another supra build - the slow way

[nuevo]

Thought I'd share another Supra build.... at a little slower pace than the molded one. ;)

If you want to build your own, see the plans here:
http://www.charlesriverrc.org/articles/supra/supra.htm

I'm building two Supras over the winter. One for me, and one for a friend.

Here are some photos of one completed rudder.

[nuevo]

A few photos of the 2nd rudder in progress. I decided to add .060 carbon rods to the vertical fin of both. Why? Tom Kiesling did it, and I liked the idea. But after building one, and thinking how that "highly sheered" kevlar works to reduce bends, the rods are probably not needed. Weight penalty of the .060 rods is about 1.5 grams, plus a little glue.

On the first rudder, I glued the rods in when bagging with very little epoxy. This one, I glued them in ahead of time and applied a little putty.

In the photo below, you can see the edges are rounded, and have been sanded back to airfoil shape. It is ready to be vacuum bagged. The cloth is cut out & waiting, too.

[nuevo]

Cores for both planes came from Laszlo Horvath at http://www.compufoamcore.com/

Here are a few photos of raw stab cores, just to show how they are when I get them.

[nuevo]

Load of kevlar pieces. 1.0 oz kevlar, Clockwise from upper left are pieces for wingtips , stabs, and rudder. Takes a long time to cut out all that cloth. Each piece of cloth is attached to wax paper with an extremely light mist of 3m77 before cutting it out with a pattern.

[nuevo]

Today's work was on the stabs.

[nuevo]

rounding and sanding the stab cores. See description with each photo.

[glderguy]

Thanks Jon, your build thread, at least for me, is greatly appreciated.
Looking forward to watching your progress this winter! I am not familiar with
the weights that can be obtained, by an experienced person, building your own Supra. Think Ive heard anywhere from 41 to 64 ounces RTF, is that correct? Is it true this plane was designed to best fly at really low weights/wing loadings?
Keep up the good work.
Walter

[AkiP]

Thanks for the thread! I will be following this one with interest.

Aki

[Robglover]

All I can say is ...YIPEE!

Rob - aka Lucky Friend

[Phil Barnes]

Think Ive heard anywhere from 41 to 64 ounces RTF, is that correct? Is it true this plane was designed to best fly at really low weights/wing loadings?Mark's original Supra weighs 48oz. Kiesling's Models weigh 58oz and 64oz. Mine weigh 62oz.

For mark's comments on the effects of weight read this thread.

[glderguy]

Oh yeah, forgot about that thread, thanks Phil.
walter

[Badger]

No problem getting under 55 oz even with a stronger spar, 2" more span and a heavier boom. My light one is now under 53 ounces. I never ballast over 10 oz. so a molded Supra would be for maximum wind conditions only. 41 ounces would be great, but I don't see how it could be done.

Using a Walba boom and my new pylon pod, another 1.5 ounces can be taken out. Jon, are you using the Walba boom?

[glderguy]

Badger, do you sell the pods or do you just make them for yourself? If you do
make them commercially, could I get the info needed to purchase one? I have been thinlking about dusting off the equipment and bagging up a Supra for some time now. Jons build thread may be the catalyst to get it started. Building the wings for me isnt an issue, cutting a plug, making molds for a fuse is.
Thanks, Walter

[Badger]

Sorry, I don't sell pods. I will acknowledge that the majority of my molding information has come from this site, so it isn't that hard.

There are others who sell pods and booms, including Kennedy I think.

[markdrela]

Today's work was on the stabs.
I suggest sanding a flat spot into the balsa on one side, where it sits on the V-mount platform. This will reduce or eliminate the need for potting the stab against the V-mount so it sits tight without rocking.

[nuevo]

I suggest sanding a flat spot into the balsa on one side, where it sits on the V-mount platform.
Thank you. I had not thought of that.

[nuevo]

a few folks commented on weights. I purchased a new scale for this build. Beware this is a dangerous piece of equipment, and can drive you to heights of insanity unknown if you purchase one. ;) The scale is accurate to 0.1 grams.

Be aware I am no expert builder. I did build an Aegea wing last year, which raised new challenges. This build will also force me to learn new things, too. Hopefully we can all learn together. For one, I have never worked with 1.0 oz kevlar, but had worked with .75 oz FG before. Basically, if you just look at the cloth wrong, it will stretch on you. I use wax paper with 3m77 as Phil Barnes mentioned some time ago, to keep the cloth from distorting while handling.

I weighed my completed rudder. Mine came out at 27.5g. The plans showed the target weight at 23.5g. The exra carbon rods I put in were 1.5g. I'm guessing the carbon rods and a little glue came out to 2g, so I would have been 2 grams over the target weight. I feel pretty good about that, so far.

[Badger]

Sheesh, I have (and use) an electronic scale that is accurate and repeatable to 0.01 grams. Usually the last digit is ignored.

[nuevo]

Badger, I decided not to pay extra for that last digit. :D Seriously, .1g is enough for me to see where weight is going. Heck, I added .4g just gluing the end-grain balsa pieces for the stab together. And the wood itself was 1.4g. Yikes. 28% increase in that one piece just from glue. This is the kind of insanity I was warning folks about. ;)

Well, I can see where some of my weight budget went, relative to Mark's. Paint. :eek: I painted the mylars for stab #1 and rudder #2 tonight. I thought I put the paint on just heavy enough so I could not read any newsprint under the mylars.

Rudder 1.3g, vs 1.0 target weight
Stab 1.8g, vs .8g target weight

On the stab, the top white was 1.2g, and black bottom was .6g

[Badger]

I'm not suggesting that a 0.01 scale is useful, just sorry that it leads me to even stranger compulsions about where weight is added. I must learn to always ignore the last digit, it's better not to go down this path. The only real value of the scale is (maybe) when mixing batches of epoxy under 10 grams.

I apologize for any implied slight.

[nuevo]

no slight perceived or taken. I was just making a joke on the .01g resolution.

[nuevo]

Jon, are you using the Walba boom?sorry I missed this question. Yes. The boom is from www.tailboom.com

[oakman7004]

Badger, How about building a SUPRA and sell it then? :)

I would love to get the hands into a world class TD plane after all years of F3B flying...

Cheers Jonas Ekman, Sweden

[nuevo]

I've despised tape hinges for years. The tape only lasts one year, and then is a mess to get all the old adhesive off. I migrated to silicone or bagged-in (integrated) hinges several years ago.

I am impressed how the hinge turned out for the rudder. I've bagged integral hinges before, and then faced the inner surfaces after bagging. It's a lot of work, and gets a little messy. Cut the surfaces off. Face the surface and the wing, then put the hinge back together with silicone or tape.

The technique Mark outlines in the hinges.pdf (http://www.charlesriverrc.org/articles/supra/all%20PDFs/hinges.pdf) file is straightforward and elegant. I believe Ib Jensen (http://www.rcgroups.com/forums/showpost.php?p=2052535&postcount=77) gets the credit for coming up with this technique.

"Closing the triangle" with bias oriented cloth makes the surface stiffer against twisting. Ib's technique also makes the hinge more durable, as it no longer relies on the bond of the skin to the foam. No downsides that I can see. Only a small amount of additional effort before bagging. I'll show some photos of the steps before I bag rudder #2.

[oakman7004]

Hi,
No twisting is in many aspects the key to good flight performance. Doesn't matter if it is just a aileron or a entire wing...This is the (in my opinion) the drawback of bagged vings, the aileron/flap tend to flex more torsionwise compared to moulded ones. Go for what you think is strong yet lightweight...

If you go for Lb Jensens version please give us feedback of the feeling of the aileron torsionwise, I have never seen a finished aileron built like this, yet...

