7 ms·
Also see @salawat above on nacelle lift-induced pitch instability. We now have airliners that begin to resemble the F-117 in their dependence on computers for
by everybodyknows 8y ago
Also see @salawat above on nacelle lift-induced pitch instability. We now have airliners that begin to resemble the F-117 in their dependence on computers for controllability in flight.
- ousta 8y agowhere did the comment go
- fsloth 8y agoThere's nothing wrong with unstable airframes. Military craft have required computer assisted flight for decades. What's problematic with 737 MAX is that the stability assist was obviously installed as an after thought, and even worse, it's dependent on instrumentation that does not have several layers of fault tolerance as would be the case if it was actually built with the care that planes actually require to be as dependable as they are. So the instability is not a problem. That boeing tried to get around it with a cheap patch of jury rigged software that is depending on input from non-fault-safe instrumentation, is.
- deleted 8y ago[deleted]
- qrbLPHiKpiux 8y ago> There's nothing wrong with unstable airframes. For military purposes, I agree. There is an end objective to fighter jet development. To own the sky at all costs. With civilian aircraft, there is too much to lose. You can't have an unstable air frame for civilian commercial mass purposes.
- pps43 8y agoHelicopters are inherently unstable at low speeds, yet there are civilian models.
- macintux 8y agoAnd if they carried 500 people we’d think that was a bad idea.
- qrbLPHiKpiux 8y agoHelicopters scare the hell out of me because they resemble nothing that's natural.
- dreamcompiler 8y agoI guess you've never seen a maple seed then. https://duckduckgo.com/?q=maple+seed&t=ffab&iax=images&ia=images https://duckduckgo.com/?q=maple+seed&t=ffab&iax=images&ia=im...
- Drdrdrq 8y agoMaple tree seeds?
- serf 8y ago(to other repliers) maple tree seeds resemble auto-rotation in a helicoptor, not powered flight. It's a nice mental model as to why you won't fall out of the sky necessarily with a loss of power in a heli, though.
- CydeWeys 8y agoWhy does that matter? There are plenty of natural things that are scary as hell, and plenty of unnatural things that aren't. The reason you don't see it in nature is that macroscopic organisms never evolved a continuously rotating joint. It's a hard problem for evolution to solve; how would blood vessels cross that boundary? But of course machines have no such constraints, and the wheel and axle is hardly a new (or scary) invention.
- thaumasiotes 8y agoYour shoulder comes close, on a ball-and-socket model rather than wheel-and-axle.
- rujuladanh 8y agoWhat? Of course there is somethig wrong with unstable airframes. This is not a figther jet where it may make sense to maximize maneuverability, it is a bloody airliner, where stability and reliability should have been the design goals.
- brianberns 8y agoInherent instability in a commercial aircraft seems like a very bad idea. Safety has to the top priority. The military has very different priorities.
- InclinedPlane 8y agoI disagree that "instability is not a problem". In military aircraft that are unstable it's common for everyone in the plane to be in an ejector seat or to have a parachute. Can you provide statistics on how many (or what proportion of) military planes with inherent instabilities routinely carry passengers without parachutes?
- ceejayoz 8y agoI suspect military planes have ejection seats not because of built-in instability, but because they have a tendency to get shot at.
- InclinedPlane 8y agoIt doesn't matter why they have them, the fact that they are there changes the overall risk equation for the aircraft. Many military planes have bailout procedures because everyone has ejection seats or parachutes. Civilian planes universally do not. What is an acceptable risk in one circumstance is not necessarily acceptable in another.
- ceejayoz 8y agoIt does matter why they have them, if you're using their having them as evidence that something else is a problem. Swap out "instability" for "drab paint jobs" and see: > I disagree that "drab paint jobs are not a problem". In military aircraft that have drab paint jobs it's common for everyone in the plane to be in an ejector seat or to have a parachute. Can you provide statistics on how many (or what proportion of) military planes with inherent drab paint jobs routinely carry passengers without parachutes?
- yarrel 8y agoYes we can make an argument incoherent by swapping words in and out. In this case the argument is not that a is evidence of b, it is that a ameliorates b. And that the risk of b is therefore priced differently where a is also present. So where a is not present, we must price the risk differently. How that assertion relates to the historical record may be of interest.
