Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Pseudoscience-is-malarkey on 27/08/2022 14:02:18
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Given how powerful lightning is, and how hard it is to know when or where it will strike, is it not really much of a direct threat to aircraft? Obviously avoiding lightning clouds is good practice, but it can't always be easy to do so, especially large, less nimble aircraft. I don't know.
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Most aircraft are built to survive being struck by lightning.
Edited to remove obvious typo.
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Most aircraft are built to survive being struck my lightning.
Given how powerful lightning is...
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It's a common occurrence at intermediate altitudes (generally below 10,000 ft) and can be a bit of a bugger but all the metal bits of the airframe are carefully bonded together so that the machine is effectively a Faraday cage with no potential difference between the parts. Radio and radar aerials that project through the shell are protected by discharge tubes or semiconductor shunts.
The real problem is the attendant turbulence, which can indeed destroy an airframe or distort it into an unflyable state, and violent "downbursts" of descending air and precipitation that can push you into the trees on approach (there's no excuse for taking off in a thunderstorm, but what goes up must come down). Rapid icing and hail damage in and around cumulonimbus should be survivable if it's not avoidable, but there are limits to what can be done or avoided by any particular aircraft, and no limits to what nature can throw at you.
I recall being struck twice on approach to Dublin. Lots of noise and shaking for a fraction of a second, but the real problem was landing on 2 inches of hailstones.
A friend got sucked into a cu-nim in a glider which rapidly disintegrated. He extricated himself from the still-rising wreckage and had the presence of mind not to open his parachute until he fell out of the cloud. He now wears a little gold caterpillar lapel badge, the emblem of parachute survivors.
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Metal box, no problem. What concerns me is composite construction techniques, commonly used in current aircraft. This product consists of a material which is essentially nonconducting at normal temperatures but will become a poor conductor at elevated temperature(due to decomposition) all covered in a thin metal film. If lightning hits a poor conductor the damage is usually extensive, eg a steel pole will at most suffer scorch marks while a tree will often burst open in flames. The engineers involved say "no problem" but they also said the same for the dc10 rear cargo door or the 737 max software. I'm not convinced, the energy dissipation in such a material could be catastrophic.
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They are aware of the problem.
https://www.compositesworld.com/articles/lightning-strike-protection-strategies-for-composite-aircraft
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Thanks for that article, bored chemist, it was interesting. I know they are aware of the dangers but I remain unconvinced. Because weight reduction is the main criterion for using composites the weight of conductive material added must be kept to a minimum. I suspect a sufficiently powerful hit will simply vaporise said metal film and then conduct through the composite with massive i²r heating.
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When you say
weight reduction is the main criterion
Do you mean
Why is aircraft destruction via lightening extremely-extremely rare?
:-)
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Significant parts of airlines suffered severe damage after being struck by covid.
But what really takes them down is a lightning strike by baggage handlers... ;)
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Why is aircraft destruction via lightening extremely-extremely rare?
The maxim of aircraft design has been ascribed to practically every pioneer from Henson (1838) to Rutan (1974): "Simplicate and add lightness." Nobody has ever managed to do so.
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Does a helium balloon design count as "adding lightness"?
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Someone in my daughter's scout troop tried this. She tied some party balloons to her rucksack but found the additional drag when walking upwind was more disadvantageous than the overall weight reduction. But full marks for ingenuity and experimental guts. And a merit to the lad who offered to carry other people's light, bulky clothes in exchange for his cans of baked beans and corned beef.
The classic example was the R100/R101 airship saga: the later machine was overweight (yes, they even added an extra ballonet to lighten it!), crashed on its maiden voyage, and transferred its reputation to airships in general, so the entirely satisfactory R100 was scrapped.
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Aircraft designers also know the parts most likely to be struck are pointy parts, like the top of the tail, the wing tips, the trailing edges of the engines, the nose and the rear stabiliser. Thus those parts, even those that move, do have rather substantial metal parts at the tips, and also have ground bonding flexible straps to bond them to the outer metal skin, and also the designers add in discharge brushes, rods of bare carbon fibre that will reduce charge build up and dissipate it, and thus reduce potential difference between the ambient and the aircraft, so reducing the chance of it being the cause for a discharge. The wheels also have the discharge brushes, and thus will shed charge as they get close to the ground, so there is no charge as the aircraft lands. Plus the same heavy flexible bonding straps, to conduct across the bearings so they do not suffer erosion from arc discharge.
