Avro Anson Mk. I – trailing aerial

Hi.
Regarding the wind in aerial on the Anson. When I was doing my square bashing at Bridge North, one day we senior men were told to report to the Guard Room for a dirty job at Cosford. At the Guard room we boarded an Anson
On the pan was an Avro Anson ticking over and we were duly put on board. I ended up next to the pilot amid jeers from the lads! The pilot said, “You’r not going to sit there doing nothing, you can wind then undercarriage up. There was a huge wheel at the side of my seat, it took thirty turns down and thirty turns up!

Ken.

Thank you, powerandpassion. When first I posted my ‘obscure question’, I never imagined that it would elicit such detailed and erudite responses!

What might have happened

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I experienced a lightning strike on the trailing aerial while flying in an Anson T22. The trainee AEO using his TR1154/55 left the trailing aerial out when we flew close to a thunderstorm. There was a hell of a bang as though some fool had discharged a shotgun and some smoke from the AEO position. He sat there stupified with his radio u/s and the aerial winch shattered beside him. I was in the nav station a couple of seats behind. Of course the aerial had gone.

Attached is a photo of RAAF trailing aerial on Anson I frame, not sure if this is a direct copy of trailing aerial in use in RAF prewar.

These trailing aerials are a bakelite ‘fishing reel’ supported in a steel frame – I would guess that the original designers contemplated some form of insulation for a conductor line that would in greater probability impact with overhead powerlines rather than be involved in a lightning event. Perhaps the weakness of the design was that it did not then contemplate a lightning event. My understanding is that lightning strikes on aircraft were being grappled with in the 30’s and the science as yet not fully understood. There is a paper here

http://www.muzeumlotnictwa.pl/index.php/digitalizacja/katalog/1112

at the Kracow museum which deals with ‘why aircraft dropped out of the sky after lightning strikes’, trailing aerial strikes and eyewitness descriptions of strikes which would make loss of aircraft control self explanatory. I have a friend who was proximate to a car when struck by lightning who described ‘fireballs’ floating through the interior of the vehicle and a loud noise, which he realized later was him screaming !

My understanding is that lightning starts from the ground, thin leaders search upwards until a connection is made then the major discharge passes down the leader to the ground. The ‘hair standing on the head’ of the golfer in the open about to get struck by lightning is the invisible leader working its way up…

So it is likely that the leader made its way via the path of least resistance, the trailing aerial, to the bakelite winding reel, isolated from the warren truss steel tube airframe. The radio operator would have started to ‘feel funny’, as the leader searched for a pathway, dancing around his body, finally connecting to the radio set and table. The microphones would have started to crackle, a frown developing on the radio operators face…then the EXPLOSION, as the leader finally found a full pathway to the opposite potential in the cloud mass and the main discharge occurred…

Picture this : a miserable, dark winter with driving snow piloting an Anson I, freezing air blowing through every miserable crack in a flying seive, your hands underneath thick mittens are frozen and you can barely think, except to ruminate on what a dumb idea it is to be there, and how you can’t even open the thermos. Suddenly there is a blinding light and huge explosion : the wings have come off !? Before you can take control of your instant reptile panic a grotesque vision expands before you : fireballs dance towards you and the navigator/radio is frozen standing up with hellish flames shooting from the headphones around his head : calmly maintain height ? Drop 1000 feet while only 300 feet off the ground ?

Thank you, nonetheless, for your efforts, Deryck.

.. The radios in use were the usual T1154/R1155.

Not in 1937, though..

Avion Ancien, I checked through both of our Anson manuals and although the have info on the Trailing Antenna they do not mention the amount of antenna line carried. The radios in use were the usual T1154/R1155.

I checked with a couple of our wireless guys and they said the amount of antenna lowered was a function of the frequency they intended to use. As to the amount of antenna on the drum – they guessed at around 300feet.

Phew. I’ve got to the point of just sitting back, reading, learning and being mightily impressed. Thank you all.

I experienced a lightning strike on the trailing aerial while flying in an Anson T22. The trainee AEO using his TR1154/55 left the trailing aerial out when we flew close to a thunderstorm. There was a hell of a bang as though some fool had discharged a shotgun and some smoke from the AEO position. He sat there stupified with his radio u/s and the aerial winch shattered beside him. I was in the nav station a couple of seats behind. Of course the aerial had gone.