Cheers, Jonas Ekman

[Badger]

Badger, How about building a SUPRA and sell it then? :)

I would love to get the hands into a world class TD plane after all years of F3B flying...

Cheers Jonas Ekman, Sweden

Talk to Jon about adding one to his schedule. I don't know anyone else who is willing to put in this much effort for a fellow hobbyist. These things take me over 60 hours of effort to produce, and mine are simplified.

[nuevo]

Rudder 1.3g, vs 1.0 target weight
Stab 1.8g, vs .8g target weight

On the stab, the top white was 1.2g, and black bottom was .6g
What a difference a day makes. I reported the weights above when the paint was 20 minutes old. Now, after a day to dry...

Rudder 1.0g vs 1.0g target weight
Stab 1.5g vs. .8g target weight.

The weights above are just the paint on the mylars.

For those who have never built a bagged wing, I have included a photo of the mylars below. The .014" (.355mm) thick mylar is waxed a few times, then sprayed with your choice of paint. When the epoxy cures in the vacuum bagging step, the paint will completely adhere to the part. At least that's the plan. ;) The mylars must not be handled, or handled very gently. I have already created two small pinholes in those below from not being careful.

[nuevo]

Jonas, I'll let you know. I have faced bagged ailerons before, just by wetting out 45-degree bias FG cloth on the open faces. After partial cure of the epoxy, trim off any excess. These steps make an obviously noticable increase in torsional stiffness. I don't have the equipment to measure, but it was easy to notice the change.

Sorry, there's no room on my schedule. I have to build an Oly 2 this winter, also. I've never built more than 1 plane a year in the last 10 years, so this winter should keep me busy.

It's been a nice week with temps in the 70's during the daytime. I didn't have to heat up the whole garage just to paint the mylars. Undoubtedly I will later, as winter temps arrive. Here's a spash of color I found yesterday around town.

[oakman7004]

No worries,
I just had to pop the question...one can never get more that a no. :)

Hey, building 2 SUPRA and a Oly will definately keep you busy. :) ..guess I'll have to settle for the moulded SUPRA then.

I will follow your thread with great interest and maby in the future I'll find some buildingtime for myself.

KEEP UP THE GOOD WORK (both building and informing us wannabees) ;)

Cheers Jonas

[rdwoebke]

Jon, your work looks great as always. I look forward to seeing more of your progress. This and your photo overview of the build of your Aegea are inspiring. Perhaps enough to get me to try this some day.

Ryan

[Radian]

What a difference a day makes. I reported the weights above when the paint was 20 minutes old. Now, after a day to dry...

Rudder 1.0g vs 1.0g target weight
Stab 1.5g vs. .8g target weight.

The weights above are just the paint on the mylars.



Jon, I see by the picture you are painting the bottom of the stabilize black. If you are really intent on achieving the target weights, how about just leaving the bottom natural colored?

Radian
www.phflyers.com

[nuevo]

Radian,

I'm not overly concerned about meeting target weights. Just reporting progress, both good & bad.

If you notice the earlier weights I posted "white was 1.2g, and black was .6g". If I eliminate the white paint and achieve better results.

Your observation made me go run a few #'s. Let's say I build the rest of the stab exactly at the target weight, but end up with the stab .7grams over the target weight.

From the 3-view drawing, I estimate the stab to be 38" behind the CG, and the nose lead to be 17" in front of the CG. Meaning the stab is 2.24x the moment arm that the nose lead has. To me this means an extra .7 grams will require 1.56 grams in the nose to balance it out. Total of 2.26 grams (0.08 oz) extra in the plane, just because of this paint. An extra 2 grams is no big deal, but brings up an interesting point. This is just paint. The builder has to watch everything, as all these little things add up.

[nuevo]

Oops hit the send button too soon.

These things add up. I reported completed rudder #1 as 4g over the target weight. From the 3-view, I'm guessing the rudder's weight is centered 44" behind the CG. Thus, 4g in the rudder means an additional 10.4 grams in the nose. Or an additional 14.4g (.5 oz) for the entire plane.

None of these "overages" bother me. Just pointing out that these little things, add up. Attention to a lot of little details required.

I'm learning a lot as I go.

[nuevo]

BTW, I have already made other design choices that will cost me more than a few grams. I have no expectation that my plane will come in at the weights posted in the plans. I'll cover those choices when I get to building those components.

[markdrela]

None of these "overages" bother me. Just pointing out that these little things, add up. Attention to a lot of little details required.
At the tail, and especially at the wingtips, the main concern is yaw inertia.
For example, if you add 5g to each wingtip, you will...

Increase weight by 0.7%
Increase inertia by 5% :eek:

So it really does pay to shave grams at the wingtips. As a reward you will get a glider which handles better and signals lift better.

[nuevo]

Worked on one set of stabs this week. Made a few goofs too. My technique for cutting the slots for the carbon rods is a small ball-end cutter in my dremel tool. It happens to cut just the right width for the .060" rods. I use a practice piece of foam to get the depth right. I use a metal ruler as a guide for the tool. If I do the slots perfect, like on rudder #1, no putty is needed.

I guess my track record is not so great. The ball shaped bit dug a little "odd" with a slight sawtooth pattern on rudder #2. So I added a little putty to make the surface smooth. I changed to a different bit for the stab, and get a smooth cut. On stab #1, I goofed and jogged the path about 2mm for a bit.

If I had cut the slots perfect, I had planned on gluing the rods in at the same time as I vacuum bag the stab. Now I have to glue the rods in ahead of time, then apply putty. Just another step along the way, becuase of my goof.

I don't know if the dremel is the best way, but it allows me to control the depth and width of the cut very accurately... assuming I don't goof up.

Here are a few photos before the slots were cut.

[nuevo]

Here's rudder #2, ready to go in the vacuum bag. It will be at least a day before I get to that task. I plan to bag the rudder and stab #1 at the same time.

I used 1.0 oz kevlar and .75 oz FG on the LE and top. Phil Barnes mentioned this technique. The strips were .75" wide. I think I'll cut it down to .5" for the next stab. I also have .75 oz FG facing around both sides of the hinge line. Finally a strip of kevlar on the bottom edge of the rudder for durability.

The insert between the hinges is covered with tape & waxed. Note the lightweight putty around the carbon rods, as mentioned in the previous post.

[nuevo]

Here's a few photos of the stab with the rods glued in. I tried to show a few voids where I got the dremel router off track a little. It takes a little nerve to cut up those pretty CNC cut cores the first time. :o

In the big picture, these are not a big deal. They can be fixed with lightweight putty. Just showing off some of my goofs. :o

[markdrela]

Minor dents in the core can be removed with the boiling water trick. Easier than filling and sanding.

[nuevo]

Mark,

Excellent suggestion. I have read about the technique, but have not yet added it to my repertoire. I have already filled the dent in the photo with lightweight spackling putty & sanded smooth. There were several other dents in the core, that I thought too shallow to fill with putty. Got rid of them all in 3 minutes with a wet paper towel and a monokote iron.

Thank you!

[nuevo]

Well, today I got stab #1 and rudder #2 in the vacuum bag. Here are some of my steps. Don't know if they are ideal, but here's what I did. Comments under each photo.

[nuevo]

I mentioned that I scraped the resin out as best as I could without applying undue pressure with the gift card. Too much pressure can distort the kevlar. I also mentioned I put the layup in the vacuum bag for a few minutes with two paper towels in between. I think this is a technique Phil Barnes mentioned. A local friend, Lars, reminded me of the technique this weekend, so I thought I'd try it. Here's a shot of the paper towel after that brief session in the vacuum bag. Sure looks like quite a bit more resin came out.

