So 3/4 through my training we got a plane and with it a hundred pounds of manuals detailing the parts of the plane, etc
Then I got to the page where I exclaimed "Wait, you mean to tell me the wings just bolt on. There is not an I Beam running straight from one wingtip to the other???
This is when I started to dislike Steep turns or any positive G maneuvering for that matter.
Also I recently learned my buddy's aerobatic plane has a wood wing spar.
"Holy crap! Really??" was my response to learning this.
So
1. This is probably the most critical part of the plane right?
2. How do they fail? (snap and plunge or "hey that one wing looks strange" and land)
3. Our annual is next month. Do they go in there with a camera ans inspect that spar?
So I am curious about wing spars, failure rate, how they are engineered, how lift is distributed along the length of the wing, How strong the connection between wing and plane is, etc
Examination of SixPapaCharlie's wing connections:
This is VERY long, you were warned.
Also, I have not run any calculations so I'm operating on the assumption that these connections were designed properly. Obviously any structure can be improperly designed. I'm only going to touch on the merits of bolted vs. welded or solid wing spars.
Let's start with the wing spars running from one wing to another:
Your wing spars are connected together by what's called a bolted moment splice. It's a very standard method of connecting two shorter beams into a longer beam. It's used extensively in bridge girders (see below), buildings, aircraft, and elsewhere.
Properly sized a bolted splice can be just as strong than the actual structural member. This type of connection has a number of advantages in an aircraft:
- Bolts are removable thus allowing dis-assembly and replacement.
- Bolts allow for sections to be fabricated in smaller pieces, reducing costs.
- Bolted splices may provide further ductility in a structure than just a straight beam. (Ductility is effectively the amount of "distress" a structural member shows before failure. Metal bends significantly before fracture so it's considered a "ductile" material. Glass does not bend significantly before failure and thus is considered "non-ductile".)
We'll see below that bolted connections are highly ductile. Also, here's a great (AND REALLY COOL) example of a non-ductile compressive failure of high-strength concrete:
- A welded connection in aluminum alloy will lower the strength gained by tempering in the alloy. Thus bolts (and rivets) offer a connection method that does not lower the strength of the structural member. This is different than most steel members which typically do not lose strength from welding.
- This connection allows the wing spars to be straight but still allow the wings to have a dihedral shape (the "v" shape of the wings).
A few disadvantages to not having a solid spar between wings:
- Any connection (either welded or bolted) will cause a fatigue point (see below).
- A solid or welded connection will be lighter and take up less space.
- A solid or welded connection are generally easier to design and a solid spar may be cheaper to build.
- A solid connection may be stronger than bolted connections.
- Lower maintenance and somewhat easier inspection.
Overall the advantages outweigh the disadvantages.
Failure modes in splices vary but most common is an overload of the splice will cause buckling in the top splice plate first and a fracture of the bottom splice plate or beam in a crack that runs through the bolt holes. Shearing of the bolts will occur as well but overall the bolt holes should deform prior to bolt shear.
Here's two good videos showing you how bolted connections (such as in a splice) will fail. The first one shows failure of the bolted plate at the bolt hole, this is called shear yielding of the bolted connection:
This next video shows the plate itself failing at it's weakest point (at the bolt hole). Note how this failure is gradual, the plate deforms greatly. This is called a tensile yielding failure:
There's another common failure mode and that's tensile fracture which is a sudden break of the material and a sharply defined crack. This is probably more common with aircraft aluminum.
So, to answer your first question: "This is probably the most critical part of the plane right?". You would be correct, at least from a structural standpoint. However, the design of the splice and other connections will reflect their importance.
For your second question: "How do they fail? (snap and plunge or "hey that one wing looks strange" and land)" That's really something that could go both ways.
I'd GUESS that snap and plunge is more common, however this ignores the likely many that were avoided due to timely inspections. Cracking in the attachment points would likely be caught hopefully before failure. Most failures of this type are likely due to fatigue or light and repeated overload. Thus, a large deformation failure is unlikely.
However, it does happen. There's actually a great AOPA and Air Safety Foundation video recently that discusses an aircraft that experienced extreme aeroelastic flutter.
