Ultralight struts/cantilever/additional weight

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maya.ayoub.32

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1700# at 4g, OK (gross weight 425#?). 4g is your limit load factor, which is the maximum you expect to see in flight. That gets multiplied by a 1.5 safety factor to get the design load, so you're actually designing for 6g or 2550#, or 1275# vertical load per side. The actual tension in the strut is that vertical load divided by the sine of the angle, so if, say, the strut is 30°, 180/sin(30) = 2550# tension. Assuming two bolts per attachment equally loaded, that's 1125# per bolt. "But wait, there's more!" For bolted or riveted connections, an additional 20% safety factor must be applied to allow for things like misdrilled or misaligned holes, so now you're looking at 1350# per bolt.
Hi Dana!
I was actually going to ask what a recommended safety factor would be so thank you for the 1.5! Thank you for pointing out my mistake, I was assuming all the lift bearing bolts would be counted in to the equation, which was obviously not the way to do it.

I definitely wasn’t thinking of converting between vertical load and tension, so it’s very good to know that there is an equation for this. Thank you for a walk through bearing stress!

We haven’t finished our load distribution or spar placement calculation yet, so to be conservative I’ll assume 23 degrees. With this assumption and the conversion to tensile strength, it would require 3 bolts (Of course, I’ll recalculate this with our actual degree once we calculate it) So I’m very appreciative of the time you put in to helping me through this, or this attachment may have failed!

Sorry, I should have been more specific, but I the thickness I was worried about the distance from the edge of the bolt to the bottom of the spar. So the shear stress on the spar, which I’ve calculated is fine as well. (I used our bearing stress on the attachment and divided it by inches to the bottom* wall thickness).

Thank you again!
 

maya.ayoub.32

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we are discussing two different things. My reference is to a picture of what RESEMBLES a Legal Eagle spar but appears to be cable braced due to the fitting bolted in. Eagles use wood compression struts and diagonal braces. Not cables.
I stand by my statement that none of the 4 Eagle variants have strut fitting mounting holes drilled into the spar caps. If this is indeed an Eagle then a LARGE deviation from the drawings was made. jb
Gotcha! Sorry about that! Looks like I need to refresh my knowledge on the Legal Eagle plans
 

Dana

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I should point out one thing... I talked about determining the loads on the bolts, but the load on the strut isn't half of the aircraft's weight times the design load factor, it will be less since the wing root is supporting some of the load if the strut attaches more than halfway out to the tip, typically it'll be somewhere between the halfway point and 2/3 out so you have to work out the sharing of the load. Also you deduct the wing weight from the strut calculation since the wing is carrying itself; the strut doesn't support the weight of the wing except during landing, which will be a lower load case than inflight negative g.
 

maya.ayoub.32

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Apr 6, 2020
Messages
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Adding to what Dana has said, the closer your strut angle gets to 90, the closer it gets to whatever the actual half span loading is, and the contrary is also true. The shallower the strut angle, the more the strut tension and corresponding bolt stresses. The relationship is, as Dana said, sinusoidal and not linear. There is also a corresponding columnar compression loading to the spars inboard of the strut attach. In your analysis, don’t forget to consider this compressive load as it is simultaneously being applied in conjunction with the transverse loads associated with lift production. Euler has some methods to analyze that combined loading. Think of the trick where your friend stands on an empty soda can, and one tiny sideways deformation collapses the whole thing.
Very good point! I was intrigued by how the graph would look like since it's sinusoidal, so I graphed it using variablized measurements. Here is a very variablized desmos graph for anyone who wants to calculate their own bearing stress: Spar-Strut Attachment
Our physics team learned column bucking a little bit ago, but we haven't done any of the major calculations yet. We will definitely take spar column compression and buckling into consideration while deciding strut placement, and attachment methods! Thanks!
 
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maya.ayoub.32

Active Member
Joined
Apr 6, 2020
Messages
28
I should point out one thing... I talked about determining the loads on the bolts, but the load on the strut isn't half of the aircraft's weight times the design load factor, it will be less since the wing root is supporting some of the load if the strut attaches more than halfway out to the tip, typically it'll be somewhere between the halfway point and 2/3 out so you have to work out the sharing of the load. Also you deduct the wing weight from the strut calculation since the wing is carrying itself; the strut doesn't support the weight of the wing except during landing, which will be a lower load case than inflight negative g.
Oh interesting! So the tension force on spar near the strut attachment will actually be around half, but we're going to have to calculate the exact forces. So looks like we're going to have to add more bolts to our wing root attachment since it's designed very similarly to the affordaplane. Similar in that it's two U brackets attached to main and aft spars. I'll definitely calculate how many bolts as soon as strut placement is finalized.
Our final weight will be 484lb (including our truss, ballistic parachute, pilot, and supplies) and our wing will weigh somewhere around 60lb, so we've been using 424lb in the strut calculations. However, this is subject to changing as the design changes! Thanks for your help!
 
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