Weird paranoia about wing spars

I am not so worried about design faults, but corrosion and fatigue are concerns. My airplane is struted, so it does not have a spar going through. It does, however, have a lower carry-through, made out of a square steel tube. Its inspection is difficult, because I have to pop the floor, held in place by cherry rivets.
 
SixPapaCharlie said:
"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???
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Mooneys have a single, carry-through spar that goes from wingtip to wingtip. One has never broken up in flight.
 
MAKG1 said:
...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...
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Last summer I set out to build a shed to keep the rain off my yard tools

ended up with a tornado shelter....
 
dell30rb said:
I was just about to post that pic!
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Here's another one - okay, they're women but still...

wings4.jpg
 
Henning said:
My friend and first aerobatics instructor Lisa was in the right seat of one of those C-130 that folded up...:(
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The US Forest Service is to blame for that. They bought those airplanes from the USAF after they were retired and sitting in the desert. Then they proceeded to work them harder than the air force ever did, while not performing maintenance (and structural inspections) to anywhere near the level of the USAF maintainers. Years before the crash the FAA and USAF raised an alarm about the relaxed inspection standards of the forest service, but nothing was ever changed due to the USFS objection to the costs.
 
dell30rb said:
The US Forest Service is to blame for that. They bought those airplanes from the USAF after they were retired and sitting in the desert. Then they proceeded to work them harder than the air force ever did, while not performing maintenance (and structural inspections) to anywhere near the level of the USAF maintainers. Years before the crash the FAA and USAF raised an alarm about the relaxed inspection standards of the forest service, but nothing was ever changed due to the USFS objection to the costs.
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Yeah, pretty sad. She was an Aeronautical Engineer and had just taken her first 'real' (non CFi) flying job.
 
Henning said:
Yeah, pretty sad. She was an Aeronautical Engineer and had just taken her first 'real' (non CFi) flying job.
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It is sad. If the C-130's were carrying passengers the FAA wouldn't have let the issue slide.
 
I vaguely recall some comments from an instructor that said certain models had known spar issues and were ordered to be replaced but this and a handful of others were sold for non military purposes before that was carried out.

Someone here probably has better details on whatever I just typed but I do recall discussing it with someone. That crash always bothered me because there was nothing the pilots did to cause it or could have done to survive it.
 
Dan Thomas said:
<snip> 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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This is a bit of a myth, There is a reason for the 1.5 Safety factor. It has to do with the breaking strength vs the Yield Strength of Aluminum*. Turns out it is about a 1.5 factor. Wings are designed to not Yield at the Limit Load (6G for a Citabria) and not to fail below the Ultimate load (9G for Citabria). So at 7G's, you may bend and permanently damage the wing but it won't fail.


For those that think airplanes are over built I did some work with a former Cessna engineer that was doing some work for us as a DER on the Thunder Mustang. He stated that when Cessna designed a wing they would create the design and then test it. If it passed on the 1st test, the engineer that designed it would be fired for building it to heavy. It was much easier to repair and strengthen the places that failed than it was to try to remove unnecessary weight.

Composite aircraft are often designed with larger Limit vs Ultimate load values 2.0 often. This is due to composite quality control being more difficult than Metal or Wood aircraft. Also do the fact that there is very little difference in the Yield and Failure strength. IE, they don't permanently bend, they just fail.

Brian


*I haven't worked with wood much but my minute google search seams to indicate that Spruce has about 1.5 Yeild vs Fail Factor also.
 
So get on NTSB's website and see if a TB-9 has ever had an inflight breakup. I don't recall one. Any aircraft I've thought of buying, I've researched it's accident history including breakups. As far as GA, you'll see there's been some high time PA-28s breakup but they were heavily used in areas such as forrestry service.

One of the reasons why I like composites is there is no stress memory. My Glasair is stressed to + 6 Gs and my Velocity + 9. You'd be hard pressed to find an inflight breakup on either model. I know of one Velocity breakup because the guy flew into a thunderstorm but that's about it.

Pretty sure there isn't a detailed spar inspection on GA aircraft unless your talking T-34s. My AA-5 had inspection panels, but that's nothing more than looking in there for corrosion which by the way is a problem where the gear bracket attaches to the spar. Still, Grummans (tube spar) are known for their strength and I've never heard of one breaking.
 
Velocity173 said:
So get on NTSB's website and see if a TB-9 has ever had an inflight breakup. I don't recall one. Any aircraft I've thought of buying, I've researched it's accident history including breakups. As far as GA, you'll see there's been some high time PA-28s breakup but they were heavily used in areas such as forrestry service.

