Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.
That's a somewhat different form of division with 2 declination bands, not 3. If a constellation was exactly on the equator it wouldn't work. Equatorial constellations just have to have the equator in them, they could be mostly in one hemisphere instead of roughly equal. The ecliptic constellation list is more famous: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Ophiuchus, Sagittarius, Capricornus, Aquarius, Pisces (prograde order, not alphabetical) Sagittarian Milky Way (talk) 08:53, 29 November 2017 (UTC)[reply]
If one were to drill 3/16-inch holes in a regular rectangular pattern (1 1/2 inches apart horizontally and 2 1/2 inches apart vertically) over an area of drywall which is 12 1/2 inches across and 10 inches high, would that significantly weaken that part of the wall, and if so, by how much? 2601:646:8E01:7E0B:9559:4F0F:AF97:BC2A (talk) 07:46, 29 November 2017 (UTC)[reply]
It would significantly weaken the drywall, especially to small area impacts over that area. There's also the aspect that drilling drywall is likely to crack it, so the chances are that these holes will be joined up internally by a network of cracks.. As the drywall is already pretty weak, that's not likely to weaken the wall by much though. Any strength the wall has will be coming from the wooden studding behind it.
I wouldn't do this. Instead I'd drill a sheet of plywood and either place that over an undrilled piece of drywall, or else cut a rectangular hole into the drywall and set the plywood into that - probably by making it wide enough to span two studs. Andy Dingley (talk) 10:57, 29 November 2017 (UTC)[reply]
Yes, to a significant degree. As others have pointed out, a reinforcement would be needed. This is apart from the ragged edges and general mess associated with making holes in drywall. Are you doing this for ventilation, or are you trying to attach something? Acroterion(talk)18:06, 29 November 2017 (UTC)[reply]
Drills for flat, thin metal like parts have a special flat form with only a center tip standing out. For wood, stone and alike - thus for your job - a so called "Core drill" is used which is basically a normal drill in the center connected to a pipe with sawing teeth. --Kharon (talk) 18:24, 29 November 2017 (UTC)[reply]
Your description detail of "normal drill in the center" rather than just the drilling pipe sounds like you mean a hole saw not a core drill. Either one would be overkill for 3/16" holes in plywood or sheetrock—I don't think I've even seen a hole saw or core drill that small for these sorts of materials. A regular twist or spade drill bit is fine. The cutting edge of the spade rather than the pulling of a twist might help keep the surface smoother? DMacks (talk) 19:15, 29 November 2017 (UTC)[reply]
Sorry, im not used to english professional terminology. Best use would be a "brad point" drill or bit then. That is what i tried to describe as "flat with center tip". They dont need allot of force to drill a hole and i would disagree on the weakening. A pattern of holes will of course help a crack to spread along the pattern, once it started, but the force to start a crack will be nearly the same, with or without a pattern of holes, as long as the holes have some distance from each other.
To keep the entry and exit of the holes tidy and position as well as angle proper, simply fixate 2 wooden planks, the front one with an already drilled hole, on both sides of the drywall, where you want to drill the hole. --Kharon (talk) 20:34, 29 November 2017 (UTC)[reply]
No worries. I linked the article so you can see the pictures (brad point makes sense for wood, similar to a small spade). There are probably interwiki links to articles in your native language also. I doubt most people care about the appearance of the back of a piece of sheetrock. I also doubt most people can even access it to place and securely hold a piece of wood there. DMacks (talk) 02:27, 30 November 2017 (UTC)[reply]
Are you sure the holes wold help a crack propagate? I think I read some years ago that putting lots of small holes into timber could make it more resistant to cracking and splitting. Dmcq (talk) 01:33, 2 December 2017 (UTC)[reply]
I just happened to see at our biceps article that, in addition to what we (or at least I) have normally thought of as the function of the biceps, namely to pick stuff up with the palm facing outward, it also supinates the forearm.
If the biceps supinates the forearm, does the triceps pronate it?
I'm guessing yes, based on a little experiment. If I hold my right fist in my left hand, so it can't move, and try to supinate my right forearm, the biceps seems to flex, whereas if I try to pronate it, the triceps looks like it's doing something. And no, this is not in any way remotely a request for medical advice, notwithstanding the fact that I have referred to myself here. --Trovatore (talk) 10:19, 29 November 2017 (UTC)[reply]
I have been through almost-all all such net pages trying to explain the phenomenon, but mostly not getting much.
