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Scaling of dimensions

Old 05-06-2013, 07:56 AM
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familyyy123
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Default Scaling of dimensions

hi all,this is my first post.i do not know if this is the right place to post this or not but i am desperate for information and this forum looked really promising.

i was searching for designs for RC planes,most of them have wing span of 2-3 meters,but i want to build a 5-6 meter wingspan(yeah i know it is a lot).
the question is:is it normal just to multiply the dimension by the desired scale? i guess the answer is NO,due to change in Reynolds number and stuff but i need to know how can i do it , and what will change exactly after scaling the dimensions to a bigger scale?

Note:i checked some threads in this forum but i could not find an answer ,plz answer ...i do not want a 5-6 wingspan airplane to crash .
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Old 05-06-2013, 06:53 PM
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fhhuber
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For the outline of the aircraft and control surface sizes you can just multiply all dimensions by the same factor.

The structure gets a bit more complicated. Some structureal members will need to be strengthened vs just multiplying the cross section's dimensions. (using the same material) Some can often be made with the original thickness material (in the smallest dimension such as wing sheeting and ribs)
Changing materials changes the needed dimensions but can save significantly on weight.

Wing area is proportional to scale squared. (2X wingspan = 4 X the wing area)

Weight of the aircraft tends to go up a bit more than scale squared but not quite cubed (2X wingspan ... about 6 X the weight)
Assuming you want similar aerobatic performance.
We can control weight to have it go up almost directly with with area, especially when using modern composite construction.
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Old 05-06-2013, 09:23 PM
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familyyy123
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first of all THANK U SO MUCH, this information will help me a lot,
but i have some questions:
1)by "control surface" you mean all the flaps,rudders,... right?

2)" Some structureal members will need to be strengthened vs just multiplying the cross section's dimensions."
do u mean that i have to use my judgment in order to decide if i need to strengthen the members or not??

3)of course after scaling i will need a new ,bigger ,more powerful engine.....do i calculate drag over new areas and then calculate the thrust required or is there a scale factor for that too(thrust factor)?

4)in general cases..does the new wing area usually provide enough Lift for the x6 weight or should i do as u said and search for other materials?

5)thanks again, i feel that i have come to the right place and sorry if i sounded a bit noob.....THX
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Old 05-06-2013, 09:58 PM
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fhhuber
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Originally Posted by familyyy123 View Post
first of all THANK U SO MUCH, this information will help me a lot,
but i have some questions:
1)by "control surface" you mean all the flaps,rudders,... right?

Correct

2)" Some structureal members will need to be strengthened vs just multiplying the cross section's dimensions."
do u mean that i have to use my judgment in order to decide if i need to strengthen the members or not??

Its not really a matter of judgement. You have to anticipate the weight and then using tables listing the strengths of the materials you calculate the dimensions of the material to be used to get the strength required.

Some really experienced modelers can take one look at the materials used in a structure and tell you if it will be adequate. But it takes years of building models to learn that.


3)of course after scaling i will need a new ,bigger ,more powerful engine.....do i calculate drag over new areas and then calculate the thrust required or is there a scale factor for that too(thrust factor)?

Some parts of drag are squared factors others are cube factors and a few are direct linear.
Again we need to use years of experience to judge what is needed to get the performance desired... or go to a program that will help us calculate it such as MotoCalc for electric power


4)in general cases..does the new wing area usually provide enough Lift for the x6 weight or should i do as u said and search for other materials?

The 2X span 4X area and a bit higher airspeed will easily support the expected higher loading for flight, but the spars may need to be made of different dimension than direct scaling and/or different materials, plus other stiffening such as (sheer webbing" may (or may not) need to be added.

5)thanks again, i feel that i have come to the right place and sorry if i sounded a bit noob.....THX
It really helps to have built kits or from plans of many different scales when you are attempting to enlarge or reduce the size of a model.

Without that experience and the knowledge of what to do in the process its better to see if you can find plans of what you want in the scale you want.
There are thousands of different aircraft plan sets out there already.
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Old 05-09-2013, 08:57 AM
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familyyy123
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thanks again.....and i hope u do not mind that i ask u some more questions if i face any problems during my project...and again...thank you.
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Old 05-11-2013, 05:55 PM
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quorneng
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familyyy123
Just remember that as things get bigger the strength of materials works against you.

Try making a paper dart 10 times the size with paper 10 times as thick and see how flexible and heavy it is.

But as you get bigger you have more space to improve the detail of the design to use the material just where it is required to carry the load. This does mean that to achieve the same strength to weight bigger planes tend to more complicated to construct.

However on the plus side as size increases the aerodynamics tend to work in your favour, particularly where drag is concerned.

Do you have any details of what you propose?
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