# Lift Strut and Cabane Strut loads

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#### wsimpso1

##### Super Moderator
Staff member
Log Member
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!
The force at the leading edge - What is the source of it? What is the point of it? Why do the vertical forces not add to 1000?

Let's get the forces in the x and y axes of the wing, so we can do structures. We can then add in pitching moment, and get back to reactions at mounts on the spars.

Billski

#### Fenix

##### Well-Known Member
HBA Supporter
The force at the leading edge - What is the source of it? What is the point of it? Why do the vertical forces not add to 1000?

Let's get the forces in the x and y axes of the wing, so we can do structures. We can then add in pitching moment, and get back to reactions at mounts on the spars.

Billski
If referring to the lift at the leading edge of 67 LBS it is just the lift that is carried by the drag truss and the 931 is the lift that is carried by the spars in their designed axis. Putting it at the leading edge probably makes it look like it would cause pitching up of the airfoil but I just put it there because it was indicating it was related as the vertical component of the 258 pound force parallel to the chord. I'm sure the illustration does not conform to "engineering standards" but that is because I don't have that background.

The vertical forces add up to 998 instead of 1000 because for this exercise I chose an aircraft weight of 998 instead of 1000. Why? Good question. because on my excel data table the weights add up to 998 and I wanted it to "match those numbers". Why does my Excel table add up to 998? Because when I did the elliptical lift distribution starting with 1000 LBS I "lost 2 lbs" in the "sum of the chunks" and I did not add that 2 pounds back in somewhere, although it seems the "missing lift" would best be "assigned" near the tip because this is where the ellipse curve is extreme and where the "average height" of the section does not reflect the true "area" of that section. So the "lost lift" it seems would come from near the tip.

#### BBerson

##### Light Plane Philosopher
HBA Supporter
If the wing is sand bag tested inverted with a 14° nose down fixture, that similates the high angle g load and applies a forward force to the wing while loaded.
See Basic Glider Criteria:

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#### wsimpso1

##### Super Moderator
Staff member
Log Member
If referring to the lift at the leading edge of 67 LBS it is just the lift that is carried by the drag truss and the 931 is the lift that is carried by the spars in their designed axis.
You just did a full tilt on my bogosity meter ...

Since the lawn mower/roller are out and rigged and the MF230 works (we just had the steering pumps out) and the brush hog is on it, I have a few minutes for this.

The discussion of the last few pages is just trying to get the total wing forces correct. The data we have for estimation of lift, drag, and pitching moment are the standard equations:

L = q*S*Cl
D = q*S*Cd
M = q*S*MAC*Cm

q comes from flight conditions of air density and the gross velocity of air coming to the foil. S and MAC are from the wing area and shape. Cl, Cd, and Cm are back derived from knowing q and S and MAC when the tests are run. The measuring devices are placed at (or very close to) 0.25c on the chord line. So, the place where we have to put these loads is the known place from the tests: At 0.25c on the chord line.

Distribution of loads is then to internal structures (the spars and internal bracing) and then supporting structures comes after all that, and was the discussion for the first few pages of this thread. But start with lift, drag, and moment acting at 0.25c, and turn the lift and drag into loads that will try to shear and bend the wing upward and will try to shear and bend (wrack) the wing forward.

Then, you can accumulate lift, drag, and moment from the tip towards the root, distribute it to the forward and aft spars, figure out how big (and what direction) the strut loads are, and what the forces and directions are of forces that find their way to the cabane.

Billski

#### Fenix

##### Well-Known Member
HBA Supporter
I think one thing that is tilting your meter is that I am putting the original forces (lift and drag) on the diagram together with the "vector components" and I now think that is not typical convention in such drawings. It makes it look like the forces are all there twice. But again, I don't have much exposure to "typical" or "standard" drawings depicting such things.

We all form basic models of understanding of what we observe early in life. Usually these models are not exactly the way the world really behaves. For example, most of us observe the sun circling the earth (or at least crossing the sky) and conclude the sun is moving. Several years later we learn that the earth is rotating (and is round) and that the sun is actually stationary (at least relative to the solar system). Though I have for many decades now understood the earth rotates and the sun is stationary, it is still intuitive to watch a sunset and feel stationary while the sun falls into the sea, or behind the mountain, etc. It is easy to even attribute the earth's gravity to the downward motion of the sun!! It takes a deliberate effort to remember that the sun is really static and I am sort of "falling over backwards" as the earth rotates. The point is we develop models of understanding the things we observe, and although they are usually not entirely accurate, they persist, at least intuitively. For me the key to better understanding is not only to understand how it "really works" but ALSO to understand why it appeared to behave according to the erroneous model and thus understand why the model makes sense and also why it is invalid. This process of "tearing out" the erroneous models of understanding is where/why I draw these "diagrams". I went ahead and attached them thinking they might be helpful to anyone who is confused, although it seems they are also confusing or confounding to those that already understand - and most notably have a well establish structure or format for illustrating certain situations.

I'm not trying to change that convention, only trying to assist my mental processes with sketches that help me "see" things. Please pardon that my drawings err from convention.

Moving fwd: The discussion on q is helpful, as I've been thinking about it a bit these weeks. Before I understood that it was "indicated airspeed" but never really considered it from a "fluid dynamics" concern. In some future exercise I will attempt to understand the creation of the forces of lift and drag from q, but for now I will start with what I know lift must be (the weight of the aircraft in question and its G limits) and continue to apply these to the lift struts and cabane structures.

