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Wing Strut Attachment

 
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John Black



Joined: 23 May 2016
Posts: 7

PostPosted: Thu Jan 04, 2018 8:17 am    Post subject: Wing Strut Attachment Reply with quote

Today I am worrying about how to attach the Struts.

What are you guys doing to attach the Struts ?

The top Strut fitting is especially challenging.

I am using a Carlson Aircraft extrusion for my struts.
[img]cid:21CD3E1D-CCAC-4D81-A3F6-EE7BBA85302F(at)wavecable.com[/img]
To attach the strut, Carlson sells a 1”x3/4” aluminum extrusion that slides into the end of the strut. Then you bolt it in place. I cut this extrusion into 8” lengths. I put this 8” piece in the lathe and end drilled and threaded a 7/16 hole 2” deep into the strut.
[img]cid:AF7B0EE5-59A4-43DC-A495-64912225D768(at)wavecable.com[/img]

On the lower end I plan to use this wing strut adjustment fork from Aircraft Spruce. https://www.aircraftspruce.com/catalog/appages/strutadjfbsets2.php
[img]cid:1D8486E5-D3E3-4EF5-8FE2-6FFB345B512B(at)wavecable.com[/img]

The top end is more challenging. The wing may move forward or back to adjust weight/balance. This movement changes the angle that the strut meets the wing… so the attachment needs to be flexible. I see that some builders have used a helm joint. This is a simple solution, but I wonder about the loads on the fitting. The strongest Helm joint that I could find is rated at 14,000 lbs :

[img]cid:D756EA10-469A-41CA-AB3A-619CA2916B86(at)wavecable.com[/img]

That is a little weaker than the strut.

What are you guys doing ?

John


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PostPosted: Thu Jan 04, 2018 1:19 pm    Post subject: Wing Strut Attachment Reply with quote

John, actually the Heim rod end is stronger than the strut.  Note that the cross sectional area of the strut is indicated as 0.431 square inches (si).  According to the numbers you reported, this means the ultimate strength is 18,012 psi * 0.421 si = 7763 lbs; the yield strength is 15,947 psi * .431 si = 6873 lbs; and the shear strength is 11,637 psi * 0.431 = 5016 lbs.  I have a chart that shows the maximum shear strength of sitka spruce as 1150 psi, which means the wood where the brackets attach the struts to the spars is likely to be weaker than any component in the struts that you are considering.   And that wood is still plenty strong if the wings are built to plans.  Cheers, Ken 

On Thu, Jan 4, 2018 at 9:14 AM, John C Black <john(at)jcblack.com (john(at)jcblack.com)> wrote:
Quote:


Today I am worrying about how to attach the Struts. 
What are you guys doing to attach the Struts ?
The top Strut fitting is especially challenging.   
I am using a Carlson Aircraft extrusion for my struts.  
[img]cid:21CD3E1D-CCAC-4D81-A3F6-EE7BBA85302F(at)wavecable.com[/img] 
To attach the strut, Carlson sells a 1”x3/4” aluminum extrusion that slides into the end of the strut.   Then you bolt it in place.    I cut this extrusion into 8” lengths.   I put this 8” piece in the lathe and end drilled and threaded a 7/16 hole 2” deep into the strut.  
[img]cid:AF7B0EE5-59A4-43DC-A495-64912225D768(at)wavecable.com[/img]
On the lower end I plan to use this wing strut adjustment fork from Aircraft Spruce. https://www.aircraftspruce.com/catalog/appages/strutadjfbsets2.php
[img]cid:1D8486E5-D3E3-4EF5-8FE2-6FFB345B512B(at)wavecable.com[/img]
The top end is more challenging.  The wing may move forward or back to adjust weight/balance.  This movement changes the angle that the strut meets the wing… so the attachment needs to be flexible.   I see that some builders have used a helm joint.  This is a simple solution, but I wonder about the loads on the fitting.  The strongest Helm joint that I could find is rated at 14,000 lbs :
[img]cid:D756EA10-469A-41CA-AB3A-619CA2916B86(at)wavecable.com[/img]
That is a little weaker than the strut.
What are you guys doing ?
John


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Dan Helsper



Joined: 28 Dec 2016
Posts: 18

PostPosted: Thu Jan 04, 2018 1:39 pm    Post subject: Wing Strut Attachment Reply with quote

Lower end I used the J-3 fork from Wag Aero, threaded into: drilled and tapped a piece of 7071 aluminum from Mcmaster-Carr. Upper end also uses a 7071 insert held fast with two AN-4 bolts. "Small" Carlson struts.

