STH7 - 'Kahu' Flying Wing (now with flight vid)- 70inch - Foam core and Fiberglass. - RC Groups
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Sep 09, 2012, 10:58 PM
Kiwi in Australia
steveobhave's Avatar
Build Log

STH7 - 'Kahu' Flying Wing (now with flight vid)- 70inch - Foam core and Fiberglass.

Having recently built a Divinity II and getting a huge amount of joy from it, I decided that it was time for the next step. Divinity is a great flyer and it has really served it's purpose but she doesn't slow down well and is a little ballistic on landings. I suspect that this is down to a combination of using papered foamboard and having a camera mounted on her in addition to a 2200mah battery.

Subsequently I have started work on a full composite wing. It's 1800mm wingspan and around 420mm root chord. She'll be getting full winglets based around the Long-EZE design which is a little special if you know anything about winglets. Long-EZE winglets are flying and actually add to the net thrust of the plane. they are also slight canted inward to the centre of the fuselage and make a drastic different to the performance of the plane.

The primary reason for doing this is to have another go at the vacuum bagging process that went somewhat awry in another ongoing build found here:

The size constraints are based on the foam that I had at hand which also accounts for the rather different way of adding sweep. The centre section is of a denser foam to hopefully help with structural integrity when I have to cut into it to install the electronics.

Cores cut

Centre secion added and flashing removed

Sweep added for better pitch control

Ends squared and trimmed and sections glued

Foam covered with micro to help create a chemical bond between the foam and the glass

Wetting process - I've gone with a 45 bidirectional weave with a unidirectional saddle added for longitudinal strength. This will have no carbon in it at all. The skin is the structural strength.


Pump attached and then added to the tent to cure

Extracted from the tent and it'll be left to complete curing till next weekend!
Last edited by steveobhave; Dec 16, 2012 at 05:13 PM.
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Sep 10, 2012, 11:54 AM
Chess club geek with wings
blunight's Avatar
Subscribed! Will you be posting dimensions and lay up details?
Sep 10, 2012, 05:11 PM
Kiwi in Australia
steveobhave's Avatar
For sure if there is interest of course. I'll be more than happy to give full details including the airfoil used. I'm kinda winging it (sorry) at the moment. It's all been based off the bits of foam I had lying around. I'll do a drawing of the layup specifics when I'm on a more capable computer (using a tablet at the moment).
Sep 11, 2012, 10:05 PM
Registered User
gazza2003au's Avatar
hey steve i have a vac bag set up but have never vac bagged i got some help setting up my machine in the aussie forum some very helpful people on RCG people also said the best way to glass a wing is the cut the leading edges off and use balsa later on as glassing the leading edge creates all kinds of problems also u should use the wing beds to sandwich the wing cores in once its bagged and weigh it down to stop the wing cores from warping and the give the glass a good adhesion to the cores
Sep 11, 2012, 11:52 PM
Kiwi in Australia
steveobhave's Avatar
Yeah you could well be right about the wing beds but the way I have cut them didn't allow for reusing the offcuts. They were cut stacked so the individual pieces wont fit around the wing. To be honest there is little you can do in terms of adding weight for adhesion because the pressure from the bag far exceeds any pressure you can achieve otherwise. Warping would be the biggest concern but the thickness of the cores doesn't leave much space for flexing. In a glider wing you'd be dead on.

Leading edges were a bit of a pain because I didn't overlap the glass as much as I would've liked but instead of allowing for the release film to go straight out I've wrapped them around. This will likely lead to a crap leading edge but it's all experimental at this stage. Worst case scenario I'll just sand it back and then put another layer of bidirectional around it then use peel ply to finish it.

Adhesion is hopefully taken care of by using micro on the foam. The Micro already has the epoxy as a component so in theory the wet glass should happily bond under the vacuum pressure. Of course I could be way off target and I could end up with a pretty rubbish looking wing and another failed glass layup. We'll see this weekend.
Last edited by steveobhave; Sep 12, 2012 at 01:38 AM.
Sep 12, 2012, 07:18 AM
Chess club geek with wings
blunight's Avatar
Steve I'm not familiar with the product your using called "micro" could you explain it?