My only concern is that the layup is not too dry (aka resin starved). Only time will tell on that. If I had not used any paint, I could probably tell by visual inspection.

[Phil Barnes]

Sucking out the excess epoxy with paper towels and a vacuum bag is a technique I picked from Kevin Kuczek.

You should add some extra epoxy back into the TE region before bagging or you'll end up with soft, weak trailing edges that will crush and debond too easily from normal hangar rash type incidents. With really dry layups it is also generally recomended to add an extra stripe of epoxy on the hinge line to prevent debonding there but that may not be needed with the pre-facing method that you have used.

If you have an accurate gram scale then you can weigh the paper towels before and after the vacuum mopping operation to see exactly how much weight of epoxy has been removed. You can even go back later and weigh the used paper towels after zeroing the scale with an equal amount of fresh paper towels if you wanted to check the weight savings as an afterthought.

So far your build looks real nice. Keep on posting.

[nuevo]

I thought about extra resin at the hinge line & the LE, and left the cloth already there a little bit wet. I thought, if the outer cloth was too dry, it would draw resin away from the LE & hinge. I did not think about the TE. :o We will have to see, when it comes out.

Just for fun, I'll weigh the soaked towel & a new set this evening.

Thank you for the constructive criticism, and encouragement. Any & all always welcome. As everyone can plainly see, I'm still learning new things, as I progress.

[Phil Barnes]

You shouldn't need the extra stripe of epoxy for the LE. I just roll epoxy on the core LE and then wipe off excess with a paper towel, I don't add the extra stripe of epoxy on the mylar for the LE.

[nuevo]

I weighed the paper towels from yesterday. They took 3.1g of epoxy out of the layup. That much surprised me.

Next set of parts are done. For those keeping tabs, rudder #2 is 25.8g, and stab #1 is 25.5g. A little better on the weight, as rudder #1 is 27.5g. Target weights for the rudder 23.5g and stab 24.4g.

Pictures below.

[davidleitch]

Jon

I'll never have the time or skill to build a Supra this way. However its fun to read the thread and I congratulate you on the clarity and quantity of the photos. Its a lot more work documenting a build and I for one am enjoying following the progress.

[nuevo]

Thank you. I hope to document as much as I can. To encourage others to build, and to demystify the process for those who have never tried. I also hope to learn by gaining wisdom from others. It's kind of different taking photos while I'm working. I find I am thinking how to present the information, as I progress.

[soarcat]

I am a fairly experienced builder and have completed other scratchbuilds such as a Magic, and others of my own design. I am still picking up a lot of new tips and techniques from your photo's and text, and am beginning to work my way toward a Supra build as well. It is not easy to maintain what you are doing, but it is definitely appreciated by those of us, both experienced and inexperienced. Thanks for your efforts, this is one special sailplane.

[nuevo]

"is not easy to maintain what you are doing"
What do you mean? If you think I've left something out, ask. If you think some part I have written is unclear, please ask. I'll try to clarify.

I'm no expert. Most every technique I show here I learned from others. I am glad my experiences might be helpful.

[Hostage-46]

It is not easy to maintain what you are doing.....

For the peanut gallary it would be worth looking at the available Supra Plans Jon is working from (and I believe referenced in this thread).

Jon thanks for taking the time to document your progress, we'll all benefit your build.

Dan

[rdwoebke]

What do you mean? If you think I've left something out, ask. If you think some part I have written is unclear, please ask. I'll try to clarify.



I think the poster was giving you a comment (at least that is how I took it). I read this as "it is difficult to keep up this kind of pace in a build and take pictures at the same time so I applaud you for being able to do this".

Jon, I'll second the other guys "thanks" as I appreciate seeing how this kind of thing is done. You take good pictures and do a good job of explaining things.

Ryan

[nuevo]

Those pesky leading edges.

I have vacuum bagged several planes, but not that many. My friend, Rob Glover, taught me to vacuum bag a balsa skinned wing long ago, and I've been on my own ever since. I have never been able to make leading edges as good as the Masters, and have never figured out why... until last night.

After all the painful sanding on the LE of my stabs and vertical fin last night, I decided to view Phil Barnes' vacuum bagging video (http://home.paonline.com/hayman/PAGE2.htm) again. Specifically the parts about leading edges. Long ago, I remember Phil writing "my mylars almost touch at the leading edge". I thought, how can this be? The mylars will never completely conform to the airfoil shape. So I ignored his advice, and went along my own way, not knowing why he did that.

Picture #1 below show how I have done my cloth and mylar layups. I can't intersperse pictures and text here, so see that diagram now. The skin and LE fabric in this case are kevlar.

The results of my technique are: 1. There is a sharp bump where the mylars end, and some epoxy pools up. 2. There is a bump where the breather cloth forms a crease, right at the LE. So I have 3 bumps to sand off. If you've ever tried to sand kevlar and make it look good, you know how frustrating this is. The more you sand, the more fuzz you get. Uggh. The one saving trick I heard is to put foam safe CA on the fuzz & let it cure. The glue makes the kevlar stiff enough that you can now sand it off. It works, but not perfect. Saved me in a few spots last night. :o


Now, remember where I said Phil's mylars can't conform to the LE curvature. Well, I was right. They don't. But it doesn't matter. Look at figure #2 below. In this picture, both the skin fabric and the LE fabric are carbon. This is how I interpret how Phil bagged wings when he used to use a carbon LE (as shown in the Fusion wings on his video). The trick is that carbon skin at the front of the mylars does not attach to the LE of the wing. After curing, the overhaning cured carbon skin gets sanded off. Easy and results in a pretty and smooth LE. The front part of the LE probably does not need to be sanded. But this can't work with kevlar, because the sanding will leave more fuzz.

Finally, I have figure 3 below. It is my interpretation of how Phil does his layup for an all kevlar wing. I'm adapting ideas I saw him do when laying up the XP3 wing. Again, the LE fabric and the skin fabric are both kevlar. The difference is the skin fabric stops about 1/4" short of the front edge of the mylar, but must slightly overlap the LE fabric. The mylars again almost touch at the LE, but do not fully conform to the airfoil curvature during bagging. What remains after baggins is almost no sanding required. I expect a small bump of resin, right where the mylars part from the surface, will need to be sanded. With care, you never get to the kevlar, and... no fuzz.

I have not tried the technique in picture #3, but those are my plans for the next stab. Comments or suggestions anyone?

[YHSC_Chris]

#3 is how I have started doing it, except I additionally try to taper the Mylar, like Mark does. I get much better results than I used to. The outer layer of LE fiberglass leaves a nice finish. I think I also stop the mylar a little further aft from the exact LE than they do. The vacuum bag leaves just a tiny ridge at the point where it comes together.

[soarcat]

Jon,

As Ryan pointed out, my comment that it is "hard to maintain" was absolutely a compliment to your efforts, time and energy as I have been asked to build and document as you are doing, and "life" usually gets in the way.

Go with picture #3, and as suggested, sand the edges of the mylar down to reduce the stiffness, this will result in very little effort to sand the LE ridges. Where you don't like the results you can apply some foam safe CA and resand, doing this multiple times if necessary.

Rex

[Phil Barnes]

Jon

Your diagrams are exactly correct. I can now simply add some comments to further explain things and to point out how some of my methods have changed/improved since the video was done.