Dan Helsper
Loensloe Airfield
Puryear, TN


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PostPosted: Thu Jan 04, 2018 4:58 pm    Post subject: Wing Strut Attachment Reply with quote

John and others, I received the following as a private message this afternoon:

"I sometimes lurk on the Piet List and saw your posting today. The Carlson numbers are the area times the various strengths of the 6061-T6 aluminum. As such they aren't stresses but strengths and their units ought to be pounds force instead of psi. They neglect thickness or area moment of inertia and represent nothing more than the strengths of a short thick bar. By ignoring shape effects, they are inappropriate for use in design. Sorry, but this does make your posting incorrect. Additionally, the equivalent round tube sizes are best ignored completely. They are of no help. It would have been helpful if Carlson had posted the area moments of inertia, but they didn't. Who the heck am I? I'm an old retired aerospace stress analyst."
Note that the old retired aerospace stress analyst doesn't offer a bottom line conclusion as to whether the Heim rod end is sufficiently strong to use or even whether the Carlson struts are sufficiently strong or that the whole thing would still be stronger than the wood to which it is attached, only that my posting is incorrect.  
Nevertheless, I'll defer to superior authority and retract what I posted earlier.  
Ken
On Thu, Jan 4, 2018 at 2:19 PM, Ken Bickers <bickers.ken(at)gmail.com (bickers.ken(at)gmail.com)> wrote:
Quote:
John, actually the Heim rod end is stronger than the strut.  Note that the cross sectional area of the strut is indicated as 0.431 square inches (si).  According to the numbers you reported, this means the ultimate strength is 18,012 psi * 0.421 si = 7763 lbs; the yield strength is 15,947 psi * .431 si = 6873 lbs; and the shear strength is 11,637 psi * 0.431 = 5016 lbs.  I have a chart that shows the maximum shear strength of sitka spruce as 1150 psi, which means the wood where the brackets attach the struts to the spars is likely to be weaker than any component in the struts that you are considering.   And that wood is still plenty strong if the wings are built to plans.  Cheers, Ken 

On Thu, Jan 4, 2018 at 9:14 AM, John C Black <john(at)jcblack.com (john(at)jcblack.com)> wrote:
Quote:


Today I am worrying about how to attach the Struts. 
What are you guys doing to attach the Struts ?
The top Strut fitting is especially challenging.   
I am using a Carlson Aircraft extrusion for my struts.  
[img]cid:21CD3E1D-CCAC-4D81-A3F6-EE7BBA85302F(at)wavecable.com[/img] 
To attach the strut, Carlson sells a 1”x3/4” aluminum extrusion that slides into the end of the strut.   Then you bolt it in place.    I cut this extrusion into 8” lengths.   I put this 8” piece in the lathe and end drilled and threaded a 7/16 hole 2” deep into the strut.  
[img]cid:AF7B0EE5-59A4-43DC-A495-64912225D768(at)wavecable.com[/img]
On the lower end I plan to use this wing strut adjustment fork from Aircraft Spruce. https://www.aircraftspruce.com/catalog/appages/strutadjfbsets2.php
[img]cid:1D8486E5-D3E3-4EF5-8FE2-6FFB345B512B(at)wavecable.com[/img]
The top end is more challenging.  The wing may move forward or back to adjust weight/balance.  This movement changes the angle that the strut meets the wing… so the attachment needs to be flexible.   I see that some builders have used a helm joint.  This is a simple solution, but I wonder about the loads on the fitting.  The strongest Helm joint that I could find is rated at 14,000 lbs :
[img]cid:D756EA10-469A-41CA-AB3A-619CA2916B86(at)wavecable.com[/img]
That is a little weaker than the strut.
What are you guys doing ?
John





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taildrags



Joined: 29 Dec 2009
Posts: 1591
Location: Medford, OR

PostPosted: Thu Jan 04, 2018 8:44 pm    Post subject: Re: Wing Strut Attachment Reply with quote

Oh, I love this!! Area moments of inertia! ;o) The easy way to determine that value for just about any shape you can draw is to draw it to scale in model space in AutoCAD and then use the 'massprop' command. That's how I determined the moment of inertia for the Carlson extruded aluminum spar that Terry and others are using. Draw the outline of the inner and outer surfaces of the shape as Plines. Make the area a region by selecting the lines that comprise the cross-section. Enter 'massprop' and select the shape, and you should move the centroid to the 0,0,0 point to do that. Massprop will spit out the area moment of inertia faster than you can say "Pietenpol Air Camper" ;o)

When I was analyzing the extruded wing spar, what I really needed was the section modulus, which for the spar is equal to the moment of inertia divided by half the height. The spar being symmetrical and us looking for the maximum stress (which occurs at the outermost fibers farthest from the centroid), that's half the height. It could be taken anywhere along the section if needed for your particular analysis, especially for an asymmetrical shape.

