Comfy enough?

By Jiri Stipek

In our sport we concentrate heavily on the safety of our equipment – for a good reason. Anything going wrong with it can hurt. Comfort seems to be a secondary issue and however we find flying in comfort more pleasant; we may not pay enough attention to small details. Sometimes, it may turn against us in a nasty way – if we like it or not, comfort and safety are closely connected. Example - how many times you lost direction, almost running into somebody while trying to adjust your harness? Never? Well, either you are a prodigy or you are fooling yourself.

Once in the air, you want to have everything at the right place, accessible and comfy. You don’t want anything restricting your movements and visibility or having to perform contortionist exercise to get to your radio or drink. Some people are getting more annoyed by discomfort than other. I, unfortunately, belong to the group of people who have to have everything a 100% and wouldn’t stop until they have it. A zipper on my flight suit scratching my neck can spoil my day to the point I rather land if I can’t fix it. Some of these nuisances can be detected during a pre-flight check. Some you can find out only when you are airborne and it is too late to do anything with it for the duration of the flight. In that case it is important to take a mental note of it and make a firm resolution: “I will rectify that bloody problem before my next flight.” I admit I sometimes don’t keep that promise given to myself – just to regret it bitterly later. Often the excuses can be: “It is not going to happen again.”

For instance, the cable to my headset kept getting caught between my body and the flightdeck every time I got in a landing position in my harness. Not a major problem – until you find out you can’t lift your head to look upwards. Every time it happened, I was convincing myself it was an odd chance not likely to be repeated. And indeed, whenever I tried to achieve it under simulated conditions with the harness suspended from a beam, the cable ran free every single time. Obviously, I forgot a small detail – the relative wind. Anyway, I allowed this nuisance to spoil my whole season before the cable finally broke. Replacing it with one 10cm shorter took me 15 minutes…  Problem with any cables getting under tension in some situations should be solved ASAP for the sake of the cables and the gadgets they connect as well. Something is going to snap eventually causing problems during the flight and incur some expenses later.

Besides of the above mentioned cables the most common sources of discomfort and distraction are – besides of ill fitting underpants (very serious):

Poorly fitted or designed instrument panel. If you have to fiddle with it each time you want to look at your instruments, it is time to do something about it.

Radio at a place you can’t see or operate it comfortably. Especially if you have to reach somewhere to pres PTT, the radio can bring more trouble than benefits.

Camera - a cool pilot is not complete without one. Just getting it from your pocket in flight is often source of a major distress, not to mention taking pictures and then trying to put the camera back. If you want to use camera, make sure you are well organized. A helmet-mounted camera with a remote control of a sort is one of the best solutions. Beware of the safety issues concerning the helmet though.

Ill fitting helmet. Helmet should fit snugly without moving around on your head or, on the other hand, causing headaches by being too small.

Zipper scratching your neck. I find this one especially distractive. A neck warmer is not only good for your health, but it will also deal with this nuisance.

Ill fitting gloves. If too big, they make operating fiddly bits difficult. Too small ones will cause numbness of your fingers after a short while.

Speedbar you can’t get to without using our hands. If you have one of these, get yourself a proper one.

Mobile phone. Take it with you, by all means, and have it somewhere you can reach it without getting off the harness. But: have it switched off to eliminate the temptation answering an incoming call during flight. No call is likely to be worth the trouble.

Harness you have adjust after takeoff. In general, ANYTHING you have to adjust after takeoff. Also, if the harness has the habit of catching cables or lanyards of your instruments, camera etc on its buckles needs your urgent attention.

I’m sure, you can add a few items to the list yourself. Do not underestimate these “trivial” details. They can become big issues in some situations and it is not worth the risk trying to cope with them. Get comfy – get safe!

Drag Rocket!

by Jiri Stipek

For simplicity sake, the weight of the canopy (fabric only for static or fabric + air inside for dynamic analysis) has been omitted throughout the article. The load produced by the weight of the wing acts in a different way than the main (pilot induced) load and can cause changes in AoA. As the weight of the canopy represents only 5% of the weight of the whole system, these changes are negligible in the situations covered by this article. They will have to be taken in account if an extreme drag is applied (reserve).

An interesting article has been published in the Cross Country magazine a few moths ago. It was dealing with problems of drag caused by PG pilot and his harness. This part caught the most attention and it is still being discussed in some forums at the Internet: drag at the pilot’s end is not necessarily a bad thing. It can actually cause an increase of airspeed!

