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Old Nov 19, 2012, 12:51 AM
Kiwi in Australia
steveobhave's Avatar
Australia, NSW, Carlingford
Joined Mar 2012
245 Posts
Unfortunately I have no good news at this stage. The small glider was an abject fail. The main problem that I can see is that once it reaches a certain angle of attack on the bungee launch it utterly stalls out and becomes a pancake dragged through the air. After about 4 attempts to get it launching true one particularly hard 'landing' snapped one wing clean off. I'm going to fix it but it will take some tinkering because of the carbon spars snapping clean through. I theorise that part of the problem is that with a normal glider there is a horizontal surface keeping the plane flying through the air whereas with this there is nothing to do that other than the wing continuing to fly... which it doesn't do.

Just for reference this is the one I'm talking about.


Kahu unfortunately has equally bad news. I decided to have another go at chucking it and discovered that in hollowing out some of the foam that it has structurally compromised the nose. End result is that after too many nose down landings the overall root chord got reduced by close to an inch... woops.

Here's a vid of how not to launch Kahu. The first toss wasn't too bad - the second... well you can see for yourself.
Kahu test flight fail (1 min 45 sec)


Plane damaged and quite disheartened I left it for a week. Coming back to it I've done some repairs which should both make the nose somewhat more 'crash resistant' and also give me something to anchor the much needed bungee hook to.



What you can see is the already completed saddle patch which is 3 layers of Uni running front to back with one layer of 45 degree bi-directional to regain a little more overall structural strength. The aim is to push the stress from landing back over the surface of the wing away from the holes cut for the battery bays. The white fabric is peel-ply to give the epoxy and glass a nice smooth finish that requires a minimum (if any) sanding. It just peels off once the epoxy has gone off.


Now I know that I said that I wouldn't build anything else till I had finished some of the other planes but I got so knocked back by my crashes that I needed a couple of quick 'wins'. I built a flying wing out of another abandoned project. I give you Buster Kestrel It flies brilliantly!







And a rangi airboat... this one doesn't fly but it's a bit of fun for tearing around on the ground.
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Old Nov 23, 2012, 12:28 PM
The only R/C pilot in the city
Scorpio9999's Avatar
Kazakhstan, West Kazakhstan Province, Aksay
Joined Sep 2012
310 Posts
Launch method

Just an idea.
I'm not sure if this acceptable for your "Kahu".

Discus launch glider / F3K demonstration (5 min 11 sec)
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Old Nov 25, 2012, 02:57 PM
Kiwi in Australia
steveobhave's Avatar
Australia, NSW, Carlingford
Joined Mar 2012
245 Posts
Quote:
Originally Posted by Scorpio9999 View Post
Just an idea.
I'm not sure if this acceptable for your "Kahu".
Heh - yeah I tried a DLG launch, unfortunately it didn't have any inclination to stop flat spinning after letting it go... Nah it seem that the Bungee launch is the way forward. I have an interesting video that I need to edit up and I'll post here. Turns out that it was way too tail heavy. I'm thinking that a redesign is on the books. I need more scope to get weight further forward so I think that it's noses days are limited
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Old Dec 02, 2012, 03:36 PM
Kiwi in Australia
steveobhave's Avatar
Australia, NSW, Carlingford
Joined Mar 2012
245 Posts
Progress report! If it can be called that - it's been a bit of a case of one step forward, two steps back, but I think I've made a modification that is going to solve a number of issues including the rapid increase in overall weight.

Where we left off last time was repairing the nose after a good wallop collapsed it in a bit. The repair went well and added a bit more core strength to the wing. As I was to discover a little later, this isn't always a good thing.

Peel ply removed and in all not too shocking a job.


I mentioned that my other experiment in progress, the DLG flying wing become bungee launch wing, didn't turn out as well as I'd hoped so I thought I'd better get that fixed before chucking anything else into the air.

Main wing hot melted to a 'splint'.


Micro balloons used as 'glue' to stick the bits back together.


Not too shocking - clean it up and glue some CF rods back in.


Reglassed and ready to fly/gracelessly fall out of the sky again.


As a side note, there are some definite issues with the design that I have plans to rectify. After talking to my father, who is usefully an aircraft engineer, he suggested two things. One, that the leading edge on the airfoil is too sharp leading to sudden loss of laminar flow at high angles of attack leading to the nasty stall tendancy and two, that a bit of twist in the wing will help dampen both the stall and the spin. It's still a work in progress but as a learning tool it's been really useful!

