FTS Projects

[AchimEngels]

The first coat of dope.


The second coat of dope.


And this shot was taken a few minutes after the fourth coat of dope was applied.

...

[AchimEngels]

The following is a rough translation of an article published in 1918 in the German aviation circular "Flugsport". The original text was published in "Flight". Unfortunately I do not have the original text, so please take my applogy for the rough translation which I did quite a few years ago for another thread that was published here.

I admit it does not deal exactly with the topic of all this, but I think it will nevertheless be of some interest to some of you, who do not know the "Flight" article.

Aircraft Dopes and their Application
(Flugsport No.4, 1918, page 86 according to forign descriptions)

Some 6 to 7 years ago in Germany for the first time it was begun to impregnate aircraft fabrics. In the beginnings cotton and silk fabrics have been treated with oil or varnish or one used balloon fabrics that have been covered with a coat of gum. The results have not been very satisfying and it was difficult to attach these fabrics tight enough using these methods onto the aircraft frames. Apart from that the fabric suffered from oils and fuel stains which came from the engine. Today´s methods of dopeing, without which the present performances of aircraft could not be reached, are still young.

Fabrics: Before we will deal deeper with the impregnations, we will first take a short look at the applicable fabrics, since the usefulness of the dope in a main part depends on the quality of the fabric itself as well as on the way the fabric is treated before the dope is applied. At present the most popular material is linen fabric. It is strong, close and does not tear that quick. Besides this special woven cotton fabrics are offered today. The main disadvantage of the last ones is that they tear off too easy and they do not take the dope that good as linen does But for certain there will be developed special paints in the future that will solve that problem. The weight of the linen fabric is between 120 gr. and 160 gr. per square meter while the tensile strength is between 1100 and 1800 kg per meter according to the weight. With cotton the same numbers are 50 to 140 gr. per square meter at a tensile strength of 450 to 1400 per meter. As a rule one should use only such fabrics that show equal thread counts in both directions.

The method of covering is well known and must not be explained here. At least the fabric is attached to the frame first and then treated several times with the dope. The main propose of which is to close the fabric pores and to give the fabric skin a smooth and tightened surface which should resist all influences provided by weather and Fuel or Oil. The application of dope increases the tensile strength at about 10-60% depending on the stretching the fabric received during application to the frame before the treatment with dope.

Fire resistance of the varnish: The varnishes used at present can be divided into 2 groups: 1.) into such made from cellulose nitrate or pyroxylin and 2.) into such, which are made from cellulose acetate. In the two falls the substance is solved in a suitable solution. Chemicals which increase the limpness of the substance or change breaking in the substance which have a similar regulation as the kampher in celluloid are added occasionally. The essential difference between cellulose nitrate and cellulose acetate consists in the fire resistance of the the latter whose production costs for it are a little larger. One has found a way out regarding this by carrying out at first three pyroxylin spreads and then only two or three acetat spreads, also obtaining a fire-resistant wing through what. Tests showed that when acetate varnishes are used, the fabric will not catch fire until the dope is completely burned away. At pyroxylin use the whole spread suddenly is in flames and the fire immediately spreads to the fabric. The test with petrol drops which one brings on two differently soaked fabric samples is still more instructively. They burn down on acetate varnish without the fabric being hurt somehow. The whole spread immediately catches fire on pyroxylin varnish however which also spreads to the fabric. The acetat varnish is therefore much more suitable with respect to fire resistance.


To paintwork ways there is a whole number of methods. Some factories prefer to draw the fabric as tightly as possible on the frames and then bring a varnish of only low shrinking fortune to use. Others which make use of the instep ability of the varnish stretch the fabric less tightly. Still others take middle courses. This differences of the procedures find their reason into this that one has given the paintwork little scientific attention till now.


Qualities of a good main plane wing: Agreement, however, is in the opinions about it how a good main plane wing surface must be after drying.

