Help Support HomeBuiltAirplanes.com:

maya.ayoub.32

Well-Known Member
HBA Supporter
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

Well-Known Member
HBA Supporter
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

Super Moderator
Staff member
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

Well-Known Member
HBA Supporter
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!

Last edited:

maya.ayoub.32

Well-Known Member
HBA Supporter
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!

reo12

Member
View attachment 92236

note the clear Tedlar covering aft of the D tube, attached with tenacious double sided tape. (Although the tired restoration project pictured clearly shows some of that has let go)
The Tedlar film is very durable stuff. However - the acrylic adhesive of the tapes used to secure it to the aircraft are greatly effected by UV rays. Both from the sun above and from ground reflection. Decades ago I looked at buying Solaris - which was the Argentinian Falcon built by Carlos Pereyra before he came to the USA and started building the Aventura. The Solaris was also covered with Tedlar but they added strips of aluminum foil tape over every area that had tape securing the wing cover. This added years of additional life to the underlying structural tapes.

ElectricFlyer

Well-Known Member
HBA Supporter
Your question is not silly at all; the French APEV used ladders on her Pouchel:
View attachment 92137

Here you will find the reason, why they were obliged to change:
www.pouchel.com The website of the APEV

"..... Due of the success of the Pouchel (over 120 plans sets sold to members of the APEV), our ladder manufacturer became concerned about his legal liability and no longer agreed to sale us some potentially flying ladders ! Therefore, we launched the design study for the Pouchel II. ...."
Love it -- any pusher designs done of this - didn't see any on the web site
Cheers

Protech Racing

Well-Known Member
Mine was pusher with ladders for spars .