Stress corrosion is an evil wicked thing. It can rip through metal at alarming rates, and rip through metals that aren’t even supposed to corrode or crack in ordinary use.
Yeah. If stress is x and corrosion is y then the combo yields wear at x[sup]y[/sup]. Or is that y[sup]x[/sup]? I can never remember. Either way it’s Baaad news.
Here ya go.
There will be a quiz.
Understanding the New Widespread Fatigue Damage Rule
On Jan. 14, 2011, a new FAA rule (14 Code of Federal Regulations [CFR] 26 Subpart C) became effective requiring airplane manufacturers to make available service actions necessary to preclude the onset of WFD and to establish operational limits, known as limits of validity (LOV), of the maintenance program that effectively define an airplane’s usable life. It is important that operators become familiar with the rule so they can prepare for changes to airworthiness limitations that will limit how long an airplane may be operated in terms of flight cycles or flight hours.
This article describes Boeing’s approach to complying with the new rule and its impact on operators of Boeing airplanes throughout the world. It addresses the imminent future changes to airworthiness limitations, how those changes were developed, and how Boeing will assist operators with rule compliance.
http://www.boeing.com/commercial/aeromagazine/articles/2012_q4/2/
Exactly. The advantage of a hot-air, hot-helium or helium/hot-air hybrid balloon is that you can control the buoyancy by varying the gas temperature, without having to drop ballast or release helium. But if you are using waste heat from the engines to heat the gas, then you don’t have a lot of control over the gas temperature. You can’t just stop the engines during the day, and run them full power at night. When you are coming in for a landing, you want to reduce lift, but also want the engines running to control the flight path.
Excellent find Leo; you should be writing this stuff up, not me. In any case it seems the numbers I ballparked were about right … back in the 1970s. Newer aircraft last about 2x as long.
For those not inclined to read the article, Boeing says that per the latest regulations and analysis, reasonably late-model 737s are good for 100,000 cycles or 125,000 flight hours, whereas longer haul 767s are good for 60,000 cycles or 150,000 hours. Note the differing cycles/hours ratio between the short- and long- haul fleet. Also note that in most airlines’ route structure, the hour limit will be reached before long the cycle limit is.
Also, these limits are not where the airplane falls apart. It’s where it becomes impractical for the engineers’ strength predictions to remain statistically valid. IOW, by that age, something somewhere is probably starting to fail and we can’t be sure *enough *that the structure is still as strong as it was when it left the factory. So it’s deemed no longer safe enough for the uber-safe standards of first world passenger & cargo service.
If an airship used this scheme, it would surely include a good method of control (not difficult to achieve).
For example, engines could be water-cooled, and the heat from this cooling system would either be directed to the gas within the envelope or to surrounding air, as appropriate. When engines were not running but heat was called for, it could come from a simple burner.
Before you tell me that the engineering is to complex and that there are no materials in existence that could accomplish the task, I would point out that My car has a heater that uses excess heat produced by the engine. When it’s hot outside I turn it off and the engine can still run.
From this, I hypothesize that doing the same thing for a blimp. Is allowed by the laws of physics.
LSLGuy, about the different cycle limits (excluding time) and aircraft design, using your examples:
Short haul/737/100,000
Long haul/767/60,000
By short haul, it implies a narrower body airplane, so a stiffer tube, which can stand being pressurized and depressurized more times, as opposed to a barn-sized airplane, given similar construction, simply from first principles of structural engineering. Right?
Because “long haul” viewed alone as a term, for a layman, is just time aloft.
ETA: 747s for short haul?
Sure, it’s possible. I was just pointing out that you can’t just get all your heat from the engines.
Also I think all modern airships use air-cooled engines, presumably for their weight advantage.
It’s hard to imagine a less true analogy.
Martin Eberhard built the Tzero, the precursor to the Tesla roadster. It’s batteries sucked. He hooked up with Marc Tarpenning and founded Tesla. The chose the Elise body the roadster was founded on, attracted investors, built prototypes.
The batteries they needed to make the car have a reasonable range did not exist yet. They counted on a “Moore’s law” of batteries believing that battery capacity might double every ten years.
They developed their car on an act of faith that worthwhile batteries would become available in the future. The numbers weren’t run. There were none to run. Elon Musk came in later, as an investor, and took over.
The idea that he ran some numbers and decided that you could build a great electric car is false.
Elon Musk is not exactly a big guy on making sure his engineering is in place before going forward with an idea. Both spaceX and Tesla’s business plans have depended and continue to depend on inventions and improvements that don’t exist yet, and may never come to exist, but that they are guessing will.
when they spectaclarly crashed and burned.
As I recal, it was the bad weather (and design faults) that killed the R101, but it was the crash and burn that killed the crew.
Where did I mention the Tzero, or even Tesla Motors? Musk financed Tesla because he had come to the same conclusion that Eberhard and Tarpenning had, which is that the existing li-ion tech was already good enough for a fast and long-range car.
