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Author Topic: Keil Kraft Polaris  (Read 727 times)
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Jasco
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« on: November 04, 2020, 01:23:17 PM »

Recently I have had a lot of luck getting some modern glider designs to fly and I was attracted by the curvy lines of the KK Polaris. I'm a sucker for these "vintage" designs, but I'm leery of the aerodynamics.  And they seem like they have way too much wood in them.

At any rate, I cut out the pieces the other day: The wings and tail group from my finest "C" grain 1/8" and the fuse from some nondescript 1/4"
After I mocked it up to gaze lovingly at it I realized that after the airfoil is carved, I am going to have to take out a triangle of material in the center  and the wings are going to be swept back. I built a Zaic design long ago that had sweep-back in the wings: it was essentially a lawn dart. I admit I didn't choose my wood carefully, or use a Stanfoil-style airfoil, but it made me skeptical of the feature.  I don't think the Polaris will need any additional "spiral stability" sweep back may offer, so I suspect it is purely an aesthetic decision on the part of the designer.  I'm toying with the idea of leaving the wing straight and foregoing the simple dihedral in favor of a polyhedral wing as in the KK Comet.

I don't fly in contests, I just want a good flying glider to play with.

Also, I put this in the HLG area because it's a HLG design.  I can't throw a paperwad into a trashcan, so this will be launched with a catapult.   Sorry if I've broken the forum.  Grin
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TheLurker
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« Reply #1 on: November 04, 2020, 01:49:03 PM »

Quote from: Jasco
I'm toying with the idea of leaving the wing straight and foregoing the simple dihedral in favor of a polyhedral wing as in the KK Comet.
Build two, or rather build another one?  One as plan, one as you describe and see which flies better?  Not a lot of wood or time required. I think lots of bods here would be interested to see how they compare.

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dosco
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« Reply #2 on: November 04, 2020, 01:56:26 PM »

Hmm. Not sure that your previous lawn dart experience would be tied to the airfoil or sweep.

Will be interesting to observe your results.

-Dave
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Jasco
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« Reply #3 on: November 04, 2020, 02:53:30 PM »

I discovered this sheet of C grain in the bin at Michaels and I've been waiting for the right project to use it on.  I don't think there is enough left for another wing.  I COULD buy more...My wife eyeballs every 36" package that comes from VMS!

I have done a fair amount of woodworking in the past but I don't recall ever making the connection between quarter-sawn wood and C grain balsa. I'm just a slow learner, I guess.  This balsa is clearly superior to the crap I've been using.

I am encouraged by the fact that the parts weigh 18.6 grams, which is the same weight as the AUW of similarly sized gliders that are finished.
There is a lot of wood to be taken off in the shaping, so I need to keep the glue and finish to a minimum.

I also cut the fuse so the wing and stab have no intentional decalage, which differs from the plan.

I'll cogitate on the sweep when I'm carving the wing.  Dosco's response gives me encouragement to build this as per plan.  I think we have discussed the overwhelming merits of light weight elsewhere. Cheesy
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RalphS
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« Reply #4 on: November 06, 2020, 05:52:47 AM »

Here is one I made earlier  - 1945 or 1946!  We didn't know anything about trimming.  Just threw it as hard as possible and loved it when it looped and came down and hit the thrower in the back of the neck.

The glider is the Frog Vanda and the twin is from one of the Astral kits.  I didn't make that, it was donated by an older boy who was leaving school at the time.

Ralph

ps  camera was Kodak Hawkeye Ace, 127 film.
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Jasco
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« Reply #5 on: November 07, 2020, 08:04:55 AM »

I cannot thank you enough for sharing that picture. Is that really you?

I built another old design a while back and looping was a dominant feature I'm hoping to avoid.
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dosco
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« Reply #6 on: November 07, 2020, 09:11:51 AM »

I discovered this sheet of C grain in the bin at Michaels and I've been waiting for the right project to use it on.  I don't think there is enough left for another wing.

Thanks for mentioning that! I found some very nice "C" grain balsa (it was not the usual Midwest stuff) at the local Michael's over the last few years and bought quite a bit of it. Unfortunately it seems the COVID situation has somehow resulted in a shortage ... there's none to be had lately (I did see 1 sheet of 1/4 in thick stuff that appeared to be Oak(!) it was so dark and heavy).

