C No Ohmen

[Chris North 3]

A couple of house and country moves has meant that it has been a while since I have manged to get the building board out but at last there is the chance to get it back in action.

I liked the look of Peter Miller's “The Ohmen” since seeing it in RCM&E and so decided that I would give it a go. I don't plan on making too many changes to it although I would like to have removable wings for transport as I am currently without a car and thus relying on taxi's and public transport. This means I may also have to think about changing the undercarriage as the plan has these fixed into the wing.

If I change the single wing locating dowel in the leading edge into a tab on each root rib, then I can create a slot in F3 to hold these tight. Replacing the single single wing retaining bolt at the trailing edge with a bolt in each wing will mean both wings should be securely fastened.

I know that this is ab aerobatic model buy to be honest my flying skills are such that it will only ever be doing the odd loop, roll and spin; most of which will be intentional!

This will be a scratch build as I can't be fussed having the short kit sent over from the UK. Unfortunately I still will only be able to manage a couple of hours a week and so, if the Batty build is anything to go by, this could take a few months. Still, the fun is in the building so lets see how this goes.

[Trevor Crook]

Chris, I have one of these and the u/c is fuselage mounted with a torsion bar arrangement. It is held in place with 4 screws.

Are you sure you need to make the wing 2-piece? It's not a very large model.

I am a very average builder but had no trouble building mine from scratch, no pre-cut parts used. It's a great flier.

[Peter Miller]

Hi Chris, Welcome to the OHmen. My current favourite aerobatic model.

I would suggest that the wing is small enough to carry in one piece, Heck! the thing goes in my tiny Aixam microcar in one piece and way back I used to take big one piece control line stunters (much bigger than Ohmen) all across England by train and taxi and bus. THat was in my RAF days.

Removing the wing as it is will not be too much longer than the fuselage.

I am puzzled that you say the undercarriage is fixed in the wing. No, itis fuselage mounted as has been said above

I am sure that your flying skills will improve rapidly with this model, it just asks to be thrown around and is most forgiving.

 I definately reccomend a solid sheet tailplane from medium to soft sheet.

Have fun

 

 

Edited By Peter Miller on 29/04/2019 08:25:53

Edited By Peter Miller on 29/04/2019 08:28:36

Edited By Peter Miller on 29/04/2019 08:30:39

[Levanter]

Hi Chris

Where are you now based?

I shall be looking out for my next Peter Miller build but have to finish Oodalally and Grumpy Tigercub first.

the Ohmen has great reviews.

Levanter

[Chris North 3]

Hi Trevor - Thanks for the feedback - I was impressed when I saw your model in one of the other threads and its amazing how different a model can look in a different colour scheme. It seems that a number of people have been on the look out for your pilot!

Peter - Thanks for the encouragement. my mistake regarding the undercarriage - I see that it is fuselage mounted so I will see once I get going whether i need to make these removable or not. Plenty of time to figure this out as I plan to start with the wing. As for the tail feathers - if building them from sheet would you recommend just cutting from a single sheet or building in some cross grain to prevent warping?

Levanter - I am now based in Singapore. There is much more opportunity to get building supplies here than there was in Thailand but it is still a case of finding the right places.

More cutting out tonight!

[Peter Miller]

Hi Chris

You can just cut the tailplane from ordinary sheet. I do this on all my designs normally with no problems.

I actually had to add a little lead in the tail so obvipously use that weight more profitably.

I actually broke the tailplane later and had to add struts to support it.

I expect you know that there is a build blog for my Ohmen on this forum

[Nigel R]

Pete, do you bother having a 1" strip of fore/aft grain at the tips, or all of it from one sheet?

[Peter Miller]

Nope! On large surfaces from thinner wood I have done. I do on my Little Ship but they are only 1/8" sheet

However you can if you feel happier dling it that way....I am just lazy and it works!!

[Chris North 3]

I have had a couple of copies of the plan made at the local print shop - one to work from so I don't damage the original and a second one to chop up. First task then was to cut up the fuselage sheet so that i could make a full plan and elevation.

With that done the rest of the sheet was sliced and diced so get a set of fuselage formers and well as wing rib templates. The wing rib template will be transferred to ply so that i can make the ribs using the sandwich method.

Last thing for the evening was to stick the templates to suitable wood stock using a glue stick, taking note of the grain direction. Once dry these can then be cut out.

The only thing i need to be careful of is that my wood sheet is metric while the plan is imperial and so there will be a few minor thickness differences along the way - no major deal for the formers other than the fact the cut outs such as for the side sheeting, need to be adjusted to match the metric sheet to make sure everything fits properly.

I have a funny feeling this may not be the last time I mention this particular issue!

The tail, rudder and fin will all be made from solid sheet as suggested by Peter - I just don't have a sheet wide enough in my left overs so will need to head to the LMS at the weekend.

[Trevor Crook]

Exactly what I did Chris, except I didn't use the "sandwich" method for the ribs, just used the ply template and a scalpel for each one. Personal preference, I tell myself it's less wasteful on sheet balsa!

[Peter Miller]

I agree on using a ply template and scalpel for the wing ribs.. IT doesn't take long and you have the template if you ever need any spare ribs.

I certainly would not worry too much about the slight variations between imperial and metric wood.. I recently was measuring some of my wood stocks with a micrometer and it does vary a bit anyway.. I suppose there will be some noticeable difference between 1/4" sq and 6 mm sq. but generally it won't matter on sheet.

