The Ohmen

[Ron Gray]

Interesting RB, which program are you using, I use Fusion 360 which I find is incredibly powerful plus has lots of YouTube vids explaining how to do things but am always interested in other software.

[RedBaron]

I too have been using Fusion360 and very good it is but I have recently changed to Rhinoceros.

It is always a struggle changing to a new UI. The Cowling was my first Rhino project.

[Piers Bowlan]

Huge respect Red Baron, very impressed.

Why don't you submit an article for inclusion in RCM&E as 3D printing has become so popular and has not featured very much in the magazine. I tried a beta version of Rhino on my Mac with a few exercises but found it very challenging. Perhaps I should have persevered.

[Nigel R]

Nice work RB.

[RedBaron]

This last post has nothing to do with The Ohmen, it is more to encourage people with printers to use them more and those without to get a basic one. After this Peter I promise to stick to the subject and thank you for your patience.

I mentioned above about non modelling uses, such as replacing broken and irreplaceable household items. Well I should not have tempted fate. For about twenty years we have had two wooden geese (carved from bamboo root I think) that live in the garden next to the bird bath.

This afternoon I went into the garden for some fresh air and noticed one of the geese lying on his side. I went over to stand him up and found to my horror that both of his feet had completely rotted way - no wonder he was lying down.

Well it seemed to me that rotten feet qualified as irreplaceable. So after 15 minutes of designing and about 1.5 hours printing Mr Goose had a new pair of orange feet. Imagine doing that by carving lumps of hard wood.

[RedBaron]

Further to the comments about Fusion360 above. I decided today to start exporting my designs from Fusion360.

And........

There is no way to export designs AND drawings from Fusion360 unless you have one of their paid for options. In fact the drawings and associations to the solids are in the wind. If you are a Personal Use Only user then you are locked into a proprietary format with no escape for your work!!

I have no desire to be locked into any product or format which makes me very happy to have recently chosen Rhino

I can export the Fusion360 solids via STEP format, which I will do but I will not have the associated drawings!! For most designs this isn't too bad. Then I will close down my Fusion360 account.But a major swindles as I do nor remember being told this at the beginning.

[RedBaron]

Back to the Ohmen - my build is complete except for a short list of minor things to do before covering.

Top of the list today was the matter of the windscreens, no matter how hard I looked there was not a template on the plan. Now whilst one can make guesses and after endless trimming with fine scissors arrive at a good result I much prefer to start with an accurate template.

My method today.took longer than the old fashioned approach but it was an investment in understanding what Rhino can do.

See the pictures:

1 I constructed a model in Rhino of the upper body and front windscreen from careful measurement of the plan.

2 Show a pretty neat feature of the program - one can unroll a surface to create a template! This picture shows the curved screen and flat template

3 I tried to upload a jpg of the template such that it can be printed and the template cut out. I have cut out the original image on thin card and offered it to the fuselage and it looks almost OK The unrolling process was not quite perfect and I don't know why yet. However I think the image is useable.

I cannot guess whether or not the uploaded image preserves the actual size. If anyone tries to use it and the size is wrong, then just scale uniformly so that the distance between the two pointed tips is 125mm

[Peter Miller]

Hi Red Baron.

Sorry! No templates. I just cut mine until it looked about right. Thex good old TLAR

The thing is that if one looks at pictiures of similar aircraft they all have variations frpm the simple curved shape to fancy one with metal frames. I have even seen a fullsize Tiger Moth with both types fitted.

Then one can even do really fancy ones like my Destiny.

[Martin Harris - Moderator]

Another variation has emerged at our club - yet to fly but I’ve little doubt that it should be at least as good as the original. 
 

It’s been scaled up to about 62” span and certainly dwarfs my little 40” GLOhmen!

 

The camera angle exaggerates the difference but it isn't photoshopped!

 

I’ve nicknamed it The Grohmen although the builder favours Big Ohmen.  The covering on the new one will be tightened before it flies.

[Peter Miller]

On 13/04/2021 at 17:50, Martin Harris - Moderator said:

Another variation has emerged at our club - yet to fly but I’ve little doubt that it should be at least as good as the original. 
 

It’s been scaled up to about 62” span and certainly dwarfs my little 40” GLOhmen!

 

The camera angle exaggerates the difference but it isn't photoshopped!

 

I’ve nicknamed it The Grohmen although the builder favours Big Ohmen.  The covering on the new one will be tightened before it flies.

Fellow club member Mick King built his Ohmen to a much larger size. Well over 50 inches span. He flew that for ages but now he has an enlarged Chaos which he loves even more.