One of the reasons why I like composites is there is no stress memory. My Glasair is stressed to + 6 Gs and my Velocity + 9. You'd be hard pressed to find an inflight breakup on either model. I know of one Velocity breakup because the guy flew into a thunderstorm but that's about it.

Pretty sure there isn't a detailed spar inspection on GA aircraft unless your talking T-34s. My AA-5 had inspection panels, but that's nothing more than looking in there for corrosion which by the way is a problem where the gear bracket attaches to the spar. Still, Grummans (tube spar) are known for their strength and I've never heard of one breaking.
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Most RVs have access panels to get at the spar and as part of the annual condition inspection they are inspected for cracks, and the bolts re-tourqued.

Several have had the wings folded up, but all were stupid pilot tricks. Several early 3's folded their wings when the drivers would do a high speed pass and pull a 10 G departure. One had the wings fold because the builder modified the wings and spar off one model and put it on another. :nono:

If you operate airplanes within their "envelope" you have a very good chance of dying in an old folks home.... Or shot by a jealous husband. ;)
 
Don't fear wood.

Wood never fatigues contrary to metal. It doesn't corrode either. It can however get cell structure deformation and it can rot, but both are visible and preventable. Wood is excellent structurally and is in many ways much stronger and lighter than metal for aviation structures. Modulus of Rupture for Birch is actually 1.7 times stronger than aluminium for the same weight, which people don't believe. We've been ingrained into thinking metal is stronger. It's not always the case. Look at the MT props as an example. They're much lighter. Sure, wood hasn't got the impact resistance of metal as the material doesn't yield, but props are not designed to hit things.

First generation Mooneys had wood spars. Bellancas have always had wood spars. And many, many other aircraft through history. None of them have broken up any more than metal planes.

Did you know that the Kaman K-Max helicopter, one of the true heavy lifters of this world as an aerial crane, has wood cores in their composite rotor blades? All the Kaman's have had wood cored blades. The Bell 47 also had wood rotor blades without a time limit. When the metal blades were offered they had to put a time limit on them as they fatigued.
 
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colojo said:
Mooneys have a single, carry-through spar that goes from wingtip to wingtip. One has never broken up in flight.
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Oh yes, at least one has. It was one flown by a former student of an instructor friend. This guy was on a commercial charter with a Mooney and flew straight into a thunderboomer and came out in pieces. The Mooney guys looked at the recovered bits and said they calculated a 20G load at breakup. The passenger and his seat had punched straight out the bottom of the fuselage.

Dan
 
Velocity173 said:
So get on NTSB's website and see if a TB-9 has ever had an inflight breakup. I don't recall one. Any aircraft I've thought of buying, I've researched it's accident history including breakups. As far as GA, you'll see there's been some high time PA-28s breakup but they were heavily used in areas such as forrestry service.

One of the reasons why I like composites is there is no stress memory. My Glasair is stressed to + 6 Gs and my Velocity + 9. You'd be hard pressed to find an inflight breakup on either model. I know of one Velocity breakup because the guy flew into a thunderstorm but that's about it.

Pretty sure there isn't a detailed spar inspection on GA aircraft unless your talking T-34s. My AA-5 had inspection panels, but that's nothing more than looking in there for corrosion which by the way is a problem where the gear bracket attaches to the spar. Still, Grummans (tube spar) are known for their strength and I've never heard of one breaking.
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Ok, I looked em up. So not many TB9s around so a small list.
only 1 fatal the others all just running off the end of runways

This one is interesting "I was taught to do what the tower instructed"
Geez and this guy probably flew my plane. I am 546PC and he is 544PC. We got ours fro his school:
http://www.ntsb.gov/aviationquery/GenPDF.aspx?id=CHI99LA087&rpt=fa


This is a horrible way to go. I imagine they didn't know what to do so just pulled up to get out of the area:
http://www.ntsb.gov/aviationquery/GenPDF.aspx?id=MIA95FA224A&rpt=fa
 
stratobee said:
Wood never fatigues contrary to metal.
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Not true, wood and practically every material can and does fatigue. However, you are correct that wood's fatigue stress range is typically close to it's failure stress so you're unlikely to fatigue it to failure without outright overloading it to failure.

stratobee said:
Wood is excellent structurally and is in many ways much stronger and lighter than metal for aviation structures. Modulus of Rupture for Birch is actually 1.7 times stronger than aluminium for the same weight, which people don't believe.
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Not even close to true.

