Ultralight struts/cantilever/additional weight

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crusty old aviator

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I wonder if anybody has rigged an overweight UL with RC gear and just flew the rig around as a model? I’ve never heard of it being done, but I don‘t get around much in modeling circles.
 

TFF

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I think many of the caught illegal ULs were either the early years of 103 where the FAA had a new reg and were checking, or the two seaters flown as singles without being designated an instructor. I remember talking to a guy in the 80s at a fly in, that his angle was being an instructor only so he could legally fly his wife around. He didn’t instruct. I’m sure plenty of fat ULs were grounded when an FAA caught someone refusing to get an LSA conversion along with a license to match. Saying no to licensing being more important than being tagged.

I know a couple of times the FAA was either talked into calling something a UL to stop a report or were misdirected that the incident was a UL when it was not. Off airport is much easier than on. A pile of parts that a retired E-6 or F4 mechanic can’t identify is ripe. A flying one is only seen if at an airshow, if you are a smart owner, you don’t stick around to answer questions, you can’t slip up. All assuming you are trying to skirt rules.
 

maya.ayoub.32

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Hello Airplane enthusiasts!

Maya here, from the San Francisco bay Area. I was doing some research on the attachment between the wing spars and the support struts but I didn’t feel like I could consider the designs correctly, as I don’t know why the designs were chosen per aircraft. I was hoping to clear up some confusion around this attachment. It looks like there’s two main attachment methods: U bracket and fingerstyle.

For a U bracket style attachment, I was wondering how much we need to worry about the U brackets rotating along our circular spars. Essentially, the U bracket would be attached at one line, which I thought might cause it to rotate with the shear forces during flight. However, I noticed that a great many esteemed ultralights, that also house circular spars, use this attachment, such as the Parasol. The Legal Eagle XL also uses these U brackets, however they have rectangular spars, so this rotation is less of a concern.


I then was redirected towards the Affordaplane which uses a finger style attachment by one of my team members, however this attachment feels very specific to their design. The spar and strut must be aligned on at least one side for one of the gusset-like steel plates to be straight. Same goes for the Legal Eagle, who used triangular gussets, however their gussets remain straight as they have an enlarged connection at the end of their strut. I was also wondering if this sort of attachment would work if the spar and strut of different sizes were centered, leading both gussets to be bent? If both were curved, I would be concerned that the tension from lift would cause the finger sheet metal to extend and potentially deform.

While I’m sure both of these attachment methods would work just fine (once our physics team calculates the required material thickness, etc), I was hoping that someone who may have experience with them could shed some light on any pros/cons that I may be overlooking. Thanks in advance for all the help!


 

radfordc

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The problem is that the slightly illegal ultralights were only slightly grounded, so documentation is somewhat iffy.

BJC
I would even settle for : "I know it happened because I was there and it happened to someone I know personally and here are some of the details." Hard to find even this much evidence that UL look-alikes are targeted by the FAA for enforcement.
 

Dana

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Maya, let's take your images in order. First one, dunno what it's from. I don't care for the way it's bolted on the tangent, as you say it could rotate. It's probably OK, but I would feel much more comfortable with a curved saddle that matches the spar tube:
1588361751011.png
Second image, the yellow one: Looks like a conventional wood spar; if so, it's not a U-channel but plates bolted either side of the wood spar; they probably extend diagonally up nearly to the top of the spar, with perhaps three bolts through the sandwich, a time tested technique. Note that all the loads from the strut tension are in line with the plates. That's a Carlson streamline extrusion, BTW, with the flats inside so a simple square bar can be bolted in as the end fitting. It probably looks like this inside:
1588363964621.png

Third, the Affordaplane drawings. There are many aspects of the Affordaplane design that are, shall we say, questionable, and I don't like this one. The offset strap is a bad idea, all the load will be taken by the straight one. It's got a plug inside the strut tube, which is good, but I don't see any reinforcement inside the spar.

The third one (is that the Legal Eagle?) is a lot better, though I would prefer to see the centerline of the strut (in assembled position) fall between the two bolts through the spar; as it is it's putting much more load on the outboard bolt, to the point that the inboard bolt may be pushing upwards on the spar, adding to the load the outboard bolt has to carry.
 

maya.ayoub.32

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It's probably OK, but I would feel much more comfortable with a curved saddle that matches the spar tube:
Hi Dana!
Thank you so much for the clarification and explanation on all the attachments! We didn’t even think of saddles! The first one is a picture off the Texas Parasol, and what I’m getting, the U attachment with the saddle is superior to (bent or not) gussets.

