Confusion on aluminum chassis on cars...

[dem00n]

My major isn't aeronautical eningerring, though i do go to an aeronautical school. There's always talk that airplanes with aluminum structures (fuselage?), basically have a expiration date. After they log a certain amount of flight hours, the aluminum is no longer deemed safe and the plane goes off to the boneyard, etc etc.

Wouldn't this be the same thing for sports cars with aluminum chassis? Wouldn't all the stress from the chassis flexing eventually deem it unsafe for racing or even daily driving?

[wbradley]

Sorry I'm not professionally qualified, but herel goes:
Airframe life is a measure of safety for a typically complex structure with many junctures, rivets and welds. Airframe failure even at the most minute level could have catastrophic results. Metal fatigue results eventually from a predictable number of loading cycles.

Automotive applications for the most part wouldn't result in the same ,magnitude of loading cycles and the statistical likelihood of catastrophic failure is probably far less, hence no need for a component service life. Not a significant public safety concern.

[Dimman]

Originally Posted by dem00n

My major isn't aeronautical eningerring, though i do go to an aeronautical school. There's always talk that airplanes with aluminum structures (fuselage?), basically have a expiration date. After they log a certain amount of flight hours, the aluminum is no longer deemed safe and the plane goes off to the boneyard, etc etc.

Wouldn't this be the same thing for sports cars with aluminum chassis? Wouldn't all the stress from the chassis flexing eventually deem it unsafe for racing or even daily driving?

Has to do with fatigue limits of aluminum, which cannot be changed. So yes, sort of the same. But as mentioned safety factors in planes are way more serious, plus they are built to minimize weight. There's no reason you can't design an aluminum part to have a fatigue life beyond what is expected of the vehicle, but it will generally be bigger and heavier. The plane 'best before' bit is probably a performance/safety compromise, whereas I hope an NSX chassis is designed to last long enough for be to buy a nice one someday...

[wheelhaus]

Airframes are made from thin sheet metal, and then riveted, welded, or sometimes bonded together. These parts are typically built to use the minimum possible material to save weight, but still pass stress tests (130+% beyond normal conditions, etc). The sheet metal is often bent, formed, and drilled. Rivets introduce hundreds of thousands (sometimes many millions) of holes, which can be additional sources of cracks. Airframes must be as light as possible, and have strict limits on allowable Gs, landing forces, load capacity, etc. The entire airframe will flex, bend, heat, cool, and swell (with a pressurized fuselage) an estimated number of times for each flight hour. So, it's somewhat predictable when an airframe will be considered unsafe, but also why such frequent inspections are necessary, and nearly an entire teardown is required every 1000 flight hours or whatever is required. If a wing spar fails, people will die. Part of a car's chassis failing would more than likely just be an inconvenience.

A car chassis on the other hand doesn't see pressurization cycles, or insane temperature shifts every time it's run, and it certainly doesn't flex as much. They're designed to be as rigid as realistically possible. It's also made of thicker welded sections and complicated preformed shapes that aren't riveted a bajillion times in a ladder-type structure of spars and ribs. If the chassis is aluminum, the subframes are likely steel (I believe this is the case with the 86). Car chassis also have to comply with some pretty gnarly crash standards, so for simple structural integrity they are pretty overbuilt. Also, being so rigid and overbuilt, the stresses are going to come from specific hardpoints (suspension links, engine mounts, subframe mounts, etc) so the chassis isn't strained in the same manner as an airframe.

[litemup]

Ok, but technically, you are asking about a material property called the "endurance limit." This is the maximum amplitude at which a material can endure an infinite number of stress cycles.

Steel has an endurance limit. Aluminum does not. No matter how low the cyclic loads, aluminum will eventually fail.

Now, consider that in the context of @wheelhaus' valid points. An aircraft must be as light as possible (duh) a wheeled vehicle does not.

And, like he says, when a wheeled vehicle fails, it doesn't fall out of the sky.