At first it must be smooth of course. The air resistance offers a measure for this in the examining channel. Tests have shown that in the comparison with flat glass as a measurement unit painted or varnished wings have a value of 1.2. Such fabrics like wire gauze or burned cotton have been found to have a unite value of 1.35 to 1.70, unburned cotton was still more highly. What concerns the influence on the speed, the wing friction changes for very smooth wings with the 1,8th to 1,85. power of the speed just like glass and lacquered wings. The exponent is generally all the higher, the rougher the wing is and reaches the value 2 at hairy surfaces.

The next quality, the good wing must have, is this one that the varnish adheres to the fabric well. The reason for it is clear: If the wing tears at all a cause, mustn't the rip transfer itself to the paintwork so much so that this peels itself off. Furthermore the wing must be tight, waterproof and petrol and oil-proofed. The necessity of these qualities does not require any explanation one needs only to think of an aeroplane in the rain or imagine about they are frequently exposed to quantities of petrol and oil during their use.

As noticed the wing must be tight, i.e., it may not form any bumps. On the other hand it is desirable, however, that the wing gives way to a certain degree under the load. In this case namely the appearing tensions are reduced while an inflexible wing is rather breaking. Tests in England have shown that one wins a performance of the pliability if one measures a piece of the material to be examined of certain quantity below increasing load and records the stretch curve. The expenditure of work which is necessary to take the material to break results from it. At not soaked fabrics, the stretching of the approximately 305 mm long and 25.4 mm wide stripes was generally greater than at soaked ones up to a load of about 9 kg. A certain room to move which at first is exhausted below easy load apparently is available at raw fabrics. As soon as this has happened, the stretching continues slowly. Furthermore one thought that flax wall (linen fabric) which was treated without tension with varnish showed a much bigger stretch than tensely lacquered. In some cases it required 2.5 times so much work as the same fabric without spread to make a lacquered flax wall break without tension. On the other hand the expenditure of work which was required for tearing one under tension lacquered linen piece was approximately the same like for raw linen without paintwork and the lacquered linen seemed nevertheless to have a higher tensile strength. With other words: Linen which is unpainted and painted under tension has approximately the same grade, however, lacquered linen has a 2.5-fold grade without tension. As noticed above the fabric usually is stretched a little bit while be pulled onto the frame before the paintwork is applied. The degree of the stretching in the different factories has been different. It recommends itself to stretch the fabric as little as possible because a greater extensibility is able to smoothen the use caused by an intense gust of wind and to protect the wing from injury, if the use is not too big, after projecting.

[AchimEngels]

Procedure to the obtainment of a good main plane wing: At first what the smoothness concerns, it can be obtained so of course even more easily the smoother the fabric is itself. Furthermore one must meet caution rules to keep dirt away from the varnish before one uses it and of the wings before they are dry. As a rule, four till six times is spread; in some factories the old paint still becomes rubbed off with sand paper to increase the smoothness in front of every new spread. It is also useful to work in the first application of paint well into the fabric to reach a good adhesion of the dope. The following layers are applied with long lines better except for the last layer which is added more thickly to improve the strenght of the paint.

The varnish is often put on the fault for this that it be little liable while the fault is lying on the part of the fabric. Generally varnishes do not penetrate into stiffened fabrics well. But there are exceptions to this rule and the reason for it may lie in it that there, where varnishes penetrate into stiffened fabrics well, the stiffness means gets solved in the varnish. Another fault that might play a role here is the character which the varnish solution has; the more colloidal it is, the lower is its inclination to penetrate into the fabric. Every varnish is able as tests have proved to penetrate better, last better, to be more liable, to resist atmospheric influences better, if it is applied on unstiffened sfabrics, as if one applies it to stiffened fabrics. This probably can be declared with that that the varnish penetrates there where one applies it on unstiffened fabric into the substance fibres instead of only into the stitches. In the dry state it forms such a part of the fibre. Since the stiff means can be removed mostly by boiling in pure water or a soft alkali solution, it is not hard so to obtain a good liability.

