# Lift Strut and Cabane Strut loads

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#### Fenix

##### Well-Known Member
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The formal training and book learning on this uses a particular very specific taxonomy that allows us to know what we are saying to each other. I suspect that words I know the meaning of are not meaning the the same thing to you. My basis for this belief is that I explain something, and you come back with a description that does not really fit... Sort of like playing the telephone game. So, I try to explain the topic a different way in the hope of being understood on what can be a tough topic to grasp

A fabric covered wing has two long slender spars that have ribs connecting them together, and all of these elements are pretty soft in torsion, so the two spars do not particularly move together, and twist from end to end is pretty easy to get until you connect the lift struts. Put a skin around the front of the wing, making a D-tube and now the front spar is torsionally stiffer, but the aft one and the connections between them is still soft, so the spars do can easily not be parallel. Lift originating as locally lowered pressure outside the skins everywhere is reacted to the ribs and then the spars. The difference between suction on the bottom and suction on the top is lift, and it is calculated at 1/4c from Cl and S and q and spanwise lift distribution. The chordwise distribution of just lift between the forward and aft spar is simply lever rules with load in at 1/4c. In the simplest system, one spar is at 1/4c and carries all of the lift. Nothing more fancy than that. Then pitching moment is calculated using Cm, S, c, and q. Moment absent lift (which is what we are doing) is two forces in the opposite direction some distance apart. the magnitude of the moment is F*L where F is the force and L is the distance between the two instances. Since the only thing picking up this load is the two spars, the distance between them is L, and one force is down at one spar, the other is up at the other spar. Nothing more complicated than that. Oh, and fore-aft loads (drag and anti-drag) is occurring too, but since drag is much smaller, it is easily neglected. Once you can get low alpha stuff figured out, you can do high alpha and add in the forward component of lift that is substantial.

These loads start at the tip and accumulates as you go toward the root. Since we simply hinge the outer panel at the cabanes, we hang a downward force on the wing at the strut mount. It pulls down equal to all the bending moment calculated from all of the lift applied to that spar about the hinge line at the cabanes divided by the distance from cabanes to the lift strut. Since the strut can not connect to anything straight down, we use an angled strut. The direction of the strut tells us the proportions of vertical and lateral and longitudenal forces, and since we know the vertical, we can figure the lateral and longitudenal forces. Put the strut right underneath the spar it carries and fly it at low alpha, and the longitudenal component is small. Slide the mount forward or aft and you can see where you are generating longitudenal forces in the strut.

The difference between total lift on a spar and the strut vertical force is the force that has to be reacted at the root. There is no bending moment in the spar at the hinge at the cabanes. Lightest spar possible is designed by placing the strut so the positive moment at the strut mount is equal to the largest negative moment between strut and cabance mounts. This minimizes the maximum moment in the spar, so you design the spar for one place and use it spanwise. You can tailor the spar, but most do this only outboard of the strut mount. That takes care of lift and pitching moment finding their way to struts and hinges.
I believe I understand the above, at least well enough to move forward, and have applied it in the most recently attached computation spreadsheets.

Drag and anti-drag remain. The forward component of lift can be around 25% of lift, distributed along the span like lift is, but pointed forward. then subtract local drag. This force times the arm from cabane hinges and summed up over the span is trying to rotate the wing forward around the combined centers of the hinges at the cabances. It is resisted internally by the diagonal bracing found in fabric covered wings, usually wires plus a compression rib where each wire intersects a spar. Without the wires or some other form of bracing, the wing will easily wrack out of square. The wing is now stiff and strong against wracking, and the moment generated by drag and anti-drag is reacted at the cabane hinges, with equal forces out and
in and calculated in a way that should now be familiar.

One last load on the cabane hinges - the actual sum of drag and antidrag for the outer panel is a longitudinal load on the cabane hinges. The lift struts do little for you on this - the root fittings usually do the work. Exactly how it is split is tough to figure as it is usually indeterminate. Now if one cabane is triangulated (stiff) and the other is a simple column (quite soft), all of the longitudinal load will go in the triangulated one. If they are equally stiff and perfectly fitted, the load might be split 50-50. and so on.
From past discussion I had a grasp on the drag / anti drag forces and how to quantify them. What you have written here gives further clarification. I will not attempt to summarize what I think you have said but instead perform and example computation. What I do and don't understand should be evident from how I perform the math.