They did not, as you say, require a non-existing Moore’s law of batteries. Although today’s li-ion cells are better than what they had then, the difference is not dramatic. They were using laptop batteries virtually identical to the ones in the Model S.
The engineering doesn’t have to be in place as long as the physics is in place. That’s the whole point. Although “it works on paper” is by no means a guarantee of success, “it doesn’t work on paper” is very much a guarantee of failure.
I’m sorry. You must be referring to the other electric car company Elon Musk runs. What was the name of the one you were referring to? You’re so knowledgable about Elon Musk, I hope you don’t mind my picking your brain.
Not at all. Ever hard pitched the concept to Musk as a machine that could go 300 miles, Elon asked him if “he could really build the electronics for the proposed amount of money.” Eberhard said he could, and that the car would cost less than half a competitive sports car.
He couldn’t do either, and it’s pretty clear, he knew it. Musk didn’t run any numbers. He wrote a 17 million dollar check to Eberhard off of that 2 hour presentation.
Yes. They did. I didn’t make that up. “Moore’s law of batteries” is a term Eberhard used to describe the concept that new battery capacity was growing at 7% year and was doubling every ten years. What he said was that it was “a Moore’s law for batteries except slower.” If he wasn’t depending on it, like you say, than it’s surprising he’d be telling people that that was what was going to make the cars real world workable.
Ahh. I see. That’s what you’ve been arguing this whole time, is it? Because a vaccum envelope blimp has the physics in place. All we need is the materials to improve for it to become practical, just like the Tesla batteries.
If you are such a fan, of going ahead with ideas before the engineering has been worked out, why were you arguing against me?
Can’t have it both ways.
“And then a miracle happens.”
No one’s denying that if you had unobtanium, you could build a vacuum balloon. Here, “unobtanium” probably means a molecularly perfect, low-density diamondoid material. We are a long way from having such materials.
Furthermore, there is a threshold effect. The vacuum balloon is completely useless until your materials become vastly better than they are now. And they have to become much better than that to just match helium balloons. Due to scaling laws, partial vacuum balloons do not help you at all. No matter what the differential, the mass you add (with conventional materials) will always exceed the gains in buoyancy.
Once molecular nanoassemblers are ready, we can start to talk about vacuum balloons being worth it.
Or even being able to show they are possible.
The electric car analogy would fit better here if a practical electric car were known to require a battery that stored antimatter. And even then, though we have no idea how to make practical use of antimatter, we at least know it exists, and some of its properties.
We don’t know even that much about VBSU (vacuum balloon shell unobtantium).
We’ve had electric vehicles for a long time now … golf carts, scissor lifts … the basic technology existed when Tesla started out. With this helium/hot air balloon machine, we just don’t have the material to allow an under-pressured shell, not even close. This problem has been being worked on, diligently, especially in rocket technology. If this material is ever developed we may be seeing a major change in many many areas of manufacture.
I have been graciously reminded that my drift on pressure vessels has been upgraded to a hijack and should be terminated, and that my hostage, LSLGuy, be released from his bondage.
Thanks to all for your forbearance. Good thread.
“Perfect materials” - the ideal battery would be a superconducting coil. Unfortunately, far too many practical difficulties stand in the way right now. (such as what happens when the temperature climbs above the superconductor limit…)
Similarly the problem is that even though the physics says a vacuum airship would float, the same physics says there is no known material even close to able to hold that vacuum. So now we know where you need to direct your research.
It’d probably be easier to invent a material with negative gravitational mass than it would to invent a material with the strength to withstand wimpy Earth atmospheric pressure. It’d sure have a lot more useful applications.
Then you just pump enough of that material into your ordinary gas-tight fabric envelope to have neutral to slightly positive pressure. You could even get by with real small aerodynamically efficient balloons if your material was anti-dense enough.
See? Easy peasy.
There was some great quote from the original Back to the Future where Dr. Brown says something like [that’s the wonder of being a scientist! You can make it happen!] Or something. I tried to Google it but was unsuccessful. Anyhow it seems applicable here. Dream yuge young man! Dream yuge!
Well… Maybe not. Implosions seem to be pretty nasty things, probably high on the list of things you want to avoid when building an airship. So, unless it’s truly fail proof you probably would not go this route.
What I keep thinking is that lets say you have your airship filled with helium, or whatever that weighs Xlbs, and has a useful cargo capacity of Ylbs. How do you increase Y?
You can make a bigger blimp and scale the whole thing up, you could heat your lifting gas, or you could cut weight. Blimps are pretty lean already, so cutting weight is a tough route unless…
We know we can’t make an envelope that can withstand a vaccuum, but we also know that we make some that can withstand some negative pressure or buckling stress as they encounter this when maneuvering or bearing weight or encountering winds, or what have.
If our rigid enveloped blimp could be made to maintain an interior envelope 1% less the pressure on the outside you could dump a ton or two of lifting gas and thus improve your carrying capacity by that much. Of course, reinforcing your envelope is going to add weight, but I suspect it could be done.
I know you can’t lift a blimp with vacuum, but you might be able to use the principle a little bit and reduce weight with negative pressure.