The Ohmen

[Geoff S]

Looks not only ready but really nice.  I hope those doors open  away from the model and no-one steps out of the rooms on each side of the corridor or you'll be building new wings ?

[Martin Harris - Moderator]

Nice job. Hope it flies as well as it looks - and having flown a good number of different Ohmen builds with variations in skills and design modifications which have all been excellent performers, I’m sure it will!

[Peter Miller]

That looks very smart.

I shall look forward to hearing how you get one.

Weather is looking pretty calm in the south this week

 

 

[EarlyBird]

Tomorrow was going to be the day but as predicted earlier I can't get the CofG.?

I can if I add 40g in the cowl. ?

I made three changes, shortened the nose, moved the servos to the tail and used a smaller motor. If I had done any two of these I would have got away with it.

As it is with no lead it balances on the spar, I was tempted to give it a go based on my Ballerina experiments when this is as far back as I went but as expected the elevator was too sensitive and not fun to fly. Plan B then. Easiest is to put the larger motor in and modify the cowl. The larger specified motor is 40g heavier. Remove the cowl, change the motor and ESC to suit, test fly then modify the cowl and refit. Extending the nose as designed is also an option while modifying the front. 

 

As I have created even more fun in the workshop tomorrow for a maiden is cancelled. ☹️ Probably too cold anyway. 

 

Trust me experimenting is fun even though there are expected disappointments .

 

[Trevor]

Why not just add the noseweight, do the maiden flight and fine tune the cg. Then you’ll have a clear target to work to with any subsequent modifications.

[Peter Miller]

Very wise to make sure the CG is where specified.

There was a certain forum member who complained that one of my designs didn't fly at all well. 

Then he said that when he moved the CG to the specified location it flew beautifully.

I use that particular example when ever anyone queries my CG location.

I have no objection to adding lead in the nose if that is needed but I much prefer adding to the tail as it normally needs a lot less.

[EarlyBird]

51 minutes ago, Trevor said:

Why not just add the nose weight, do the maiden flight and fine tune the cg. Then you’ll have a clear target to work to with any subsequent modifications.

? yes I could add the weight and see how it flies with the small motor. ? If it lacks power then make the changes.

[Martin Harris - Moderator]

I maidened an Ohmen for a club mate - who’d assured me that the C of G was as per plan (which wasn’t to hand).  I did a quick thumbs job and as it was forward of 1/3 chord I thought it should be fine.  It was quite a handful which I was relieved to get back on the ground in one piece and after moving the balance point forwards over a couple of flights, found that it was then just like any other Ohmen…at what turned out to be 25% chord. 
 

Basically, on this one, believe the designer - and I would strongly advise against any attempt to go behind the point shown on the plan. I think I recall a very experienced flyer recounting a similar experience on the forum. 
 

[EarlyBird]

1 hour ago, Martin Harris - Moderator said:

Basically, on this one, believe the designer

I will second that and if like me one must experiment then start at 25% and move back in very small increments because of how lively the model becomes. Trust me it's not a pleasant experience.

[EarlyBird]

Today was the day. ?

A bit of thinking and some changes to make.

60g nose weight to balance on the mark.

Too much power ? flew on 1/4 to 1/3 throttle which was very sensitive.

Trimmed out and did a few circuits.

More to do and more to come but I am more than happy.

As has been said before flies on rails.?

[Martin Harris - Moderator]

Great news.  Good, aren't they!

 

Propping down a bit?

[EarlyBird]

37 minutes ago, Martin Harris - Moderator said:

Propping down a bit?

That's one of two options. 10x6 and 4S at present my thoughts are try 3S first, if they will fit. I have 10x5 would be easiest though or 8x6.

Still thinking though. ?

[Peter Miller]

  I only ever use a 2200 Mah 3S battery I use a 10X6E prop

[EarlyBird]

2 minutes ago, Peter Miller said:

  I only ever use a 2200 Mah 3S battery I use a 10X6E prop

Thanks Peter that confirms my thoughts.

[Martin Harris - Moderator]

They are a hoot on 4S but 3S can provide plenty of fun - even if most mere mortals don't build as light as Peter.  I've flown several examples of both but I can't comment from long term experience as my own one is 85% scale and powered by an OS 20 four stroke engine...

 

 

P.S. Here's some info from another thread that I posted a year or two ago and gives a few facts and figures which might be useful:

 

"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."

 

...and

 

"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."

