|Dec 19, 2013, 04:07 PM|
Heat Seeker, 3 Meter, 120", Thermal Plank.
Following on from my previous 2M Plank, I have started building a 3 piece, 3 meter wing in EPP and EPS foam.
Specs as follows:
Blended center section.
PW 106 airfoil.
300mm wing root.
180mm wing tip.
6 servo wing.
600mm outer elevons.
400mm inner elevons.
Weight, much more than I would like I'm sure! Maybe 2kg?
Taranis Tx, my brand spanking new self Xmas present.
I like to experiment with the new ideas that wake me up in the middle of the night, so I tend to make up things as I go. Being a mechanical machinery breakdown specialist has given me the experience needed to think on the fly so I will be using plenty of build techniques that I haven't tried before.
The blended centre in lieu of a fuselage appeals to me, so this time it will be made from cheap EPS with fiberglass skin. The foam will be hollowed out in the middle for electronic gear and motor.
By spraying the glass mat with contact spray I was able to cut it accurately with no fraying edges and then tightly wrap it to the foam starting with one layer all over then a further 2 from the spar forward. Being tapered the nose ended up with multiple layers which will help with frontal impact, which is bound to happen where I have to land.
100mm epp foam was sliced into 3 to make 30mm thick wing cores. 12mm mdf was glued to both sides of each piece to provide a flat strong surface for the wing beds as they are so thin. 50mm beds would have been easier but it costs a fortune to ship it here from USA so this method yields 3 cores rather than 2.
Aluminium templates were fabricated and the surfaces were sanded and polished to assist with snag free hot wiring. The templates were sat in specially made formers and the double sided tape onto the foam. A 12 volt carry battery hooked up to 24 gauge NiChrome wire was used to cut by far my best set of wing cores to date. Totally snag free with a smooth surface.
My satellite internet connection is slow so I will upload this and continue later.
|Dec 19, 2013, 08:39 PM|
This will be a cool wing. For thermalling, it should fly the pants off of the 2.2 m version and be easier to see. Bigger wings carry the weight better too. I already want one.
Programming the 6 servos should keep you busy.....lots and lots of options. This plane will be a good test bed to test any number of control theories. Maybe you could use a crow function between 2 control surfaces on one side a get a rudder.
BTW, I've never bothered with flap protection on my light weight SW-2 and regularly land on deflected flaps. It didn't take long to break just about everything. First the 2-56 linkage, then the servo started making an odd noise, then tore the kevlar hinges. I too chose to leave them deployed once near the ground.
I'd be interested in seeing a few photos of your spar system when that time comes around.
|Dec 20, 2013, 04:03 PM|
After glassing, the center section weighed 300 grams and is very strong. Even though I went to a lot of trouble lining all the holes up they still required a bit of tweaking to get the joiner tube and all 4 incidence pins to line up simultaneously. The joiner hole required filing the hole a little offset. After I could slide the arrow in easily I applied several coats of car polish to the arrow then epoxied it into the filed holes in the wing root formers. When the epoxy had set a quick twist and the arrow slid out with a really firm fit.
Kent, the wing spar system was the result of several tests. The photos will describe it better, but the wings were cut on the bandsaw tapering from 12mm at the root to 8mm at the tip. 200mm long strips of 1mm ply were glued vertically from the wing root and an 8mm thick piece of ply was glued between them to receive the wing joiner, sort of like a sub spar.
Due to the dihedral angle the wing joiner arrow is not as long as I would like but I'm hoping the spar system will help distribute the load. The arrow is 7mm dia and I'm hoping that on impact the arrow will bend and provide some shock absorption instead of breaking stuff......time will tell.
After gluing in the ply spar sections and lining up all the holes the inner and outer panels were joined then 1/16" vertical grain balsa was glued in to the tapered cuts with PU glue and set in the wing beds to cure.
After breaking all the spruce spars in the last wing I wanted to try something that would hopefully flex on impact rather than snapping so fiberglass seemed to work well in my test samples. A 1mm deep trench was routed over top of the balsa/ply and 5 layers of glass were epoxied in. Extra attention was taken wetting out the first layer to make sure the balsa end grain would have good contact with the glass fiber. Both bottom and top spar caps were wet out at the same time and covered with baking paper then put in the beds and weighted down for the night.
The result is quite stiff with a little bit of flex when weight is applied and after the double D box 5mm laminating is applied it should really stiffen up. That's the plan anyhows.
The sub trailing edged was trimmed at the 30% chord line and 6mm, 1/4" , spruce drag spars were glued on using shoe goo. First time using this glue. Worked great for this application. Dries quickly but allows enuf time for positioning.
