Faro
Topic Author
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### Voltage Induced Across Wingspan In The Cruise

Just out of curiousity, what magnitude of voltage is created between one wingtip and another by an airliner's winspan cutting across the lines of the earth's magnetic field at cruising groundspeed. Presumably this would be at a maximum near the polar areas, would it not?

Also, is this the main reason for the static dischargers installed on wingtips?

Faro
The chalice not my son

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### RE: Voltage Induced Across Wingspan In The Cruise

Static dischargers are to discharge static from picking up or losing electrons while flying through charged areas. The entire airframe's potential rises and falls as it picks up and loses static charge.
You'd have a hard time measuring any voltage induced by flying across magnetic fields since the whole plane is a big, honkin conductor and there's no way you could get a significant difference of potential between parts of it from those small currents.
Planes are made to take lightning strikes without incurring any serious differences in votlage between components. Induced current would be totally insignificant compared to that.
Anon

DocLightning
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### RE: Voltage Induced Across Wingspan In The Cruise

the question is a valid one. We had to do this one in HS physics. First, the direction of movement must be perpendicular to the magnetic field AND the wings must also be perpendicular to the field lines. So, basically, you could only develop a significant voltage at the magnetic poles where the field lines are vertical. I forget the voltage we calculated for a 727 wingspan, but it was small. Less than 10V. Maybe less than 1. This is because Earth's magnetic field is really weak.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan

tdscanuck
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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting Doclightning (Reply 2):I forget the voltage we calculated for a 727 wingspan, but it was small. Less than 10V. Maybe less than 1. This is because Earth's magnetic field is really weak.

What was the assumed resistance for that? 1 V across a conductor as large as a 727 structure would be an absolutely enormous current, with concurrent I^2R power losses and drag.

Tom.

Caryjack
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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting Nomadd22 (Reply 1):the whole plane is a big, honkin conductor and there's no way you could get a significant difference of potential between parts of it from those small currents.

 Quoting Doclightning (Reply 2):Maybe less than 1.

That'd be my guess.

 Quoting Tdscanuck (Reply 3):1 V across a conductor as large as a 727 structure would be an absolutely enormous current, with concurrent I^2R power losses and drag.

The voltage is a result of the wings cutting magnetic lines of force, right? So the drag comes from cutting these lines to generate power? What power loss are you refering to? Not that this actually happens.
Thanks,
Cary

tdscanuck
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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting Caryjack (Reply 4): The voltage is a result of the wings cutting magnetic lines of force, right?

Yes. If you move charged particles through a magnetic field, they experience a force. In a metal the electrons are free to move, so they get pushed one way until the charge build up is just enough to balance the magnetic force. The charge buildup causes a voltage.

 Quoting Caryjack (Reply 4):So the drag comes from cutting these lines to generate power? What power loss are you refering to?

As soon as you have moving electrons, you get a magnetic field due to their motion that reacts with the original magnetic field. This is basically analogous to the back-EMF in a motor. This is the physical principle behind an eddy-current brake...you convert kinetic energy to thermal energy.

 Quoting Caryjack (Reply 4):Not that this actually happens.

It actually does, but the effect is so small compared to airplane thrust that it's functionally zero. Magnetic forces on satellites can be significant, and are an entirely practical way of satellite control.

Tom.

Caryjack
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### RE: Voltage Induced Across Wingspan In The Cruise

OK, I got it.

Given

 Quoting Tdscanuck (Reply 3):an absolutely enormous current,

the energy required to produce this current would result in

 Quoting Tdscanuck (Reply 3):power losses and drag.

Looks like I skipped over the "enormous current" assumption.

I was trying to make the point that

 Quoting Tdscanuck (Reply 5):the effect is so small compared to airplane thrust that it's functionally zero

 Quoting Tdscanuck (Reply 5):Magnetic forces on satellites can be significant, and are an entirely practical way of satellite control.

Interesting, first I've heard of that.
Thanks,
Cary

DocLightning
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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting Tdscanuck (Reply 3): What was the assumed resistance for that? 1 V across a conductor as large as a 727 structure would be an absolutely enormous current, with concurrent I^2R power losses and drag.