Ah, I see!

Thanks Mark, now I get your drift 😀

I altered my post afterwards to add that I have niggling doubts about my workings.. I may be missing a ‘vector’ element to how the forces act against each other?

Now don’t over complicate my back of the napkin by asking me to allow for drift as well, I have nearly got away with “ignoring” drag – smiles.

regards

Mark Pilkington

Just for the record, the Anson Mk I has a steel tube fuselage with a fabric covering and one piece wooden wings.

The Anson Mk V has a moulded wood fuselage and 3 piece wooden wings.

I suspect that the Mark V was still tied electrically together by all that plated copper earthing strips tacked all over the wooden parts and that would have bonded the external metal parts to the radio rack, and of course conducted 1 Billion Volts and 80KA for a little while smiles. (a very little while).

Regards

Mark Pilkington

Ah, I see!

Thanks Mark, now I get your drift 😀

I altered my post afterwards to add that I have niggling doubts about my workings.. I may be missing a ‘vector’ element to how the forces act against each other?

Great stuff, Mark

But I would still take issue with your assertion about forward velocity. This is based on – forgive me – a schoolboy error.

It is incorrect to state that velocity has any effect. As I said before, think of something hanging from the roof of Concorde. While the aircraft is neither accelerating or decelerating it is vertical, at 200mph or Mach 2.

The same with the aerial wire. Your ‘worst case’, the one that ignores drag, is vertical. A 200ft cable drops 200ft. Plain and simple. Newton. Plotting a velocity against a force in this way is meaningless. Velocity has NO EFFECT. Drag does.

Yes, drag increases with velocity.

To show my working.. Drag is a force. This force is also dependent upon the density of the medium, the shape of the object (represented by a ‘drag coefficient’), and the effective (presented to the force) surface area.

At a given velocity, all the variables remain static except effective surface area. This is free to vary according to the angle of the wire.

The wire will therefore assume the position – in ‘O’ level physics terms ‘come to rest’ – in a position at which the Drag force (made up of velocity, density, drag coefficient and effective surface area) is equal to the opposing gravitational force.

The equation worked backwards give a surface area for each velocity. Knowing the width of the wire gives us its ‘effective’ length – ie, the length of the drop, the bit facing the slipstream.

I like the rest of your analysis though!

Smiles, I’m not suggesting Drag isn’t having an effect, I’m just saying that my vector diagram wont be trying to calculate its contribution to decreasing the angle of the dangle, but instead will calculate a theoretical worst case angle that in real life the cable could not reach due to drag.

(Yes of course without any drag, the cable would hang directly below the aircraft, but we know that’s not a real world outcome).

I’m proposing that you consider the end of the Aerial ie the weights are stationary in free air.

Apply a force to move it forward horizontally applied at the end of a 200 feet cable, and a force to move it down vertically at the weight itself (ignoring the weight of the cab ble along its length, the ratio of those to forces on the weight will describe a trajectory that is not horizontal or vertical, ie a triangle.

I would expect the triangle will represent the “best” angle of the dangle the cable and ignoring trying to calculate the drag force in the other direction which would tend to flatten the angle of the dangle up toward the horizontal.

I suspect if I consider the velocity of the aircraft to instead be the initial zero standing start and then to reach cruising speed, then I have an acceleration value that will give me a rough worst case angle of the dangle that the cable cannot actually ever exceed, ie the maximum height for length that it could possibly ever achieve with the aircraft at cruise velocity.

As we know F = Mass times Acceleration, and obviously the mass of the weights is a constant in both.

Acceleration is the rate in change in velocity over time, and Gravity is already an acceleration value, my standing start gives me a rough acceleration value based on cruising velocity.

I haven’t got out my graph paper to see if I can plot something to make sense of the numbers but I am very sure the cable will drag below the horizontal flight line of the aircraft, due to the forces discussed. and will drag at an angle even when the aircraft is cruising at a constant velocity, it will be in equilibrium between the forces acting on the cable. Drag, Gravity and forward motion.

Clearly if I troll a weighted lure behind a boat, the slower I travel the deeper the lure runs below the boat, but if I travel at too high a speed the lure will be dragging along the surface.