The 48" sloper that I made suffered from impact damage and the lam film came loose from the foam over large areas. Still flew well but surely had extra drag. Several tests to improve lam film adhesion were carried out. PU glue applied to a small section of foam at a time then spread by holding a credit card vertically and dragging across the surface with very light pressure results in a fairly even, thin layer of glue. Following this the surface is rubbed vigorously in all directions. I did a quarter of each wing then covered with baking paper as smooth as I could get it and put the cores back in weighted beds for a couple of hours. Using the baking paper gives a really smooth finish with little sanding required. Even tho' the glue later is thin it penetrated up to 10mm into the foam.
Spray adhesive will be applied over the PU finish when laminating time comes along. The tests I tried using this method resulted in tearing the foam away in chunks when I tried pulling the film off. That should take care of torsional stiffness.
Yesterday arvo I routed out the servo cavities and cable tracks. The servos will be mounted in the top of the wing because its a real pain to turn such a large wing upside down to work on.
A test elevon was made yesterday using EPS foam sanded to shape with 1 layer glass applied on the bias. The result is overly flexible lengthways but with reasonable twist resistance. I don't think it will be as stiff or light as covered balsa tho' so that will be the first thing to do today.
Hopefully I can make the control surfaces today and spend some quality time drinking wine this arvo while I pulling my hair out trying to program the new Taranis Tx.
Maiden in a few days I hope.
|Dec 20, 2013, 04:28 PM|
Photos didn't attach for some reason last post.
|Dec 21, 2013, 02:33 PM|
I would not be concerned about the seemingly short joiner. As long as the joiner sleeve / spar connection is wrapped, it can be quite short, as is the case in full sized gliders.
I like the glass cloth spar cap. It should provide a good mix of strength and flexibility. At a recent aerotow there was a 53 lb., 40% glider that had a 1 1/8" diameter glass joiner rod. He chose glass instead of carbon because of the realistic flex it provided and flex is what he got. Even on a gentle aero tow, you can see some flex.
Your 7mm carbon fiber arrow shaft joiner will no doubt be the highest stressed component in this system. I would expect significant flex at the joiner when you pull a high G load loop. 3m is a pretty big wing. But to be honest, I have very little feel for what the minimum spar might be for any particular wing. I prefer a over designed spar system just because I like designing and building them, so I never get much experience at the lightest possible spar system.
If you are concerned about the joiner, as an upgraded , you could have one that is filled solid with carbon tow. Also, a solid glass joiner rod might be the perfect match for your flexible EPP wing. It would be interesting to have a variety of joiners on hand for the first test flights. I hope that you able to shoot some good video as you did with the 2.2m version.
|Dec 21, 2013, 03:25 PM|
Thanks for your comments Kent. Yeah, the wing joiner is a concern. If it doesn't have enuf rigidity I may have to join the wings permanently for this build. I would try a different technique if I was starting over again. This was my first attempt at a wing joiner and could definitely be improved.
Solid balsa elevons were fabricated and covered with 5mil laminating film yesterday. Surprisingly the torsional twist resistance is similar to the foam/glass one made the day before. Both are comparable for robustness as well. The foam/glass method is a cheaper alternative to balsa with a similar work input.
Programming with the Taranis went well. Full span elevator and aileron but all at 100% throw at the moment till control surfaces are hooked up. I tried to put the crow on the slider for graduated deployment but due to the fact that the slider has a middle detent I can only get half servo throw. Not sure if the slider can somehow be programmed to give full throw. I put the crow function on the throttle stick for now so I can set the servo horns in the correct positions prior to skinning.
Today will be potting the servos and soldering cables , mpx plugs etc. Hopefully skin the wings and hook up control surfaces as well.
I will include a video of the 48" sloper that was made prior to this build. The throws are on low and not set up for good axial roll. Dialled in much better after this flight. The wind was only about 10 knots as can be seen by the lack of white caps. It is a joy to fly in bigger wind with great energy retention and good roll rate. A small motor is fitted for flatland testing and flaps for landing. It has copped a good beating so far!!!
|Dec 21, 2013, 06:09 PM|
Nice video Greggy....720p, bright sunshine. Looks good full screen. It's seems that you are somewhat familiar with that little sand bowl. You'll have a harder time trying to fit the 3m into that same downwind turn. Of course do-able, but I bet you really notice the difference.
|Dec 22, 2013, 06:05 PM|
Still thinking about your joiner..............and indeed mine too.
