Preformed leading edge for bagged wings

[Zephyr41]

I've been happily making bagged wings for a while using Phil Barnes' techniques, adapted for available materials, and all has gone well except for the leading edges. Rather than the 8020 carbon Phil uses in his video I've been using uni-web which is very difficult to wet out properly when it's stuck to the foam core. I just can't exert enough pressure to force epoxy through the web and more often than not my LEs need some remedial work after initial trimming of the overhanging skins.

To remedy this I decided to try pre-forming a leading edge using the technique employed by free-flighters for built-up wings with hollow carbon D-boxes. For a test piece I used the LE section of a scrap core trimmed to 300mm long and about 50mm chord, mounted it on a piece of MDF. I laid a release film over the LE and laid-up a strip of uni-carbon and a strip of CF cloth on a 45deg bias on a piece of 0.010" mylar. I thought the thinner mylar would conform better to the LE than the stuff I use for bagging. I didn't pre-bend the mylar although some of the FF guys seem to do this. They're working with much smaller LE radii than me...

I put it in the bag with the mylar centered up on the LE radius and loosely taped in place. As the pressure came down I sort of helped it into place but it was really OK on its own. While it was still pretty green I de-bagged the piece and trimmed the edges. The result is a nice LE 'cuff' that is about 12mm wide and marginally thicker than the 3.7oz carbon uni-web I usually use for skinning. It has the 'bling factor' of the shiny CF weave too...

The trailing edges of the cuff were chamfered slightly and I applied a section of skin as normal, overlapping the cuff by about 4mm. I rollered some epoxy onto the LE of core and put the cuff in place which is a whole lot quicker and easier than wetting out a strip of CF (unless you're Phil Barnes!).

After taking it out of the bag there's just a slight ridge to be sanded down- a minimal amount of carbon has to be removed and I didn't need to pay any attention to the LE.

This technique looks like it will work for me, the extra effort needed to make the LE cuff is about equal to the amount of sanding and cleaning up I usually have to do and the finished result is much better. I whacked the test piece as hard as I could on the edge of my bench and dented the ply bench-top quite deeply without making the slightest dent on the LE...

Of course if the mylars are painted there will be a wider strip at the LE to touch up.

Is anyone else doing this? I did a quick search and didn't come up with any hits. This is my first attempt and I'd be interested to know if anyone has a better way of doing it.

No photos I'm afraid, but I'm going to do this on my next wing and I'll take pix as I go.
Cheers
Rich

[geostomp]

Pics would be great.

[Air Head]

I was thinking up the same sort of plan. I want to wrap the leading edge and cant get the Mylar around the bend. Any sutiable solution for this. I was thinking of just using bagging film over the core and making a mold of the leading edge and then molding it from that. I know its more work but I am thinking of making a tail fin this way that would be hollow with the structure inside and a taco shell molded piece over that.

Keep it coming I like it

Thanks

Rick

[GaryO]

Paint the mylar you used to make the cf LE.

[Zephyr41]

Paint the mylar you used to make the cf LE.

That would work- if one was careful when sanding only a 5mm wide strip of CF would be exposed. I must admit I don't usually paint mylars anyway, preferring to paint the wing when I have finished the LE and inspected the skin for flaws.

[Zephyr41]

I was thinking up the same sort of plan. I want to wrap the leading edge and cant get the Mylar around the bend. Any sutiable solution for this. I was thinking of just using bagging film over the core and making a mold of the leading edge and then molding it from that. I know its more work but I am thinking of making a tail fin this way that would be hollow with the structure inside and a taco shell molded piece over that.

Keep it coming I like it

Thanks

Rick

I like that idea! This page (http://www.gregorie.org/freeflight/cf_wing/cf_wing.html) shows how FF builders do it and I just noticed another thread on this forum discussing the technique.

I don't think 0.014" mylar would work too well. The thinner mylar I tried was fine although the wing section I used has a 220mm chord and I don't know how well it would conform to really small radius LE on a tip panel. My next trial will be a 2.6m electric glider wing with a 60mm tip chord so I'll find out soon...

The mylar I used took a definite set in the vac bag- it's permanently bent where it went around the LE. You could try bagging a piece of mylar over the form for a couple of hours, applying heat if possible, to see if it conforms better.

[DKChris]

How about laying a medium thickness sheet of polyethylene plastic foil (it'll conform more easily to the shape) over the layup on the core and using some sort of negative shape elastic foam arrangement to press it onto the core ?? I've seen wing joining done that way (maybe check out the HJZ predator DLG build thread under hand launch gliders) with good result. If it works its likely to be a fast and simple method.

[flyonline]

Try the laminating films, it's much thinner than mylar and will go around very tight curves but will give you a better finish than the poly film.

Steve

[heli_bee]

Zephyr41,

any updates on your method of doing the leading edges? It seems simple enough to do but have you managed get it moulded around a curved leading edge on a wingtip? If so how? I have been trying to think of a way!!

