The Ohmen

[Martin Harris - Moderator]

Posted by Peter Miller on 12/07/2020 10:49:11:

I rely on George at 4-Max to tell me what power combination I need for any given model.

Even my larger heavier models use the same power train and work well. I simply give George the specifications and he says use X.

Yours does seem considerably lighter than the average build coming out of our exercise if the figure of 2 lbs 10 oz on the 4Max site is correct - although I've only weighed my 85% version, possibly incorrectly remembered anecdotal reports seem to be coming out only a little below 3 lbs but I know of another on a 4S which is supposed to weigh 3.3lbs ready to fly - which it seems to accomplish very well...

I may have to take my digital spring balance to the field...

Edited By Martin Harris on 12/07/2020 17:08:47

[Chris Walby]

I flew Sue's again this morning on 3S and its running a 11x5 prop (28A 300W WOT) and IMHO it flies perfectly well.

Later on I watched Dave fly his (4S ohmen) and it was markedly faster (like a scolded cat) and hurtled around taking up large amounts of sky. It didn't look like it was any better/worse at aero's, just got there a lot faster (okay if you have the skills to handle a ballistic model and assuming it does not fail with the additional stress induced).

If you want manic 4S

If you want non manic 3S

I understand that the 4S guys are running 4S3300 which is very nearly double the weight of a 3S2200 which will add to the AUW.

[Peter Miller]

I had mine up this afternoon. I was doing square loops and my party piece, sqquare loops with a half roll on each leg and my other one, clover leeafs. (four loops with a 1/4 roll between each loop but you really need no wind to do that one!!)

JUst my 2200Mah 3S batteries.

There was no real wind. In fact most of the time the wind sock was hanging vertically.

Mick, another club member is flying a 130% Ohmen and he only uses four cells.Don't know what motor he is using

[Martin Harris - Moderator]

Some further information has come to light. The motor he's fitted, a Turnigy 3542/6 1000kV, while appearing close in specification to the 4Max recommendation, according to Motocalc only produces 266W as opposed to 305W for the combination you're using.

It appears that a small change of prop might give some decent results - e.g. a 10 x 6 is predicted to produce 350W, well within the power train capabilities and although it would far short of the theoretical 555W it flew so well on with 4S, the weight will be reduced significantly. Another factor might be that the 10 x 5 prop is an APC lookalike and not the genuine article.

There remains the slight anomaly that the C of G was possibly a little on the edge for the owner with the battery as far forward as possible but we may be able to work with that when he finally gets his hands on the sticks. Maybe the tailwheel he fitted might have to come off in favour of a lighter skid.

Perhaps some of this info will be of benefit to other builders who might have departed from the exact equipment recommendations...

[Peter Miller]

I use a tailplane of 15% of wing area which does need that CG at 25% chord.

I am using a 10X 6 APC prop on mine.

[Martin Harris - Moderator]

Posted by Peter Miller on 13/07/2020 12:15:14:

I am using a 10X 6 APC prop on mine.

Thereby may lie the explanation of why your model flies so much better than his!

This isn't what is shown on the 4-Max site where they specify 10 x 5 and there's a very significant power increase (266 - 350 watts) predicted by Motocalc for the 10 x 6 on his motor.

Other builders who've used the 4-Max recommendations may find this info beneficial...

P.S. It is a hoot on 4S though, and carries the extra weight very well!

Edited By Martin Harris on 13/07/2020 13:27:59

[Peter Miller]

I have just double check and Iwas wrong.I am using an APC10 X 5.

I also just checked the weight with a Turnigy 2200 3S battery. 2 lbs 11 ounces.

There is no substitute for low wing loadings

[Martin Harris - Moderator]

I collected some figures today. His model's empty weight is 2 lbs 15 oz - or 3 lbs 5 oz with a 3S 2200 fitted which seems to suggest that our scales are calibrated differently unless he's incorporated some cast iron into the structure!

As a smaller battery would have resulted in a rearward C of G - something I can confirm is not desirable from my first flights with his model - I tried it with a 3S 3200 and 10x6 APC which gave 9660 rpm and a current draw of 28A. This resulted in a reasonable aerobatic performance although understandably much less dramatic than on 4S. It seems that the motor he's used gives a similar output on a 10x6 to the 4-Max on 10x5.

I think he'll find this more than acceptable.

[Peter Miller]

So that makes it 10 ounces more.

That is an awful lot.

One silly question.When I fit my battery it is completely forward so only the last 1/4" is showing in the hatch.Is his any further back?

[Martin Harris - Moderator]

I can see where you're coming from Peter but, except for 1/4" of high density foam to protect the battery from the motor mounting screws, the battery is against the firewall. If I recall correctly, your motor has a shaft protruding through the firewall which you built a box around? If that's right, his battery is at least as far forward as yours...