I'm no pro but here is how I've been doing my leading edges:

Cut 1" wide (25mm) bias cut strips of fiberglass; cut enough strips to go around the whole plane, dust them with the 3M77 then stick them to the part. This way you have the edges done before you begin mixing epoxy.
The 3M77 keeps the strips in place and will disolve once saturated with epoxy.

If your concerned about the epoxy and fiberglass sticking to the foam, paint the foam with water based acrylic paint before fiberglassing - it will help the bond and allow you to color your work of art.

I've inclueded photos of my current project- the foam I used was pink Dow 25 psi
Sep 12, 2012, 07:43 AM
Kiwi in Australia
steveobhave's Avatar
Very nice indeed.

Micro is a mix of Microballoons and epoxy, it's extremely light and great for filling holes and finishing glass before primer. It's like lightweight spackle I guess but because it has a matching chemical makeup I know it plays nice with epoxied glass. I'll post pics of the wing when it comes out of the bag.

Lovely work on the paint BTW - looks great!
Last edited by steveobhave; Sep 12, 2012 at 07:59 AM.
Sep 12, 2012, 10:17 PM
Kiwi in Australia
steveobhave's Avatar
We have a name It is going to be called 'Kahu' - it's the Maori name for the New Zealand Harrier Hawk.

Photo credit:
Sep 12, 2012, 10:41 PM
Registered User
gazza2003au's Avatar
Originally Posted by steveobhave
We have a name It is going to be called 'Kahu' - it's the Maori name for the New Zealand Harrier Hawk.

Photo credit:
nice steve when u make more wing cores mate check out my video my cores are made from styrofoam or extruded polystyrene the same stuff u used for your wing my wings come out smooth no ripples at all if u need a hand shoot me a PM in also in sydney

cutting foam flying wings with hotwire cutter (3 min 43 sec)
Sep 12, 2012, 11:01 PM
Kiwi in Australia
steveobhave's Avatar
Thank you - I appreciate the offer I know why mine came out a bit ripply, the wire wasn't tight enough but we cut them both at the same time. A few ripples aren't the end of the world anyway, nothing that a little sanding can't fix. I have a pretty experienced person working with me - he's building one of these:

And he's also worked with Dave Ronneburg who's responsible for the Berkut:

The complexity is coming from the bagging process but we're feeling it out as we go I appreciate the comments though, it's all great stuff and food for thought.
Sep 16, 2012, 08:04 AM
Kiwi in Australia
steveobhave's Avatar
Another fairly productive weekend done. Arriving Friday afternoon at the hanger and of course the first thing we did was go check out the bagged wing.

There were a couple of lessons to be learned from the result. The first being, vacuum bagging with release film does not produce a beautifully smooth finish. What it does do though is wick off a great deal of the excess epoxy.

All in all I was pretty stoked with the outcome. The other lessons learned were, don't overlap the release film, don't leave gaps because the wicking material sticks to the glass like poop on a sheeps bum and make sure that the bag is FLAT! Any wrinkles lead to epoxy ridges - DOH!

Nothing a little bit of sanding didn't fix pretty quickly.

All the flashing cut off and the rough bits sanded back. I was properly stoked with the state of the trailing edges, the glass adhesion and also the overall finish. It's so far ahead of my last bagging attempt. The work that will be required to bring this to a paint ready finish is incomparable.

The next trick was winglets. The winglets were cut out using the technique demonstated by Gazza2003au! Cracker of a way to do it especially when you're doing the work alone. Made for a lovely result - thanks for that Gazza.

Glassed - I've opted for a very light standard weave glass here as an experiment - again I've used a thin application of micro for adhesion and it really makes all the difference for layup because the glass sticks to the foam but is easily repositioned.

Bagged and this time the bag was FLAT!

Joining area cut on the wing. I'm nesting the winglets which is probably asking for trouble if I ever crash it and snap one off but a blended winglet is way prettier than an endplate, also more functional aerodynamically.