[Nigel R]

"my wood sheet is metric while the plan is imperial"

Welcome to wood building!

You're overestimating how accurately balsa sheets are cut - they're rarely the same thickness from one side to the other. It also absorbs water like a sponge and will change size quite dramatically, warp and bend when you cut it, or, sometimes just because it feels like it a few hours after various internal tensions are released.

Recently I cut a sheet of 1/2" for some ailerons. One stayed perfectly straight. The other instantly bent by about 1/8" in the centre, in two directions. Provided you expect this sort of thing, it is no problem, just part of working with natural materials - in my case I cut a bit oversize (width-wise, and the sheet was slightly over-thick for the aileron) and was able to sand back to straight.

Make stuff fit as you go, this is always the way with wood of any kind.

[Charles Scott-Knox-Gore]

Having built a beautiful tailplane as per the plan, I was rather dismayed to see Peter Miller's comment about making it from solid sheet. Can I ask Peter, how you broke your tailplane, as I should have thought the made up one would be strong enough? Should I abandon the one I have built and go for solid at this stage? I am inclined just to carry on, but would appreciate your further comment.

[Peter Miller]

Hi Charles.

Don't worry. Mine is still flying happily with the built up tailplane and as I have said, it only flies straight and level when inverted, the rest of the time it is being tied into knots.

I had had a bit of a thump with mine. I simply added struts under the tail and it was fine.

I added the comment about solid sheet simply because it is simpler and allows for any thumps.

[Charles Scott-Knox-Gore]

Thanks for that Peter, I am much relieved. I have greatly enjoyed building this model, my first from a plan.I am now about to tackle the covering, with some trepidation. Will let you know how I get on.

[Peter Miller]

I willlook forward toseeing your model.

One of our club members is building an Ohmen at 130%, about 60"

[Chris North 3]

Well I have made a start cutting out the various formers and parts. Not a lot to say here really as it has just been a case of cutting out around the plan and then using the duragrit block to make sure the edges are straight and square.

I have found in the past that the duragrit block does not have grit all the way to the edge which means that if the sanding block and the part being sanded are on the same level, there can be a small section at the bottom that is not sanded. Therefore I find it works best to have the duragrit block on the bench and the component on the cutting mat. This way the who edge is sanded.

[Chris North 3]

The hatch formers F3A require 4 pieces to be made so, in readiness for the wings ribs, I thought I would try Peter's suggestion of using the one blank to cut the rest.

Since this items is balsa, I cut one item using the plan and then cut another 3 blanks of approximate shape, secured them together using two pins and then sanded to shape using the sanding block. Due to the length of the pins I could only stick three together at a time and so completed this in two batches

[Chris North 3]

As noted before, some of the fuselage formers will need to be adjusted to account for the metric sheet being used so for now these have all been cut out to full size and put to one side while I sort out the wings.

So instead of the sandwich method I followed Peter's advice and made a rib blank from 3mm ply then ran the box cutter around this a few times to create a set of 20 ribs plus 2 partial ribs.

Step one cut rib template and pin to balsa sheet (note its pinned down backwards to help minimise wastage.

Then cut around

Creating a pile of blanks

With this done the spar slots were cut into the ply template. The cut blanks were then pinned together in groups of 4 or 5 using long T-pins and then sanded to shape. Once all of these were done the spar slots were cut from the ply template and the slots transferred to the ribs using a 6mm duragrit spar slotter - when I bought this I was not sure if I would get much use from it but I have to say it have proved very useful.

[Chris North 3]

At this point I realised that I only had 2 lengths of 6mm Sq stock while I needed 3 per wing section. While I have not had a lot of success stripping thick sheet stock I thought this was a better option of paying through the nose for someone else to do it for me.

The problem in the past has always been trying to cut too deep with the stripper and ending up being forced off track by the balsa grain. So the solution seemed to be to make sure the cut was performed in shallow cuts.

So first up was to set the blade cutting width. While this can be by ruler, I simply too a tpece of 6mm stock as a guide and closed the blade until it was just gripping the stock. With that set I tool a scrap piece of 3.5mm sheet and placed it under the cutter thus reducing the cutting depth. After making a very easy pass, I reduced the scrap sheet under the cuter to 2mm and made a second pass. Finally I removed the scrap sheet and made a final pass to full depth. The result - straight cut 6mm square stock with not cutting difficulties.

[Chris North 3]

First pass with 3mm scrap

2nd pass with reduced thickness scrap

Final pass

and the end result...

[Chris North 3]

With the spars cut I then went on to strip the T.E sheeting ready to to set-up the first wing panel.

[Piers Bowlan]

Great stuff Chris.

Personally I incorporate the spar slots into the ply blank and use a Swan Morton knife with a no. 11 blade to cut them out before I remove the blank from the piece of balsa. I find it doesn't really take any longer to produce a set of ribs this way and avoids the extra process of sanding the spar slots, but each to their own. I also make the blank from 1mm or even 1/32 (.4mm) ply, which I find adequate and easier than a thicker piece of ply to cut around.

Edited By Piers Bowlan on 06/05/2019 06:51:51

[Former Member]

[This posting has been removed]

[Peter Miller]

You certainly produce parts to a far finer tolerance than I do!!!

I like the way you have found for stripping hard thick sheet. I could have used that a few days ago when trying to strip 3/16" sq.from 3/16"sheet.