Things I found in storage - Trainer Foam conversion and Build

[SteveOBHave]

Servo holes cut and hollowed out. I still haven't decided how I'm going to seat them, I'm thinking that a light ply insert or some sheet fiberglass might do the trick.

I've tunnelled out foam from the servo recesses and also through to the electronics bay. I've also made recesses for the servo connectors and the wires so that if I need to change them it's just a matter of lifting the elevons and swapping them out.

Both servos and control horns installed but not permanent yet. I will use a standard two part epoxy on the control horns but I need to be able to replace servos if they die.

Kahu will be pretty clean when she's all done and I'm still trying to decide if I want to put a duct forcing air both into the electronics bay and also to the motor of if I leave the motor in the wind. Again it's a case of making sure that I'm able to easily replace parts if they fail. I still need to figure out how to make the motor mount removable. The final thing that I will need to decide on is if the trailing edge in front of the prop needs to be streamlined - I'm guessing that if I want her quiet that a nice trailing edge will be needed.

This will be it till I can get back to the hangars because I need to clean up the elevon interfaces, probably with some micro, before I install the hinges and then it'll be test flight time!

 

[SteveOBHave]

Final(ish) CG calcs done using http://wingcgcalc.bruder.com.br/en_US/?

I gave myself a bit of a fright by not measuring properly the first time and the CG came out a good 25mm or just shy of an inch forward of what I though it was but then after some rather concerned and extremely anal remeasuring it came out exactly where I was expecting it.

First test throws followed by the first test flight will be this weekend. I am quietly pooping myself as it's a whole lot of plane and she's no foamy...

 

[earthsciteach]

Cool cg program! Haven't seen that one before.

Here's a question for you. In your experience, would it be better to attach the winglets and then glass, or glass the wings and winglets separately and then attach and bridge with glass?

 

[SteveOBHave]

Personally I'd glass the winglets first and then glue them on and use the glass as a strengthening medium. Make sure that the direction of the weave is going to compliment the forces that they're likely to encounter. My main reason for suggesting this is that it reduces complexity and any structural differences will be negligible.

I should ask though Teach, what style of winglet are we talking about? If it's just an end plate then as above but if it's a blended winglet then you might want to consider doing it as one piece.

 

[earthsciteach]

For the prototype, I'm just doing a flat plate. The blended winglet was more pain in the butt than I was willing to deal with. I just want to get this one done, bang out one more with slight improvements, then get on to the Grumman Goose scratchbuild I want to tackle.

 

[SteveOBHave]

Then yeah for sure - glass then glue. If you have some lightweight spackle make some fillets between the wing and the winglet and then add the glass. The glass probably won't like going around a right angle corner.

 

[SteveOBHave]

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.
http://youtu.be/qToH3cpOkTo

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.

 

[earthsciteach]

ARRRRGGH!!! NO, NO, NO! This is not allowed. I had reached a certain level of comfort with the maiden of my wing. You've now put me back in the Zone of Fear.

 

[SteveOBHave]

ARRRRGGH!!! NO, NO, NO! This is not allowed. I had reached a certain level of comfort with the maiden of my wing. You've now put me back in the Zone of Fear.

You won't like the next video that I took then

 

[SteveOBHave]

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 (From RCGroups) 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...

http://youtu.be/Lbi_xW9QW5o

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. The nose area is also an extension of the primary airfoil so it's effectively a lifting body hopefully adding to the overall lift of the wing. 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!

 

[earthsciteach]

I built my first non-sheet wing with a snub nose, as you did with yours. It definitely made it very cg sensitive. I had to add 6 AA batteries to the nose of mine to get it to fly correctly.

My fpv wing will need a fair bit of nose weight to get the cg correct. I chose a wing sweep that helps out with the cg. Greater sweep equals less cg sensitivity. My HUGE winglets are definitely adding rearward weight unnecessarily.

I also used the same cg calculator that you did, but am uncomfortable with the result. I'm going to push it forward a bit. I'd rather have it be nose heavy.

 

[Carbon]

Honestly Steve, your work is outstanding! very inspiring!

 

[SteveOBHave]

I built my first non-sheet wing with a snub nose, as you did with yours. It definitely made it very cg sensitive. I had to add 6 AA batteries to the nose of mine to get it to fly correctly.

My fpv wing will need a fair bit of nose weight to get the cg correct. I chose a wing sweep that helps out with the cg. Greater sweep equals less cg sensitivity. My HUGE winglets are definitely adding rearward weight unnecessarily.

I also used the same cg calculator that you did, but am uncomfortable with the result. I'm going to push it forward a bit. I'd rather have it be nose heavy.

Yeah, it's definitely preferable to start with a forward CG and work your way back. I wonder what it is with that specific calculator or if it's down to the inclusion of the winglets in the calc. I'll see where the CG ends up after this mod and refer it back to the calculator possibly with removing the winglets from the equation. It's likely more down to my lack of understand of how to apply the tool rather than the tool itself being wrong.

Honestly Steve, your work is outstanding! very inspiring!

Thanks Carbon, it's been a serious learning experience that's for sure!

 

[earthsciteach]

I do think it has a lot to do with the winglets.

 

[SteveOBHave]

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!

In other fun I built myself a biplane since I've always wanted one. I've yet to do more than glide test it as it was too windy. In as far as the glide tests went, it's pretty docile and doesn't seem to have any odd or unfavorable tendencies. I've called it The SSW - The Stevens Stagger Wing. It only has ailerons on the bottom wing at this stage because constant coupling and decoupling is added complexity so I think I'll see if it needs it before having at it again.

 

[Foam Addict]

Cool stagger wing.

 

[SteveOBHave]

Thanks! Hopefully it flies well!

 

[earthsciteach]

Very cool looking. Its like a Beech Staggerwing and a Quickie had a baby!

 

[SteveOBHave]

Yeah it does have that kind of feel I was going for a modernised Beech Staggerwing. I needed something to give me some breathing space after my wing ended up doing its lawndart impression again.

 

[SteveOBHave]

Well I have utterly heartbreaking news with Kahu. I totally stacked it. It was entirely my fault, I still had too much throw on the control surfaces and it was causing serious proverse yaw.

It's a little hard to see but the left wing has creased which has totally compromised its strength. Repair is pretty easy but I'm about done with this iteration. The nose is also completely stacked.

The nose absorbed the lions share of the impact as it went in from an attempt at a full power climb out of a spin from around 10-15m or 30-45 feet but I managed to throttle off before it hit the ground.

The failures are clearly related to where I have cut into the structure but on the whole it is actually a pretty straight forward repair.

I will say this, it's not the end of the Kahu project. I've loved every moment of building this beastie and learned a great deal from the process.

Kahu 2.0 will be looking at the following:
- Likely be a 60-70% build to eliminate the need for the bungee launch.
- I will put twist into the airfoil to add yaw stability
- I'll retain the winglet structure as they have not only proven to be seriously strong but they add a good amount of aerodynamic advantage.
- I will cut the sweep specifically rather than altering the sweep by angling the root, changing the profile of the airfoil I'm sure has reduced the effectiveness of the design. In actual fact it changes the airfoil and I'm sure Mr Martin Hepperle would be shaking his head at my shoddy application.
- I will use a much lighter layup technique possibly even implementing a central spar configuration rather than relying on the skin across the middle of the wing to support the lion share of the stress. The reason for this is that every time I cut into it for electronics I was effectively reducing the strength of the wing.

I'll start design and construction on this pretty soon. I've gotten over flying my beautiful wing into the ground and I'm ready to stop fixing a flawed design!

Crash video