Question building polyhedral wing, and wings in general
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Guys I'm building my 2year old a old gilder kit (he's been begging for a airplane and transmitter for christmas ) i've had sitting on the shelf for years (a bridi tercel).
I've had this "problem" with every kit I've ever built and although it doesn't really affect things it just drives me nuts.
heres the setup...
on the polyhedral portion of the wing it tapers.. the trailing edge is straight but the leading edge tapers back.
so I build the outer wing section on the plans and everything is perfect, I then set the two ajoining ribs to the proper angles for the polyhedral, then I join them together, everything fits perfect, except for the fact the trailing edge is no longer perfectly straight inline with the trailing edge on the center panelas on the plan...
if I do make the the trailing edge perfectly straight with the center section then there is a gap at the leading edge of my ajoining ribs..
to put it into perspective if I laid the wing on a table that was agains the wall, then slid it all the way back so the trailing edge is against the wall the center section would follow the wall perfect, but the polyhedral (swept back) section the trailing edge tapers away from the wall...
why is this?
I've had this "problem" with every kit I've ever built and although it doesn't really affect things it just drives me nuts.
heres the setup...
on the polyhedral portion of the wing it tapers.. the trailing edge is straight but the leading edge tapers back.
so I build the outer wing section on the plans and everything is perfect, I then set the two ajoining ribs to the proper angles for the polyhedral, then I join them together, everything fits perfect, except for the fact the trailing edge is no longer perfectly straight inline with the trailing edge on the center panelas on the plan...
if I do make the the trailing edge perfectly straight with the center section then there is a gap at the leading edge of my ajoining ribs..
to put it into perspective if I laid the wing on a table that was agains the wall, then slid it all the way back so the trailing edge is against the wall the center section would follow the wall perfect, but the polyhedral (swept back) section the trailing edge tapers away from the wall...
why is this?
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I'm having trouble visualing what you're doing but perhaps this will help.
At any dihedral joint half the angle needs to be on each butt joined piece. Otherwise the part with all the angle will not fit against the part with a 90 degree face. Trigonometry says that the angled face will be longer and won't match the face done at a right angle.
Is that what you meant? If not then what about a good close up picture?
These days I build the dihedral breaks and panels all in one go by using a pair of building boards angled upwards to match the dihedral angle. For polyhedral I start on one side and work across to the center and then over to the other tip by only cutting the spars and leading and trailing edges to length. THis way I build around the dihedral joints and can fit everything quite finely.
At any dihedral joint half the angle needs to be on each butt joined piece. Otherwise the part with all the angle will not fit against the part with a 90 degree face. Trigonometry says that the angled face will be longer and won't match the face done at a right angle.
Is that what you meant? If not then what about a good close up picture?
These days I build the dihedral breaks and panels all in one go by using a pair of building boards angled upwards to match the dihedral angle. For polyhedral I start on one side and work across to the center and then over to the other tip by only cutting the spars and leading and trailing edges to length. THis way I build around the dihedral joints and can fit everything quite finely.
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Sounds like what you need is a tapered wall ![Smile](https://www.rcuniverse.com/forum/images/smilies/smile.gif)
I think that the problem is that you start getting some complex angles when you combine even small amounts of wing twist and dihedral (or polyhedral). You may want to temporarily bond the wing panels together as designed then check for twist by sighting down the wing or some other method. There is one method in particular that comes to mind, but I am having a difficult time finding the link for it right now. If I find it, I will post it. In any case, if the wing appears twisted, you can fix that with the covering, and that may pull the trailing edge back in line.
Good luck! I hope your son enjoys his first plane!
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I think that the problem is that you start getting some complex angles when you combine even small amounts of wing twist and dihedral (or polyhedral). You may want to temporarily bond the wing panels together as designed then check for twist by sighting down the wing or some other method. There is one method in particular that comes to mind, but I am having a difficult time finding the link for it right now. If I find it, I will post it. In any case, if the wing appears twisted, you can fix that with the covering, and that may pull the trailing edge back in line.
Good luck! I hope your son enjoys his first plane!
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ORIGINAL: BMatthews
These days I build the dihedral breaks and panels all in one go by using a pair of building boards angled upwards to match the dihedral angle. For polyhedral I start on one side and work across to the center and then over to the other tip by only cutting the spars and leading and trailing edges to length. THis way I build around the dihedral joints and can fit everything quite finely.