What you explained about "redundant load paths" or "redundant structure elements" also makes sense. It also gives me a clue as to what is meant by "indeterminate" (a term I have come across many times but never understood). But for now there is no point in getting bogged down in the nuances of this topic. So onward with the calculation of drag and anti drag loads and then applying these forces to my cabanes, which is the stated objective of this entire thread.

#### wsimpso1

##### Super Moderator
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I have to say that the readback approach is useful, and I am not complaining. My difficulty is in knowing if you have understood or not, and then if finding a different way to describe the physical system so that it can be understood. When the terms we use and the definitions for those terms differ, it becomes more difficult. This is why each field has its own taxonomy. The nouns and verbs are specific. It is not intended to exclude folks, but to make the language clear. To the outsider, it seems like the secret handshake. Sorry about the learning curve, but the language makes it possible to have someone else check your work or clear up the understanding.

The difficulty on hba.com is twofold. We have trained folks used to using the words and relationships as defined in the field, and we have untrained folks trying to figure it out using their own words and analogs, with resulting confusion when we attempt to confirm knowledge. Then we have people who are successful specialists in one related area but incomplete in other related areas, and having trouble because of it. The classic is the aerodynamicist messing up on structures. And I am trying to convey the topics to both types.

Billski

#### wsimpso1

##### Super Moderator
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I know of one Pitts (no, not mine) whose owner broke the upper spar by tightening the flying wires too much.

I also have watched the flying wires turn into a 5 or 6 inch blur from slackening at -6 g.

Metal javelins look cute, but locally fatigue the wires, so stick with wood.

BJC
Broke a spar. Significant overtightening there...

There is also the one of the guy who looked at the internal wire torque spec (in in-oz) but tightened to the same number (in in-lb), and the wing went all wonky with alternating twists.

Then there is undertightening, which causes fatigue and has resulted in a certain bird being donated to an aviation museum after a certain engine mount bolt broke in flight two different times. The aerodynamicist builder thought he must have overtightened said bolt, and made sure after the first failure that he did not overtighten the replacement. No torque wrench, but I sure wish he had used one the first time. I wonder how many of these events have wrecked stuff.

So, are the flying wires supposed to vibrate that much, or were the wires undertensioned?

Billski

#### Pops

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Like I said, A very good teacher.

#### TFF

##### Well-Known Member
I think -6g is telling the story.
Pitts are probably the most pushed design out there. It is probably one of the most experimented planes too. Wire tension maintenance is variable with the Pitts crowd. Some go by plans and some will set to plans and test fly for g loading and tighten the wires until they don’t vibrate. +6-4 are just numbers.

#### Pops

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My old body knows the difference.

#### BJC

##### Well-Known Member
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So, are the flying wires supposed to vibrate that much, or were the wires undertensioned?
Wire tension maintenance is variable with the Pitts crowd. Some go by plans and some will set to plans and test fly for g loading and tighten the wires until they don’t vibrate.
Tension was set per Pitts specifications, but, as TFF said, there are lots of opinions about what is optimal.

BJC

#### Fenix

##### Well-Known Member
HBA Supporter
I have to say that the readback approach is useful, and I am not complaining. My difficulty is in knowing if you have understood or not, and then if finding a different way to describe the physical system so that it can be understood. When the terms we use and the definitions for those terms differ, it becomes more difficult. This is why each field has its own taxonomy. The nouns and verbs are specific. It is not intended to exclude folks, but to make the language clear. To the outsider, it seems like the secret handshake. Sorry about the learning curve, but the language makes it possible to have someone else check your work or clear up the understanding.

The difficulty on hba.com is twofold. We have trained folks used to using the words and relationships as defined in the field, and we have untrained folks trying to figure it out using their own words and analogs, with resulting confusion when we attempt to confirm knowledge. Then we have people who are successful specialists in one related area but incomplete in other related areas, and having trouble because of it. The classic is the aerodynamicist messing up on structures. And I am trying to convey the topics to both types.

Billski
I've long recognized and admired the specificity of terms in engineering (and other sciences). Sadly, I don't "speak the language" yet but it is coming to me as I begin to see what is happening and hence, what needs to be described. Thanks for your patience. I really do feel like I am gaining some ground whereas in the past I was only becoming acquainted with a growing number of terms I did not understand. Yes a lot of it is because I began reading books about AE and not about physics - but at the time I didn't know any better.....

So I have attempted an analysis of the longitudinal forces on the wings and struts in cruise flight. So in this case it is only drag.
Attached is my understanding of these forces. It is a combination of equations and narrative "readback".
Hopefully the combination will help the engineers here see whether or not I "get it".