In regard to the distribution of the forces, yes we covered that in the first part of this thread but I sort of "artificially" considered it at zero AOA. But that does not negate the process I learned. I think in my last Excel attachment I completed a table of the "forces in the x and y axis" (or something like that) at 15 degrees AOA.
Next I will (making some assumption that my last data table is correct) go back and again distribute these forces at hi AOA to the spars, struts and cabanes using what I learned early in this thread.

In the above thread I became familiar with the accumulation the moments of lift and anti drag from the tip inward. I used Cm to distribute the lifting load percentages to the fwd and aft spars but have not examined Cm in terms of an accumulated moment, so perhaps that is an element that is still lacking in my understanding.

If my attempt and determining the forces discussed above goes goes well I think the remaining element (there may be more than one though) is to understand the application of the lateral forces from asymmetric lift, and whatever other factors may generate lateral forces.

Vertical forces - Lift and Gravity
Longitudinal Forces - Drag and Anti Drag
Lateral forces - Asymmetric lift and ???

Are there others?

#### Fenix

##### Well-Known Member
HBA Supporter
Attached is my attempt at computing the loads on the Lift and Cabane struts of a Pietenpol at 15 degrees AOA and 4.4 G's.

Still lacking is any asymmetrical lift loads (which I don't think occur if not "rolling" and anything I am still not aware of....

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#### Fenix

##### Well-Known Member
HBA Supporter
I have began to study the lateral forces on the cabane and lift struts.

Part 23 Appendix A states that for Normal and Utility Categories the wing and carry through (center section in my case of cabanes) must be designed for 100% load on one side and 70% load on the other side. The attached spreadsheet is my attempt at quantifying the effects of this asymmetry. Primarily it appears that this will change the compression loads in the spars so that where "before" they were equal and pushed against each other in regard to the left and right wing, now they are unequal, causing the entire "wing system" to have a force causing it to shift left or right laterally across the fuselage.

I'd appreciate any comments in regard to the way I have calculated this force.

I got to wondering, however, if one wing is lifting less (at only 70%) (I supposed due to an upward deflected aileron) does not the other wing have to lift more than the original 100% to keep the total lift equal to the total aircraft weight? If this is so, then the loads on the "more lift" wing would exceed the previously calculated loads for its strut system as it would exceed 100%. It seems that instead of looking at a 70% and 100% lift per wing situation a more practical method would be an 85% vs 115% situation. But this is not how it is described so apparently this is not the correct way. I am curious I guess as to why the 85/115 logic is not correct. In terms of asymmetry and lateral forces is does not appear to make a difference (see attached), but just in total loads on the strut system it does. This is illustrated in the second table attached.

Once I get these lateral loads resolved I think it will "complete" this examination of the primary strut and cabane loads.

Thank you to those who have contributed!

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#### BobDaly

##### Well-Known Member
The specified loading in the simplified design criterion occurs at VA, point A of the Flight envelope - Wikipedia . Another good reason to define the envelope for the Pietenpol. So one wing is lifting its share of about 3g while the other is lifting its share of 4.4g for a utility category airplane. Greater than 100% lift isn't available to the 4.4g wing, it is on the verge of stalling. So one designs for an unbalanced 30% of the 4.4g load. For a Pietenpol with a strut-braced wing, the center section has to be braced for the unbalanced axial loads in the spars. That's the job of the X'ed wires between the cabane struts.

The asymmetric loading can also occur from an oblique, upward wind gust where one wing encounters the gust before the other. In either case, there is a sudden momentary difference in angle of attack between the wings. Having initiated a turn and established the bank, the ailerons are neutralized and the lift returns to symmetry, back pressure on the elevator supplies the increased angle of attack needed to hold altitude. Above VA one should avoid abrupt control inputs and, if strong gusts are encountered, slow down to VA. So, the simplified design criteria makes sense.

#### Fenix

##### Well-Known Member
HBA Supporter
Ahhh yes Bob, that makes sense. At Va the lift cannot exceed 100%. I guess at higher speeds it could, but then it would also be exceeding its rated G limit.

In many parasols the x wires between the cabanes are only on the front cabanes, either crossed to the other cabane or down to the firewall and top longeron location. In this case the lateral forces on the rear spar are not reacted by a pair of diagonals or wires so it would seem that the center section must internally transfer these loads from the rear spar to the front spar where they can be reacted by the front cabane lateral bracing. Usually the center section has a fuel tank there so no drag/anti drag truss. I suppose it is the center section floor, which is usually substantial plywood, which is transferring this lateral load of the rear spar over to the front spar?

Yes the gust makes more sense. It seemed to me that a 30% split (70 vs 100%) from aileron deflection would be a lot. Perhaps ailerons can apply this much differential, but the other factors make sense too.

So I think I've now quantified the loads on the struts, as I started out to do. Now I need to do it at Vd and for negative G's but it seems the principals will be the same. One thing I was wondering still: Is differential drag something that needs considered. It seems this would come from aileron deflection (and of course ANY differential lift will also create differential drag) but it seems to me that differential drag in regard to the cabanes would just be an aft longitudinal force on one side and the opposite on the other set. Differential drag (aka adverse yaw) seems it would be more of an issue related to side loading of the aft fuselage due to the vertical stabilizer and rudder countering the adverse yaw/differential drag.