There is a potential difference between any long conductor oriented perpendicular to the magnetic lines of force and also traveling perpendicular to the magnetic lines of force. This is because charged particles traveling perpendicular to magnetic lines of force experience a sideways push. Isolated particles will move in circles (which is how accelerators work) and particles in conductors will move to one end of the conductor.

Thus, there is a potential difference of less than one volt between the wingtips on a 727. That potential difference will disappear as soon as the magnetic field becomes more parallel to the plane of the wings.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan

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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting Tdscanuck (Reply 5):It actually does, but the effect is so small compared to airplane thrust that it's functionally zero. Magnetic forces on satellites can be significant, and are an entirely practical way of satellite control. Tom.

Hubble has bars that react with Earth's magnetic field to generate enough force to desaturate the reaction wheels, so it doesn't need any type of control thrusters.
Anon

David L
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### RE: Voltage Induced Across Wingspan In The Cruise

Damn! I had grand intentions of postion a diagram of the Right Hand Rule and asking if that's what you mean but, sadly, could not find one with the middle finger in a comical orientation. I was even prepered to add motion lines.

(Yes, I'm in a "Saturday night but I've got nothing to do" mood.)

NoWorries
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### RE: Voltage Induced Across Wingspan In The Cruise

I'm a little rusty at this, but

According to wiki -- Earth's mag field at the pole is .6 Gauss ...

... so B (flux density) would be 6e-5 webers/sq m.

Assumung a wingspan of 33m, and a velocity of 200 m/s and assuming travel completely perpendicular to the field

A very simple formula E = vlB (assuming all linear relationships to make math easy) gives an answer of about .4V

 Quoting Doclightning (Reply 2):First, the direction of movement must be perpendicular to the magnetic field

Any movement not completely parallel will generate some emf. In fact, the cosine of the angle between field and travel gives the relationship -- at 90 degress it's one -- and slowly drops (in the form of a sine wave) until it reaches 0 at zero degrees. This is why AC generators produce the classic sine wave current.

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### RE: Voltage Induced Across Wingspan In The Cruise

just to add - earth magnetic field is mostly parallel to the ground outside polar areas.
so, wingspan is not really a factor here - rather body height is important.

DocLightning
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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting NoWorries (Reply 10): Any movement not completely parallel will generate some emf. In fact, the cosine of the angle between field and travel gives the relationship -- at 90 degress it's one -- and slowly drops (in the form of a sine wave) until it reaches 0 at zero degrees. This is why AC generators produce the classic sine wave current.

Fair point, but at most points along the planet, the magnetic field is roughly parallel to the ground. Since planes fly roughly parallel to the ground, the current induced would be so tiny that the inherent resistance in the metal would probably kill any potential difference that could arise.

Were the wings made of superconducting material, they would generate a teensy, tiny, weensy voltage in such a situation. Not enough to kill a gnat, most likely. But they aren't made of superconducting material.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan

NoWorries
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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting DocLightning (Reply 12):Were the wings made of superconducting material, they would generate a teensy, tiny, weensy voltage in such a situation. Not enough to kill a gnat, most likely. But they aren't made of superconducting material

Superconducitivity wouldn't change anything -- the magnitude of the voltage is dependent only on the field strengthn, conductor length, and velocity.

DocLightning
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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting NoWorries (Reply 13): Superconducitivity wouldn't change anything -- the magnitude of the voltage is dependent only on the field strengthn, conductor length, and velocity.

There's also the issue of resistance in the conductor. For example, a length of rubber won't generate much of a potential difference because the resistance is too high to the flow of electrons.

An aluminum wing is a pretty good conductor, but if you're flying at 3° to the local magnetic field, the force felt by the electrons is so small that I can't imagine you'd really get a detectable potential.

Remember, Earth has a very large magnetic field, but a very weak one.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan

NoWorries
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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting DocLightning (Reply 14):There's also the issue of resistance in the conductor.

Resistance doesn't affect induced voltage (as long as it is a conductor), only the amount of current that will flow as a result of the induced voltage. In this case, since there isn't a closed loop, an excess of charges will pile up at one tip, and there'll be a defecit at the other.

 Quoting DocLightning (Reply 14):An aluminum wing is a pretty good conductor, but if you're flying at 3° to the local magnetic field, the force felt by the electrons is so small that I can't imagine you'd really get a detectable potential.

