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Jan Hudec

 Discussion on answer by g s: What tec…

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Jan Hudec
Apr 2, 2022 23:48
@piet.t, it is not the current definition, but it is the original definition and each newer definition is done so as to be the same value within the accuracy the previous one could be established, which was always better than the requested 10¯⁴ so using the older definitions is good enough for the case.
 

 Discussion on answer by J. Murray: Do…

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Jan Hudec
Dec 22, 2021 16:18
@J.Murray, the guidance says you need it above 40,000 ft, but because most airliners are only certified to 41,000 ft or 43,000 ft – to avoid having to have them for everybody – it isn't the reason airliners have such masks for pilots. Fighter pilots of course do have them for that reason, but airliner pilots have them for a different reason (and the overpressure is probably lower, too).
Jan Hudec
Dec 22, 2021 16:18
The positive pressure masks in airliners are there for case of air contamination so the pilots breathe just the oxygen from the tanks and none of the possibly poisonous smoke or fumes. The regulations do say aircraft occupants must not be exposed to pressures less than corresponding to 40,000 ft except in extremely unlikely (less than once in 10⁹ hours) events, but transport airplanes satisfy it by not flying above 41,000 ft (some 43,000 ft) so the bit they can descend in case of decompression before the pressure fully equalizes is enough.
 

 Discussion on answer by fraxinus: Can…

Imported from a comment discussion on aviation.stackexchange.c...
Jan Hudec
Aug 29, 2021 00:28
@jamesqf Chinook is a helicopter (with normal swashplates and common transmission connecting the rotors), not a multicopter (with fixed-pitch rotors controlled by differential power).
Jan Hudec
Aug 29, 2021 00:28
@fraxinus the swashplate mechanism is so ingenious that it's really, really hard to beat. A step motor on each blade is more complicated and has more modes of failure.
Jan Hudec
Aug 29, 2021 00:28
A multicopter needs even number of rotors, half spinning one way and half the other, to also balance torque, so minimum is four.
Jan Hudec
Aug 29, 2021 00:28
Control algorithm of a variable-pitch rotor with swashplate is trivial—you just tilt the swashplate in the direction you want the rotor to tilt. It is indeed mechanically complex though.
 

 Discussion on answer by Berend: Why n…

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Jan Hudec
Apr 30, 2021 04:59
“MCAS is designed to function only during manual flight (autopilot not engaged), with the aircraft flaps up, at an elevated AOA.”, via here, from the Lion Air flight 610 accident report
Jan Hudec
Apr 30, 2021 04:59
Really? The MCAS is not active when flaps are out, so it does not do anything either during take-off or go-around.
Jan Hudec
Apr 30, 2021 04:59
… if that position is not actually usable, why isn't the stabilizer travel range more limited. Or is it actually usable with centre of gravity near the aft limit and only a problem with forward centre of gravity?
Jan Hudec
Apr 30, 2021 04:59
@Berend what you mean by correct trim and counter it with controls? You don't need to trim nose up while pushing nose down here, no? You only need to trim up when you are pulling up. My understanding instead is that the mechanical trim simply requires too much force when the aircraft is further out-of-trim. And of course in every aircraft that trims by moving the stabilizer the two commands add, so when you are trimmed all the way nose down, your nose up authority is limited.
 

 Discussion between Noorul Quamar and …

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Jan Hudec
Apr 21, 2021 12:50
And the reason it decreases with speed is that the faster the aircraft flies, the more mass of air rushes past the wing per unit of time, so it does not have to be deflected as much to produce the desired lift and the force is less tilted aft. It is also the reason induced drag decreases with span as larger span allows affecting air in larger area.
Jan Hudec
Apr 21, 2021 12:47
And since it is adding downward component of velocity, it must reduce the aft component to maintain the magnitude, so the force applied to air must be down and somewhat forward, and the forward component creates aft reaction on the wing, the induced drag.
Jan Hudec
Apr 21, 2021 12:45
The induced drag on the other hand decreases, again with dynamic pressure. The reason for induced drag is that in order to produce upward force, lift, the wing must (principle of action and reaction) apply downward force to the air. This force accelerates the air downward. But, viewed from the reference frame of the wing, the wing is not doing any work, so the kinetic energy, and therefore velocity, of the air cannot change, only the direction.
Jan Hudec
Apr 21, 2021 12:41
Above speed of sound the coefficient of drag decreases again (the drag still increases, but not so fast). The main source then is that there is no pressure recovery above speed of sound, so there is high pressure in front and low pressure behind the aircraft.
Jan Hudec
Apr 21, 2021 12:39
The gist is that at slow speed the air avoids the moving body by accelerating, which allows it to squeeze through less space, but as the speed nears the speed of sound, this causes decrease in density, which means the air needs more space again and has a problem to avoid the body. This causes a peak in drag at the speed of sound.
Jan Hudec
Apr 21, 2021 12:35
In the transsonic region, compressibility becomes an issue and the drag increases rapidly up to M=1. This is the “sound barrier”. As far as I know there is no simple mathematical model for it, but there is a nice qualitative explanation from Peter Kámpf somewhere on the site.
Jan Hudec
Apr 21, 2021 12:29
(grr, formulas work in normal comments; it's a bit silly they don't work here)
Jan Hudec
Apr 21, 2021 12:29
Parasitic drag has two causes: friction (in the fluid between the surface and the free flow) and imperfect pressure recovery (the pressure behind the object does not raise as high as in front of it). Both forms are proportional to the kinetic energy of the fluid, that is the dynamic pressure $q = \frac12\rho v^2$.
Jan Hudec
Apr 21, 2021 08:44
No, parasite drag is proportional to square of velocity. It's all dynamic pressure terms $q = \frac12\rho v^2$, so no odd powers may be involved. Unless $T$ indicates thrust (the usual symbol for drag is either $D$), where $T = \frac{P}{v}$ and power is approximately constant for engine driving constant-speed propeller.
Jan Hudec
Apr 21, 2021 08:44
All the coefficients are defined as the corresponding quantity divided by dynamic pressure and reference area.
Jan Hudec
Apr 21, 2021 08:44
Does this answer your question? What is the formula for induced drag?
Jan Hudec
Apr 21, 2021 08:44
I am offering aviation.stackexchange.com/questions/36062 as duplicate. It asks generally why the induced drag formula looks like it does, and contains a detailed derivation.
Jan Hudec
Apr 21, 2021 08:44
There are also How is the induced drag calculated for a wing with elliptical planform? and Is the induced drag independent of wing span? with further derivations of induced drag.
Jan Hudec
Apr 21, 2021 08:44
That's Capital-C-coefficient-of-lift, the one that decreases with square of speed because we are talking about constant lift case (lift balances weight). I am pretty sure there is already a good derivation somewhere around here, so I'll try to link it instead of explaining all over again (the related questions search is unfortunately utterly inept; IME it is a bit more useful in the composer though).
 