These days I build the dihedral breaks and panels all in one go by using a pair of building boards angled upwards to match the dihedral angle. For polyhedral I start on one side and work across to the center and then over to the other tip by only cutting the spars and leading and trailing edges to length. THis way I build around the dihedral joints and can fit everything quite finely.
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So you are saying your single tapered wing ( the leading edge) turns into a double tapered wing ( the leading AND trailing edge)?
I build all four panels flat with a steel rule at the trailing edge to ensure that it is straight. Then the tricky part, sanding and fitting the joints while keeping the TE straight.
I have learned the hard way GO SLOW! a few strokes with the sanding block, check fit, repeat. Might take me all day to fit the poly and dihederal joints but I am usually less than 1/32 off some times less!
BM's way takes all the guess work out of it! A hinged board I am assuming? Sure would like to see pictures. Could be another winter project.....
Joe
I build all four panels flat with a steel rule at the trailing edge to ensure that it is straight. Then the tricky part, sanding and fitting the joints while keeping the TE straight.
I have learned the hard way GO SLOW! a few strokes with the sanding block, check fit, repeat. Might take me all day to fit the poly and dihederal joints but I am usually less than 1/32 off some times less!
BM's way takes all the guess work out of it! A hinged board I am assuming? Sure would like to see pictures. Could be another winter project.....
Joe
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ORIGINAL: thermaler
BM's way takes all the guess work out of it! A hinged board I am assuming? Sure would like to see pictures. Could be another winter project.....
BM's way takes all the guess work out of it! A hinged board I am assuming? Sure would like to see pictures. Could be another winter project.....
Once upon a time, in a much earlier time (before retirement), in a place far, far to the NORTH, in a world of snow and cold (back when I lived and worked in Minnesnowta), I used to build a glider wing a week. I had a 2m with cleverly strapped on wings, and the Eppler airfoils were just being discovered. I wanted to test airfoils and focusing on one wing planform seemed a sensible thing to do. I was developing the glider so had about 3 fuselages that were basically interchangeable so why not make wings that'd interchange as well.
The builder was still constructing houses in the neighborhood and the trash bins often had fairly large pieces of usable stuff. I used to layer my work table with insulation board. It took pins better than cork board. When it got too cut up, I'd rotate it 180 and keep building. I had some surplus and some structural paneling and slapped the thing pictured together one night. Used liquid nails. It weighs a ton.
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Hinging can be a problem.
Thick material in the top cramps your hinge location. When I first setup my building table for polyhedral building, I used two sheets of that homosote board. I think that's what it was called. I glued a strip of scrap cotton cloth along the top of the seam between the two boards. You need to have no gap right at the polyhedral break because you've got a lot of building to do right there. The hinged outer board then had to be supported firmly at whatever angle you were building. The scrap bed sheet did a good job as a hinge and allowed me to use T-pins.
I built that board after seeing how little was to be gained from greater or lesser polyhedral angles. Also, built it solid because all I wanted it for was fast production of a bunch of similar 2m wings. I figured I'd throw it into one of the construction site dumpsters when I got through with it. That was 30 years and 3 different states ago.
Dragging it out brought back memories...... Gotta go fly that glider soon.
Thick material in the top cramps your hinge location. When I first setup my building table for polyhedral building, I used two sheets of that homosote board. I think that's what it was called. I glued a strip of scrap cotton cloth along the top of the seam between the two boards. You need to have no gap right at the polyhedral break because you've got a lot of building to do right there. The hinged outer board then had to be supported firmly at whatever angle you were building. The scrap bed sheet did a good job as a hinge and allowed me to use T-pins.
I built that board after seeing how little was to be gained from greater or lesser polyhedral angles. Also, built it solid because all I wanted it for was fast production of a bunch of similar 2m wings. I figured I'd throw it into one of the construction site dumpsters when I got through with it. That was 30 years and 3 different states ago.
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#8
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I believe to build one today, with today's materials, would give a much more transportable board.
I use foam insulation board today for lots of things. It's sold in big sheets that're different thicknesses. The most useful is just under an inch thick. I made my wife a "quilter's ironing board" with the stuff a few years back. It's a "board" that lays over the standard ironing board. It's much wider and has a square planform. The insulation board wouldn't be strong enough by itself so I glued it to thin sheet plywood. That combination would be rigid enough for a wing board. It wouldn't be for the q-board, so I glued longerons and such to make the q-board work for her. She covered it with iron board stuff and it works great. The backed insulation board would work great for our size and shape wing board, and you could get her to pay for all the materials if you made her an ironing board with the wasteage.