Thank you for confirming our concern with the Affordaplane’s gussets before we got too far along the way! Sounds like bent gussets are too dangerous.

The third one is indeed the Legal Eagle, however it’s hard to avoid the gussets bent with this design because of our different sized spar and strut tube. From your comments about the Affordaplane, bending gussets for this attachment is out of the question. But if we do need to use this type of gusset, and work out how ensure they are straight, we will definitely secure the spar in line with the counterpoint between the two bolts.

Thank you again for your response and information on saddles!
 

Dana

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The problem with the U-channel bolted to the bottom of the spar is that the bolt holes through the spar remove material at the most highly stress points (top and bottom). The side gussets as on the LE, OTOH, have the bolt holes on the neutral axis of the tube so its strength is not as severely impacted.
 

maya.ayoub.32

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The problem with the U-channel bolted to the bottom of the spar is that the bolt holes through the spar remove material at the most highly stress points (top and bottom).
Whoops I forgot about that! Looks like we need to do strength and drag analysis, since an attachment to our spar tube add more weight weight but it would make our gussets straight and as you said, on the neutral axis.
 

maya.ayoub.32

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*Sorry the straight triangular gussetted strut attachment is actually the Excalibur, a kit plane who machines their own strut-spar adaptors.
 

maya.ayoub.32

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Hello again!

We took everything you said into consideration and cadded up a new gusset attachment! We decided to compromise a little weight for a safer spar-strut attachment, and created an adaptor for our one inch tube. Below is a side profile of our gusset. Because the strut would be in tension most of the time, the force vectors won’t be straight upwards. From this assumption, we decided to have the two left holes in a line. Is this safe to assume? We were a little confused by
The problem with the U-channel bolted to the bottom of the spar is that the bolt holes through the spar remove material at the most highly stress points (top and bottom). The side gussets as on the LE, OTOH, have the bolt holes on the neutral axis of the tube so its strength is not as severely impacted.
Should we move the spar attachment bolt in the middle of the two spar attachment, or should the strut line fall between the two spar bolts?

The extended strut line falls between the two bolts, and the farthest left side is perpendicular with the strut, so there are as many contact spots as possible. On the inside of this gusset is an attachment which we will drill from aluminum.

Will this work? Any suggestions or comments are greatly appreciated, and thank you in advance!
 

Dana

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Ideally, the strut centerline will be centered to the hole pattern (could be any number of holes) on the strut:

1588674647619.png

Here, the green hole is new, and the gusset plate can be chopped off at the red line. Now, both bolts through the spar are carrying the same load. This is only an example. You might need three holes, one on the centerline, one each side. You will need to look at the allowable bearing stress (load on the bolt at each point divided by the bearing area, which is bolt diameter times wall thickness) on the spar tube to know how many bolts, and of what size, you need.
 

Swampyankee

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A high wing will have the struts in tension during flight; this will mean they are lighter and smaller. They will also tend to cause less interference with the wing than struts on a low-winged aircraft, which will need to be in compression in flight, so their size will be determined by buckling criteria, not strength, and their intersection with the upper surface of the wing will tend to cause more drag.

In either case, the presence of a strut may permit an increase in span or decrease in root thickness for the same wing weight or a decrease in weight for the same thickness and span, due to a reduction in the bending moment that has to be withstood at the root. The design of the fuselage structure where the strut joins the wing is also important. Where to put the strut on the wing is dependent on the lift distribution along the wing, but also on considerations like getting into and out of the airplane and, for an aircraft that needs to be transported by road, how the wing is going to be folded or removed.
 

JohnB

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I looked at all the pictures posted and could find NO pictures of Legal Eagle spars. The one wooden spar shown was close but no cigar. There are NO holes in the spar caps at the strut on any of the 4 flavors of Eagles. They are all into blocking between the upper and lower caps. if there are holes in the caps the builder needs to start another wing
 