You are correct that I’m putting in some approximate values, but I think for the exercise they would suffice?

If I can find an example in the shed over the next few days I will give you the cross sectional thickness of the cable and its weights and perhaps you can devise a calc inclusive a drag formula.

However, I am pretty sure that no matter what angle of the dangle I might guestimate the cable sits at relative to the horizontal line of flight, or what ever reduction in altitude the end weights are travelling at, the likely situation was that they weren’t necessarily required to be in contact with the ground to cause a discharge and heavy current to pass through the Anson.

regards

Mark Pilkington

Just for the record, the Anson Mk I has a steel tube fuselage with a fabric covering and one piece wooden wings.

The Anson Mk V has a moulded wood fuselage and 3 piece wooden wings.

Great stuff, Mark

But I would still take issue with your assertion about forward velocity. This is based on – forgive me – a schoolboy error.

It is incorrect to state that velocity has any effect. As I said before, think of something hanging from the roof of Concorde. While the aircraft is neither accelerating or decelerating it is vertical, at 200mph or Mach 2.

The same with the aerial wire. Your ‘worst case’, the one that ignores drag, is vertical. A 200ft cable drops 200ft. Plain and simple. Newton. Plotting a velocity against a force in this way is meaningless. Velocity has NO EFFECT. Drag does.

Yes, drag increases with velocity.

To show my working.. Drag is a force. This force is also dependent upon the density of the medium, the shape of the object (represented by a ‘drag coefficient’), and the effective (presented to the force) surface area.

At a given velocity, all the variables remain static except effective surface area. This is free to vary according to the angle of the wire.

The wire will therefore assume the position – ‘come to rest’ – in a position at which the Drag force (made up of velocity, density, drag coefficient and effective surface area) is equal to the opposing gravitational force. This is in principle similar to ‘terminal velocity’.

The equation worked backwards gives an effective surface area for each velocity. Knowing the width of the wire gives us its ‘effective’ length – ie, the length of the drop, the bit facing the slipstream.

I am a bit concerned about this model working ‘around corners’ whereas the terminal velocity equation I borrowed for it assumes diametrically opposed forces, but my brain starts to melt when I try to work out how that works, or doesn’t. Any REAL physicists out there able to help?

I like the rest of your analysis though!

In view of that, Mark and Beermat, are you able to express an opinion as to what height might have been “sufficiently close to the ground” for the Anson to serve as a conductor for lightning to an extent such that “traces that the aeroplane had been struck were found” and that “these traces were ….. round the wireless operator’s seat and table”? Furthermore is there likely to be substance in the assertion, made at the inquest into the deaths caused by the crash of the Anson, that “the lightning probably came up into the trailing aerial through the aerial winch and then discharged itself in the cabin”?

Like a bird sitting on a High Voltage Powerline, you don’t get electrocuted if there is no current flow, hence the images above of a metal airliner seemingly suffering a strike and the passengers inside happily surviving it.

I would suspect that airliner may still have suffered some skin damage, but the currents were conducted on the external monocoque structure, and everything in side would have been lifted to the same potential and so no current flowed between passengers and the skin of the aircraft.

Once the leader from the airplane meets a leader from the cloud, a strike to the ground can continue and the airplane becomes part of the event. At this point, passengers and crew may see a flash and hear a loud noise when lightning strikes the airplane. Significant events are rare because of the lightning protection engineered into the airplane and its sensitive electronic components.

After attachment, the airplane flies through the lightning event. As the strike pulses, the leader reattaches itself to the fuselage or other structure at other locations while the airplane is in the electric circuit between the cloud regions of opposite polarity. Current travels through the airplane’s conductive exterior skin and structure and exits out another extremity, such as the tail, seeking the opposite polarity or ground. Pilots may occasionally report temporary flickering of lights or short-lived interference with instruments.

Airplane components made of ferromagnetic material may become strongly magnetized when subjected to lightning currents. Large current flowing from the lightning strike in the airplane structure can cause this magnetization.

While the electrical system in an airplane is designed to be resistant to lightning strikes, a strike of unusually high intensity can damage components such as electrically controlled fuel valves, generators, power feeders, and electrical distribution systems.