The one element you have missed is that when I do Kevlar LEs I use a "composite fabric" that is composed of a layer of 1.7oz Kevlar which is bonded to a layer of light fiberglass. This "composite fabric" is cut into strips with the weave at a 45 degree bias to the length and these strips are stuck to the core LE. I believe you can see this technique in the video also. Having the fiberglass on the outside of the kevlar also helps to avoid sanding into the Kevlar when you sand the LE. I'll do a separate post with links and updates to this technique.

One small change that has happened since the video was shot: I don't run the mylars quite as far forward as I used to. Now I try to have them still come far enough forward so that there will not be a ridge where the mylars end. The mylars still pretty much come to LE of the wing but usually aren't long enough to actually come together as in your third sketch. This is probably about the way YHSC_Chris does it. Also I learned that most of my mylars are actually tapered in thickness near the LE. They got that way by being scraped with a single edge razor blade as you see me doing in the video. The purpose of the scraping was to remove epoxy and spray adhesive from the back side of the mylar which would collect there after each use but it turns out that this scraping which was most heavily done near the edges of the mylars also had the effect of thinning the edges of the mylars.

If you do a dry layup and use a paper towel to mop excess epoxy off the LE fabric then there will be very little to sand off the LE after bagging. I will still use the scraper as shown in the video to quickly knock off any epoxy blobs or any fiberglass skin that overhangs the LE but there is very little work for the scraper to do. I then use a 100 grit sanding block to get close to the final shape followed by hand sanding with 220 and 320 grit paper. You should be able to sand out the LE with almost no Kevlar fuzz being raised. You typically only see small spots here and there and they are easily sanded out with the fine sandpaper.

[markdrela]

Go with picture #3, and as suggested, sand the edges of the mylar down to reduce the stiffness, this will result in very little effort to sand the LE ridges.
Agreed. But mylar is very tough to sand. I used a planer-type setup, using a Dremel 1/8" cylindrical carbide bit jigged up at a shallow angle at the edge of a board. The edge of the mylar was then passed under the bit. This worked OK, but not great. If someone can come up with a better method let us know.

My bevels are roughly 3/8" wide. The edge is as thin as possible. The thin edge might end up being a bit ragged in a few spots, but this doesn't cause any problems.

PS
Before the Dremel method, I used the razor-scraping method. Easier than sanding, but still a lot of work. Using a knife with a big handle, like a box-cutter, will save your fingertips.

[markdrela]

It is possible to sand kevlar without any fuzz if you follow a few rules:

1) Use only fresh sandpaper, one grade finer than you normally might use on fiberglass.
2) Use only light pressure, with rapid motion if necessary to increase cutting speed.
3) Use wet-sanding if possible. When the sandpaper gets clogged with kevlar sludge, just wash it out with a drop of detergent and keep going.

[Jose E Bruzual]

JThey got that way by being scraped with a single edge razor blade as you see me doing in the video. The purpose of the scraping was to remove epoxy and spray adhesive from the back side of the mylar which would collect there after each use but it turns out that this scraping which was most heavily done near the edges of the mylars also had the effect of thinning the edges of the mylars.


Phil,

not to get off topic, but how do you clean your mylars now a days, I use 10 mil for my stuff, and clean the excess the same way you did, I am about to start using my second set of mylar and if there is way to make them last I would be interested

Thank you

Jose

[Phil Barnes]

I still scrape mylars with a single edge razor blade to clean the back side between uses. I almost never retire a mylar due to old age. I get many dozens of baggings from a single mylar. I use 14mil mylar and the edges end up with a very nice taper after a few uses. I find it very easy to bevel mylars with a razor blade. I didn't even know it was even happening until I just happened to notice it one day. The fact that Mark finds it taxing while I find it easy might just be a question of scale :) :)

[nuevo]

Soarcat. thanks for the clarification. Sorry I missed your intent.

Phil, I do use the "composite fabric" you outlined for my LE's, but I left it out of those drawings.

On composite fabric, I've been using a variant on what you suggested. I cut the 1.0 oz kevlar at 45-degree bias. But I then bond with 3/4 oz FG that is 0-90 orientation. What this achieves is a strip that is easy to handle and does not distort. Is noted earlier, 1.0 oz kevlar will distort if you just look at it wrong. ;) I see from your video that you used all fabric on a 45 degree bias. I missed that point before. :o After seeing how easy the fabric goes around curves, I am convinced to go with your way, from now on. My strips had to be cut with darts to lay down when going around curved LE's.

Mark, I have my mylars thinned at the LE, too. I just never brought them as far forward as Phil. I thought of using a razor plane, but feared a blade digging into the mylar and possibly ruining the edge. My bevel is about 1/4" wide at the LE. I used the razor scraping method... and yes my arms got tired.

Like Mark's rule #1 above, I found that using very fine sandpaper is easier on the finishing of cured kevlar. I was using old 220 grit to cut down the rough spots. I quickly migrated to 400 grit paper, which worked fairly well.

Phil & Mark, I presume you don't bring your mylar LE's that far forward (nearly touching) around the tip curvature. There are compound curves at the tip, and the only result I have seen is waves in the surface as a result. I leave my mylars short of the LE, when the sharp tip curvature starts. Comments?

A few more notes on my just completed rudder hingeline cuts, with insights gleaned from Phil's video. A new blade is best for cutting cured kevlar with no frays. If you make one cut, making another cut 1/16" away is difficult to do right. The cloth wants to fray. Making the cut right the first time eliminates much of the need to sand.

When I made the foam wedge inside the rudder hingeline, I waxed the outside of the tape covering. Much easier to remove after bagging. On the first rudder, it was difficult to isolate & remove the tape, so I just left it. On rudder #2, once I found the tape, it was easy to remove, and leaves a fairly smooth surface for the hinge facing.

[nuevo]

Cleaning the backs of mylars? I've never done it. But then again, I've never built the same plane twice before. :rolleyes: There's a little epoxy on the back of mine, but nothing sharp that would puncture the bag. If I were in production, I could see the need to reduce buildup.

[KevinSharbonda]

See these posts...

http://groups.yahoo.com/group/SALglider/message/8854

[Phil Barnes]

Making the "composite fabric" LE material with both fabrics on a bias will make the strips easy to handle but still allow the fabric to easily go around compound curves at the tips.

You are correct about leaving the mylars a bit shorter at the tips.

More vacuum will help with forcing the mylar around corners quite a bit.

Why clean the back side of the mylars?

So far you have only done tails with a very dry layup and you have never re-used a mylar. If you do wings or other layups that are much richer with epoxy you will likely find that epoxy builds up on the outside of the mylar near the edges. This is just epoxy that oozes out of the layup. If I am doing a layup that does not have any glass fabric that extends past the mylar edge then I will tack the mylar down to the bench with a spray of 3M77 spray adhesive. This keeps the mylar from moving around on the bench during the layup. This spray adhesive also builds up on the back of the mylar and needs to be removed after bagging. I always want my mylars to be very smooth so that they lay flat on the bench. This is so that I can use a hard roller to mash out the fibers in some fabrics to close gaps. This hard roller technique is very beneficial to the style of uni-carbon that I use and also would help that 1.0oz Kevlar that you have been using. You can get a 1" wide wooden wallpaper seam roller in the wallpaper section of many home improvement stores. Use that roller while the layup is still somewhat wet (prior to the vacuum mopping operation). The roller will mash out the fiber bundles in the 1.0oz Kevlar fabric and help to close up the weave. This works best when the layup bench is real flat and smooth and the Mylars have no bumps and lumps on the back side.

Cutting Kevlar skins

I like to use one of those snap-off knives for this. It does work better to have a sharp blade and with a snap-off blade you get a new tip by just snapping off the dull one. Also the shape of the snap-off blades is better for skin cutting than exacto blades.

