FlyDave

Hello !

I want to build a semi-scratchbuild pattern/aerobatic airplane with detachable wings.

I want to use the phenolic+aluminium tubes to join/separate the wings, but i don't know if there is a "magic formula" to choose the tube size or if i simply should select the biggest size that can be put inside the wing.

The wingspan of the aircraft is 1.4mt (or ~55 inches). The weight of the airplane should be around 2kg (70oz)

I've seen tubes from 12mm to over 50mm , as my airfoil maximum height is 44mm what's the best choice ? i wanted to go with the little one (12mm) , but i want your opinion before...

Thank you !
David

Lnewqban

David,

The tube that supports the wing will "feel" some bending and shear stress.

How much of both types of loads each half wing will transfer to the fuse via the tube?

During strong 3-D aerobatics your model may reach, let's say 15 G's.

If not stalling before, each half wing will feel a force of 2 Kg / 2 X 15 = 15 Kg, which will be applied perpendicularly on the centroid of the area.

Then, shear stress = 15 Kg and bending stress = 15 Kg X (Distance centroid-fuse side)

I know that all that doesn't respond your question, but in order to calculate a minimum diameter for the tube, we need to know the thickness of the wall and the strength of the material.

Remember that the wing's material surrounding the tube needs to be strong enough to transmit these forces to and from the tube.

FlyDave

Thank you Lnewqban for the help !

The material is called "Strongal", it's a combination of aluminium tube inside a phenolic tube.
Wall thickness is 2mm.

This is the google-translated description of the material :

"Strongal is special wing-with very high tensile strength of 630 N / mm ² can. This is no need for additional amplification (for special applications, however, possible and appropriate) which allows weight savings.
The tubes are anodized gold."
Link : [link=http://shop.lindinger.at/product_info.php?products_id=66803]Strongal product at Lindinger[/link]

It is correct that i have to measure the area of the aluminium tube, and with this value i have the "force" that the tube can handle ?

EDIT : opsss... TENSILE strenght... they don't talk about the flex strenght...

wildwillie3

David,

I had a Groovy 50 F3A with a 53.5" wing span and it had a 16mm Dia wing tube. I never had a problem with it. It was made of aluminum and kind of heavy. Good Luck

Bill

Lnewqban

David,

You are very welcome.

Please the attached PDF file with the calculation for your preferred tube of diameter 12 mm.
I have tried to describe the process for you to keep for future projects.

Note that the calculation is for the critical section of tube next to the fuse, and disregarding the help of the tight plastic bolt(s) (like if the bolt(s) was loose).

Also note that I have considered the thickness of the wall as 1.5 mm in lieu of 2 mm as the catalog shows.
I have preferred disregarding the outer shell of phenolic material (assuming ~0.5 mm).

As you see, that critical section of the tube is able to resist many times the shear load, as well as 1.8 times the bending load of a 15g loop.

Now, practical considerations:
1) Accidental loads like hits against the ground or cartwheeling may increase the max calculated load of flight.
2) The wood around the tube will push back with the same forces; hence, they must be strong and rigid. No chain is stronger than its weaker link.

In summary, your plane will be safe with the 12 mm tube.
The next size in that catalog is 16 mm, which is an overkill for your plane.

Best regards and luck with your project!

FlyDave

Hi Lnewqban , thank you so much for taking the time to do the calculations and to give those precious informations !!

I will keep this document on my book of wonderful tips.

I forgot to say that i will use a S8035 airfoil, but I've seen that you have choosen the right airfoil thickness, so... what i can say... PERFECT !!

thank you again!
David

Lnewqban

You are very welcome, David.

I worked with the data you showed in your first posts, including the height of 44 mm for the airfoil.

Best luck with your project!

FlyDave

Ah....upsss... I forget even what I say... eheheheh