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The Ohmen

My first electric powered aerobatic model

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Martin Harris12/07/2020 17:07:59
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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 Walby12/07/2020 18:03:24
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I flew Sue's again this morning on 3S and its running a 11x5 prop (28A 300W WOT) and IMHO it flies perfectly well.

20200516_100836.jpg

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 Miller12/07/2020 18:17:22
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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 Harris13/07/2020 11:55:53
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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 Miller13/07/2020 12:15:14
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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 Harris13/07/2020 13:26:08
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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 Miller13/07/2020 13:34:00
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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 Harris14/07/2020 18:43:50
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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 Miller14/07/2020 21:16:10
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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 Harris14/07/2020 22:57:44
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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 Miller15/07/2020 07:14:37
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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 Crook15/07/2020 22:36:23
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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 McIntosh05/08/2020 14:51:57
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Just received my "Ohmen" kit from Sarik as a first kit built. That's my winter building project sorted.

Peter Miller05/08/2020 18:21:21
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I will watch with interest.

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