Osama bin Laden, Mahmoud Ahmadinejad, and Dee Rice are civil engineers.  Yasser Arafat was a civil engineer. He's not on line anymore. 

In a geodetic wing do the angled-in ribs and angled-out ribs contribute equally to antiwarp stiffness? (assumes they are in the same location spanwise).

The ribs that are in compression would contribute more to the stiffness I would think but if its all tied together well enough it shouldn't really matter much since the strength is needed in both directions anyway.

And in wings that actually have to use geodetic structures a D box of any size is not usually possible, but I have seen a South African stunt model that was an egg box construction and there was no way it could have been made stiffer for what it weighed.

Here's another picture. 

There is no stiffer wing than a geodesic layout.  Twisting this one is like trying to twist a 2x4.

Here is a pic of Graham Swallow's Wind Dancer from South Africa. It was designed by a free-flighter, Graham's late father John Swallow, who designed some egg-box free-flight models. I've felt this wing before it was covered in Monokote, and it just does not twist at all. Graham has two of them and I've flown them. The last one was powered by a Discovery Retro 60. It's a very good design and looks really pretty.

Building one is another story however as the wing uses around 24 sheets of 1/16" balsa. You build it with rectangular ribs and then sand it to shape with a special sanding block. The curved leading edge is laminated from 1/16" sheet as well. This is a real "builders" project. I suppose that these days the ribs can be laser-cut, so this could save all that sanding. Graham and I looked at this some years ago, but then he stopped flying and this never got any further.

Keith R

That's the model I was alluding to Keith!

Is a 'D' tube going to better this for stiffness? I kinda doubt it!

Inspired by this discussion, I made a lattice of 1/16" balsa to fill the stab of my new stunter.  This piece, which will fill one side, is really stiff in torsion, wimpy in bending, and weighs 4.24 gm.  It appears to be stiff enough to cut and sand to shape. 

Inspired by this discussion, I made a lattice of 1/16" balsa to fill the stab of my new stunter.  This piece, which will fill one side, is really stiff in torsion, wimpy in bending, and weighs 4.24 gm.  It appears to be stiff enough to cut and sand to shape. 

Howard, thank you for posting this picture. It turned the lights on in my head and I now understand and acknowledge the geodetic structure can indeed produce a wing that is stiff in torsion. Of course this arrangement of ribs looks nice, but it's not just pretty; it's very clever. Seeing the structure in unsanded form as you have shown helped me realize how this works.


Now if I can explain what I see. . .

We know a torsion tube like the D-section I mentioned earlier is strong in torsion. The analogy I like to use is the tube within a roll of paper towel. Twist the tube and it is strong in torsion. We could also make a square tube from card just like the round one and it too would be stiff in torsion. If the square tube were only an inch long and we glued it between two sheets of cardboard and gave it a twist it too would be stiff.

What Howard illustrates in his photograph is a series of these square torsion tubes; but they are vertical, the tubes is not aligned with the wing. How can that work? In this case, it does not matter because when you twist the wing at the tip, the shear flows around these torsion tubes in the same way as if you were twisting them.

How come they they don't apply this idea to real aircraft structures? Actually they do.
Honeycomb panel are extremely stiff in both bending and torsion and the torsional rigidity is due to the same principle.
http://imgs.tootoo.com/ec/2c/ec2c53f194ea36b165bae833591292ca.jpg

When Howard's structure is sanded down into an airfoil shape the workings are less obvious, I didn't see the lattice at work, and that's why I didn't realize the benefits previously.

Damian

When Howard's structure is sanded down into an airfoil shape the workings are less obvious, I didn't see the lattice at work, and that's why I didn't realize the benefits previously.

This will go into a modified Impact stabilizer, sandwiched between sheets of 1/16" balsa.  It has a constant-thickness airfoil with semicircles at the front and back, so not much sanding is required.  That sounds aerodynamically awful, but Mr. Walker tried a bunch of configurations and this works as well as any.  I reckon that the horizontal tail on that airplane is sized for stability, rather than pitching moment, and the way he rigs the elevator keeps the laminar-turbulent transition from moving around a lot when the airplane is flying level.

I considered using Nomex honeycomb, but balsa saves 10 grams.   

This will go into a modified Impact stabilizer, sandwiched between sheets of 1/16" balsa.  It has a constant-thickness airfoil with semicircles at the front and back, so not much sanding is required.  That sounds aerodynamically awful, but Mr. Walker tried a bunch of configurations and this works as well as any.  I reckon that the horizontal tail on that airplane is sized for stability, rather than pitching moment, and the way he rigs the elevator keeps the laminar-turbulent transition from moving around a lot when the airplane is flying level.

I considered using Nomex honeycomb, but balsa saves 10 grams.   

That's going to be superb. Honeycomb sandwich is something else. Who can forget picking up a piece of CF honeycomb for the first time; it's like a material from a different planet.
Having an airfoil shaped stab as opposed to a flap plate has the similar effect of increasing tail volume without increasing the tail arm or area. At least that's how it pans out in theory, though I am told it's debatable point. . .