You must always make the Kevlar skin cut in the correct, desired location on the first cut. You never want to cut it over size planning to sand it to the exact size later as you could with a carbon or glass skin. I've never done the type of prefaced hinge line that you have done but I can see where there would be some possible difficulty in doing a knife cut at exacty the correct position where it would not need further trimming after the cut. I'm thinking I might try cutting the skin dead center in the middle of the hinge gap. Then trim the remaining Kevlar skin off with small curved scissors. Kinfes and scissors that are modified to cut Kevlar will do a clean job of cutting kevlar. Sanding Kevlar is something to be avoided at all costs.

Maybe Mark can tell us how he opens up the hinge gap on those prefaced hinges after bagging without making Kevlar fuzz mess.

[nuevo]

Wow, great insights Phil !!

Mere mention of a hard roller made your point loud and clear. I will make sure the back of my mylars are clean from now on. I have used some old mylars for use in this project. [1] I never got much epoxy on the back of my mylars in the past. I taped peel ply cloth and then paper towels along the leading edge of my wings. [2] Any extra epoxy got soaked up in the paper towels. But all that extra thickness made getting a good fit in the beds impossible. After today's conversations, I plan to omit the peel ply & towels. I will make sure I clean the backs of my mylars after use.

I meant to cover my use of the roller in post 41 (http://www.rcgroups.com/forums/showpost.php?p=4586909&postcount=41), but forgot to mention it. The weave on 1.0 oz kevlar is very loose. I got a bit of pinholes near the TE on rudder #1. After that, I remembered Mark's mention of using a roller to mash the threads together before bagging.

I did exactly as you mentioned with my rudder hinge cut. I made one center cut in the hinge slot, then made the other two cuts. The cuts get awfully close to each other, near the top of the vertical stab, as the gap narrows. I also found it difficult to find exactly where these cuts should be, and didn't want to cut too much. I ended up having to sand 1/32".

A continued thank you to everyone contributing to this thread.


[1] Pseudo Quote from Phil Barnes "Old mylars never go away, they just get smaller". :)

[2] Aegea 130 build thread (http://www.rcgroups.com/forums/showthread.php?t=312944). See last picture in post #7 (http://www.rcgroups.com/forums/showpost.php?p=3064147&postcount=7) and post #8 (http://www.rcgroups.com/forums/showpost.php?p=3064159&postcount=8) for how I've used peel-ply & paper towels.

[Phil Barnes]

For simplicity and to help in getting the cores located in the beds I definitely recomend not doing the peel ply breather stuff at the LE.

I use a breather and release system that minimizes the thickness of stuff that comes between the part and the vac bag. I use ordinary, inexpensive 4mil drop cloth plastic bought at Home Depot for the release film. Attached to that is some very thin "batting" material that I bought at the fabric section of WallMart. I don't know what it is actually called but it is the thinnest, white batting type material that they had. It's definitely thinner than 1/32". The batting is attached to the release film with a spray of 3M77. I make two strips of batting, each about 10" wide on the release film. The batting strips are separated by an inch or two. This release/breather is wrapped around the part. One breather strip ends up on top and one strip on bottom of the part. Only the plastic wraps around the TE of the part. The only thing that ends up between the mylar and the bag is one layer of the release plastic and the very thin breather. This makes it easier to locate the part in the beds for curing and is exactly why I do it that way. To help aid vacuum distribution I add a very thick strip of breather next to the part but still sitting on the thin breather. The vacuum nipple always lands on the thick breather and the thick breather is folded under the release film at the end to connect the top and bottom thin breathers.

I never think in terms of using a bleeder of any sort to soak up excess epoxy when doing vac bagged wings. I always think in terms of getting the correct amount of epoxy in the layup before putting the part in the vac bag. Whatever oozes out of the part under vacuum is very minimal but just enough to need cleaning off the outside of the mylars sometimes.

I'll have to go back and read that Aegea build thread. I think that was done before I started following this particular forum.

[target]

Cleaning the backs of mylars? I've never done it. But then again, I've never built the same plane twice before. :rolleyes: There's a little epoxy on the back of mine, but nothing sharp that would puncture the bag. If I were in production, I could see the need to reduce buildup.


I've found that if I wax BOTH sides of the mylars ONCE before the first use, the excess epoxy comes off the backside easily.
Phil-
Any reason that I SHOULDN'T wax the backside?

Thanks,
Target

[markdrela]

Maybe Mark can tell us how he opens up the hinge gap on those prefaced hinges after bagging without making Kevlar fuzz mess. The extra kevlar is easy to cut off with a new single-edge blade, because it is very firmly supported by the edge of the glass facing. Using a sawing motion with the blade seems to help.
In the situation you described, the two adjacent cuts are difficult because the kevlar is supported only by squishy foam.

[dougalert]

Thinning leading edges of the mylars really makes final finishing easier. The best thing that I have found for doing this is with a cabinet scraper. To see if this will work for you, take a new Stanley knife blade (a single edged razor blade is too thin and brittle) and draw it back and forth across a round, smooth piece of steel such as a large screwdriver. Start at 45 degrees and work your way up to 90 degrees. You will feel a burr develop. This is your cutting edge. Just pull the blade along the edge of the mylar and is will cut a thin curl with each stroke. The Stanley blade is hard to hold and will only last for so long. If you find that it works for you, google "cabinet scraper" to find out where to get one and how to sharpen it. Easy to hold and it will last forever.
Thanks for all the great info.

[Phil Barnes]

Any reason that I SHOULDN'T wax the backside?Not that I can think of. That's just one of the things I've never tried.

[nuevo]

Thought I'd show a few photos of the rudder & stab I just made. Not the prettiest. Here they are up close. Hopefully the next parts will be better, after all the great lessons I learned today.

Sorry the focus is not top-notch. Hard to get the autofocus to behave.

[Salto]

Thanks Jon for a great thread, and thanks Phil and others for the update on bagging leading edges. I've just finished a Supergee (and am now doing a second one for a friend), and I'm planning a Supra next so this is all timely information for me.

Like others, I find the leading edges the most challenging aspect of bagging a wing. I understand the logic of the different methods shown in Jon's 3 sketches, but neither method seems the "right" solution to me.

On the Supergee, I used method 3, with composite fabric strip, thinned Mylars, no peel ply, full vacuum etc. Generally it worked OK, but in some places the composite strip bulged forward into the cavity left between the Mylars. I'm confident that I can avoid this with practice, and probably by getting the strip in place sooner so the 3M bond to the foam is stronger, but there seems to be fundamental problem here in that the Mylars will either be pushed onto the LE and leave a ridge, or will create the cavity which effectively prevents the release film and bag from directly pushing the LE strip onto the foam.

So, I'm wondering if there is a better way. If turnaround time was not an issue, do you think that a two step process would work where you bag just the LE strip onto the core (without Mylars), possibly using peel ply or just smooth release film, then in a second operation bag the skins with the usual small overlap onto the LE strip. This would result in the LE strip conforming exactly to the foam core at the LE, with no possibility of bunching or blistering forward away from the foam. The Mylars would still be thinned at their leading edges, and would extend just past the forward edges of the skin in the normal way so the skin bagging operation would result in a very smooth tapered end to the skins right over the LE strip. Hopefully, no sanding or trimming would be necessary.

Any comments? Has this been tried before?

Graham.

[Phil Barnes]

Hinge line fuzziness

Next time you might avoid that fuzziness by using Mark's method of sawing the skin off from the hinge line area with a razor blade. It sounds like that would work as long as there is some skin overhang still rather than just fuzziness like you have now.

