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Author Topic: Ellipse 16in.  (Read 2446 times)
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Tapio Linkosalo
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« on: July 07, 2019, 11:23:58 AM »

Catapult gliders seem to be really popular in Finland this summer. So obviously I have to participate. And when I bumped into a picture of Tmat's new wing for Minimat, with elliptic outlines on the wing, I decided that that is the way I want to try. So after drawing the outline for a 16 inch wing, it was time to produce some cutting. The elliptic wing gives a natural wash-out for the tips if you cut the wedge shape on the top first before tapering the trailing edge. But I did not want wash-out on the center sections, so I modified the trailing edge to straight between the dihedral breaks.

Tail feathers also repeat that same elliptic shape, and I took the dimensions roughly from Hooosier Kitty (same span and root width for both surfaces).

On to the fuselage. Front-pivoted DT is the way to go. Last winter I had drawn a nose pod to be 3D-printed, but felt that it turned too heavy. I then tried to print a mold so I could make the nose out of carbon, but that turned out to be quite tedious. So for this project I re-drew a slightly more simple nose, which turned out at a bit less than 4 grams. Weighting some plywood suggested, that wooden nose would not be much lighter, so I decided to give the printed version a go. And instead of making the wing hold-down of plywood, I printed that too. All in all it was success. The nose pod weights less than 4 grams, and the wing hold-down less than 2. The nose is really rapid to use, as there is ready hole for the timer, tailboom is easily glued and tied to a groove at the aft. And it turned out that the weight of the pod was just perfect, the model balanced without any extra ballast! The wing holddown needs a bit more work. In principle you can print threads to the parts (in this case 2 M2 threads to attach the wing to the holddown and a third thread for a screw adjusting the decalage). but in practice you need to use a tap to open up the threads. Not a big job after all.

I went to test fly the model this morning. No surprises here. Find the proper setting for rudder (about straight) and reduce decalage until model climbs with minimal spiral, and I was done.

That "tumbling pigeon" is really a curious way to DT!
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Ellipse 16in.
Ellipse 16in.
Ellipse 16in.
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tross
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« Reply #1 on: July 09, 2019, 09:52:22 AM »

Very nice. Smiley
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Instructions: Step One...Assemble the pile of sticks shown in pic "A" to look like the model airplane shown in pic "B"........
OZPAF
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« Reply #2 on: July 09, 2019, 09:26:49 PM »

Neat Tapio - that should make you very competitive.

John
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Tmat
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« Reply #3 on: July 11, 2019, 10:40:54 AM »

Very cool Tapio!
I like the nose. Hope it flies as well as it looks!

Tmat
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Tapio Linkosalo
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« Reply #4 on: July 12, 2019, 05:32:54 AM »

I tried another color. This also shows the parts in better detail. The rim to attach the parts to platform while printing needs some cleaning.

I have not made any longer test flights yet, but trimming was straightforward and climb altitude is good, so this should be a performer.
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Re: Ellipse 16in.
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flydean1
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« Reply #5 on: July 12, 2019, 09:24:27 AM »

Looks like something you could sell!!
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Tmat
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« Reply #6 on: July 12, 2019, 02:13:35 PM »

Which material did you use for printing Tapio? Does it sand well?

Tmat
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frash
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« Reply #7 on: July 12, 2019, 03:14:21 PM »

Tapio,

I think that you could sell or post these 3D printer files for your cat glider front hook and wing mount, and there would be customers for such nice work!

No 3D printer at my house, but the local public library prints for only the cost of the filament. Some razor blade miter boxes and E36 motor mounts for me were good. Some props were borderline, but that was only a first try.

Fred Rash
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Tapio Linkosalo
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« Reply #8 on: July 13, 2019, 03:57:16 AM »


The material I have used is PLA, the most common 3D print material. Made of corn starch, biodegradable, is said to be sensitive to UV but I do not plan to store my models outdoors. Probably not as durable as some other forms of printable plastic, but popular as it does not have much of odor when printed. Sands and cuts with blade pretty well. Works OK for items such as this nose. I have also printed molds for indoor model VP hub parts from that same stuff.