 Discussion on answer by Peter Kämpf: …

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Jan Hudec
Mar 16, 2021 13:56
@xxavier, as far as I can tell, the transition zone is way, way too large to carry significant inertia across. You can look how the jet streams look e.g. here. Horizontally the change is smooth over hundreds of kilometers, and while vertically it is over only a few kilometers, it is still smooth and you can't build up enough kinetic energy to zoom-climb several kilometers.
 

 Discussion on answer by user14111: Ol…

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Jan Hudec
Aug 29, 2020 09:19
@Beta Effectively it does inflate it. Since everybody needs more money to pay off their interests, the money will have to come into existence, which they will, because these days ultimately all money come into existence by being borrowed from the central bank.
Jan Hudec
Aug 28, 2020 15:29
@Mark, yeah, it's complicated, because the things are getting better (in some sense) without getting more expensive and the exactly same things are getting cheaper (but obsolete).
Jan Hudec
Aug 28, 2020 15:29
And of course average interest will include returns from all kinds of investments, so on a bank account it will always be lower than average. Which means the amount on the account will become lower and lower fraction of the total money, so it will end up being able to buy very little no matter what the number will end up.
Jan Hudec
Aug 28, 2020 15:29
@Beta, taxes make very little difference in the overall picture. When and where interest is not exempt altogether, it will just reduce the interest a little. But it won't change either the geometrical growth of the numeric value, nor the fundamental reasons why interest rate on a bank account can't beat inflation in the long run.
Jan Hudec
Aug 28, 2020 15:29
Interest rates and inflation are actually interconnected. Money only get its worth from what they can buy, and all the money in the world can't buy more than there is to be sold. Therefore when you inflate the numeric amount of money with interest rate, the prices inflate to match simply by law of supply and demand. So average interest can't beat inflation in the long run.
 

 Discussion on answer by ThoriumBR: Ar…

Imported from a comment discussion on security.stackexchange.c...
Jan Hudec
Jul 6, 2020 10:56
@ddyer, there is myriad of ways a site can stop working, and expired or revoked certificate is just one of them. That's why every production site needs some monitoring that will alert you of any failures and impeding failures and you just have to include certificate checking in it. And you can have certificates renew themselves using letsencrypt's certbot like literally half of the internet these days anyway.
 

 Discussion on question by Damien Pham…

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Jan Hudec
Apr 23, 2020 19:18
@Alex, for this question, stored procedure are not a solution anyway.
Jan Hudec
Apr 23, 2020 19:18
@schroeder, yes, it is definitely the word I meant to use. Useless, fake, effectively dangerous.
Jan Hudec
Apr 23, 2020 19:18
@schroeder, that recommendation sounds bogus. 1. while less natural for them, stored procedures still can build queries by string concatenation and therefore suffer all the same issues and 2. the statement calling the stored procedure can be target of injection itself.
 

