Sunday, November 26, 2017

Wing covering with adhesive films

Wing covering with adhesive films

 By Pierre Rondel

(Pictures: Gerard Prat, Thierry Martinet and Pierre Rondel)

INTRODUCTION

About 20 years ago, Thierry Martinet, a friend of mine who is graphist/designer decided to investigate with Gerard Prat, an experimented modeller, the possible use, for foam/veneer wing covering, of a material he was using everyday for his job: colored vinyl (adhesive film). This material is usually used for vehicle lettering, light-box advertising, decoration of flexible-face material or windowpanes, and those for both indoor and outdoor use.

The main advantages of the vinyl are the following: Brilliant shades and a large color selection, excellent life-spans and stability (between 5 to 7 years which is much more than the average life expectancy of our gliders).In addition they have good plotting and application characteristics. The thickness is between 0.07 and 0.09 mm and includes the glue.

The main drawback is its weight, which is around 90 gr/m2 where classical covering films are from 50 gr/m2 to 70 gr/m2 (Solarfilm, Ultracote, etc…). So we can tell that the extra weight is about 20 to 40 gr/m2. But now consider the absolute extra weight for a 3 meters glider for example:

The covering area for such glider will be: 60 dm2 x 2 (under and upper side) = 1.2 m2 The extra weight will be around: 30 gr/m2 x 1.2 m2 = 36 grams And finally, these 36 grams corresponds only to 1.5 % of the final weight of the glider (2.4 kg).

In conclusion, I would say that the excess load is minor compared to all the advantages of this covering material. But, I agree that at the beginning, it’s not really obvious to imagine that this material can cover entirely a wing, especially the wing tips. So, my objective with this article is to share with you the expertise of Thierry and Gérard (and they encouraged me doing so!). I just hope that septic people will be convinced at the end of this paper!

WHAT REFERENCE TO USE?

There is lot of different brands and references of adhesive films, but they are not all well adapted to our application. The main vinyl brands we use are JAC and ORACAL. The best ratio price/quality for our use if the Oracal plotterfolie serie 651. Lot of different colors are available in this range. You can also have a look on the JAC Serilux product range.



The standard packaging is a roll of 25 m x 0.5 m if you order directly to the manufacturer. I recognize that it can be a little bit expensive if you just want to cover a rudder or a tailplane ! For 5 years now, french kit manufacturers sell this vinyl retail at a reasonable price (~ 2.4 £ for 1m x 0.5m).

In the operating mode, we can distinguish 3 main steps, which are:
  1. Preparation (maybe the most important step!)
  2. Covering
  3. Finishing

PREPARATION

Like classical covering materials (i.e Solarfilm, Oracover....), the surface quality obtained will depend of your skills, of the wood type and the vinyl quality.

A very good surface quality will be difficult to obtain with balsa (which is too soft), but will be obtained easily with wood veneer. The best result is obtained with the Aningre (Latin name: Aningeria ) which has the particularity to have a very strong surface and thin grain (its color is dark brown). Between these two extremes, the Obechi give also really good result, with a little preparation.

Do not bypass this stage because the final result is directly linked to the care taken during the preparation.

For the preparation of the wing surface, we usually use abrasive paper (commonly use with water but that we use dry, of course). It is very resistant and can be vacuum-cleaned some times before being thrown.

But before sanding, it can be necessary to apply some filler cement, either to eliminate a large defect, or to make the wood fibers almost invisible. The ideal filler cement exists: Its aspect is comparable to the toothpaste. In all cases, the objective will consist in depositing what it is just necessary to fill in spaces between wood fibers. You will thus avoid a tiresome sandpapering. Unfortunately, I won’t give you any references because they are certainly different in each country.

Small detail, which helps for the finishing: We intercalate between the hold and the abrasive paper, a piece of foam, very flexible, of approximately 10 mm thick. Thus we obtain perfect curves (even visible to the eye under oblique light) and an impeccable surface quality by using a grain from 320 to 350 for balsa, 250 for samba and 220 with Aningre. A sanding with thinner sandpaper does not bring anything better to the finish after the covering.

If you have installed elastic flaps (using nylon ribbon), cut out and articulate the control surfaces (ailerons, flaps), installation of the A.F, etc.

During its handling, the vinyl attracts any kind of dust (static electricity). The only solution is to transform yourself into a beautiful cleaning lady: Pass the vacuum cleaner everywhere in the room where you plan to operate. I personally use an ironing board as worktable because the cotton sheet, which covers the table, is less sensible to the dust.


Outdoor or in another room, clean conscientiously your wings (and wing blanks) with an air compressor or a vacuum cleaner and finally with a clean rag by “whipping” the wings (and not by “rubbing”) in order to avoid new static electricity. Never use a wet rag to fix the dust, because the wood fibers will react immediately by inflating, and you will certainly have to redo the previous sanding operation.




For the very final cleaning of the surface of the wings, simply use your hands: This is, for the moment, the best tool that we found, and moreover which is equipped with very efficient sensors! Actually, you will feel under your hand the least dust or roughness, which, if you insist, will disappear. The hand’s skin acts like a thin sandpaper (grain: 400 or 600). Of course, all these advises are also applicable with any type of covering materials, not only adhesive films.

THE TOOLS

A minimum set of tool is necessary to complete the covering:
  • A (preferably) non synthetic rag which does not become fluffy
  • A cutter equipped with a brand new blade
  • An iron for solarfilm.
  • A hairdryer.
  • A soldering iron

COVERING

Thanks to Thierry Martinet and Gérard Prat who made an exceptional job by doing all these pictures, I propose to you to see how to operate, step by step, as if you were there!