It would be interesting to see a plot of the aerofoil of that one at various locations. I bet it looks unconventional!

You might note the 1/2 (really closer to 3/4) ribs aligned perpendicular to the LE between the angled ones...to maintain the airfoil. 

It's actually slightly heavier than a conventional layout.  But much stiffer.  Don't build a warp into it 'cause you'd never get it out.

Speaking of civil engineers, the new prime minister of Egypt is one.  This one's OK, though; he went to Purdue.

Kim,

I'm not an engineer (civil or un-civil), but I do have access to some good software that can help visualize what you are asking.  I modeled 3 different wing designs in SolidWorks and run an FEA or stress analysis on each.  I tried twisting and then bending each design and let the computer calculate the resulting stresses and amount of deflection in each.  I tried to make each design as simple as possible but all the exact same size - 1.5 in thick x 7.5 in wide and 31 in long.  I made a geodetic design, an I beam design and a fully closed D tube design.  All the forces applied are the same for each test.

To answer your original question, the first picture shows how the stresses are spread evenly across all the ribs when the geodetic wing is twisted. In the other designs the stresses are more along the trailing edge (I beam) or the wing tip (D tube).

The second picture shows the amount of movement or displacement of the twisted geodetic wing - .088"
The third picture shows the amount of displacement of the I beam wing - .437" (the geodetic wing is a lot stiffer)
The fourth picture shows the displacement of the D tube - .015" (Damain is right, the D tube is the most rigid)
The fifth picture is the amount of displacement of bending the geodetic in the middle - .170"
The sixth picture is the amount of displacement when bending the I beam wing - .080" (the geodetic is not so good at bending)
The seventh picture is the amount of displacement of bending the D tube - .002” (the D tube easily wins that one)

The geodetic wing is the heaviest design simply because it contains the most balsa.  The D tube weighs less and the I beam the lightest.  Although weight has a lot to do with selection of materials and construction techniques.

Don’t be so quick to right off the geodetic design based on these calculations.  If you want to construct a fully sheeted wing the geodetic design is probably the most rigid and can be built light because no spars or stingers are required inside.

The last picture shows the bending displacement of a fully sheeted geodetic wing - .017” (and that’s with a very thin (.020’”) covering).

Who needs engineers when you got the computer.

Paul

Howard,
Premusably the 1/16th sheet + covering/finish will dewimp the bend.  What density balsa are you using for the sheeting?  I'm curious cuz working on a similar flat-airfoil eggcrate sandwich stab myself at the moment.

Paul,
Thanks for the excellent graphics and data.  Very helpful in visualizing what is happening with stress distribution in the various construction methods.

All,
I think my original post was a bit obtuse--not unusual.  I was looking at the popular planform which combines a D-tube with ribs from TE to spar angled alternately Toward and Away from the foosylodge (moving from TE forward)--no rib overlap/interlock.  It occurred to me that the Towards may be more helpful than the Aways, because this direction undergoes greater compression during a warp.  So for example, if I have a wing with straight ribs, it seems to me that if I add a pair of Toward diagonal braces from TE to spar, top and bottom, to the outermost, say, 4 to 6 rib bays, I will get more bang for da buck in terms of stiffness vs weight than if I add alternate Towards and Aways to that same wing.  Note I'm not comparing with LE sheeting, shear webs, etc in this example. 

The overlaping/interlocking geo setup looks interesting if the labor and weight could be kept reasonable.  Not to mention the totally killer cool look through transparent covering.     

FWIW: Back in my R/C Glider competition days I designed and built a 12 foot cross country/thermal ship that I called the "RHOMBUS"  (Equilateral parallelogram) It had a "D" tube LE with egg crate ribs to the laminated TE. It also had a glass fuse that took me a year to make, including the molds and then the fuse. I only built one. I'm a slow learner!

Here's what happened; During a CC run I lost lift and had to do an off field landing about a 1/4 mile from the road. A 1/4 mile not to far? It is when you are walking through a plowed field! When I got to the ship I noticed that ALL the ribs were broken at the TE on both wings. Took it home and repaired it. Same thing happened again.

Finally I figured out that with NO flex in the wing, something has to give when flexed! And guess what? It's the ribs! Nope, haven't rebuilt it and have no intention of ever doing so.

Would I build a wing with geodetic ribs? No way in **#L. I'll stick to "D" tube and capstrips.

Good luck ALL, Jerry

How about an O-beam spar?   http://clcombat.info/sosspar.html

Take a look at Bobby Hunt's Lost foam wing set ups.  I have seen and held one of his geo-detic wings.  Very light and stiff.  That is the wing called the Geo-bolt.   Need to get Bobby back over here from the electric threads.   

Update

Bin Laden no longer a Civil engineer or on-line.

Norm