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C No Ohmen

Build blog for my attempt at the Ohmen

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Chris North 310/06/2019 01:25:32
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243 forum posts
380 photos

Well 3 weeks away working in the UK and Norway has meant that there has been very little progress on the Ohmen. The upside was that since I was passing through the UK I managed to get the recommended 4Max motor and speed controller so the model should have plenty of power once finished.

With a number of household items to catch up after being away I only managed a couple of hours this weekend but I did manage to get the false LE shaped on the right wing panel ready for the top sheeting, false TE edges on both wing panels, scrap balsa installed at all hinge locations, the false TE shaped to match the ribs and the TE top sheeting installed on the left panel. A paper tube was made up and installed in the right panel but I have left it out of the left panel; for now as it is missing the root rib which is installed later.

I see that on the plans both panels have the paper tubes running to the root rib and top sheeting over the center section so I am assuming that this needs to be modified a little to let the servo leads exit the top of the wing. Once I am a little further along with both panels I will see about stopping the paper tube at the 1st rib and making a hole in the top sheeting.

A few photos to follow

Chris North 310/06/2019 01:33:13
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243 forum posts
380 photos

false LE attached to right panel

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False LE, scrap for hinges and paper tube in right hand panel

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False trailing and leading edges attached to left panel

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Trailing edge top sheeting on left hand panel

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Chris North 310/06/2019 01:38:42
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243 forum posts
380 photos

Unfortunately, despite noticing it on the plans at the start, it was not until I had installed the false TE that I realised I did not not leave sufficient overhang to cover the wing tips - I will have to fix this later but hopefully it will not be too much of a problem as there is sufficient LE strip to help secure the wing tip.

I'll put this down to jet-lag!

Well most of this week I'll be working in Bangkok so it will be another slow week on the build.

Peter Miller10/06/2019 08:26:08
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9929 forum posts
1160 photos
10 articles

Looking very nice.

Chris North 317/06/2019 02:51:41
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243 forum posts
380 photos

Managed to grab a couple of hours this weekend so managed to get a progress on the wing panels - although looking at the photos it doesn't really look like it!

Anyway they are coming along and getting closer to top sheeting which will help them look as if progress is being made.

The root rib in the left panel was attached and glied vertical as i could see this as a bit of headache later on given my wing joiner solution.

The lower LE sheeting was lifted and glued to ribs on left hand panel using a combination tap, pins and cyano to tack it in place before all the joins were run with superphatic. While drying shear sheeting was cut for both wing panels and then glued to the spars and bottom sheeting. The sheeting was left just over length and then sanded down level with the top of the upper spar once dry.

The paper tube for the servo leads was installed in the left wing and then both tubes were cut flush with the servo bay and root ribs.

As noted earlier I have changed the dihedral brace design into a combined dihedral brace / wing joiner and so cut this from ply using the same dihedral angle as suggested on the plan / build article. This was then test fitted in both wings resulting in a really nice a snug fit.

Deviating from the suggested build sequence a little, I removed both panels from the board at this point and trimmed the sheeting and spars flush with the root rib. Then, having propped up the wing tip by 20mm I put the sanding block on its side on the table and sanded the root rib until it was vertical. This was repeated with the second panel.

A test fit was then made with the wing brace joiner and the two wing panels together resulting in a nice neat and solid fit. Happy with this, the panels were separated, the joiner glued to the left wing panel and the separate panels pinned back down on the board ready for top sheeting.

Chris North 317/06/2019 02:52:58
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243 forum posts
380 photos

Wing joiner / brace trial fit

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Peter Miller17/06/2019 08:30:07
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9929 forum posts
1160 photos
10 articles

Coming along nicely

Chris North 318/06/2019 01:41:45
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243 forum posts
380 photos

Thanks for the feedback Peter - I know the building is slow but it supposed to be a hobby not a race...

Managed to get half an hour in tonight so cut the 1.5mm top sheeting for the first wing panel. This was cut slightly over length and little over width. Using super-phatic the back edge of the sheet was aligned with the back of the top spar and then the sheet curved along the ribs and secured to the lower sheeting using masking tape to hold everything down. The back of the sheet was weighted using a steel rule and some car tire balance weights and left overnight.

Hopefully I will be able to put the sheeting on the second wing panel this evening before looking at how and where to run the aileron servo cables up through the wing sheeting. I suspect I may cut the paper tube back to the second rib and leave an access hole in the top center sheeting.

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If this blog is showing too many steps then let me know

Peter Miller18/06/2019 08:20:13
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9929 forum posts
1160 photos
10 articles

Hi Chris.

Your work is excellent. and no need to worry about "too many steps". The more information and ideas the better as others always can learn from them and say "Oh, I like that way better"

I agree that cutting the tubes aback a little and a hole in the top centre section sheet is the obvious way to go, in fact the only way to do it.

When I am gluing down the top leading edge sheet I clamp it to the spar with a paper clip in each rib bay. THen I pin it down to the leading edge with map pins every couple of inches. THis just my way of doing it and what ever system works is fine.

Piers Bowlan18/06/2019 15:08:16
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1774 forum posts
42 photos

For the sake of clarity (probably unnecessary) the 'paper clips' Peter is referring to are sometimes referred to as 'fold back clips' (or even Bulldog Clips, which are very similar). Whatever one calls them it is a good tip yes.