No, it wasn’t an April issue.  Follow the logics behind it as presented:

We all know, when you want to make a glider fly faster, you simply increase its negative pitch – point its nose down – and gravity will do the rest. So, if you increase drag at the pilot’s end, you effectively apply a torque to the system and it will rotate. The system will tilt forward until a new equilibrium between the pilot’s drag and his pendulum is reached.  The pitch of the wing will become more negative and – BINGO. We’ll fly faster! Or, will we?

It is interesting how many principles from GA can’t be applied to an aircraft with a strong pendular stability such as a paraglider. To see why, we have to establish some less obvious (or less known) facts first:

The first thing we have to realize, paraglider is not being loaded directly by the weight of the pilot. Our wing is being loaded by a force in the direction of the lines (direction best visible on the risers just above hang points). This force is a vector of pilot’s weight and his drag. It is always slanted forward against horizontal, as a result of the pilot’s drag.

Again, there were lengthy discussions on some forums, if the drag adds to the load or reduce it. To make it obvious, try to apply an extremely high drag – a large drogue chute, for instance. It is easy to imagine the pilot being suspended now between the chute and the wing, its weight divided between these two. A simple vector diagram confirms our suspicion: the force on the lines is ALWAYS lower than pilot’s weight.

Another fact, rather hard to absorb, is: the angle of attack (AoA) of a paraglider is given exclusively by the trim of the glider – by the length of the respective lines. It is vital to understand this statement, as it is the key for virtually everything in PG flight mechanics. Doesn’t matter if the PG is in a steady flight, being deployed or towed: the direction of the “lines” force relative to the canopy and thus AoA of the glider remains the same - As long as we don’t change the length of some of the lines (applying brakes or speed system), that is. The magnitude of the force may change, but not its direction relative to the canopy.

Still we have to understand, the ONLY factor determining the airspeed of a given wing, flying at a given AoA, is its load: the bigger load, (or, more accurately, the square root of load), the higher airspeed and vice versa.

Now we are ready for the close: we add drag at the pilot’s end. The glider pitches forward. At the same time the glide path (L/D) becomes steeper. As result, AoA stays the same (we know it can’t change anyway). The added drag reduces the force on the lines thus the load of the wing. Now we have the same wing, flying at the same AoA, with a less load. The net result: it will be SLOWER. Period.

If you don’t won’t to bother about all the stuff above, just follow my intuitive reaction on reading the “Drag Rocket” (as I call it) theory:

“So, if hanging in a strong wind just above the launch, going nowhere, I should ask somebody: ‘please, pull me back a bit, I need to penetrate!’ And, I idiot, always ask people to push me forward…”

By the way, pushing is not the best solution either. The right words in this situation are actually  “Pull me down!” A forward force in the direction of glide path (at pilot’s level) also causes reduction of load and consequently reduction of airspeed – but the glider will be climbing in this case. If you ever tried, you know! A vector diagram would show you why.

The gist of this whole article: if you found somewhere a piece of advice suggesting to stand up in you harness (to increase drag) if you are in the danger of being blown back, think twice. I have all the reasons to believe it doesn’t work.

Myths and Physics

 by Jiri Stipek

In the last issue we’ve established that in the world of wings we rarely get what we see. In an environment like this, it is no surprise, a number of myths or misconceptions appears and often holds for many years. Some of them are harmless other can cause serious problems or even accidents in some situations. We have to admit there is still a lot to be discovered in aeronautics and there are parts nobody fully understands yet. On the other hand: it would be foolish to believe the mechanics of flight has some exemptions from the general laws of physics.

For this reason there is no such a thing as a “compression zone” on the top of a hill. By using this term we are contradicting a lifetime work of an 18th century physicist, Daniel Bernouli. He discovered the pressure exerted by fluid is in reverse proportion to its speed. A hill causes a virtual narrowing of the cross section available for airflow, forcing it to move faster. As a net result the pressure on the top of a hill is lower!

Very much the same mistake are making people, trying to explain Ground Effect as a result of “air compressing between wing and the ground”. The exact opposite applies! A venturi effect would actually cause the wing being “sucked” towards the ground – if it ever comes close enough. Ground effect is caused by blocking off wingtip vortexes.