As noted in the calculation, it was performed at the pole where the field would be perpendicular to level travel. The number is the number -- maybe I made a mistake in my calculation, it's very easy to do that of course, but AFIAK the calculation says around .4V.

DocLightning
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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting NoWorries (Reply 15): As noted in the calculation, it was performed at the pole where the field would be perpendicular to level travel. The number is the number -- maybe I made a mistake in my calculation, it's very easy to do that of course, but AFIAK the calculation says around .4V.

Yup. For a vertical magnetic field I remember doing that calculation and getting <1V.

Not exactly a stunning, vapor-igniting figure.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan

Faro
Topic Author
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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting DocLightning (Reply 16):Not exactly a stunning, vapor-igniting figure.

Not at all vapour-igniting, no. Gadget-wise however, potentially a good means to cross-check your groundspeed. If one could set up a voltmeter across the wingtips with sufficient sensitivity and link the output to your FMS position (which would also be fed with a detailed map of the earth's magnetic field strength), you could then compute an independent indication of your groundspeed based on the voltage reading. The advantage would be that this is linked directly to earth's magnetic "ether" so to speak, a most compelling proposition!

Just a gizmo off the top of my head though, I doubt that it would be very practical.

Faro
The chalice not my son

NoWorries
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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting DocLightning (Reply 16):Not exactly a stunning, vapor-igniting figure.

No matter how we slice it -- definitely small potatoes. It would be bigger on an A380 -- a 787 on the other hand ...

ThirtyEcho
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### RE: Voltage Induced Across Wingspan In The Cruise

I am shocked, shocked at this topic.

I order all airplanes grounded at once!

-Claude Rains

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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting ThirtyEcho (Reply 19):I am shocked, shocked at this topic. I order all airplanes grounded at once!

Good luck finding 7800 nm long ground wires at Home Depot.
Anon

DocLightning
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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting NoWorries (Reply 18): No matter how we slice it -- definitely small potatoes. It would be bigger on an A380 -- a 787 on the other hand ...

According to your statement, the 787 would have an induced voltage because the resistance in the conductor doesn't matter.

Remember, all materials are conductors. It's just a matter of the voltage you have to apply across them before they will conduct.

Anyway, if the A380 has twice the wingspan, then the voltage will be 0.8V.

One wonders if this could be stored in a capacitor so that we can run the conveyor belts for takeoff...
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan

NoWorries
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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting DocLightning (Reply 21):According to your statement, the 787 would have an induced voltage because the resistance in the conductor doesn't matter. Remember, all materials are conductors. It's just a matter of the voltage you have to apply across them before they will conduct.

Well sure, but the potential (.4V or whatever it works out to be) isn't nearly enough to turn the 787 wing into a conductor -- so under these conditions it would still be an insulator and there would be no current flow.

Until an insulator crosses the conduction threshold, it doesn't doesn't have resistance -- it has a dielectric constant. Resistance R = E/I, and if the current is zero, division by zero is undefined.

The voltage at which an insulator starts to conduct is called the breakdown voltage -- I don't know what it is for a 787 wing -- I'm guessing it's high since they embedded metallic mesh in the CFRP to disperse lightening strikes.

Fixed a typeo

[Edited 2009-05-21 18:30:54]

Again -- thought I spell-checked this puppy

[Edited 2009-05-21 18:32:48]

NoWorries
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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting NoWorries (Reply 22): I'm guessing it's high since they embedded metallic mesh in the CFRP to disperse lightening strikes.

this post is turning into a typing disaster for some reason -- so if the wings have the embedded mesh then they will conduct as well. Even if they don't, and the fuel tanks are metallic, then they would conduct also.

ThirtyEcho
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### RE: Voltage Induced Across Wingspan In The Cruise

Going to add, in my next flight plan, "request no routing or vector perpendicular to the earth's magnetic field."

Better safe than sorry.

-Sparky

Faro
Topic Author
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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting NoWorries (Reply 22):The voltage at which an insulator starts to conduct is called the breakdown voltage -- I don't know what it is for a 787 wing -- I'm guessing it's high since they embedded metallic mesh in the CFRP to disperse lightening strikes.