For Aluminum 2024-T3 (a common aircraft aluminum alloy), the ultimate tensile stress is[SIZE=-1] 70,000 psi with a density of 173 pcf[/SIZE][SIZE=-1][SIZE=-1]. A good high value for the ultimate strength of Hickory (a typically strong wood) is around [/SIZE][/SIZE]20,000 psi with a density of 37 to 58 pcf (approximately 3.6 times lighter). So, if we take an aluminum and wood block of the same size, the aluminum block will have 3.5 times the fracture strength and 3.6 times the weight of the wood block.

However, these are ultimate strengths. Design strengths for aluminum would be approximately 1/2 of the ultimate tensile stress. Wood unfortunately is a very variable material, thus we have to use a much higher factor of safety to account for differing material values. Thus, the NDS design values for select structural grade BEECH-BIRCH-HICKORY wood for buildings is 1,450 psi. :eek: At this value aluminum is 24 times stronger and only 3 times heavier.

In addition, stress in a beam is dependent on the beams geometry. This is why I-beams are shaped the way they are. This shape puts the most material the farthest from the middle of the beam allowing for better strength per weight ratio than a rectangle of the same dimensions. This is why aluminum spars and structural members are shaped the way they are. However, wood is costly and time consuming to form to I-beam shapes. Structural I-beams are starting to become more common in homes but it would be really costly to shape each individual spar for maximum strength to weight ratio for an aircraft as compared to aluminum.

Finally, wood's strength is VERY unidirectional. Wood has NO design value for cross-grain bending. It literally can't take load across the grain to any significant amount. Just think how easy it is to split wood with an axe through the grain than perpendicular to the grain. Aluminum and steel are mostly an isotropic material.

Thus, wood is more expensive, time consuming, heavier, weaker, and harder to design. Don't get me started on wood connections.

Now, for props we have different design criteria. We really don't need the strength of metal as the prop isn't going to be stressed as much as a wing or strut and the stresses will mostly be along the grain. So, for that we can get a lighter, cheaper, and in many ways better prop using wood. However, we do lose strength and sacrifice the low maintenance of metal props. Personally metal props are my pick but both have advantages.

Short version: for items that don't need the strength of metal, wood can and does make sense. Wood can also be easier to work with if time isn't an issue. Finally, wood does weigh less than metal so if you just need to make a shape that isn't structural then it can make a lot of sense to use wood.
 
Geico266 said:
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.
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Generally, safety factory is 50%, not 100% for aerobatic airplanes. Your 6G RV is expected to deform at 9G, not 12G.
 
stratobee said:
Don't fear wood.

Wood never fatigues contrary to metal. It doesn't corrode either. It can however get cell structure deformation and it can rot, but both are visible and preventable. Wood is excellent structurally and is in many ways much stronger and lighter than metal for aviation structures. Modulus of Rupture for Birch is actually 1.7 times stronger than aluminium for the same weight, which people don't believe. We've been ingrained into thinking metal is stronger. It's not always the case. Look at the MT props as an example. They're much lighter. Sure, wood hasn't got the impact resistance of metal as the material doesn't yield, but props are not designed to hit things.

First generation Mooneys had wood spars. Bellancas have always had wood spars. And many, many other aircraft through history. None of them have broken up any more than metal planes.

Did you know that the Kaman K-Max helicopter, one of the true heavy lifters of this world as an aerial crane, has wood cores in their composite rotor blades? All the Kaman's have had wood cored blades. The Bell 47 also had wood rotor blades without a time limit. When the metal blades were offered they had to put a time limit on them as they fatigued.
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Brantleys just are a thin skin of aluminum glued to foam.
 
Geico266 said:
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.
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14,500 isn't all that hard to get. And if most of it is done doing pipeline patrol, then you might have an issue. Tomahawks have an 11,000 hour time limit, plenty out there timed out and basically junk.

Cherokee's have a corrosion SB(SB1006 I think it is) on the fuel tanks for dissimilar metal corrosion that can @#$%^ the spar up and some wings have come off. I wouldn't consider buying a Cherokee unless SB1006 was complied with.

Now, as far as structural failure, I'm far more concerned with flutter.
 
tehmightypirate said:
Not true, wood and practically every material can and does fatigue. However, you are correct that wood's fatigue stress range is typically close to it's failure stress so you're unlikely to fatigue it to failure without outright overloading it to failure.