I could probably draw the Carlson extruded strut section in CAD from the catalog graphic and the dimensions provided, and extract the area moment of inertia pretty easily from that, but then I would be depriving someone else of the fun ;o)


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jarheadpilot82



Joined: 21 Mar 2011
Posts: 696
Location: Athens, GA

PostPosted: Fri Jan 05, 2018 8:57 am    Post subject: Re: Wing Strut Attachment Reply with quote

Oscar,

I know that you love this stuff, so how about doing all of us non-engineer types a favor and do the math. I have done my initial w&b and my cabanes definitely lean back so I am most likely going to need to use a ball joint rod end as well for connecting at the wing strut end.

And Oscar, FWIW, my daughter is a Junior at Alabama and is a Metallurgical Engineering major there. I told her that I couldn't even spell metallurgical engineering without spell check, much less study it. And a 3.3 GPA to boot! (Statics and Physics beat her up pretty good last semester.)


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Bill Church



Joined: 21 Feb 2008
Posts: 932
Location: Ontario, Canada

PostPosted: Fri Jan 05, 2018 11:25 am    Post subject: Re: Wing Strut Attachment Reply with quote

The data posted on Carlson's website definitely appears to be suspect. The raw material of the struts, 6061-T6 Aluminum has an Ultimate Tensile strength of about 42000 psi, which, when applied on a cross-sectional area of 0.431 square inches would translate into an ultimate load of just over 18000 pounds (assuming that the listed 0.431 square inch area is correct.)

The topic of appropriate size for rod ends on lift struts was discussed several years ago. This link should answer the original question:
http://forums.matronics.com/viewtopic.php?t=50044&start=0

Bill C.


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John Black



Joined: 23 May 2016
Posts: 7

PostPosted: Fri Jan 05, 2018 3:38 pm    Post subject: Wing Strut Attachment Reply with quote

THANK YOU EVERYONE for great posts, but I still don’t know exactly what I should do.
I have read as suggested…. learned a lot…. I take back my errors…. Thank you again.

Here are the questions that I still have :

1) Is Aluminum 6061-T6 adequate for the 1”x3/4” Bar Stock that inserts into the Carlson Small Strut and is end drilled plus tapped to accept the 7/16” 20 male end fittings ? Or do I need to use steel which about triples the weight of the Bar Stock ? I get lost trying to understand if at least 1” of 7/16”-20 threads in 6061-T6 aluminum is strong enough ? Or Is steel needed ?

2) Does the side wall of the Carlson Small Strut need to be reinforced ? If so, Is it enough to weld aluminum washers at the bolt holes or should a 3/4” or 1” plate be welded on that spans all 3 of the 5/16” bolts that I plan to use ?

3) Who has a Tension Test machine ? I am beginning to look for a Tension Test stand. I want to build a test piece and test it to destruction.

Thank you Ken Bickers for reminding me that the aluminum strut arrangement probably is adequate if it is stronger than the Sitka Spruce struts that are flying around.

John
On Jan 4, 2018, at 8:14 AM, John C Black <john(at)jcblack.com (john(at)jcblack.com)> wrote:
Today I am worrying about how to attach the Struts.

What are you guys doing to attach the Struts ?

The top Strut fitting is especially challenging.

I am using a Carlson Aircraft extrusion for my struts.
[img]cid:21CD3E1D-CCAC-4D81-A3F6-EE7BBA85302F(at)wavecable.com[/img]
To attach the strut, Carlson sells a 1”x3/4” aluminum 6061-T6 bar that slides into the end of the strut. Then you bolt it in place. I cut this bar into 8” lengths. I put this 8” piece in the lathe and end drilled and tapped a 7/16 hole 2” deep into the strut.
[img]cid:AF7B0EE5-59A4-43DC-A495-64912225D768(at)wavecable.com[/img]

On the lower end I plan to use this wing strut adjustment fork from Aircraft Spruce. https://www.aircraftspruce.com/catalog/appages/strutadjfbsets2.php
[img]cid:1D8486E5-D3E3-4EF5-8FE2-6FFB345B512B(at)wavecable.com[/img]

The top end is more challenging. The wing may move forward or back to adjust weight/balance. This movement changes the angle that the strut meets the wing… so the attachment needs to be flexible. I see that some builders have used a helm joint. This is a simple solution, but I wonder about the loads on the fitting. The strongest Helm joint that I could find is rated at 14,000 lbs :

[img]cid:D756EA10-469A-41CA-AB3A-619CA2916B86(at)wavecable.com[/img]

That is a little weaker (I was wrong… apparently stronger) than the strut.

What are you guys doing ?