In the wing platform I have drawn a couple of M2 threads. For these the printer resolution is not quite sufficient, but the threads need to be tapped to take the bolts. Thus the parts need some finishing before they are ready to use. Therefore I would also need to think if the parts would be ok for selling, or how much finishing would be needed. But I guess I should thoroughly test the nose first!
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Tapio Linkosalo
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« Reply #9 on: July 23, 2019, 01:18:59 AM »


Interesting second outing of the new model last Saturday. I switched  over to 1/4" grey FAI rubber, and noticed that it gives higher initial acceleration, leading into changing trims. First of all the higher initial speed increased the roll towards the glide circle, thus ending up in to a A-patterned climb. Also, the model showed a tendency to bunt, which made trimming more difficult and resulted in a couple of crashes. It also seemed as the amount of bunt was not consistent, so it made me wonder if that was due to higher sensitivity to trims, or some aeroelasticity. My potential reasons for the bunt are:

- the tailplane has slightly lifting airfoil (flat bottom, max. thickness 1.5mm) so maybe that lifts too much at launch?
- maybe my spring that holds the wing down is too weak and lets the wing TE lift ever so slightly during launch?
- most likely the tailboom is stiff enough, but I have to verify that.

I also noticed that the 3D-printed wing saddle is a bit too flexible at the front, so I need to re-design that. I plan to replace the wing pivoting pin with a piece af aluminum tubing, threading one end so that it screws to the saddle, and glueing a flange to the other end, so that the pin will "lock" the front of the wing saddle, making a more firm attachment.

Unfortunately I never got the launch sorted out so that I could have glided the model all the way to the ground, so I do not what what my still-air time would be. But the best launch altitudes looked good, and the glide much better than with my smaller "Rakt upp" -model.
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Tmat
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« Reply #10 on: July 26, 2019, 11:36:29 AM »

Tapio,
My experience has been that the only time I've had a glider bunt it's been due to aero elastic effects (something flexing). Tailplane airfoil is not likely to be the culprit.

Tony
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Tapio Linkosalo
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« Reply #11 on: July 29, 2019, 01:17:10 PM »


Thanks. If it is not the tail airfoil, then most likely my wing hold-down is not stiff enough, and I need to add more pins to route the line around, to add more friction. The tailboom is made of carbon over a steel mandrel, in best FF practice, so I'm convinced that it is not flexing.

But the model is fixed and set up to go once weather co-operates. I'll report then.

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Stan
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« Reply #12 on: July 29, 2019, 03:31:16 PM »

You have reached a speed that, coupled with your wing airfoil center of lift to move aft (please excuse if I am using the wrong terms). Pitching moment increases. The wing is trying to wash-out. Depending on stiffness of parts wing may flutter (tips not rigid enough) or it may bunt (mains not stiff enough). A boom that is too weak can also cause this problem.

Very unlikely to be stab issue.  The problem is reduced by using sufficient up-sweep in wing airfoil, airfoil progressing to near symmetrical near tip
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Tapio Linkosalo
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« Reply #13 on: August 10, 2019, 03:28:38 AM »

Problem solved. The issue was with too weak DT line. I had it originally attached to the wing, and routed 90 degrees around a peg in the fuselage towards the button timer. I now modified the wing saddle to extend past wing trailing edge, and attached the DT line to the fuselage, so that it goes around the wing saddle extension to the pin. Now my launches do not bunt any more.

Also interesting to see how much better this model glides compared to my previous "Straight Up" models. Much slower glide, much lower sinking speed. Ellipse weights 20 grams while the Straight Up is 16, so the wing loading is about the same. The major difference must be in the airfoil. I did some flying this morning and constantly made 55 to 60 seconds in still air. My transition to glide still was not perfect, as the model pulled a bit on the back and dived a bit following the stall/flip.

This model is 400mm (16 in) span, the wing is 80mm wide as my balsa sheet is only 5mm thick. I am tempted to try to stretch the span to 450mm (18") to see how that would go.
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Tapio Linkosalo
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« Reply #14 on: August 13, 2019, 01:39:21 AM »

Well. I pulled out my drawing in order to stretch the wing to 18 inches, only to realize that it is already 18in (450mm flat, 420mm span with the D/H breaks). So not my dilemma is: should I try even longer span (like 20" flat) to get even better glide, or try shortening the span for 16 to get higher climb and maybe more agile transition? Mind you, the wing is narrow (80mm) and thin airfoil (5mm), so I'd guess the drag is quite low for the climb. Then again, the glide is also rather floaty, very different from my "Straight Up" -glider, that glides and sinks considerably faster.

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OZPAF
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« Reply #15 on: August 13, 2019, 04:05:39 AM »

I would try say a 4mm thick wing section at the root. I have used 4.5mm on a 520 span HLG - 89mm root chord, and it worked well. The thinner air foil will help to give you a better launch and should still be strong enough.
To improve the transition I would consider if it is possible to thin the wing out even further towards the tip.
Anyway good luck.
John
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Tapio Linkosalo
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« Reply #16 on: May 18, 2020, 03:33:05 AM »

My latest development has been trying to route the Ellipse CLG wing with a 3D router. First attempt (the left wing in picture) had some errors in the program, with the router bit trying to move through the wing - hence the glued balsa strips to fix those errors. The right wing was better, but now there is another issue - that wing is some 0.2mm (1/12") thicker. I still need to learn how to position the router bit with sufficient accuracy. The problem is that the part to be routed needs to be moved during the process, and I need to figure out a way to ensure the parts to be positioned properly. I first route the bottom of the airfoil. Then flip the part over and route the top. Then change the router tip from ball-end (for routing the surfaces) to a sharp tip, which is then used to route the dihedral breaks and the wing outlines. Also I route the left and right wing separately (as my router is so small that the whole wing does not fit). I do have some ideas though how to make the wings accurate enough.