Or tell her the basement needed some insulation over in the corner. Lean your wing board in that corner with the foam showing and..... ![Wink](https://www.rcuniverse.com/forum/images/smilies/wink.gif)
The hinging would still be a design challenge. If you wanted no gap and a pinnable upper surface that is.
I use foam insulation board today for lots of things. It's sold in big sheets that're different thicknesses. The most useful is just under an inch thick. I made my wife a "quilter's ironing board" with the stuff a few years back. It's a "board" that lays over the standard ironing board. It's much wider and has a square planform. The insulation board wouldn't be strong enough by itself so I glued it to thin sheet plywood. That combination would be rigid enough for a wing board. It wouldn't be for the q-board, so I glued longerons and such to make the q-board work for her. She covered it with iron board stuff and it works great. The backed insulation board would work great for our size and shape wing board, and you could get her to pay for all the materials if you made her an ironing board with the wasteage.
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The hinging would still be a design challenge. If you wanted no gap and a pinnable upper surface that is.
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OK, back to the main topic of this thread,
99GrandT,
If your joint is acceptable, and the outer section of wing lines up properly, don't worry about any minor amount of TE sweep. There is nothing magic about keeping the TE in a line from root to tip. Heck, a planform with TE forward sweep might make for better performance. Truth is, any performance change good or bad will be miniscule.
99GrandT,
If your joint is acceptable, and the outer section of wing lines up properly, don't worry about any minor amount of TE sweep. There is nothing magic about keeping the TE in a line from root to tip. Heck, a planform with TE forward sweep might make for better performance. Truth is, any performance change good or bad will be miniscule.
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The hingeing issue for a nice building board had me stumped as well until I got the idea of KISS. Here's the results in a little schematic. I'd show pictures of them but the boards are packed in a box in the garage behind a huge number of other boxes. The boards and rest of my model stuff won't see the light of day until the basement shop renos are done. And I mean T bar ceilings, proper lighting and electrical, finished walls and even click together engineered wood flooring along with kitchen like cabinets for storage.
In the meantime here's a sketch of what I'm using that works superbly.
In the meantime here's a sketch of what I'm using that works superbly.
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I know I'm splitting hairs on this one as it probably won't affect anything but i'd still like to learn how to do it properly...
Here are some photos, in progression showing the outer panel and then inner panel on the plans...
then showing them together and the difference in the trailing edge....
if I were to rotate the adjoining wing rib on the outer panel any to compensate then the spacing between the ribs would be off / not even from leading edge to trailing edge.
also as far as the wing rib/joints at the polyhedral the only way I could figure out how to do it was to put the inboard panel at 90 and adjust the outer panels adjoining rib to the proper angle to give me the amount of polyhedral I need.
if someone has photos showing how to do it "properly" where they both have the same angle I would love it so I can learn.
P.S The camera angle makes it look like some of the ribs are outa place / skewed I assure you there not, its cause its not a flat bottom wing and the leading edge is raised also I only have it held down on the trailing edge and the wax paper under it isn't smoothed out.
Here are some photos, in progression showing the outer panel and then inner panel on the plans...
then showing them together and the difference in the trailing edge....
if I were to rotate the adjoining wing rib on the outer panel any to compensate then the spacing between the ribs would be off / not even from leading edge to trailing edge.
also as far as the wing rib/joints at the polyhedral the only way I could figure out how to do it was to put the inboard panel at 90 and adjust the outer panels adjoining rib to the proper angle to give me the amount of polyhedral I need.
if someone has photos showing how to do it "properly" where they both have the same angle I would love it so I can learn.
P.S The camera angle makes it look like some of the ribs are outa place / skewed I assure you there not, its cause its not a flat bottom wing and the leading edge is raised also I only have it held down on the trailing edge and the wax paper under it isn't smoothed out.
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I don't remember but one thing I did to make kit wing ribs fit. I remember cap stripping one once long ago. When I started drawing my own wings and going from there, I never looked back. When scratch building, that outer rib would often get remade with some extra height and the new fat one would then get sanded to match.
I preferred to cap strip that joint when scratch building too. It gives the covering a lot more area to grip in a place that benefits a lot from being wider. It looks better too. I made up a number of special shape sanding blocks years ago (they're still working today) and one was rounded at just the right curve for my 2m polyhedral's one cap strip in that joint. But you really don't need any special curved sanding block. Wrap sandpaper around your finger and use the area of finger that is shaped the curve you need. It's not critical.
The cap strip can wind up rather thin in places until you get the feel for fitting it.
