Wikipedia:Reference desk/Archives/Miscellaneous/2016 November 1

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November 1

F-15 — Vapor trail from single wingtip?

This weekend I observed two F-15s from a park near my house. They made a wide turn overhead at low altitude, each trailing some sort of vapor from a single wingtip (I think, but am not certain, it was the left wing). A few minutes later, I saw them flying on the approach path for the local commercial airport, with no vapor trail evident. A friend who works at the airport later passed along that they had landed for a maintenance issue. My question is, what might the vapor trail have been? Both aircraft were standard military gray, not marked as a demonstration squadron (and definitely not an F-16 with the Thunderbirds), though there was an airshow about 100 miles away that weekend in Rome, GA. The turn was not severe enough for that style of contrail to be a credible explanation, and the vapor was definitely only from a single wingtip — not from both, and not from the aircraft centerline. Any ideas? — Lomn 00:23, 1 November 2016 (UTC)

What makes you say "The turn was not severe enough for that style of contrail to be a credible explanation,"? Contrails can and do form when aircraft are in stable level flight. You don't need a turn, even a severe turn. Moriori (talk) 00:28, 1 November 2016 (UTC)

Because explaining "observed in a turn" suggests severe maneuvering as a possible cause. The day was hot, clear, and dry, and the aircraft were (subjectively) below 1000 feet AGL. Conditions were not suggestive of natural contrails, particularly given the one-wing-only nature coupled with no contrails on the second pass at roughly the same altitude and speed. — Lomn 01:56, 1 November 2016 (UTC)

They may have been simply fuel dumping before landing. --Aspro (talk) 11:25, 1 November 2016 (UTC)

At 1 min 36 secs into this video it shows where the venting valve are and during deployment on the F-15. Did the OP witness 'exactly' the same thing? F-15 Eagle - Undefeated Combat Fighter . Atmospheric vapour condensation tends to form nearer the root (closer to the fuselage). This is because the air flows out towards the tip on the underside of the wing and towards the root on the upper side. Hence modern passengers plans having those little wing-lets at the tips to counter act this lose of lift. So the greater adiabatic cooling the more likely condensation will appear nearer to the roots of the wings The cost of an F15 (and crew training) is such, that it is better to dump a few thousands dollars worth of tax payers money in the air, rather than risk making a right mess of the runway. --Aspro (talk) 12:10, 1 November 2016 (UTC)

I'm not sure; at 1:36 I see a still of a both-wingtips contrail, and I didn't spot a single-wingtip contrail elsewhere (though I just spot-checked the preview window). Fuel dumping was a possibility I considered, but both aircraft were creating the vapor trail; I would have assumed that the fuel dump would only have been for the aircraft actually undergoing the emergency to reduce fire hazard. Perhaps it was the weight limit issue mentioned in the article. — Lomn 14:23, 1 November 2016 (UTC)

Agreed and got interrupted before finishing editing. The winglets on modern passenger planes are there to reduce the lose of lift form the higher pressure airflow leaving the tip and getting sucked on to the low pressure area over the top of the wings. On high performance fighters, the wing profile is different. They don't have to produce high lift at low air speeds in level flight (when flying slow the pilot just increases the angle of attack). The profile moves the adiabatic cooling towards the tips. This video is probably a better example as it show that on turn where one wing is contributing more + g the vapour trail is visible on that wing only and when both are contributing then both show trails. They normally have to be close to transonic for this to appear unless the RH is very high.Great Sounding F-15 Eagles Flying In Wales " The Sound Of The USA ". So make up your own mind as to which was which because we were not watching. What we really need is a F15 pilot to say how it dumps fuel to discount wing tip dumping.--Aspro (talk) 20:52, 1 November 2016 (UTC)

Definitely not close to being transsonic, nor was humidity very high. Found a picture of one aircraft in question (right wingtip contrail, not left as I was thinking -- outboard side of turn). Local media reports that one of the aircraft was down one engine. Also, it appears that the F15 does have its fuel dump port on that wing, so that appears to be the case (Google searches overwhelmingly showed F-111s lighting off their fuel, so I'd assumed that centerline was the usual location). Thanks for your assistance. — Lomn 21:39, 1 November 2016 (UTC)

Gymnastic Splits

Is it possible to do/learn splits at the age 31 or 32, or am I planning for hospital stays (lol)? 103.230.104.30 (talk) 18:29, 1 November 2016 (UTC)