The wing surface seems quite rough in the picture, but actually would only need the lightest touch of sanding paper for a finish.

 
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Re: Ellipse 16in.
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OZPAF
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« Reply #17 on: May 18, 2020, 04:30:26 AM »

Interesting approach Tapio. Perhaps if the wing can be temporarily attached to a bed which has uses external index datum points for the CNC router -one for the left panel and one for the right. The bed - thin plate of MDF would be indexed against external guides - pegs or rails etc.

The accuracy of alignment can be increased by using a larger base plate.

good luck

John
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Tapio Linkosalo
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« Reply #18 on: May 21, 2020, 07:05:50 AM »


The solution for the reference was twofold. First I learned how to place the origin of routing to the place I want in Fusion, so I can put it now to the bottom of the wing, at root, center. Thus for routing the top I can reference to the (plywood) bed that the balsa sheet lies on. For routing the bottom of the wing (where I need to flip the balsa sheet over) I cannot use the bed, as then I would get bias from the variation in the balsa sheet. Also I do not want to take it directly from the balsa, as it is soft and thus not easy to get the height accurately. Instead I glued a smaa tab of plywood against the balsa and the bed when the sheet is upright, so when flipped for the bottom, this gives me accurate reference for the sheet bottom height.

The resulting two wing halves are not of similar thickness. Maybe a tad too thick (but then again thinner wing might flutter), but it will be easy to adjust that by moving the tool tip 0.5mm down when cutting the upper half of the wing.
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Re: Ellipse 16in.
Re: Ellipse 16in.
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Tapio Linkosalo
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« Reply #19 on: May 21, 2020, 10:02:25 AM »

Correction - once I sanded the routing blurr away, the wing thickness turned out to be just right. TE is 0.8 to 0.9mm thick, and I would not go thinner if shaping the wing by hand. So it is just fine as it is. I have the other hald of the balsa sheet left, so I suppose I'll make another wing, now that I'm on the run...  Cheesy

Funny how coarse the surface seems on a photo, as it feels nice and smooth to the hand. I suppose I need to sand it a bit more between coats of dope.
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OZPAF
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« Reply #20 on: May 22, 2020, 05:29:33 AM »

Not bad Tapio. It will be very easy to produce more wings now.

That was a simple neat solution to your indexing problem.

John
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Tapio Linkosalo
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« Reply #21 on: May 22, 2020, 06:18:40 AM »


Tell me about it (easy production...). Now that I got the router files done and routines established, I decided to try another wing (finished this morning). And as I found one more sheet of 5mm balsa, I made a new setup for 16in wing (the current is 18"), which is currently being routed. Then I'm out of 5mm balsa wood, have to go to shopping for more!  Cheesy Grin
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Tapio Linkosalo
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« Reply #22 on: May 22, 2020, 08:45:39 AM »

Here is a picture of my routing jig. Base plate of 4mm plywood. A list at the back of the picture is glued to the bottom and works as reference (it is strictly in X-axis direction). The bottom of the plate has the wedges to attach the balsa sheet firmly to place. The two small ply pieces to the left, the leftmost is glued to the bottom and act as reference point, the other part is glued to the balsa sheet to provide firm point for locating the balsa sheet but also reference point of the bottom of the sheet (being on top when the bottom of the wing is routed).

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Re: Ellipse 16in.
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OZPAF
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« Reply #23 on: May 24, 2020, 06:06:12 AM »

That looks fun Tapio. I worked as a factory hand loading CNC routing machines cutting patterns out of plastic sheet, for a friend. He used a 6mm dia dowel rogethedr with a back fence to index the sheets of plastic on the base board in a similar way to what you are doing.

Don't you miss the hand shaping Smiley

John
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Tapio Linkosalo
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« Reply #24 on: May 25, 2020, 01:45:20 AM »


I can always try hand carving props (have not yet figured out how to draft them in 3D, but working towards that). Meanwhile, 3D routing makes replicating and modifying wings pretty easy. I'm now out of 5mm balsa wood, but have two new wings of the original 18" size plus one 16-incher. Now just tails and fuselages, and I'll have a lot of trimming to do!

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