It's fiddly work, but really looks good later on, and pretty planes fly better.
I preferred to cap strip that joint when scratch building too. It gives the covering a lot more area to grip in a place that benefits a lot from being wider. It looks better too. I made up a number of special shape sanding blocks years ago (they're still working today) and one was rounded at just the right curve for my 2m polyhedral's one cap strip in that joint. But you really don't need any special curved sanding block. Wrap sandpaper around your finger and use the area of finger that is shaped the curve you need. It's not critical.
The cap strip can wind up rather thin in places until you get the feel for fitting it.
It's fiddly work, but really looks good later on, and pretty planes fly better.
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The picture showing the joined trailing edge is fuzzy around the trailing edge becuase it focused on the clamp so I can't see the problem you describe. But doing it the way you are with the inner panel rib upright and the outer panel rib with all the dihedral angle is one reason why you're having trouble getting the trailing edge to fit smoothly. The angle needs to be split evenly between both so the parts fit smoothly. Done the way you did it you're bound to get a step in the upper surface of the trailing edge where they meet. See the sketch below.
In addition to the purely geometrical issue a couple of construction technique factors can come into play. First is that if you block sand the wing faces in prep for doing the joint in an attempt to get a nicer fit you need to support the ribs better so they don't just push inwards and not get sanded enough. Also you want to use the edge of a table or custom sanding board to guide the block so the trailing and leading end grain sections don't get abraded more than the face grain of the rib. The end grain will tend to cut easier so it's important to watch for that. You can make it all work out easier by using fresher sandpaper so it cuts easier and use minimal pressure so the grains of the paper can cut without being forced.
In addition to the purely geometrical issue a couple of construction technique factors can come into play. First is that if you block sand the wing faces in prep for doing the joint in an attempt to get a nicer fit you need to support the ribs better so they don't just push inwards and not get sanded enough. Also you want to use the edge of a table or custom sanding board to guide the block so the trailing and leading end grain sections don't get abraded more than the face grain of the rib. The end grain will tend to cut easier so it's important to watch for that. You can make it all work out easier by using fresher sandpaper so it cuts easier and use minimal pressure so the grains of the paper can cut without being forced.
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Use a bit of the scrap to make angle sanding blocks or jigs. Use only half of the total angle to make the block or jig then sand both sides as demonstrated. Some kits have these included.
If you have a table saw, or a bud with one, make a custom block with half the dihedral angle and half the polyhedral angle and use a biit of contact cement to stick on your paper.
You will want to use glue that lets you peel the old paper off.
If you have a table saw, or a bud with one, make a custom block with half the dihedral angle and half the polyhedral angle and use a biit of contact cement to stick on your paper.
You will want to use glue that lets you peel the old paper off.
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99
I think what you are seeing is an optical illusion. When you raise the tip on the polly break it will automaticlly change the trailing edge line. (and the leading edge line) As long as it's the same on both sides it should not matter. How it lines up against the wall means nothing.![Wink](https://www.rcuniverse.com/forum/images/smilies/wink.gif)
Marc 540[8D]
I think what you are seeing is an optical illusion. When you raise the tip on the polly break it will automaticlly change the trailing edge line. (and the leading edge line) As long as it's the same on both sides it should not matter. How it lines up against the wall means nothing.
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Marc 540[8D]
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There is yet another strategy to deal with getting the ribs sanded to match. Don't have two ribs.
For a number of wings I built, I produced a thicker rib for that location. It seemed to me that all you actually need at that rib location is a thicker surface area for the covering so it won't pull away in the depression it sees. There is no real value to having two halves glued together for strength. You can't beat the strength of one piece. What matters is getting the spars, LE, and TE connected strongly. Where that rib (or two) mate to the LE, TE, spars matters in that it helps support the butt joints, and having just one wider support is often easier to match up.
The wing sections got built with the thick rib pinned in to insure a more uniform wing, then pulled out and used to glue up the other part of the wing.
I use a right angle when framing both halves. Leave some extra length to the LE, TE, spars and when joining the polyhedral break, sand to fit. The right angle insures the halves join with no front or back "sweep" to the outer part. Paper plans will sometimes be "tilted" or have a "lean" to the print. It's always worthwhile to check for that with a big right angle. You still will need to bring the two halves of the wing together with a sanding block. Getting 4 points to match up perfectly isn't going to happen on the 1st fit. Having an extra thick rib to back each butt joint has always seemed stronger to me. And easier to do.
