex cable[/link].
10. RadicalRC NiMH battery pack. A 5-cell, "AAA-sized" with 1000 mAh capacity. Surprising light for the capacity.


The kit comes nicely packagaed (including the sports section from a Memphis paper).

chemie

Step 1 was to unpack, inventory, and label all parts. I like to "bag" parts but this one has very little in the way of loose parts (in stark contrast to last winter's GP 300 Extra).

I had an inventory sheet from Paul's site and noticed a few differences:
1. The 1/16" x 1" sheeting was 36" long versus noted 24"
2. The 3/16" sq. was 24" long versus noted 36".

Also, the original instructions had you laminating some 1/4" ply and 1/4" balsa to make 1/2" mount mount and root rib spacers but these came already as 1/2". I guess Paul found a source of 1/2" stock. Anyway, that knocks two steps off the instructions.

chemie

I will use TBII glue almost everywhere (allergic to CA).
I did use 6 min. epoxy to glue in the wing tube to the motor mount. Also, for the main spars, I used 6 min. epoxy to laminate in the carbon sheet and the Basswood strip. (Photos #1-2)

I made the root rib on the plans to ensure the motor mount and wing tube were square and everything lined up while building out the root assembly. (Photo #3-4). To be honest, I am not sure it helps to have the motor mount on there, I mostly lined up the pieces based on the slot holes and edges anyway.

chemie

I put the root rib down and squared up the spar (both ways). I do like the tip about using scrap to align and pin the ribs; I will use this in future builds! Assembled ribs to bottom spar and glued. Since I was using TBII glue, when I came back after ~1 hour, I noticed the bottom spar had sagged and dropped down away from some of the center ribs. I had to cut away the areas and lift it back again. Obviously a down-side to wood glue but I can not use CA much.

Photos #2/3 shows top and bottom spars in place.

Photos #4/5 is after the LE and sub spars in place.

For the TE, I had to use CA as there was no easy way to pin it to those thin ribs.

Next was the shear webs. The instructions say to cut 6 but the blue prints say to install 4 on each side (see Photo #6). However, the details on the drawing only show 3 each side so I went with 3. I wanted the webs to be flush to the one rib so I just put the piece on the building board and marked to cut the top. This leaves the bottom extending over the spars (more as you moved out). I will just cut these when it gets flipped over. This seemed easier than trying to mark and cut the bottom too (see photo #7/8). See photo #9 to see what I mean about extending below the bottom spar.

Photo #10 shows progress to this morning.

chemie

I ran into my first problem today. When measuring for the LE sheeting, I noticed one wing was 1/8" long than the other! It took a while to figure out why. The problem is that the drawing shows all ribs as 1/16" thick. Mine were 3/32". With two pieces used for the root rib, this means the root rib is 1/16" thicker than shown on the drawing. When I spotted the root rib, I put the right had side flush to the right hand side of the line for the rib. I did not notice this meant that the left side was actually 1/16" over. Effectively, I shifted the center line to the left by 1/16". Then, the usual slop added another 1/16" but unfortunately it added to the long side for a total of 1/8" of extra length. I ended up pulling the wing up off the board and re-setting the center line. I then had to cut out the last two ribs from the right side and re-spot and glue. It took a couple of hours to carefully cut the ribs and get them back again. Almost felt like a crash re-build! I spotted with the right hand outboard rib on the drawing location and this left me still with about 1/16" extra on the right side and I figure that is within the normal slop.

Having fixed the errors from yesturday, I proceeded to make two more today. First, when cutting the taper on the LE sheeting, I measured the 5/8" inset but did it on the 36" length. I had forgotten to cut to the required length first. I noticed the mistake on the first piece and could re-cut it since the taper was steeper when doing it correctly. However, this meant I "lost" some of the sheeting for the center sheeting.

The next mistake was attaching the LE sheeting. I glued it to the main spar. Luckily, I noticed this right away and pulled it up and re-glued. In this mistake, TBII helped because I had lots of time to pull it up and re-mount. CA would have been bad with this mistake.

Anyway, I hope I have all the goofs out of me.

Photo #2 shows the center sheeting butt joining. Becuase of the mistake on the sheeting, three pieces were only 10.5" long versus normal 12". Since instructions mention, and measuring confirmed, you need about 12.5" of length, I used some of my shop 3" stock to get me back to the right length. Photo #2 you can see the "short" mistake-piece of sheeting and the extra new piece (whiter color on the bottom).

*JCB*

My Feedback: (9)

save yourself some time and use a couple of washers instead of that angle plastic that you have in the pics for mounting the motor.

also, don't worry too much about the difference in wing length...it's not a precision aerobatic airplane. It will generate most of the lift from the prop rather than the wing...keep us updated!

chemie

I bought the thrust plate because it was $2 so I figured I could decide later. I does seem a little "finicky".

chemie

Photo #1 is with rails in. Photo #2 the center sheeting. I need to use less sand bags; I got a bit of a sag in the middle above the rails. I had to re-wet and weight down from the other side (after flipping over step) to push the sheeting back out.