John


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taildrags



Joined: 29 Dec 2009
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PostPosted: Sat Jan 06, 2018 10:07 am    Post subject: Re: Wing Strut Attachment Reply with quote

John; I'm running some calculations to determine where the weakest link is in the string of things between the wing attachment fitting for the lift strut and the attachment to the fuselage at the bottom end. I'll post the results here when I'm done.

In the meantime, I'm intrigued by your mention of building a test stand to load-test a sample to failure in tension. This is easily done if you want to "backyard" it. Simple hardware and lumber, but be prepared to be surprised at what it takes to fail the assembly. Let's just pick one of the numbers that you threw out there, the Heim joint at 14,000 lbs. If you have some nice heavy-wall steel pipe out there, you can develop that kind of load by using a 5 ft chunk of that pipe, mounting one end on something very sturdy using a bolt or other attachment that will allow the pipe to pivot, and then attaching your test piece 1 ft away from the pivot end of the pipe. You now have a 4:1 lever and you can get 14,000 lbs on the test piece by loading the long end of your pipe with 2800 lbs of "something". One such "something" is a fairly common 2-ton "come-along" (Maasdam Pow'r-Pull). I have one in my truck. Using a come-along, you can gradually load the test piece but since you don't know what's going to fail or when it's going to fail, you should use an extension on the come-along handle to get you some distance away and put up a plywood shield or something you can work behind when you conduct the test.

If you have something stronger than pipe, you can make the lever arm longer to get more advantage. For example, if you have an 8 ft section of steel I-beam, you could develop a 7:1 advantage and only need to put 1,750 lbs on the long end, which you can now do with just a 1-ton come-along. If you wanted to know how much load you were putting on it as you loaded it, you could attach a 200 gallon container to the long end and start filling it with water (8.34 lbs/gal), with marks on the container to let you know how much load was accumulating. I know, this is getting ludicrous, but you get the point... you don't need a special load testing apparatus to run your test.

I prefer to just run the calculations instead of conducting exciting load tests like this, especially since there are so many Air Campers flying around safely out there with all sorts of lift strut arrangements.


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taildrags



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PostPosted: Sat Jan 06, 2018 10:41 am    Post subject: Re: Wing Strut Attachment Reply with quote

John: also, in regard to thread engagement, there are some general rules of thumb as to how many threads need to be engaged in order for the full strength of the connection to be developed. The question you're asking has to do with a steel fastener threading into an aluminum bar, so the question has two answers... one for the steel fork end fitting and another for the bar it's threaded into. The *general* rule for a steel part is that it must be engaged at least 1x its diameter in order to develop full strength. The fork end in question has a 3/8-24 thread, so at least 3/8" has to be engaged in the threads of the mating part for the fork end to develop its full strength in tension. For a 24 threads-per-inch threading, that would be at least 9 threads engaged.

However, for *aluminum*, the rule of thumb is 1.5x the diameter due to the lower shear strength of the material. Again for the same thread pitch and diameter, that would be about 11 threads engaged, or roughly 1/2" of full engagement for the aluminum bar to receive the full load being transferred to it by the fork end. More threads are better though ;o)

Speaking of rules for threads in general, the first thread always takes the most load and it drops off with successive threads as you come out. Here's the theoretical breakdown as you add threads: 1st thread takes 34%; 1st and second combined take 57%; 1st, 2nd, and 3rd combined take 73%; 1st through 4th take 84%, 1st through 5th take 91%; 1st through 6th threads take 98%. It would seem, then, that all you would need is 6 or 7 threads engaged even in aluminum, but the problem is the shear strength because when a threaded fitting fails by stripping, once the first thread strips the following ones strip in rapid succession so it's important to prevent a pull-out failure by thread stripping. If you've worked in the shop any length of time, you know that sickening feeling when you're putting some muscle onto a ratchet on a nut and it strips. It all goes mushy and that fastener is done for.


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taildrags



Joined: 29 Dec 2009
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PostPosted: Sat Jan 06, 2018 10:50 am    Post subject: Re: Wing Strut Attachment Reply with quote

John: found just the thing for your test rig: Northern Tool has a 3-ton hydraulic jack for only $60! No problem developing a couple of thousand pounds of load on that test rig ;o)

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Dan Helsper



Joined: 28 Dec 2016
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PostPosted: Sat Jan 06, 2018 2:04 pm    Post subject: Wing Strut Attachment Reply with quote

Here is an example that has been flying for 7 years. Upper strut attachment. Carlson "small" strut, and Mcmaster-Carr 7071 alloy insert, with two AN4 bolts.