(Kindly note that we are talking about "real" helicopter that's got single roter on the top and a fan on tail to fight-off it's (main-rotor's) counter effect)
Once pilot's taken it adequately up, he wants to move it forward. What exactly happens when he moves the control:
1. Does the whole main-rotor ''tilt'' forward ?
2. Or does the main-rotor gets manipulated in such a way that when a blade comes to front, or nearly front, it changes it's "angle of attack" on air but gets back to normal when moves away from that position. (normal: "angle of attack" on air just to keep levitating.)
If second option is the answer, then it must be extremely difficult to built. Changing the angle of blade of such a fast moving rotor must be the hell of a job for manufactureres. One'd like to known how exactly it's done. And what measures are taken to ensure that Murphy's Law gets least chances or none. (BTW, at what RPM is the main-rotor moving, typically? ). Jon Ascton (talk)12:37, 29 November 2017 (UTC)[reply]
Your option 2 is correct. See helicopter flight controls. The mechanism is clever and simple: there is a rotating disc to which the edges of the blades are attached. When the disc is tilted left, right, forward or backward (the "cyclic" control) the angle of attack of the blades changes at different positions of their rotation. This allows the helicopter to move in any compass direction regardless where its nose points. When the disc is moved up or down (the "collective" control), the blades grab more or less air, making the helicopter rise or fall. 91.155.192.188 (talk) 12:57, 29 November 2017 (UTC)[reply]
Rotations and constructions are different on each model. A MBB Bo 105 main rotor will rotate at around 400 rpm and the tail rotor over 2000 rpm, when the engine is at 95%. Helicopter main rotor heads are usually build in solid titanium and the rotor blades are made of composite materials, usually including titanium and/or carbon fiber. These materials are very capable of withstanding massive forces, vibrations and even collisions. The main construction difficulty is actually not in the field of simple forces but in fluid dynamics, or more precise in cavitation, because the blade tips get near the "speed of sound" when you add up the rotation speed and the vehicle velocity and the air flow may become chaotic if the flow speed is to fast (see Reynolds number), which then causes cavitation, which will "grind down" every material over time. --Kharon (talk) 17:37, 29 November 2017 (UTC)[reply]
Cavitation is not an issue for helicopters normally, as it requires a liquid, which, due to low pressure, either evaporates or releases gasses. The collapsing bubbles then cause pressure pulses close to the surface.
Physics which break helicopter propellors are more likely to be either impact or centripetal forces. The blade tips can fail due to tension if the speed is high enough, but that is not a problem at the swashplate, where heating/friction would likely be more of an issue. Rmvandijk (talk) 14:40, 30 November 2017 (UTC)[reply]
@Rmvandijk: You are wrong. See cite from Fluid:"Fluids are a subset of the phases of matter and include liquids, gases, plasmas, and to some extent, plastic solids." Aerodynamics is actually a sub-field of of Fluid dynamics!
Also Cavitation is actually caused by "bubbles" or "voids" of vacuum that implode, not of "gases". The "gases" from cavitation in water are in fact the simple physics of water, which evaporates or boils at much much lower temperatures in vacuum. These vacuum voids are caused by the fluid movement unable to keep up with the wing, blade, impeller, propeller or whatever forces the flow (again see Reynolds number(which btw. of course also applies to water and air)) and the resulting micro-implosions wear down every material, nomatter how hard or elastic.
Now, this is crucial in aerodynamics not somuch for lift but especially for rotors or propellers because they are a fine balanced construction, where every unbalanced weight causes considerable forces on the bearings that hold the Shaft (mechanical engineering). Any seriouse harm done there and you eventually will even loose the ability of Autorotation. --Kharon (talk) 15:57, 30 November 2017 (UTC)[reply]
@Kharon No, from your own second link: Cavitation is the formation of vapour cavities in a liquid, small liquid-free zones ("bubbles" or "voids"), that are the consequence of forces acting upon the liquid. (emphasis mine).
While Aerodynamics is certainly part of fluid dynamics, not every effect observable in liquid occurs in gas and vica versa, especially effects to do with phase transitions. For more info on cavitation please see the article on the Rayleigh–Plesset equation.
I fear we are drifting off topic here, if you want to continue this discussion please do so on either mine or your talk page. Regards, Rmvandijk (talk) 18:42, 30 November 2017 (UTC)[reply]
That part of the Article is wrong aswell and i already thought about correcting it. "Sidenotes" does not make an answer off topic if it manage to come back to the topic, which i this time, hereby do by providing a reference: [1] --Kharon (talk) 21:29, 30 November 2017 (UTC)[reply]
"Cavitation" does not appear in the Helicopter Flying Handbook; I am not aware of any helicopter-specific curriculum that includes training about cavitation in theory or in practice.