Not even close to true.

For Aluminum 2024-T3 (a common aircraft aluminum alloy), the ultimate tensile stress is[SIZE=-1] 70,000 psi with a density of 173 pcf[/SIZE][SIZE=-1][SIZE=-1]. A good high value for the ultimate strength of Hickory (a typically strong wood) is around [/SIZE][/SIZE]20,000 psi with a density of 37 to 58 pcf (approximately 3.6 times lighter). So, if we take an aluminum and wood block of the same size, the aluminum block will have 3.5 times the fracture strength and 3.6 times the weight of the wood block.

However, these are ultimate strengths. Design strengths for aluminum would be approximately 1/2 of the ultimate tensile stress. Wood unfortunately is a very variable material, thus we have to use a much higher factor of safety to account for differing material values. Thus, the NDS design values for select structural grade BEECH-BIRCH-HICKORY wood for buildings is 1,450 psi. :eek: At this value aluminum is 24 times stronger and only 3 times heavier.

In addition, stress in a beam is dependent on the beams geometry. This is why I-beams are shaped the way they are. This shape puts the most material the farthest from the middle of the beam allowing for better strength per weight ratio than a rectangle of the same dimensions. This is why aluminum spars and structural members are shaped the way they are. However, wood is costly and time consuming to form to I-beam shapes. Structural I-beams are starting to become more common in homes but it would be really costly to shape each individual spar for maximum strength to weight ratio for an aircraft as compared to aluminum.

Finally, wood's strength is VERY unidirectional. Wood has NO design value for cross-grain bending. It literally can't take load across the grain to any significant amount. Just think how easy it is to split wood with an axe through the grain than perpendicular to the grain. Aluminum and steel are mostly an isotropic material.

Thus, wood is more expensive, time consuming, heavier, weaker, and harder to design. Don't get me started on wood connections.

Now, for props we have different design criteria. We really don't need the strength of metal as the prop isn't going to be stressed as much as a wing or strut and the stresses will mostly be along the grain. So, for that we can get a lighter, cheaper, and in many ways better prop using wood. However, we do lose strength and sacrifice the low maintenance of metal props. Personally metal props are my pick but both have advantages.

Short version: for items that don't need the strength of metal, wood can and does make sense. Wood can also be easier to work with if time isn't an issue. Finally, wood does weigh less than metal so if you just need to make a shape that isn't structural then it can make a lot of sense to use wood.
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You seem to know what you're talking about and I don't want to argue with your numbers, but if we expand the process with compound wood construction it is very easy to get to higher than aluminium strengths. It's an excellent core material. Take your I beam for instance. You can lay that up with wood core, then skin with resin infused s-glass or carbon fibre and you will get both a stronger and lighter construction than aluminium. It's cheaper as well. Ask any canoe builder and they'll tell you that with the exemption of pure carbon fibre sandwich, the lightest canoes can be made with resin and s-glass covered cedar strips or thin plywood. The metal canoes don't stand a chance when it comes to weight. Or the DH Mosquito - resin covered plywood and balsa sandwich construction which produced one of the lightest and sturdiest airframes of WWII (at least when they figured out the glue problem they had in the beginning ;) ).

We've just been told for decades that wood is old and bad. We associate it with bygone eras. If you incorporate wood into modern engineering, in conjunction with resin and/or in advanced sandwich construction, it's a great and very strong material and also impervious to rot or any of the the things traditionally associated with it. It's a natural composite.

de-havilland-mosquito1.jpg
 
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tehmightypirate said:
In addition, stress in a beam is dependent on the beams geometry. This is why I-beams are shaped the way they are. This shape puts the most material the farthest from the middle of the beam allowing for better strength per weight ratio than a rectangle of the same dimensions. This is why aluminum spars and structural members are shaped the way they are. However, wood is costly and time consuming to form to I-beam shapes. Structural I-beams are starting to become more common in homes but it would be really costly to shape each individual spar for maximum strength to weight ratio for an aircraft as compared to aluminum.

Thus, wood is more expensive, time consuming, heavier, weaker, and harder to design. Don't get me started on wood connections.
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Yeah, I-beams are great... until you try to shear the beam. Then, not so much.

And when would you *ever* have cross-grain bending in an aircraft spar?:rolleyes:

BTW- Wood is a great material for spars since it is effectively "stronger" under shorter duration loads. Upside? Overstress a wood spar with a maneuver and it will splinter and bend, but is unlikely to shear completely off.