Peter Miller18/06/2019 18:12:43
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9929 forum posts
1160 photos
10 articles

Thanks Piers.I should have posted this picture showing exactly what I was talking aboutballerina const 013.jpg

kc18/06/2019 18:53:51
5901 forum posts
168 photos

An interesting wing joiner arrangement -it seems to be glued in one side but what does it go into on the other panel? Will the glued in side be reinforced where it meets the spar webs? If not it would appear the joiner is effectively only the 1/16 balsa webs on that side.

It also appears that instead of a dowel the centre ribs ( presumably both panels ) form a peg which seems a neat way of connecting both LE together. I like that.

Chris North 324/06/2019 01:28:52
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243 forum posts
380 photos

Slow and steady wins the race - Well this weekends activities included: cutting the servo lead tubes back one rib at the root, filling the TE of the first rib bay with scrap balsa, joining some sheet to make a strip wide enough for the top and bottom sheeting, cutting and applying the top and bottom sheeting (remembering to cut a hole for the servo leads), applying rib cap strips top and bottom.

For the cap strips I have not applied any at either of the servo bays yet as I want to figure out how best to mount the servos. The plan shows a simple bottom mount while Peter's build article suggests building the servo into the wing so only the control arm is protruding. While I think this is a neat solution, is does means that I have to make servo access hatches if I ever want to get to them. While I could do this, I think that following the plan will give a simple bottom attachment and will allow access if I ever need to change them.

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Top sheet under pressure..

Chris North 324/06/2019 01:33:59
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243 forum posts
380 photos

With the sheeting fixed I thought it was about time to figure out the wing tips.

First up I transferred the pattern from the plan to some 3mm balsa using baking paper and a pin. While I could have made two identical tips I thought it easier to flip the paper and make a reverse so that I would have the stiffener marking on the top of each tip.

Tip traced onto baking paper, pinned to balsa and outline transferred

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Mark the dots...

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Join the dots....

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repeat.....

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Chris North 324/06/2019 01:43:27
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243 forum posts
380 photos

You will note that I cut the tips oversize with bot the LE and TE areas intact - this is because I had not left enough length of TE and LE on one of the wings to cover the tips!

Anyway with both tips cut out they were sanded to make a matching pair and then the long edge was sanded square.

Now the plan shows a tip profile with the wing tip evenly spaced on the tip rib with equal height gussets top and bottom, From what I could see there was no way of achieving this without bending the tip and so instead I simply marked a center line from center of LE to center of TE and then ran two parallel alignment lines 1.5mm either side. Once happy the tip was held square and tacked with cyano before a bead of superphatic was applied top and bottom.

The same procedure was repeated for the second tip although this time no trimming was required as there was no LE &7 TE !

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Chris North 324/06/2019 01:50:31
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243 forum posts
380 photos

Ok so the wings are starting to look like wings but still plenty to do.

So far I have only applied the false LE and so the top and bottom sheeting needs to be trimmed back and the LE applied to both wing panels.

Similarly, the top and bottom TE sheeting needs to be trimmed back against the false TE before the trailing edge and ailerons are attached.

The ailerons are made from 5/16 sheet (8mm) which I don't have so rather than try to find some I improvised by cutting some blanks from hard 6mm sheet and laminating these with 2.5mm sheet. This will give a slightly oversize block which can then be trimmed down for a nice neat finish.

Waiting for the laminations to dry......

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Lets see what this week brings.....

Peter Miller24/06/2019 08:28:53
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9929 forum posts
1160 photos
10 articles

The hatches are very simple.

1) glue a srip of 1/4" sq. hardwood along each rib flush with the edge of the rib where you want the hatch.

2) cut the hatch from 1/16" ply and glue on two small pieces of 1/4" thick hard wood to suit the servo. I like to add a screw to hold the wood to the ply.

3) cut a slot of the output arm.

The servo is screwed to the block with the arm through the slot. The ply plate is screw to the strips along the ribs.The rib capstrip bring the ribs up tomatch the hatch.

The picture shows the inside view of such a hatch although in this case I had to use a pushrod to the bellcrank.

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Nigel R24/06/2019 09:47:22
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2858 forum posts
469 photos

Build is looking good Chris. Your tip treatment sounds spot on - straight line from LE to TE is always how I've done it (and seen it done).

Hatches are a little extra work over the 'bottom mount' setup, but look much nicer when done.

I find 1/8 liteply makes a good hatch, too. A touch easier to work than birch ply.

Or, if you feel lazy, SLEC make a neat moulded servo hatch arrangement, in several sizes for micro or standard servos.

Edited By Nigel R on 24/06/2019 09:50:23

Chris North 324/06/2019 10:11:17
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243 forum posts
380 photos

Thanks Peter and Nigel for the feedback, its appreciated - it looks like I'll be making hatches again then!

I did use hatches for a neat finish when I built the Batty a few years ago so I can easily do a similar thing here again. I must admit they turned out well and made everything very sleek

Just trying to cut corners embarrassed

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kc24/06/2019 10:53:12
5901 forum posts
168 photos

Peter did not actually mention whether the bearers for the servo hatches are recessd by the thickness of the ply, but it would seem necessary although perhaps the capstrips provide the clearance.

In an article by Sid King he said he uses a method of fixing wing servos by wrapping in 3 turns of masking tape and epoxying the wrapped servo to the ply. If the servo needs removing the masking tape is cut and peeled back. This seems better and cheaper than the SLEC servo mounts for side mounted servos. I have not used the tape method myself so perhaps others can comment.

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