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As an aeroplane accelerates through the Earth's B-field, it experiences a changing flux $d\phi \over dt$ and a potential difference is induced along its wings.

Given the wings are made of metal and act as a continuous conductor, current flows along the wings in a direction perpendicular to the vertical component of the Earth's B-field. Does this mean that the aeroplane would experience a Lorentz force?

What other effects could the building up of charge have on an aeroplane, and what difference would it make to this process if the aeroplane's wings were made of an insulating material?

Also, in terms of bigger effects, what would happen to an aeroplane in a thunderstorm?

Update: I discovered that aeroplanes have something called "discharge wicks" that are designed to counter this problem (see pic). However. the build up of static charge seems more significant than the buildup of charge due to the effect I described as Gupta pointed out so these are probably used to dissipate static charges.

enter image description here

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  • $\begingroup$ Earth's field, and its field gradient, is fairly small, so it seems likely that the voltages and forces exerted would also be pretty small. $\endgroup$ – DumpsterDoofus May 11 '14 at 12:28
  • $\begingroup$ @DumpsterDoofus I know that but when it comes to aeroplanes, even small effects can have significant consequences. For example, most plane crashes do not occur due to a single big cause — engine failure, thunderstorm — but follow as a series of small ones. $\endgroup$ – hb20007 May 11 '14 at 12:33
  • $\begingroup$ @hb20007 Doesn't mean that every single small fault will cause trouble. $\endgroup$ – Apoorv Khurasia May 11 '14 at 12:47
  • $\begingroup$ @Monster Truck If an aeroplane accelerates rapidly in a short amount of time, such as during take-off, $d\phi \over dt$ can be significant. $\endgroup$ – hb20007 May 11 '14 at 12:53
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    $\begingroup$ @hb20007: Similarly, a wire loop moving away from a magnet at constant nonzero speed has a nonzero rate of change of flux, even though the acceleration of the loop is zero. $\endgroup$ – DumpsterDoofus May 12 '14 at 13:12
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Nothing would really happen that may be of any significant consideration at least for normal sized passenger planes from private jets to Boeing.

Lets see two major steps, I have considered landing and take off similar, and flight as separate.

First Lets consider take off/landing

enter image description here

The blue lines are supposed to be magnetic field lines of earth, they are not normally so straight , but this would be a good approximation.

Lets assume a $30^o$ angle between the field and plane's body, the earth's magnetic field being $0.65G$ at maximum that is a mere $6.5 \times 10^{-5} T$. While The largest wingspan of an aeroplane till now is $97.51m$ we'll consider is $100m$ just for our ease.

So even if an aeroplane had been a square of such a large side, its area would be $10^4m^2$, and a decent $30^o$ angle with magnetic field would give you a feeble $0.56weber$ of flux.

Even if it reaches this much change of flux in just one second which it doesn't, then we would have a potential difference of about $0.5V$ across the wings, now with the resistance the current would just become too negligible and already these numbers are too large because we considered the plane a square and a constant angle of $30^o$ that the area vector had with the horizontal while its not like that and is much over $45^o$

Thus concluding that at least while take-off and landing we do not need to care about any induced currents and voltages

Now lets consider the flight

enter image description here

This is a very typical route for a flight, But still at this point you can rotate the plane in all directions and still get a near $90^o$ angle between the magnetic field and area vector in nearly all places where aeroplanes can land and/or take off! Same calculations with just a difference of angles when applied in this case give even smaller and much less noticeable currents.

Now you might think of Antarctica where the lines are pretty much perpendicular, but even for that if you calculate for $90^o$ you will get negligible results only.

Now I am going to take an impossible scenario just to show that any real life flux changes are easily negligible. The minimum that a flux can be is $0weber$ and the maximum it can be even for the largest square sheet aeroplane is $0.65weber$, even if any plane makes this amount of flux change in a mere second the potential difference developed would only be $0.65V$ this would give negligible currents and hence can be neglected over all.

Now to the case of thunderstorms, the maximum trouble that any aeroplane would receive is well known to arise from air turbulence, during thunderstorms these would create a danger far greater than any lightning bolts etc can produce, lets take a look.

During a typical thunderstorm the potential difference between the clouds and earth is more than $1\times 10^8$, the total resistance of the plane would be negligible as compared to that of the air, the major current that runs between the cloud and earth to establish a channel for the discharge is of the order of $100A$, now lets calculate the energy density that will arise in the plane due to Joules heating,

The energy generated would be for the square sheet plane $U = I^2 \frac{\rho l}{A}$ and energy per unit area would be $$u = \frac{I^2 \rho l}{A^3}$$

For our square plane, where sides are equal it would result in $$u = \frac{I^2 \rho}{s^5}$$ Lets assume an impossible current of $1000A$ for our square plane of $100m$ side, and check again$$u=\rho \times 10^{-4}$$ clearly negligible energy density

For a small plane, like the drones we have nowadays or even any thing with wingspan less than $10m$ the energy density increases rapidly, but I don't think those planes are operational under such harsh weather conditions!

PS: I am not 100% sure about the thunderstorm analysis, but I am sure about the analysis for magnetic flux

Addendum : the "discharge wicks" you show in your picture are 100% for static discharge with as simple a reason as their other name is "static discharger". I won't increase the length of the answer explaining their working etc but you can easily check it out here or simply do a google search of "discharge wick".

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  • $\begingroup$ Nice detailed and quantitative answer! $\endgroup$ – hb20007 May 11 '14 at 16:17

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