Prior to the beginning of the operations, cut your 4 vinyl panels with the correct planform of your wings plus a few extra centimeters of margin (width and length). The longer your wing is (or if it is you very first covering using vinyl) the larger the margin will be. For example, 6 cm on each side (leading edge + trailing edge = 12 cm) for a half wing of 2 m is a good value. Begin with the covering of the under-surface. You can use the wing core blanks to avoid your wing to slip on your worktable, but this is not mandatory. Vacuum-clean also the two faces of the vinyl panels before spreading it over the under-surface of the wing.


STEP 1

Center and align the vinyl panel and maintain the whole in place on the wing by using magazines (your entire QFI collection for example !) or phone directories.







STEP 2

Begin the covering by the wing root. For that, separate the vinyl from its protection sheet on 30 cm. Then, stretch the vinyl panel and apply it progressively starting from the root cord. This is the most delicate step to start the covering, because it is necessary to well align the panel and have no surprise when arriving at the wing tip … if you see what I mean! If needed, don't hesitate to ask some help for this particular stage.

STEP 3

To stick the vinyl on the wing, rub it with the rag by starting each time from the middle of the cord towards the leading edge and from the middle of the cord towards the trailing edge. To avoid any fold that would be tragic for the result, progress gradually towards the wing tip and stick the vinyl on the LE and TE only upstream of the point where you have sticked the vinyl at the middle of the cord. If you progress too rapidly on the LE and TE, you are certain to have wonderful folds… Progress that way by steps of 30 cm by separating the vinyl from its protection sheet and by making it slip on the wing. The adhesive used on the film behaves like all other adhesives and reaches its maximum adherence only after a certain period of time. If something wrong occurs during the covering, peel off immediately the vinyl and eventually heat it a little bit with the hairdryer. However, even if you limit the damage on the wood, you certainly will have to throw the panel of vinyl out and do it again with a new one.





STEP 4

Wing tip covering: The funniest stage is the covering of the wing tip: The vinyl is now completely removed from its protection sheet, and widely exceeds the wing tip. Immobilize the wing on the worktable with something heavy: Then with a hairdryer in one hand, warm carefully the film while pulling it with the other hand. You will see that the vinyl becomes soft and can easily follow the wing tip shape.





STEP 5

Cut the extra vinyl and With a cutter cut the excess vinyl around your wing. Once done, apply the iron or soldering iron to perfect the sticking at the periphery of the adhesive film. To fold back and stick the vinyl against the root, first cut the vinyl with a small margin (3 to 4 mm), then vigorously rub the vinyl on the edge (with the rag or directly with the finger) to heat it and it will take the angle almost alone. Eventually, you can sandpaper the join line to reduce it at the maximum before to start the covering of the upper side.

STEP 6

Upper side covering : The under side of the wing being covered, repeat the previous operation (Steps 1 to 5) with the upper side. This time cut the vinyl with a small margin before to wrap the leading edge and obtain a join around 5mm large. On the trailing edge, you can cut the vinyl very short or let a margin and wrap the trailing edge. Personally, I prefer to cut very short and avoid a double layer of vinyl at the junction. Then, I use the iron to perfect the fixing of the vinyl on all edges.



     

FINISHING

Clear the servos holes, air brake slots. Depending of the building technique used, you will have to cover the control surfaces separately, and install them with hinge tape. If control surfaces are articulated with nylon ribbon during the construction (this is often the case today), free them by cutting the vinyl underside and at each extremity. Then apply the nail of your soldering iron along the control surface slit while taking care to operate the control surface in order to widen the slit. Repeat the operation 2 or 3 times.




DECORATION

You have finally finished to cover your wings? Congratulations! You can now think about how to decorate them. CorelDraw, interfaced with a cutting machine, is the perfect configuration. This software allows to vectorize any drawing and then to pilot the cutting machine. The only limit is the range of color you have and your imagination. I joined one or two pictures to show you some decoration examples.

If you don't have access to a cutting machine, you can also by wall stickers (Ebay, or amazon) and find some nice ones, in different colors and combination and create your own color scheme. This is what I did on my Excel 4004 recently, and I'm satisfied of the result. Just make sure the stickers are coming with a transfert tape which is not aways the case.

It is also possible to plot (with special inks) on the vinyl (before the covering of course!) and obtain incredible results like the one shown on this picture. Unfortunately, this printing technology is not accessible to us yet and the cost price is still prohibitive.



MAINTENANCE / CLEANING

You can use Acetone, white spirit, and other distillate without any problem. If you have to repair some damages on your wing, a small vinyl patch will be fine and remains discreet. In case of removing entirely the covering, use an hairdryer to warm up the film and then peel it off with precaution. However, a new sandpapering will be necessary before covering again.


CONCLUSION

I think that you now know everything about the topic. The vinyl is certainly not perfect, but with a little experience, you will manage a covering in record time to obtain a very good finishing for your wings. The surface quality obtained will surpass easily conventional covering materials and the preparation and operating mode is accessible to everyone and does not necessitate special tools. Also The durability of this covering material is just great and will last for years


Tuesday, November 21, 2017

RC Soaring Digest December issue


December issue of RC soaring Digest is available with plenty of interresting articles. Just follow the link below !

Sunday, November 19, 2017

Large Scale Glider meeting of St Jean en Royans


Mid october, I stopped few minutes at the GRP meeting of Saint Jean en Royans. Lots of very nice sailplanes, tow planes. Weather was perfect with plenty of thermals, with vultures, no wind. The airfield of Saint Jean en Royans is superb for this type of meeting. Below my photo album.





Video of my Excel 4004 I built in September and flown few time. More to come.


Mistral Soaring - Rognac

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