What the tightness concernes it can be stated that all dopes which are in use today do shrink the fabric well, although it must be noted that do shrink more and some do shrink less. At first all of them are, probably waterproof, too, they offer resistance to the constant action of the water in this if they are exposed for example to the weather in a rain period. Only the degree is different. So an aircraft which had done job at the North Atlantic sea coast for 6 weeks in a very unfavourable season had been exhibited in the last Aero show in New York. It had come back from duty ice-covered and the fabric covering anyway was in a good constitution still. One had treated it with acetate varnish and then still spread one or two spar-varnish layers. Indeed some companies today use to cover their aircraft surfaces with a additional protection coat of spar-varnish to make it more resistant against the influence of water, but the main disadvantage of this procedure is that holes in the surface can not be repaired that easily, since the spar-varnish does not accept glue and needs to be removed completely before repairs.

Till now, mention has not been done to the illness of the varnish space which speaks in the formation of white spots on the lacquered wings yet. It more usually happens with pyroxylin dopes than at acetate dopes and arises from the condensation of the humidity when drying and to be more precise only on humid or rainy days. One can remove the spots by sweep with a varnish solution at the best of an acetate varnish solution which at the same time also makes the wing fire-resistant. The formation of the white spots can be avoided even on wet days if one regulates the humidity content of the air in the varnish space. However, this requires usually a ventilation system worked out well.


Opacity: Straight away cannot be answered the question about the opacity of this or that varnish since the "personal equation" of the worker plays a role and the behaviour of the different fabrics has been different. A certain varnish contains so and so much integral, i.e. not brief constituents on the litre and therefore can depending on this whether it is spread thickly or thinly, less or more wing muckrakes. On the other side, different dopes with the same percentage of integral constituents are different in their opacity when compared with repesct to the amount required to create a usefull wing surface. One can generally say at this complexity of the influences: About 0.45 to 0.9 sqm per litre cover the varnish. If they are applied alone, without a final protection coat of spar-varnish. This performance is obtained with 4 to 6 spreads depending on spread practice in detail, in any case the weight of the ready coating will be the same roughly at a certain varnish and a certain degree of the end treatment, however. This weight moves between 50 and 120 gr. per sqm.

Long-lastingness: It is also difficult to answer the question about the length of the time at which a well lacquered wing lasts to some extent exactly. Of course the most defining factor is the varnish itself, if how we have already seen a series of other circumstances, also play a role here. Some details on it are contained in the reports of the Advisory-Commitee of 1916. They have recognized that the acetate varnishes are generally far superior to the nitrate varnish at application to cotton: i.e. the tearing resistance of the lacquered fabrics caused by weather influence at acetate varnish does not be so great than at nitrate varnish. The same also applies apparently to use of flax wall, if also not in such considerable measure.

Usually the varnished fabrics which are to be tested are stretched onto boards and are exposed to the elements for a while. This is a harder treatment than aeroplanes learn usually, though. The first of the valued qualities that disapears is the suppleness of the treated fabrics. The length of the time up to the entering of this phenomenon can be accepted as a measure for the relative value of the different varnish. It is not possibly to accept a normal duration as a particular time because of the far parting differences of the atmospheric ratios depending on the place and the season of the examination. One can say only quite generally that it might be necessary to renew the dope every 3 to 5 months.

If the translucide and fire-resistant wings actually do not belong in the chapter "varnish" either, then it will be of interest to hear something of these anyway. The material being considered is produced from cellulose acetate. In a strength of 0.25 mm it weighs 300 g/sqm and has a tearing resistance of 1300 kg/m. It is claimed that an aeroplane from a certain height equipped with such wings is almost invisible. The visual field of the leader should at all events be extended considerably. A little pleasant quality of these sheets consists of the fact that they can not be fastened very easily and tear up very quickly if they have got once a rip. One has suggested to strengthen the transparent sheets by a fabric like an in bulk woven page to avoid around tearing up. If such a fabric does not have great strength to itself either, then it serves to prevent a tearing up further anyway. The disadvantage hereby is then again that the transparency decreases, though.