His is built with a solid fin and tailplane but built up control surfaces - it does seem odd that it struggles to get the C of G forward enough.

10 ounces is an awful lot of balsa/ply/glue/covering so it seems likely to me that there's a calibration error in either or both of our scales. The only visible difference affecting the C of G is that he's fitted a tailwheel but it looks very lightweight to me.

Anyway, it now seems to have virtually the same power now as yours and is quite enjoyable to fly. My view of it may be slightly coloured by my 85% IC version - that weighs 2 1/2 lbs, does very large loops and will just about prop hang - equating with a theoretical 270w if an online calculator using its static rpm is to be believed...

[Peter Miller]

Actually I always cut the motor shaft off close to the motor.

I do have a second set of scales soI will weigh it on those.

Now I am offto do early morning shopping in the Supermarket. YUK!!!

[Trevor Crook]

Weighed mine today on digital luggage scales - 3lb 2oz with a 3s 2200 battery, so a few ounces heavier than Peter's. I got the correct cg with my battery protuding back into the hatch area by 1", handy for wrapping a velcro strap around it. I built my tail as per plan with an open structure. Flies great.

[Steve McIntosh]

Just received my "Ohmen" kit from Sarik as a first kit built. That's my winter building project sorted.

[Peter Miller]

I will watch with interest.

[RedBaron]

I have just about got my build complete.

When I got to the cowling construction I just could not face all the balsa shavings and sanding dust so...

I 3D designed a cowling and printed it. The accuracy was very good and the profiles match the plan nicely. I also designed a back plate for the cowling, this screws to the firewall and the cowling screws to the backplate.

[Peter Miller]

THat sounds really good,

Sorry but no pictures showed up.

You could probably sell a few of those

[RedBaron]

Wowww don't know where the picture went, it was there when I posted the message?!?

Try again two pictures now

1 the printed parts

2 the parts fitted

[RedBaron]

And just for good measure the parts before printing as the appear in the design program:

[RedBaron]

And another thing whilst on The Ohmen and 3D printing.

The Ohmen wing has quite a nice profile and I am not very good at shaping leading edges.

So

I scanned a wing profile from the plan and used it in my trusty design program to make a custom sanding block. It then needed an appropriate grade of garnet paper adding and retained with a few blobs of UHU.

This was quite successful as it very quickly created a perfect and correct profile:

[Peter Miller]

THat is a very smart cowling indeed. I am sure that a lot of builders would love it.

I also like your profile sanding block.very clever.

[RedBaron]

Thanks Peter for your kind remarks.

I originally meant to just show the picture of the fitted cowl - but over my evening meal it occurred to me that some folks might be wondering how one gets from the free plan to the finished design. So it might be worth pondering that for a moment.

As they say a picture saves a 1000 words

[EarlyBird]

I have just ordered The Ohmen and if you are taking orders then one of each please.

Cheers

Steve

[RedBaron]

Bother I posted that without completing the thought:

The first step was to scan key parts of the plan,. import the scans into the 3D program and set them up as shown above.

It is then relatively easy to construct drawings and then to make the solid parts within the program. The parts are then exported as .stl files to ones PC desktop or anywhere.

The .stl files then need to be processed in another program to produce the toolpaths for the 3D printer and there you are.

I have been playing at 3D design for a couple of years but this cowling was the first project in a new design program (purchased)

Does it cost a lot well yes and no. Where I have got to after a few iterations is:

The free plan thank you Peter.

The design program, there are lots of free ones but many are not up to the job, the one I have just acquired was wincingly expensive but less than the cost of a middling moulded glider

The program that makes the tool paths, there are lots of free ones but I have settled on a purchased one I think it was about £50

The printer, there are lots of cheap ones but I have now settled on one that cost much as a medium sized electric model. Do not buy one that locks you into the manufactures consumables! For filament choose an established supplier and buy a medium to higher range filament, cheap filament will not do.

So the whole lot cost a biggish moulded glider.

I do enjoy making stuff in wood. But... Once you have designed say a servo mount it is very easy to modify it for another type of servo or another model. The same if you make a mistake. So I believe that embracing the new technology helps one to be a more efficient builder and it is a different sort of fun

On the more mundane side of things stuff around the house often breaks and the plastic parts are completely unobtainable now I can design and print replacement parts in a lot of cases.

[Peter Miller]

Amazing. I am lost at the complexities.

I do have a Dremel 3D printer. A freebie from Amazon Vine. About all that I have managed is a few pilot figures from it...So glad that I didn't have to pay for it because they would have been the most expensive pilots in the world!!

Sounds like you have your first customer!!!

Edited By Peter Miller on 23/11/2020 21:03:48

[RedBaron]

If I may say so - not complex, just a load of simple sequential actions.

If you have a printer all you need is practice, decide on some simple projects like design and print a servo mount to go in the Ohmen? and when that works another project. But from my own experience regular use is good

The more you do the more you can do.