Re: Precipitation Static
Thanks Ed. The friction between a metal surface and air is indeed very real. The triboelectric series if broken into thirds would have air at the top for most positive in the series and metals would be 2/3 of the way down from the top for most negativity. Hovering helicopters dropping lines must allow the line to touch the ground (or water) first before anyone can touch it. As a side note: One of the theories about charge generation in thunderclouds states when the water vapor of a cloud freezes any free electrons existing interstitially due to the polarity of the water molecules get pushed out when the molecules lock into crystal formation. Large charge measurements on the order of 600V/m (IIRC) have been measured in the lab with this phenomena. Thus, flying a plane through a cloud could conceivably have more of an effect than just tribo-electric friction. As a further side fun note: That's why it's a little inconceivable that the the crew of Star Trek could survive flying through a "plasma storm". Passing a large star ship at very high speeds through a plasma would cause a large current to flow in the outer skin of the ship. The ship would burn up. Physics is still physics even in the 24th century. -- > From: ed.pr...@cubic.com > To: bruc...@gvl.esys.com; cet...@cetest.nl > Cc: emc-p...@ieee.org > Subject: RE: Precipitation Static > Date: Friday, March 06, 1998 5:07 PM > > > --- On Fri, 27 Feb 1998 16:47:45 -0600 bruc...@gvl.esys.com wrote: > > Jeff: > > The Air Force Design Handbook DH 1-4 gives some info on P-Static. > > > Precipitation static is a phenomenon that occurs on aircraft in flight. > > Friction between liquid water > > and also ice crystals, sand, dust and particulates > > > and the aircraft skin causes charge to > > build up. This can have two effects on the system , arcing and coronal > > discharge. The arcing often occurs between non-metallic components and > > metallic surfaces. Examples include, nose radome to structure below > > wind screen and engine inlets to personnel on ground after landing. The > > broadband noise from the spark can interfere with radio reception as > > well. The second effect is the corona > > called St. Elmo's Fire by really old guys > > > which can occur along sharp > > edges, usually the trailing edge of the wings, horizontal, and vertical > > stabilizer. While the sparks may cause squelch breaks, only an > > annoyance, the corona can obscure radio reception. Interference with > > radio navigation aids can and has caused loss of vehicle and crew. > > The noise spectra ranges from "a few Hz to the Gigahertz area and is very pronounced in the VHF and UHF bands" > > > To > > dissipate the charge build up static dischargers are positioned on the > > trailing > > > edges mentioned above. The dischargers are often referred to as wicks. > > The wicks are just megaohm resistors protruding into the airflow. > > Not quite. The discharger consists of a conductive mounting foot, typically screwed or riveted to the trailing surface. A conductive, but probably painted or plastic coated, wand protrudes out from the base. The wand is about 1/4" diameter and about 4" long. At the end of the wand is a replaceable tiplet. The tiplet is conductive, often painted high visibility yellow (for ground crew safety), and also his some tiny spikes or bristles to enhance the discharge effect. (It looks a bit like a rifle bore brush.) > > I am not aware of any series DC resistance. > > > The > > controlled dissipation of charge does not produce the aforementioned > > effects. > > The following is extracted from DH 1-4, Design Notes 7B2 and 7B3. > > Don't install dischargers closer than 12" apart on an edge. > Locate outboard discharger as close to wing tip as possible. > Five dischargers per trailing edge is commercially OK. > Minimum of two dischargers per trailing edge. > Mount to frames; watch out for ungrounded parts. > > And, a formula for subsonic flight: > > N=V x S / 12,400 > > where N= number of dischargers (round up) > V= knots air speed > S= span, in feet > > There's also a reference to Mil-S-9129, but that spec may be a dead by now. > > > Triboelectric charging is very real. The model for human ESD is a 150 pF capacitor, charged to about 20 kV, discharging through about 1000 Ohms. I wonder what is the equivalent capacitance of an aircraft or helicopter? > > A Coast Guard Chief told me that standard helicopter rescue procedure calls for a ring (or harness), to be dipped into the water and dragged to the victim. He pointed out that the victim'
Re: Precipitation Static
I just want to throw in my two cents... ESD from rescue helicopters has killed more than one rescuee as they reached for the cable. That is why the choppers drag the cable to the person rather than dangle it above. So the scene from Red October is over dramatic in the sense that no-one executes rescues in that manner any more!! Ah, Hollywood - Where would my education be without it?? LOL Charles Grasso EMC Engineer StorageTek 2270 Sth 88th Street Louisville CO 80027 MS 4262 gra...@louisville.stortek.com Tel:(303)673-2908 Fax(303)661-7115 Symposium Website URL: http://www.ball.com/aerospace/ieee_emc.html