Dan Helsper
Loensloe Airfield
Puryear, TN


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Location: Medford, OR

PostPosted: Sat Jan 06, 2018 3:45 pm    Post subject: Re: Wing Strut Attachment Reply with quote

I’ve taken a look at the forces in the wing lift struts of the Air Camper. To run the analysis, I’ve used the dimensions given in the Orrin Hoopman (1933-34) plans to create the geometry, but there’s not a lot of difference in any of the others that are commonly used. The main lift strut length is given as 89-3/4” between the bolt holes, the front cabane length is given as 21-1/4”, and the distance from the cabane mounting point to the lift strut mounting point at the bottom of the fuselage side is 22-3/4”, so that side of the triangle is 44”. With those two dimensions, we find that the angle that the lift strut makes with the bottom of the wing is 29.4 degrees.

Assuming an aircraft gross weight of 1,088 lb and a design loading of +3.8G (normal category aircraft), we get a total load to be supported by the wing of 4,134 lb, and each half of the wing thus has to support 2,067 lb. According to the source quoted in my spar strength article in the BPA Newsletter (Noel Becar’s analysis in a 1963 Sport Aviation), the main spar in a strut-braced monoplane at a high positive angle of attack carries about 84% of the total load, so that results in 1,737 lbs that each front lift strut carries.

If the load on the spar where the lift strut attaches is 1,737 lbs and the strut is at an angle of 29.4 degrees to the wing, the tensile force in the strut will be 1737/(sin 29.4) = 3,538 lbs. This is the force that every element in the lift strut assembly must meet or exceed in order for the wing to sustain +3.8G loading.

Let’s start with the item that everybody is asking about, which is the “small” Carlson extruded aluminum strut itself. I have graphically checked the value that Carlson gives for the cross-sectional area of aluminum material in the strut, 0.431 sq.in., and my number comes very close to theirs so I’ll use theirs. Assuming that there are no obvious stress concentrating aspects of the cross-section and that it all gets evenly loaded by transfer from the attachments at the ends, and that the tensile strength of 6061-T6 aluminum is 45,000 psi, the strut should be able to carry a tensile load of 19,395 lbs. The Carlson literature gives it as 18,012 lbs so we’ll use their number. This is about 5 times more than what it will need to carry at +3.8 G of wing loading. The shape is important in the other regime of *negative* G loading (compression) and in bending, but that’s a separate topic from just the loading in tension under wing lifting condition. I agree with the person who stated that the information that Carlson gives about round aluminum tubing is confusing and of no use in strength analysis. They compare their strut to a 12.75” diameter, 0.065” wall tube... absurd. The closest match to their strut and a round tube with that wall thickness is a 2-1/4” OD 6061-T6 tube, which has a metal area of 0.446 sq.in., a weight of 0.533 lb/lin.ft., and a tensile strength of 20,079 lb. It is obvious by inspection that the Carlson strut will have significantly lower aerodynamic drag than a round 2-1/4” tube, which along with cost is just about the only useful comparison between the two.

Moving along the strut to the ends, the fork ends won’t be a problem... with nominal AN6 ends (3/8”) and 125,000 psi steel strength, they’re good for at least 13-14,000 pounds in tension... about 4 times the needed strength. The catalog touts them as high strength, but no strength value is given. The Heim ends appear to have the same strength in tension as the forks, so they shouldn’t be a problem either.

The trickiest part is the attachment between the fork (or Heim) ends and the lift strut. Looking at the 1” x 3/4” 6061-T6 bar that Carlson uses, when undrilled it has a cross-sectional area of 0.75 sq.in. so it’s good for about 33,750 pounds in tension but at the spot where the first mounting hole is drilled through it to attach it to the strut, the cross-sectional area is reduced to about 0.5625 sq.in. and the capacity drops to 25,312 pounds. Still about 7 times what’s needed. The aluminum bar is just fine; steel is not necessary.

Where the fork ends are threaded into the bar stock, the edge distance of the hole in the end of the bar is about 3/16” on the short side, so that shouldn’t be a limiting factor. If a sufficient number of threads of the fork end are engaged into the threaded hole in the bar, that won’t be the weak point either. The problem then comes down to the end of the bar that slides up into the hollow end of the strut and how the two are connected. Since the bar is much thicker than the wall of the lift strut, the weakest point of the connection is at the mounting bolt holes through the sides of the strut. Although there are rigorous methods for determining the load distribution between the bolts in a multi-bolt connection, a fairly safe assumption in this case is that if the bolts are not placed too close to one another or too close to the edges of the material, at ultimate loading all of the bolts will be carrying about the same amount of load. Let’s look at the load at the bolt holes through the strut wall nearest to the end of the strut. There is no need to look at shear strength of the AN4 connecting bolts.... they will be far stronger in shear than the thin walls of the strut.