I have no doubt that some research, somewhere, has studied the effects of cavitation on rotorcraft; I know that a lot of people study cavitation as it applies to propellers in watercraft; but cavitation is not a major item that would be in the top ten physical effects I would be talking about when describing rotorcraft (helicopter) aerodynamics.
To directly answer the original question: "If the pilot pushes the cyclic forward, the rotor disk tilts forward, and the rotor produces a thrust in the forward direction. If the pilot pushes the cyclic to the side, the rotor disk tilts to that side and produces thrust in that direction..." (H.F.H. 1-5). The entire rotor "tilts" forward, under mechanical control of the pilot, actuated by the extremely complicated mechanism at the root of the rotor drive shaft.
Pilots of certain specific helicopters must be extremely careful to avoid catastrophic failure caused by "mast bumping," a disastrous condition that occurs in low- and negative- G loading, in which the main rotor drive shaft tilts so far off the vertical that it slams into other structures. This can cause total vehicle destruction and has resulted in many fatal accidents. "Mast bumping" is specifically called out as a topic of mandatory training for helicopter pilots. For example, see the Robinson safety alert; and this exaggerated cartoon-ish diagram, Negative G and Mast Bumping, part of the supplemental training materials from a flight instruction school.
So: if there's any doubt left about the rotor disk: yes, the entire rotor disk does actually tilt. You can read all about that, with real photographs and instructive diagrams, in the official textbook published by the FAA - and in the supplemental publications from any major helicopter manufacturer - which are pretty darned reliable sources. If that's not enough, exactly one year ago we had almost the same discussion: Helicopter Yaw Control (November 29, 2016); and I linked to several more reliable sources, including flight training videos.
@Kharon: Thank you for that link, I stand corrected that there is no effect called cavitation in aerodynamics. I am not convinced however that this is the same phenomena as mentioned earlier, which I'll refer to as liquid cavitation for now. Your source does not mention negative effects other than reduced performance loss due to velocities approaching the speed of sound, while if effects similar to liquid cavitation would occur, they should be worth mentioning. Rmvandijk (talk) 16:33, 1 December 2017 (UTC)[reply]
@Rmvandijk: Im prowd of you! Engineering is a way to big field of special applications, even for retired senior Engineers, to own an expertise, or even have a clue, about every technology and knowledge that was developed and established in it. Please, allow me another link/try to convince you:[2] --Kharon (talk) 20:12, 1 December 2017 (UTC)[reply]
Really? That's your cited source? "Bearing the long period impact and blast, the fatigue crack formed in the end the blades, coming into being the cavitation." (sic)
That paper is hardly the bastion of reliable research. It is a conference paper with all the hallmark evidence of poor-quality translation into the English language - and it has nothing to do with helicopters!
Googling "what is a white weasel" yields this definition: "Ermine (Mustela erminea), also called stoat, short-tailed weasel, or Bonaparte weasel, northern weasel species in the genus Mustela, family Mustelidae. The species is called ermine especially during its winter white colour phase. Sep 15, 2017" ←Baseball BugsWhat's up, Doc?carrots→ 17:39, 29 November 2017 (UTC)[reply]
Multiple weasel species switch their fur twice each year to adapt. See Snow camouflage#Seasonal polyphenism. It might be that multiple different weasel species are termed "white weasel" when in their winter fur or "coat". Ferrets can also turn mostly white in the winter (i dont mean the albino ones). --Kharon (talk) 20:59, 29 November 2017 (UTC)[reply]
Hence the great equatorial fur-coat exchange held on the equinoxes, when Northern and Southren Hemisphere weasels migrate thousands of miles to swap pelts for the coming season. μηδείς (talk) 02:53, 5 December 2017 (UTC)[reply]
I'm trying to figure out, for a beam of Ag atoms being sent through an electromagnet for a Stern gerlach experiment, what the speed of such atoms would be. The experiment in question is the original S-G experiment, though an order of magnitude would suffice. Moreover, what kind of magnetic intensities would be seen, if an electromagnet is being used? The number I've been given is a vertical gradient of 500 T/m (assuming the electromagnet to produce a vertical field), but that doesn't tell me anything about the intensity of the field in the whereabouts of the particles' passage. Thanks in advance. 7dare (talk) 19:49, 29 November 2017 (UTC)[reply]
I'd love to, but I can't seem to be able to find an English version, and not a free one either (I can't pay 40 euros for two numbers...). 7dare (talk) 20:16, 29 November 2017 (UTC)[reply]
What is the idea of giving outer space objects such bizarre names?
