Basic structural questions when we are designing an aircraft...
well generally I do over design ! :D
yet I usually do a static test I always use carbon tube wing joiners which those are also selected over design (25 -30 mm tubes with 1mm wall thickness) . I will put 1.5 times the take off weight at 75% of the wing span . I will usually do a multi spar design with a main spar being a box design with 3x15 to 3x20 birch plywood as top and bottom flange and 1.5mm balsa on both sides as webbing for a 2500-3000mm wingspan and 7000-1000gr of weight. other smaller spars 4x3-6x3 are distributed along the rib top and bottom which generally support the 1.5mm balsa sheeting of the wing .
I've created an excel spreadsheet to approach this subject... refer link.
If anyone needed help in understanding the so called calculator, just ask. The main sheet is all, while the next 2 sheets is only material property... and only the gray shaded box requires inputs...
I can provide further discussion on the approach to optimize (not to overly design things) the design of an RC aircraft structure... however the forum here decided not to hold further discussion, so I believed we let it rest. Thanks.
the group is young and has not got running quiet well .these discussions will gain much more attention in the near future . yet thanks for your contribution
The excel spreadsheet requires access approval. Can you share it directly as an attachment with the group?
You can download it from that google drive link... can you try it and let me know, thanks.
Got it. Thanks!
Just to start the conversation…
In 45+ years of model flying I can’t recall seeing a wing fail due to a simple spar failure. If a wing fails in flight or even in a hard landing it’s usually at a point of stress concentration or glue joint failure. What that tells me is for the most part, spar load capacity, as we typically see in the average model, is probably many times greater than is what is necessary. Failure usually occurs at points of stress concentration. Like where the fuselage meets the wing or landing gear mounts. Or it occurs at the wing center where spars are spliced by various methods.
I have completely rebuilt kit planes from scratch quite a few times just to revamp the wing design. I’ve never felt the need to change the spar size or type. What does get changed every time is the type and placement of the shear webs between the spars. Shear webs are generally unnecessary on the outboard 20% of the wing. It doesn’t save much weight to eliminate them but sometimes every gram counts so why add unneeded weight. On the inboard end of the wing, the stresses rise exponentially. Doubling up on the shear webs or even changing to plywood that spans the center section is advisable. Adding extra bracing where stress concentrations exist can eliminate the additional stresses on the spars that they wouldn’t normally be expected to absorb. These stress concentrations may often produce latent damage that may cause failure long after the damage actually occurred.
A good example was a Morris Hobbies Profile Hots. Built from a kit the fuselage was essentially just a flat plank. The built up wing had a very thick airfoil and all the radio gear was stuffed in the center of the wing. The wing was permanently attached to the fuselage. To give room for the battery and receiver, the shear webbing was to be omitted from the very center of the wing. All it took was a hard landing and the wing spars would crack right where they crossed the fuselage. There were several other minor design issues with the kit and after making a dozen repairs mainly due to the design deficiencies over the first year of its life, I threw it in the waste bin. I built a second version in a 24 hour marathon building session. The main change was a plywood shear web on that center section and the elimination of the outer shear webs. That was 20 years ago. The plane is routinely put through extreme 3D aerobatics and it’s survived its share of rough landings. other than a few pin holes in the covering, it has never required any repairs. (It is on its third generation of radio gear.) it is lighter and obviously much stronger than the original. The spars were the weak link in the original yet I kept them the same size!
To keep all this in context I don’t think it matters very much to know the specific spar load capabilities until most other spar design considerations are satisfied. Not that I’m not interested to know the spar load capabilities. But what I think might be more important are the construction techniques. Where are the loads applied? Where are the highest and lowest stresses. They obviously are not evenly distributed across the width of each wing panel yet we tend to build the wing the same from one end to the other. What failures have you seen and what did you do about them? Once all these points are covered then I think fine tuning the spar size would become the obvious next step.
What other considerations must be given to the wing design that effect spar performance in addition to the basic spar design itself?
Yes, the load on the wings is not the same from root to tip, it depends on the local thickness of the wing and its local chord length. Furthermore at tip region you'll have losses in terms of high pressure air escaping to the top of the wing thus create the dreadful tip vortices i.e. drag...
I have seen wings snapped in 2 mid-flight and yes failures typically at the inconsistent glue joints and rarely material failures. Most of the wing designs from old generations kits is some what over designed i.e. it can carry much higher load than necessary.
Contrary to the standard thinking of the spar web is adding the strength to the spars, yeah, in a way it does, but more to it is to add the resistant to shear loads that occurs between the top and bottom spar. Take 2 pieces of 6 inch metal ruler, place it one of top of the other and align it properly. Bend it, you'll see the bottom and top will be miss-aligned and the same can happen between top and bottom spar of the wing under load. This is why, the spar web typically will use cross grain cutting of balsa... i.e. the grain will be aligned vertically between the spaces between each ribs. As you rightly put, this high shearing loads will be more pronounced at the root and therefore only the first few bays we'll have this web. It will also be economical in terms of weight if the spar can be reduced in size as we move towards the tip but for practical reason, majority of the designs it'll remain the same.
Later, we'll discuss about failures in other part of the aircraft, at this moment enough to say that for every failures, there must be a cause to it, it can be either aerodynamic load or it can also be impact loading...
I believe the fact that our planes are subjected to all kind of off limit forces and dont have a documented maintenance routine and on the other hand they are made by wood which has a wide range of mechanical properties fluctuations sample by sample , requires an over design in its nature
dear bob you have pin pointed rather fine design areas ! and that is the importance of webbing that is a place where material is chosen to be minimal (usually 1.5mm balsa for wide box designs and 2mm if there is only one web as an I beam design . as we all know the term balsa would refer to a wide variety of wood from 120gr/m3 up to 300gr/m3 which is a drastically wide range of mechanical properties difference . the other fact is webbing is glues to the spar and gluing is where fluctuations occure one joint might be well bound and other might be weak due to lack off enough glue or not enough bonding pressure and time on miss placement of parts and gaps
yet i have always calculated that a high S.F over designed wing with wider spars and harder and thicker webbing would only be 10% heavier than a marginal design (100gr for a 1000gr wing of a 3000 mm piper cub for example) which is totally affordable weight and should be considered worthy !
i should mention that by spar i mean the sole structure of each wing panel and not the joiner on the two since for the joiner i always use 2 carbon tubes one in 25% as thick as my rib thickness would allow me and other smaller in 70% on the chord
i personally prefer to reduce weight as much as i can from the ribs i use hollowed 4.5mm rim ribs on that 3000mm wing spam piper where it has 3x25mm top and bottom spars with 1.5mm webbing on both sides as box
i dont have very much experience as you and bob might have yet i have found out myself the best and most effective way on weight reduction is through good use of material selection and variety rather than slimming the structure .
I use built up ribs too. Especially when the airfoil is very thick.
In terms of overall weight, the contribution of the spar itself is usually quite small. So overdesign is well tolerated. The shear webs must be considered part of the spar structure. Without them, the total tensile and compressive strength of the upper and lower spars is barely realized. (Hence your example with the rulers.) I would be interested in finding out the optimum shear web arrangement at the root, mid section, and tip for a constant spar size. (A tapered spar isn’t necessarily difficult but payback in weight savings is marginal.) Based upon my observations the shear webs have been the weak link. So optimizing shear web design is as important as optimizing spar size.
As a side note, vertical grain shear webs seem to be most common but any design that couples the shear forces between the spars fits the narrative. Diagonal grain shear webs might be most efficient. A lattice structure might be more applicable on lighter models. I’m sure there are many other methods but this becomes part of the discussion of shear web optimization.