Aluminum Tube & Gusset Airbike / Legal Eagle / Parasol Thread

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

Ollie Krause

Well-Known Member
Joined
Feb 26, 2020
Messages
77
Wow, what a great and informative thread. I just spent the last hours or two reading through the first couple pages which answered all but one of my questions. First a little background though: I'm currently in the process of designing a truss fuselage based off the SkyBike and Legal Eagle. I'm planning on using 6061 T6 circular tubing and gussets (material TBD but likely not 6061) to hold it together. My question is whether or not I need to cope all of my beams where they intersect. Instead, I was thinking I could just cut them straight and instead have the gussets carry the full load. My thinking is since the tubes aren't welded, maybe most of the load is being carried by the gussets and not the intersection points. A design like this would substantially decrease design complexity and remove the hassle of complex coping at busy intersections. Any help would be greatly appreciated! I've attached a couple images below to that might make my question make a little more sense. Thanks!!!

No Cope.jpg
No coping ^
Cope.jpg
Coping ^
Fuselage 2.jpg
My super rough current design ^
 

Victor Bravo

Well-Known Member
Joined
Jul 30, 2014
Messages
7,042
Location
KWHP, Los Angeles CA, USA
Ollie, I believe that you can "square cut" the tubes without coping them, but it becomes far more important to have gussets on both sides of the joints. The reason is that a closely coped pair of tubes will automatically contact each other and serve as a "stop" against movement in one direction. So this would significantly reduce the amount of flexure and movement that the joint would see. If you have gussets on both sides, the gussets will prevent that movement or flexure, even better than a coped joint.

This same general principle is accepted and used widely in the construction of "wooden stick" wing ribs. Gusseting on both sides of the joint guarantees that the only loads put on the glue joint are in double-shear, which is where a glue joint is at its strongest.

f you imagine blind rivets as being much better at withstanding shear than other types of loads, and that they will stay safe as long as you don't introduce bending loads (trying to pry the heads off the rivets), you can see how a riveted double-shear joint would have a much longer service life.
 

mcrae0104

Well-Known Member
HBA Supporter
Log Member
Joined
Oct 27, 2009
Messages
3,208
Location
BJC
I was thinking I could just cut them straight and instead have the gussets carry the full load. My thinking is since the tubes aren't welded, maybe most of the load is being carried by the gussets and not the intersection points.
The gussets will need to carry all of the load. Even if the joint were in compression, you'd never get the coping perfect enough to distribute the load evenly across the coped surface, resulting in stress concentrations. And even if you could get an absolutely perfect fit, would that load be distributed across a large enough area on the "receiving" member? Maybe, maybe not. Gussets spread that load out. Finally, depending on different load cases, many, many joints on a truss fuselage may experience either compression or tension, and you'd need to design the gussets appropriately for tension anyhow.
 

ScaleBirdsScott

Well-Known Member
Joined
Feb 10, 2015
Messages
1,195
Location
Uncasville, CT
I second the concept of having at least two gusset contact points (double-shear) per tube.

To simplify the concept to the minimum, you can easily twist a tube that has been riveted to a plate along only one line of rivets. Imagine that if you rock that tube back and forth it'll eventually work that hole open and your rivets are no longer doing anything but acting as measly hooks. In general practice even if the rivets became bolts, the same principal more-or-less can apply.
 

Ollie Krause

Well-Known Member
Joined
Feb 26, 2020
Messages
77
If you went to square tube, as per my LMA build, you can 'stagger' the rivet line, this will stop any "hinge" type flexing, between tubes.View attachment 95320
We tried square tubes but it made the tapered geometry of our fuselage difficult to construct with gussets because beam rotation also became an issue. We ultimately decided on circular tubes to just keep things simple.
 

Ollie Krause

Well-Known Member
Joined
Feb 26, 2020
Messages
77
Thanks for all of the help, everyone! I'll go adjust my design to remove any coping and instead use two gussets to ensure every joint is in double shear. If anyone's interested, you can find our latest designs for our fuselage (and entire airplane) on our public Onshape document. Thanks again!
 

ScaleBirdsScott

Well-Known Member
Joined
Feb 10, 2015
Messages
1,195
Location
Uncasville, CT
I like the idea of it. Give it all-aluminum wing frames vs wood and I'd possibly be interested in throwing one together.

In the Onshape link, I don't think we can turn things on/off so all we can see is the fuselage with a big orange box inside? Are there details for the wing/etc?
 

Ollie Krause

Well-Known Member
Joined
Feb 26, 2020
Messages
77
I like the idea of it. Give it all-aluminum wing frames vs wood and I'd possibly be interested in throwing one together.