It's possible, but there is indeed a loss of flexibility with age. Suggestions are to do non-weight-bearing splits, and/or to have a spotter. That is, you can practice splits while lying down, so that your body weight is not forcing the splits deeper: [1]. This makes injury far less likely. A spotter can also help you up or get medical help, if you do get stuck in a bad position and/or injure yourself. StuRat (talk) 19:02, 1 November 2016 (UTC)

(ec)Jana Kramer, who's about to turn 33, did several splits in her routine on DWTS last night, so clearly it's possible. Whether it's a good idea or not is going to depend on your physical condition. You should see a doctor ahead of time rather than waiting until after the fact. ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:03, 1 November 2016 (UTC)

Halloween

When is Haloween? – 00:00 31/10/2016 or 00:00 01/11/2016? 103.230.104.30 (talk) 18:29, 1 November 2016 (UTC)

Hallowe'en is the eve of All Hallows (All Saints' Day). All Saints' Day is 1 November, the "eve" (which here means the day before) is 31 October. 86.147.210.2 (talk) 18:39, 1 November 2016 (UTC)

The related Day of the Dead starts October 31, but extends until November 2. StuRat (talk) 20:57, 1 November 2016 (UTC)

Winning in Florida and North Carolina: Does it ensure the victory beyond any reasonable doubt?

HOTmag (talk) 21:00, 1 November 2016 (UTC)

If won by Hillary, yes, but not if won by Trump, since he is farther behind in the rest of the states. See [2]. Specifically, using those numbers, Clinton has 258 of 270 electoral votes locked up, and the 15 and 29 electoral votes from North Carolina and Florida, respectively, would put her over by 32 electoral votes. Thus, for Trump to win, if he lost those states, he would need to win every other toss-up state listed (8 full states and 2 half-states), and also 32 electoral votes from Hillary states (meaning either California or 2 or more of those states). On the other hand, for Hillary to win, if she lost those states, all she would have to do beyond winning what she is currently projected to win, is to get 12 electoral votes from toss up states. Georgia or Ohio would do it alone, or any 2 of Arizona, Utah, Nevada, Iowa or Wisconsin. (New Hampshire would only provide enough electoral votes in combo with Wisconsin or Arizona, or we would need 2 or more additional toss-up states.) StuRat (talk) 21:03, 1 November 2016 (UTC)

See 270 To Win where you can play with the electoral college yourself. It's fun if you like that sort of thing. Herostratus (talk) 21:09, 1 November 2016 (UTC)

As for Clinton, you are right. As for the trumpet, if he wins in Florida and North Carolina, then he will probably win in all of the other toss up states, according to the projection in this map. That's why I mentioned FA [FL] and NC only. HOTmag (talk) 21:35, 1 November 2016 (UTC)

(FL is Florida's postal abbrev.) That site seems to have divided most of the toss-up states from my source between Trump and Hillary, such that Hillary already has 32 more electoral votes than she needs to win. Nevada is the only other state they didn't call yet. So, that supports my assertion that winning those states in no way ensures a Trump victory overall. StuRat (talk) 23:48, 1 November 2016 (UTC)

• If you really want to research all the various scenarios and winning conditions for the candidates based on the best currently available data, then you should check out FiveThirtyEight.com, which has the analytics to answer this, and many more questions. They have an analytic called "Tipping Point Chance" as you can see here which is a decent metric for the importance of each state in terms of "tipping" the election from one candidate to another. According to them, Florida is indeed the single most important state, but North Carolina is fourth on the list. The second most important state is either Pennsylvania or Michigan. It should be noted also that based on their metrics, and who is currently projected to take each state, Trump would need Florida, North Carolina, and at least one other sizable state, and still keep all of the states he currently has the lead in. --Jayron32 12:15, 2 November 2016 (UTC)

It seems like you missed out the link indicated in my last response. HOTmag (talk) 13:46, 2 November 2016 (UTC)

So I did. Mea culpa. --Jayron32 01:42, 3 November 2016 (UTC)

In theory,you could win the top 11 states in terms of electoral college voters,and lose all the other 39 and still make it in as President... Lemon martini (talk) 18:02, 8 November 2016 (UTC)

Actually, winning the bottom 23 guarantees a more lopsided victory, as the share of the popular vote per elector is greater in smaller states than in larger... --Jayron32 18:10, 8 November 2016 (UTC)

Wrap Speed Animated Wallpaper

I'm unable to find a wrap speed animated wallpaper that starts from a normal motion to the wrap speed motion. 103.230.105.9 (talk) 23:53, 1 November 2016 (UTC)

Do you mean warp speed? See Warp drive for the fictional concept. AllBestFaith (talk) 00:41, 2 November 2016 (UTC)

I'll bet you're right. I didn't know what the OP was talking about until you proposed that. StuRat (talk) 03:42, 2 November 2016 (UTC)

For warp speed, that would probably mean Star Trek theme, although Star Wars has the similar "hyperspace". Some other sci-fi franchises use wormholes for their FTL travel.