Composite has the metal mesh embedded in it, or a foil of metal to act like charge spreader, and the large area means the unit current per square is low, plus brief, so the metal does not heat up much moving the charge across it. You might suffer damage, but it will survive. Of course you also do not fly into active storm cells, more due to the sudden steep air flow gradients being able to exceed the stress allowed on the airframe, there have been a good number of airframes written off for surviving flight through a storm cell, and the airframe exceeding it's maximum load limits multiple times, and the resultant stress deforming parts, so that, after landing safely, the airframe has been written off, because the number of failed rivets is uneconomical to repair.
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Most aircraft are built to survive being struck my lightning.
It is not your lighning, it also mine.
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Someone in my daughter's scout troop tried this.
Aproximatley 1m3 can lift a kilo ish , I think that whomever dreamt it up must have their head in the clouds.
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Probably for the same reason that powerline workers can work on the electrified cables as long as they are not connected to the earth. I should think a lightning bolt would have greater trouble passing to an ungrounded plane and back again than continuing its path through the air surrounding the plane.
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I think that whomever dreamt it up must have their head in the clouds.
Yes, that's one of the occasional joys of ballooning. But chilly.
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I should think
Indeed.
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The charging of an aircraft seems to be a great factor in lightning strikes, .
[ Invalid Attachment ]
A plane travelling through the air creates the obvious static, plus the fact that a plane is travelling through a magnetic field, that of earth.
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Unlikely to separate charges as the front and rear are connected by a conductor. There are certainly circulating currents, however.
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Unlikely to separate charges as the front and rear are connected by a conductor. There are certainly circulating currents, however.
I believe that the image is the static, but if the magnetic field of earth imbues a pd between the plane and the cloud it increace the chance of a strike. I should not think the chance is that great as the plane is ungrounded.
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You can't separate + and - static charges on a conductor.
The magnetic field induces circulating currents in the aircraft but doesn't alter its potential difference to the earth or cloud.
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You can't separate + and - static charges on a conductor.
How does this work then?
http://physics.bu.edu/~duffy/sc526_notes01/charging_induction.html
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The net charge distributes itself over the surface of the conductor.
(+) + (-) = 0
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The net charge distributes itself over the surface of the conductor.
(+) + (-) = 0
You seem to have forgotten to say anything useful.
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You can't separate + and - static charges on a conductor.
The magnetic field induces circulating currents in the aircraft but doesn't alter its potential difference to the earth or cloud.
You can have static on one end of a conductor if it is in motion through another air.You can't separate + and - static charges on a conductor.
The magnetic field induces circulating currents in the aircraft but doesn't alter its potential difference to the earth or cloud.
By default there would have to be aninbalance inbalance during build up of static due to the process. It would be most severe in the drag inducing surfaces, if not no static would ever happen. During motion a static imbalance is bound to occur. Plus of course there is the paint.
A conductor through a magnetic field will produce a PD
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The biggest points of strike are engines, as there you have very hot air, with plenty of ionised atoms in it, to initiate a discharge. Then the pointy bits, all of which sport static discharge points so as to protect the metal, and those are relatively easy to replace, and are part of your pre flight checklist for damage.
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Your diagrams show a negative static charge accumulating at one end of a conductor and a positive charge at the other. The definition of a conductor is that charges flow freely through it, therefore opposite charges cannot accumulate anywhere - they will flow and cancel each other.
It is entirely true that you can induce a potential difference between the ends of a conductor when it is moving through a magnetic field, but the EMF is induced between the wingtips, not from nose to tail, and is unlikely to exceed 0.5 volt for an airliner flying at 500 kt near the poles.
Static charge may accumulate on the entire airframe but it will be of one sign only, and distributed evenly over the smooth surfaces, with concentrations at sharp points. The use of conductive trailing wicks ensures that the concentration is sufficient to ionise the surrounding air and leak the charge away through that conductive path.
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A conductor through a magnetic field will produce a PD
The Earth's magnetic field is pretty weak. The PD will be much lower than the breakdown voltage of air, which is around 700,000 Volts/meter (depending on whether it is raining or not). [Overlap with alancalverd...]
