Monday, March 30, 2009

Wing vacumn bagging

Hi There
There will be this and one more stage in the completion of these wings.
Basically the structure is complete and it is the wing finishing and control surfaces that need to be completed.

The wing tips are made with the correct dihedral when joined with some strips of glass cloth.

As seen in this photo the alutube are cut through and these tipswill now align perfectly. The tubes are sealed and the blue foam sqaushed in a little , a mix of micro-balloons and resin is used to fill these hollows and makes a hard end rib once cured.

The hollows have been filled , the whole wing water-papered with 360 grit and the final coat applied the results are very shiney and I am pleased.
The glider is starting to come together and these wings will be a big part of its performance.
Each wing weighs about 1850 grams and the aircraft all up weight should be between 7 and 8 kgs with an estimated wing loading of 60-70gr per dmsq quite acceptable.

Saturday, March 28, 2009

Andy's Ju 87.

Sunday, March 22, 2009

An elegant Spitfire, Tim's I think

Francois' Mistral takes off on aerotow behind Stuart's tug

Wednesday, March 18, 2009

Wing Bagging part 5

We left off last time with the wing in the vacumn bag overnight, I had a very good seal on this new bag and my vacumn pump (controlled by an electric switch coupled with a car advance vacumn switch)cycling about every 6 to 8 mins , after an hour I checked the vacumn/compressor set up and it was running ice cold.

Next morning everything looked ok except that I had misaligned the bottom skin at the root, but only some carbon cloth was short, when scratch building models "re-engineering " is often required , in this case I dont believe strength will be compromised so It will be filled as will all the hollows, waterpapered down and the whole wing given a final spray of 2k.

The second wing has gone into the vacumn bag and its time to clean up and trim wing number one. I always use a slight overlap and the mylars can move slightly , so first of its time to cut the trailing edge to shape where I have plenty of overlap.
I mark using a permanent marker and a straight edge the line to cut, then I mask this line with masking tape , the line shows through, this is important otherwise the cutting blade can slip and veer off damaging the surface, with a new sharp blade you cut with a series of cuts. This has a carbon skin which is really tough , I broke a sweat cutting this edge .


The leading edge was carefully trimmed back to the rounded section that was formed during the core preparation.

The wing now looks great but has a few blemishes and a black leading edge , some sanding , filling and when both wings are ready , final paint. "IMPORTANT MESSAGE"
Sand the leading edge with a block and not your hand and sand paper trying to make a rounded shape, last time I did that the paper snagged the carbon tows , lifting a splinter that went right through the middle of my finger and resulted in a trip to hospital for removal, this wasted a whole evening of building time and some money.

So the next post is the finishing of of the tips and cutting the control surfaces
the servo wiring and servo placement. The fuselage is also well underway and if anyone is interested in how that is being built I will do a build sequence on that
No one has asked what the heck I am building (its a bit of a secret)its going to have its maiden flight at the upcomming Sungazer Slope event at Volksrust.
Cheers For Now

Saturday, March 14, 2009

Denis and Peter. I think they were giving first aid to Denis' arm Peter's Extra in the foreground.

Thursday, March 12, 2009

Wing bagging part 4

We left off the last post getting ready to mix epoxy, it is important that all the items needed for the next steps are pre-pared.
This includes the vacumn bag and compressor which was pretested to ensure no leaks
a length of peel-ply material to prevent any escaped epoxy sticking everything together(can be substituted with a length of paper towel.

I have estimated that about 300/350 grams of glass cloth for both top and bottom skins. I will need about the same in resin , 30 percent less if you work carefully. Resin binds the components together ,excess resin adds no strength only weight, so work it in well and spread the excess with a plastic card (I use telkom telephone cards ,which I collect free from some pay-phones near my office.

