Existing single cantilever tubular spar wing designs?

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cluttonfred

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In a homebuilt context, Mike Whittaker’s various MW-series designs use plywood ribs bonded to tubular spars with fiberglass and polyester (not epoxy) resin. There are two rivets per rib just in case. See Plank Progress.
 

Riggerrob

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Grumman American Lynx, Traveller, Cheetah, etc. all started as the BD-1 kitplane. Jim Bede designed a variety of kitplanes with tubular wing-spars: BD-1, BD-4, BD-5, etc.
Dozens of BD-4s were built from plans and partial kits and are still flying today. BD-4 wings start with a tubular aluminum spar with fiberglass wings sections glued on. Each wing section includes one rib bonded to both top skin and bottom skin. These wing sections can be blanked off to become fuel tanks.
 

sohailshaikh

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Hi All,

My first post in this form hoping someone might have an answer or point me in the right direction for this newbie. I am designing a trike type single seat LSA with preliminary weights 200 lbs for the pilot, 200 lbs for engine and gas. My question is related with strength of main spar which I am thinking of using aluminum tube 6061-T6 for this purpose. The question I have is how do you determine what diameter and wall thickness for this tube based on the weight and is there any guidance, calculation, theory behind this determination? I have attached a sketch to explain what I am trying to do. I saw BD-5 used this type of tube for main spar.

Thanks
Sohail
 

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cheapracer

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Information I happen to have ..

The Grunman AA1 series spar outside diameters are 6.625” for the carry through and 6.75” for the wing.

In metric, 168.3mm for the carry though, with a 5mm wall thickness, and 171.3 mm for the wing panel, with wall thickness 2.6mm.

On the AA1, AA1B and AA1C both carry through and wing were 2024 T3.

On the AA1A, the carry through was 2024T3, and the wing was 6061T6.



The ends of the carry through and wing spars where the wing slides over the carry through are sized (expanded) to provide a precise fit, instead of relying on the tube manufacturing tolerances.

The clearance allowed is 020” or 0.5mm, and if it is more than this, shims have to be used. The overlap is 290mm long.



When I carried out a structural analysis to get clearance to do aerobatics, I did not find theoretical margins as big as 16.8 g.

I used the method of analyzing for ultimate load and testing to limit load, because the whole exercise was for an existing aircraft, and was to be a one-off modification.

Whereas the utility gross weight for the aircraft is 1600 lbs, I worked on an aerobatic weight of 1300 lbs, and test flew it to 6 g at this condition.



It is tricky to predict the ultimate failure load on a thin walled tube, because the first failure mode is generally local crippling of the wall.

I used curves from the Boeing Design Manual, which give allowable ultimate bending moments for thin walled tubes in terms of geometry and material.

Working the bending moments back to stresses gives values well below the material yield stress.

I think the curves are very conservative, and allow for a worst case situation.



Basing the ultimate load on the ultimate strength of the material will give a much higher margin, but there is no guarantee that the spar will carry that load without the walls buckling!
 

TFF

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To do it all the way, you figure out how much everything weighs and the G loading it needs to take. Then you take that information and figure out how strong the wing needs to be. You take that information and start matching it to the tubes strength. You probably will
have to figure out the tube strength in the directions that matter for a wing.
There is no table or chart to match what you are after. The only other way is to have plans and measurements of like aircraft, and guess what matches your needs.
 

TFF

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The AA1B and C got thicker spars over the AA1 and A. Outboard or inboard or both I don’t know.
 

Victor Bravo

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Also, do not forget to calculate/include the concept that the ribs serve as external tube stiffeners, delaying or reducing the deformation (flattening) of the tube that precedes buckling. Making the ribs "structural" in this capacity, and adding reinforcing "rings" in between the ribs, can possibly improve the performance of the spar by some significant amount.
 