RE: Precipitation Static
Ed Price wrote: >> I asked him whether any helicopters ever used any active charge dissipators. His reply was that he "had seen those things, and that nobody he knew ever thought they worked!" They work. You find out when they're broken. There was an accident a few decades ago where an Army CH-47 was lost due to an inoperative active discharge system allowing a charge to build up on an aircraft. Don't recall the details, but there was apparently a fuel vapor/air explosion. As I recall (it really has been a few decades) the system was written up inoperative in the logbook. Note that this antistatic system is NOT a low-power box. Cortland
RE: Precipitation Static
> Precipitation static is a phenomenon that occurs on aircraft in flight. > Friction between liquid water and also ice crystals, sand, dust and > particulates and the aircraft skin causes charge to build up. I don't think so ... My understanding is that the electrostatic charge buildup is due to friction with air molecules. In fact, the drier the air is, the bigger the buildup. Perhaps you mean that this built up charge discharges to liquid water, etc., and causes the precipitation static. > Perhaps you may remember the scene in "Hunt for Red October", in which a sub > crewman is zapped by a static charge during a helicopter to sub personnel > transfer? I always thought that was a bit over-dramatic, but h... In a previous incarnation, I used to work in the oil patch up in the Canadian Arctic. In the winter, because of the extremely dry air (often 10% relative humidity or less), ESD was a major problem with all our printed wiring boards. And when a helicopter brough us a load hanging on a sling, we made damn sure to have a heavy ground cable that we could touch to the sling first. The arcs we drew from this arrangement were quite impressive, to say the least. Being of a scientific bent, we were all curious, of course, about exactly how much energy was involved in this phenomenon. Unfortunately, we couldn't find anybody stupid enough to volunteer, so there is no test data to confirm if this is lethal or not. Pity ... :-) Cheers, Egon Varju
RE: Precipitation Static
--- On Fri, 27 Feb 1998 16:47:45 -0600 bruc...@gvl.esys.com wrote: Jeff: The Air Force Design Handbook DH 1-4 gives some info on P-Static. > Precipitation static is a phenomenon that occurs on aircraft in flight. > Friction between liquid water and also ice crystals, sand, dust and particulates > and the aircraft skin causes charge to > build up. This can have two effects on the system , arcing and coronal > discharge. The arcing often occurs between non-metallic components and > metallic surfaces. Examples include, nose radome to structure below > wind screen and engine inlets to personnel on ground after landing. The > broadband noise from the spark can interfere with radio reception as > well. The second effect is the corona called St. Elmo's Fire by really old guys > which can occur along sharp > edges, usually the trailing edge of the wings, horizontal, and vertical > stabilizer. While the sparks may cause squelch breaks, only an > annoyance, the corona can obscure radio reception. Interference with > radio navigation aids can and has caused loss of vehicle and crew. The noise spectra ranges from "a few Hz to the Gigahertz area and is very pronounced in the VHF and UHF bands" > To > dissipate the charge build up static dischargers are positioned on the > trailing > edges mentioned above. The dischargers are often referred to as wicks. > The wicks are just megaohm resistors protruding into the airflow. Not quite. The discharger consists of a conductive mounting foot, typically screwed or riveted to the trailing surface. A conductive, but probably painted or plastic coated, wand protrudes out from the base. The wand is about 1/4" diameter and about 4" long. At the end of the wand is a replaceable tiplet. The tiplet is conductive, often painted high visibility yellow (for ground crew safety), and also his some tiny spikes or bristles to enhance the discharge effect. (It looks a bit like a rifle bore brush.) I am not aware of any series DC resistance. > The > controlled dissipation of charge does not produce the aforementioned > effects. The following is extracted from DH 1-4, Design Notes 7B2 and 7B3. Don't install dischargers closer than 12" apart on an edge. Locate outboard discharger as close to wing tip as possible. Five dischargers per trailing edge is commercially OK. Minimum of two dischargers per trailing edge. Mount to frames; watch out for ungrounded parts. And, a formula for subsonic flight: N=V x S / 12,400 where N= number of dischargers (round up) V= knots air speed S= span, in feet There's also a reference to Mil-S-9129, but that spec may be a dead by now. Triboelectric charging is very real. The model for human ESD is a 150 pF capacitor, charged to about 20 kV, discharging through about 1000 Ohms. I wonder what is the equivalent capacitance of an aircraft or helicopter? A