Back to Kahu!

Having had such an abject failure with getting the plane in the air I had decided that using a bungee was the way forward. My intention is to use a 'slingshot' bungee configuration as it seems like the easiest way to get double the pull out of one bungee. Now Peter M suggested that my CG looked about 30mm rearward. I had used one of those nice online CG calculators to generate my CG location so I was willing to try that out first. Hmm... I'll leave it to the video to see who was right...

Kahu second test flight (3 min 58 sec)


Eh... yeah. It seems there is something to be said for listening to the voice of experience! Peter M, I bow down to your wisdom on this one - The CG was seriously out of whack and I suspect that you're almost dead on with your 30mm call.

As I alluded to earlier, my previous patch job had only served to push the damage to the next weaker point on the airframe. It also shows that wherever you cut the holes, it's going to weaken the overall structure. Back to the repairs!

Got rid of the useless glass.


More patching - yay! On the plus side, it's structural weight rather than dead weight forward of the CG.


Taking the wisdom of Peter M on board I have reset my CG projections to about 30mm forward of the original and approached the problem with a broader modification allowance.

I've hot melt glued a block of foam core onto the nose and used it to gauge what it was going to take to get the CG to the projected point without needing to used deadweight ballast. The two 2200mah batts plus the washers add up to an overall weight that equals two 3000mah 45C 3C batteries plus a Contour video camera.


With this in mind I had an idea as to how far forward I would need to get a nose. The problem that I personally foresaw was twofold. I'm about to wreck my lovely wings 'purity' and how do you put a nose on a glass wing without damaging it's structural integrity?

The initial idea was to whack on what amounted to a square nosed extension of the root airfoil, essentially like cutting out the entire middle section of the front nose and then gluing an extended version on but this didn't appeal to me at all aesthetically. After going away and having a think about it I remembered the Boeing Lifting Body X-48B aesthetic and thought that I kinda liked it.



Solution in mind I set to with first cutting out a template and then transferring it to a block of foam.


Cut it out and then took to it with a belt sander to carve out a shape that I found fairly pleasing then cut out the leading edge shape. After cutting it out I glued some sandpaper to the leading edge of the wing and used it to make the fit a bit more true.


And there she is!


Hole drilled in the leading edge to run the battery harness through.


Glued onto the leading edge.


Glassed! Peel Plied!


And jobs a Goodun!




Finally I've cut in and hollowed out the battery bay.


Now as a solution I think the end result has played out to make it a slightly more interesting looking plane and ultimately it has a far more useful payload area. With the added leverage forward of the CG I can also afford to move the motor further back to quiet her down and to up the efficiency of the prop. My only concern is how well my solution would survive a lawn dart incident but we never plan to crash do we... well not often

Unfortunately I'm left with about a two week gap before I can get back to working on this but as far as progress goes, I'm pretty happy. I'll say this though, I'm damn glad that I didn't make Kahu all pretty before test flying, that would've likely completely broken my spirit!
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Old Dec 02, 2012, 04:03 PM
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Joined May 2012
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This project is getting more and more interesting...
I really want to see this wing in the air.It looks beautiful.Are you not worried about the weight?
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Old Dec 02, 2012, 04:15 PM
Kiwi in Australia
steveobhave's Avatar
Australia, NSW, Carlingford
Joined Mar 2012
245 Posts
Yeah for sure - every bit of added weight is less flight time. I've been a victim of my own design approach. Every fix is more weight but it's been a huge learning experience and things to think about with my next build. It's still hovering around the 3KG mark and I've had to step away from the foamy comparison - they're just not the same class of model.
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Old Dec 10, 2012, 09:39 AM
The only R/C pilot in the city
Scorpio9999's Avatar
Kazakhstan, West Kazakhstan Province, Aksay
Joined Sep 2012
310 Posts
I'm a beginner in this hobby and don't know many things.
But what I'm thinking when I see your project that ply walls is needed in all bays. They will strengthen the structure in a weak places and will not add much weight. IMHO

