Weird paranoia about wing spars

SixPapaCharlie

SixPapaCharlie

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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
 
Check out a local glider club someday. You'll get a good look at quick-connect spars there, and many are aerobatic rated.
 
They rarely fail. Even rarer if you do not exceed their design loads.

Generally, spars are tested in place by loading the wings with sand bags until there is deformation in the structure, and then failure. They cut that load in half and there is your G factor limit. If a plane is rated at 6 Gs is means it was tested to at least 12 before failure. This is a generalized statement.

Wood makes an excellent spar if properly maintained. It absorbs repeated forces very well.

Always good to question things though. :yes:
 
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This is why I was afraid of turbulence and steep turns.

But the wings have since demonstrated to me that they are more then capable of handling the stress.

1.414lbs x gross weight= how much they are rated to hold in a steep 45* turn. And you have to feel confident knowing there's a big safety factor.
 
Your airplane is rated to 3.8G if you keep it in W&B limits and the flaps up. There is also a 50% design margin.

I strongly suggest you go up with an aerobatic instructor and feel what 3+G feels like. It's a lot more than you think. Or go up yourself and do 60 deg steep turns (that's only 2G -- still a lot more than you've probably felt to date).

I tend to overengineer, too. That comes from not being an ME. I once built a camera stand as an intern, and later found one of the techs using it as a stepstool. Yes, it was that overdone. Steady as heck, and you could probably jack your car up with it.

Every mechanical device has a yield limit. The balcony on your house will break if you put a swimming pool on it. The engine in your car is probably held in by no more than four bolts -- and that through rubber donuts. The wheels on pickup trucks are often held on by ONE nut each (not the lug nuts, but the hub and bearings).
 
I went up in a pitts last month for an aerobatic flight.
Holy crap. 4 G's was tremendous (to me anyway).

I have never tried a 60 degree turn.
I will put that on my to do list.
 
Sometimes they bend, sometimes they break.

But, unless you are into teaching yourself aerobatics, flying into thunderstorms, and/or totally loosing control in IMC, it's pretty hard to get either failure mode.

Most annuals don't specifically involve inspecting the spar unless there is a known problem with that particular make / model.

They are engineered to a specific weight / G load that depends on how the aircraft is to be operated. There is typically the specified maximum G (at some specified load) and about a 1.5 times safety factor beyond that - ymmv.

Lift distribution tapers off as you get towards the tips. A lot depends on the wing planform, twist, and any changes in the airfoil as you go outboard. This would be included in the analysis of the loads on the spar.

The connection between the wing and the airplane has to be strong enough. How strong depends on how the structure is designed. In my airplane there is one 3/8 inch diameter bolt at the main spar and a 5/16 inch bolt at the rear spar. Those fittings do not carry any bending loads - just compression (positive G) / tension (negative G). The strut carries all of the actual vertical loads back to the fuselage - it's attached at each end with a 3/8 inch bolt. If one of those fails, someone is going to have a bad day.

The wing spar is the last thing that I worry about. Fuel, weather, engine, etc. are way higher on my list.
 
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SixPapaCharlie said:
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
Click to expand...

Maybe a mechanical/structural engineer can explain how incredibly strong such types of bondings really are.

btw - if you don't like bolts, don't go in skyscrapers.
 
Capt. Geoffrey Thorpe said:
The wing spar is the last thing that I worry about. Fuel, weather, engine, etc. are way higher on my list.
Click to expand...

I worry about those things too but they all have a a plan of action.

This just seems like an unknown to me and likely 100% chance of ending my life if it fails. So time to get some knowledge about it just to get some comfort.
 
SixPapaCharlie said:
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
Click to expand...


You need to look at a plane being built. It will really open your eyes. It never ceases to amaze me how a bunch of parts can be bolted together and it will fly. Never gets old, never gets boring. It is always cool to me anyway.

When you build a wing on an RV for example, you can see how the load is distributed along the main spar to the tip of the wing. There is also a forward spar and an aft spar to secure the ends of the ribs. Together, it is a very strong structure. Individually, the pieces will scare the hell out of you. FYI, all Vans spars are factory made, they don't want anyone building them but factory trained craftsman.