...unless it falls out of the plane that's carrying it. <doh!>

[serialk11r]

I think the answer is a yes, but you don't worry about it for the same reason you don't worry about fatigue destroying your engine block or pistons. Cars are a lot more rigid than planes, and the stress on the chassis itself is pretty spread out.

[VacantSky]

All of the above, planes see a lot more stress than cars. Each time an aircraft flies and lands without doing a quick turn while its initial preflight is still valid: It's inspection time. I'm airforce and we have 4 different inspection types on the c-130 variant I work, for daily flightline use. Post Flight, Pre Flight, BPO/PRE (post flight and pre flight combined) and a thru flight. The most gruelling inspection by the book is the post flight/pre flight. Though if you read into the rules of following our maintenance data all of these inspections must be completed the same. Every single mm of exterior and interior part of the plane that is visible by eye (flash light assistance for dark areas) has to be looked at.

Each type of inspection has a time limit on how long it's good for before it has to be accomplished again. Even if the plane doesnt fly.




BUT if it makes you feel any better, we recently started retiring our AC-130H fleet here at cannon afb. They were made in the mid/late 60's, that's 50 years of service on the airframe, and at least 30 of those years were with a set of cannons and guns hanging out the side of it putting even more irregular forces onto the structure. so I'm pretty sure our little aluminum cars can last a very long time if treated right

Complimentary night vision pic I took while we were somewhere, at some point....

[dem00n]

Thanks for clearing it up guys, makes a lot more sense. It's overkill to compare the stress that car will see and then jump to an airplane, should of thought of that first.

[wheelhaus]

Originally Posted by VacantSky

BUT if it makes you feel any better, we recently started retiring our AC-130H fleet here at cannon afb. They were made in the mid/late 60's, that's 50 years of service on the airframe, and at least 30 of those years were with a set of cannons and guns hanging out the side of it putting even more irregular forces onto the structure. so I'm pretty sure our little aluminum cars can last a very long time if treated right

Hey small world, are you with the 16th SOS? Right when/after the 16th moved into Cannon, the company I worked for at the time was hired to install a bunch of AV gear, I think it was in 2009.. We installed all the flat panels and av systems in the (main?) building with the auditoium and office/briefing rooms. I am no longer with that company, but I loved seeing the AC-130 gunships out on the ramp...
[/threadjack]

[VacantSky]

Originally Posted by wheelhaus

Hey small world, are you with the 16th SOS? Right when/after the 16th moved into Cannon, the company I worked for at the time was hired to install a bunch of AV gear, I think it was in 2009.. We installed all the flat panels and av systems in the (main?) building with the auditoium and office/briefing rooms. I am no longer with that company, but I loved seeing the AC-130 gunships out on the ramp...
[/threadjack]

Ha yeah small world! No I work aero repair and crash recovery. Mainly rigging, removing and replacing flight controls and landing gear. Then the second half is self explanatory, but we don't get many crashes. Occasional drone but that's it for that half!

My neighbour is in the 16 SOS but they just merged with the 73rd SOS and have moved over to brand new buildings and hangars on the far side of the runway where the new flightline is.

There's only about 4 H model gunships left now, sad to see an iconic plane slowly go away.. several have been sent to the bone yard and we towed one to the main gate back in march.

[strat61caster]

Building of VacantSky's post (and of course everyone elses, lots of quality in this thread), a manufacturer for an airplane can rely on competent ownership, tight scheduled maintenance and thorough quality inspections to ensure the aircraft is within specifications.

When's the last time you could say that about a car? Even the most meticulously maintained vehicles will almost never see the same level of detail that an aircraft demands. Sure an auto manufacturer may be able to design an aluminum car chassis that can last 1 million miles of regular use but that one outlier case where a handful of cars fail at 150k could be disastrous.

The change is that OEM's are to the point where the amount of data and testing performed has made them feel comfortable enough using Aluminum for primary structures and the advantages are big enough to be almost impossible to ignore.

Get ready for the deluge of 'we designed our car to aircraft standards' as everyone forgets Saab's marketing... Oooh revival of BMW's connection to the Luftwaffe inbound!