I mix the epoxy in paper cups and have 3 x premeasured with 50 grams of resin in each
The first cup has the harderner added (slc 30) in this case with 30 mins pot life
and its "game on" .
The Mylar is still on the news paper with the two layers of cloth, I fold back "carefully the 163gr about halfway , smooth out the wrinkles in the first 106gr fine cloth and pour on about 2 x teaspoons of resin. I only work with a plastic card
no brush, and spread the resin as soon as a reasonable area is wet out , the glass stays in place and you can add more resin and spread/scrape it to the edges, working back and forth very soon the whole area will wet out, carefull near the edges that you dont pull the threads apart in the cloth.
You want it to look shiney and it can be a little resin rich.

I used about 30grs resin to this point, I now fold the top layer back down onto the wet out area and rub it down onto the wet out cloth under neath very soon it starts
soaking up the excess resin and you can see the areas change from white to shiney clearish. If a lot of resin comes through start working it in near the middle and add a little resin as you work outwards.The 163gr needs more resin to wet out and frays easily on the edges.

Soon the mylars are both wet out I used about 100gr epoxy per skin to this point
and its time to lay on the UD-carbon. The latex gloves which by now are all sticky come off and on go new ones, the carbon can become like spidersweb ,it comes of the backing especially where cut at an angle and can be a nightmare. You basically line it up on one corner and lay t down onto the wet below dont touch it unless you are happy with the possition.
Now you can press down on it and work it with the card through the pink plastic backing (not many people use UD carbon so if using any other woven cloth you would just wet out as per normal)
Shorty you will see the areas that have resin, I spend lots of time trying to pull the excess resin up into the carbon.Its worth the effort, we are about 2hrs into the proceedure.

The backing is lifted off and and some fresh resin mixed and poured on (sparesly)
This is worked in and then its time for a short break (5 mins) .
After 5 minutes when spreading more epoxy in you will notice the white binding threads that hold the carbon material together start to release and can be removed.
It takes some effort but the epoxy melts some sort of glue holding them and the come off, there are some lengthways threads as well which I also remove but which could stay in as they lie flush and wont show in the wing surface.

An off-cut piece of 163gr goes over the carbon at the wingroot to strengthen this area which also holds locating pins/ the ply rib etc. I have also put a piece at the very tip for the winglet .

At this time I have used all 300gr of epoxy and its time to possition the wing on
to the bottom mylar, the leading edge must line up , plenty of overlap on the trailing edge.

I now make up a mix of 50gr resin with micro/balloons about 20 percent to 80 percent
Cabosil/Aerosil this is a thickening agent mix in until you get a consistency of peanut butter ie quite thick paste.
This is spread along the trailing edge which now looks quite ragged, add extra where pieces are missing and spread with the plastic card.

, "morefresh new lattex gloves on" the top skin is lined up on the trailing edge and then in one motion flipped over, it can be possitioned to line up to the leading edge
I then attach some masking tape fron the top skin onto the bottom skin in three places tips middle and root ,I then slide the peel-ply cloth under the wing and slide the whole thing into the open end of the bag.

The possition of the skins is checked to see they havent moved , a second piece of peel ply is layed over the top and the bag closed and vacumn pulled. I then release the vacumn and spread out any wrinkles in the skins/peelply , vacumn re-applied .

Now some builders place the foam core outershells under and on top of the wing and weight the wing down while it cures overnight, I no longer use the outers and weight it down on a flat surface(floor) I position the weights carefully if the wing has built in washout as in this case.A good workshop cleanup and we are done for the day

The next day no1 comes out the bag and number 2 goes in , part 5 hope it all works.

All materials used in this project are available from A.M.T. who are most helpfull
and can advise on the products they supply . Speak to a MR. E. Shaw there.

Tuesday, March 10, 2009

Wing bagging part 3

Well after the epoxy has cured , we should now have a fairly rigid wing core with it's tubes and shearweb all integrated with the top and bottom carbon spars.
All the hollows are filled and any high spots sanded, unfortunately minor dings do occur and will show up in the end product , try handle the foam with care.

