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Author Topic: Wing Taper Methods  (Read 324 times)
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High Point
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« on: May 10, 2022, 08:09:28 PM »

After being away from gliders for awhile, I thought I'd like to start back up building the SWEEPETTE-ETTE 14 MK. II CLG. The problem that I'm having is how to taper 1/8" from the CL down to 3/32" at the dihedral break, then down to 1/16" at the tip. I figure this should be precise. I have no problem with shaping the airfoil, but could use some input on the taper step.

Thanks,
Curtis   
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lincoln
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« Reply #1 on: May 10, 2022, 08:29:13 PM »

If you sand the flat on the back first, at a constant angle, and then sand the front back to the high point shown on the plan, you will get a consistent shape.

A trick I used once, on an Apogee hlg wing, was to make two narrow, hard wood rails that I carefully sanded to the right thickness. Then I stuck a piece of fine sandpaper to the table, with the rails on each side. The sanding block, which was bare near the ends, ran on those rails. I made two tapered blanks this way.

If you go to charlesriverrc.org , you can find a page about the Apogee, including a link with instructions for accurate shaping of balsa surfaces. There are similar links, I think, for the tails of the  Allegro Lite and Bubble Dancer.
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High Point
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« Reply #2 on: May 10, 2022, 08:54:05 PM »

Lincoln,

"If you sand the flat on the back first, at a constant angle, and then sand the front back to the high point shown on the plan, you will get a consistent shape"

Are you saying in your above statement that the taper will work itself out and no initial taper is needed? I'm still real fuzzy about the process. Thanks for more detail if you have time.

Curtis
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stupid
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« Reply #3 on: May 11, 2022, 12:44:12 AM »

   Another technique is to sand the two wing halves at once side by side, with the bar straddling them. cut your one wing blank in half. By repeatedly swapping the wing halves side-by-side. As you get close to the right thickness, they will end up flat and rectangular in X-section. Tape a strip to the table against which the tips are butted when you swap the wings. Don't let sanding dust build up underneath the wing. Cutting the wing planform while the wing halves are together back-to-back. When you cut the wing blank in half to taper it,  glue it back together again with permanent stick glue for  sanding the airfoil. The butt joint should be perfect if the saw cut was clean! It's easier to hang onto the wing when it's in one piece.

   Courtesy of Mark Drela........ modified by Bob

http://charlesriverrc.org/articles/construction/markdrela_improvingsandingaccuracy.htm
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VictorY
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« Reply #4 on: May 11, 2022, 09:04:43 AM »

Lincoln,

"If you sand the flat on the back first, at a constant angle, and then sand the front back to the high point shown on the plan, you will get a consistent shape"

Are you saying in your above statement that the taper will work itself out and no initial taper is needed? I'm still real fuzzy about the process. Thanks for more detail if you have time.

Curtis

Yes, it will work itself out because as the plan form tapers towards the tips, so will the thickness if you sand towards the high point on the top side.
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VictorY
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« Reply #5 on: May 11, 2022, 09:13:35 AM »

I have a blank that has been stuck at that stage for a few years if you want to see an example. Pics will have to wait until this evening though.
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Tmat
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« Reply #6 on: May 11, 2022, 11:24:48 AM »

I've done it several ways.
For a one-off, I just put a piece of tape on one side of the centerline, and mark out the dbreaks on the edges of the blank.
Then use a razor plane to taper towards the tips. I check often with a Vernier or Micrometer. Then I use a hardwood sanding block with coarse sand paper (80 grit or so) and finish the span wise taper checking with a Vernier. Do the opposite side to match.
Another option is shown in the attached pictures. I made a tapered balsa jig with a piece of music wire glued to the top surface. With a bit of calculation I was able to sand both the span wise taper AND the high point to trailing edge taper at the same time. A Sweepette layout should work for this also.
Any questions just ask.

Tmat
Attached files Thumbnail(s):
Re: Wing Taper Methods
Re: Wing Taper Methods
Re: Wing Taper Methods
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Tmat
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« Reply #7 on: May 11, 2022, 11:32:16 AM »

Here are a couple of more sketches with details of how I sanded the two tapers. I always build CLGs or HLGs on glass plates fwiw.
This method worked very well. After tapering the blank, I just had to carve the front of the airfoil from the leading edge to the high point.