To deal with the fuzziness I would first see how much I could snip off with small curved scissors. CST sells these small, inexpensive ones, part #E811 (https://www.cstsales.com/CuttingTools.htm). Just modify them for cutting Kevlar in the usual way. Run each blade across a spinning 60grit sanding disk with the blade about perpendicular to the disk. This will rough the blade up and square it off which prevents the Kevlar from slipping between the blades rather than cutting. This method of making kevlar scissors works with almost any stainless steel scissor, preferably with a tight fitting hinge to keep the blades tightly pressed together. Any remaining fuzz should sand off with 220 or 320 grit and light sanding. You might need to use some epoxy or foam safe CA on some fuzz if it does not come off before sanding into the stuff you want to stay. After hardening the fuzz up with the adhesive, it should sand off.

Stab leading edges

It looks like the Kevlar skin has extended too far forward and is also not bonded to the LE of the core. Next time keep the skin further back on the mylar so it doesn't need to be trimmed back. I would try sanding off the loose skin first. If you can sand the kevlar back a bit further until you get to where it begins to be bonded then that would be a good move. Sand with new 220 grit to remove the loose skin. Once you get back to where it is bonded switch to light sanding pressure in an attempt to remove as much fuzziness as possible. Then switch to 320 grit and finer grits. You may need to harden some areas of fuzz with CA or epoxy then resand.

If you can't sand the skin back far enough to get to a bonded area then you need to think about using more vacuum, more thinning of the mylar LEs or more epoxy in the LE area next time. For this stab you will need to work epoxy or CA under the loose edges to bond them then proceed with sanding.

Next time

Make sure that you won't need to sand the main Kevlar skin by keeping it far enough back on the mylars. With a new mylar, here is what I do; I cut the mylar to size and then lay it on the core which is to be bagged. I line the mylar up exactly as it will be when I bag the core. I can see through the mylar and see where the LE fabric on the core is. I then just make some little tick marks on the back of the mylar that will mark the midway point of the LE fabric. I can then use a straight edge and my sharpie marker to connect the dots and mark the exact location where the skin fabric needs to stop. This will ensure that the skin hits the LE fabric but does not extend too far forward. If you are using a painted mylar that you can't see through then you can do the same sort of line but you'll need to draw it on the paint with a red sharpie. The red probably won't show on the finished part, the black would bleed all over the place. In practice I don't generally need lines like this. I can judge the correct distance from experience but for the first few parts it might be a good idea.

When you see unbonded skin at the LE as in your stab photos, it indicates a problem of some sort. My first reaction is to sand the skin further back. This is fine with carbon and glass skins. My first reaction if I see a Kevlar skin like that generally involves profanity, sanding it back comes later. If the sanding back process begins to go farther than I like then for me it generally means I had low vacuum on that bag and the mylar was not forced down tight enough to conform as far forward as usual. I usually run 12 bags at a time on a single pump with a series of valves and hoses. My vacuum gauge shows me the vacuum at one point in the system. If one particular bag has a big leak then the vacuum in that bag could be far lower than the gauge says. That's when I start changing out bags so it won't happen again. For me, the problem has never been too little epoxy at the LE but I can see how that could be a problem if you didn't carefully saturate the LE fabric.

[hohensej]

This is a great thread, I have been using method 1 for the longest time but with fiberglass skins. I have two questions:

1) after tacking the leading edge composite, is it wet out or do we assume that it gets wet out as epoxy gets sucked out of the skin material?

2) Why is kevlar being used as a skin material? Is it only because it is tougher? Isn't it heavier then fiberglass/carbon?

Thanks

[Phil Barnes]

Graham's LE problems

The problem you are having is that the LE fabric is not sufficiently bonded to the core before you bag the core. This is a problem that is caused by not using enough spray adhesive or taking too long to get the LE fabric in place so the adhesive is too dry and does not transfer to the core.

You might also try Mark's idea of ironing the LE fabric in place after sticking it on with the 3M77. The heat will reactivate the adhesive and make it stick better.

The mylars should always be sized such that they cannot leave a ridge at the LE, they always extend to or slightly beyond the LE point. If they do stop just slightly aft of the LE then the small ridge is easily sanded off. You should never be thinking of the bag or release film being used to press on the LE. I should say that the above applies to my personal bagging methods. Other guys have success with other methods also but I'll leave them to describe their methods.

There have been a few occasions when I've wetted out a LE and then realized that for some reason I'm not actually ready to bag that particular core. In those cases I've just set the core aside to let that LE epoxy cure. I have bagged those cores at some later time in a separate operation and had no problem with it. So your idea of curing the LE in a separate operation should work. I don't think you should need to bag it though. Just wet it out, maybe wipe excess epoxy off with paper towels, and set it aside to cure.

[markdrela]

I'm confident that I can avoid this with practice, and probably by getting the strip in place sooner so the 3M bond to the foam is stronger, but there seems to be fundamental problem here in that the Mylars will either be pushed onto the LE and leave a ridge, or will create the pocket which effectively prevents the release film and bag from pushing the LE strip onto the foam.
Soon after you apply the LE strip, iron it with a 160-170F iron. This melts the 3M-77 and makes the strip adhere very firmly to the foam. I also iron the hinge-facing glass, and also the glass strips holding the front and flap pieces of the core together.

With the integral hinges and the wrapped spar, there is a big time investment sunk into the cores before bagging. You don't want to waste it by having the LE strips, hinge strips, or connecting strips coming off all over the place during the wet-out and wrecking the layup. The ironing goes fast, and is very cheap and very valuable insurance.

As further insurance, I wet out the LE strip first, but only along the LE radius, leaving the strip edges dry initially. Same with the hinge glass strips. I wet out the dry edges as the very last step, preferably with a fresh small batch of epoxy for fast saturation, and immediately put the wing in the bag.

[Phil Barnes]

1) after tacking the leading edge composite, is it wet out or do we assume that it gets wet out as epoxy gets sucked out of the skin material?Wet out the LE fabric also. See cautions in my response to Graham.

2) Why is kevlar being used as a skin material? Is it only because it is tougher? Isn't it heavier then fiberglass/carbon?It is tougher but it is also stronger/stiffer than fiberglass of the same weight. carbon is stronger/stiffer than Kevlar but it is not available in the lighter weights that kevlar cloth comes in.

[ASK]

Like others, I find the leading edges the most challenging aspect of bagging a wing. I understand the logic of the different methods shown in Jon's 3 sketches, but neither method seems the "right" solution to me.

On the Supergee, I used method 3 ...Generally it worked OK, but in some places the composite strip bulged forward into the cavity left between the Mylars... but there seems to be fundamental problem here in that the Mylars will either be pushed onto the LE and leave a ridge, or will create the cavity which effectively prevents the release film and bag from directly pushing the LE strip onto the foam.3M77 can and should be used pretty liberally on LE strips. I used to think "mininum required, so it won't block the epoxy." Now I'd say it isn't necessary or even beneficial for the epoxy to penetrate the LE strip fabric all the way to the core. If the epoxy can get that far, you haven't used enough 3M77! (But you still need to wet out the LE with epoxy)

More than any other cause, I think it's not using enough adhesive that contributes to the LE strip releasing from the core and "bagging down" to the leading edge. Or as Graham says, "bulged forward into the cavity between the Mylars".

As to mylar overhang, what Phil says.

[Salto]

Phil, Mark and ASK, thanks for your responses.

I'm using 3 or 4 VERY heavy coats in a row on the LE strips, and I estimate I get them fully on the core within 1.5 - 2 mins of putting the spray can down. I find handling the strip with tacky 3M on my fingertips very fiddly and more than a touch frustrating, and I'm not too confident of being able to do this any fatser. But I think the heat iron trick will be just the right solution!!