Besides the suggestions in post #6 regarding making your joiner rod stronger, there are some possibly stronger options. One would be to find a 7075 alum. joiner. Here in the States, they can be found at Onlinemetals.com. Next stronger would be music wire and after that would be a so called "stressproof" equivalent steel. Also from online metals. Music wire has the advantage of being bendable under load. Even if you bend it beyond it's elastic limit, it will not snap. In that case you have the pleasure of landing with a lot of dihedral. Carbon, 7075 and stressproof when bent enough, will snap, although it is hard to imagine your spar ever breaking a stress proof joiner even at the modest 7mm diameter.
One way to visualize whether or not a joiner rod is strong enough is to imagine that the joiner is as long as the full wing span, then load it at the center with the full fuselage (flight pack) weight then multiple that load by 40 if you plan on pulling a 40G load. Clearly, the bare arrow shaft tube joiner will have to be limited to light flying loads.
Another safe alternate would be to fill the existing arrow shaft joiner with a big music wire and epoxy. The would provide a stronger joiner that would have the benefit of failing safety....that is it would not snap. The carbon would snap but the music wire core would not. There are some who design their joiners not so much for strength/rigidity but rather for how they fail.
Finding an alternate joiner that fits properly may be a problem, so choose the biggest joiner that will fit, then epoxy on a few layers of glass and sand to fit.
Also, if the joiner is very stiff and strong you may get a failure where the spar caps over lap the joiner sleeve from the prying action. This is solved by wrapping the spar caps AT the joiner ends with something very strong, such as kevlar thread. I believe miniphase had this problem eventually with his 3m+ ship.
|Dec 22, 2013, 09:30 PM|
Thanks once again Kent.
Re the video, the sand bowl as you call it ( yes sand does get in everything!! ) is a great place to fly to get close in video. I probably wouldn't take a thermal plane there again as I prefer the fly something faster and more nimble on the slope. Whales were jumping the last time I was there. Its an hour and a half drive and the wind has the be almost due east preferably at least 15 knots so combining that with my days off I don't get there too often.
Kent your thoughts on the joiner are much appreciated. I would have done more testing prior to the build but I just couldn't fully envisage the centre section until I had made it.
I usually build with what I have lying around as specialty items from hobbyking for example take up the 6 weeks to arrive and I'm too impatient to get on with the build to wait for that.
I'm tossing up whether to proceed with the build and hope that lots of tape over the join might stop the inevitable, or hack into the wing and center and somehow incorporate a beefier joiner and try to blend it in as best I can.
I like the idea of fitting the joiner in between the spar shear webs. I have some aluminium sheet 3mm and 10 mm and may use some of that as a rectangular joiner. I can see the reasoning behind wrapping the spar caps with kevlar thread for example. Due to the flat centre section and the wing dihedral I think I will need two joiners, on for each wing, as a bent one won't go through the centre.
Anyhows, I like a challenge so this will give me something to contemplate.
|Dec 23, 2013, 12:48 AM|
I was thinking that this might be a slope ship facing big air and high speed stunts.
If instead you plan on gently motoring up then floating around in thermals, the existing arrow shaft joiner may be fine. If you do over load the arrowshaft, it will flex a lot, giving you a visual warning, before it snaps. So you should be able to safely have a test flight or two and then decide whether or not to over ride the joiner by making it a one piece wing ( if you could still transport it ). This would be in lieu of the upgraded joiner.
I would think that the existing spars could be connected at the center blended wing portion by adding another spar cap over the top of the existing spar caps, thus creating a spar across the blended wing portion.
|Dec 24, 2013, 04:25 PM|
Change of plans.
I posted yesterday but it disappeared into the ethernet.
I appreciate the effort you've made to help me out Kent. The Aus warehouse doesn't stock a lot of stuff but its great when they've got what you need in a hurry.
Much deliberation has gone into the joiner. A test with the wing all fully assembled resulted in flexing in the middle section that was very flimsy. I finally decided that to incorporate a more substantial joiner would be best done by starting from scratch. I can already see several design/build changes that I will make in the next version. Kent, I like the glass box joiner that you posted on Youtube and setting it in place as part of the build.
I calculated that an 10mm thick aluminium joiner would weigh 300 grams plus the joiner and spar upgrade. I didn't want to add that much weight to a wing that I will mostly fly at home. Extra weight make for more brutal impacts
So,.... I decided to glass over the join. This can be cut through if the need arises later on. The epoxy cured overnight and yesterday I ironed on the laminating film.
The film added 200 grams. AUW is going to be close to the target 2kg.
The wingtips were hotwired as well and will be glued on today and skinned.
Getting close now.
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