This is why I love experimentals. You can study the pieces and learn how things are made, and why they are designed the way they are. Get involved in aviation! There is a difference between between being a pilot, and being an aviator. Learn how the airplane you fly works, and how it was built. Then study more and different airplanes. Ultralights, powered parachutes, gliders, acro, helps, spray planes, jets. It never ends and it never gets old.

Join EAA and get involved with observing builders in your area. Dallas has TONs of RV builders. Fly down to Hicks Field and have a talk with Jay Pratt, or any RV driver there for lunch.
 
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3 categories of aircraft load factor. Normal: 3.8G, Utility: 4.4G, Acrobatic: 6G(+). All Bonanzas are rated in utility category at gross weight. Some were rated at acro. I think, but am not sure that the Tampico is rated for Utility category at some weights, and at Normal category at max gross. I could be wrong though.

Spar failures are exceptionally rare. Review the history of the T-34 Mentor for some info on spar issues. It took a long time, and a lot of cycles before there was a problem.
 
SixPapaCharlie said:
I worry about those things too but they all have a a plan of action.

This just seems like an unknown to me and likely 100% chance of ending my life if it fails. So time to get some knowledge about it just to get some comfort.
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You are a pilot, now you need to become an aviator. ;)

Will a broken spar end your life? Not if you jump up right before impact. ;)
 
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Spars are deathtraps.:rolleyes2: That is why I fly aircraft without wing spars.:D
 
ClimbnSink said:
Spars are deathtraps.:rolleyes2: That is why I fly aircraft without wing spars.:D
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What aircraft are you flying without wing spars? Powered parachute? :confused:

Even most helo rotors have wing spars running through them. :dunno:
 
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Don't worry, the thin aluminum in the spar will let go long before that high strength steel bolt will fail.
 
Henning said:
Don't worry, the thin aluminum in the spar will let go long before that high strength steel bolt will fail.
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This is why wood makes a great spar. Light weight, strong, absorbs vibration. Termites love them! :eek: ;)
 
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Bob Noel said:
Maybe a mechanical/structural engineer can explain how incredibly strong such types of bondings really are.

btw - if you don't like bolts, don't go in skyscrapers.
Click to expand...

Okay.

I would be less concerned with steel bolt connections failing that I would be concerned with the structural aluminum spar sections themselves failing due to cyclic overloading, and/or stress corrosion cracking. Which isn't to say I'm worried about the spar section failing, because I'm not.

I read an NTSB report not too long ago about an Arrow that landed hard enough to shear the gear mounts off the mains (trust me, the gear on those things are STOUT and can take a pounding) but the wings themselves held together.
 
I am worried about all of it because it is unknown to me.
My plane was built in 1994 but has close to 8000 hours on it (cringe).
It was owned by Parks Aviation College in St Louis so lots of students beating it up I am sure.

I read that the wing spar is rated for 14,500 hours
 
SixPapaCharlie said:
I am worried about all of it because it is unknown to me.
My plane was built in 1994 but has close to 8000 hours on it (cringe).
It was owned by Parks Aviation College in St Louis so lots of students beating it up I am sure.

I read that the wing spar is rated for 14,500 hours
Click to expand...

That's why I wanted a parachute for the longest time. If the wings fail, your chances for death are almost 100% unless you have a parachute.... lol which is why my wife wants a SR22.
 
Jhernandez04 said:
That's why I wanted a parachute for the longest time. If the wings fail, your chances for death are almost 100% unless you have a parachute.... lol which is why my wife wants a SR22.
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You are a strong guy Jesse, you could just grab hold of the wing and fly it in.
 
SixPapaCharlie said:
I am worried about all of it because it is unknown to me.
My plane was built in 1994 but has close to 8000 hours on it (cringe).
It was owned by Parks Aviation College in St Louis so lots of students beating it up I am sure.

I read that the wing spar is rated for 14,500 hours
Click to expand...

So that means you have 6,500 hours left. Average 100 hours a year and you have 65 years left. Trust me, no one has hit 14500 hours yet, and if and when they do it can be extended.
 