- TecoAndJix 8y agoAs an unrelated side note, I have always said “jerry rigged” and wanted to know which of us have been saying it wrong. Turns out it’s both! https://www.merriam-webster.com/words-at-play/jerry-built-vs-jury-rigged-vs-jerry-rigged-usage-history https://www.merriam-webster.com/words-at-play/jerry-built-vs...
- war1025 8y agoOr the rather less tasteful version starting with N that was common with many of the older people while I was growing up.
- qaq 8y agoYou can eject from most of those mil aircraft
- VBprogrammer 8y agoThat's a huge stretch. The fix was to work around a certification requirement that the control forces shouldn't decrease as the aircraft approaches the stall. This is a long way away from the aircraft being inherently unstable.
- kraig 8y ago"Fehrm said Boeing must have added this system on the MAX because when the angle of attack is high this model is less stable compared to prior 737 variants. That’s because the MAX has bigger, heavier engines that are also cantilevered further forward on the wing to provide more ground clearance. That changes the center of gravity." Fehrm, a former jet-fighter pilot and an aeronautical engineer: https://www.seattletimes.com/business/boeing-aerospace/faa-evaluates-a-potential-design-flaw-on-boeings-737-max-after-lion-air-crash/ https://www.seattletimes.com/business/boeing-aerospace/faa-e...
- deleted 8y ago[deleted]
- VBprogrammer 8y agoIt seems like you think that contradicts what I said. The system by design adds a little trim in very specific circumstances. That's not the same as being needed because the aircraft is aerodynamically unstable. That is a term which has an actual definition. That said there are airliners out there which are unstable in the longitudinal axis and require a yaw damper to prevent Dutch roll.
- fsloth 8y agoThanks, you are quite right. My intention was not to claim 737 MAX had the same aerodynamic nature as, say, F-16, but in the scope of the discussion point out that fixing instabilities with flight software can be made to work reliably. The main argument was that it was inherently a bad idea to require computer assist to maintain stability and decades of military planes demonstrate that you can engineers such systems to be more or less fault safe.
- usrusr 8y agoA combat aircraft can trade reliability for superiority, you increase one kind of risk to lower another. This trade-off simply does not exist in civilian aviation.
- abfan1127 8y agois that the case? The Osprey certainly tried and failed that trade off, at least in the beginning. https://en.wikipedia.org/wiki/Accidents_and_incidents_involving_the_V-22_Osprey https://en.wikipedia.org/wiki/Accidents_and_incidents_involv...
- wazoox 8y agoHere's salawat's comment on a flagged thread: https://news.ycombinator.com/item?id=19382641 https://news.ycombinator.com/item?id=19382641
- lutorm 8y agoAs I understand it, this effect is only noticeable at very high angles of attack, and is not present in normal flight regimes. (Which is why the MCAS system only activates near the stall.) Having an unrecoverable stall mode is not a desirable flight characteristic, for sure, but I don't think it's unheard of. For sure it's nothing like inherently unstable airframes as used in fighter jets.
- borkt 8y agoIm pretty sure the problem is that a sensor failure is causing it to believe it is at a higher angle of attack than it actually is, but I'm only skimming.
- lutorm 8y agoYes, that's the present problem. I'm referring to the inherent instability issue that was alluded to. The stick pusher / MCAS is designed to prevent the plane from getting into a state where it's unrecoverable.
- salawat 8y agoThe effect I refer to as being unheard of in previous 737 hulls is the positive pitch instability. I don't want to repost the whole comment, because I think that's frowned upon, but someone did provide a link. My rather limited understanding is that the airframe certification path Boeing used was one meant for fast-tracked recertification of minor configuration changes of the same hull. The main idea being that the manufacturer need only prove that the reconfiguration meets the same or similar flight characteristic requirements as the hull it was derived from. This lets them produce/sell a technologically refreshed product without having to pass the rigorous testing normally associated with type-certifying a new airframe. The important characteristic of this is that the onus for this certification is left to the manufacturer. I do not know if the FAA actually independently confirms the recert is justified, or just plays it via honor system. Old 737 hulls definitely didn't have this characteristic onstability, or do-or-die reliance on functioning AoA sensors, so while I agree there isn't anything wrong with incorporating instability into a design to satisfy some specifications per se, I'd still contend that the addition of that instability should disqualify an airframe from being recertified as a reconfiguration of an older one, and should require more extensive testing/training before pilot's are cut loose on it. If anyone else has more accurate understanding of type certification, I'd be thrilled to learn the specifics.