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'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 hmmmm... > > -------------------------- > Ed Price > ed.pr...@cubic.com > Electromagnetic Compatibility Lab > Cubic Defense Systems > San Diego, CA. USA > 619-505-2780 > Date: 03/06/98 > Time: 14:07:33 > -------------------------- >