Mark, that's also a good point not to let the epoxy sit too long and soften the 3M bond. I had been soaking the strips on the LE first off in my bagging sessions, so I'll now do that later in the process.

Phil, after reading your comment about just leaving the LE strip to cure on it's own, I may try the 2 step process sometime. But in the mean time I'm bagging a Supergee wing half tonight, so I'll try out all the new suggestions - thanks again guys.

Graham.

[Salto]

Hinge line fuzziness

With a new mylar, here is what I do; I cut the mylar to size and then lay it on the core which is to be bagged. I line the mylar up exactly as it will be when I bag the core. I can see through the mylar and see where the LE fabric on the core is.
Phil, with the above method, when you place it all in the bag how do you ensure that the TE of the Mylar gets placed correctly relative to the foam core TE. Do you size the Mylars exactly to the core TE? If this is the case, after curing how do you trim the TE to where the foam core ends. There would be an epoxy ridge here which would make it difficult to trim at that point.

I've been sizing my Mylars about 1/2" longer at the TE so I can get a clean knife trim line just aft of where the cores end. But maybe I'm doing this wrong?

Graham.

[Phil Barnes]

More than any other cause, I think it's not using enough adhesive that contributes to the LE strip releasing from the coreThis is what I've said in countless posts in many forums. It is the point that many people just don't understand. There are tricks that can be used to make up for insufficient adhesive or to help guarantee there won't be a problem with the LE fabric getting loose. These tricks include ironing or special ways to apply the epoxy like Mark pointed out. But if you learn how to use the spray adhesive then you don't need these tricks.

Here is something I've said before also: There is no such thing as too much spray adhesive on the LE fabric. If you start to apply the LE fabric to the core and it swims around on the core instead of sticking then that is the point when you have applied a bit more adhesive than the LE fabric needs. It still isn't too much because all you need to do is wait a few seconds and the fabric strip will begin to stick as the adhesive dries.

I agree with ASK, the epoxy certainly does not penetrate all that adhesive and get to the core. If it did you would have the nightmare scenario where the LE strip falls off as you wet it out with epoxy.

I find handling the strip with tacky 3M on my fingertips very fiddly and more than a touch frustratingThis then is the problem. I will attempt to describe the correct technique although this is something that is screaming out for pictures or better still a video. (http://home.paonline.com/hayman/video.htm)

I hang the fabric strip in the back of a tall, narrow spray booth which is just a cardboard box standing on end. I spray about 3 or four passes with the 3M77 spray adhesive on the fabric strip. It is quite a heavy coat of adhesive. The fabric strip is just loosly tacked to the spray booth (very loosely) so that I can pick up one end of the fabric strip and peel it away without distorting the fabric. I hold the core in one hand and the end of the fabric strip in the other hand. The fabric strip needs to hang down freely without touching anything. I hold the core up vertically and begin attaching the fabric strip at the root end of the core which is held up highest. I hold the core and fabric strip such that the fabric strip can only touch the root end of the core on the point of the LE. The core angles back away from the fabric strip and the strip is attached carefully along the point of the LE. This is the critical part of the operation. You need to work from the root end to the tip until the fabric strip is tacked to the point of the core LE and is centerred. Once that is done you just need to press the rest of the fabric down around the core. OK, I give up. There are some things that words cannot describe adequately. This really is one of those things that is quite easy to do once you know how but is very hard to describe.

when you place it all in the bag how do you ensure that the TE of the Mylar gets placed correctly relative to the foam core TE. Do you size the Mylars exactly to the core TE? If this is the case, after curing how do you trim the TE to where the foam core ends. There would be an epoxy ridge here which would make it difficult to trim at that point.When I place the cores in the mylars I look at the LE and align the mylars there. That is the critical alignment that needs to be checked. The mylars are sized such that they extend past the TE some distance. I aim for 1/8" to 1/4" past the trailing edge of the core. This dimension is far less critical and can vary quite a bit. The only downside to having a lot of TE overhang is that it makes it a bit harder to keep the TE of the part straight and harder to locate the part in the beds for curing.

Trimming the TE after bagging

This is done with a snap-off knife blade and a straight edge or template. I trim 1/16" aft of the core TE.

[Salto]

When I place the cores in the mylars I look at the LE and align the mylars there. That is the critical alignment that needs to be checked.
Once again, thanks for your comments Phil. This is indeed how I understood you were doing it, but your earlier post made me think you had a new method based on TE registration or something.

I have watched your video many times, and found it immensely useful. I just need more practice at handling the LE strip. Tonight I bagged the second wing half, having first ironed down the strip and I'm a lot more confident that I've got a neat leading edge.

Jon, I'll stop asking questions now and let your thread get back to more Supra build stuff :). Keep those pictures and descriptions coming!

Graham.

[BrianSmith]

While all this is "WAY" over my skills and abilities, I'm enjoying it to the max. Great job Jon. Brian Smith

[nuevo]

Graham, Your questions not are off-topic for the discussion. No problem.

Brian, building like this is not beyond your abilities. I hope seeing all the photos shows lots of folks who've never tried vacuum bagging how straightforward the process is. Now there are lots of finer points, as Phil, Salto, and Mark have discussed recently.

Still, a highly functional and good looking plane can be made without the "finer points" for the perfect leading edge. All of my past bagged planes, which you've seen, are an example. I just didn't know a better way to do the leading edge. I'll tell everyone a story about my first composite skin vacuum bagged plane soon.

Phil, Mark, Aradhana, and other contributors, We are indebted to you. I believe we've been given a PhD course in the finer points of vacuum bagging this week. It's been many years since I've had graduate level courses. Hope I pass the class. :o

Everyone, thanks for your encouragement, PM's, and advice.

[nuevo]

OT: long story of a plane gone bad... or not.

Ok, here's the story. I had vacuum bagged one balsa skinned plane befor. I am now about to vacuum bag my first carbon skinned plane. I've made a few small sample pieces and think I have the process figured out. Even painted the mylar on some of the samples. My father-in-law, also an RC pilot, wants to come over and observe/help. So on the appointed evening he comes over, and I begin wetting out the cloth, and all the steps.

My goal for this plane, which I designed and built some 5-6 years ago, was to be a lightweight 3m plane for TD competition, but still not sacrifice cruise ability. Light weight improves minimum sink and can lower landing speed. L/D gets me across the sky & back from downwind. If weight is kept down, then lower camber required to generate required lift. So I chose a thin, lower cambered airfoil. To the Drela design fans, these themes should sound familiar. :) I had never heard of any Mark or any of his planes at that time.

Anyway, I started the wetting out of cloth. Pushing the resin around with an old plastic card, and scraping the extra off for the first wing. Unlike my previous tries, this time the resin seemed to not wet out the cloth well. And I had to work a lot harder to get the extra resin out. I was trying to keep up my pace before the resin starts to set, so I kept working. Well, a while later, we get the whole wing done, and finally in the bag. THEN my father-in-law calmly says, "You used a lot of epoxy. Look at all those cups you used over there." Oops. :o :o

We had been working in a 55-60 degree garage and I never thought to heat the garage or the resin. It was too thick for proper wetout & removal. In hindsight, I probably could have gotten out a heat gun, or moved the mylars indoors and gotten the extra resin out. I just did not think of it at the time.

When the epoxy had cured and the mylars came off, the wing had a beautiful shiny finish. It should, it had plenty of epoxy. :rolleyes: Well, the carbon has been paid for, and I now have one wing. On with the next. I learned my lesson about temperature and went on to build the next wing, and kept the weight down. The 2nd wing was ~5 oz lighter than the first one. :eek: I can't remember the exact weights of the finished wings. Something like 13 oz and 18 oz. Quite a bit of difference for a 122" plan plane.

