Building the new Bushbaby SAFARI

[ystervark7]

Well now that the weather is delaying oupa, I will write something about wiring.
First about wire selection, simple enough most people will say, wire is wire. I will take a look at more commonly or sometimes not so commonly available wire types (or more correctly the isolation used for the wire).
PVC
Let us start with the normal off the shelf Nylon/PVC type. It is easy to get and probably used in most home builds. It does have a big problem, it is flammable and the gases from burning are extremely harmful. It also can not tolerate very high temperatures. The temperature rating for different types of PVC/Nylon wire also varies a lot from as low as 70degrees C up to 125 C with about 80 C the most common. In an open cockpit none of this is probably a big thing but in a closed cockpit I do not think that you would like all those gases during a fire. Also keep in mind that the temperature inside a closed engine compartment is much higher than you think. I can not find any numbers specifically for airplanes engine compartments but automotive manufacturers must design all components that is inside the engine compartment to handle temperatures of 120 degC. Only the best Nylon/PVC insulation can handle these temperatures.
As an interesting fact, PVC has been used in commercial aircraft: Douglas DC-9s, Boeing 707s, 727s and early 737s (it is also suspected that it is still used in some smaller commercial aircraft). Some of these are still flying and it will probably cost more to rewire them than what they are worth. There has been incidents where the wiring played a role including 1983 Air Canada DC-9 in which 23 people died due to smoke inhalation before the plane landed and a fire in AirTran DC-9 in 2000.

It is late, so that is it for now. In the future I will also look at Silicon, Teflon and Tefzel.

[Ranger]

At our last club meeting Oupa-G was chatting to me about wiring, crimping and so on. I found it very interesting and informative. The simple little things like what the actual wire is coated with and the huge differences this makes to voltage drop and life span etc. The fact that Oupa-G researches any aspect of what he tackles to the slightest detail first impressed me a great deal. On a number of postings he has mentioned that we should strive to be even better than what CAA and what the NTCA regulations require has my full support. Hats off to you Oupa and thanks it was a great conversation.

[justin.schoeman]

It is late, so that is it for now. In the future I will also look at Silicon, Teflon and Tefzel.

Looking forward to that post! Will be doing the wiring on my plane soon!

-justin

[Oupa-G]

We can listen to Ystervark7 he is very clued up on these sort of things I do not know if he is connected to Vliegvark or Bosvark but like in Jeremy Clarkson's program nobody knows who is the "Stig" . I have a clue I think Ystervark7 knows something about the Eboxi. We will prompt him to talk more about wiring crimping earth straps and common mistakes and myths in wiring. Meanwhile we have got the first templates of the baffles Waterjetted by the famous WaterJet company in the fairest.One set went to Loeriesfontein for Safari II it will be managed by the now famous Boet. The other set Vliegvark and myself left on Friday afternoon to Knysna it will be adapted to the BMW engined Whisper. We also dropped a Eboxi dual system at the builder of a Zodiac 601. It is always nice to see the enthusiasm and ability of home builders. The trip all happened in a long rainy drive but I'm back in the Cape ready for my voluntary RedX duty tomorrow. At the Knysna Yacht Club we had breakfast with Derek Kershaw on the finer details of the baffle system of the Whisper and also details of the airflow around the engine, it is all about proper heat management the worst are undiagnosed hot spots. I can report that with the ground running of the Bosvark the oil temp was never higher then 90C and there was a temp drop of 30C between the oil cooler inlet and outlet.I packed out the various components of the baffle plates on a table in the Knysna Yacht Club the photos are not a good quality but I did not have my normal Camera with me.Time is late and I'm tired.

Cheers Oupa-G

[ystervark7]

Wiring chapter 2
Silicone wire insulations ability to handle very high temperatures up to 200-300 degC is its main feature. Its drawback is its very low tensile strength, in fact, it is so low that you can strip it with your nails. To ensure that it is at least reasonably strong, the silicon isolation is very thick which probably makes this wire the heaviest. If used for general purpose wiring it should be kept clear of any edges, even if they are not sharp and even cable ties might cut through it. I am not aware that it has been use in any commercial aircraft for the general wiring. It is used for special purposes in aviation including engine ignition wires as well as used for wrapping tape due to its non-flammable nature, e.g. FlameGuard(TM).

PTFE and FEP (Teflon(TM))
Characteristics: Handle high temperatures up to 200 degC continuous, non-flammable, very flexible, resists moisture absorption and atomic oxygen erosion. The insulation is very thin. PTFE is difficult to strip but FEP is supposed to have poor cut through properties. Sound pretty good but it does have some nasty properties: it cold flows over time when bent over tight radius or when tied too tightly and it can split longitudinally. It was used in B747's but has been banned by manufacturers since 1983. It has been blamed for some aviation disasters including Apollo 13 and in TWA 800 fuel tank wires. Teflon also breaks down at very high temperatures and releases hazardous byproducts. (I think that the hazardous byproducts will be the least of your problems at those temperatures. )

Next time I will look at Tefzel (TM) and I will also say something about Kapton (my heavy thinks that Kapton must be related to Kripton so it should not be used if you bargain on Superman to save you)

