Bolted Connections / Gussets.

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Matt G.

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I'm not sure how you are getting fastener loads from what you have posted. Starting from the 2nd picture in post 18, you have an applied load from the horizontal stabilizer, and nothing else. What about loads from the structure ahead of the tailboom that you've left out? What about the wing? You need to determine where those loads are being reacted out (create a balanced free-body diagram) and then determine the forces and moments in each member, then take a section cut at each joint and figure out the loads being transferred across the joint. Then you can analyze the individual fasteners.

Be aware that a consultant is probably going to run you in the neighborhood of $70-100+/hr...
 

Topaz

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And don't forget torque loads on the boom added by the rudder and vertical stabilizer, from rudder deflection, yawed flight, and lateral gusts (if you want to take it that far). FAR23 (the now "old" version) is a goldmine for this sort of thing, as is EASA CS-22, the latter specifically for gliders and motorgliders.

These loads can occur simultaneously to loads imposed by the horizontal stabilizer and elevator, and the worst-case scenario where both are operating is likely to be the load scenario that drives the ultimate design of the parts related to your boom. Or, for a downward deflection of the boom, it could be a hard landing scenario. You won't know until you run all the cases.
 

proppastie

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Ok thanks I am sneaking up on the idea that I have to know the loads at every joint and then calculate the loads on the bolts for that gusset. As to the wing... it is joined at the center, and the fuselage hangs from it, so I am not sure if wing loads apply. Here are my analysis concept of air loads
and landing load.

gus7.jpg
 

Topaz

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I'm going to let someone else look over your load diagrams; someone who is better at structures than I. I've gotten so far as to have some notion of the breadth of load-cases that have to be examined, but statics for me is so far back in my history that I'd rather let someone else help you there than shoot my mouth off and lead you astray. I'm going to have to re-learn it all myself, and I'll be asking similar questions to what you're doing here, when I get to that point.

That said, yes, you'll want to calculate and tally up the loads at every joint and on every structural member, and then look for the worst-case scenarios where the part is at its highest loading or combination of loadings. Once you have that number, it should be relatively straightforward to apply Peery or Bruhn and size the part appropriately. At least that's what I'm hoping! :grin:
 
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pictsidhe

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Flabel goes through this. Suggest his book and engineering course (I went through it a long time ago and Mr. Flabel lives in my hometown and publishes the book from here). But when you do please don't post illegal scans from his book. Nor Peery's. When I was looking for an Aircraft Structures answer key I was in contact with the Peery family. They are very nice and the book is still in print and helpful for aero structures engineers so they should be supported. Mods, can you pull the attachment?
I think 3 pages from a 560 odd page book counts as 'fair use'. My copy of Peery is across town, so being able to see the scan was helpful. Others here with an interest in structural design but who don't, yet, have a copy, can still understand and participate in the thread. Tasters like that will encourage people to buy the book, people will assume that the other 99.5% also has good information. (It does BTW!)
I'm not a mod, so I could be wrong, but if it was my book, such snippets would be fine.
I miss going into a bookstore and spending 10 minutes sizing up a book to decided whether I want it or not. Google books with it's partially censored copies works pretty well, though.
 

Matt G.

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Two main observations:

1. The first rule of constructing a free-body diagram: The sum of the forces and moments must equal zero. Those do not, so that is not a valid FBD.

2. You appear to have a selection of critical loads from various load cases all combined together. This makes no sense and will not work- you need to develop individual balanced FBDs for each critical load case, and then analyze your structure for each load case separately. You are on the right track with what kinds of loads you are applying, but they have to be loadcase-consistent. For example, for the critical tail load, I would expect the wing reactions to be the opposite direction, no ground reaction, and an inertial force from the engine (not just torque...the engine isn't weightless). For what you are trying to analyze, lumping the structural mass at a point is not accurate; you would need to use distributed (running) loads. You may find that if the structural mass of the tubes has a very small effect on the loads, you can neglect it.

This is not necessary, but easier for people trying to help- standard coordinate system is +X aft, +Y out the right wing, and +Z aft.
 

proppastie

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Sorry for the confusion. Those diagrams were not meant to be a balanced FBD, but rather a depiction of the loads as I thought they should be.

What you seem to said, of significance to that thought was the wing load is in the opposite direction. Shown here is addition of engine inertia load 15x12G= 180 lb
and reverse direction of the wing load. Structure load will be distributed or ignored. (engine load is pretty small too) :

gus10.jpg
 
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proppastie

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My mission as regards this thread, is to design the tubes/gussets so the tubes/joints will not fail.....To that thought I believe it is necessary to know the proper loading before I can begin to try to draw FBD of each case. So going back to the Basic Glider Criteria I have pulled diagrams of what I believe are the major/largest loads acting on these tubes. If the list is willing and this makes sense I will try to mount the balanced FBD for each case. The words page 75 give a hint as to what the FBD is to look like and how (more importantly) it needs to be tested :

Case 1Basic Glider Criteria p37.jpg Case 2 Basic Glider Criteria p39.jpg Case 3Basic Glider Criteria p40.jpg Case 4 Basic Glider Criteria p21.jpg
Basic Glider Criteria p75.jpg
 
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proppastie

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Easiest Case 1, assume wheel at CG: Classic book on table FBD
FBD1.jpg
Which translates to
FBD2.jpg
 
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proppastie

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Remember I am doing this because I am not sure I am right. I decided to do a weight and balance, and the design is changing. Shown is new configuration. The pilot is shown here 30" forward 20% MAC (CG) I am not sure if this is good or not but gives me plenty of room to grow fat. (adjust the seat back)

Shown case 1 and proposed test of case 1

gus11.jpg
gus12.jpg

This case is not as interesting (difficult ?) as case 2
 

Matt G.