Enjoy!

Achim

[Macca]

Achim,

Very nice. It almost looks as though it's a taught as a drum yet it still has not been doped.

Ian.

For those, who are interested in the history of German aircraft industry 1918:

Baumwolle (cotton) Albatroswerke, Johannisthal 0,088 kg/qm
Baumwolle (cotton) Luft-Fahrt-Gesellschaft 0,098 kg/qm
Leinen, roh (linen, coarse) Mech. Weberei, Ravensburg 0,157 kg/qm

Fokker used coarse linen because it was cheap. Fokker used Cellit Z based dope because it was cheap.

The result you can see when studying the photographs of Fokker D VII's in use.

Fokker was warned, more than twice, but he was protected by some higher authorities.

(Source: BLV, TM, DZA)

ZAK

[AchimEngels]

So you say it was done that way at Fokker, although it was forbidden?

[AchimEngels]

Today the first of the three Fokker D.VIII seats is in the frame now.







The seat cushion is not the final one. It will be replaced later.

The entire seat with cushion weights 2.5 kg while the frame weights 2.0 kg, so the whole assembly is 4.5 kg ( 9.9 lbs)

The fabric back will be doped tomorrow.

Enjoy!

Achim

[womenfly2]

WOW!!!! Looks fantastic.

Questions:

1. Is there any cushion material between the aluminum seat back and the fabric?

1.a. Why did they use aircraft fabric and not say, leather or another more longer wearing material?

1.b. Why dope it?

2. The "U" brackets, are they for the lap belt?

Thanks for posting the pictures. Truly wonderful!

WF2

[AchimEngels]

Hello Womanfly2,

1. There is no cushion between the fabric and the Aluminum shell, except for the air between them.

1a. They used aircraft fabric, most probably, because it was light, cheap and easy to handle. It can also be doped.

1b. It was doped for three reasons: first it gives the entire seat shell most of its stiffness. Second it is weather proved. And third, it makes the seat much more comfortable. Since this is was acts as a cushion on the back. The doped fabric does not attach the aluminium shell at all but goes the shorter way along the round contour keeping it tight and away from the aluminium shell. There is no need for any aditional cushion that way. It really feels comfortable.

2. These U-brackets are the only confirmed way the leap belts have been attached to the seat of a service plane I know of. Actually I do not know and have no confirmation it was done that way on Fokker war build planes. But it is how it was done on the early post war Swiss Fokker D.VIIs before they changed their planes to carry another and entire different seat deign. It is at least a safe and prooven and contemporary design.

However, there are evidiences that Fokker used to attach the leap belts to steel wires attached to the frame, passing through holes in the aluminium shell and attachimg to brackets at le leap belt bottom end. There is photographic proof it wa done that way on earlier planes and indications on the modified seat of the Caproni D.VIII in Italy.

We are however fine with this way of attachment until we have found other proof and may do it different on subsequent planes.

Cheers!

Achim

For those, who are interested in the history of German covering practice in 1918.

An Idflieg engineer:

"Zum Kleben von Stoffteilen aufeinander wird nicht mehr normaler Spannlack, sondern besonders konzentrierter "Klebelack" verwendet...die Festigkeit der Klebestelle ist dann bei etwa 5 cm Überlappung größer als die der anderen Stellen...die zu verbindenden Stoffteile müßen vorimprägniert werden."

That means 5 cm overlapping by patchworking, preimpregnated and a special kind of dope (Klebelack).

Fokker was infamous for his bad workmanship - and his prices. As the prices even for the different yarns he used is known, the cost for his coarse linen per meter (width 1,30 cm) was 6,90 Mark - if bought from the Idflieg (Hauptkasse Döberitz) it was 5,50 Mark for the German treasury, by a width of 150 cm, bleached, and of a much higher quality and with less weight.

ZAK