However, in a lightning strike, the resistance of an airplane fuselage is much lower than the resistance of air (especially before breakdown), so lightning is very likely to take that path...
- I guess that means that an aircraft in level flight is slightly less likely to be struck by lightning that one that is flying directly up or directly down...
- And an aircraft flying well above the clouds is less likely to be struck than one flying within the clouds, or between clouds and ground. But you would need to beware of blue jets...
https://en.wikipedia.org/wiki/Upper-atmospheric_lightning#Blue_jets
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And an aircraft flying well above the clouds is less likely to be struck than one flying within the clouds, or between clouds and ground.
see reply #3 above!
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I'm still waiting for a sensible reply to this.
You can't separate + and - static charges on a conductor.
How does this work then?
http://physics.bu.edu/~duffy/sc526_notes01/charging_induction.html
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Your diagrams show a negative static charge accumulating at one end of a conductor and a positive charge at the other. The definition of a conductor is that charges flow freely through it, therefore opposite charges cannot accumulate anywhere - they will flow and cancel each other.
They will flow, but for an instant there has to be an inbalance, otherwise no static would ever form? With a moving body continually producing static coupled with the paint I would think some inbalance does exist. A conductor through a magnetic field will produce a PD
However, in a lightning strike, the resistance of an airplane fuselage is much lower than the resistance of air (especially before breakdown), so lightning is very likely to take that path...
I thought the air was supposed to be to greater insulation to enable lightning anyway? The breakdown plasma channel etc of lightning would mean that the bolt would have to transition twice, from air to metal and back plus overcome any contact resistance at the interface, most of the time the bolt would encounter less interference flowing around the plane, unless the plane actually just flies into it.
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Bored chemist, what alancalverd said was that on a unitary conductor you can't have a potential difference between two zones(in the absence of a time varying magnetic field). Sure you can induce charge as in Faraday's ice pail experiments but that charge will be evenly distributed with some minor anomalies around sharp points and hence no potential difference. If this butting-in by me is inappropriate , I apologise.
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I'm still waiting for a sensible reply to this.
If you ask a sensible question, you will get a sensible reply. Try reading the article carefully. At no point does it suggest that you can separate charges on a conductor. If you think about it, that even defines a conductor. But Gauss, Coulomb and Cavendish were idiots, weren't they? To say nothing of those fools who build airplanes or power lines.
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I am aware that this does not answer whether a static charge can be in balanced in a conductor, perhaps one that is passing through the insulation fluid at 500mph but it's an example of static on a plane
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what alancalverd said was that on a unitary conductor you can't have a potential difference between two zones(
That may well be what he meant to say,
It's probably what he should have said.
But what he actually said was
You can't separate + and - static charges on a conductor.
And you can.
It is, for example, how the electrophorus works.
https://en.wikipedia.org/wiki/Electrophorus#/media/File:Electrophorus_cycle_en.png
In the picture numbered 3 you can see the charges are separated, positive on the bottom; negative on the top.
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The Wikipedia diagram is interesting. It suggests that earthing the top of the plate will result in a net positive charge on the plate. So what happens if you earth the bottom of the plate? Why doesn't it?
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So what happens if you earth the bottom of the plate?
You can do the experiment and find out.
But, in the meantime, do you understand that, if you use an electrophorus, you are proving that charge separation happens?
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That charge separation only occurs because of the contrived situation where a powerful electric field is at play. Lift the conductor up and the separation disappears. Yes you are correct in that both an electric field (or a time varying magnetic field) can induce temporary charge separation. Alancalverd was dismissing the idea of charge separation occurring on an aircraft where neither process of induction is present.
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That charge separation only occurs because of the contrived situation where a powerful electric field is at play.
No
Even a weak field will cause some separation.
And the fields involved in lightning strikes are, clearly, higher than those involved in an electrophorus.
They must be- the lightning fields are strong enough to make the air conduct.
Alan's just wrong.
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So what happens if you earth the bottom of the plate?
You can do the experiment and find out.
The problem with the given explanation is that if the metal plate is very thin, the potential gradient between the upper and lower surfaces of the plate will be very large, so the charges won't separate. But the result is the same.
