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Author Topic: Ellipse 16in.  (Read 807 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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