“A less loaded paraglider will stall easier than a more loaded one…” A proper interpretation is needed here. Your glider will stall at the same position of brakes, regardless of its load. The truth of the initial statement will appear only if we apply it to a turbulent air. The less loaded wing will be slower in both, horizontal and vertical direction. Sudden changes in airflow will more easily change its angle of attack and stall will be more likely to occur.

“If we overload a wing, it will stall.” Not true. Stall is a result of an excessive angle of attack. In a paraglider the angle of attack is given by the trim and can be increased by pulling on brakes. Again, angle of attack will remain the same regardless of load. If we apply more load the sink rate will increase. But the forward speed will immediately increase by the same rate and angle of attack will remain the same. We can get some problems in extreme cases, where the overloading approaches the limits of structural integrity. Nothing changes on the above principle, but the changes in the shape of the wing (stretching of lines etc.) can change angle of attack.

“The stall speed (or any other speed) of this wing is xxx kph.” A meaningless number. This statement is valid only if a load is specified for that particular instance. Wing can stall at any speed – in dynamic situations even at very high speed – if the critical angle of attack is reached. It is true: within the weight range the differences in speed are rather insignificant. But we have to realize a load is not always only the static force exerted by the total weight.

“Weight shifting in the direction of turn will make turning more efficient.” This belief survived about 2 decades and only the birth of Acro Paragliding helped to dismiss it. Acro pilots need to turn more efficiently and safely than CX pilots. They discovered soon, the accepted technique doesn’t work. If you look at the mechanics of it, you’ll easily understand why:

In a turn, you want the outer wing to fly faster than the inner one. In order to make it fly faster, you need to put more load on it. Furthermore, you want to maintain the correct angle of attack. If you reduce the load at a given speed, angle of attack will get lower and the wing will eventually tuck.

The inner wing, on the contrary, has to fly slower and you can guess: loading it more doesn’t exactly help the cause. We also want it to stall as late (at as low speed) as possible. A funny thing… the higher the loading, the higher the stall speed is.

So, now: why, for god’s sake, should you transfer more weight to the inner wing?

It doesn’t work the other way? Who says so? Have you ever tried?

Despite of the physics of it was clear to me from the day one, it took me almost 2 years to learn the new technique and find it perfectly natural. The reason it took so long: I believe we have some instincts inherited from riding a motorbike. Takes a while to get over it.

At this stage I apply more weight in the direction of the turn only for a brief period needed to initiate the turn and associated bank. Why it is needed and why it works at all is a bit of a puzzle. I discussed this phenomenon with some highly involved people, including the designer of Gradient gliders, Vaclav Sykora. Nobody was able to give me a full explanation. The whole thing seems to be several factors working together. Changes in the shape of the wing seem to be one of them, but I wouldn’t try to elaborate on this problem myself.

Anyway – from the point of initiation of the turn I transfer my weight immediately to the outside wing while braking on the inside – and bingo: everything works exactly the way the theory says.

A word of warning: By using this technique you can get results you never dreamed about before. Applied during a spiral you go down more than 20m/s during the first turn. In a wingover you’ll find yourself above the canopy before you know it. Do NOT use this technique for any kind of radical manoeuvres unless under supervision of an instructor and in a safe environment (above water). For improving the efficiency of your “normal” turns however, it is perfectly safe. Tuck of the outside wing will become a history and negative spin will be hard to induce. Do not push it too far though and discuss the matter with your favourite instructor before trying at home.

“The number of collapses and their severity is directly proportional to the strength of thermals.” Not true. Wing has no idea how fast the vertical movements of the air are. It will fly exactly the same way in a parcel of air rising 20m/s or falling at the same rate. It does take a notice of the speed of the changes. Acceleration will cause a change of its loading (“F=m * a” - where “F”[N] represents the load, “m”[kg] the mass of the aircraft and “a”[m/s^] acceleration). If the change of vertical speed is fast the loading will change dramatically. Just an example: if you enter a 10 m/s thermal within one second, the wing loading will DOUBLE during this period of time. Due to the pendulum effect the results in a paraglider are especially unpleasant. However, as soon as the wing adopts the new ground related vertical speed, its load and resulting airspeed (both vertical and horizontal) will become the same as in a still air. The problem is not in the absolute speed of the air movements. It is in the horizontal distribution of them and related speed of the changes applied to the glider. Especially if one part of your wing finds itself in an air moving vertically only a fraction of m/s faster than another, the change of angle of attack will cause a serious tuck or collapse. Flying in broken thermals of speed of 1m/s or so would become a frightening experience, while entering a 10m/s boomer can be a pure pleasure if the transition is slow.

