Wikipedia:Reference desk/Archives/Science/2024 June 22

= June 22 =

Boeing 707
Several questions: (1) Can a Boeing 707-120B take off at MTOW from Runway 04/22 at Greater Rochester International Airport? (2) How much payload (if any) would have to be taken off for the same jet to take off from Runway 10/28 at the same airport, with a 45-knot headwind and below-freezing temperatures as would be the case in a severe blizzard (and would this be possible at all)? (3) How much payload (if any) would have to be taken off for said Dash-120B to fly nonstop from ROC to Rome Fiumicino Airport with acceptable reserve fuel remaining, assuming no tailwinds along the route (and would this be possible at all)? (Questions inspired by the original Airport movie.) 2601:646:8082:BA0:649B:7753:3C84:C70D (talk) 12:27, 22 June 2024 (UTC)


 * 1: According to the official documentation from Boeing, if it's equipped with the original JT3D-1 engines, the takeoff runway length requirement at 500 feet above sea level, standard temperature and maximum take-off weight is 8500 feet. Those Boeing documents are in US units. Your runway is 8001 feet, so it won't fit. At least, not with the proper safety margin. If equipped with the more modern JT3D-3 engines, the takeoff runway length requirement is 7800 feet, so that fits.
 * 2: Under normal circumstances, assuming JT3D-3 engines, maximum weight on a 6400 foot runway is 232,000 pounds, 26,000 pounds below MTOW. Freezing temperature would take the density altitude down to below sea level, which the documentation doesn't tell about, so I'll assume sea level. The 45 knot headwind reduces takeoff groundspeed to about 70% and required runway length to a bit more than 50% of the original, so about 4000 feet should be enough, even at MTOW. Unless there's snow on the runway. Ploughing through snow may increase drag enough that it can't take off at all, but it also reduces braking performance, so you need more length for an aborted takeoff.
 * 3: The distance from ROC to Fiumicino is 3780 nautical miles. Taking off at MTOW, it can carry a payload of 25,000 pounds, assuming JT3D-3 engines and a standard cabin configuration. This is slightly less than a full payload of 137 passengers and baggage, 28,000 pounds. Maximum payload for short flights is 42,000 pounds, passengers, baggage and cargo. PiusImpavidus (talk) 09:54, 23 June 2024 (UTC)


 * OP: Why did you choose Rochester? Airport_(1970_film) is set in Chicago, so wouldn't O'Hare International Airport or Midway International Airport be a better reference?  RudolfRed (talk) 18:24, 23 June 2024 (UTC)
 * Because I'm doing some very early work on a rescue simulator game, so for this mission I wanted to move the departure airport further east so as to speed things up (it wouldn't do to have some of the players, in particular the one playing the medic, sit around for 2 hours or more doing nothing) -- but I see this probably won't work, so I guess I'll go with Boston Logan Airport instead (I had rejected it at first because I thought it would have sped things up too much, but on second thought it wouldn't be the case because the flight would be flying parallel to the shore at first). Not happening any time soon, though -- for one thing, a simulator would be way over my head at this point (and I'd have to put together a whole studio, because it's far too much work for just one person, no matter how highly skilled), and also for this particular mission I'd have to first get permission to use the likenesses of Dean Martin, Jacqueline Bisset, Helen Hayes and Van Heflin as Vernon Demerest, Gwen Meighen, Ada Quonsett and D.O. Guerrero respectively! 2601:646:8082:BA0:649B:7753:3C84:C70D (talk) 21:34, 23 June 2024 (UTC)
 * And there was me thinking you meant Rochester Airport, which is certainly further east. Alansplodge (talk) 14:56, 24 June 2024 (UTC)
 * I'll admit to having to read the original comment twice. The thought of a 707 attempting to take off from Rochester Airport's  grass strip is a bit alarming! Martin of Sheffield (talk) 09:04, 25 June 2024 (UTC)

Stepper motor driver and power saving
Hi. Some motor drivers have an energy saving feature; for example, in the TMC2209 it's called "CoolStep".

I'm guess that this is accomplished via some very clever control circuitry inside. For example:

1. if a stepper is holding its position, and there's 0 load on it (just a bare stepper motor sitting on a desk connected to a TMC2209), then 0 current is needed

2. if I use my finger to try to turn the stepper shaft, a back EMF is generated in the stepper. The TMC2209 senses this voltage, and supplies a current in the opposite direction to counteract it

3. as a result, my finger will feel a "torque" from the stepper, and the stepper will hold its position, despite my applied force

(This is my best understanding of how it works.)

Is it possible to apply this same technique, or an analogous technique, to drive a solenoid?

I'm asking because the two situation are almost analogous to each other:

A. a solenoid is connected to a solenoid driver. A permanent magnet is stuck to the solenoid via magnetic force. There is 0 movement, so 0 induced voltage. There is 0 force, so 0 current is needed to hold this position

B. if I use my finger to try to move the magnet, a back EMF is generated in the solenoid. The solenoid driver senses this voltage, and supplies a current in the opposite direction to counteract it

C. as a result, my finger will feel a "force" from the solenoid, and the magnet will hold its position, despite my applied force

Is this kind of control actually possible in reality? In my head, I'm imagining something like this is possible in theory, but I don't know enough electronics to know whether it's actually possible or not.

Is there any commercial solenoid driver that can accomplish the above described power-saving feature? I could not find any myself. Since manufacturers use different marketing terms (such as "CoolStep") to describe their proprietary technology, it's possible that such a driver exists, but I don't know the right keyword to search for so I cannot find it. OptoFidelty (talk) 21:24, 22 June 2024 (UTC)
 * The TMC2209 Datasheet says thatCoolStep's operation relates to the StallGuard4 feature, which it describes as being based on back-EMF. But "CoolStep is not able to measure the motor load in standstill and at very low RPM". One application of sensing linear motion and counteracting it (effect: holding something in approximate position with minimal required force) is damping...lots of ways of implementing it. DMacks (talk) 21:53, 22 June 2024 (UTC)
 * Is there any commercially available IC that can achieve this damping effect?
 * I'm guessing that there are at least a dozen solenoids any new car, so presumably they have some very smart solonoid drivers inside them to control the drive current.
 * Currently I'm just using a constant current to drive the solonoid, which is very wasteful, since 99% of the time, 0 current is needed. I don't know how to achieve this back-EMF sensing feature on my own.
 * It'd be helpful if I can find a commercially available solonoid drivers IC that has this feature built-in. OptoFidelty (talk) 22:48, 22 June 2024 (UTC)
 * Not my field (ha!), sorry. DMacks (talk) 02:39, 23 June 2024 (UTC)
 * No worries. Thank you for the help so far.
 * At least I know it's physically possible now. Just not sure where to find any commercial solutions for it, if such a thing exists. OptoFidelty (talk) 06:40, 23 June 2024 (UTC)
 * The back-EMF gives a velocity sensor. Holding a position with only a velocity sensor is possible with a perfect sensor, but in reality sensors aren't perfect and some creep will happen – which is why the example mentioned above doesn't work in standstill. You need a position sensor, or apply a brake. PiusImpavidus (talk) 10:17, 23 June 2024 (UTC)