Wing Tube joiner stress calculations 1

[Pfielgood]

Hi
Designing a 25% Commonwealth CA-1 Wirraway flying scale replica.
Need some assistance calculating beam strength of 6061-T6 Aluminium tube which will be used as wing joiners for outer panels to centre section.(The outer panels have to be removable for transport)

Each side (wing panel) will use two poles in parallel, one 1.25” dia located at thickest part of wing section, another of .75” dia just in front of the rear spar.

Both tubes will be 6061 - T6 with .035” wall.

Given the model will weigh approx. 60lbs, will the T6061 tube be up to the task ? and what length of pole would be required to tolerate flying (and landing) stresses.
Other data
Centre Section Wing:-
Span 30.75”
Cord 25.25”
Area (Less Fus)=454 ins2
Outer Wing Panels:-
Span (Less Tips)=48”
Cord at wing joint=25.25”
Cord at tip =12.5”
Area (each)=906 Ins2
Projected weight of model 50 – 55lbs

Thanks and regards
Chris

 

[rb1957]

a Wirraway ! ... not too many people know anything 'bout them these days !!

whilst there are probably lots of guys nearby (model flying clubs would be a start) we'll need a bunch for info to give you good answers ...

are these tubes local joining pieces (like at a transportation joint between the wings and fuselage) ?

or the primary bending elements of the structure ??

if 1), then how would they attach to the wing structure ?

without knowing, i suspect that these are the primary structure of the wing so at least the larger one should span the wing. you'll probably want to taper it ... cut a wedge out along one diameter and weld (or, more simply, crush the tube to the height you want).

6061T6 will be fine, easier to work with and presumably more available to you. making a structure that's strong enough is easy, the trick is to make the structure light. you'll need ribs in the wing, presumably some sort of polystyrene foam, glued to the spar. for a 1st stab at sizing, 60lb plane design for say 10g (what sort of roll rates ?), so each wing supports 30*10 = 300 lbs, 30" span ... assume CP for the wing is 16" from the root ... 4800 in.lbs ... 6061T6 tube (fcy = 36ksi) OD = 1.25" ... required t = 0.16" ... weight of the spar ... 30"*0.55in^2*0.1 < 2lbs ... optimising the wall thicknes would save a little weight, but too difficult ? ... and we're not using the smaller tube ... that'll work mostly at locating the wing on the fuselage, 0.06" wall would be plenty.

if there is a transportation joint at the wing root (as i suspect) you get into some tricky sizing ... the two pieces have to fit like a socket and presumably quick-release pins ... but then maybe you already know this !?

good luck !

 

[Pfielgood]

Hi rb1957
Thanks for your post. Excellent. Nice to meet someone who knows what a Wirraway looks like )
I have a wealth of information I can share.
Yes, the tubes are planned to be the primary bending elements of the structure. The entire centre section of the wing will be constructed from a/.c grade ply ribs and sheeted in balsa, then fibreglassed. the centre section, of course, will be permanently fixed to the fuselage.
This "centre section" will be home for the retracts and facilitate easy transportation whilst the wheels are locked down. As you can imagine, transporting a 60lb model with 138" wingspan is not easy, hence the need for removable outer wing panels. The outer panels alone will each span 48" - so this is a very large model.
The centre section is completely flat, constant chord. This is where the fun begins. The outer panels have dihedral, which commences at the centre section break lines either side, of course. If outer (female) tubes are nested into the centre section in a straight line, the dihedral causes problems.....as you can imagine, if the you extrapolate a line outward from the centre section panel either side, the tube exits the underside of the wing panel rather quickly. What we would like to do, is have a long length of tube in the *panel* which would be epoxied in place during construction of said panel (ply ribs, box spar, full webbing etc) then introduce this panel and its protruding wing pole to the centre section....here's the trick, the dihedral would have to be achieved by angling the outer tube in the centre section. Its a little hard to explain, so I will organise some sketches. Our main concern is whether or not the t6061 will perform under load for both flying, and landing!
Now, if there was just a way of emailing you.. )
I will be back with some sketches.....
thanks for helping out.
Chris

 

[rb1957]

g'day (no piss take) chris,

you can probably see that i messed the moment calc ... only looked at the center wingspan. read your post 138" span ... 48" outer wing ... CP from joint more like 24", 50% increase in moment ... increase wall to something like 0.25", tip-to-tip weight = 10 lbs ... too much, you'll need to "optimise" it some (what machining can you do ?). and it'd need to be heavier on the inbd wing. maybe 10g is abit excessive, maybe 60lbs is conservative ?

why tube stock ? you have bushell's of it ?? square tube ? (would be easier to work with). how about a a break press (for bending sheet). my suggestion would be a clevis type of fitting. alternatively a welded tube fitting, just hate the idea of welds at the point of maximum load.


try me at rblake1225ATrogersDOTcom

 

[Compositepro]

If you are fiberglassing the wings then the skins must be load carrying. Otherwise there are much lighter skin materials.

 

[rb1957]

and the fiberglass resin is probably enough of a bond to attach a formed sheet metal web. then "heavy" fittings at the joint, "lick of paint"

 

[Pfielgood]

Thanks guys,

The fibreglass is .75oz cloth - more grain filling and hard wearing finish than anything - most large models use this technique for finishing - we vacuum bag it on so its a pretty good finish. Yes, the balsa skins are certainly load carrying.
60lbs will be pretty close to the final weight, could I suggest 5g ? If the tube length was 15" into the panel, and say 15" into the centre section, thats a 30" T6061 tube either side.....would tube with wall .035" handle 5g?
Thanks, I'll be in touch rb1957

 

[berkshire]

Pfielgood (Aeronautics)
Since the Wirraway is an Australian development of the North American N16 which was later followed by the T6 (Harvard to commonwealth types) and there are several 1/4 scale versions of T6'S around, could you not use the arrangements used on those aircraft?
B.E.

 

[Pfielgood]

Hi B.E - I haven't seen any 1/4 scale T6's on the market...would you have an example we could check out? cheers, Chris

 

[berkshire]

Pfielgood (Aeronautics)
Well I guess I opened my mouth and put my foot in it.
We had one guy in the San Diego area who had a model of an SNJ but I now find it only has an 80" wingspan which is not 1/4 scale.
A Google search produced just about anything in 1/4 scale except a T6 or variant thereof.
B.E.

 

[Pfielgood]

Hi B.E - I couldnt find anything either, at 138" wingspan it quite unique. ANyway, we were just hoping to find out what the shearing strength of Au. tube is at 1.25" dia and .035" wall with a 50-60lb aircraft...rb1957 has kindly offered to do some calcs for us. Thanks.Chris.