Question(s) about Steel Tube Fuselage Design

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Tom Kay, Jan 21, 2010.

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  1. Jan 28, 2010 #21

    Autodidact

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    Tom, sorry about the bad information. The info about trusses is in Schaum's Engineering Mechanics: Statics and Dynamics and buckling of columns is in Strength of Materials. That's two books and at the end of the day, two books is better than one, but not as enjoyable. Sorry 'bout that! The first pic below is a sample page from Statics and the other two are from Strength:
     

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  2. Jan 28, 2010 #22

    Tom Kay

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    Autodidact;

    Thanks, and don't worry. I make lots of mistooks.

    The first page of Schaum's on trusses looks interesting. I was thinking of starting small, with what should be a simple example, like 3 tubular members (similar to your figure 7-38, but with a point load, not what appears to be a uniformly distributed load), two of them hard-grounded, and the free node being acted on by some downward force, much like Dan Raymer's engine mount examples. But then, well, what from there? I'll get there eventually!

    Rome wasn't built in a day. That's a LOT of math.

    Cheers, Tom.
     
  3. Jan 28, 2010 #23

    Starman

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    I haven't read through this whole thread, but did anyone point out that most of the vertical tubes shown in the first and second posts are unneeded, useless weight?
     
  4. Jan 28, 2010 #24

    Autodidact

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    Tom, take it one cell at a time. The key to finding the compression load in ED (Fig. 7-38) is to find the perpendicular distance from ED to point C. The load is evenly distributed so that half is on point D; the other half on point C creates no moment about point C. The moment for this cell is 1/2 the load times distance CD. Thus, the compression force in ED times the perpendicular distance from ED to point C equals 1/2 the load times distance CD. Angle CDE is arctan of (i.e., the angle who's tangent is) 12/24 i.e., arctan(1/2) or 26.565°. The perpendicular distance from ED to point C is 8*sin(26.565°) or 3.5777 ft. So, 1/2 the load times distance CD = 2,000*8 = 16,000 lb/ft. 16,000 = the compression in ED times 3.5777. Thus,

    compression in ED = 16,000/3.5777 = 4,472.147 lb.

    But the distributed load is on a four wheel cart, therefore there are two trusses side by side and so the actual answer is

    compression in ED = 4,472.147/2 = 2,236.07 lb

    or

    approx. 2.24 K(thou) C(compression)

    Oh, what fun! :grin:
     
  5. Jan 28, 2010 #25

    Autodidact

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    Ah, now thats quite possible. But, it stands to reason that a truss w/twice the number of tubes could be made of tubes of half (or something like that) the size, and be more or less the same weight and possibly stiffer, as well. But without being able to calculate the forces on the truss tubes, theres no way to know what size they can be. There is probably a chart somewhere with good rule of thumb for truss size/number of tubes on it.
     
  6. Jan 28, 2010 #26

    Starman

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    That's true, but did you check out those drawings? The verticals as shown in them aren't contributing anything except for weight.
     
  7. Jan 29, 2010 #27

    Autodidact

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    Yeah, I see what you mean now! The diagonals are in Warren strut configuration so the verticals aren't really needed - it'd be like putting verticals into the Sukhoi in the later post. Sheesh, I gotta learn to become more detail oriented :emb:. In the first post they would help the longerons resist buckling, but in the second post it doesn't look they're needed at all.
     
  8. Jan 29, 2010 #28

    Tom Kay

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    Hi Starman;

    Thanks for offering an opinion. If possible, it would be helpful for you to expand a bit on what you've pointed out. I certainly have no experience at designing tube trusses, but my logic for adding the verticals was that they constrain the otherwise long spans of horizontal tubes from buckling under compression.

    Let's look mostly at the tail area, based on the pictures in my initial post, not so much the second post that shows smaller "cells." My thinking was that, during a hard pull up, the tail would bend down like a fishing rod with a large bass on the hook (not quite as bad) and the top longerons would be in tension while the bottom ones are in compression. If the span of the bottom longerons is too long, I assumed that this would make it easier for those longeron tubes to start buckling. By adding a vertical, this constrains the bottom tubes in the middle, making buckling less likely. That was my thinking, at least.

    Now, this is a pure guess on my part. Obviously I could be wrong. That's why I'm hoping that others will join this thread.

    The 3 verticals in the wing-mount area are to help the bottom longeron resist bending upon taking the landing and flight loads.

    So please help me by indicating why the verticals, at least in the tail area, would not be serving as anti-buckling constraints. I admit, that I have seen tube frame fuselages that do not have the verticals, especially from cockpit to tail. Having done no math on these, I always wondered why the long unconstrained spans of some longeron members was acceptable.

    Thanks, Tom.
     
  9. Jan 29, 2010 #29

    Tom Kay

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    Autodidact;

    Anyone who tries to teach me math is to be commended, so thanks. I keep saying math, but since most people can add, it's really the derivation that eludes me for most of this stuff.