And why is the ground considered to be positively charged? You can do the same experiment with a different dielectric that retains a positive surface charge, so the earth suddenly decides to be negatively charged. How does it know?
Beware of naive explanations of what is a rather more complicated phenomenon.
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Beware of naive explanations
The first thing I did was ask for a better explanation.
What are you waiting for?
How does this work then?
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so the charges won't separate.
Why not?
But, looking on the bright side, if you say that for a thin plate the charges won't separate, at least it looks like you have recognised the fact that they will separate for a thicker plate.
So the next question is how thick (or thin) does the plate need to be compared to say... an aircraft?
Is the length of a plane less than the thickness of an electroscope plate? (I think the ones I saw at school were made from coffee tin lids).
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If there were active lightning in the area, even without a direct strike, I would expect enormous field gradients to exist and hence charge separation would occur. What started this controversy was petrochemical's suggestion that charge separation leading to a voltage differential between the front and rear of the aircraft would occur by reason of forward movement through the air: I don't see this happening, at all.
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If there were active lightning in the area, even without a direct strike, I would expect enormous field gradients to exist and hence charge separation would occur. What started this controversy was petrochemical's suggestion that charge separation leading to a voltage differential between the front and rear of the aircraft would occur by reason of forward movement through the air: I don't see this happening, at all.
PC talks a lot of twaddle; his signature says so.
But charge separation in a conductor is real
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. What started this controversy was petrochemical's suggestion that charge separation leading to a voltage differential between the front and rear of the aircraft would occur by reason of forward movement through the air: I don't see this happening, at all.
Why not? Do you think that static electricity generation on a balloon is immediately equalised even though only one side of the balloon is rubbed?
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It seems that Alan doesn't understand the charge distribution an a conductor, but he's still ahead of PC who has yet to work out what a conductor is.
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You need to distinguish between charge separation, a static phenomenon, and charge flow, which is dynamic.
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Petrochemicals, a balloon is an insulator unless metalized and hence could have different charge distributions. If one removed the trailing static wicks from an aircraft I could imagine a large static charge building up but it would not lead to a differential between front and rear as you asserted.
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Petrochemicals, a balloon is an insulator unless metalized and hence could have different charge distributions. If one removed the trailing static wicks from an aircraft I could imagine a large static charge building up but it would not lead to a differential between front and rear as you asserted.
Did I not mention paint on a plane yet? Plus someone I believe has mentioned magnetic fields. If it is in motion the electrons are being blown by the wind down the plane to the extremities in simplistic thinking.
Alan's conductor distribution whilst at rest is a different kettle of fish. Wherever ever electrons are moved or removed is an inbalance until the system is at rest and the charge equalises.
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Did I not mention paint on a plane yet?
Yes you did.
But the interesting article from bored chemist about metallic mesh built into aircraft laminates already addressed this issue - the laminate itself is an insulator, as is the paint. So the combined thickness of both must be limited.
Here is the link again, for your convenience:
https://www.compositesworld.com/articles/lightning-strike-protection-strategies-for-composite-aircraft
If it is in motion the electrons are being blown by the wind down the plane
What is the wind resistance of an electron?
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But the interesting article from bored chemist about metallic mesh
Regrettably, I didn't say anything that interesting.
I just pointed out that Alan was talking hogwash when he said
You can't separate + and - static charges on a conductor.
because, you clearly can.
About 8 minutes in here.
https://www.khanacademy.org/science/physics/electric-charge-electric-force-and-voltage/charge-electric-force/v/conductors-and-insulators
[Edited to add the vid link].
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Here's the problem
Dielectric materials are poor conductors of electricity because they do not have any loosely bound or free electrons that may drift through the material. Electrons are required to support the flow of an electric current. The current flows from the positive to the negative terminal and, in the opposite direction, as free electrons that flow from the negative to the positive terminal.
Dielectric materials support dielectric polarization, which enables them to act as dielectrics rather than conductors. This phenomenon occurs when a dielectric is placed in an electric field and positive charges are displaced in the direction of the electric field, while the negative charges are displaced in the opposite direction. Such polarization creates a strong internal field, which reduces the overall electric field within the material.