If/when I am correctly managing simple low alpha drag I will take the next step into the more significant forces at high alpha.

For those who want to contribute, whether you add narrative or just re-arrange my equations to make them correct, I will probably learn something either way.

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#### Fenix

##### Well-Known Member
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In preparation for dealing with anti-drag forces I considered a case of slow flight near a stall at 1G. I examined this in the attached diagrams.
Some of them may have some things right, but I'm pretty sure some of them have some things wrong as some just doesn't "seem intuitive".

I knew that analyzing the struts would introduce me to more than just truss theory and that is reflected in the attached diagrams.
Remembering however that the focus here is to arrive at cabane loading some of the errors in the attached diagram may at this point be best just identified as incorrect reasoning but perhaps the "correct reasoning" does not have to be fully explained at this point if it is not needed to arrive at cabane loads. I don't want to drift from the stated goal of this thread.

Anyone who is inclined to strike through my errors in the attached is welcome to do so. Perhaps I began "on the wrong foot" at step one and the entire contents here are nonsense and need to be just abandoned and started over.

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#### Fenix

##### Well-Known Member
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I just realized that in the hi alpha example I failed to use a Cl and Cd that are practical for Hi Alpha so the product numbers will be unrealistic but given my purpose in this particular exercise is just one of learning the correct methods it should not affect the usefulness in this exercise.

#### BobDaly

##### Well-Known Member
There is drag of the wing and drag of the whole airplane. The wing drag is composed of form and friction drag as well as induced drag from producing lift. The drag coefficient of the wing can be broken down into a zero-lift drag coefficient plus an induced drag coefficient. When you triangulate the lift and drag of the wing, you're only getting the induced, lift-dependent, drag.

Thrust more or less equals drag of the entire airplane in unaccelerated level flight. Usually, when the airplane is level, the wing has a positive incidence angle so it can produce the required lift. If the airplane is not level and the thrust is aimed upward slightly, thrust contributes to lift supporting the airplane. If the airplane is in a glide, thrust is aided by gravity and can be less than the drag.

It is important to separate the wing drag from the airplane total drag for designing the structure. Wing drag will pull the wing back. Lift at high angles of attack pulls the wing forward.

#### wsimpso1

##### Super Moderator
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Log Member
In preparation for dealing with anti-drag forces I considered a case of slow flight near a stall at 1G. I examined this in the attached diagrams.
Some of them may have some things right, but I'm pretty sure some of them have some things wrong as some just doesn't "seem intuitive".

...

Anyone who is inclined to strike through my errors in the attached is welcome to do so. Perhaps I began "on the wrong foot" at step one and the entire contents here are nonsense and need to be just abandoned and started over.
Try your thinking on it this way. The drag is in there too.

Cl, Cd, Cm as measured in the wind tunnel at high alpha look like the top illustration.

Then look at it from the way the structure sees it. Structure only knows which way the loads are applied. The lift at high alpha does have a forward component. The drag decreases it some, but usually the drag is on the order of 1% of the lift while the forward component of lift can be on the order of 25-30%. And the lift that is perpendicular to the chord line is around 97% of the lift straight up.

I hope that this helps.

Billski

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#### Fenix

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Thanks a lot guys. Yes, that helps! Suddenly it is all falling into place - I think.

I just have one more bothersome element that is "not conforming" to my mental model.

I will soon post a revised version of this segment of the analysis.

I think I'm just a couple steps away from the complete answer to strut loads this thread begain with. : 0 )

#### Fenix

##### Well-Known Member
HBA Supporter
Well I think I understand this now. I had quite a few major errors in my previous submission on this. Billski's simple diagram showing how wind tunnel data is measured and how this applies to a structure really got me started down the right path. But still I had a real problem not transferring the fwd component of lift (anti-drag) to the center section/fuselage and having it show up as thrust. I have long heard about this force and how it caused some early airplanes to shed their wings forward and I have pondered from time to time just how drag can act forward. I never really dedicated myself to understanding it. So it seems this can be a difficult concept to grasp, at least for me. For the benefit of those readers who might also struggle with this attached is a diagram that shows how it finally came to make sense to me. The AE's may find it an obtuse way to look at it, but perhaps it will help some of the confused to get a place to grab it and resolve it further for themselves.

Attached again is the spreadsheet analysis of these forces and a couple diagrams that accompany it. Hopefully this time it is close enough to what is really going on to be more useful than the previous one. As always, any modifications or re-arrangements of it to make it "match reality" are appreciated.