The two ply end ribs are cut to shape and made to fit over the joiner tube which stands out by a few millimetres. It adds strength if this can be "built in " with the wing skins. I have also glued in two thin aluminium tubes which will be part of the removable wing tips/joiner.

The mylar sheet is cut to the size and outline of the wing, I leave about 3/4cm overlap on the trailing edge. I vaguely recall this is medium tickness mylar 200 micron thick? I think.there is a right hand top and bottom I mark the outside thus with a permanent marker. These will reverse for the other wing.

The mylars are waxed with release wax and polished/buffed off , the mylars are sprayed with a white 2k (only just enough thinners) about 3 passes gives a nice solid white. The mylars are sprayed the same day the first wing is to be layed up, I generally start early in the morning.
Once the paint has flashed off and is touch dry , I put thye mylars down on some news paper and I start cutting the various layers off glass cloth.

The first cloth is layed with the weave running length ways with the span, it is a fine weave 106gr cloth and it is cut to the mylar shapeas a template and left in place on the mylar.

The second cloth on this particular wing is a 163gr cloth which is cut so its weave/strands run at 45 degrees to the wing , this resists twisting of the wing while flying, it has to be done with two pieces and distorts easily , so should not be pulled by the corners at all, this layer is also trimmed to the mylars and left in place at this time.

Now in most biggish wings 4mtr upwards I would lay on a second 163gr at an opposite 45 degree bias, but this is a high aspect ratio wing and I want realistic wing flex and used slighty less carbon tows than normal.
So the next cloth is a 100gr uni-directional carbon cloth which makes up for less tows, this comes with a binding thread and a pink plastic backing film. It is difficult to work with but here goes, I lay it over the mylars , carbon down and then cut it to shape, use really sharp scissors,(or even a roller blade if cutting on a decent work surface) once cut it is carefully layed to one side.
Its now lunch time , after that we start mixing epoxy and then its non stop until one wing is in the bag , you need about 3hrs plus if working alone .
Adding the epoxy in the next post.

Monday, March 9, 2009

Vacumn bagging part 2

The joiner Tube and shear web is next.

The aluminium tube and spruce strip (3mm thick on this wing)are measured against the core and 2 x slots cut and sanded to shape,. This glider will have a shear web to about 1.3 mtrs out from the root , I use a sharp new blade and a straight edge for the shear web slot , the foam sliver is lifted out with a thin flat screwdriver. The tube slot is made with a rounded sanding attachment on my Dremel tool. It is important that slots are cleaned down to the existing carbon spar and the shear web fits with a little play, the shear web (spruce ) is trail fitted and marked so that it can be sanded to a taper and flush with the pre-sanded spar groove.The aluminium tube is sanded with some rough sandpaper for extra grip and the open end filled with some tissue or a foam plug to keep epoxy out.

The aim of the shear web is to ceate an I beam , it must hold the spars apart (under compression)This is the most common failure on long glider wings, the top surface/spar fails. It must also keep the top and bottom skins together (under tension). The spruce is strong and resists flexing in the verticle moment , but could split along its grain. So a carbon braid (with the strands running at 45 degrees) Is pulled tight over the spruce and onto the aluminium tube. I tack this in place with cynano glue. Another way is premade sheets/strips of glass and end grain balsa, light and strong but a little more time consumming, or even light ply-wood.
The next steps all take place as one exercise so it is important to have the next set of carbon tows already precut for the bottom spar.

The carbon braid is wet out with epoxy resin (I mix about 50gr at a time)and set aside , I then paint resin with a small brush against the exposed carbon top spar inside the slot. At this time there would still be a fair amount of epoxy left in the pot, to this I add glass flocks(cotton flox) until I achieve a runny mix (like syrup) A bead is layed into the slot with a large syringe as well as plenty in the area for the joiner tube, which is wedged in possition to create some di-hedral.
The shear web and tube is now pressed into possition, some mix "goop" should ooze out
then you know it is well bonded.
Any excess can be scraped off, thickened with more "flox" and a thicker mix used to fill hollows , especially around the joiner tube area.