Over an operational lifespan of 20 years or more of wing flexing, does the metallic mesh remain intact or would breaks in the mesh lines gradually develop? I know that CFRP fibres can break with repeated flexure but individually only so that the overall strength of the material is conserved. Is it the same thing with the metal meshing so that conductive capacity is preserved?

Faro
The chalice not my son

NoWorries
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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting Faro (Reply 25): does the metallic mesh remain intact or would breaks in the mesh lines gradually develop?

Presumably/hopefully the Boeing engineers took that into account. Since it's only job (AFAIK) is to disperse lightening (or other static?), small cracks probably wouldn't matter that much. I guess it could be more serious if somehow an entire section of the wing or fuselage became completely electrically insulated from the rest of the plane.

NoWorries
Posts: 493
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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting ThirtyEcho (Reply 24):Better safe than sorry.

Every few hundred thousand years the mag poles flip -- some scientists are saying we're on the verge of another flip (maybe 1500 years away). There could be periods of wide variability in the field before that happens -- maybe it's best just to stay on the ground. If man were meant to fly ...

tdscanuck
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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting Faro (Reply 25):Over an operational lifespan of 20 years or more of wing flexing, does the metallic mesh remain intact or would breaks in the mesh lines gradually develop?

It depends very strongly on what the mesh is made from. If the mesh is substantially more elastic than CFRP (which is very likely for most common conductors) then the stress will be very low and the fatigue life should be really really long.

Tom.

AverageUser
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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting DocLightning (Reply 12):Were the wings made of superconducting material, they would generate a teensy, tiny, weensy voltage in such a situation.

Superconducting material practically repels ALL magnetic fields, so no current at all.

 Quoting NoWorries (Reply 10): Assumung a wingspan of 33m, and a velocity of 200 m/s and assuming travel completely perpendicular to the field

Somehow everyone seems to have discarded the fact that only a changing magnetic field will induce any current. As the Earth's apparent magnetic field will hardly change over the distance over time rate of even fast jet aircraft, there will be hardly any induced current. This just might be different if you flew trough a field of some strong aurora borealis/australis.

NoWorries
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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting AverageUser (Reply 29): Somehow everyone seems to have discarded the fact that only a changing magnetic field will induce any current. As the Earth's apparent magnetic field will hardly change over the distance over time rate of even fast jet aircraft, there will be hardly any induced current. This just might be different if you flew trough a field of some strong aurora borealis/australis.

It's exactly what happens when when a coil of wire passes through a "static" magnetic field inside a generator -- also known as the Lorentz force law.

[Edited 2009-06-23 14:56:54]

Caryjack
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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting NoWorries (Reply 13):Superconducitivity wouldn't change anything

The higher the conductivity the less voltage developed along that conductor. If a wing was a perfect conductor, there would be no voltage developed between the tips.

 Quoting DocLightning (Reply 14):For example, a length of rubber won't generate much of a potential difference because the resistance is too high to the flow of electrons.

It takes a conductor cutting through lines of force (in this context) to generate the potential. Insulators can't generate anything.

 Quoting DocLightning (Reply 14):An aluminum wing is a pretty good conductor

Aluminum is an excellent conductor followed only by gold and silver, IIRC.

 Quoting DocLightning (Reply 14):Remember, Earth has a very large magnetic field, but a very weak one.

For the purpose of generating power by flying an airliner through it, sure.

 Quoting NoWorries (Reply 15):Resistance doesn't affect induced voltage

Induced voltage will be proportional to the load resistance, a wing in this case. Such "a big honkin conductor" would require an "absolutely enormous current" to generate anything close to a 1 volt drop. That's what these earlier posts were referring to:

 Quoting NoWorries (Reply 15):an excess of charges will pile up at one tip, and there'll be a defecit at the other.

This is a pretty good definition of a voltage drop (potential difference, EMF) across a wing.

 Quoting DocLightning (Reply 21):Remember, all materials are conductors. It's just a matter of the voltage you have to apply across them before they will conduct.

I'd disagree here. Conductors and insulators, semiconductors too, have physical properties that make them good at what they're designed for. An over voltage will destroy an insulator as surely as an over current will destroy a conductor. Once you've done it, you can't undo it.

 Quoting NoWorries (Reply 22):Until an insulator crosses the conduction threshold, it doesn't have resistance

They usually don't have much resistance once they've started conducting either. In fact they often arc over destroying the insulator and many thing in the area.