P.S. The nose is nice.
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Old Dec 12, 2012, 02:52 PM
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United Kingdom, England, Portsmouth
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just a small note, but i thought that when checking the cg on a flying wing you had to turn it upside down to check the cg?
i may be wrong but i did read something on this forum about it.
great wing great idea.
RCT
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Old Dec 12, 2012, 07:58 PM
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conbones's Avatar
United States, FL, Panama City
Joined Jun 2012
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with my wings i check cg normally " top up" and have had perfect results but im sure you would get the exact same results upside down to. the cog is the same on both sides.
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Old Dec 12, 2012, 10:37 PM
Kiwi in Australia
steveobhave's Avatar
Australia, NSW, Carlingford
Joined Mar 2012
245 Posts
Quote:
Originally Posted by Scorpio9999 View Post
I'm a beginner in this hobby and don't know many things.
But what I'm thinking when I see your project that ply walls is needed in all bays. They will strengthen the structure in a weak places and will not add much weight. IMHO

P.S. The nose is nice.
Thanks! I've thought about the ply thing and I'm considering making a box to stick in there but I'll see how I go with it as it is, there is zero flex in it as it stands.

Quote:
Originally Posted by rcta15 View Post
just a small note, but i thought that when checking the cg on a flying wing you had to turn it upside down to check the cg?
i may be wrong but i did read something on this forum about it.
great wing great idea.
RCT
Thank you, nah it's fine to measure a wing from the bottom.
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Old Dec 14, 2012, 12:48 AM
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Newcastle, New South Wales, Australia
Joined Sep 2007
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Keep at it Steve, you're nearly there!

Can I suggest you tone down your throws a bit more?

Also since your flying weight is around 3kg and you need between 4 to 5 times your flying weight in tension for the bungee to safely launch, I think making yourself pvc conduit launch ramp and foot release will also make your life easier.

Regards
Peter
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Old Dec 16, 2012, 03:50 PM
Kiwi in Australia
steveobhave's Avatar
Australia, NSW, Carlingford
Joined Mar 2012
245 Posts
Quote:
Originally Posted by Peter M View Post
Keep at it Steve, you're nearly there!

Can I suggest you tone down your throws a bit more?

Also since your flying weight is around 3kg and you need between 4 to 5 times your flying weight in tension for the bungee to safely launch, I think making yourself pvc conduit launch ramp and foot release will also make your life easier.

Regards
Peter
Thanks Peter - yeah I've had the rates set at 20% for flying and that's turns out to be about right so I'll deffo be shortening the servo arms. The ramp is a really good idea and I think I'll be sorting that when I get the time and funds.

I've had the first successful flight and it flies really nicely although the end of the flight was not pretty. I think I had a failure or a brown out somewhere that ended up with the plane lawn darting from about 8-10m. I've got the video uploading now. I'm going to order some more reliable servos as these ones are not brilliant and are cause for some nervousness.
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Old Dec 16, 2012, 04:11 PM
Kiwi in Australia
steveobhave's Avatar
Australia, NSW, Carlingford
Joined Mar 2012
245 Posts
First successful flight of 'Kahu'. Due to my Contour camera deciding that it wasn't going to fire up we ended up having to use a mobile phone. But in all I'm pretty happy with its handling. I've got some good quality digital servos on order which will hopefully give me finer control. I'm also going to re-route the RX wiring out of the bay with the ESC in it to one of the wing hatches just in case it was RF interference that cause the control failure.

I'm still not 100% that it wasn't the start of a spin as it all happened pretty quickly but it wasn't slow and it wasn't a high angle of attack turn, at least no higher angle than the previous one, so I'm erring more toward a control failure than an aerodynamic failure. Having said that I'm thinking that moulding in a little wingtip twist could have some advantages. I'll see after I replace the servos.

Next test flight will be in a larger field as well, such a solid sized plane shouldn't really be flown on a playing field. Power wise it seems fine, motor wasn't hot, ESC was still cool so I'm not worried about it being over propped.

Without further ado - here's the video!
Kahu first successful flight (0 min 51 sec)
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Old Dec 17, 2012, 05:37 AM
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Well done!
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Old Dec 17, 2012, 09:08 PM
Kiwi in Australia
steveobhave's Avatar
Australia, NSW, Carlingford
Joined Mar 2012
245 Posts
Quote:
Originally Posted by Zoltan Scotland View Post
Well done!
Thanks fella - we're almost there. I need to do some reconstructive surgery on a winglet and also on the nose but it's so close to being ready to paint that it's not funny.
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