Moving right along while everything is still wet , get the carbon tows wet out and layed into the pre-sanded groove. Using a plastic phone card you can flatten and spread the epoxy through these, until they are all nicely wetout and flush.
Dont panic if the tows are below the groove edges , this is easy to fill later, you dont want to sand off part of the carbon if it's proud.

Place the core on some paper ,on a flat surface, wet side up and weight it down to prevent a bow in the wing,
I do both wings at the same time and needed to mix extra epoxy in 50gr batches , mixing more on a hot day is looking for trouble with an exo-thermic runnaway, where
the epoxy curing accelerates generating so much heat it can catch fire(been there done that).

Now pour a long cold one and start thinking about the the next step , the mylar preparation and glass composite wing skins. (part 3)


Sunday, March 8, 2009

Adrian and Howard with the new glider tug.

Plenty of power there.

Thursday, March 5, 2009

A how to make a vacumn bagged wing

Hi Guys,
Making vacumn bagged foam wings has been around for a long time , but unless someone shows you the steps , many builders are not keen to tackle this method which can produce a light strong wing in a short time, like all things it's easy when you know how.

I have been asked by some of the gliding guys to explain, but might add that this type of construction is even more suited to power planes with thicker airfoils and shorter spans.

This build thread then will detail all the steps and show my latest build , but there are variations to bagging techniques , anybody who has any questions are welcome to ask in the comments section, Or drop me an E-mail.
As always wear gloves when working with epoxies, and work in a well ventilated area
, make sure your work surface is flat and even.

I am not going to cover the template making and foam cutter but needless to say you need a set of cut foam cores and I prefer extruded polystyrene foam , in this case Dow blue foam but local ISO-Board available from the cornice moulding shops also works well, and can be ordered in longish sheets for a few hundred Rand, generally you can only hotwire cut panels accurately to about 800mm in length.

Phase 1.
The preparation of the cores.
The cores are cut in segments and in this case glued together end to end with a thin layer of 5 minute epoxy (Alcolin the type in the syringes)its a tapered wing , so I used the outer shells to hold it accurately while the epoxy glue set (10 mins).
Polyurethane glue also works but seems to be off the store shelves.

The spars are marked out and then masked with tape, this wing has a tapered spar.

I made a simple sander and sand out a groove between the tape , about 3mm deep and with a square edge. I sand the top and bottom grooves and blow the dust off with compressed air.

The carbon tows are wet out on a piece of mylar this wing has 2 x 60k tows to the tip
and then the wing is divided into equal sections and 2 x tows cut and layed up to about 75% then 2 x shorter ones to 50% and then 4 x short ones to 25%
This now means 10 x in total out to 25% which is where the greatest load is.
On a power plane I normally make a constant spar groove and carry the carbon spar throughout.
The next step is to sand the leading edge, a small con-cave block can be used to get an even profile, but this wing is tapered so I sand by hand and eyeball the shape. I then cut some strips about 5cm wide and about 30cm long out of some straight weave
104gr glass cloth, enough for the full length of the wing, I use LR20 epoxy resin and SLC 30 harderner (available from AMT) I have now sanded a nice round leading edge and wet this with a thin line of resin , to this I adhere a single 12k carbon tow , I wet this out with a small brush then add 2 x more tows , one above and one below the leading edge centre line, these are also wet out.
Then I pre wet out the glass strips on some mylar sheet and lay them over/around the leading edge carbon strip. I leave the wings upright while this cures overnight.

These cores now with top carbon spars and carbon/glass re-inforced leading edge are left to cure.
Next steps include the joiner tubes ,Shear webs , end rib and bottom spars.
Time spent so far 1 x weekend.

Sunday, March 1, 2009

A Flying Saucer over JOMAC?

On Saturday I was flying my Minij with a Flycam attached. Lo and behold, irrefutable evidence.