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The Ohmen

My first electric powered aerobatic model

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Peter Miller03/05/2018 12:48:34
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Over the years I have designed a few electric powered gliders and a scale model. plus the other odd model

I have now decided to try an electric powered model.This is pretty much like manyof my powered designs but a bit smaller at 47" span.

Before anyone says that I am giving up i.c power let me say that you will have to wrench my four strokes from my cold dead hands.

However tempus fugit means that I don't always feel up to lugging all my gear across a field and up a hill in less that perfect weather conditions.

I will say that it is an interesting operation as there are some things that I have never had to consider before. For example, cooling tp the speed controller and then the cooling air exit. Also providing an extra hatch for access to the battery

I asked George at 4 Plus about the power and he suggested a 3541 1070 motor turning a 10 X 5 prop and a 40 amp ESC

So lets make a start.

The fuselage is pretty conventional. I did cut lightening holes in the 1/32" ply doublers and selected very light wood where possible

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The styling is basically Ryan STA influenced.

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The battery platform is well forward, Hope that is not too far forward.

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I found that the leads were not long enough so had to open the hole in the firewall up so the ESC could go in through the front.

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Under carriage mount is my standard arrangement but only using 10 SWG wire

The Wright Stuff03/05/2018 12:51:03
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What a fabulous name for an electric model! laugh

Tom Sharp 203/05/2018 13:13:15
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Posted by The Wright Stuff on 03/05/2018 12:51:03:

What a fabulous name for an electric model! laugh

Oh Man! cool

Biggles' Elder Brother - Moderator03/05/2018 13:31:40
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HI Peter,

I very much doubt the battery will be too far forward. One of the standard catches with people who understand IC very well, but less so electric, is that they don't always appreciate that the weight of equivalent IC and electric systems might be very similar but the weight distribution is very different.

IC focusses all the major weight, the engine heaviest single element, right up front, maximum leverage on the CoG.

But a motor is rarely the heaviest element in an electrical set up - it's usually the battery. So to get basically the same total weight to give you the same CoG you've got to get that battery well forward.

Even major manufacturers get this wrong. One well known outfit released an updated IC/EP version of one of their models. They made a big hatch and said "put the battery in the fuel tank bay". Those who did that found that they needed 3lb of lead (in an 11lb model) in the nose to achieve the CoG! There's more to converting a model to electric than just making a big hatch! As you are discovery and have observed yourself Peter! I wish all "IC based" designers were as careful and diligent.

Anyway, she look great and I'll be following with interest. I too think the name is very clever! Nice start.

BEB

Peter Miller03/05/2018 13:37:40
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Thanks BEB

I did think about the motor being light. I have also kept the tail end very light.

I spent a lot of time trying to find the right name. and ended up going through the OM section of the dictionary.

I had a design [published in the USA which I called "The Ohmsick Angel"

Peter Miller03/05/2018 13:47:23
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Next stage.

dscf3987.jpg

The rolled sheet decking has now been added.

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Keep the tail light. THat is obvious and I alwasy try to but for electric I think it is even more important and BEB's comments confirm that.

dscf3979.jpg

The wing follows my standard sequence which guarantees a warp free wing. Built over the lower sheet.

dscf3977.jpg

The right wing is then covered with the top sheet before lifting from the building board.

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It is joined to the left wing and then the left wing has its top sheet added.

Note the use of clamps to hold the sheet to the spar. That is why my webs go in front of the spar.

Nigel R03/05/2018 14:29:15
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Looking good Pete

What size lipo are you using? (I'm guessing a 3S3000 going on motor and prop combo)

I completely agree about four stroke, BTW. They are things of beauty.

Electrics, marvels of technology and so forth that they are, are things of convenience.

"I did think about the motor being light. I have also kept the tail end very light."

I found on my RM Trainer build, that I ended up nose heavy, and couldn't get the lipo far enough back! As long as you have space to shift it backwards AND forwards, you'll be ok.

 

 

Edited By Nigel R on 03/05/2018 14:34:25

Andy4803/05/2018 14:37:48
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I have converted a few plans and kits to electric, and BEB is, as usual quite correct. Almost invariably the battery is close to the bulkhead.