BoKu

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...I am designing a trike type single seat LSA with preliminary weights 200 lbs for the pilot, 200 lbs for engine and gas. My question is related with strength of main spar which I am thinking of using aluminum tube 6061-T6 for this purpose. The question I have is how do you determine what diameter and wall thickness for this tube based on the weight and is there any guidance, calculation, theory behind this determination?...
****Warning: I am not an engineer. I've done a lot of design, and some engineering under supervision, but I have only a little bit of formal engineering training.****

The simple answer is that there isn't a simple answer. To have any degree of confidence you'll need to do some engineering, or have it done for you. In the case at hand, what you do is evaluate your proposed wing planform to determine its spanwise lift distribution. For a conventional wing, I'd use Schrenck's Approximation, but I have no idea whether that would be valid enough on a delta or Rogollo planform. Given that, then you can use a bit of calculus (or MS Excel) to find the bending moment at any chordwise slice through the wing at any given load factor. From there, you use conventional beam theory to find the combination of outside and inside diameters of tubing that has adequate bending strength and stiffness everywhere along the span to resist the bending applied by the lift at some reasonable load factor plus some factor of safety. At that stage you'll take into account any external bracing and its effect on both the bending and shear distribution.

That process gives you the basic parameters for the main spar. From there you need to do a bunch of detail design to figure out how all the parts attach to each other and to the airframe.
 

rbarnes

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I always wondered why no one else used glue bonded aluminum skins. Seems like a great idea for cutting down production time/cost and reducing drag. I loved my Tiger I owned for years. Zipped along faster than anything else in class, and heck faster than some retracts.
 

sohailshaikh

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****Warning: I am not an engineer. I've done a lot of design, and some engineering under supervision, but I have only a little bit of formal engineering training.****

The simple answer is that there isn't a simple answer. To have any degree of confidence you'll need to do some engineering, or have it done for you. In the case at hand, what you do is evaluate your proposed wing planform to determine its spanwise lift distribution. For a conventional wing, I'd use Schrenck's Approximation, but I have no idea whether that would be valid enough on a delta or Rogollo planform. Given that, then you can use a bit of calculus (or MS Excel) to find the bending moment at any chordwise slice through the wing at any given load factor. From there, you use conventional beam theory to find the combination of outside and inside diameters of tubing that has adequate bending strength and stiffness everywhere along the span to resist the bending applied by the lift at some reasonable load factor plus some factor of safety. At that stage you'll take into account any external bracing and its effect on both the bending and shear distribution.

That process gives you the basic parameters for the main spar. From there you need to do a bunch of detail design to figure out how all the parts attach to each other and to the airframe.
Thanks.
 

Victor Bravo

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I always wondered why no one else used glue bonded aluminum skins. Seems like a great idea for cutting down production time/cost and reducing drag.
It can be done safely and reliably in a controlled factory environment, but it's not simple or reliable in a homebuilder environment. I was informed that there are chemical conversion processes that make aluminum reasonably bondable, but it is my understanding that without that process... aluminum absolutely hates to be glued to anything.
 

rbarnes

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It can be done safely and reliably in a controlled factory environment, but it's not simple or reliable in a homebuilder environment. I was informed that there are chemical conversion processes that make aluminum reasonably bondable, but it is my understanding that without that process... aluminum absolutely hates to be glued to anything.
Well Grumman definitely had some bonding issues with certain glues on AA5's, but I've also seen an A&P try and remove the top wing skin on AA5 wing and the metal wanted to tear before the glue would let go.
 
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trimtab

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Is anyone familiar with how the torques on the spar were constrained on the Grummans/Bedes? The photos I have seen appear to show only a thru-drilled clevis. Is this the ONLY torque reaction in the design? I ask because of the large moment differences that seem to show up in my napkin calculations during slow flight vs cruise.

For the questions regarding sizing of spars, I'd recommend Gere & Timoshenko for a text on how it is done (calculating principal stresses with bending and torque inputs, Euler stability, etc). It really does not take a great deal of study to understand these things for simple prisms like a circular tube, and they are the basis of further understanding of buckling.
 

BrianW

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I reached for "Build Your Own Airplane" which I bought at a Californian book store around 1978. It's Bede's design manual for the BD-4. The BOM specifies 2024-T3, 6.410" X 6.680" 10 ft extrusions plus a 6.00 X 6.40" extrusion 5.5 ft long for the center section. the rib/surface panels were epoxied to the tube. That was the spar specified for 1600 to 2000 lb gross structures. It was quite a swift airplane, by all accounts.
 

TFF

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Tubular spar screams Bede. I guess the only one that did not have one was the BD10. It’s smart for simplicity. From BD1/Yankee to his last, he must have had stock in the aluminum tube factories.
 
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