    I will go through your solution, admittedly at my reptilian pace, and try to grasp it, so that this doesn't elude me forever. I was thinking that this could be solved using a vector method, but I have yet to try it. What I mean, is that for any pin-jointed member, the force only acts along the axis of the member (members in tension could actually be replaced by a strong rope). That means that if a member is taking a 5000 pound load in tension, and it's situated on an angle, there would be some pure vertical component and also a pure horizontal component to the 5000 pound load. I could use right angle triangles to figure out how big each vert/hor component is. Again, early thinking, and maybe cumbersome.

    Thanks. There are some examples on Youtube, and elsewhere on the web. Along with your example, I'll study these things and see if I can get the light to go on. I know I can do this, it's just a question of burning through that 30 years of post-university fog.

    Cheers, Tom.
     
  10. Jan 29, 2010 #30

    Autodidact

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    Yes exactly, right triangles, and not necessarily cumbersome! What you are talking about, of course, is trigonometry. Early thinking, indeed; goes all the way back to Pythagorous! See pic below:
     

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  11. Jan 29, 2010 #31

    Tom Kay

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    Yes that's largely what I was thinking. I'm not sure how to translate this approach to a multi cell, or multi node truss, but this is a start, I guess.

    Again, I'll probably lurk on Youtube or Google to find some tutorials about trusses.

    Thanks again for the input. Tom.
     
  12. Jan 29, 2010 #32

    Starman

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    That is a valid reason for using those verticals as you show them, however if you do that those tubes can be much smaller and lighter than the main tubes and if you did that you would need to cross brace those points from side to side as well. It all depends on the details of things like mainly how wimpy the longitudinal tubes are.

    I haven't looked into truss built craft much, but I've seen plenty that don't use those verticals like that, so if the longitudinal tubes are really wimpy then you could decrease the cell spacing like you did in the second post and eliminate the verticals.

    As you say verticals could come in handy in some spots like over the wing but think about what each end is pressing against. If either end is pressing against a lengthwise tube it is still essentially wasted unless it incorporates a hard point.

    What I suggest is using stronger tubes in the wing to engine area and using weaker tubes from there back and lose the verticals.

    I'm going to be making a steel tube truss for my design and haven't looked into it yet, but I prefer to gravitate to the simple and stout end of the spectrum. Some planes use teeny tubes and have a zillion of them like a spider web, some use stout tubes with fewer welds and connections.
     
  13. Jan 29, 2010 #33

    orion

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    One thing to remember in all this and that is simply to make sure that you're thinking in three dimensions. So, if you include a vertical member to reduce the length of a compression longeron so as to reduce the likelihood of buckling, that will work only in the view you're looking at if you also do not include a cross piece at that point going horizontally across the truss assembly. Simply said, you can constrain it all you want for up and down deflection but it may still buckle to the side.

    Then, keep in mind load resolution. Those vertical members may actually be doing little for you since a perpendicular member (one that attaches in a perpendicular manner to a longeron) has no ability to resolve the load into said longeron, so it is truly there only to aid stability. But if that's the case, it doesn't need to be as big as the diagonals. But either way it really is weight that you don't need - the Warren Truss configuration (diagonals only) is significantly superior to this type of load resolution.
     
  14. Jan 29, 2010 #34

    bmcj

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    Not to mention, too many members increase the complexity of the force vector calculations and may even drive you into a statically inderminate state. Static indeterminacy occurs when the number of unknown forces exceeds the number of equilibrium equations used to statically balance the forces and moments. To borrow from http://www.engineering.uiowa.edu/~design1/StructuralDesignII/Chapter5-ForceMethod.pdf,
    [FONT=Times New Roman,Times New Roman][FONT=Times New Roman,Times New Roman]Statically indeterminate structures [/FONT][/FONT][FONT=Times New Roman,Times New Roman][FONT=Times New Roman,Times New Roman]are the ones where the independent reaction components, and/or internal forces cannot be obtained by using the equations of equilibrium only. To solve indeterminate systems, we must combine the [/FONT][/FONT][FONT=Times New Roman,Times New Roman][FONT=Times New Roman,Times New Roman]concept of equilibrium with compatibility[/FONT][/FONT][FONT=Times New Roman,Times New Roman][FONT=Times New Roman,Times New Roman]. [/FONT][/FONT]
    Which goes on to say,

    [FONT=Times New Roman,Times New Roman]
    [FONT=Times New Roman,Times New Roman] [FONT=Times New Roman,Times New Roman][FONT=Times New Roman,Times New Roman]Advantages[/FONT][/FONT][FONT=Times New Roman,Times New Roman][FONT=Times New Roman,Times New Roman]. There are several [/FONT][/FONT][FONT=Times New Roman,Times New Roman][FONT=Times New Roman,Times New Roman]advantages [/FONT][/FONT][FONT=Times New Roman,Times New Roman][FONT=Times New Roman,Times New Roman]in designing indeterminate structures. These include the design of lighter and more rigid structures. With added redundancy in the structural system, there is an increase in the overall factor of safety. [/FONT]
    [/FONT][/FONT]

    [/FONT]

    But with that, comes the problem of determining exactly how the loads will distribute so that you can properly size the tubes. This will require you to include calculations of material properties and deformations to resolve the loads.