Good luck with the build.

-Dave
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MKelly
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« Reply #7 on: November 07, 2020, 10:19:54 AM »

There's been a lot of C-grain in the 1/32" sheet at our local Hobby Lobby stores this year.  Not particularly light (mostly 8-10 lb stuff), but useful on occasion.

Mike
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dosco
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« Reply #8 on: November 07, 2020, 11:32:54 AM »

There's been a lot of C-grain in the 1/32" sheet at our local Hobby Lobby stores this year.  Not particularly light (mostly 8-10 lb stuff), but useful on occasion.

Mike

If you hold it up against a bright light, you might be able to cut the light stuff away from the heavy stuff ...
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Jasco
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« Reply #9 on: November 09, 2020, 11:08:04 AM »

It must be my isolation from the outside world that is making me marvel at this sheet of wood.  I spent an enjoyable morning planing and sanding the flying surfaces.  I do enjoy planing wood and getting those long thin shavings.  There must be something I can do with them!

I ended up gouging the wing over on the right side toward the T.E. It didn't go all the way through the wood, but it's way too thin.
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Jasco
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« Reply #10 on: May 10, 2021, 08:43:57 PM »

After reading some not-too-complimentary posts about this old design, I decided I didn't want to waste the "good wood" on a glider that didn't have a chance, so I pulled out the "That-Looks-About-RightDesign Handbook” and set out to proportion the ship a little more like ones I already knew flew well.

To wit: More tail boom, less tail area, 0-0 decalage, an attempt at a “Stanfoil” airfoil, and a pull tab for my right hand to grab for the launch.

I got all fancy and ordered the special floral spray paint that the folks that seem to know what they are doing use for my paint scheme.  I was inspired by the colors of the old Sleek Streek, and happy with the way my masking job turned out.  I also printed out a water slide decal with the Keil Kraft logo.  It is really a tail flash, but using it as such puts weight on the wrong end.

To say it flew right off the table would be a lie. It loves to spiral in to the right and I lost the wing twice during trimming.  I finally put two 1/8 x 3/16” “shoulders” at the wing-fuse joint.  That took care of that problem, but the finger grip is not guaranteeing the tail doesn’t hit my hand at launch.  I’ve lost the stab several times as a result.

You also have to give it a very healthy launch.  I couldn’t get the transition on video, but you have to give it enough “oomph” to get it to complete a roll at the top of it’s launch arc or it will fall into a right spiral dive to Terra Firma.  The early consequences of half-hearted launches are shown.

In the end, I was very happy with the final flight profile. I want to address the tail-strike situation and do some math to determine the tail volume, which I understand is an important number.  It’s also what I changed to make it fly the way it does and I want to see if I can make the transition as trouble-free and consistent as some of my other planes.

Had a beautiful calm morning Saturday before Mother's day.  I see I'm not the only one posting videos!

https://youtu.be/7buof0fv-aY
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dosco
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« Reply #11 on: May 11, 2021, 10:48:05 AM »

I like it!

You mention having to really heave it ... what's her mass? (I noticed the same thing with the Zweibox).

Also the spiraling bit ... I assume you've set her up with cross-controls? Right roll and left yaw?

Cheers-
Dave
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Jasco
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« Reply #12 on: May 11, 2021, 11:21:25 AM »

This plane is my heaviest glider so far...It weighed 21.0 grams when I brought it home from the field.  I mentioned earlier that although this is an hlg design, I have it set up for catapult launch.  I can't throw anything.  The catapult I'm using is 2-9" loops of 1/8" rubber on a handle.

Also in my previous post, I mistakenly insisted that it spirals to the right.  It spirals to the left.  It turns to the left when it transitions properly and spirals to the left when it doesn't.  Yes, I have it cross controlled with stab tilt, a 1/16 sq. gurney flap on the left side of the rudder and one on the underside of the left wing.

It required the tiniest gurney on the top surface of the stab.  It's only a piece of 1/32 x 1/16 about  1/2"x long.  I wouldn't think it's doing much, but I've never gotten it to fly without it.

I noticed in the video that after the long first flight, it started stalling around a little more.  Perhaps another pea of clay on the nose when the air starts warming up and rising.

I definitely wish I had mounted the wing further aft.  I'd be able to leave more clay in the flight box.

It's exciting to fly and I can hardly wait to take it out again!
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