 

His Ohmen was also in a RAF type colour scheme. I don't have a picture at the moment

[BrianRH]

Hi Peter, I just completed my lockdown build project and thoroughly enjoyed the experience - thankyou for providing another great design (I built Bootlace a few years ago, a truly beautiful aircraft)! I'm no good at coming up with colour schemes so just copied yours. Just waiting for an opportunity for a maiden flight now, let's hope the weather in June is more favourable than that of May!

[Peter Miller]

Hi BRian

 

You have done a great job on The Ohmen/ It looks really smart.

 

Sunday is looking great so I hope that the weather will play ball fro you too.

 

My Ohmen lives in one piece and is my "go to" model for a quick trip up to the field  for some fun although it is looking a little tatty round the edges these days.

 

I will look forward to hearing how you test flying goes.

 

Have fun

 

Peter

[BrianRH]

Hi Peter,

 

Rather unexpectedly after reading about how lovely she flies from everyone, I have struggled to achieve a successful maiden flight yet, having had three attempts all ending in a prang (fortunately, with minimal damage in each case). I suspect the main problem has been the long take off run needed to get her airborne, meaning I've had difficulty seeing and reacting properly to what goes on when she leaves the ground. I suspect this may be due to the fact that she is a little heavier than the figure you mention in the magazine article, weighing in at 3lbs 6 oz, or 1.54 kg. Not sure why this should have happened as I followed the plan meticulously (though I am quite heavy handed with the glue). I'm planning to fit an 11 x 5.5 prop instead of the 10 x 5 for the next attempt in the hope I'll get a bit more thrust and shorter take off run. I don't know if you have any thoughts on this? I may also try hand launching.

 

You mention in the article the tendency of the model to swing hard left on take-off. I'm wondering if this has also played a part in my difficulties as I have not really been alert to the need to counter this with rudder - all my previous models have had built-in engine thrust line offsets to counter the effect. When experiencing non-straight climb-aways my natural instinct is to apply roll, I presume this would be an effective counter if that is indeed part of the problem?

 

Will be interested in your comments.

 

Cheers

 

Brian

 

[Martin Harris - Moderator]

I’ve flown at least half a dozen Ohmens - built by novices and more experienced builders and at various scales - and some not built lightly by any means. All of them have flown well with straightforward handling. 
 

There will be a roll couple associated with yaw but yaw will increase with more power. The ideal way to counter it is with the rudder stick as, especially at low airspeed, use of aileron will create adverse yaw, increasing the amount of correction required. 
 

If you’re unable to coordinate the rudder correction confidently yourself, a throttle to rudder mix might assist you. Do you have an experienced club mate who could help trim the correct compensation?

[Peter Miller]

For a start my take off runs are about 15 yards or less.  I would not have expected  an extra 12 ounces to make the run so much longer.

Any swing is corrected with rudder, not aileron as Martin says.

 

Are you using the specified motor? a 3541 1070 with an APC 10 X 5E prop?

 

The amount of glue would not add significant weight but hard balsa will and that is probably the grade of wood used in any precut set of parts.

 

I will just mention that none of my designs use side thrust. They may swing on take off but once in the the air they fly straight and level.

I work on the principle that I have never come across a full size aircraft with side thrust and many aircraft need a boot full of rudder to control the swing on take off.

 

The bigger prop should give you more thrust.

 

[kc]

A weight of 3lb 6 oz for Brian's model seems quite good for a 47 inch span model - if it includes the Lipo.   Peter's model was stated to have been 2lb 10 oz by RCME and I  always thought that was very light if it included the Lipo.  But of course Peter is one of the most experienced builders !

It's unlikely the weight is a problem but it could be any number of other things - maybe small wheels and  long grass could cause a slow take off run?    Checking the CG might be a next check and especially check that the lipo is not moving around.

 Using a wattmeter to show many watts the motor gives with the 10 by 5 and then with an 11 by 5.5 prop might reveal something.

Personally I have found that when moving from a model that leaps off the ground easily to one that lumbers along the ground slowly I have forgotten to use the rudder.   It's easy to get used to not needing to use the rudder on take off!

 

[Peter Miller]

2 lbs 10 ounces was my weighed weight with the Lipo.  I have a thing about   low wing loadings. 

 

The CG in my designs is always set at 25% of the chord back from the leading edge.

 

[EarlyBird]

This has made me concerned about the weight of my build so I have weighed the covered frame and added the weights of 2200 4S lipo, ESC and U/C. The result will be in the region of 2 lbs 10 ounces ?. That's a relief as I am using a 3536 motor giving some 120 watts/lb at that it should leap off the ground at full throttle but there is only one way to be certain and that is fly it.