Tmat
Attached files Thumbnail(s):
Re: Wing Taper Methods
Re: Wing Taper Methods
Re: Wing Taper Methods
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Olbill
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« Reply #8 on: May 11, 2022, 03:16:22 PM »

If you sand the flat on the back first, at a constant angle, and then sand the front back to the high point shown on the plan, you will get a consistent shape.

A trick I used once, on an Apogee hlg wing, was to make two narrow, hard wood rails that I carefully sanded to the right thickness. Then I stuck a piece of fine sandpaper to the table, with the rails on each side. The sanding block, which was bare near the ends, ran on those rails. I made two tapered blanks this way.

If you go to charlesriverrc.org , you can find a page about the Apogee, including a link with instructions for accurate shaping of balsa surfaces. There are similar links, I think, for the tails of the  Allegro Lite and Bubble Dancer.

I've done a few wings with the Drela tangent sanding method. It takes some time - especially if you've got to figure out the angles as well as doing the actual work. It took me 30 hours to shape my Slow Poker wing.
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High Point
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« Reply #9 on: May 11, 2022, 05:13:47 PM »

All, thanks for the helpful information; I think I'm getting the idea now. Using the gigs for accuracy and a lot of sanding.

Tmat, the diagrams and pictures are great. Very nice looking wing too.
 
VictorY, I would like to see your photos and anything else you think would be helpful.


Curtis
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VictorY
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« Reply #10 on: May 11, 2022, 05:55:09 PM »

Here's that pic of my stalled CLG project. The thickness taper isn't quite as pronounced in planforms with near constant chords for the majority of the span, but the more chord tapers, the more the thickness will taper with this shaping method.
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Re: Wing Taper Methods
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dosco
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« Reply #11 on: May 12, 2022, 11:17:30 AM »

Here's that pic of my stalled CLG project. The thickness taper isn't quite as pronounced in planforms with near constant chords for the majority of the span, but the more chord tapers, the more the thickness will taper with this shaping method.

Stalled project? Don't you mean "long term project" ...?? lol.

(I understand all about that)

What is it?

Best-
Dave
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High Point
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« Reply #12 on: May 13, 2022, 02:49:46 PM »

All,

I'm starting to set up a tool described above and still reading a lot of posts to get a good grasp on wing shaping. Please review this past post I found from back in 2008 by Graham.

Hey Dan and all,

Marks designs and airfoils are well regarded by the RC glider guys. He has done much work on the aerodynamics of these types and yes, bigger and faster and in a different realm. Mark was (maybe still is) a free flighter as well. Had a few designs for peanuts published in Model Builder magazine many years ago.

My "light bulb" moment came when I was reading how Phil Ball described how he carved/sanded out the wings for these HLG/CLG's and showed some drawings. At first it didn't make sense just carving the back end of the wing to the same angle and getting a tapered root to tip shape as well. But then I mentally followed along and then cut the curved leading edge back and voila - tapered from root to tip just as described. Dead simple once you grasp what is happening and I have made CLG/HLG wings that way since. No muss no fuss and no measuring.

Airfoil shape is another topic for good discussion. I also belive the straight top (from peak to trailing edge) shape is the better choice although I tend to think so because it removes a lot of dead wood that otherwise isn't doing much. Also, a nice shinny smooth bottom wing and a somewhat rough curved leading edge bit for some turbulence.

I'm trying understand the "light bulb" moment paragraph above. Would someone shed light on that for me? Any photos on diagrams would be appreciated.

Curtis     
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High Point
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« Reply #13 on: May 13, 2022, 11:35:26 PM »

All,

I'm trying the method Tmat posted using the tapered balsa rail with a length of music wire glued on top. Instead of trying it out on my wing blank, I used a test piece the same width about 3" long. After some sanding, I could see that I need to move the wing forward from the edge of my board (no glass yet) due to the very thin TE.
Am I on the right track?

Curtis
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lincoln
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« Reply #14 on: May 13, 2022, 11:42:42 PM »


I've done a few wings with the Drela tangent sanding method. It takes some time - especially if you've got to figure out the angles as well as doing the actual work. It took me 30 hours to shape my Slow Poker wing.

It's been a long time, but I seem to remember 10 or 12 hours for a 36 inch wing. One of these days I should go back and finish the model, but whatever I do is likely to make the wing less pretty. Doing the job outdoors in bright sunlight probably made it easier. And less nasty for my lungs. A wing with a constant chord center section and tips with a straight taper would probably be quicker. I'm sure the flat back of the airfoil saves a lot of work, too.
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