My second question is about this animation[3]. What is the time frame? How fast did the asteroid really move in space? It's been about two months since its discovery and is it already gone? Is the speed of all the objects in the video scaled up proportionally? An association with a 6-mile wide meteorite that killed the dinosaurs 65 MY ago begs to be highlighted. This piece of space rock is only 2 football fields wide but still it probably had a potential to land in the middle of some industrial center. Per video, it has very nearly come to it.
When the purple planet, Jupiter, completes 1 orbit that's 11.862 years. When the purple and yellow planet (Saturn) and the Sun are in a line (which would be hard to tell from this perspective) is late 2020 AD. That gives an idea of the time frame. Sagittarian Milky Way (talk) 00:45, 30 November 2017 (UTC)[reply]
The solar escape velocity from Earth's orbit (1 AU from the Sun) is 42.1 km/s. For comparison, even 1P/Halley moves at 41.5 km/s when 1 AU from the Sun, according to the formula v = 42.1219 √1/r − 0.5/a, where r is the distance from the Sun, and a is the major semi-axis. Near-Earth asteroid 2062 Aten only moves at 29 km/s when 1 AU from the Sun because of the much smaller major semi-axis.
10 AU
2019
29.51
100 AU
2034
26.65
1000 AU
2196
26.36
2300 AU
2430
26.32 (normal comet at this distance: much slower)
It dimmed so quickly since it wasn't discovered till it already passed Earth's orbit outbound and Earth was also passing it. And it dims fast cause it's moving away from the Sun at hyperbolic speed and if other things are equal, anti-Earth velocity x mph dims faster the closer to Earth you're at. If the Earth was in a different part of its orbit when the Sun flyby happened it could've easily been multiple times further away from Earth at closest approach to Earth. Then it'd have to be bigger or more reflective to reach the same brightness but would've dimmed slower. Sagittarian Milky Way (talk) 02:51, 30 November 2017 (UTC)[reply]
As to the trajectory, it is definitely hyperbolic. This is known because the orbital eccentricity is greater than 1. See the infobox in the ʻOumuamua article. Note that the uncertainty is mcuh smaller than the amount that the number is greater than 1. (A circular orbit has an eccentricity of exactly 0, an ellipse is between 0 and 1, a parabola is exactly 1, and a hyperbola is greater than 1. Real-life orbits are never exactly circular or parabolic.) --69.159.60.147 (talk) 00:59, 30 November 2017 (UTC)[reply]
@Sagittarian, thank you. It is interesting. Is the table for this Hawaiian asteroid? Your picture of the orbits on the black background differs radically from the video I referenced. AboutFace 22 (talk) 02:08, 1 December 2017 (UTC)[reply]
The strange names can be attributed in part to "cultural diversity"; inevitably, representing more cultures means having more names. More specifically, however, as mentioned above, the language is Hawaiian - this reflects not merely that Mauna Loa telescopes were part of the discovery effort, but that their presence is a highly contentious political issue (Opposition to the Mauna Kea Observatories) and the ability to build the next big telescope at that location is in doubt, with other alternatives viewed as less satisfactory. But also bear in mind that a very large number of names have been used for astronomical objects, and they're not supposed to be used twice, so people are really reaching out there for new ideas. Wnt (talk) 14:33, 1 December 2017 (UTC)[reply]
@Sagittarian and @Wnt, thank you. The table is especially impressive and the diagrams too. The name is of course bizarre, Hawaiian or not, probably a handful of people in the world can pronounce it and this is what counts. You simply cannot use it in a conversation. Astronomers who name their discoveries this way should be censured. AboutFace 22 (talk) 15:06, 1 December 2017 (UTC)[reply]
Well at least it's easier to pronounce than Jupiter and other shit like that, so that's a positive. 15:33, 1 December 2017 (UTC) Nil Einne (talk)
Our article gives a sound file to follow. [4] Think of the slowest cow in elementary school trying to give an answer. "oh... mooo ... uh ... mooo ... uh..." Wnt (talk) 21:42, 1 December 2017 (UTC)[reply]
Note: The redirect term White weasel does not appear in the target text. Is it really a synonym?--Herfrid (talk) 16:13, 29 November 2017 (UTC)
![[3]](https://commons.wikimedia.org/wiki/File:C2017U1.gif){kind=link}