In the next issue we will have a closer look at the units and terms we are using to measure and calculate some of the aspects of our flying.

Flying and optical illusions 

Most of our experience at perceiving the world around us comes from living as earthlings. So, we know that “down” is towards the earth, “up” is towards the sky “forwards” “backwards” and “sidewise” is always towards the horizon. As soon as we leave the earth surface and apply a bit of dynamics, all this can change dramatically and all our previous experience becomes a burden. If you keep relying on visual references like horizon or the ground in dynamic situations, you’ll come to grief sooner or later. It is hard to overcome our natural instincts and it may take a long time of conscious effort before pilot stops judging angle of attack (for instance) from the position of the wing to the horizon. Towing is the best example, how deceiving this reference can be.

Two interesting, and never-ending, threads popped up on the famous Xtreme Big Air discussion group recently. Both of them became so complicated, because some of the participants refused to accept the above. One Scottish gentleman has developed a completely new theory of a paragliding flight based on his observation that wing appears to fly in front of the pilot. As lines can transfer only “pull”, not a “push”, his conclusion was simple: Wing is producing a force (a component of lift) that pulls the pilot thru the air. He claimed, he’s discovered a 30 years old mistake in the theory of flight - causing a ferocious verbal battle between his followers and opponents.

He fell in a simple optical illusion trap - using horizon for a reference. He watched the situation from the human perspective. Paraglider is different from a human - it can’t see. Our wing senses only two aspects of the surrounding world: the pull on the lines (it is NOT vertical) and the relative airflow (glide path) that is NOT horizontal. Any other concepts, like the horizon, are entirely foreign to it and wing behaves only in relation to the two references above. So, if we look at our vector diagram from the perspective of the wing (using the glide path as a main reference), we see the PILOT is IN FRONT of the wing. A component of gravitation force applied to the pilot pulls the wing forward generating the required speed. For a better understanding see caption 1.

Effect of the weight of the wing and drag on the lines were omitted as negligible for our purpose. Pilot’s drag is responsible for the slight visible declination of the lines from the vertical. A reaction for this drag is the pendulum effect (not drawn) as the pilot is continuously trying to swing directly under the wing - so, he is being pulled by it after all! Well, in a sense, yes. Depends on the frame of reference we are using - either conclusion is correct. But the "prime mover" is always gravity.

Another mayhem on the forum was triggered by somebody’s query: “Can wing stall while shooting forward of the pilot?” A seemingly stupid question: We all know, wing will stall while well behind the pilot. Or does it?

The first thing to take in account is, it is not always the movement of the wing that causes the apparent change of mutual position of the wing and pilot. It is more often the swinging motion of the pilot (the surrounding air has to be used as a reference to get the right picture) in relation to an almost steady wing that creates this impression. We are getting sucked into perceiving this as movements of the wing as we use the pilot’s position as a fixed observation point.

The second aspect: go back to the beginning of this article and forget the horizon. And you have the answer. Yes, you can stall your wing while it is

                                 capt.1

“shooting in front of you” if you apply enough brakes - exactly the same way as if the wing is above or behind you. Look at caption 2. Everything fits in the picture, as you know it. Wing stalling behind the pilot – before you look at caption 3 and realize caption 2 is only an inverted crop from caption 3… 

The story of this picture (I talked to the pilot after taking it): This pilot decided to abort a loop (felt like he didn’t have enough speed) and stalled the wing by applying brakes while the lines were approx. 45 dg up. But, there was plenty of speed after all and the wing-pilot system kept rotating another 45 dg, almost causing the pilot falling in the canopy. He missed it by inches…

So, beware: In the air things are rarely exactly as you can see them!

            cap.2                                       cap.3

                                                 Marinated Electronics
                                                                                                By Jiri Stipek

We all like flying. Unfortunately, sometimes things do not go exactly as planed and we end up with a totally different activity: bush walking or swimming, for instance. Landing in the water, Needles to say, is a health hazard. But even such an innocent event like tripping on a beach landing and rolling in an ankle-deep puddle can be very painful - for your pocket. An average hang glider or paraglider pilot carries at least $1,000 worth of electronic equipment on him nowadays. These things do not need much salt to be rendered worthless; as little as a few drops of sea water and that often is it.

