Wikipedia:Reference desk/Archives/Miscellaneous/2014 November 15

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November 15

RC watch with no controls

I'm after a radio-controlled watch with no crown and no buttons, that adjusts itself in line with time signals only. Does such a watch exist? If not, is there one with but a single button to tell the watch to synchronize with some time signal?--Leon (talk) 14:18, 15 November 2014 (UTC)

You might find something suitable from La Crosse Technology, see here [1]. Some seem to have very few buttons, not sure if any have zero buttons. SemanticMantis (talk) 19:07, 15 November 2014 (UTC)

Or perhaps there might be one with a hidden panel, so that no buttons are visible unless it is pried open. Would that be acceptable ? StuRat (talk) 23:02, 15 November 2014 (UTC)

It would!--Leon (talk) 14:33, 16 November 2014 (UTC)

I think this one works like that: [2]. StuRat (talk) 15:42, 16 November 2014 (UTC)

Picture in Manila, Philippines

Does anyone know, where exactly in Manila this pic was taken? 112.198.90.141 (talk) 14:19, 15 November 2014 (UTC)

How much energy to instantly stop helicopter rotor blades?

I saw a video on YouTube showcasing an invention for a table saw that would instantaneously stop the saw once the blades detected that it made contact with a human finger. This saw used some kind of destructive arrestor mechanism that instantly stopped the saw from spinning, somewhat destroying the saw and table in the process.

How difficult would it be to employ a similar mechanism for a helicopter's main rotor blades? Let's use a UH-60 Blackhawk for example, how much energy would it take to instantly stop the main rotor blade from spinning while it was spinning at max speed? Acceptable (talk) 19:06, 15 November 2014 (UTC)

This is not the trivia desk. You'd need to provide mass distribution specs for the blades for anyone to do any sort of calculation. There's no such thing as instantaneous stopping. Anything that did so very quickly would make your blades chip and shatter. There's simply no way any system is going to prevent decapitation once a blade has struck someone's forehead. μηδείς (talk) 19:35, 15 November 2014 (UTC)

That is not an attempt at a referenced answer. Rmhermen (talk) 20:00, 15 November 2014 (UTC)

Nor is your tendentious response, Rmhermen. I am telling the OP that his question can't be answered as stated, that he has some ideas wrong and has not given us enough information to provide an answer. Then he would be asking us to do highly speculative equations based on other unspecified assumptions. Dragon's flight below has said exactly what I said, and it's not our place to suggest various helicopter models to the OP, African or European. μηδείς (talk) 22:03, 15 November 2014 (UTC)

Since they gave us the model of chopper, all that info is out there somewhere, if anyone cares to look it up. Perhaps you don't want to (I don't either), but there's nothing wrong with them asking. And if they said we could pick whatever model chopper we wanted, that would be fine, too. StuRat (talk) 23:00, 15 November 2014 (UTC)

This gives 5×10⁵ J for the rotational energy in a small helicopter. As Medeis said, there is no such thing as instantly, so you would have to say how fast you want it to be, though I have trouble imagining any fast stop process that doesn't tear the blades apart and create lots of shrapnel. Dragons flight (talk) 20:11, 15 November 2014 (UTC)

how much energy would it take to instantly stop...? The answer is none, or rather a negative amount, ie, in principle it would be possible to build a device that stops the rotor blades and converts their kinetic energy into useful work (see, regenerative brake). Now there are certainly engineering questions about how quickly the blades can be stopped without damage; how efficient such a energy recovery device can be; etc but I wanted to correct the apparent premise behind your question that "the greater the energy of rotation, the greater the energy needed to stop it". Abecedare (talk) 22:24, 15 November 2014 (UTC)

Note that if you used some type of brakes affixed to the helicopter, that the helicopter itself would then start to spin (unless it was anchored to the ground at the time). So, you'd need to actually reverse spin the blades a bit, relative to the chopper, to ensure that they stopped moving forward, relative to the ground. StuRat (talk) 22:56, 15 November 2014 (UTC)

I don't know about the energy requirement, but flying your helicopter into a hard object (like a building or cliff) tends to stop the blades pretty quickly. Of course, it destroys the helicopter so it is not much use after. Astronaut (talk) 16:07, 16 November 2014 (UTC)

I'm not a physicist, but I would think the stresses resulting from the stated scenario (max speed to zero instantaneously) would necessarily tear the helicopter apart, unless you had something like Abecedare suggested. That's a lot of kinetic energy and it has to go somewhere. ‑‑Mandruss ☎ 19:33, 16 November 2014 (UTC)

The whole scenario is absurd. There's no practical way it could slow down quickly enough to be of any use. And just how would a blade detect such an obstacle? Clarityfiend (talk) 22:21, 16 November 2014 (UTC)

SawStop claims 5 to 10 milliseconds to decelerate a table saw blade without damaging a finger. The detection is done by electrical changes. Rmhermen (talk) 23:27, 16 November 2014 (UTC)

Yes, which explains the impracticality of the helicopter question--the many orders of magnitude in difference between the angular momentum of a helicopter's blades and a table saw blade, and the impracticality of mounting proper sensors. μηδείς (talk) 00:46, 17 November 2014 (UTC)

Wow, I'm impressed by SawStop. Based on Mandruss' comment, the numbers for SawStop don't work out. Now, where's my flying car? Clarityfiend (talk) 07:42, 17 November 2014 (UTC)

The Kamov Ka-50 helicopter has an ejector seat - and uses the unusual trick of blowing explosive bolts at each rotor hinge and relying on centrifugal force to fling the rotors outwards before the seat leaves the aircraft. It's an interesting alternative to trying to stop rotation. SteveBaker (talk) 01:05, 17 November 2014 (UTC)

Re SawStop, I just added this to that talk page. Feel free to comment there to avoid that tangent here. ‑‑Mandruss ☎ 08:21, 17 November 2014 (UTC)

I explained over on that page - but briefly, it's a misunderstanding of what the Sawstop actually does. Sure, it does stop the blade - but that's not how it works. It harvests the rotational energy of the slowing blade to pull the entire blade down under the table. The blade literally vanishes below the table in the blink of an eye. So it's rather irrelevant how fast it stops - what matters is that it pulls the blade away from you at a speed of a dozen feet per second or so. In one of their videos, you can see that only one or two teeth of the saw touch your skin before it starts to pull away. SteveBaker (talk) 20:22, 17 November 2014 (UTC)

Eddy current brakes could be used in this application. If done with permanent magnets, it would require no power. Power enough magnets could slow the blades down to a fraction of their initial speed within several milliseconds, though not to a complete stop. Mechanical brakes could do the last bit. The axle or spindle would heat up extremely quickly, and that may cause damage. The blades would also be subject to large forces, which may or may not break them. It depends on the kinetic energy of the blades and the strength of the material they are made from.--79.97.222.210 (talk) 22:51, 18 November 2014 (UTC)