Winglets out of the bag - again properly stoked with the outcome. The bag does a great job of pulling the glass around edges and curves. Well impressed.

Cleaned up and looking good - it's thin glass so I've had to be very careful with my sanding.

As you can see here, these are not your run of the mill winglets - they are actually flying winglets with a genuine airfoil. The running story with these is that they actually provide a net addition to the thrust of the plane as well as additional lift similar to if you were to extend the wing the same distance as the winglets are tall but without sacrificing the space.
Last edited by steveobhave; Sep 16, 2012 at 06:40 PM.
Sep 16, 2012, 08:10 AM
Kiwi in Australia
steveobhave's Avatar
Micoballons (Micro) and epoxy mix at the ready as a glue to attach the winglets and to also allow me to sculpt the wingtips before applying structural glass.

Micro applied

Basically I have just squished the suckers on then blurped on a bunch of micro to form both the transitions between the wing and the winglet and also to form up the wintip so that it blends nicely aerodynamically.

And she's looking ready to go... to the emergency room... It ain't pretty but it serves a purpose. The winglets need to be at a fairly precise angle for a couple of reasons. First, because they are flying winglets they need to be 'lifting' in toward the centre of the wing, second they need to have a slight inward 'cant' to counter any dihedral effect they may have. To retain manoeuvrability it's important to not have the winglets creating too much stability. Leaning them in toward the inside of the wing reduces this effect... apparently...

Because I'm going to a fair amount of effort to get this looking tidy, I am a bit concerned about belly landing it (especially if it has a nice paint job) so I've opted to put a single recessed wheel just forward of the CG similar to what you'd see on a glider. I'll also put some small skids or similar on the wingtips to protect them.

To make the fairing for the wheel I'm going to use a mould and glass it. This is the mould base:

Rough blend done with micro - it'll be sanded to look nicer next weekend. Following sanding I'll cover it with tape and then apply the glass over the top. Once it's cured I'll ping it off and then cut out the hole for the wheel to poke through. Slightly less than half of the wheel will be recessed in the wing. I'm hoping that the advantage of a nice roll out on landing with outweigh any aerodynamic losses.

This is the wing following it being removed from the curing oven and after a little bit of sculpting. Not looking too bad if I don't mind saying so myself!

And finally the mould and the wheel sitting on the wing.

All of this is really just a precursor to re-visiting the motor glider and the trainer in my other thread.. Not being comfortable with the techniques I decided to make some test pieces. Not wanting to waste anything I've ended up with 3 separate projects - I'm turning into the worlds best starter... hopefully I can finish some of these some time soon More next weekend!
Last edited by steveobhave; Sep 16, 2012 at 06:41 PM.
Sep 16, 2012, 10:12 PM
Kiwi in Australia
steveobhave's Avatar
Some dimensions and initial layup details.

After doing some calculations and some estimations on payload I've come to an AUW of around 3kgs which is a little hefty (Kahu weighs 1.6kgs now so I'm accounting for paint and electronics of around 1.4kgs or around 50oz. I don't know if this is too conservative or not.) but it has an effective wing area of 1060sq inches. I could've made her a bit lighter by using lighter glass but as Kahu has no internal carbon or glass I'm using the skin as the structural strength much like many composite aircraft do.

Using the venerable Zypher II as a yardstick she'll have a wing loading of 14.26 oz/sq. ft compared to a fully laiden ZII of 14.96oz/sq ft so if the way that the Zyph flies is anything to go by this should be pretty happy in the air.

Sep 17, 2012, 03:34 AM
the answer 42 is
You should use Mylars for the lamination process, in that way your finish is perfect out of the bag saving you weight as well

my two composite cents

Sep 17, 2012, 03:50 AM
Kiwi in Australia
steveobhave's Avatar
Yeah I was wondering about that - when you use mylars does the excess resin make its way to the edges or do you just have to be really careful about not wetting the glass out too much? Also I'm not sure I have wide enough mylar to cut single pieces nor is it flexible enough to make it around the leading edges? I know mylars work well for thin airfoils like sailplane wings but I would've thought it'd be a bit harder on a large thick wing?

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