 Discussion on question by Primordial:…

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Jan Hudec
Mar 9, 2020 17:48
Sensors are not some magical thing that can be fooled with things naked eye can't, they are simply telescopes. They work on exactly the same principle as eyes (well, they are probably reflectors rather than refractors, but that only makes them harder to jam) and look for exactly the same kind of signals (and many others, because eye only sees this visible light band and the telescope can have selection of bands from radio to gamma) with higher resolution and sensitivity. So the only way he can see something out of a window the sensors don't is that the sensors are rigged.
Jan Hudec
Mar 9, 2020 17:48
Also note that radar jammer is simply a bright light that overdrives the radar sensor so it can't pick the weak signal it's looking for. Kind of like the Sun makes it impossible to see stars during the day because it's so much brighter. But besides being (quite literally) blindingly obvious it's there, it does not work as well in space as due to severe shortage of scattering medium you can easily observe faint objects right (from your point of view) next to the sun by simply blocking off the sun with suitable stop. So there is no realistic way the jammers could work.
 

 Discussion on answer by Loren Pechtel…

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Jan Hudec
Jan 9, 2020 01:07
@LorenPechtel, yes, but water is also much denser than feather pillows or straw. The point is that you can jump to water legs first from much higher than flat, and that will apply to other materials that can move out of the way to the sides as well (for a net it won't make much difference).
Jan Hudec
Jan 9, 2020 01:07
The deceleration can be decreased by jumping legs first and pushing the pillows aside. That way they'll need to move a lot slower to get out of the way, so they'll absorb less energy. They are still denser than the preferred material to fall into, the cardboard boxes though.
Jan Hudec
Jan 9, 2020 01:07
@LorenPechtel, I think he will, just like those who jump into water do. Straw would probably be better than pillows though since the cloth wrapping the feathers of the pillows will make them spread non-uniformly.
 

 Discussion on answer by IndigoFenix: …

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Jan Hudec
Jan 9, 2020 01:07
@SRM, plausbility of hiding is 0, because you can't hide it's gravity and you can't actually hide its thermal radiation either—it isn't called “black body radiation” for nothing. So it will only look stolen to normal people, but not to astronomers and people of power will know well what is going on from astronomers. I also doubt anybody will actually care—so you've painted moon black, and?
 

 Discussion on answer by Morris The Ca…

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Jan Hudec
Dec 24, 2019 11:57
@Matthew, humans are far from best (bio)engineering solution, even as generalists. They are just good enough and within the very limited set of techniques available to our evolution. Not only does the cell-based construction not allow any rotating mechanisms, but it seems organisms are not even able to re-evolve body segments they've lost, so since vertebrates only began with four appendages, we are left with two legs after converting two to hands even though four would be better (six are even better for stability, but four are best for speed with utilisation of flexible back).
 

 Discussion on answer by Paul TIKI: Mo…

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Jan Hudec
Oct 5, 2019 11:39
It is all limited by the economy. But Lord Gary can start by using his skills to improve that first. As useful information is for warfare, it is even more useful for trade, so Lord Gary can start by offering services of his messengers and other communication methods to his merchants, basically setting up a post service. Better information would make the economy run more smoothly and that would give him edge in being able to supply his army better than his opponents.
 

 Discussion on answer by Renan: What c…

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Jan Hudec
Sep 1, 2019 03:30
@berendi, that sounds like somebody either did it intentionally… does it have a license key? In such case I can imagine somebody putting in a time range check, and either not knowing about the wrap around (the grunt doing the work never sees the actual broadcast format) or leaving in for whatever reason anyway. Still does not make a plausible reason that would be “[…] unknown to the best and the brightest” as the question requests.
Jan Hudec
Sep 1, 2019 03:30
@Renan, as far as we are talking about time wrap-arounds, there is no problem that is hard for any theoretical reasons. The only problem here is how to force that other register that keeps time in different format to reset so the times match again.
Jan Hudec
Sep 1, 2019 03:30
@Renan, then just reset the device's own date to the one it's receiving. And if it can't be done because there is some check that you are not trying to set time before what was already there, the maker was probably actively evil and trying to force you to buy a new one. If the stakes were high—like in the case in the question—somebody would surely come up with a way to do a complete enough reset to work around it.
Jan Hudec
Sep 1, 2019 03:30
GPSs that don't understand they are across a new epoch, unless they contain explicit check for the roll-around that breaks it, which is borderline malicious thing to do, are far from useless. They are just showing incorrect date, but they most of their functions are still fine. And that will apply to any other device on time roll-over. Because computers don't understand what time it really is. Most programs don't care at all. Some need it to be increasing, and these will have to be reset after the roll-over. And online may care it matches the time on server, which can be worked around there.
Jan Hudec
Sep 1, 2019 03:30
@berendi, it no longer acquires position? Because that would really, really surprise me. It will show date that will be off by 7168 days, but since the value will still be consistent between the time signals, ephemerides and almanacs, so position can still be acquired. And even the time and day of week will be correct, because the time format is week number + seconds into it. So unless they did something really stupid (or malicious) it should only show one wrong value, and not the most important one. Far from being a paperweight!
 

 Discussion on question by Philip Thom…

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Jan Hudec
Jun 2, 2019 14:54
See also what-if.xkcd.com/58
 

 Discussion on answer by JBH: What doe…

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Jan Hudec
Apr 30, 2019 14:50
The lightning channel is not a pure, unadulterated electron flow, it is extremely hot, conductive plasma. It is hot, therefore it expands, which is what causes thunder, not the air rushing to fill it back in. That makes the rest of the conclusions irrelevant.