Note that such a wallpaper would be essentially playing a repeating movie clip, unlike many animations which are far simpler. Therefore, it may negatively affect system performance. StuRat (talk) 03:42, 2 November 2016 (UTC)

There's no particular reason that this needs to be a video. Star field simulations are fairly trivial to implement and don't require a lot of computing resources, since only a relatively small percentage of pixels need to be touched. Star field screen savers existed back in the 1980s using standard consumer class computers of that era. They simply drew each star individually and updated the position of each star in real time. I've written a couple myself and the per-star computation is quite trivial. The limiting factor of course is how many stars you want to depict. I would guess that many animated wallpapers, and certainly many if not most screensavers, are far more computationally expensive. CodeTalker (talk) 02:22, 3 November 2016 (UTC)

Those simulate constant speed motion, but the "going to warp speed" part means the stars need to move faster and faster, and then instantly become lines, or something similar. There might be a way to do that with a vector graphics approach, but it will need more processing power than just dots on the screen. StuRat (talk) 03:38, 3 November 2016 (UTC)

Going faster doesn't matter, it will use exactly the same resources regardless of the speed -- you just draw the stars farther away from their previous position in each frame. The frame rate doesn't increase. Depending on the available graphics hardware and software, drawing lines is not much more expensive than drawing circles and might even be cheaper. On almost any modern computer, line drawing would be handled by the graphics card and wouldn't use any of the main CPU. Even in the unlikely case that there is no hardware line drawing support, there are very efficient line drawing algorithms, such as Bresenham's line algorithm, which doesn't even require any multiplications. CodeTalker (talk) 14:19, 3 November 2016 (UTC)

If you draw the "stars" moved farther in each frame, with the same frame rate, that won't look like smooth motion. You will need to keep the distance constant and up the frame rate, to make it look smooth, and that does increase processing costs. Lines could actually be drawn using fewer resources, since the frame rate can be lower, but you can't go directly from slow moving "stars" to lines, you need a transition between them, with fast moving "stars". StuRat (talk) 16:32, 3 November 2016 (UTC)

I don't see how such a variable frame rate technique could work for a star field simulation, or what the benefit would be. No matter what your simulated speed is, even during the "slow" part, the stars at the edge of the screen are moving much faster (on the screen) than the ones near the center of the screen, so you always have the problem of low frame rate producing non-smooth motion. The distance each star moves cannot be constant, because stars at different positions move a different amount (on screen) each frame. Normally one uses a constant frame rate, say 60 fps, sufficient to produce the desired effect through the whole animation. Do you have a citation suggesting a benefit for using a variable frame rate in this kind of animation? CodeTalker (talk) 21:47, 3 November 2016 (UTC)

Here "constant distance" means constant at each location on the screen. To see the problem with showing a fast moving object at a low frame rate, just view sports on a 1080p screen, at 24 fps, without any interpolation. 60 fps should be enough, but it's not necessary when the motion is slow, or after we go to lines, so it's using up more resources than are needed. And if we are talking about using an animated GIF, I don't believe you can reliably get 60 fps out of those. From our article: "Internet Explorer slows down GIFs if the frame-rate is 20 frames per second or higher and Microsoft reports that Google Chrome and Safari also slow down some GIF animations." So, that leaves us with a proper video format, which uses up more resources, especially when full screen and at 60 fps. StuRat (talk) 17:15, 4 November 2016 (UTC)

Please fasten your seat belts. AllBestFaith (talk) 17:33, 2 November 2016 (UTC)

Yep, that's an animated GIF. It doesn't start from normal motion, though. StuRat (talk) 03:40, 3 November 2016 (UTC)

Here is something that is not the desired animated wallpaper file type [3] 103.230.105.6 (talk) 08:38, 3 November 2016 (UTC)