Edited November 23, 2021 by Martin Harris - Moderator

[Martin Harris - Moderator]

51 minutes ago, Peter Miller said:

  I only ever use a 2200 Mah 3S battery I use a 10X6E prop

 

Sorry to drag up the past but have you changed props again Peter?

 

On 13/07/2020 at 14:33, Peter Miller said:

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

[Peter Miller]

• •  11k

Posted 2 hours ago

  3 hours ago, Peter Miller said:

  I only ever use a 2200 Mah 3S battery I use a 10X6E prop

 

Doyou know I am now so confused I Will have to go and look at it.....

 

Sorry, Yes, 10 X 5E.

 

It has been a tiring day and just had the club sec on the phone for 45 minutes

 

Sorry to drag up the past but have you changed props again Peter?

[EarlyBird]

Having slept on the issue I have decided to go back to the drawing board and extend the nose, probably as the original design, this will give more battery options and get rid of the 60g nose weight. This is becoming a very interesting experiment and giving me opportunities to learn. First lesson could be not to mess with the design ?

The motor has ample power on either 3S or 4S so I will keep that. We will see.

Thanks everyone for the help. 

[Peter Miller]

I never object to people modifying my designs.

 

It is quite possible that they could improve them and if they don't improve them they have learned that old classic saying "If it ain't broke don't fix it!"

[Graham Davies 3]

49 minutes ago, EarlyBird said:

Having slept on the issue I have decided to go back to the drawing board and extend the nose, probably as the original design, this will give more battery options and get rid of the 60g nose weight. This is becoming a very interesting experiment and giving me opportunities to learn. First lesson could be not to mess with the design ?

The motor has ample power on either 3S or 4S so I will keep that. We will see.

Thanks everyone for the help. 

Apart from the pleasure taken in making the mods, it seems like a huge amount of effort for very little gain. 60g is not a lot in the grand scheme of things, and you have plenty of power on the batteries you have. I'd try your 10x5 and just fly it. At lower throttle with your current pack, you'll get nice long flights. If it's still too much, try another prop. If it's still too much, well, you have a throttle stick...

 

As Peter says, modifying is good. However, I always work to keeping the rear light and weight up front. I rarely modify to increase tail weight. I usually make sure I have room to move the flight battery around before putting in stops to secure it once I know where it balances. And in any case, you add a lot less weight at the back if it does turn out nose heavy.

 

Graham

[EarlyBird]

Just back from the field ?

I tried an 8x6E and it flew perfect on half throttle. Plenty of power for me and controllable more to the point. 

I think I will leave it as it is and get some more flights in. I could try a 9x5E as part of the experiment.

I have only two 2700 4S batteries that fit easily three 2200 4S that are a very tight fit. Modifying the front would give more battery options as well as losing the 60g.

Based on my results so far and as advised by @Graham Davies 3I will leave well alone.

Thanks for the help everyone.

 

[EarlyBird]

After sleeping on this I realise that if I leave it as it is the experiment will be over ☹️

I made a few mistakes right from the start, using 3/32 LE bottom sheet, I should have taken that as a bad OHmen?, along with using a motor and battery I had. At this point I made the decision that this build would be an experiment and modifications would be made as I went along.  

 

I now have a model that fits in the car made up and always on hand for a quick flight. It flies like a dream, despite my changes. This makes it ideal for a go to model.

 

I am either going to modify this one or buy another one to build and modify while I play with this one. There is a chance that I could end up with two but based on past performance it won't be for long.?

 

The experiment continues. 

 

 

[Peter Miller]

Mine is my go to model It lives in one piece down stairs in the "Drawing office" and goes in the car in one piece.

[Chris Walby]

IMHO, Just fly the Ohmen and enjoy it and if you want to build then I would recommend an electric Grumpy Tiger Cub...another of Peter's designs and a really nice flyer for a twin.

 

Lots of room for the odd mod and adjustment for electric, George at 4Max did the electric set up with runs on 4S3300 and best of all it fits in the car fully assembled

 

Lots of post on here about different GTC builds and message me if you are interested as I have a spare canopy + other info

 

A club mate and myself built one each.

 

 

Edited November 25, 2021 by Chris Walby

[Peter Miller]

Mine was ic and everyone was amazed at how safe it was on one engine.it was hard to tel which engine had stopped and it was safe to turn into the dead engine.

 

Of course this doesn't apply to electric as it is always on two motors

 

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