explosive fracture,courtesy,safety,please!

[aerohead]

Hello all! As I was trying to track down the formulas associated with the "mechanism of flow separation" I ran across something I've experienced, on at least four occasions.Its mentioned within my text on aeroelasticity,and is supposed to be a phenomena of supersonic flight,although it seems that the sky has reserved a special version just for me.Its in the section on "surface skin panels",has to do with "panel flutter",and the term is "explosive fracture".And it dovetails a bit into basjoos discussion on the German V-2 rocket nose.And I'm going backwards here a bit so please forgive me.I had made a comment about the the V-2's nose potential accoustic signature.Actually,the early nose configuration was responsible for panel flutter which led to "explosive fracture",where the sheetmetal of the nose section guite literally exploded off the rocket.The article says this is different from "conventional lifting surface flutter".On four occassions,some of my creations "exploded" off the vehicle,once on the Dodge,and three times on the T-100.As I do not have a proper shop facility,and budget restraints create a situation where I'm working when the weather permits, streamlining my daily driver,I'm often in a position where I must operate the vehicle while I'm in the middle of cobbling something up.So where I'm going with all this,is that unless we all make our modifications robust enough,we run the risk of "losing" our modifications while testing or enjoying them.And this has potentially disastrous consequences! Since we don't have private test facilities (if you do you have a new best friend!)we have to share the road with everyone else.When your fabricating,please think about the other motorist driving behind you,and what it might be like,should some part of your car end up on their windshield.I have been relatively fortunate,that my structural failures occurred in situations of light road use.I did lose a hood blister off the Dodge and it crashed right into the cab of a following 18-wheeler,disintigrating on contact! It was pretty terrifying! And I thank the driver for not doing a "drive-by" with me.Please don't rely on luck.Its not in the do-it-yourself-manuals.Build it aero,build it strong,build it safe.End sermon.

[trebuchet03]

Just taking an intuitive stab here as I've never heard the term "explosive fracture" before... But, because you mentioned flutter - I imagine it's a fancy term for fatigue failure when there's a high cycle/second count... Am I on the right track?


As far as making things robust.... The easiest way to determine if there's something bad coming your way (at least, a very telling symptom).... Listen for noise. Noise is vibration, and vibration can lead to fatigue or other failure (fasteners coming out etc.). Interestingly, on my drive across alligator alley today, I hit some large bugs that blasted their way through my grille block It was made using a wig makers mold technique

But ya, things flying off is a pretty good reason to avoid sharp edges, points, etc. If it's going to fly off, make sure it's got a soft landing

[Gone4]

I would suspect you were operating you modifications too near their natural frequency. Even if you merely pass through, components with a low fatigue life, such as aluminum, will break eventually unless you use a high safety factor in your design. For a simple beam the calculations are trivial but on something like a nose cone you are best left to testing it. In some applications you can pass through multiple resonating frequencies - we often encounter this in turbomachine design. If yu want to investigate attach some accelerometers to oscilloscopes and read them as you drive. If you notice peaks in the cycles, you have a problem.

[aerohead]

Originally Posted by GenKreton

I would suspect you were operating you modifications too near their natural frequency. Even if you merely pass through, components with a low fatigue life, such as aluminum, will break eventually unless you use a high safety factor in your design. For a simple beam the calculations are trivial but on something like a nose cone you are best left to testing it. In some applications you can pass through multiple resonating frequencies - we often encounter this in turbomachine design. If yu want to investigate attach some accelerometers to oscilloscopes and read them as you drive. If you notice peaks in the cycles, you have a problem.

GenKreton,I think your right on the money.These were the beginning of what was to be heavier structures,however,while in their embryonic stage,lacked the wall thickness to stand up to air induced perterbations encountered and the havoc they played with these thin shells.When reproduced and laminated to their final thickness,while a bit heavier,they were nevertheless robust enough to survive the rigors of the road and thunderstorm wind spectra that we can experience here in Texas and the desert Southwest.It would be great to instrument everything.What an education that would be.Thanks for the input!

[slopemeno]

If you guys want to see just how violent flutter can be, go to Youtube and search "dynamic soaring". Its a way of flying model sailplanes off the back, or "lee" side of a ridge. Speeds have gone through the roof in the last 5 years, with the latest record being 357 mph with a 100" sailplane.

Back when they were just getting through the 200 mph range, planes would just "poof"- on second its screaming around the circle, the next it's confetti. The bottom line was seemingly solid control linkages were not, and what was strong enough for 100 mph frontside flying wasn't enough for the backside. The wing would flutter and disintegrate before the pilot could react.

NOW- guys have the number of what it takes to make one strong enough- sometimes
3X the layup of the first generation of the same plane.

Bottom line, the forces can be much higher than we think the are.

[aerohead]

Originally Posted by slopemeno

If you guys want to see just how violent flutter can be, go to Youtube and search "dynamic soaring". Its a way of flying model sailplanes off the back, or "lee" side of a ridge. Speeds have gone through the roof in the last 5 years, with the latest record being 357 mph with a 100" sailplane.

Back when they were just getting through the 200 mph range, planes would just "poof"- on second its screaming around the circle, the next it's confetti. The bottom line was seemingly solid control linkages were not, and what was strong enough for 100 mph frontside flying wasn't enough for the backside. The wing would flutter and disintegrate before the pilot could react.

NOW- guys have the number of what it takes to make one strong enough- sometimes
3X the layup of the first generation of the same plane.

Bottom line, the forces can be much higher than we think the are.

Yeah,the road confetti really sucks!

[kevlar]

I'm no engineer, but I do believe in a solid, tight fitting type of framework under your panels before you cover stuff.

[Bruce]

Good post, aerohead. I also ride motorcycles and this kind of stuff could kill a motorcyclist.