    Bruce :)
     
  15. Jan 30, 2010 #35

    Starman

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    Tom, I just saw your design in the P51 Replica thread and see the one here is the same design. You have the horizontal tubes going side to side where the verticals are, which is good if you are going to use that method, but there should be Warren truss diagonals going to them in the top and bottom views also. You need the triangular bracing in all directions in order to take rudder loads and any twisting torsional loads.
     
  16. Jan 30, 2010 #36

    Tom Kay

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    Hi Guys;

    Great feedback all, thanks. So, let's get to it:

    Starman, valid point about too many tubes, like a spider web, and too many welds. I can also see each cluster getting a little crowded, and tough to weld well.

    Orion, I haven't shown the top or bottom in this thread, but I had indeed added cross pieces in my model thus far. What I haven't drawn in, yet had fully intended to, are the diagonals in the top and bottom of the fuse frame. Having built that Murphy Renegade fuse frame out of simple pine wood, I did learn that diagonals on all faces of the truss, increase twisting resistance enormously. So again, I am definitely intending to add them.

    What I really should do is pick a frame concept that I like, model it with all members that should be there, and then ask for critiques, after providing several views from different angles. Ever notice that a single view of any truss fuselage is very hard to show all tubes that are present? Tubes hide behind other tubes.

    BMCJ, although I really don't have a solid feel for what statically indeterminate means, I do recall the term from Ryerson. I also understand that the more members I add, the greater the complexity of load calculations. That in itself may not be a good reason for having fewer frame members, but ultimately, it would be good to have a pretty firm understanding of what each frame member is doing. And I guess that means statically determining each load value and direction.

    I could say that I was surprised to read that there are advantages to designing a statically indeterminate structure, but then I considered that the Wellington bomber (Wimpy) must have been pretty complex to analyze. Remember that Geodisic (spelling??) structure? Strong, almost like rigid mesh.

    Starman, yes again on the upper and lower diagonals. That was just me being too lazy to finish the model before show-and-tell day. There are a few ways this could happen, such as zig-zag (my observation of the Warren truss) or all zagging in one orientation on the top, and zigging in the other direction on the bottom, like the Renegade biplane.

    I think I'll go finish my model, or this "version" of finished, at least. Having just learned that Rhino can calculate areas, volumes and centroids of any 2D or 3D item, I might take an early stab at the weight of my truss concept as well.

    Appreciated once again, all. Tom.
     
  17. Jan 30, 2010 #37

    Autodidact

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    Tom, I also looked at the 3d pics in the other thread, one thing occurs to me about the diagonals is that if you arranged all of the diagonals in the rear fuse (sides) to run from the top front of the cell to the bottom rear of the cell, then all the diagonals would be under tension when the aircraft is pulling positive g's. As you have it now, half of the diagonals are in compression, and since the fighter-like plane is designed to pull more positive load than negative, those diagonals under compression during positive g's would need to be a little bigger than if they were only under compression during negative g's, i.e., you could pull more positive g's by flipping half the diagonals so that they all went the same way, given the same tubing weights.

    By the way, those 3d dwg's are awesome!
     
  18. Jan 30, 2010 #38

    GESchwarz

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    It is my understanding that a design that is composed of a large number of smaller tubes is generally a stronger structure than a design composed of a smaller number of larger tubes. Is there any experience out there that can prove or disprove this?

    I suppose the the limit to the quantity of small members how much time you want to put into it.

    Are there any guidelines regarding the use and size of gussets?

    Are there guidelines on the span of longerons between crossmembers? I would imagin that someone will say that shorter is better and of course that is true.
     
  19. Jan 30, 2010 #39

    Tom Nalevanko

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    A large number of small tubes is called a composite. The threads are the small tubes. But it really depends on the application and you really can't make a general statement. For example, sometimes one single large diameter fastener is better than two smaller ones.

    Guidelines for use and size of gussets involves the load transfer and stress reduction between and among members. Guidelines on span of longerons is generally a consideration on buckling. There are many books that address these subject.
     
  20. Jan 30, 2010 #40

    Tom Kay

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    Autodidact;

    I have vacilated many times about whether I'd go with the Warren truss (what I affectionately call zig-zag) or just diagonally braced truss with the diags all going the same way. The Renegade biplane has all the side diags oriented as you said, from top to bottom as you progress aftwards, making them all in tension in a hard pull up. Great for positive G's, as this biplane is capable of 10 G's +.

    I thought I had some logic for trying a Warren approach, but I'm not sure if there really is an advantage to it. Thanks for the compliment about the 3D models, but Rhino can take most of the credit. I like the software more every time I use it, despite a few axes gaffs.
     

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