I've seen enough of these. Our swimming pool served as a regular first aid for canopies of those pilots who didn't quite make it on the east coast. Unfortunately, by the time they arrive, it is far too late for their high tech gadgets. Knowing what to do after flooding your toys by that solution of sodium chloride can save you a fortune - but not always. The little factor called luck is always needed here as well. The actions described below are acts of desperation, but they are worthy of trying.

Once the item has been in salt water, there is very little left for you to spoil...

The most important part just after the flooding is the speed by which you remove the batteries. Do not waste time by switching the gadget off. Not only it is useless (salt water is highly conductive and will bridge the contacts regardless), but you may even destroy the switch! And worse: if the instrument case is almost watertight and there is a couple of minutes delay, the electrolytic process will fill it with an explosive mixture of hydrogen and oxygen. Your action will produce the last important ingredient for every good blast: A spark. The first objective is to REMOVE the batteries. In fact, you often have only a few seconds to perform this task and still have some chance of success. All the following effort will help only if you were fast enough on this primary task! The batteries safely out, take the unfortunate gadget and drop it unceremoniously in a bucket of fresh water. Leave it there for some 15 minutes, stirring occasionally. Even if it doesn't help, at least you'll attract a crowd of spectators and you can charge some moderate fee for the show. Then you can relax a bit.

Drive to the nearest petrol station and buy a couple of litres of distilled water, by which you replace the existing brine. From now on the treatment will vary for each different kind of equipment. Devices with some mechanical and optical components (cameras) shall be left in the water, the container filled to the top, made airtight and delivered ASAP to a specialized service facility. Look for people specialized in drowned equipment - they advertise in SCUBA diving magazines. A regular service technician might send you, well, somewhere else. If the camera was a modern SLR (the past tense here is fully justified), don't bother. Give it to your 3-year-old. If you get a smile in return, smile too. You've made a bargain! With radio gear, varios and similar you have a good chance even without professional help. Leave the items the distilled water for about 2 hours and stir a few times. After that, put them briefly in methylated spirits, blow off with a low pressure compressed air and place them in an oven (not microwave!) heated to 50 dg C for about 4 hours. After cooling down, spray all switches, contacts, potentiometers and metal parts by a water displacing lubricating agent, like CRC2/26. Keep this stuff well away from RF parts (especially varicaps) pressure sensors and soft push buttons. It can cause some problems there. Now you are ready for the moment of the truth.

In most cases everything will work like a chime! Often only a speaker, with a paper diaphragm, suffers and has to be replaced; otherwise it will sound like, well, my own radio. A very cheap experience indeed! If it doesn't work at all, see professionals.

The rechargeable sealed battery packs need special attention too. Take the word "sealed" with reservation. The casing will positively let the salt water in, SEALING it perfectly inside - hence the name. Such a pack will become a serious fire hazard and it will fail shortly, often with spectacular sound and light effects. To give you some idea what it can do, I can show you (for a small fee) my own radio. To eliminate this time-bomb factor, DISCHARGE THE PACK THOROUGHLY ASAP through an automotive globe or a similar load. Never shorten it! Until it is completely flat, wear glasses and gloves as it can explode at any moment. Then break it open, wash and dry thoroughly in similar fashion as described above and glue it back together. If you do not want to bother with the above, GET RID OF IT!

Beware; none of the drowned items is as healthy as it looks now: DO NOT FULLY TRUST THEM AGAIN! The most effective, but highly unethical solution is to sell them quickly to an unsuspecting enthusiastic. If you want to keep them, replace all insulated wires inside (usually, there are not many of them) within about a month. The salt water seeped under the insulation and there is no way of getting it out. After powering the gadget again, it will start eating the core at a surprising speed. The terminals or soldered ends will usually fall off within a few weeks... After the wires exchange you can expect ALMOST the original reliability.

Finally, for god's sake, do not fall for some much faster and simpler treatments, unless you want to make sure nobody will be able to fix your gear ever again! One of the recipes circulating here is, to wash the items in VINEGAR, which NEUTRALIZES!!! the see water! Mere spraying them with some moisture-displacing agent is, at least, almost harmless, but equally useless.

And the last advice: if you are being offered a second hand electronic device, look for the telltale greenish-blue deposits of copper chloride around the positive terminals. If these are present, politely decline the offer, even if it looks like a deal of the century!

                           FLAT BATTERIES? - NOT AGAIN!
                                                                       Czech original by STANDA HLAVINKA. Translated by Jiri Stipek.