[Peter Miller]

A 4S Lipo???  I only have a 3S 2200 Lipo in mine!!

[EarlyBird]

Yes 4S because I have them and the motor is 910 kv again because I have it going spare after the demise of my WOT4 Foam-E. 

[BrianRH]

Hi Peter and everyone,

 

Thanks for all your suggestions and advice. Peter, yes I am using the kit supplied by 4-Max which includes the specified motor and battery. I already have bigger wheels fitted to minimise drag from the grass but I have been using a mown field recently. I have checked the C of G more carefully and was a tad nose heavy so have corrected that too. Having read all the comments I will, next time round, feel a lot more confident. I am very much at the novice end of the scale compared to you guys so that is probably part of it too. I am not a member of a club and enjoy teaching myself (albeit the hard way, sometimes). A few years ago, I built a couple of Tony Nijhuis design SKY40s having seen it in the RCME. But before flying them, I bought an E-Flite Apprentice to teach myself to fly and to give myself the confidence to try flying the SKY40's that I'd spent so long building myself.  This plan worked a treat, the Apprentice is such an easy model to fly and I soon became quite proficient with that, even capable of a few aerobatic manoeuvres and have since had many excellent flights with my SKY40s which are a treat to fly and very robust so survive the odd prang! 

 

I then saw Peter's Bootlace in the magazine and absolutely feel in love with it - such a gorgeous looking aeroplane. It took me a long time to build Bootlace from scratch, no kit of parts being available. I was determined not to fly it (and risk damage) until I had become proficient with low-wingers. So, then I saw The Ohmen, another gorgeous looking model and built that. Before flying The Ohmen which also involved quite a lot of effort to build, I thought I'd better learn the ropes of low-wingers using an ARTF so purchased an Arcro-wot Mk2 Foam E. I've had about a dozen or so successful flights with this and so thought I'd got the hang of low-wingers was ready to try The Ohmen. Maybe, I'll try a few more flights with the Acro-wot before attempting to take to the air with The Ohmen again! Will keep you posted, and thanks again for all the support! Brian

 

PS: If anyone has any other suggestions for ARTF low-wingers to "cut my teeth" on before further risking Peter's beautiful models,  I'd be grateful to hear them. I'm not that impressed with the design of the Acro-wot, it it relatively flimsy and prone to shedding its main wheels given anything other than a perfect landing - the Apprentice is a far superior design in that respect and will tolerate lots of abuse associated with learning.

[Graham Davies 3]

Hi Brian,

 

I'm a warbirds man, but have many, many low winger hours. Having flown several of Peter's models and currently putting a great many hours on his old Ballerina, I don't think you'll find better flying, or more forgiving low winger's than Peter's designs. They are far tougher than ARTFs, and fly very well because they are properly designed, and designed for real world use. I understand that you don't want to damage a model you've invested so much time into, but genuinely can't think of better first low wingers.

 

Graham

[john stones 1 - Moderator]

Spacewalker, plenty out there.

[EarlyBird]

6 hours ago, BrianRH said:

PS: If anyone has any other suggestions for ARTF low-wingers to "cut my teeth" on before further risking Peter's beautiful models

This was my thinking also but having built a HK Spacewalker kit and flown it there is no comparison to the Ballerina build, strength and flight characteristics. Now I have decided that my Peter Miller builds are the only way to go because I know they are built and flown by a very experienced designer, I suspect some kits and ARTFs are created by some young lad on a computer using copy and paste who has never built or flown anything. My thinking has changed more in line with many of my club mates in that the build is as much a part of the fun as the flying to the point where the flying proves the quality of the design and build. Build fly and have fun is the way to get the most out of this hobby. 

 

[Peter Miller]

Well thanks for all the votes of confidence.

 

I took The Ohmen out yesterday and tried to get video of the take off but I am still getting used to a new camera. Next time!!! My Ohmen is now four years old and is still my go to model. It lives in one piece so I can just  throw it in the back of the car and go.

 

We have a club member who started off on an Apprentice and who still flies it years later.

 

One of the good points about building ones own from plans is that if you do have an accident you know how to repair it and they are repairable. 

 

I build models and some only get flown once. It is the building.  One recent one was a kit for a Sterling Piper Tri Pacer converted to electric. I built one in the 60s for free flight and won the FF scale at the RSAF Championships.  I did NOT like the kit this time round but it flew OK on electric power and the channel radio