I would say because there are fewer ribs and they are in a Warren Truss arraingement with is "stronger".

I'm sure others here can give better answers. 

IMHO I believe that the advantage aside from weight (?) is the resistance to torsional movement. When you orient the grain (strength ) with the direction of stress it reduces the allowable movement along that axis.

Bill,

A triangle is much stronger than a square or rectangle.

Bill,
First, this is my observation, I am open to correction by someone who knows better, but this is what I have learned and experimented with so it is my own understanding.Your mileage may vary
the strentgh of wood lies along its grain. To maximize the use of its inherant strength you want the grain to run in the same direction as the stress. for example, when you twist a wing it wants to flex diagonaly from the trailing edge to the leading edge. If you orient the grain of the wood so that it lies parallel to the motion of twisting, (the axis or rotation) it will more efficiently resist the torsional movement. by aligning the ribs at an angle to the leading and trailing edges it more accuratly aligns the strength (grain) of the wood  with the twisting motion in the wing. The leading edge sheeting resists the bending of the tips up and down because the grain is aligned with the direction of motion, or axis of rotation. In a conventional wing configuration the ribs and sheeting are perpendicular to each other and have comparativly limited resistance to torsion, twisting. The sheeting still resists the tips movement vertically as long as there is enough strength seperating the skins to prevent them from moving in relationship to each other. This is where the shear webs come in, they prevent the wing skins from collapsing. By incorparating diagonal ribs, we are allowing the strength of the rib to align more closely to the expected axis of rotation,(twist as well as fore and aft flex, tip moving front to back)
This principle is also why putting glass cloth or carbon fiber at the center of a structure will Not increase its stiffness, it will increase its strength with regards to flexability. In order to increase the stiffness of a structure you need for the material you are using to be the maximum distance from the axis of rotation, (the center of the structure in most cases)  which increases the levarage, or moment arm that is acting on it. so in other words, if you want to use carbon fiber or glass to stiffen a structure, it needs to be as far towards the outside as allowed by the structure. If you want to increase the ability to flex and not fracture, then put it at the exact origin of movement, (or axis of rotation ) To hopefully make this more clear, say for example a bow, as in bow and arrow, you want the bow to be strong, but yet flex. the more of the glass reinforcement you put on the outside of the bow the more it will resist bending. the higher percentage that remains in the core of the bow the more flexible it will be. the arrow is a hollow shaft of composite because it needs to be very rigid, therefore any weight in the center of the shaft contributes little for rigidity(versus its added weight) so it is eliminated.

I've never seen a REAL airplane with the ribs angled.

The major downside I see (of the angled ribs) is that it leaves some large areas of covering unsupported.  The normal fore-and-aft rib placement gives a more consistant support of the covering.  I've seen a few "taperwings" where the rib placement increased toward to tip, thereby making the square inches of covering per panel equal.

Things such as leading edge sheeting, trailing edge sheeting, and covering do a fine job of keeping the wing square.  No need for angled ribs.

We can discuss the airflow over the angled ribs later.

I think the British Wellington bomber had geodetic structure.  You see geodetic and Warren truss used quite a bit in high-tech free flights.  Maybe I should say moderate tech.  I do think that the wing cross section is weirdly affected. 

For stunt purposes a geodetic wing, ala' Werwage's P-47, is a bit stiffer for the reasons Mark S. states, and a bit lighter because it uses a little less wood.  Any airfoil effects from the angled ribs don't matter much in a stunter.  The planes usually have so much excess lift and power  that slight airfoil differences really don't enter in to the equation. 

The biggest thing is the stiffness.  If the plane doesn't bend when you turn it you can fly it much more consistently.  Make the wing as stiff as possible.  Make the fuse as stiff as possible.  Make the stab/elevator very stiff.  Box in the rear of the fuse so it can't twist.  And as Brett Buck pointed out somewhere, even mount the stab on some small 1/64 in. ply or glass doublers so the wood under the stab doesn't flex.

I am not an expert on aircraft structures, either full size of for models.  I have read and studied a bit on the subeject.

It was good to see the post and the picture above by Ray Stone (Minnesota modeller) with the geodetic construction on his 1/2 a model.

Geodetic wing construction helps considerably with the torsional stiffnes of the wing.  There is one factor that seems to be lost in some applications seen in recent years on some control line stunt designs.   Maximum rigidity from geodetic construction is achieved when the ribs are oriented to each other at 90-degrees.  (See the 1/2 A above).  Any free flight design that has been published with geodetic wing construction will universally show the 90-degree rib orientation.  There is a very good reason for that and that is increased resistance to twist which is kind of important considering the loads encountered by modern free flight designs and their thin, undercambered airfoils.

There is probably a trade-off by those who have designed stunt ships with angled ribs.  It will take more ribs (meaning more wood = more weight) to maintain a reasonable airfoil for the entire wing planform using a 90-degree orientation than if the rib angle is not so "pronounced".  (Reference is made to the Geo XL and its derivatives.)  So the trade-off is between a "satisfacory" increase in torsional stiffness while not increasing the weight of the structure an "unreasonable" amount.  The Geo XL type wing can be built very light.  One of the reasons is those wing strucures just do not have that much wood in them.  And those designs have definitely proven themselves at the highest levels of competition.

This is not written to be critical of any particular design.  It is merely a statement that maximum geodetic wing torsional rigidity is achieved when the ribs are oriented 90-degrees to each other.

Food for thought.

Keith Trostle