We hate to spend more money than we have to. In the case of batteries we often do - only because we "follow the book". The problem is; The "Book" is often written by an expert in the marketing area and not by somebody who is familiar with the technical side of the problem. In the "Batteries Department" there is a whole score of myths worth investigating.

Form them or not?
One of the myths seems to stem from some strange analogy between NiCd or NiMH cell and a hard disk of your computer. The later, as we know, is useless without formatting. So, a new battery has to be formatted as well. The lifetime and performance of it will depend how well we perform this initiating ritual. As there is no smoke without fire, we are not looking at a total nonsense but at a mere misinterpretation of facts. By definition we are looking at a "confusion of slightly inaccurate truth with a blunder - nevertheless a very accurate one" Cell, either new or not used for a long time, isn't capable of delivering its full capacity. The deficiency is somewhere around 10%. Only after 2nd or 3rd cycle the cell is capable of delivering the full 100% of its energy. Only those who can't stand the thought of having a battery with mere 90% of its capacity during the first couple of cycles should go through the trouble of formatting it. As a 10% difference is hardly noticeable, it seems to be wise let the battery form in our instruments during their normal use.

Discharge before charging?
Another myth often misused by marketing experts is the "memory effect" Buy the more expensive charger which discharges your batteries before every charging. Sounds smart - and the price doubles at once. You can throw your money through the window - and be better of, because you'll save on the batteries! Memory effect NiCd or NiMH cell's capacity does depend on the degree of previous discharge. Again, it is not a total nonsense. If we discharge cells repeatedly (tens or hundreds of cycles) to let's say 50%, at the next discharging process they will not deliver 100% of their capacity. As if they remembered the level of the previous discharges - hence "memory effect". We are looking at only few % of the total capacity and the problem will disappear after one or two deep discharges. It looks like the answer lays in a total discharging every single time. Watch out, it is not that simple.

Number of charging cycles - life span
There is another important relation here. It is the relation between the maximum number of cycles achievable and the depth of each cycle. During the normal use the useable capacity is slowly rising (by a few %) with the number of charging/discharging cycles. This is true up to the number of 2 or 3 hundred. Then it will become declining irreversibly. The maximum number of useable cycles is usually defined as a point, where the capacity drops below 60%. Up to now, we were talking about deep cycles - 100%in, 100% out. We would tend to believe if we use the battery twice on only half of its capacity, we have 2 half-cycles, which should equal 1 full cycle. In the real world the situation is much better. For example, if we keep discharging our batteries by only 25%, the maximum number of useable cycles won't increase only 4x, but approximately 10x. This phenomena is being used in satellites, where the power system is designed to discharge the batteries only by some 10% at each cycle. Changing batteries in orbit is not a mean feat! So, now we have a drop of capacity (by maximum of some 20% and reversible too) against increase in useable charging cycles and thus lifespan of the cell by hundreds of percent. The conclusion is clear. We should charge our batteries after every significant use and avoid discharging close to empty whenever possible.

Deep discharging
There is another important reason why discharging batteries to the very bottom is not desirable.
Batteries in most instruments consist of two or more cells connected in series. Their voltage and power ads together (not capacity). This applies to sealed packs as well. Even if most instruments are designed to switch off when the supply voltage drops below a certain level, there is not a benchmark for this voltage and some instruments will simply keep discharging the battery to zero. No two battery cells are exactly the same. After a period of time inevitably the capacity of individual cells in our pack will show significant differences. And that's where things will start to go wrong. While the battery is gradually discharging, the cell with the smallest capacity gets flat. The rest of the battery is still delivering current to our instrument. The weakest cell is done for. As the current flows through it, the cell is forced to reverse its polarity. The chemical process during the inverse charging of the already weakened cell is causing another irreversible damage. During the next discharging this cell will get flat even earlier. And so on. We have a chain reaction and that cell is as good as dead. The higher the number of cells in the pack the higher the chance of this problem developing. At the same time, if we avoid deep discharging, it may not happen at all.

Conclusion
If we use our batteries only little they will last a long time. The longest will last batteries with no use at all. Test by company Sanyo show, their unused batteries lose only 10% of their capacity over a 10 years period of time. Here goes the myth the lifespan of NiCd batteries is only 3-5 years anyway. The determining factor is the number and depth of charging cycles. That is, if we do not overheat the cells. A continuous use at 40dg C shortens the lifespan by 50%. At 60dg C by shocking 90%. Know your enemies!