Most of the external parts of legacy airplanes are metal structure with sufficient thickness to be resistant to a lightning strike. This metal assembly is their basic protection. The thickness of the metal surface is sufficient to protect the airplane’s internal spaces from a lightning strike. The metal skin also protects against the entrance of electromagnetic energy into the electrical wires of the airplane. While the metal skin does not prevent all electromagnetic energy from entering the electrical wiring, it can keep the energy to a satisfactory level.

By understanding nature and the effects of lightning strikes, Boeing works to design and test its commercial airplanes for lightning-strike protection to ensure protection is provided throughout their service lives. Material selection, finish selection, installation, and application of protective features are important methods of lightning-strike damage reduction.

Areas that have the greatest likelihood of a direct lightning attachment incorporate some type of lightning protection. Boeing performs testing that ensures the adequacy of lightning protection. Composite parts that are in lightning-strike prone areas must have appropriate lightning protection.

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http://www.boeing.com/commercial/aeromagazine/articles/2012_q4/4/

But clearly the Anson didn’t survive, something was different?

The Anson is a wooden frame with all of its metal parts tied together via metal flat ribbons nailed to the structure, and therefore a severe lightning strike could well lift some parts to different potentials than others, creating arcing between them, ie fuel tanks and lines could be at risk on an Anson in my view, as they are sitting there in a wooden structure, a poor conductor, (unless its moist).

I don’t see any faraday cage effect occurring in the Anson, the steel tube frame is just a network of large pipes to conduct the discharge through, a Faraday Cage works on the principle of an earthed screen stopping radio waves getting through it, no such role for the Anson steel tube in that role?

For the wireless operators seat and table to show arcing marks and there to be evidence of strikes on the aeroplane my view is:

The aircraft was at a greater height than the end of the trailing aerial, it will undoubtably be dangling at some angle below the aircraft and the horizontal slip stream, and we can calculate the likely worst case angle of the dangle ignoring the effect of drag to pull it back up into the slip stream.

I therefore think the airframe got hit by the Lightning, the steel fuselage frame would be the main item to become charged, and then the EHV have lept via the frame and radio mounts into the aerial, (clearances inside the radio equipment would not be sufficient to stop the lightning jumping across and into the aerial).

The Aerial being extended out and below the aircraft, would be much closer to the ground and ground potential, especially if the air was damp, (which is the typical environment for lightning) and hence the charged aerial weights would discharge into the moist air trying to reach the ground, and even if not physically seen to hit the ground, there would be a current flow to ground sufficient to have high currents flowing through the steeltube frame, the radio and aerial, and of course any humans sitting on metal seats and touching the radio, or otherwise providing parallel paths.

I would imagine the storm clouds might be at say 1000 to 2000 ft, the Anson might have been at 1000 ft to 500 ft to 300ft? to maintain VFR, and if the 200 ft of aerial was say dangling at 50 ft, then the ground may have still been 150 to 200 feet below the weights, and if there was drizzle the lightning could easily still jump that gap.

At anything below 50 feet in moist air, and at a lightning strike voltage in the MegaVolts, (and 10,000kV per inch in free air) I don’t think it wouldn’t be greatly different to the aerial dragging along on the top of the soil. (1MV is 100 x 10kV and in dry air will jump 100 inches or 15 feet, and that’s in dry free air).

But I suspect it could still discharge over the 150 to 200 feet I mentioned above.

When we see lightning strike a bolt half way to the ground, the fact we don’t see it “hit” the ground, doesn’t mean current isn’t flowing to the rest of the way to the ground.

It just means its no longer a large enough flashover voltage to create visible light. – Electricity is normally invisible, that’s why it bites so many people undertaking DIY in their homes.

A typical cloud to ground lightning flash culminates in the formation of an electrically conducting plasma channel through the air in excess of 5 kilometres (3.1 mi) tall, from within the cloud to the ground’s surface. The actual discharge is the final stage of a very complex process.[2] At its peak, a typical thunderstorm produces three or more strikes to the Earth per minute.