Regards,
Martin.

[Alex.Schweig]

I bet using PE painter's plastic would work well too, and almost nothing sticks to PE, so it's a great alternative to lam film or thin mylar.

[wyowindworks]

The downside to the laminating films and simple plastic is they will telegraph the fabric as well as imprint the breather cloth pattern into the part. Even the 10mil laminating film is too soft and causes significant imprinting.

Adam

[Zephyr41]

I haven't tried advancing the technique but I'm working on the cores and LE forms for a new wing now so I'll post some pics over the next week or so.

Curved tips are a problem, as are the 3-panel tips that I'm working on. Previously I've bagged them with a single skin but at the moment I haven't worked out a way to make the pre-formed LEs. In fact I'm not even sure if I can get an acceptable finish on the small radius LE at the extreme tip. We'll see...

[heli_bee]

Rich,
any updates yet?
Martin

[tom43004]

I have tried this in the past and pretty much gave up because I had a hard time getting the skin thickness to match up. If you come up with something workable, I'd love to see some pictures though.

[Zephyr41]

I just finished a fin LE using 1k CF cloth with light glass on a 45 degree bias on the outside and a strip of 1.7oz Kevlar on the inside. My all-carbon test pieces held up to being whacked on the bench but close inspection shows a lot of the fibres are breaking so I decided to try the Kevlar.

I should say that I design, build and fly UAVs for agricultural work and I need tough LEs as we spend a lot of our down-time on field trips repairing minor damage. Sometimes you don't have the luxury of a good landing area! If I was building to fly for fun I wouldn't bother with the Kevlar.

The Kevlar is smaller than the carbon all round so there's no need to cut it when trimming the part. I also tried painting the mylar this time. I thought the paint might wrinkle or tear at the tip but it worked OK.

The result is all right- the part is useable but not quite as nice as I would hope to achieve. The fin tip chord is 85mm and the 0.010 mylar conformed to the LE radius perfectly well. I wouldn't want to go much smaller though.

Pics show the form, the mylar and cloth ready for the layup, and the finished piece. The form has to be slightly over-length as obviously the ends get crimped. Yes, I could have cut the mylar to fit the form perfectly...

It's quite difficult to get the form absolutely straight when gluing it to the MDF support. If the gluing surface isn't flat the LE wanders and you get a slight S-bend in the part. I cut the form in the pics with a knife but I'll try doing it with the hot wire next time- should make a straighter cut.
Rich

[TomFlyer]

Your thread caught my eye because I have been on the same journey to find a way of building durable, accurate leading edges in an efficient manner. This link will take you to my Blog where I have links to the 4 different methods that I have used to date. http://www.rcgroups.com/forums/member.php?u=62063

[TomFlyer]

I think that one could use a properly hot wire cut foamn core as the form by puting some packing tape over the LE of the core, wrap a 2" wide strip of CF and a 1" wide strip of glass over the LE and bag just that part with the LE vertical in the bag with mylar sheet over for a smooth finish. Once that is cured, and out of the bag, remove the packing tape from the LE. Then I would place the finished LE on the core to size the fiberglass that will abut the CF and a second layet that will go over the CF and abut with the FG nose strip. So the LE molded part has a lap seam with the FG that goes over the wing. The mylar laps the joint and the excess epoxy ridge will need to be sanded off down to the LE mold.

Only question is, will the bag pull against the LE enough to form properly.

This sounds like it would be worth a try.

[TomFlyer]

I am wondering if anyone has tried this for bagging the LE only?

[Zephyr41]

That should be OK but you'd have to protect the trailing edge somehow I think.

I've almost finished a fin using the pre-formed LE. I trimmed the part and gently sanded the edge to as fine a taper as possible. The LE was then attached to the core with 5min epoxy. When laying up the skins I lifted the edge of the LE and brushed some epoxy under them. I also sanded the LE lightly to ensure the skins would stick.

The mylars are about 5mm short of the LE which allows the skins to overlap the moulded LE around 5mm past the point where I want to feather them out when sanding.

I painted the mylars this time, something I don't usually do, and it came out of the bag looking OK. Today I'll sand the skins back at the LE and take some photos.

Problems: The LE former crushes a bit and bends in the bag. The moulded LE is slightly distorted but I was able to get it into shape when glueing it to the core. The tip end of the form needs some support though. Probably not an issue with wing LEs due to the thicker section.

I got a sort of crimp in the back edge of the LE part. Not sure why but it could have been because of the form bending under vac. This resulted in a slight dimple in the finished fin.


Photos of the finished fin to come.
Rich

[TomFlyer]

It might be better for a straight LE wing to cut off the front 2" to use for my LE mold and then glue on an 1/8 inch thick strip of wood to replace the 1/8 inch saw blade cut. Glue the 1.875" LE back onto the wood strip for final bagging.