Wikipedia:Reference desk/Archives/Science/2016 July 9

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July 9

Voltage generation

Copied from talk page:Talk:Moving magnet and conductor problem

I was looking for, but did NOT find it: A real experiment involving an electromagnet that is switched on and off, a short piece of copper wire or a coil situated within the magnetic field, and a sensitive voltmeter connected to the two ends of that wire. As that electromagnet is switched on a voltage pulse would be generated as shown on the voltmeter in one direction, and as the electromagnet is switched off that would result in a voltage pulse in the opposite direction. What quantitive relations can be measured? And how about a different experiment during which the magnetic field is measurable and variable and so is its rate of change in both directions. 70.27.152.243 (talk) 00:39, 9 July 2016 (UTC)

This is the basic principle of the flip-coil magnetometer - an old-fashioned device that was very difficult to obtain accurate readings with, but which had the advantage of simplicity. See these lecture notes for a circuit diagram and mathematical treatment. Tevildo (talk) 01:08, 9 July 2016 (UTC)

Bootleg ground

What, exactly, is the danger associated with a bootleg ground (a "ground" to the neutral wire)? The article says it's very dangerous, but despite this it was a common practice until recently, and apparently is still legal in some parts of Europe -- so why is it so dangerous? 2601:646:8E01:7E0B:ECDB:601F:4E1E:971F (talk) 01:02, 9 July 2016 (UTC)

Never mind -- I've just re-read the current version of the article, and it says this is because it won't trip the GFCI (which can indeed be very dangerous in wet conditions). 2601:646:8E01:7E0B:ECDB:601F:4E1E:971F (talk) 01:05, 9 July 2016 (UTC)

That's not the only reason. If you ground the frame to neutral and the neutral line has a bad connection anywhere along the line, your frame get the full voltage from the hot wire and could easily kill you. --Guy Macon (talk) 17:00, 9 July 2016 (UTC)

Need help identifying Southwest U.S. plant

This plant spontaneously popped up in a planted garden. I'm assuming it's a weed, but I'd like a positive ID before I recommend cutting what could be something beautiful. This is in the Southwest United States. Can anyone ID this? WDavis1911 (talk) 04:58, 9 July 2016 (UTC)

Unless it's causing problems crowding out other plants, why not leave it until it blooms, and then decide whether to cull it for the following year? My mother has commanded me (she's rather elderly) to remove such "weeds" as a rose and a mulberry. Can't personally identify the plant, but you always post again when it flowers. In the meantime, it's not poison ivy, oak, or sumac. μηδείς (talk) 18:23, 9 July 2016 (UTC)

Propeller orientation and placement

How do they choose the orientation and placement of a propeller when designing a ship or an aircraft? It appears to me that all ship propellers are placed in the back and point backwards. On the other hand, almost all aircraft rotors point forward and can be placed either in the front, middle or tail.--Llaanngg (talk) 13:23, 9 July 2016 (UTC)

I don't know why ship's props are almost always at the back. For aircraft there are several contradictory preferences. Tail mounted implies long,or rearmounted undercarriage and poor efficiency because the prop is in the disturbed airflow. Mid mounted needs extra structure and drag from a twin boom fuselage and again operates in disturbed airflow. Front mounted gives higher drag due to higher velocities over the fuselage.Greglocock (talk) 14:25, 9 July 2016 (UTC)

Some information at Tractor configuration, Pusher configuration, and Push-pull configuration in regards to air planes. For an examples of planes with mid-mounted propellers, see Royal Aircraft Factory B.E.9 (aka Pulpit) and Focke-Wulf Triebflügel. As far as ships go, Ross Winans experimented with various propeller configurations on his 'Cigar Ships' (more details: here, including some drawings of the various configurations). WegianWarrior (talk) 15:19, 9 July 2016 (UTC)

• They're mostly there because of tradition. The tradition remains established because it's the best known way to do it, and it's hard to find another new way that is already clearly much better from the outset.

Ships and planes are different because of the different Reynolds numbers for air and water. Water is dense and viscous compared to air and ships are larger. Initially ships put their propellers near to their rudders, because this aided their steering, especially at slow speeds. It's also convenient to put the heavy machinery in "the blunt end", not the pointy bow. The bow is a terrible place to put anything on a ship, as it suffers worst in storms.

Modern ships have become more flexible. It's hard to drive a large ship with a single large propeller, especially a fast ship like a passenger liner (slow tankers and container ships are optimised for efficiency and an efficient, slower cruising speed instead, so they continue to use one huge prop). So we see the big cruise ships now using podded propellers with electric transmission all over the place, such as two forward and two aft, to give self-docking manoueverability in small ports, and saving tug fees.

Aircraft began using front-mounted tractor propellers it from the front because they had "tail dragger" undercarriages, large propellers and this kept the propeller off the ground, especially when rotating on takeoff. There are also advantages for control, as the denser 'prop wash' airflow downstream of the propeller flows over the tail control surfaces and gives more effective control for ground handling.

Stability for light aircraft is also easier with the heavy engine mounted forward, to keep the CoG forward too. When early observation aircraft from the Vickers Gunbus to the modern Optica put the pilot or observer ahead of the propeller and engine for visibility, they ended up "mid-propellered" as the weight still had to stay forward, relative to the wing chord. When WWII aircraft with huge single engines suffered from poor visibility over the engine (especially Allied aircraft with V engines, rather than the German inverted V), one solution was to put the pilot (and a heavy cannon armament) ahead of the engine, but behind the propeller, and to link them with a long driveshaft, as used in the Bell Airacobra. As more power was needed, this approach gave way to twin-engined heavy fighters.

Single-engined jet aircraft have rear-mounted engines because long intakes are easier to provide than long hot exhausts, both to build and as a source of drag.

A better question is why so few aircraft are canards as yet. This has definite advantages for reduced drag, as the (forward) elevator can now generate lift, rather than draggy downforce from a rear tail. Canard aircraft are some of the few to favour rear-mounted pusher propellers. The rearward wing chord (and thus ability to rear-mount a heavy engine) encourages this. Andy Dingley (talk) 16:02, 9 July 2016 (UTC)

This forum thread has considerable detail on the advantages and disadvantages of pusher propellers on aircraft, not least "having a pusher prop makes egress in flight much more dangerous"; in other words, how are you supposed to bail out without being minced by the propeller blades? Alansplodge (talk) 18:42, 9 July 2016 (UTC)

If I had to bail out of (say) one of Burt Rutan's pushers (like what John Denver flue) File:NOAA-Long-EZ.jpg – I would simply stop the engine first. Second: I would post on Wikipedia help desk HELP! --Aspro (talk) 21:05, 9 July 2016 (UTC)

I think you meant "flew". John Denver's flue would have been on John Denver's fireplace. John Denver's flu, on the other hand, may be mistaken for Rocky Mountain Spotted Fever. :-) StuRat (talk) 22:54, 9 July 2016 (UTC)

...And John Denver's Floo is from his time spent at Hogwart's. --Guy Macon (talk) 05:07, 10 July 2016 (UTC)

• Unless you have variable pitch, it's hard to stop a propeller windmilling with the engine off. Also it's still a static obstruction to hit. A few aircraft have adopted ejector seats, trapeze seats (it swings on a long arm clear of the propeller) or explosive propeller shedding. Andy Dingley (talk) 09:27, 10 July 2016 (UTC)