In it I ask a few questions. Any contributions to the answers are appreciated.

Thank you!

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#### BBerson

##### Light Plane Philosopher
HBA Supporter
There is no anti-drag force. It is the strong lift from the leading edge that is somewhat forward at high angle of attack.
It can be seen in the airfoil pressure chart, if I can find it.
The word anti-drag is a misnomer.

Like this but the arrow should point forward.

Lift is forward on a glider.

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#### BobDaly

##### Well-Known Member
Your "Understanding fwd component of lift" is correct up to the last sentence about "transfer to the fuselage/cabanes". The fuselage pulls down on the wing while the wing pulls up on the fuselage. Its the same pull. The wing struts will take care of the pull(s) perpendicular to wing chord, the wing spar root connections take care of the pull parallel to the wing chord. Then the cabanes, connected to the wing root and a conduit to the fuselage, have to react the chordwise pull. The Pietenpol Aircamper does this either with fore and aft crossed wires on one side of the passenger cockpit or with angled bracing struts to the longerons at the firewall or a combination.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Well I think I understand this now. I had quite a few major errors in my previous submission on this. Billski's simple diagram showing how wind tunnel data is measured and how this applies to a structure really got me started down the right path. But still I had a real problem not transferring the fwd component of lift (anti-drag) to the center section/fuselage and having it show up as thrust. I have long heard about this force and how it caused some early airplanes to shed their wings forward and I have pondered from time to time just how drag can act forward. I never really dedicated myself to understanding it. So it seems this can be a difficult concept to grasp, at least for me. For the benefit of those readers who might also struggle with this attached is a diagram that shows how it finally came to make sense to me. The AE's may find it an obtuse way to look at it, but perhaps it will help some of the confused to get a place to grab it and resolve it further for themselves.

Attached again is the spreadsheet analysis of these forces and a couple diagrams that accompany it. Hopefully this time it is close enough to what is really going on to be more useful than the previous one. As always, any modifications or re-arrangements of it to make it "match reality" are appreciated.

In it I ask a few questions. Any contributions to the answers are appreciated.

Thank you!
Before you look at distributing loads inside the wing, let's just get the loads figured out in the first place. Since you did not get the same picture I got (and that picture is all there is to it), I am trying again on forward component of lift:

Wind tunnel frame of reference: Airflow imposed is horizontal, drag is horizontal, lift is vertical. Airfoil is rotated through a range of alpha from below negative g stall to beyond positive g stall. Lift and drag and moment are measured at 25%c. This is the top diagram back in my post. Got that?

Let's shift our frame of reference to the wing. At the same airspeed and alpha as in the case in the top picture, we are trying to figure out what the lift and the drag are doing relative to the structure. The loads are exactly same... but now we translate them into a loads perpendicular to the chord line for wing bending and parallel to the chord line for wing wracking. That is the second picture and I even gave you the math for how to compute each of the new forces... L is big. And a big fraction of L is perpendicular to the chord line, with a smaller fraction parallel to the chord line. The fractions are exactly the cosine and sine of alpha. Drag (much smaller than lift) has a fraction aft parallel to chord line and a smaller fraction perpendicular to the chord line using similar math. Drag is on the order 1-2% of lift, so some folks get by just fine without worrying over drag at all.

The big deal is that when you take that lift that looks like it is angled forward (we angled the wing up relative to the air flow) the angle puts a component perpendicular to the chord line of about 97% of lift (assuming alpha is 15 degrees) while the component parallel to the airflow is about 26% of lift. And that lift forward is trying to bend the spars forward. That is the bottom diagram in my post. And at whatever alpha you fly the wing, Lper^2+Lpar^2 = L.

Sum up the components perpendicular to the chord line and that is the load that puts shear and bending into the wing the strong way for the spars. Sum up the components parallel to the chord line and those are the forces that try to bend the spars forward, which is the weak way for most spars, so we do one of: Diagonal wire sets inside; diagonal struts inside, structural skins on the outside.

Then we can get into distributing lift and drag and moments into the cabane structure...

Billski

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#### Fenix

##### Well-Known Member
HBA Supporter
Before you look at distributing loads inside the wing, let's just get the loads figured out in the first place. Since you did not get the same picture I got (and that picture is all there is to it), I am trying again on forward component of lift:

Wind tunnel frame of reference: Airflow imposed is horizontal, drag is horizontal, lift is vertical. Airfoil is rotated through a range of alpha from below negative g stall to beyond positive g stall. Lift and drag and moment are measured at 25%c. This is the top diagram back in my post. Got that?