 Quoting NoWorries (Reply 22):The voltage at which an insulator starts to conduct is called the breakdown voltage

There are lots of devices designed to do this but normally when an insulator breaks down it is no longer useful. Just what sort of application do you have in mind? The only reusable insulation that comes to my mind is air. Others that were mentioned such as rubber and CFRP would be destroyed in an over voltage condition.

 Quoting AverageUser (Reply 29):Somehow everyone seems to have discarded the fact that only a changing magnetic field will induce any current.

A changing field into a fixed conductor, or a fixed field into a changing (rotating) conductor.

 Quoting NoWorries (Reply 30):It's exactly what happens when when a coil of wire passes through a "static" magnetic field inside a generator -- also known as the Lorentz force law.

But I did have to dust off Grob.

 Quoting David L (Reply 9):Damn! I had grand intentions of postion a diagram of the Right Hand Rule

That’s OK. You can still use the Left Hand Rule to figure out which pole you're flying over.

Thanks,
Cary

AverageUser
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### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting Caryjack (Reply 31):An over voltage will destroy an insulator as surely as an over current will destroy a conductor. Once you've done it, you can't undo it.

Not necessarily, and there's a class of insulating liquids, gases, and of course the vacuum.

 Quoting Caryjack (Reply 31): A changing field into a fixed conductor, or a fixed field into a changing (rotating) conductor.

Yep, but, there's practically no changing field, and the a/c will practically see a constant magnetic field. The Earth is a magnet, but a mighty big one, so at these rates of flying across it (any way), practically nothing will be seen across the conductor.

If you want to study plots of the Earth at any height, see here:

http://www.geo.fmi.fi/MAGN/igrf/applet.html

NoWorries
Posts: 493
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### RE: Voltage Induced Across Wingspan In The Cruise

Resistance doesn't affect induced voltage

Induced voltage will be proportional to the load resistance, a wing in this case. Such "a big honkin conductor" would require an "absolutely enormous current" to generate anything close to a 1 volt drop. That's what these earlier posts were referring to:

TThat's a good point -- I was thinking about this in terms of a circuit (a source and a sink) -- in this case the source is the sink, and so, duh, there can't be a potential if there's perfect conductivity -- my apologies to DocLightning.

Probably what I should've said is that the conductor moving through the field creates a charge imbalance in the conductor, and so there's a potential difference between the tips of the wing, but once steady state is reached there's no current flow.

[Edited 2009-06-24 05:02:04]

Caryjack
Posts: 328
Joined: Tue May 08, 2007 9:45 am

### RE: Voltage Induced Across Wingspan In The Cruise

 Quoting AverageUser (Reply 32):Not necessarily, and there's a class of insulating liquids, gases, and of course the vacuum.

I had air but rubber and CFRP were being discussed.

 Quoting AverageUser (Reply 32):If you want to study plots of the Earth at any height, see here:

Cool. Just playing with it, it looks like the magnetic intensity increases closer to the poles. Thanks.

 Quoting AverageUser (Reply 32):Yep, but, there's practically no changing field, and the a/c will practically see a constant magnetic field.

The idea is that an airliner flying through the Earth's magnetic field (fixed field, moving conductor) would produce a potential difference across the wing tips. Posters noted that this potential would be greatest at the poles where the fields are denser and perpendicular, then weakest near the equator where the fields are less dense and parallel to the conductor.

 Quoting Faro (Thread starter):what magnitude of voltage is created between one wingtip and another by an airliner's winspan cutting across the lines of the earth's magnetic field at cruising groundspeed. Presumably this would be at a maximum near the polar areas, would it not?

 Quoting NoWorries (Reply 15):As noted in the calculation, it was performed at the pole where the field would be perpendicular to level travel.

 Quoting AverageUser (Reply 32):The Earth is a magnet, but a mighty big one, so at these rates of flying across it (any way), practically nothing will be seen across the conductor.

This gets my vote, but I think we're talking about nano watts.

 Quoting NoWorries (Reply 33):Probably what I should've said is that the conductor moving through the field creates a charge imbalance in the conductor, and so there's a potential difference between the tips of the wing, but once steady state is reached there's no current flow.

Sounds about right to me. But as said above, not much will be made of it.

Thanks,
Cary

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