There are at least two possible modes of failure at the connecting bolts but I’ll look only at “shear out” of the bolt, where the steel bolt pulls or tends to elongate the hole in the softer aluminum as it yields. In that mode, the force that the bolt can carry before the hole in the aluminum shears out is equal to the shear strength of the material (extruded 6061-T6 is about 24,000 psi) times the area that the bolt bears on. For AN4 bolts and a strut wall thickness of 0.119” at its thinnest point, with the first bolt in the row being about 3/4” from the end of the strut (such as in the image Piet_construction_449.jpg that Dan Helsper shared earlier), that works out to about 8,568 lbs for the first bolt in the row. That one alone should be able to carry twice the required load, so two bolts through each end of the strut should be more than adequate but if they give you three holes, use three bolts. The distance of the first bolt hole from the end of the strut is very important though... if the edge distance is reduced from 3/4” to 1/2” (for example), the pull-out load drops to about 5,712 lbs... only about 1.6 times the required capacity. With at least two attachment bolts, the connection should be adequate, but by all means keep the first bolt hole comfortably away from the end of the strut.


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Oscar Zuniga
Medford, OR
Air Camper NX41CC "Scout"
A75 power, 72x36 Culver prop
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John Black



Joined: 23 May 2016
Posts: 7

PostPosted: Sat Jan 06, 2018 4:05 pm    Post subject: Wing Strut Attachment Reply with quote

Oscar,

WELL DONE !!

Thank God you are interested in Pietenpols !!

This really helps.

John

PS… Regarding doing a destruction test to confirm… I think I have borrowed 6 Jersey barriers weighing about 2 tons each to anchor my test piece to the ground. Several guys around here have cranes and other construction equip that can lift 10 tons. Creating a test piece is no problem. The problem is how to measure how much force is being applied as we lift. Some cranes estimate force. All this is crude. I’m trying to find a universtity that has a machine set up to do this with some accuracy.

On Jan 6, 2018, at 3:45 PM, taildrags <taildrags(at)hotmail.com> wrote:



I’ve taken a look at the forces in the wing lift struts of the Air Camper. To run the analysis, I’ve used the dimensions given in the Orrin Hoopman (1933-34) plans to create the geometry, but there’s not a lot of difference in any of the others that are commonly used. The main lift strut length is given as 89-3/4” between the bolt holes, the front cabane length is given as 21-1/4”, and the distance from the cabane mounting point to the lift strut mounting point at the bottom of the fuselage side is 22-3/4”, so that side of the triangle is 44”. With those two dimensions, we find that the angle that the lift strut makes with the bottom of the wing is 29.4 degrees.

Assuming an aircraft gross weight of 1,088 lb and a design loading of +3.8G (normal category aircraft), we get a total load to be supported by the wing of 4,134 lb, and each half of the wing thus has to support 2,067 lb. According to the source quoted in my spar strength article in the BPA Newsletter (Noel Becar’s analysis in a 1963 Sport Aviation), the main spar in a strut-braced monoplane at a high positive angle of attack carries about 84% of the total load, so that results in 1,737 lbs that each front lift strut carries.

If the load on the spar where the lift strut attaches is 1,737 lbs and the strut is at an angle of 29.4 degrees to the wing, the tensile force in the strut will be 1737/(sin 29.4) = 3,538 lbs. This is the force that every element in the lift strut assembly must meet or exceed in order for the wing to sustain +3.8G loading.

Let’s start with the item that everybody is asking about, which is the “small” Carlson extruded aluminum strut itself. I have graphically checked the value that Carlson gives for the cross-sectional area of aluminum material in the strut, 0.431 sq.in., and my number comes very close to theirs so I’ll use theirs. Assuming that there are no obvious stress concentrating aspects of the cross-section and that it all gets evenly loaded by transfer from the attachments at the ends, and that the tensile strength of 6061-T6 aluminum is 45,000 psi, the strut should be able to carry a tensile load of 19,395 lbs. The Carlson literature gives it as 18,012 lbs so we’ll use their number. This is about 5 times more than what it will need to carry at +3.8 G of wing loading. The shape is important in the other regime of *negative* G loading (compression) and in bending, but that’s a separate topic from just the loading in tension under wing lifting condition. I agree with th!
e person who stated that the information that Carlson gives about round aluminum tubing is confusing and of no use in strength analysis. They compare their strut to a 12.75” diameter, 0.065” wall tube... absurd. The closest match to their strut and a round tube with that wall thickness is a 2-1/4” OD 6061-T6 tube, which has a metal area of 0.446 sq.in., a weight of 0.533 lb/lin.ft., and a tensile strength of 20,079 lb. It is obvious by inspection that the Carlson strut will have significantly lower aerodynamic drag than a round 2-1/4” tube, which along with cost is just about the only useful comparison between the two.