Photo #3 shows the TE sheeting and the caps installed. For the TE sheeting, instructions say to make 2 from the 36" stock by "measuring carefully". However, I carefully measured the plans and the length is 18 1/8" so you are 1/4" short for two. I butt-joined an extra 1" and made two. I did the same for the other side. I was then staring at 2 extra pieces of 1" wide sheeting left over. I read ahead and do not see any use for these. I believe Paul now sends 4 pieces of TE sheeting and the instructions are no longer correct. One piece per side.

Photo #4 shows the overhang on the center sheeting. I just cut it when the wing was flipped over.

Photo #5 shows some scrap added as backer for the hinges. Since this is my first time using the GP slotting tool, I was worried I might go too depth and breakthrough the TE which would result in messy glue inside the wings. I added 1/4" scrap at each planned hinge location. I will go overboard and put 5 in; one in each wing bay.

After flipping the wing over, I moved up step 22 and added the sub spars first so they could dry before doing the sheeting. I was surprised how easily the LE sheeting wrapped around and held down despite just using wood glue to hold. Over course, lots of pins though.

The instructions say to cut some 1" triangle wedges but does not say from what stock material. There was a block of 1/4" but I did not bother to read ahead to see if this was used somewhere else. I just used some extra 1/4" stock I had left over from another build and made them. In looking at the photo, I now see I messed up the grain. The instructions clearly say "running WITH the long side" but I had in my mind 90 degress to the hypotenus. Since they are installed flush to the top and bond to the sheeting, hopefully this won't be too weak now. See Photo #6.

I added a short tube for the 2.4 antenna. I want the two antenna to be 90 separated and in different wing bays so this should work nicely. Fuel tubing works great for the antenna(photo #8)

Work on the wing continues today.

chemie

I forgot to mention, for support on the TE, I used some of the cut pieces for spar material 1/4" sq. and made a 1" x 1/4" "board" for the TE. Photo #1.

For the hatch opening, instructions say to use 1/4" wide sheeting on the sides. However, the rib caps require 100% of the supplied 1/4" cap material. I suppose you are required to cut some from the 4" sheeting scraps. Instead, I used some 3/32" sheeting left over from another build. This will provide a bit more strength/support. (photo #2)

Attached the triangle stock. (photo #3)

For the hatch covers, again you are required to use 1/4" sheeting for reinforcement. Instead, I used some balsa stick stock 1/4" x 1/8". Maybe over-engineered but I thought this would give some more strength and saved cutting the strips. (photo #4)

Photo #5 shows the hatches in place. I will need to sand down the edges to account for covering thickness but for now it looks nice and flush. I need to look around for some small screws; not sure what I have inthe shop. I am a little worried about the amount (or lack of amount) of 1/16" balsa for the screw. You have just 1/8" of basswood to screw into which leaves 1/16" on the top of the balsa sheet used on the hatch. Suggestions?

I just have to put the rib caps on and the wing is about done except for sanding.

chemie

I come from the max. strength approach vs min. wieght, so for the ailerons, I was very tempted to add diagonal cross-braces for each bay. However, since this is a well proven design, I talked myself out of it. The ailerons just seem to be very flimsy, especially on the TE around the 1/4" stock. I could not resist adding some 1/4" triangle stock in the TE joints just to make my "build to strength" side happy. Not sure it helps much though. Also added on a triangle on the inboard TE.

Also, I cut half a bay off the outboard side. Supposedly, this is to stop from catching the aileron on the grass but I suspect anytime my flying takes me that close, it will be a much closer encountered at a much higher speed such that it would not matter that a aileron "might catch"....

For the right aileron, I turned the plans over, wet the paper, covered with wax paper, and built. I am not sure why the plans could not have this drawn in as there is plenty of room on the RHS too. I taper with 80 grit sandpaper.

Photos for aileron build attached.

chemie

Fuse went easily. Photo #1 shows final. I have taped a scrap piece on the front to keep the top and bottom aligned and not stress the rudder post on the back. You can just see the landing gear ply. I used two 2-56 blind nuts and gorilla glued the ply into the balsa.

For the first curved piece on the bottom, I cut a scarf cut instead of the butt on the plans. I also used the left over carbon tape and left over 1/8" basswood to splice over the joint from the inside and glued with epoxy (photo #2). Photo #3 shows some additional 1/8" basswood used on some other joints.

Photo #4 shows how I will carry the plane. I will hold with thumb and index finger over the built up area (built using extra 3/8" stock).

Photo #5 shows where I pinned the front joint on the top portion. Gorilla glue used here.