I ended up adding ~3 oz of nails embedded in the other wingtip, to get the plane to balance laterally. So my stupid mistake ended up adding 8 oz of unneeded weight to the plane. My goal was to get the whole plane under 70 oz. It ended up at 78 oz. So much for the lightweight plane. :(

What I ended up was a plane that tracked really well :D, never upset by wind gusts. The plane was not the light air wonder that I had hoped. But launched high, and was superb on windy days. I flew the plane for 2-3 years in TD contests in my area. Overall, I flew quite well with it. I even won one of my LSF 5 contest wins with that plane, against some very talented competitors.

All this to say, even a heavy plane built by someone who barely knew what he was doing, can still perform ok.

[mlachow]

I have used a regular plane to taper mylar edges. Just make sure it's good and sharp and don't get too agressive in the cut. Low angle plane worked better.

Agreed. But mylar is very tough to sand. I used a planer-type setup, using a Dremel 1/8" cylindrical carbide bit jigged up at a shallow angle at the edge of a board. The edge of the mylar was then passed under the bit. This worked OK, but not great. If someone can come up with a better method let us know.

My bevels are roughly 3/8" wide. The edge is as thin as possible. The thin edge might end up being a bit ragged in a few spots, but this doesn't cause any problems.
.

[nuevo]

Not much activity this week. I'm prepping stab #2 for vacuum bagging.

Photo 1 shows the entire stab. I need one more application of putty to smooth around the spar rods.

Photo 2: I used a shallow angle of light to show the gouge I made with sandpaper. I was using very light pressure while sanding. Sometimes the putty "balls up" and then the ball makes a rut in the core. I often had to take 2 strokes with the sandpaper, blow off the dust, and repeat. The sanding process goes quickly, as the dried putty is softer than the core.

Photo 3: Same photo, but with shadows enhanced to show the gouge. Note I need a little more putty in the spar, in this area. Reality is not as bad as the photo shows, but using a shallow light reveals many flaws.

The gouge came out fine with a a wet paper towel & a covering iron. Took about 3 seconds.

[nuevo]

It's been a while since I've had time for building. Things like repairing the lawn mower, heat pump, etc. this weekend. :D

Now that I've seen the light, I'm adapting much of my bagging technique to follow Phil's ideas. I've made new slightly larger mylars for stab #2, and have made some breather wrappers.

It's been raining today, so it's too humid to paint the mylars. I paint in the garage, so outside weather has to be noted.

[mlachow]

Photo 2: I used a shallow angle of light to show the gouge I made with sandpaper. I was using very light pressure while sanding. Sometimes the putty "balls up" and then the ball makes a rut in the core. I often had to take 2 strokes with the sandpaper, blow off the dust, and repeat. The sanding process goes quickly, as the dried putty is softer than the core.

I use drywall sanding screen for sanding cores. Just be careful of the edges, but it usually reduces the ball-up stuff you get with regular paper. The other thing you can do is always sand in the same direction.

[nuevo]

I use drywall sanding screen for sanding cores.
Sounds good. I'll give it a try.

[nuevo]

Question for Phil.

From our previous comments, the mylar should extend slightly in front of the LE when not conformed, but not extend around the sanded edges near the tip curvature. Is this picture about right?

red - mylar
blue - core
blue at bottom - side view of core.

[Phil Barnes]

That's pretty close. Maybe a little farther out on the tip and loose the sharp corners on the mylar where it transitions from the LE to the tip.

[nuevo]

Thanks. Super! I'll do my final mylar cutting tonight.

[markdrela]

On my Supra I cut the mylars close to what you show, with Phil's mods.
But in retrospect I could have extended the mylars off the entire tip edge, i.e. treated the whole tip as a continuation of the leading edge. I've done this on my SuperGee layups and it works fine. The vacuum is strong enough to give the mylar the required slight compound curvature. I feather the edge of the mylar which probably helps a lot. The fact that the airfoil is so thin on the Supra tip also helps.

[nuevo]

Thanks Mark. I've never extended the mylar to where the tip curvature starts for fear of waves in surface, as the mylar is forced to bend in two directions. I've seen a few production parts with this flaw over the years. As you stated, thin airfoils and thinning the edges of the mylar may be the key.

[Jurgen]

Did anyone stacked (not glued !) thin mylars onto eachother to simulate a thicker mylar? This might avoid the previous mentioned "wave" because the individual layers can take a different bent form. The layers don't have to be exact same size too, so the mylar-thinning-job is also simulated in same effort. Just a thought.

[Salto]

Jurgen,
I think the reason the thicker Mylar is used in the first place is to simulate a very accurate mold surface. The stiffness of the Mylar evens out any imperfections (small, and large scale) in the foam core surface. Whilst your method would solve a potential problem at the tips, it would not give as good a result over the rest of the wing.

On the thin Supergee wings I've bagged, I used feathered .014" Mylar and did exactly what Mark suggested above, running the Mylars right off the tip. I used 25" to 28" vacuum and the tips came out perfectly. After 3 sets of wings, the Mylars are now assuming a nice compound curve in the tip area.

On another point raised in an earlier post, I've had great success cleaning foam cores before, and as I sand them using "Scotch Brite" cleaning pads. The open structure of these pads holds balls of foam and filler and carries it away without scratching the surface.

Graham.

[nuevo]

I put stab #2 in the vac bag tonight. Here are some photos.

You can also see where the bottom cloth is a little short on the TE mylar. The mylar extends beyond the core, and the cloth should be ok. I won't trim the cloth this close next time.

Changes in my process, so this is a first. kevlar does not come to LE of mylars.
trim shape of mylars & mylar positioning (discussed earlier)
a little extra epoxy on TE

This is the last time I wet out using a plastic card, at least with this lightweight cloth. Too easy to distort the cloth. I'm going with the rollers for wet out, as Phil demonstrated in his video.

The more I do this, the more I realize the wisdom of Phil's techniques.

[Phil Barnes]

Just a quick post (I hope) to outline an easier way to do that layup:


Stick mylars to the bench with light spray of 3M77 spray adhesive
Roll epoxy on the myalr with foam roller
Drop Kevlar with paper backing on the wetted mylar
Roll more epoxy on top of paper. If you are lucky and the paper is only lightly attached then the paper will roll up on the foam roller and separate from the kevlar. The paper may also simply lift a little bit from the kevlar or a corner will lift off from the kevlar.
Lift one end of the paper. In one fast motion you can pull the paper off the layup. This is the same as removing a bandage from a hairy arm. It must be done fast in a single quick motion. The paper will come off without distorting or moving the Kevlar. The kevlar is imobilized in the epoxy and cannot move quickly so the paper comes off before the kevlar can move. If you pull the paper slowly or if the Kevlar is not wetted with epoxy then then the Kevlar can move.
Finish wetting the Kevlar and roll with the foam roller until the layup is evenly wet out
Roll the layup with a hard roller to mash out the Kevlar fibers and close the gaps in the weave
Dry the layup with paper towels in a vacuum bag. (This technique recently written up in the HL forum)
Add extra epoxy to the TE area of the layup


I have posted this information before. Probably in the HL forum (http://www.rcgroups.com/forums/forumdisplay.php?f=96) but I don't have time to search it right now. I hope the short hand version above will suffice for now.

[kak8]

Is it really necessary to dry the layup? The mylars should press the epoxy out in vacuum? I`m a beginer so what do I know :-)