Coast Guard Chief told me that standard helicopter rescue procedure calls for a ring (or harness), to be dipped into the water and dragged to the victim. He pointed out that the victim's natural response is to reach for the harness, which makes the victim in the water the discharge path for the entire charge on the helicopter. Same Chief also told me that he's seen arcing as the frame of an emergency pump was helicopter delivered onto a ship's steel deck. I asked him whether any helicopters ever used any active charge dissipators. His reply was that he "had seen those things, and that nobody he knew ever thought they worked!" Another Navy Chief told me that the sparks seen spraying from the aircraft tailhooks during night carrier landings are only partly from mechanical friction. It's also due to aircraft static discharge. (Interesting problem on verifying that claim; may just have to believe him.) Perhaps you may remember the scene in "Hunt for Red October", in which a sub crewman is zapped by a static charge during a helicopter to sub personnel transfer? I always thought that was a bit over-dramatic, but h... -- Ed Price ed.pr...@cubic.com Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA. USA 619-505-2780 List-Post: emc-pstc@listserv.ieee.org Date: 03/06/98 Time: 14:07:33 --
RE: Precipitation Static
Precipitation static is a phenomenon that occurs on aircraft in flight. Friction between liquid water and the aircraft skin causes charge to build up. This can have two effects on the system , arcing and coronal discharge. The arcing often occurs between non-metallic components and metallic surfaces. Examples include, nose radome to structure below wind screen and engine inlets to personnel on ground after landing. The broadband noise from the spark can interfere with radio reception as well. The second effect is the corona which can occur along sharp edges, usually the trailing edge of the wings, horizontal, and vertical stabilizer. While the sparks may cause squelch breaks, only an annoyance, the corona can obscure radio reception. Interference with radio navigation aids can and has caused loss of vehicle and crew. To dissipate the charge build up static dischargers are positioned on the edges mentioned above. The discharges are often referred to as wicks. The wicks are just megaohm resistors protruding into the airflow. The controlled dissipation of charge does not produce the aforementioned effects. _ Jeff Bruce EME EngineerRaytheon Systems Co. (903) 457-4187 Electromagnetic Effects Fax: (903) 457-7640 Box 6056, CBN 024 bruc...@gvl.esys.comGreenville, TX 75403 The opinions expressed here are mine and not that of my employer. >-- >From: Ing. Gert Gremmen[SMTP:cet...@cetest.nl] >Sent: Friday, February 27, 1998 3:00 AM >To:'bruc...@gvl.esys.com' >Cc:'EMC POST' (E-mail) >Subject: RE: Precipitation Static > >Hello Bruce > >Tell us someting about precipitation statics ! >== >CE-test, qualified testing, >Consultancy, Compliance tests for EMC and Electrical Safety >15 Great EMC-design tips available ! >Visit our site : http://www.cetest.nl >The Dutch Electronics Directory http://www.cetest.nl/electronics.htm >== > > >-Oorspronkelijk bericht- >Van: bruc...@gvl.esys.com [SMTP:bruc...@gvl.esys.com] >Verzonden: donderdag 26 februari 1998 17:29 >Aan: emc-p...@majordomo.ieee.org >Onderwerp: Precipitation Static > >I realize this may be slightly out of scope for this group, but maybe >someone has experience in this matter. >I need to analyze how adding antennas to an aircraft will affect >p-static dissipation for the aircraft. Specifically I would like to >know how the number of dischargers required for a specific surface area >is determined. > >Any advice or references will be greatly appreciated. > >_ >Jeff Bruce >EME Engineer Raytheon Systems Co. >(903) 457-4187 Electromagnetic Effects >Fax: (903) 457-7640Box 6056, CBN 024 >bruc...@gvl.esys.com Greenville, TX 75403 > >The opinions expressed here are mine and not that of my employer. >
RE: Precipitation Static
Hello Bruce Tell us someting about precipitation statics ! == CE-test, qualified testing, Consultancy, Compliance tests for EMC and Electrical Safety 15 Great EMC-design tips available ! Visit our site : http://www.cetest.nl The Dutch Electronics Directory http://www.cetest.nl/electronics.htm == -Oorspronkelijk bericht- Van:bruc...@gvl.esys.com [SMTP:bruc...@gvl.esys.com] Verzonden: donderdag 26 februari 1998 17:29 Aan:emc-p...@majordomo.ieee.org Onderwerp: Precipitation Static I realize this may be slightly out of scope for this group, but maybe someone has experience in this matter. I need to analyze how adding antennas to an aircraft will affect p-static dissipation for the aircraft. Specifically I would like to know how the number of dischargers required for a specific surface area is determined. Any advice or references will be greatly appreciated. _ Jeff Bruce EME EngineerRaytheon Systems Co. (903) 457-4187 Electromagnetic Effects Fax: (903) 457-7640 Box 6056, CBN 024 bruc...@gvl.esys.comGreenville, TX 75403 The opinions expressed here are mine and not that of my employer.
