• Welcome aboard HomebuiltAirplanes.com, your destination for connecting with a thriving community of more than 10,000 active members, all passionate about home-built aviation. Dive into our comprehensive repository of knowledge, exchange technical insights, arrange get-togethers, and trade aircrafts/parts with like-minded enthusiasts. Unearth a wide-ranging collection of general and kit plane aviation subjects, enriched with engaging imagery, in-depth technical manuals, and rare archives.

    For a nominal fee of $99.99/year or $12.99/month, you can immerse yourself in this dynamic community and unparalleled treasure-trove of aviation knowledge.

    Embark on your journey now!

    Click Here to Become a Premium Member and Experience Homebuilt Airplanes to the Fullest!

Lift Strut and Cabane Strut loads

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Fenix

Well-Known Member
Supporting Member
Joined
Dec 25, 2020
Messages
90
The Pietenpol plans call out strut materials that are no longer available. Of course there are materials that have been “shown to work” as substitutes without failing but I don’t know that there has ever been an analysis done of the Piet struts. I am embarking on such an analysis for three reasons. I want to learn how to do this (the biggest reason) and also because it might reveal some answers that are not already well known and lastly because a Piet is on my list of future projects so these answers will eventually be specific relevance to me.

I have been studying truss formulas and I think I understand enough to calculate the load paths through the spars, lift struts and cabane struts but do not fully understand how to handle the two separate spars that are used in wooden wings. So next on the list of things to understand is the chordwise lift distribution of an airfoil which should allow me to know how much lift to assign to each the fwd and aft strut. Note I have recently discovered “Javafoil” (but not yet began to use it) which should help me some on this matter. But I am wondering if there is a “rule of thumb” for the lift distribution on the fwd and aft spar configuration found in wooden wings (at least those that are fabric covered). I know that the lift distribution will change (maybe a lot which will rule out a rule of thumb) with various airfoils and the spars are not always located at the same % chord and the lift distribution changes with angle of attack. I would expect the critical lift distribution would be the one that occurs at high AOA as it is this high G condition that is going to be where things fail. I also expect any “rule of thumb” (if such exists) may be conservative and state , for example, the aft spar generally does not exceed 45% of the total load and the fwd spar generally does not exceed 65% of the total load…. (where the totals exceed 100% - that being the conservative part). Such a conservative rule of thumb would be useful as I am not trying to optimize this design.

So the direct question: How to assign the lift loads to the fwd and aft spars of the pietenpol or of fabric skinned dual spar wings in general. Again I anticipate the lift distribution at high AOA is the relevant distribution to this analysis.

Note: I am going to assume that the total lift is transferred through the lift struts and that none of the lift is acting on the cabanes. It is my understanding that a small amount does actually act on the cabanes, but that most of it acts on the lift struts. I will be conservative and assume it all does. A future project is to learn to determine how much acts on the lift struts and how much on the cabanes…..

I will assume in this case that the weight of the wings do not act on the lift struts (I’m not sure about the center section weight and the fuel in the center sections. It may depend on whether it is a “one piece wing” or “three piece wing”. The conservative approach is to assign that weight to the fuselage for now. Another future project is to understand this though).

Once I understand the load distribution on the fwd and aft spar I can calculate the tension on the lift strut and the compression on the wing spars and the compression on the fuselage members up to the cabanes and the compression in the cabanes as “independent systems” – one being the forward and one being the aft assemblies (or at least that is my current level of understanding). Also the tension across the fuselage between the lift strut attach points on the fuselage can be calculated, but these are of less concern because the material for this can be “per plan” (white ash) unlike the struts which materials are no longer available.

Now to throw in a kink. Mostly just to increase learning, not that I’ve seen Piet’s built this way or intend to. But I also have interest in also a Parasol version of the MiniMax (V-Max specifically) where this understanding will be relevant. The Piet has parallel lift struts that attach to the fuselage in two different locations (that being of course what makes them parallel). If the rear strut were attached at the fuselage at the same point as the fwd strut (as is done on the Minimax line of airplanes) you have what I will call a “V-strut”. It is my expectation (subject to verifying mathematically) that this change in the rear strut will increase the compression on the rear spar because the angle of the strut becomes shallower as it now has “two angles”. One the original angle between the wing spar and the strut at the strut attach point of the wing and also another angle where the aft lift strut is also “swept fwd” and no longer parallel to the rear spar. I’ve not sure about the change in the compression of the rear spar (not yet done the math) but I am confident it will also create a compression load between the rear spar and forward spar where the struts attach to them. The Piet, even with parallel struts, has a compression member at this location but I think it is there due to compression loads created by the drag/anti-drag wires. (These loads are also a future project.)

What I am really unsure about is how this will change the compression loads in the cabanes. As I stated above, with the parallel lift struts I expected the compression loads of the rear lift strut to be reacted by the aft cabane and the forward lift strut to be reacted by the fwd cabane. Changing from parallel struts to a “V-Strut” seems to me that all of the compression loads created by the lift strut assy will be reacted by the forward cabane strut. If the forward cabane takes on a lot more compression in the V-Strut design I have to wonder if the fwd spar also sees an increase in compression…?

If this is true then the fwd cabane strut would have to me much stronger than in the parallel lift strut configuration and the rear cabane strut could be almost non-existent. I suppose the rear cabane may be loaded only in tension of the small amount of lift that exists at the wing root?

While we’re at this point in the “logic” I’ve been wondering about the “net loads” on the cabanes. They are obviously under compression from the lift strut loads but they also are “lifting” a small amount of the fuselage (the part that is not lifted by the lift struts). So it seems they are reacting compression AND tension loads at the same time (which seems weird to me) and the only resolution that makes sense to me is that the small amount of tension created by lifting the fuselage could be subtracted from the compression loads given a net load that is a compression load, but slightly less than it would have been had the cabane not also been doing a bit of lifting. Did I explain that well enough to follow? Is that how it works? If so, in practical application the “lifting tension” loads could be ignored and the cabanes would be conservatively designed to simply handle “all of the compression”.

Finally once all of the strut loads are calculated there must be some reference table that shows the tension and compression (and then to learn more about Euler) strength of common materials, diameters, wall thickness of streamlined and round (for the budget conscious) tubing. Wicks and AS&S have tables that show the weight per foot, but not the strength, of these materials. There are probably lots of these references. Does anyone have a favorite that just makes a good “shop manual”?

My primary goal is to learn about this so please feel free to comment on any of the above. Am I on track at all in the “logic path” or am I not even close? Any “re-direction” or “further explanation” would be appreciated. Also, I have been through a number of “so called beginner” books but even they are hard to follow. Perhaps someone knows of a book that is really geared toward the type of subjects dealt with in my post, and at a beginner level. It seems most of what I’ve read is written for engineers, who don ‘t need to read it because, well, they’re already engineers.
 
Back
Top