So we have two completely different models that give the same result! On the one hand, it is asserted that charging by induction works because conductors conduct electricity and thus sustain an internal field the cancels the applied field, and on the other hand insulators sustain an opposing internal field because they don't conduct electricity.
It is left as an exercise to the reader to explain what is going on. [hint - think fields rather than currents]
But as far as the present question is concerned, there is no external electric field affecting a plane in horizontal flight so no polarisation or charge separation anyway!
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there is no external electric field affecting a plane
Unless... maybe, it's in a thunderstorm or something.
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Did I not mention paint on a plane yet?
Yes you did.
But the interesting article from bored chemist about metallic mesh built into aircraft laminates already addressed this issue - the laminate itself is an insulator, as is the paint. So the combined thickness of both must be limited.
Here is the link again, for your convenience:
https://www.compositesworld.com/articles/lightning-strike-protection-strategies-for-composite-aircraft
If it is in motion the electrons are being blown by the wind down the plane
What is the wind resistance of an electron?
I think the laminates are the composite aircraft, very little of the airframe of a composite aircraft is metal. On a metal aircraft it has several layers of paint, this adds considerable weight to the aircraft. As paint is non conductive it effectively insulates the airframe. The breakdown voltage of the paint is probably not great. It doesn't answer the static conductor at rest in a neutral field question.
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American Airlines abandoned all-over paint some years ago and saved a fair amount of fuel by just polishing the aluminum.The only large aircraft that carry substantial amounts of (nonreflective/anti-infrared/visual camouflage) paint are military, but composites generally incorporate white pigment in the final gel coat to reflect solar heat.
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American Airlines abandoned all-over paint some years ago and saved a fair amount of fuel by just polishing the aluminum.
Yep and it probably had to be kept polished due to the friction from oxidised material. I imagine then you have the problem of damaging the integrity of the pressure Hull.
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No need to keep polishing, just have to do a weekly wash to keep salt accumulation from building up. They do however give the metal a clear coat protective finish, which bonds well to the thin surface oxide, keeping the surface shiny. You save weight, because you do not have multiple layers of heavy pigment, just a single coat to protect. Alternatively you use something akin to an automotive wash and wax liquid, that removes the dirt and salt build up, and leaves a thin film behind.
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Salt on airplanes? We get bugs on the windshield and the leading edges at low level, and I guess the navy would have a problem if the left their assets on the flight deck, but American Airlines spend most of their time above 30,000 ft.....
The stuff you really want to stick is deicing fluid, and I've always wondered why it is sprayed on at such high velocity that most of it seems to bounce off. If you deice when airborne, it can be done with a "weeping wing" that just trickles the gunge into the airflow with minimal loss, but the ground deicers seem profligate, to say the least.
I recall a winter morning when all flights from Stansted were delayed for 2 hours because the the deicing trucks were parked outside overnight, and thanks to a thunderstorm they were, er, covered with ice. If you've time to spare, go by air.
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Yep and it probably had to be kept polished due to the friction from oxidised material.
A lot of people know that many aluminium alloys essentially don't "rust"- particularly if they are anodised.
Could someone explain that to PC?
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Salt is an issue as you get salt spray even far inland, especially in winter from road deicing, so you really want to wash it off regularly. Deicing has a detergent and a gelling agent in it, to both have it adhere as a film, and to stay in a thin film for long enough for the ionic salts in it to depress the melting point of the ice far enough so it melts. That stuff is corrosive, so you really want it to only stay on for a while before it melts the ice and falls off, and then is scrubbed off in flight. Most in flight deice is done using either electric heat or hot air bled off the engines, or via rubber boots that swell to break the ice off. Spraying the deice fluid at high pressure both ensures loose ice will be popped off, and also you want to get the fluid under the ice, and use pressure to move it off. A trickle will not do, you need to have enough pressure to cause the skin to vibrate, breaking the bond between ice and metal. That is why deicing has to be careful around probes and static ports, so as to not fill them with the corrosive liquid.
Aluminium will, for almost every alloy, have a thin layer of oxide on it, to be shiny the oxide just has to be thin, so the important part of it is to not grow the oxide, so you want to keep it clean, and a thin organic wax coat, or a thin polymer clear coat, will do this easily, though salt will cause issues by either dissolving the oxide, or growing it thicker.