SixPapaCharlie said:
You are a strong guy Jesse, you could just grab hold of the wing and fly it in.
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Right but my wife has this weird thing about me reaching over her body. So as long as it's the left wing we're good. But The right would mean I get slapped before we crash.
 
Jhernandez04 said:
Right but my wife has this weird thing about me reaching over her body. So as long as it's the left wing we're good. But The right would mean I get slapped before we crash.
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It would suck to be on your way to certain doom and then also get slapped in the face.
 
The Citabrias hold their wings on at the root with a 3/8" bolt in the front and a 1/4" bolt at the rear, both in double shear. The wing struts use 7/16" at the front and 3/8" at the rear and one 1/2" bolt at the fuselage. The airplane is rated for 4.5G, IIRC. And there will be a 1.5 safety margin above that, so it takes at least 6.75G to start bending otr breaking something.

Most airplanes will show signs of damage well before an actual failure. If you see wrinkled wing skins on a metal airplane, for example, be wary. Someone might have overstressed it. Damage in composite airplanes is much harder to detect.

If you want to worry about something, you need to know that airliners have lower ratings than that.

Dan
 
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Citabria is 'Airbatic' spelled backwards. They are +6G rated.
 
Geico266 said:
It failed at 154% of design load. Perfect engineering.

Now go fly and quit worrying about it. ;)
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I want to but the weather is stuck in crap mode.
Was planning on going to San Antonio on Sat but looks like it may be a no go.
 
Geico266 said:
It failed at 154% of design load. Perfect engineering.

Now go fly and quit worrying about it. ;)
Click to expand...

My A&P has a hangar full of half assembled RVs I am going to pay him a visit and get the scoop on the engineering of wings. I like knowing how things work.
 
Bob Noel said:
Maybe a mechanical/structural engineer can explain how incredibly strong such types of bondings really are.
Click to expand...



Structural engineer reporting for duty. Give me a drawing or diagram of your wing spar and I'll tell you how terrible it is and how much my fee is to design a better one. :P (But, seriously, post a sketch or diagram or drawing of the wing spar bolted connection, I'm curious.)

Bob Noel said:
btw - if you don't like bolts, don't go in skyscrapers.
Click to expand...

Watch out for bridges, trucks, industrial facilities, hangers, prefabricated steel buildings, and swing sets as well. :lol:

One thing to note is that connections are usually designed for a greater safety factor than the structure. The wings may be designed for 1.5 safety factor but the bolts are probably 2 or more. I'd worry about fatigue failure of the connection before outright fracture of the bolts.
 
Let me know if you need more

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SixPapaCharlie said:
.........................
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???
Click to expand...
You think that's scary? With a helicopter the whole main rotor assembly is held on the mast with just one threaded nut. If the "Jesus nut" comes un-screwed the main rotor just slides right off and the aircraft assumes the aerodynamics of a rock.
 
Remember that for metals there is a "yield strength" and a "failure strength". At the yield strength the metal will permanently deform, but not break. Most wings are tested to yield, when you see the wing take permanent damage. Wing may bend, skins wrinkle, ribs deform, etc. Many times an airplane with this kind of damage can be landed. As mentioned this is usually a 50% load over the airframe load limits, so if your plane is rated at 4g then yield *may* occur at 6g. True breaking failure is somewhere beyond that.

My airplane has carbon fiber spars, and they act differently...there is no yield strength for composites. Depending on the type of load and the shape, they either stay rigid or non-permanently flex right up to failure, then they shatter in spectacular fashion. Not better or worse, but different from how metal behaves. My plane is rated to 4g positive, and I once did the math on factory numbers on ultimate load and found it to be around 8.9g, a number I am very unlikely to ever get to.
 
My friend told me about that.

He said it is called the "Jesus nut" because if it comes off, the first person to meet Jesus wins.
 
SixPapaCharlie said:
I have never tried a 60 degree turn.
I will put that on my to do list.
Click to expand...
Really? Has your instructor not had you do steep turns yet? I was taught (less than two years ago) that a steep turn was 60 degrees, and used that criteria during my checkride as well.

45 is just "Aww, crap, I need to tighten this up a little". :)
 
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