maya.ayoub.32

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""
Ideally, the strut centerline will be centered to the hole pattern (could be any number of holes) on the strut:
Here, the green hole is new, and the gusset plate can be chopped off at the red line. Now, both bolts through the spar are carrying the same load. This is only an example. You might need three holes, one on the centerline, one each side. You will need to look at the allowable bearing stress (load on the bolt at each point divided by the bearing area, which is bolt diameter times wall thickness) on the spar tube to know how many bolts, and of what size, you need.
Thank you for the very informative reply! Having the strut line midpoint the bolt line is very smart! Thank you!
Our wings would be creating 1700 lb of lift max at 4g so with two bolts per attachment, 8 more connecting strut tubes, and 4 connecting the strut to the fuselage each would be withstanding 85 pounds. I doubled this for safety considering that the strut-spar bolts will probably bear a lot more of the weight. I did the calculations then compared it to the bearing strength of aluminum and look like we can use 2 bolts!
Thank you for bringing bearing strength to our attention, as it wasn’t even on our radar (which is why this response took so long, sorry about that).
Thank you again!
PS. We’re worried since bearing strength doesn’t consider thickness so we’re calculating tear out strength right now : )
 

maya.ayoub.32

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I looked at all the pictures posted and could find NO pictures of Legal Eagle spars. The one wooden spar shown was close but no cigar. There are NO holes in the spar caps at the strut on any of the 4 flavors of Eagles. They are all into blocking between the upper and lower caps. if there are holes in the caps the builder needs to start another wing
The yellow one is taken from Les Homan's YouTube channel, and the video can be found here :
It is a Legal Eagle XL, as far as my knowledge goes, but it could be possible that he changed the design.
 

lr27

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A high wing will have the struts in tension during flight; this will mean they are lighter and smaller. They will also tend to cause less interference with the wing than struts on a low-winged aircraft, which will need to be in compression in flight, so their size will be determined by buckling criteria, not strength, and their intersection with the upper surface of the wing will tend to cause more drag.

In either case, the presence of a strut may permit an increase in span or decrease in root thickness for the same wing weight or a decrease in weight for the same thickness and span, due to a reduction in the bending moment that has to be withstood at the root. The design of the fuselage structure where the strut joins the wing is also important. Where to put the strut on the wing is dependent on the lift distribution along the wing, but also on considerations like getting into and out of the airplane and, for an aircraft that needs to be transported by road, how the wing is going to be folded or removed.
If the struts on high wing airplane can't stand significant compression, the airplane may not survive the first time it hits a strong downward gust. Unless it has a kingpost and more struts or wires.

When calculating the wing weight savings by using struts, don't forget to subtract the weight of the struts, fittings, and additional reinforcements. I'm skeptical there would be any significant weight savings on a part 103 ultralight that had a thick airfoil and a real spar. Except possibly if it had an unusually long wing with no taper.
 

Dana

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Thank you for the very informative reply! Having the strut line midpoint the bolt line is very smart! Thank you!
Our wings would be creating 1700 lb of lift max at 4g so with two bolts per attachment, 8 more connecting strut tubes, and 4 connecting the strut to the fuselage each would be withstanding 85 pounds. I doubled this for safety considering that the strut-spar bolts will probably bear a lot more of the weight. I did the calculations then compared it to the bearing strength of aluminum and look like we can use 2 bolts!
Thank you for bringing bearing strength to our attention, as it wasn’t even on our radar (which is why this response took so long, sorry about that).
Thank you again!
PS. We’re worried since bearing strength doesn’t consider thickness so we’re calculating tear out strength right now : )
I'm not sure I follow your calculations.

MADE UP NUMBERS FOR EXAMPLE ONLY!:

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.

Now, you said "bearing strength doesn't consider thickness." Yes, it does. Bearing stress is the bolt diameter times the material thickness. So let's consider, say, an aluminum tube strut of 1½" diameter and .058 wall thickness, a common size. That bolt passes through both walls, so each side sees 675#. The bearing area, assuming a 1/4" bolt, is .25 * .058 = .0145 in², giving a bearing stress of 375/.0145 = 46.6 ksi (46,552 psi). According to MIL-HDBK-5, the allowable bearing stress for 6061 is 67 or 88 ksi depending on edge distance, so that particular connection is OK. I'm also assuming that your gusset plates are thicker than .058 so they'll be stressed less.

Naturally, the above will change based on your particular dimensions and angles. This is assuming all of the load is carried by a single strut. If you have two struts per side, the rear strut will carry significantly less load, so assuming it's all on the front for this calculation is a conservative assumption. Then if you make the rear strut the same size, you don't have to worry about it. Calculation of the exact load distribution is beyond the scope of this thread, which is a nice way of saying, "I don't remember and am too lazy to look it up."
 

JohnB

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The yellow one is taken from Les Homan's YouTube channel, and the video can be found here :
It is a Legal Eagle XL, as far as my knowledge goes, but it could be possible that he changed the design.
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
 

b7gwap

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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.
 
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