In the Onshape link, I don't think we can turn things on/off so all we can see is the fuselage with a big orange box inside? Are there details for the wing/etc?
Thanks! It's still super early in the preliminary design and we are making major changes daily so check it regularly for updates. Ah, that box is just a placeholder surface which we used to create the trusses around. You should be able to hide it by scrolling down to the surfaces drop down in the lower left parts list and clicking the eye icon when you hover over it. If not, you can create a free Onshape account and can copy the doc with a single click. The wings are in the same document but in a different folder. I believe this link should take you directly to the wings though. Our current wing designs are what we would consider a "complete rough draft" so if you see anything you think we should change or anything you don't think would be safe or strong enough, please don't hesitate to let me know! Thanks to the awesome members of this forum, we learned a ton designing our wing. You can see this HBA forum thread to see all the questions we asked and the changes we made in response to all the great feedback we got.
 

Victor Bravo

Well-Known Member
Joined
Jul 30, 2014
Messages
7,042
Location
KWHP, Los Angeles CA, USA
I think that if the airplane works, several people would want to build one.

Eliminating the coping is going to save you a boat load of time and effort.

Have you decided on an engine for this aircraft yet?
 

Ollie Krause

Well-Known Member
Joined
Feb 26, 2020
Messages
77
I think that if the airplane works, several people would want to build one.

Eliminating the coping is going to save you a boat load of time and effort.

Have you decided on an engine for this aircraft yet?
We'd love to see some other people construct our airplane and give us some feedback on how we can improve it! We are actually going to make our ultralight electric but I'm sure it could be easily adapted to use a traditional 2 stroke internal combustion engine.
 

erkki67

Well-Known Member
Joined
Feb 18, 2010
Messages
1,887
Location
Romont / Fribourg / Switzerland
A couple more worthy construction shots

And remember, these gussets are merely stabilisers for the insides of the tubes, the real gusset is the outer skin, which you can imagine how easy it is to attach to the square tubes, albeit a lot of holes to be drilled for riveting. that's one part I have eliminated in my design.

View attachment 89450

View attachment 89451
Cheapracer, those square tubes your are useing to build your aircraft with, what is the quality 6061 T6 or 6060 T66 and which dimensions?
 

Hot Wings

Grumpy Cynic
HBA Supporter
Log Member
Joined
Nov 14, 2009
Messages
6,904
Location
Rocky Mountains
if you see anything you think we should change
Not 'should' but might consider:
Swap the orientation of the drag struts. The anti-drag forces, forward, can be higher at stall than from drag at high speed. The drag/anti-drag tubes will be stronger in tension than in compression.

Agree with BJC - Start your own build log with a link to an associated comment thread.
 

cheapracer

Well-Known Member
Log Member
Joined
Sep 8, 2013
Messages
5,990
Location
Australian
Cheapracer, those square tubes your are useing to build your aircraft with, what is the quality 6061 T6 or 6060 T66 and which dimensions?
The ones in the picture you quoted are not mine, but Morgan's. Longerons are 25mm x 3mm, while a few of the braces are 3mm, but most are 1mm.

They tested at 6060, I can only presume T6 temper.

I use 25mm x 1.1mm. 1mm actually if you take into account that extrusions have a 0.1mm tolerance.

6061 T6
 

Hot Wings

Grumpy Cynic
HBA Supporter
Log Member
Joined
Nov 14, 2009
Messages
6,904
Location
Rocky Mountains
Watch out for those non-triangle polygons...
What he said. Those are called indeterminate structures.
The green struts make everything nice easy to calculate triangles.... and add weight.
You - might - leave out the blue with the intention of having this portion of the structure deform in a crash to absorb energy?
The aft green carries the loads from what looks like a good spot to attach the shoulder harness?

The cabin 'entry hole' is always a hard area on a high wing to run the load paths around without having some indeterminate structure. Sometimes we just have to make a compromise or 2.
Indet.jpg
 

Ollie Krause

Well-Known Member
Joined
Feb 26, 2020
Messages
77
What he said. Those are called indeterminate structures.
The green struts make everything nice easy to calculate triangles.... and add weight.
You - might - leave out the blue with the intention of having this portion of the structure deform in a crash to absorb energy?
The aft green carries the loads from what looks like a good spot to attach the shoulder harness?

The cabin 'entry hole' is always a hard area on a high wing to run the load paths around without having some indeterminate structure. Sometimes we just have to make a compromise or 2.
View attachment 95413
Ah I totally didn't think of that! We'll do some research into indeterminate structures and will adjust our fuselage design accordingly. Thanks!
 

BrianW

Member
Joined
Jul 2, 2018
Messages
21
Location
Altus SW Oklahoma
Not 'should' but might consider:
Swap the orientation of the drag struts. The anti-drag forces, forward, can be higher at stall than from drag at high speed. The drag/anti-drag tubes will be stronger in tension than in compression.

Agree with BJC - Start your own build log with a link to an associated comment thread.
Ah, someone who remembers the fate of an early German WW1 fighter - those durn wings kept folding forwards!
 
2
Top