Source- I'm a Structural and Materials Engineer.
 
N659HB said:
Gotta love those Luscombes! Wonder how the rag wings would fare?
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Since metal and fabric wings are completely interchangeable, other than the possibly detrimental act of actually sitting on the fabric, underlying ribs and light leading edge skin they should do just fine. Of course these are all marketing gimmicks, was any damage done? Maybe not to the wing spars but how about the skin or landing gear? It's like that famous picture of BD Maule shooting straight up out of the Maule factory hangar. I've been there and that hangar is probably 500 feet long, open at both ends and with extended ramp areas at either end as well.

Pretty good marketing photo though.

clearshangar.jpg
 
warthog1984 said:
Yeah, I-beams are great... until you try to shear the beam. Then, not so much.

And when would you *ever* have cross-grain bending in an aircraft spar?:rolleyes:

BTW- Wood is a great material for spars since it is effectively "stronger" under shorter duration loads. Upside? Overstress a wood spar with a maneuver and it will splinter and bend, but is unlikely to shear completely off.

Source- I'm a Structural and Materials Engineer.
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Beam shear in a properly sized I-beam should rarely control outside of connections. I can't recall the last time I designed a steel or aluminum beam where shear controlled the beam size over flexure (excluding all the short stub beams and shear only setups).

Right, cross-grain in a spar is probably not possible. I was simply pointing out that a blanket statement of "wood > aluminum" is not true.

The load duration factor given by the NDS for wood design is 1.6 for short term loads and 2.0 for impact loads. However, constant loads such as wing loading due to aircraft weight would have use a load duration factor of 1.0. With the typical wood design safety factors load duration isn't going to do much except in extreme turbulence and such.

As far as being ductile I'd say aluminum and steel are more ductile than wood as they show much larger deformations following yield as opposed to wood which is fairly uniform in flexibility up to failure.

-Source Civil/Structural E.I.T. (2 months from getting licensed if all goes well)

stratobee said:
You seem to know what you're talking about and I don't want to argue with your numbers, but if we expand the process with compound wood construction it is very easy to get to higher than aluminium strengths.
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Oh yes, I agree. Wood composites can provide much higher strength to weight ratios over plain wood alone. Cost wise aluminum still makes sense to me for production aircraft but I could easily see a wood composite kit-built aircraft in the future.
 
tehmightypirate said:
Beam shear blah blah blah.
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More importantly, did you look at my diagrams?
Are my wings coming off?

That's 2 videos of wings coming off in flight. That s 100% of videos uploaded to this thread.

We're all doomed I tell ya.
 
SixPapaCharlie said:
More importantly, did you look at my diagrams?
Are my wings coming off?
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Without examining the calcs or having the sizes/strengths of everything, it appears to be a properly designed connection (aka- multiple connecting bolts, a normal factor of safety for capacity over demand, and the joint is stronger than the main spar).

If I were to fly in a plane with that connection, I would not be concerned about the connection beyond normal workmanship/maintenance issues with the metal spar.

SixPapaCharlie said:
Are my wings coming off?
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Could the wings come off (in-flight)? If it is properly sized, it is theoretically possible- but you'd have to work at it and the plane would give you plenty of warning unless you flew it into cumulogranite or something similar.

Source- I'm a Civil P.E. with a structures emphasis and a job as a materials engineer.
 
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tehmightypirate said:
Right, cross-grain in a spar is probably not possible. I was simply pointing out that a blanket statement of "wood > aluminum" is not true.

The load duration factor given by the NDS for wood design is 1.6 for short term loads and 2.0 for impact loads.
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So, cross-grain bending (aka bending along the long axis of the plane) is not possible; strength for 10-minute loads (wind loads, short-term maneuvering, etc...) is 60% higher than design values for durability; and we haven't even gone into the fracture mechanics yet.:mad2:

Your point was???
 
MAKG1 said:
Ya gotta admit, a video of wings not coming off would be about as interesting as watching paint dry....
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Wood spars. Wood wing ribs. Wood leading edge. Fabric covering. Metal drag wires.
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For some strange reason, the wings don't fall off.

 

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SixPapaCharlie said:
More importantly, did you look at my diagrams?
Are my wings coming off?

That's 2 videos of wings coming off in flight. That s 100% of videos uploaded to this thread.

We're all doomed I tell ya.
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Yes, but I wanted to write up something more than "looks fine to me" so you can learn how everything works in your wing spars and why it's really not an issue.

Basically I'm kinda going nuts.
 
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