Moving along the strut to the ends, the fork ends won’t be a problem... with nominal AN6 ends (3/8”) and 125,000 psi steel strength, they’re good for at least 13-14,000 pounds in tension... about 4 times the needed strength. The catalog touts them as high strength, but no strength value is given. The Heim ends appear to have the same strength in tension as the forks, so they shouldn’t be a problem either.

The trickiest part is the attachment between the fork (or Heim) ends and the lift strut. Looking at the 1” x 3/4” 6061-T6 bar that Carlson uses, when undrilled it has a cross-sectional area of 0.75 sq.in. so it’s good for about 33,750 pounds in tension but at the spot where the first mounting hole is drilled through it to attach it to the strut, the cross-sectional area is reduced to about 0.5625 sq.in. and the capacity drops to 25,312 pounds. Still about 7 times what’s needed. The aluminum bar is just fine; steel is not necessary.

Where the fork ends are threaded into the bar stock, the edge distance of the hole in the end of the bar is about 3/16” on the short side, so that shouldn’t be a limiting factor. If a sufficient number of threads of the fork end are engaged into the threaded hole in the bar, that won’t be the weak point either. The problem then comes down to the end of the bar that slides up into the hollow end of the strut and how the two are connected. Since the bar is much thicker than the wall of the lift strut, the weakest point of the connection is at the mounting bolt holes through the sides of the strut. Although there are rigorous methods for determining the load distribution between the bolts in a multi-bolt connection, a fairly safe assumption in this case is that if the bolts are not placed too close to one another or too close to the edges of the material, at ultimate loading all of the bolts will be carrying about the same amount of load. Let’s look at the load at !
the bolt holes through the strut wall nearest to the end of the strut. There is no need to look at shear strength of the AN4 connecting bolts.... they will be far stronger in shear than the thin walls of the strut.

There are at least two possible modes of failure at the connecting bolts but I’ll look only at “shear out” of the bolt, where the steel bolt pulls or tends to elongate the hole in the softer aluminum as it yields. In that mode, the force that the bolt can carry before the hole in the aluminum shears out is equal to the shear strength of the material (extruded 6061-T6 is about 24,000 psi) times the area that the bolt bears on. For AN4 bolts and a strut wall thickness of 0.119” at its thinnest point, with the first bolt in the row being about 3/4” from the end of the strut (such as in the image Piet_construction_449.jpg that Dan Helsper shared earlier), that works out to about 8,568 lbs for the first bolt in the row. That one alone should be able to carry twice the required load, so two bolts through each end of the strut should be more than adequate but if they give you three holes, use three bolts. The distance of the first bolt hole from the end of the strut i!
s very important though... if the edge distance is reduced from 3/4” to 1/2” (for example), the pull-out load drops to about 5,712 lbs... only about 1.6 times the required capacity. With at least two attachment bolts, the connection should be adequate, but by all means keep the first bolt hole comfortably away from the end of the strut.

--------
Oscar Zuniga
Medford, OR
Air Camper NX41CC &quot;Scout&quot;
A75 power, 72x36 Culver prop


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cdawson5854(at)shaw.ca
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PostPosted: Sat Jan 06, 2018 7:33 pm    Post subject: Wing Strut Attachment Reply with quote

Now if the rest of our beloved beast matched these figures ----- goodby
Sukhoi. Smile

Attached are pics of my strut ends and the jury struts on my Hemlock struts.
Just for comparison, you understand.

Clif
"Hell, there are no rules here; we're trying to accomplish something."
(Thomas Edison)

Handle every Stressful situation like a dog.
If you can't eat it or play with it,
Piss on it and walk away.


I?Tve taken a look at the forces in the wing lift struts of the Air Camper.
To run the analysis, I?Tve used the dimensions given in the Orrin Hoopman
(1933-34) plans to create the geometry, but there?Ts not a lot of
difference in any of the others that are commonly used. The main lift strut
length is given as 89-3/4? between the bolt holes, the front cabane length
is given as 21-1/4?, and the distance from the cabane mounting point to
the lift strut mounting point at the bottom of the fuselage side is
22-3/4?, so that side of the triangle is 44?. With those two
dimensions, we find that the angle that the lift strut makes with the bottom
of the wing is 29.4 degrees.

Assuming an aircraft gross weight of 1,088 lb and a design loading of +3.8G
(normal category aircraft), we get a total load to be supported by the wing
of 4,134 lb, and each half of the wing thus has to support 2,067 lb.
According to the source quoted in my spar strength article in the BPA
Newsletter (Noel Becar?Ts analysis in a 1963 Sport Aviation), the main spar
in a strut-braced monoplane at a high positive angle of attack carries about
84% of the total load, so that results in 1,737 lbs that each front lift
strut carries.

