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Voltage Induced Across Wingspan In The Cruise  
User currently offlineFaro From Egypt, joined Aug 2007, 1551 posts, RR: 0
Posted (5 years 4 months 2 weeks 4 days 20 hours ago) and read 6469 times:

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
34 replies: All unread, showing first 25:
 
User currently offlineNomadd22 From United States of America, joined Feb 2008, 1871 posts, RR: 0
Reply 1, posted (5 years 4 months 2 weeks 4 days 19 hours ago) and read 6474 times:

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.



Andy Goetsch
User currently offlineDoclightning From United States of America, joined Nov 2005, 19786 posts, RR: 59
Reply 2, posted (5 years 4 months 2 weeks 4 days 17 hours ago) and read 6443 times:

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.

User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 3, posted (5 years 4 months 2 weeks 4 days 14 hours ago) and read 6384 times:



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.


User currently offlineCaryjack From United States of America, joined May 2007, 332 posts, RR: 0
Reply 4, posted (5 years 4 months 2 weeks 4 days 12 hours ago) and read 6355 times:



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.

 checkmark 

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

That'd be my guess.  biggrin 

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


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 5, posted (5 years 4 months 2 weeks 4 days 7 hours ago) and read 6318 times:



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.


User currently offlineCaryjack From United States of America, joined May 2007, 332 posts, RR: 0
Reply 6, posted (5 years 4 months 2 weeks 4 days 7 hours ago) and read 6307 times:

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,  smile 
Cary


User currently offlineDocLightning From United States of America, joined Nov 2005, 19786 posts, RR: 59
Reply 7, posted (5 years 4 months 2 weeks 4 days 4 hours ago) and read 6279 times:



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.


User currently offlineNomadd22 From United States of America, joined Feb 2008, 1871 posts, RR: 0
Reply 8, posted (5 years 4 months 2 weeks 3 days 18 hours ago) and read 6169 times:



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.



Andy Goetsch
User currently offlineDavid L From United Kingdom, joined May 1999, 9524 posts, RR: 42
Reply 9, posted (5 years 4 months 2 weeks 3 days 17 hours ago) and read 6166 times:



Quoting Doclightning (Reply 2):

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.  biggrin 

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


User currently offlineNoWorries From United States of America, joined Oct 2006, 539 posts, RR: 1
Reply 10, posted (5 years 4 months 2 weeks 3 days 15 hours ago) and read 6135 times:

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.


User currently offlineKalvado From United States of America, joined Feb 2006, 491 posts, RR: 0
Reply 11, posted (5 years 4 months 2 weeks 3 days 3 hours ago) and read 6032 times:

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.


User currently offlineDocLightning From United States of America, joined Nov 2005, 19786 posts, RR: 59
Reply 12, posted (5 years 4 months 2 weeks 2 days 5 hours ago) and read 5812 times:



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.


User currently offlineNoWorries From United States of America, joined Oct 2006, 539 posts, RR: 1
Reply 13, posted (5 years 4 months 2 weeks 1 day 23 hours ago) and read 5769 times:



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.


User currently offlineDocLightning From United States of America, joined Nov 2005, 19786 posts, RR: 59
Reply 14, posted (5 years 4 months 2 weeks 11 hours ago) and read 5618 times:



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.


User currently offlineNoWorries From United States of America, joined Oct 2006, 539 posts, RR: 1
Reply 15, posted (5 years 4 months 2 weeks 11 hours ago) and read 5612 times:



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.


User currently offlineDocLightning From United States of America, joined Nov 2005, 19786 posts, RR: 59
Reply 16, posted (5 years 4 months 2 weeks 4 hours ago) and read 5585 times:



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.


User currently offlineFaro From Egypt, joined Aug 2007, 1551 posts, RR: 0
Reply 17, posted (5 years 4 months 2 weeks 2 hours ago) and read 5555 times:



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
User currently offlineNoWorries From United States of America, joined Oct 2006, 539 posts, RR: 1
Reply 18, posted (5 years 4 months 2 weeks ago) and read 5539 times:



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 ...


User currently offlineThirtyEcho From United States of America, joined Dec 2001, 1653 posts, RR: 1
Reply 19, posted (5 years 4 months 1 week 6 days 5 hours ago) and read 5465 times:

I am shocked, shocked at this topic.

I order all airplanes grounded at once!

-Claude Rains


User currently offlineNomadd22 From United States of America, joined Feb 2008, 1871 posts, RR: 0
Reply 20, posted (5 years 4 months 1 week 6 days ago) and read 5440 times:



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.



Andy Goetsch
User currently offlineDocLightning From United States of America, joined Nov 2005, 19786 posts, RR: 59
Reply 21, posted (5 years 4 months 1 week 5 days 15 hours ago) and read 5399 times:



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...  duck 


User currently offlineNoWorries From United States of America, joined Oct 2006, 539 posts, RR: 1
Reply 22, posted (5 years 4 months 1 week 5 days 10 hours ago) and read 5379 times:

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]

User currently offlineNoWorries From United States of America, joined Oct 2006, 539 posts, RR: 1
Reply 23, posted (5 years 4 months 1 week 5 days 9 hours ago) and read 5367 times:



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.


User currently offlineThirtyEcho From United States of America, joined Dec 2001, 1653 posts, RR: 1
Reply 24, posted (5 years 4 months 1 week 5 days 5 hours ago) and read 5350 times:

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


25 Faro : 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 de
26 NoWorries : Presumably/hopefully the Boeing engineers took that into account. Since it's only job (AFAIK) is to disperse lightening (or other static?), small cra
27 NoWorries : 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 c
28 Tdscanuck : 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 con
29 AverageUser : Superconducting material practically repels ALL magnetic fields, so no current at all. Somehow everyone seems to have discarded the fact that only a
30 NoWorries : 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
31 Caryjack : The higher the conductivity the less voltage developed along that conductor. If a wing was a perfect conductor, there would be no voltage developed b
32 Post contains links AverageUser : Not necessarily, and there's a class of insulating liquids, gases, and of course the vacuum. Yep, but, there's practically no changing field, and the
33 Post contains links NoWorries : Quoting Caryjack (Reply 31): Quoting NoWorries (Reply 15): Resistance doesn't affect induced voltage Induced voltage will be proportional to the load
34 Caryjack : I had air but rubber and CFRP were being discussed. Cool. Just playing with it, it looks like the magnetic intensity increases closer to the poles. T
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