The electrical current of the return stroke averages 30 kiloamperes for a typical negative CG flash, often referred to as “negative CG” lightning. In some cases, a positive ground to cloud (GC) lightning flash may originate from a positively charged region on the ground below a storm. These discharges normally originate from the tops of very tall structures, such as communications antennas. The rate at which the return stroke current travels has been found to be around 1×108 m/s[27])

The massive flow of electrical current occurring during the return stroke combined with the rate at which it occurs (measured in microseconds) rapidly superheats the completed leader channel, forming a highly electrically-conductive plasma channel. The core temperature of the plasma during the return stroke may exceed 50,000 K, causing it to brilliantly radiate with a blue-white color. Once the electrical current stops flowing, the channel cools and dissipates over 10’s or hundreds of milliseconds, often disappearing as fragmented patches of glowing gas. The nearly instantaneous heating during the return stroke causes the air to explosively expand, producing a powerful shock wave that is heard as thunder.

CG lightning can occur with both positive and negative polarity. The polarity refers to the polarity of the charge in the region that originated the lightning leaders. An average bolt of negative lightning carries an electric current of 30,000 amperes (30 kA), and transfers 15 coulombs of electric charge and 500 megajoules of energy. Large bolts of lightning can carry up to 120 kA and 350 coulombs.[32]

Unlike the far more common “negative” lightning, positive lightning originates from the positively charged top of the clouds (generally anvil clouds) rather than the lower portion of the storm. Leaders form in the anvil of the cumulonimbus and may travel horizontally for several miles before veering towards the ground. A positive lightning bolt can strike anywhere within several miles of the anvil of the thunderstorm.

Because of the much greater distance to ground, the positively-charged region can develop considerably larger levels of charge levels and voltages than the negative charge regions in the lower part of the cloud. Positive lightning bolts are considerably hotter and longer than negative lightning. They can develop six to ten times the amount of charge and voltage of a negative bolt and the discharge current may last ten times longer.[34] A bolt of positive lightning may carry an electric current of 300 kA and the potential at the top of the cloud may exceed a billion volts — about 10 times that of negative lightning.[35] During a positive lightning strike, huge quantities of extremely low frequency (ELF) and very low frequency (VLF) radio waves are generated.[

As a result of their greater power, as well as lack of warning, positive lightning strikes are considerably more dangerous. At the present time, aircraft are not designed to withstand such strikes, since their existence was unknown at the time standards were set, and the dangers unappreciated until the destruction of a glider in 1999.[37] The standard in force at the time of the crash, Advisory Circular AC 20-53A, was replaced by Advisory Circular AC 20-53B in 2006,[38] however it is unclear whether adequate protection against positive lightning was incorporated.[39][40]Positive lightning is also now believed to have been responsible for the 1963 in-flight explosion and subsequent crash of Pan Am Flight 214, a Boeing 707.[citation needed] Aircraft operating in U.S. airspace have been required to be equipped with static discharge wicks. Although their primary function is to mitigate radio interference due to static buildup through friction with the air, in the event of a lightning strike, a plane is designed to conduct the excess electricity through its skin and structure to the wicks to be safely discharged back into the atmosphere. These measures, however, may be insufficient for positive lightning.[41]Positive lightning has also been shown to trigger the occurrence of upper atmosphere lightning between the tops of clouds and the ionosphere. Positive lightning tends to occur more frequently in winter storms, as with thundersnow, and in the dissipation stage of a thunderstorm.[42]

When the local electric field exceeds the dielectric strength of damp air (about 3 million volts per meter), electrical discharge results in a strike, often followed by commensurate discharges branching from the same path. (See image, right.) Mechanisms that cause the charges to build up to lightning are still a matter of scientific investigation.[66][67] Lightning may be caused by the circulation of warm moisture-filled air through electric fields.[68] Ice or water particles then accumulate charge as in a Van de Graaff generator

http://en.wikipedia.org/wiki/Lightning

Powerful stuff, Nature at work, very few things we build, can survive a zap of 3 Million Volts or 300,000 Amperes, let alone 1 Billion volts or the plasma and high temperatures referred to above, and certainly a wooden Anson or its crew are not going to safely conduct the excess electricity safely out of the airframe, especially if the trailing 200 feet of aerial cable simply brings the path to ground/earth just that much electrically closer that the top of the charged cloud, resulting in those heavy currents passing through the airframe and into the trailing aerial cable.

As referred to in the wiki article, there are instances of aircraft losses due to lightning strike where clearly the aircraft itself was not a direct connection to earth, but just in the wrong place at the wrong time.