If the load on the spar where the lift strut attaches is 1,737 lbs and the
strut is at an angle of 29.4 degrees to the wing, the tensile force in the
strut will be 1737/(sin 29.4) = 3,538 lbs. This is the force that every
element in the lift strut assembly must meet or exceed in order for the wing
to sustain +3.8G loading.

--------
Oscar Zuniga
Medford, OR
Air Camper NX41CC &quot;Scout&quot;
A75 power, 72x36 Culver prop


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Ray Krause



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Posts: 450

PostPosted: Sun Jan 07, 2018 10:53 am    Post subject: Wing Strut Attachment Reply with quote

Oscar,

THANKS! Well done and well presented. Now I feel more confident that my wing will stay on the SkyScout as it flies for the first time!
Here's how I approached my attach point of the strut to the wing.


Sent from my iPad

On Jan 6, 2018, at 7:32 PM, Clif Dawson <cdawson5854(at)shaw.ca> wrote:

Now if the rest of our beloved beast matched these figures ----- goodby Sukhoi. Smile

Attached are pics of my strut ends and the jury struts on my Hemlock struts.
Just for comparison, you understand.

Clif
"Hell, there are no rules here; we're trying to accomplish something." (Thomas Edison)

Handle every Stressful situation like a dog.
If you can't eat it or play with it,
Piss on it and walk away.


Iâ?Tve taken a look at the forces in the wing lift struts of the Air Camper. To run the analysis, Iâ?Tve used the dimensions given in the Orrin Hoopman (1933-34) plans to create the geometry, but thereâ?Ts not a lot of difference in any of the others that are commonly used. The main lift strut length is given as 89-3/4â? between the bolt holes, the front cabane length is given as 21-1/4â?, and the distance from the cabane mounting point to the lift strut mounting point at the bottom of the fuselage side is 22-3/4â?, so that side of the triangle is 44â?. With those two dimensions, we find that the angle that the lift strut makes with the bottom of the wing is 29.4 degrees.

Assuming an aircraft gross weight of 1,088 lb and a design loading of +3.8G (normal category aircraft), we get a total load to be supported by the wing of 4,134 lb, and each half of the wing thus has to support 2,067 lb. According to the source quoted in my spar strength article in the BPA Newsletter (Noel Becarâ?Ts analysis in a 1963 Sport Aviation), the main spar in a strut-braced monoplane at a high positive angle of attack carries about 84% of the total load, so that results in 1,737 lbs that each front lift strut carries.

If the load on the spar where the lift strut attaches is 1,737 lbs and the strut is at an angle of 29.4 degrees to the wing, the tensile force in the strut will be 1737/(sin 29.4) = 3,538 lbs. This is the force that every element in the lift strut assembly must meet or exceed in order for the wing to sustain +3.8G loading.

--------
Oscar Zuniga
Medford, OR
Air Camper NX41CC &quot;Scout&quot;
A75 power, 72x36 Culver prop


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taildrags



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PostPosted: Sun Jan 07, 2018 11:27 am    Post subject: Re: Wing Strut Attachment Reply with quote

Clif; those are beautiful struts and fittings there! Now, just for comparison you understand, let's look at Western hemlock. Common references for mechanical properties of hemlock show a modulus of rupture as being 11,300 psi and many references estimate that the ultimate tensile strength of wood in the direction parallel to the grain is 80% of the modulus of rupture, so about 9,040 psi. Your struts look like they are somewhat larger than the Carlson small extruded aluminum struts but I've got the data for those so I'll use what I've got. The cross-sectional area of the aluminum strut is 1.8603 sq.in., so a solid Hemlock strut with the same outside dimensions as the Carlson strut should be able to carry a tension load of some 16,800 lbs.

I doubt that your struts are under-built. If you can catch the Sukhoi, see if you can out-maneuver it ;o)


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Medford, OR
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A75 power, 72x36 Culver prop
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taildrags



Joined: 29 Dec 2009
Posts: 1591
Location: Medford, OR

PostPosted: Sun Jan 07, 2018 11:39 am    Post subject: Re: Wing Strut Attachment Reply with quote

John; many cranes have a scale rigged right into the hoist cable. Some with digital readout, some with remote readout in the operator cab. If you know people with cranes, I'm sure they have scales.

You can also epoxy a strain gauge onto a section of bar stock of known cross-sectional area, placing it inline with your hoist using clevis eyes or something simple. You could read out, collect, and graph the load data as you test the piece, but what for? Unless you wanted to optimize the design in order to whittle the weight down to the bare minimum (like if you were building something for a nonstop, unrefueled circumnavigation of the earth and were looking for ounces here and there), close enough is good enough and a little bit extra is better than not enough ;o)


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Air Camper NX41CC "Scout"
A75 power, 72x36 Culver prop
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