One feature I do, is to put a liteply tray from the front bulkhead to just behind the wings, usually about a third the way up the fuselage. This allows the ESC to sit under it close to the engine bulkhead with a hatch below, and as you have done, no formers until the middle of the wing. The small servos sit in this tray in roughly the same position as yours appear to be. The wing bolts can be incorporated in this tray too. This makes a tremendously strong fuselage with plenty of space to move the battery back and forward to get the CofG right. Its also worth considering the increasing use of current and voltage sensors with electric. This requires extra space behind the battery. In the picture below (this is a Galaxy/Pegasus Musketeer) the CofG is almost exactly on the position of the former behind the battery.

musketeer 8.jpg

Edited By Andy48 on 03/05/2018 14:40:58

Peter Miller03/05/2018 15:04:06
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Actually I am using a 2200 Mah battery because that is what I have already.

It lookxs as if I may need a little more nose weight, not sure yet. The ply battery plate goes to the firewall but as I have nt chopped off the extra shaft the battery will be a little back from this.

9 gram metal geared servos on those bearers.

The bare airframe as seen in the series of pictures was 14 ounces.that is basic wing, fuselage and tail parts. I had originally expected a total weight of about 3 lbs but now expect to be well below this figure.

Biggles' Elder Brother - Moderator03/05/2018 16:24:57
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Peter, do you not have any larger capacity 3s batteries? It would be far better to have "uesful" weight that will extend your flight time rather than lead if you can.

BEB

Peter Miller03/05/2018 18:24:04
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No, I don't have bigger batteries and I am sure that these will give reasonable performance based on Georges comments and seeing a similar sized model in the club.

As for added lead. I have come up with a rather sneaky idea. I am not going to build the cowl until the model is complete. Then, if I need to move the CG forwards I will mount the motor on stand offs thus moving it forward about 35 mm. Then build the cowl.

I know, I have a twisted mind but it works quite often

Biggles' Elder Brother - Moderator03/05/2018 18:33:34
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I don't have any doubts about the performance - I'm sure the 3s will be fine in that respect. But I do have fears that you might be dissapointed in the flight times you get from a 2200. But time alone will see!

BEB

Edited By Biggles' Elder Brother - Moderator on 03/05/2018 18:34:09

Peter Miller03/05/2018 18:41:14
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Possibly that will be true. I will just have to see and maybe buy bigger batteries. I am still learning.

Ace03/05/2018 18:48:49
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If weight is needed and space allows what about paralleling up a pair of 2200 to give 4400 x 3s?

More useful weight with better duration.

Trevor03/05/2018 19:03:46
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Posted by Peter Miller on 03/05/2018 18:24:04:

No, I don't have bigger batteries and I am sure that these will give reasonable performance based on Georges comments and seeing a similar sized model in the club.

As for added lead. I have come up with a rather sneaky idea. I am not going to build the cowl until the model is complete. Then, if I need to move the CG forwards I will mount the motor on stand offs thus moving it forward about 35 mm. Then build the cowl.

I know, I have a twisted mind but it works quite often

Yes, I’ve been known to leave the fuselage sides over-long and defer fitting the firewall until the flying surfaces have all been built and covered and the model can be balanced. The length of the nose can then be adjusted so that there is enough room to shift the battery for and aft to fine tune the cg. That said, needing a bit of lead nowadays isn’t so much of a problem. Back in the days of ‘can’ motors and NiCd batteries though, adding lead to an electric model was a cardinal sin!

Peter Miller03/05/2018 19:11:06
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Space is limited I am afraid.

Sounds a good idea to leave the sides long for final adjustment

I will make a confession. I took one of my old designs and reduced it to 86% to get the size that I wanted.

Interestingly if you look at the suggested motors etc at 4 Plus my set up is almost identical to that suggested for Peggy Sue 2 and that is a heck of a lot bigger model.

Andy4803/05/2018 19:24:10
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I've started Peggy Sue 2 and based on the weight of the plane I will be using a 650W motor with a 4S 3000. An underpowered electric plane will always be underpowered, but electric motors like to run at about 2/3 of maximum power, anything above that and you find a very steep current rise for very little more performance. Telemetry is really useful at times! laugh

Biggles' Elder Brother - Moderator03/05/2018 19:29:26
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I find electrical systems like a bit of "head room" all round, motor, ESC, wiring etc.. They run cooler and more efficiently that way.

BEB

Peter Miller03/05/2018 21:08:44
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Andy.

Let me know how you get on with the Peggy Sue build please.

BEB

Well there is space round the electrics but adding a battery would be almost impossible without major surgery.Might get a 3300 battery in but I must check on the size

Old Geezer04/05/2018 00:01:13
576 forum posts

You'll usually need to mount your electric motor on stand-offs if you do a straight swap for an i/c motor, to get your prop-driver in the same place, or else you would have to shorten ( butcher! ) that nice glass fibre cowl - besides, who wants a brachycephalic Wot4?!

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