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Many thanks! Old wisdom among fliers:
After 100 hours, you know everything
After 1000 hours you realise you don't know everything
After 10,000 hours you know you will never know everything
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No need to keep polishing, just have to do a weekly wash to keep salt accumulation from building up.
Aluminium, particularly the soft ductile stuff I would imagine pressure hulls are made from for reasons of stress fracture inhibition would generally need a fair ammount of maintenance to remain as low friction as possible. Perhaps a Teflon paint job would be best, or maybe dimples like a golf ball.
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Not by any means an expert but I believe natural aluminium is not used in aircraft. I think It may be duralumin or similar alloy, considerably stronger than natural aluminium.
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"Pure" aluminium is used for foil, electrical cables and pretty much nothing else because it's too soft.
I think it's fair to say that the aircraft industry has put some thought into this.
Sometimes, they do their thinking a bit late...
https://en.wikipedia.org/wiki/BOAC_Flight_781#Initial_findings_and_reaction
And sometimes they forget to think about things that they should already know.
https://www.ft.com/content/ab798d0c-ed7f-4df7-9689-d7405e7ee365
someone did come up with this
https://en.wikipedia.org/wiki/Alclad
the best of both worlds.
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Not by any means an expert but I believe natural aluminium is not used in aircraft. I think It may be duralumin or similar alloy, considerably stronger than natural aluminium.
I would imagine what ever they use in the pressure hull it has qualities in it to avoid stress fracture as seen in the boac comet crashes, I shouldn't think it is the same structural aluminium seen in other parts of the aircraft.
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I shouldn't think it is the same structural aluminium seen in other parts of the aircraft.
As I understand it, part of the solution was to make the windows more rounded in shape. Stress tends to build up at corners of a window.
Of course, it would be much safer and cheaper to have passenger aircraft with no windows at all - just give everyone a screen with selectable views from the pilot's seat, the tail, first class (left and right), straight down, etc. I once travelled in an A380 that had about 5 selectable views, as well as the more traditional satellite view...
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The military also have the sense to face passengers backwards. Much safer when landing on a rough strip,and quicker to exit whether by parachute or jeep.
A Navy wife of my acquaintaince once managed to get a courtesy flight for herself and children to meet her husband on a foreign posting. They went in a mostly-empty RAF Hercules with a very friendly loadmaster. On returning to school one produced the obligatory "what we did on holiday" essay: "We flew backwards to America and played football on the plane". Was sent home with a note about "overactive imagination".
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Interesting. I once saw a picture of a Vickers vc10 with the seats backwards. I was wondering is this normal ?. must have been a troop carrier.
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Normally done to pack in a lot more sardines, as now you can share leg space between the 2 rows, and thus leave room for more cargo as well. On the Hercules you also had the rather interesting toilet, which was just basically a tube to the outside, where the pressurisation removed liquid faster than you could produce it. However on those I flew on we only had seating sideways, edge of cabin and central row, allowing the space to be used for cargo. My seat of choice was right by the wing root, behind the thin red line on the wall (turbine blade comes through here, as shown by a few Antonov crashes recently), so you were at the most stable point on the plane, and could do odd things in turbulence, like have a bottle of cooldrink with the bottle going wild, but the level of liquid staying still relative to the bottle side. Yes i did have a few of the clear plastic baggies with, just for the queasy souls near me, though the guys on the cargo at the rear were literally flying in the back.
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On the Hercules you also had the rather interesting toilet, which was just basically a tube to the outside, where the pressurisation removed liquid faster than you could produce it.
Also a feature of some gliders (10,000 ft with no engine heating). Known as the "Irish/English/Polish/French/German microphone" before the days of political correctness. Venturi tube instead of cabin pressure - yet another example of Bernouilli in action.
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Well they use to use conductive coating a black paint on the aircraft to spread the lightning charge across a large area with composites they use a metal mesh embeded in the composite that does the same thing.With out a conductive coat a lightning strike would put a hole in the aircraft.They also use to use a techneque called flame spray which put a thin metal coat that also spread the lightning charge. Most are composites today so wire mesh is the current tech to offset damage.I inspected many of them for a major manufacturer I should know.
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it can't always be easy to do so, especially large, less nimble aircraft. I don't know.