Here's what aeroelastic flutter looks like:
In the ASF presentation the pilot was able to successfully land the plane. The planes wing was deformed by up to 2 inches. However, the plane's spar was cracked 2/3 of the way through and the planes wing rear attachment point was cracked through. Essentially the plane SHOULD have had the spar snap but he got very lucky.
I've attached some screenshots below of the damage. Click to see the horror.
So, this is something you really aren't likely to see from the outside. More often it will be caught during annuals before it becomes a problem. However, if it doesn't get caught in time it will likely be a snap and crash failure.
At this point I'd like to stress the difference between overload failure and fatigue failure. Materials can be overloaded till they break, however this is accompanied by deformations, cracking, and other "signs" and will often not result in total failure.
However, fatigue occurs when repeated stress cycles open up a crack in the structure. All materials can fatigue but metals are most famous for it. Fatigue occurs because not all parts of the material get stressed evenly. A notch or crack will have MUCH higher stress at the corner of the notch or crack. This will "open up" the crack further or cause an invisible crack to become visible to the naked eye. Over time this crack will propagate through the material until not enough uncracked material remains and a failure occurs.
Skip to 3:43 for the good bits but the whole video is good.
Stress concentrations in connections occur at welds, bolt holes, notches, edges, etc. Fatigue always happens unless you have a VERY low stress (not likely in a wing spar) or your lifetime number of stress cycles are kept below 20,000 (again, not likely as wings bounce around many times each flight). Thus, fatigue will always happen and will always spell the death of any structural part given enough time. That's why airframe time is so important when buying a well used airplane. You might be looking at having to buy entirely new wings if your wing spar has a large fatigue crack.
Now, the most important parts, the four bolts that attach your wings to your aircraft. Note, it really is four bolts and not two bolts per side. The wings are spliced together and will act together on the airframe. So don't think that just one bolt failing will take down the aircraft.
As shown above steel bolt failure is much less likely in aircraft aluminum, the bolts are simply much stronger than the plates. Thus, there isn't much advantage of using more bolts as far as the steel bolts are concerned. However, more bolts will spread the load out more and could reduce fatigue.
In the end though you really only need four bolts. As long as the bolts are properly design there is no reason to add more. Yes it's more redundant but that's not entirely the point of engineering. Engineers design something to be safe but not so safe that you lose performance or efficiency. Thus, if four bolts meets their design criteria then why use more?
Plus, this has some advantages. Maintenance and inspection are easier as there are less things to go wrong. This also makes it easier to remove the wings if needed. Also note that the wings act like a "saddle" for the plane. The wings lift up and the plane pushes down.
Now, it's worth noting here that I'm a building structural engineer. I don't design aircraft. However, I suspect that those bolts are primarily there for twisting, turning, negative loads, centrifugal loads, and so on. The main support of the weight of the aircraft when flying is done in concert with other supports.
For your final question: "Our annual is next month. Do they go in there with a camera ans inspect that spar?"
I do not know.
I assume they do and I know some aircraft have AD's that require regular inspection of a wing spar and attachments. An A&P can obviously answer this one.
Some quick notes on wood: There's nothing wrong with wood. You can make a plane very, very strong out of wood. Connections can be problematic but not impossible. Rot and moisture and such are important for wood but definitely not impossible to deal with.
Essentially the only reason we use mostly metal and not wood is metal is stronger (see previous posts) and results in a more efficient design. Also note that metal is more durable and easier to maintain. It's also easier to design with and mostly easier to manufacture. Note I didn't say it's better, a plane made out of wood can be just as good as one made from metal. In some ways better because if you don't NEED the strength of metal your plane can be lighter. Lighter planes put less load on their structure. Nothing to worry about getting into a wooden aircraft. That's like saying you don't want to drive over that stone arch bridge because it's old and doesn't use modern materials.
Summary:
Your aircraft uses a common connection design that has worked for a VERY long time. It's robust, effective, and offers many advantages such as wing removal. Lowered fatigue resistance is about the only problem but this is a very hard problem to remove regardless of the design. Periodic inspection and avoiding repeated overload of the wings is your only big concern. The design loads that your wings can take are much more than you would have during normal high-G maneuvers. As long as you're not hitting things and diving through thunderstorms you're fine.