If you make the thick rib just twice as thick as one of the originals you are discarding, you also wind up with a lighter wing. Heck, you may save a thousanth of an ounce in glue.
That ought to result in at least an extra minute of time on your next thermal......
For a number of wings I built, I produced a thicker rib for that location. It seemed to me that all you actually need at that rib location is a thicker surface area for the covering so it won't pull away in the depression it sees. There is no real value to having two halves glued together for strength. You can't beat the strength of one piece. What matters is getting the spars, LE, and TE connected strongly. Where that rib (or two) mate to the LE, TE, spars matters in that it helps support the butt joints, and having just one wider support is often easier to match up.
The wing sections got built with the thick rib pinned in to insure a more uniform wing, then pulled out and used to glue up the other part of the wing.
I use a right angle when framing both halves. Leave some extra length to the LE, TE, spars and when joining the polyhedral break, sand to fit. The right angle insures the halves join with no front or back "sweep" to the outer part. Paper plans will sometimes be "tilted" or have a "lean" to the print. It's always worthwhile to check for that with a big right angle. You still will need to bring the two halves of the wing together with a sanding block. Getting 4 points to match up perfectly isn't going to happen on the 1st fit. Having an extra thick rib to back each butt joint has always seemed stronger to me. And easier to do.
If you make the thick rib just twice as thick as one of the originals you are discarding, you also wind up with a lighter wing. Heck, you may save a thousanth of an ounce in glue.
![Wink](https://www.rcuniverse.com/forum/images/smilies/wink.gif)
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#17
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There is yet another strategy to deal with getting the ribs sanded to match. Don't have two ribs.
For a number of wings I built, I produced a thicker rib for that location. It seemed to me that all you actually need at that rib location is a thicker surface area for the covering so it won't pull away in the depression it sees. There is no real value to having two halves glued together for strength. You can't beat the strength of one piece. What matters is getting the spars, LE, and TE connected strongly. Where that rib (or two) mate to the LE, TE, spars matters in that it helps support the butt joints, and having just one wider support is often easier to match up.
The wing sections got built with the thick rib pinned in to insure a more uniform wing, then pulled out and used to glue up the other part of the wing.
I use a right angle when framing both halves. Leave some extra length to the LE, TE, spars and when joining the polyhedral break, sand to fit. The right angle insures the halves join with no front or back "sweep" to the outer part. Paper plans will sometimes be "tilted" or have a "lean" to the print. It's always worthwhile to check for that with a big right angle. You still will need to bring the two halves of the wing together with a sanding block. Getting 4 points to match up perfectly isn't going to happen on the 1st fit. Having an extra thick rib to back each butt joint has always seemed stronger to me. And easier to do.
If you make the thick rib just twice as thick as one of the originals you are discarding, you also wind up with a lighter wing. Heck, you may save a thousanth of an ounce in glue.
That ought to result in at least an extra minute of time on your next thermal...... ![Wink](https://www.rcuniverse.com/forum/images/smilies/wink.gif)
BTW, that fat rib got sanded concave along the top and convex along the bottom after the glue had dried.
For a number of wings I built, I produced a thicker rib for that location. It seemed to me that all you actually need at that rib location is a thicker surface area for the covering so it won't pull away in the depression it sees. There is no real value to having two halves glued together for strength. You can't beat the strength of one piece. What matters is getting the spars, LE, and TE connected strongly. Where that rib (or two) mate to the LE, TE, spars matters in that it helps support the butt joints, and having just one wider support is often easier to match up.
The wing sections got built with the thick rib pinned in to insure a more uniform wing, then pulled out and used to glue up the other part of the wing.
I use a right angle when framing both halves. Leave some extra length to the LE, TE, spars and when joining the polyhedral break, sand to fit. The right angle insures the halves join with no front or back "sweep" to the outer part. Paper plans will sometimes be "tilted" or have a "lean" to the print. It's always worthwhile to check for that with a big right angle. You still will need to bring the two halves of the wing together with a sanding block. Getting 4 points to match up perfectly isn't going to happen on the 1st fit. Having an extra thick rib to back each butt joint has always seemed stronger to me. And easier to do.
If you make the thick rib just twice as thick as one of the originals you are discarding, you also wind up with a lighter wing. Heck, you may save a thousanth of an ounce in glue.
![Wink](https://www.rcuniverse.com/forum/images/smilies/wink.gif)
![Wink](https://www.rcuniverse.com/forum/images/smilies/wink.gif)
BTW, that fat rib got sanded concave along the top and convex along the bottom after the glue had dried.