Rudder is Photo #6. I added a cross-brace on the bottom section. I have not decided if I should cross-brace the rest of the bays too.

chemie

So as the framing part of the building comes to a close, a few notes on the supplied wood:

* Balsa was top quality and very well cut.
* As noted above, the instructions say to use one 36" length of the 1/16"x1" material for two TE sheetings. I now have 2 left over. You should use one sheet for each one if you get 4 in the kit like I did.
* The instructions say to cut the two triangle pieces for the root ribs. I used my own stock but now see I have an extra two 1/4" triangle in the bag. These are supplied and the instructions were wrong (I did not look too hard since it said to cut your own).
* I had a small amount of 1/16"x4" sheeting left over, partly because I had to use some 3" from my stock due to the cutting error. You will need to be careful here but should be OK.
* I had one full length of extra 1/4" triangle stick left over.
* The 1/4" cap material is fully used (less than 1" left) but layout is straight forward so just be careful.
* The 1/4"x3/8" stock must be cut carefully as I only had < 1" left. When working on the last 18" of each of the three sticks, make sure you lay it out so that you are only left with 1/2" and not 2" on the last cut.
* I ran out of 1/4" sq. stock. Partly because I used some for the rudder cross-brace and also for the hinge doublers on the TE of the wing. Also, one section was just too light and flimsy to use on the rudder/elevator. I used about one extra 36" long stick here.
* I used my own stock of 1/8"x1/4" stock for behind the 1/4" basswood on the servo rails. Also used it on the wing bay doors.
* For the machined slots (fuse, rudder, elevator), I found they were not square. I sanded the corners of the 1/4" sq stock to get a tight fit but you could also use a file to square up the slots if you wanted (my smallest file was a little too big for the slots).
* You have a small amount of extra 1/8"x1/4" basswood (which I used to reinforce the fuse). You have a small amount of 1/4" sq. basswood left too.

So I would say if you stock build, and are careful, supplied material works well. If you make ANY changes, be prepared with your own stock material. Specifically, some 1/8"x1/4" and 1/4" sq. stock.

chemie

Elevator and horz. stab now done. I did the half CB but same comments as the aileron applies....

Opened up the wing front and tube exit hole. It took a lot of sanding/filing to fine tune the motor mount to set into the wing.

flygilmore

If you want the most out of your Mojo.....get some decent servos!!!!!!!!! The 3010's or DS821's are what most use including myself on both my Mojos and Primo. I have built too many Mojos to count....they are awesome planes! Have fun with it!

BTW, next you need to build the Primo....much more fun that any Mojo I have had

FentonFlyer

My Feedback: (11)

Next time you build ailerons, just build two lefts. When you're done, flip one over and voila, its now a right!!!

Build is lookin good. I gotta get me one of these for the stash.

chemie

The problem with building two left is that the taper you need to sand would be going opposite ways. I think the taper should be the same side on LHS and RHS. It would either look funny or cause flight issues by having the taper on top on one side and bottom on the other because after tapering, they are not symetric (flat on the bottom and taper on top).

In temrs of servos, these have more torque than 3010. I hooked them up today and there was no jitter...time will tell. Like I said, it is a test.

chemie

I hinged everything using GP nylon hinges and GP hinge tool. First time with the tool but seemed to go fairly smoothly. I think I am ready for covering now.

FentonFlyer

My Feedback: (11)

I just assumed the cross section of the aileron was square on a Mojo. If you "rough frame" two lefts, you can sand them any way you wish once you pull them from the board. I scratch built alerons for my Somethin Extra this way without issue.

chemie

Fenton. That is not the case with the Mojo. The LE and cross pieces are 3/8" and TE is 1/4". You then sand the 3/8" cross pieces down to the 1/4". If you did "two lefts", the 1/4" would be on the "other side" of the 3/8" pieces.

FentonFlyer

My Feedback: (11)

Okay, I'm pickin up what you're puttin down now.
Build is looking good, BTW.

rlipsett

you are getting near the evil part now. I wished I had built it with the removeable wings just so you could build the center section and fuselage as one piece instead of trying to get it all glued together at the same time along with the tube down the center of the aircraft

flygilmore

Go to the profile site (you know where I am talking about) and look there.....super easy way to get it all glued together in the end. Not stressful or hard by any means as there is a new way to put it all together.

chemie

already have the tube in the motor mount...so I guess I follow the original.

chemie

Started to cover. Pics below of are horizontal stab and elevator. I also covered the triangle stock and mounted so everything is ready to attached to the fuse. The photos of the stab made the covering look almost black so I took one with flash and lights to show the color. It is MK medium purple.

The elevator is MK metallic copper. I just thought it was a cool color and wanted to try a scheme with it.

I am still trying to figure out my scheme for the wing top accounting for the hatch doors. As I post, you will see I suck at covering and therefore keep the scheme VERY simple.

chemie

I forgot to mention: I added a trim iron to my last TH order almost on a whim but I must say it is a great tool. This is especially handy given all the 1/4" stock used on the control surfaces. I am very pleased with the addition.

Here is rudder and ailerons. I am pissed about the left aileron. I must have used a "lighter" stick of 1/4" on the TE and the covering actually pulled the TE back in between the ribs. It is only a 3" span in length, and it only happened on one but you can see the warp on three bays and it is about 1/8"+ in the middle of the span. Any suggestions on how to fix?