1 BV = 100000x 10,000V

10kV will jump an inch in dry air, 1Billion Volts will therefore jump 8,300 Feet in dry air, the distance between the end of the trailing aerial and the ground need not concern a discharge of 1BV too much, it will jump into the Anson, out the end of the trailing aerial, jump the rest of the way to ground, and for a very small period of time unleash massive current and energy along its path from the cloud to the ground, the Anson is a high resistance conductor in some wooden parts, but still a dead short compared to the moist air, and the damage will be significant.

I will still try and dig out one of my trailing aerial reels and measure the cross sectional area of the cable and the 50 weights, but I’m still of the view that a first order answer of the worst case angle of the dangle can be easily constructed from the length in M, and the forward velocity of the aircraft and the downward gravitational force, the vector of those should create a triangle slope which would reflect the ideal angle of the trailing aerial ignoring the force that we know existing in the other direction in the form of drag.

Regards

Mark Pilkington

Not an expert, so this really is just opinion now – but I believe that aircraft are frequently struck by lightning, and without a ground, the ‘bolt’ just passes on through (or ’round’ where there is a metal skin, ie Faraday Cage). However, the heat of the arc might get you if you are caught in it. Wasn’t the rear fuse of an Anson MkI entirely wood? So no Faraday cage protection there as such – Just a bloody great lightning rod sticking out in the form of a trailing wire, and a winch – connected by conductive wires to a wireless.. so it seems feasible. Very. But I doubt it would have ‘discharged itself in the cabin’.. it is contact with ground, surely, that discharges a bolt?

Perhaps the Anson wouldn’t have needed to be particularly near the ground for this to happen – see pic

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On the other hand, being connected with the ground, IIRC, makes matters much worse? Mark?

In view of that, Mark and Beermat, are you able to express an opinion as to what height might have been “sufficiently close to the ground” for the Anson to serve as a conductor for lightning to an extent such that “traces that the aeroplane had been struck were found” and that “these traces were ….. round the wireless operator’s seat and table”? Furthermore is there likely to be substance in the assertion, made at the inquest into the deaths caused by the crash of the Anson, that “the lightning probably came up into the trailing aerial through the aerial winch and then discharged itself in the cabin”?

Yes, that’s a point! The Anson and cable just provided a path – no contact required?

These contributions are very erudite and I’m most grateful for them. The reason for my questions should be evident if you look at http://sussexhistoryforum.co.uk/index.php?topic=3471.0. I’m trying to form a view concering the height at which the Anson might have been flying when the aerial cable first struck the ground, thus providing an earth when it was, apparently, struck by lightning. Assuming that the aerial cable was not in contact with the ground for a prolonged period of time, it does seem to be exceeding bad luck to have been struck by lightning when flying so low that the aerial cable was serving as an earth so that the steel fuselage frame of the Anson failed to serve as a Faraday Cage.

Well that may make the need for accuracy in this triangle equation even less important.

Lightning clearly conducts from charged clouds through moist air to the ground and therefore discharges.

An Anson, struck by lightning, with a trailing aerial, sufficiently close to the ground but not necessarily in contact with it, may simply have acted as a conducting path, with the lightning discharging again from the end of the aerial through the moist air to the ground to complete the circuit, and of course heavy current and High Voltage would then be occurring within the airframe.

All we can determine is the approximate Angle of the Dangle unless we apply a co-efficient of drag into the equation, taking into account the cross sectional area of the cable and the weights, and then its still not certain the aerial was actually grounded at the time, but simply at a low altitude to create a relatively small airgap.

In free dry air, electricity conducts, at a potential difference of 10,000V per inch.

In moist Air, and with voltages of much higher order, Lightning strikes to ground (without an Anson and trailing Aerial to assist) over 1000’s of feet.

ie Lightning can be at voltages in excess of 10MV etc.

Hence the Anson need not have been grounded at the time, to form a conducting path.

I will have a look in my shed and see if I have a trailing aerial handy, although many of them are stored elsewhere in my Anson containers.

Regards

Mark Pilkington

Thank you, Beermat. Certainly it will be most interesting if anyone can come up with the exact figures to insert into the equation. But for the present, the indications seem to be that the Anson was very close to the ground when it was struck by lightning.