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Where are the inertial loads representing the wings, engine, pilot, tailboom/stabilizers, etc. that balance out the ground reaction at the tire? How are you going to analyze the loads on the fuselage if you sum all of the inertial loads at the CG?
 

proppastie

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Where are the inertial loads representing the wings, engine, pilot, tailboom/stabilizers, etc. that balance out the ground reaction at the tire? How are you going to analyze the loads on the fuselage if you sum all of the inertial loads at the CG?
What I though was trivial is not....oh well. Tube/structure was not distributed load here, but certainly could be. Were I going to analyze this some how.... I would want to do it with vectors.... graphically. I screw up the signs horribly if I try to calculate a FBD. Right now I am lost as to how to analyze a single joint using all these inputs. any good reference?

gus13.jpg

Edit: OK thought about it, distributed the tube loads, bending is the most important stress as concerns the gussets (I think)....start at the tail and work forward like a FBD to the wheel. At the nose go aft to the wheel. Not sure exactly how but seem to remember a little. Case 2 should be more stressful so that might be the one to actually do as regards ground load in the aircraft Z direction

gus14.jpg
 
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Matt G.

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You need to learn how to draw and solve a free-body diagram, as it is the basis for all of your stress analysis. Before you can analyze any of the joints, you need to determine the shear, moment, and axial load being transferred between members at each joint. Your drawings have a lot of unnecessary stuff on them for this purpose...all you need is the 'stick frame' of your structure with the applied loads and reactions. You also forgot the drag load on the tire. When you go to sum forces and moments about the CG, you may find you need to add a tail aero load to cancel out a pitching moment (unless the FAR pertaining to this loadcase lets you assume this is not in static equilibrium) (aero experts, what are appropriate wing and horizontal tail aero loads for this case?).

Another question: Why is there no triangulation in this structure? You have the cockpit tube cantilevered off of the vertical main tube, and then a square frame between the tail boom, engine mount, and vertical tube, with no diagonal bracing. Particularly for the joint between the cockpit tube and the vertical tube, the bending loads are probably going to be really high, and you'll have to carry 100% of that bending moment through the gusseted joint. And on the subject of that rectangular area, you'll probably have to use one of the methods in Bruhn for frames and rings to determine the bending moments.
 

proppastie

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Another question: Why is there no triangulation in this structure? You have the cockpit tube cantilevered off of the vertical main tube, and then a square frame between the tail boom, engine mount, and vertical tube, with no diagonal bracing. Particularly for the joint between the cockpit tube and the vertical tube, the bending loads are probably going to be really high, and you'll have to carry 100% of that bending moment through the gusseted joint. And on the subject of that rectangular area,
There are probably as many different possible designs as there are designers. Had I your skills and experience a frame design probably would be lighter, however I am limited by my ignorance and experience. The simple answer is today that is the design. The forward tube is down the center of the airframe and is 4" x .12 wall 7075-T6. Yield 64K. If the gussets (it remains to be proven) can carry the load it is the most simple for me, and a light weight structure. It is hopped that tube can carry all the loads, and preliminary simple bending calculations of the tube (no gussets) show it should.
 
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proppastie

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sticking with the single tube/s concept down the center of the Pod, I am finding it difficult to substitute 35K yield material (6061-t6) for 64K yield (7075-t6) and save much weight. 2-3 lb not counting gussets, and they add up. Deviating from the center tube concept brings up attachment issues with the boom. Not impossible but much more difficult for me stress analysis wise and more difficult fabrication wise. The 522 lb gust loading at the tail and 2100 lb pilot load or 2100 lb landing load, I have decided I need as a FOS make it a difficult problem for me. Today and maybe only for today (need to keep drawing) Post 34 design is at 37 # for tubes only..... no gussets. The tubes are a mix of 4 x.120 wall 7075-T6 and 4 x .049 wall 6061-T6.....Gussets, bolts, wheels, frames to carry fabric, fabric, controls will all add weight. The original Carbon Dragon paper work I have says the Fuselage is 60 #

gus16.jpg

this below with different mix is at 37 # only checking 522# gust load

gus17.jpg
 
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proppastie

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you'll have to carry 100% of that bending moment through the gusseted joint.
And on the subject of that rectangular area, you'll probably have to use one of the methods in Bruhn for frames and rings to determine the bending moments.
THE devil is in the details as they say... I am looking at the Gusseted joint and assume from what you say above that neither one of these cross sections to figure the Moment Of Inertia= I is valid? #1, #2, or back to the book Bruhn A9? #1 totally screwed, #2 works but maybe not right, Bruhn A9 looks similar to what I do for the bolts but somewhat complicated at this point.

gus18.jpg
 
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TFF

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Just a visual observation, but would the bracket not be for pulling joint apart instead of the force going down? Force going down, the bracket is just a little bit bigger than the perpendicular tube, probably breaks at close to the same point.
 
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