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Model enlargement

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fly boy314/07/2018 22:08:06
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3467 forum posts
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Hi all, if enlarge a plan by say 50 percent, and build model to that spec. would the final all up weight increase to a similar value. Thank you.

jrman14/07/2018 22:17:24
337 forum posts
3 photos

No. The weight would increase by the cube of the linear enlargement i.e. 1.5x1.5x1.5 = 3.375. (all other things being equal)

Simon Chaddock15/07/2018 00:23:31
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5390 forum posts
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fb3

The problem is that all other things are not equal!

In simple terms a beam of 1.5 times the physical size is 3.375 the weight bit it has rather less than 3.375 times the physical strength and stiffness. This means that a bigger plane scaled up and constructed in exactly the same way will actually be a bit weaker for its weight! sad

The plus side is that a physically larger structure makes it easier to incorporate a more complex and efficient structure that makes better use of the material so saving weight and/or increasing the structural strength. In addition bigger planes have more efficient aerodynamics so proportionally require less power to fly.

Scaling a plane up (or down) presents a set of very interesting variables to fly well. wink 2.

Peter Miller15/07/2018 08:26:11
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The way we used to do it was take the cross section of the wood e.g. spars in square 1/16ts and convert them that way.

For example, 1/4 sq. has 4 X 4 1/16ths = 16 1/16ths so a 1 1/5 times version would want 24 1/16ths, the nearest size would be 5/16 sq.

It worked very well

Paul Marsh15/07/2018 09:11:17
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3614 forum posts
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I find that increasing the plan by 33% works best. For example, I build a enlarged Wonder from a .15 size to .60 sized, enlarged it to 33% resulted in the right size.

super_sytky (3).jpg

Thinking to enlarging a .15 size of this to 120 size. so would go for 33% again.

Will do it this winter, as the model flies so well and will be better, larger...

fly boy315/07/2018 12:40:10
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3467 forum posts
18 photos

Thanks for the info lads. I do like Peters method. Would you say the most important parts would be wing spars, firewall u/c mount and perhaps fuz formers ? Thanks. Ps would it be possible to just measure the size of wood shown in the enlarged plan ? Cheers

trebor15/07/2018 18:22:57
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1898 forum posts
214 photos
Posted by Paul Marsh on 15/07/2018 09:11:17:

I find that increasing the plan by 33% works best. For example, I build a enlarged Wonder

What design / brand is this one ? I googled it and didnt find it.

Paul Marsh15/07/2018 18:30:02
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3614 forum posts
1008 photos
Posted by trebor on 15/07/2018 18:22:57:
Posted by Paul Marsh on 15/07/2018 09:11:17:

I find that increasing the plan by 33% works best. For example, I build a enlarged Wonder

What design / brand is this one ? I googled it and didnt find it.

Sig. American company.

**LINK**

Piers Bowlan15/07/2018 18:48:28
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1769 forum posts
42 photos

Super Sytky, Christian Moes design (free plan) from RCMW August 2015. An enlarged 120 size would be impressive.

Peter Miller15/07/2018 19:02:25
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9906 forum posts
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10 articles

Well, you could measure the enlarged plan bjut if you double the plam 1/4" Square would go up to 1/2" sq and that would be far too big.. 3/8" sq would be twice the cross section.

Tony Richardson15/07/2018 20:04:27
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598 forum posts
25 photos

If you enlarge the plan by photo copying it do you build on the middle of the line or on the inside/outside as the plan line thickness also increases in size, its something I have not tried but would like to?

Martin McIntosh15/07/2018 20:05:31
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2819 forum posts
1050 photos

I know that this has been debated on here before but you need to consider the power increase requirement for even a small enlargement. I have said elsewhere on here that I increased a 60 powered Dalotel by 10% hoping to use an OS 91FX. It would scarcely even loop without first diving it. A YS 110 cured that.

60, 11x7, 91, 14x6, 110, 16x8. Quite a difference needed for a small increase in model size to get the same performance.

Simon Chaddock15/07/2018 20:08:54
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5390 forum posts
2818 photos

The other aspect to consider is that most model planes are considerably over engineered (to provide some degree of crash resistance) so a modest increase is size probably does not need much strengthening so Peter's 1/16 method is likely to be quite adequate.

Such scaling does create a problem if the model is only just strong enough in a particular area or prone to a specific failure in a crash. Scaling up will make this weakness worse but how do you know where it is likely to be?

Few, if any, models are designed using the detailed stress analysis of full size so designing, or modifying, a stressed structure comes down to experience based on trial and error!

Martin McIntosh15/07/2018 20:11:04
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2819 forum posts
1050 photos

I have done this on a copier many times. Use the centre of the line. I think that a PDF drawing should keep the line width as per original but do not quote me on this.

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