Let's shift our frame of reference to the wing. At the same airspeed and alpha as in the case in the top picture, we are trying to figure out what the lift and the drag are doing relative to the structure. The loads are exactly same... but now we translate them into a loads perpendicular to the chord line for wing bending and parallel to the chord line for wing wracking. That is the second picture and I even gave you the math for how to compute each of the new forces... L is big. And a big fraction of L is perpendicular to the chord line, with a smaller fraction parallel to the chord line. The fractions are exactly the cosine and sine of alpha. Drag (much smaller than lift) has a fraction aft parallel to chord line and a smaller fraction perpendicular to the chord line using similar math. Drag is on the order 1-2% of lift, so some folks get by just fine without worrying over drag at all.

The big deal is that when you take that lift that looks like it is angled forward (we angled the wing up relative to the air flow) the angle puts a component perpendicular to the chord line of about 97% of lift (assuming alpha is 15 degrees) while the component parallel to the airflow is about 26% of lift. And that lift forward is trying to bend the spars forward. That is the bottom diagram in my post. And at whatever alpha you fly the wing, Lper^2+Lpar^2 = L.

Sum up the components perpendicular to the chord line and that is the load that puts shear and bending into the wing the strong way for the spars. Sum up the components parallel to the chord line and those are the forces that try to bend the spars forward, which is the weak way for most spars, so we do one of: Diagonal wire sets inside; diagonal struts inside, structural skins on the outside.

Then we can get into distributing lift and drag and moments into the cabane structure...

Billski

Billski,

That was an excellent description that should be in a textbook somewhere. I copied it to put in my notes on this project - it should not go to waste.

However your explanation was not contrary to what I understood from the original sketch you provided from which I was able to understand this - I think.
Perhaps the reason our pictures don't look the same has more to do with my art ability than with my understanding.
Attached is another attempt at a better drawing illustrating what I tried to depict in the prior one.
Perhaps I still am missing this but it all makes sense and my numbers add up. I think it is my poor illustrations.

Perhaps the new drawing is still lacking talent - my HS had an art class but I took a semester of music instead. NOOO I still can't sing - or dance!

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#### Fenix

##### Well-Known Member
HBA Supporter
Your "Understanding fwd component of lift" is correct up to the last sentence about "transfer to the fuselage/cabanes". The fuselage pulls down on the wing while the wing pulls up on the fuselage. Its the same pull. The wing struts will take care of the pull(s) perpendicular to wing chord, the wing spar root connections take care of the pull parallel to the wing chord. Then the cabanes, connected to the wing root and a conduit to the fuselage, have to react the chordwise pull. The Pietenpol Aircamper does this either with fore and aft crossed wires on one side of the passenger cockpit or with angled bracing struts to the longerons at the firewall or a combination.

Bob,
I've thought about what you described and (I think) you're exactly right. It appears I stumbled at the finish line in my "lesson" on anti-drag.

I have revised my "Understanding Anti-Drag" lesson and it is attached again here. Hopefully it is better now.

I have also revised the spreadsheet analysis to reflect this "new understanding" which actually is very similar to how I had it a few versions ago. It appears I managed to confuse myself into thinking that I had a better idea.........

As always - critics welcome!

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#### wsimpso1

##### Super Moderator
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That was an excellent description that should be in a textbook somewhere. I copied it to put in my notes on this project - it should not go to waste.
Every statics textbook and course pack covers this topic - vector mechanics is used in all engineering disciplines.

However your explanation was not contrary to what I understood from the original sketch you provided from which I was able to understand this - I think.
Perhaps the reason our pictures don't look the same has more to do with my art ability than with my understanding.
I do not have a problem with your artwork. My problem was your mechanics. Your previous posts did the vector conversions backward, which raised the lift perpendicular to the chord line. Your latest document has it correct. Your drawing still indicates to me that you do not get that breaking the lift vector into two orthogonal vectors is intimately related to the lift vector, but you followed up with the math and did all of that correctly. Bravo!

... Perhaps the new drawing is still lacking talent - my HS had an art class but I took a semester of music instead. NOOO I still can't sing - or dance!
No, but your illustrations should reflect the scheme you are going to express with mathematics. The latest one is good. If most of us put in the vector mechanics wrong in the picture and then try to program using that picture, well, it is GIGO... Which is why I stayed on with you for the correct vector mechanics. You will need them again for forces in the cabanes.

Billski