[Oupa-G]

While Ystervark7 is giving us the information on the wires I did some more practical preparation for the upcoming taxi tests. I imported a handlanger from Gauteng also a future BMW flyer he is visiting to make himself more"lus" for the future flying. Alewyn helped with the measuring of the wheel track and with the help of a laser pointed spirit level the necessary marks were made. Out came the rusty knowledge of the trig tables after multiple calculations moving the decimal points to the correct values the final answer is a tow-out of 0.5 degrees. For the first taxi tests I can live with this value. There is the age old argument if one should have toe-in or tow-out. As the airfield is a grass field I will except these figures without adjustment. I did some more low end tuning and now the Bosvark is screaming for its wings and to be tied down so that the engine can pull under full power. Thats all for tonight


Cheers Oupa-G

[Oupa-G]

Just a photo of the nose job on Bosvark

Cheers Oupa-G

[Oupa-G]

Just a photo of the nose job on Bosvark

Cheers Oupa-G

[Boet]

MET die boot cowl aan en die paint sceme klaar? Sal moerse nice lyk.

[Oupa-G]

Past 31000 hits what a wonderful response. We will keep the stories going


Cheers Oupa-G

[ystervark7]

ETFE -Tefzel(TM)
-Withstands abuse well
-Good high and low temperature properties and can handle continuous temperatures of 150 degC
-can flex and bend with fair cold flow properties
-weather resistance not as good as PTFE
There are few disadvantages, such as fameability above 30% oxygen and softening of the insulation at high temperatures, which are unlikely to be encountered in microlight flying. This wire is therefore probably the best choice if you can find it and afford it.
Note: Not really used in commercial aircraft. Tefzel is suspected to have cause or contributed to a fire on Swiss Air flight SR111 in 1998 where it was used in the entertainment center.

Crosslinked Tefzel (XL-ETFE) or so called Spec 55
I am not going to say much about the specifications because it is very similar or better than normal Tefzel. It has been used in a lot of Airbuses up to the A340 and Boeing up to the B777. It is also used on satellites and NASA used it until 1983. The problem is that it gives off a very dense toxic smoke and some people view Crosslinked Tefzel as very dangerous.

It is unclear to me if ETFE also have the dense toxic smoke problem? If somebody knows maybe they can comment on it.

Now the what do they use on Boeings and Airbuses today?
Airbus and Boeing use what they call "TKT" or sometimes Tufflite This is a wire with a layer of Teflon, Kapton and again Teflon on the outside. This is done to combine the best properties of Teflon and Kapton. Kapton is sable at high temperatures and very light wieght. I will continue with this next time.

[Tracer]

Hi YsterVark,

The insight you are sharing with us is really valuable. Could you please share some of your knowledge about the guage of wiring that is recommended for different uses in the aircraft?

[ystervark7]

Hi YsterVark,

The insight you are sharing with us is really valuable. Could you please share some of your knowledge about the gauge of wiring that is recommended for different uses in the aircraft?

There are tables to lookup the wire thickness. If you search a couple on the internet you will also find that they do not give the same values. The reason is because they make different assumptions and are used for different purposes. e.g. If you look at "http://www.powerstream.com/Wire_Size.htm" you will find that they have 2 numbers for each thickness. For AWG18 or 1 mm the limit is 16A for chassis wiring and 2.3A for power transmission.

So what is the limiting factor(s) affecting wire selection? Basically how much voltage drop the application can tolerate and how much the wire will heat up. Because wires are normally relatively short in microlights, the limiting factor is normally the heat generated. How much heat you can generate depends again on how fast the wire can get rid of the heat, e.g. the air flow and the melting temperature of the isolation. In most cases the current limit is determined experimentally for a specific isolation type as the current that will heatup the wire with 10 degC in free air.

My rule of thumb if I do not have the tables with me is about 5A per square mm. This number is adjust somewhat depending on circumstances:
1 if the current is drawn for a short time every once a while, e.g. a prime pump, this number can be increase up to 8A or even 10A
2 if it is a critical system I normally limit it to 3 or 4A
3 if the wires are in a bundle they heatup more so you have to reduce the current rating depending on how many wires etc.
4 long wires carrying high current also needs to be thicker (but this is normally not an issue for us)

Also note that for microlights you should use stranded wire with about 19 or more strands. Un-stranded wire breaks due to vibration.
You should also keep in mind that thin wires break easier than thick ones due to vibration. I will avoid using single wires less than about 0.32 - 0.4mm.

[Tracer]

Thanks, I will search a couple of sites as well, but looking at the advise you gave, I seem to be on the right track.

[Boet]

Oupa supplied me with one of the first " BMW Baffle kits" to try out. Having all the pieces ready cut sure helps. It required some minor trimming here and there, but it works. I took some pictures as I went along, to help future BMW fitters. The oil cooler is in too, and I am busy with fitting the panel. The panel will be hinged along the bottom, a nice feature for future maintenance.

My own plane is completed, wings on and all controlls in and working. Need engine. Will have to moer die spaarvarkie broken, and / or chase the wife out on prostitution...... BUT it WILL be a BMW. Anyone out there feeling generous??