Scratch built tapered wing

How do you build a tapered wing when the plans show only the root rib and the outboard rib. I saw something recently that explained the method but I have misplaced it. As I recall all the ribs are drawn using the root rib as a basis. Can anyone give me some help?

The usual way is the Sandwich method.

Make ply or ally templates for the tip and root.

You can then sandwich the correct number of blanks between and sand to shape.

Tony

You can also get Profili, the airfoil CAD program from its developer for <$20. Scan in the root and tip ribs, give it the dimensions and it will draw patterns for the ribs in between. I just got my first ribs laser-cut this way by drawing the airfoil and sending the file to Kyle Tankersly. Whoosh! Bonk! Laser cut ribs.

LAST EDITED ON Jul-20-04 AT 05:07 PM (CDT)
 
>How do you build a tapered wing when the plans show only the
>root rib and the outboard rib.

There are several ways to do this. Get a large sheet of paper about 36" long and about 18" wide. Draw a line the length of the large rib at one end of the paper showing it as a top view. I assume you know the wing span so draw the other rib at the other end of the paper the correct distance between ribs draw it as a top view too. All you need to do is draw a straight line the correct length to represent each rib. Now draw a straight line for the LE and a straight line for the TE. I hope you know the correct rib spacing or the correct number of ribs. Draw lines for all the other ribs. Now the picture will show you the length of each rib.

Repeat all the above steps drawing each rib as an END VIEW this will give you the height of all the ribs.

Now that you have dementions length and height of each rib you can make ribs. Starting with the SMALL rib and Calculate how much smaller the 2nd rib is than the rib next to it, then the 3rd rib, then 4th rib and so on.

Take the length of the short and divide it by the length of the longer rib next to it. EXAMPLE a 9" rib divided by a 10" rib = .90 which means the length of the 9" rib is 90% of the 10" rib. Write down the number in percent on each rib.

Make a drawing of the LARGE rib on a piece of paper. Be sure to include all the notches, holes, slots, etc. on the rib drawing. Take the rib drawing to the copy shop and make copies. EXAMPLE, If the #2 rib is .98% of the #1 rib then set the copy machine on .98% and make a copy of rib #1. The copy will be a pic of rib #2. Do this to all the ribs. When you are finished you will have a picture of all the ribs.

You can use the rib pictures to make tempelates if you want or place a sheet of carbon paper on top of your balsa wood and lay the copy on top of the carbon paper. Trace over the copy and it will transfer to the balsa wood. All you have to do now is cut out ribs with a razor knife and build a wing.

Its not as hard as it may seem. It takes me about 30 minutes to draw the pictures and do the calculations. It takes me about an hour to drive across town and make copies and return home again.

>>How do you build a tapered wing when the plans show only the
>>root rib and the outboard rib.
>
>There are several ways to do this. Get a large sheet of
>paper about 36" long and about 18" wide. Draw a line the
>length of the large rib at one end of the paper showing it
>as a top view. I assume you know the wing span so draw the
>other rib at the other end of the paper the correct distance
>between ribs draw it as a top view too. All you need to do
>is draw a straight line the correct length to represent each
>rib. Now draw a straight line for the LE and a straight
>line for the TE. I hope you know the correct rib spacing or
>the correct number of ribs. Draw lines for all the other
>ribs. Now the picture will show you the length of each rib.
>
>Repeat all the above steps drawing each rib as an END VIEW
>this will give you the height of all the ribs.
>
>Now that you have dementions length and height of each rib
>you can make ribs. Starting with the SMALL rib and
>Calculate how much smaller the 2nd rib is than the rib next
>to it, then the 3rd rib, then 4th rib and so on.
>
>Take the length of the short and divide it by the length of
>the longer rib next to it. EXAMPLE a 9" rib divided by a
>10" rib = .90 which means the length of the 9" rib is 90% of
>the 10" rib. Write down the number in percent on each rib.
>
>Make a drawing of the LARGE rib on a piece of paper. Be
>sure to include all the notches, holes, slots, etc. on the
>rib drawing. Take the rib drawing to the copy shop and make
>copies. EXAMPLE, If the #2 rib is .98% of the #1 rib then
>set the copy machine on .98% and make a copy of rib #1. The
>copy will be a pic of rib #2. Do this to all the ribs.
>When you are finished you will have a picture of all the
>ribs.
>
>You can use the rib pictures to make tempelates if you want
>or place a sheet of carbon paper on top of your balsa wood
>and lay the copy on top of the carbon paper. Trace over the
>copy and it will transfer to the balsa wood. All you have
>to do now is cut out ribs with a razor knife and build a
>wing.
>
>Its not as hard as it may seem. It takes me about 30
>minutes to draw the pictures and do the calculations. It
>takes me about an hour to drive across town and make copies
>and return home again.
>
>Gary Weaver [email protected]

Hmph...seems kinda labor-intensive, not to mention problems with constant-size spar or LE cutouts...you wouldn't want to scale those down. Whatever melts your butter, though.

LAST EDITED ON Jul-21-04 AT 07:06 AM (CDT)
 
What your working for is the correct geometrical rib size and shape. Naturally the slots and holes in the drawing will be reduced too by the copy machine. The slots and holes will be in the correct location and it will be up the the builder to cut them out the correct size. I like to make cardboard templates. Cardboard from a cerial box, shoe box, back side of a note pad works great. I transfer the xerox copies to the cardboard with carbon paper and a pencil. It takes me about an hour to transfer and cut out a good set of templates with sissors and a razor knife. Templates are good if you want to make sure all the ribs in the left wing are the exact same size as the ribs in the right wing. I can trace around all the templates with an ink pin and cut out all the balsa ribs in about an hour with a razor knife. I cut out a complete set of 22 ribs with a razor knife in about 2 hours that included making the templates. I save the templates just in case I want to build another set of ribs some day. Once the templates are made 1 hours work is not bad for 22 ribs. I use a new razor blade in the razor knife and there is almost NO sanding required.

Another way is to use the Bob Hunt lost foam method to generate the rib shapes. That works very well.

When I have used the sandwich method of rib production, I use the pinned together stack of blanks with a template on each side of the stack, but change the alignment of the stack depending on what part of the ribs I am shaping. I got the idea from an article written by Claus Maikis some time ago in Aeromodeller. The reason for this change is that if you just clamp the stack together and shape away, because the stack is considerably shorter than the wingspan of the finished plane, the ribs all end up with a very bevelled edge. Especially at the leading and trailing edges.

The slight difference in my system is to lay the block of blanks on the plan before shaping to check the three major points of alignment. First with the root rib template aligned in plan view with the root rib on the plan, I check that the spar notch on the tip rib template is over the spar on the plan. This sets the correct angle for the spar whether swept or not. I then shape the middle part of the ribs above and below the stack. You can cut the spar notches at this setting too, or after the ribs are fully sanded. Then push the stack back and repeat the setting procedure with the TE of the root and tip templates lined up with the TE. This sets the exact angle for the TE even if swept forward. At this setting shape the back part of the ribs up to where you have already done. Now push the stack forward and repeat the setting so that the LE of the templates line up with the LE and shape the front of the ribs, and if required the notch for the LE.

This method gets the angles of the LE and spar notches exactly right for a perfect fit against the spar and LE. There are no more of those LE cutouts that have sharp angles needing gap filling glue. I hope you can follow what I am getting at.

LAST EDITED ON Jul-20-04 AT 10:42 PM (CDT)
 
L.L.

Thought I would post a couple of pictures so you could see how it's done with the "stack and sand" method.

It's really not all that hard, but having a scroll saw or band saw to make the two end rib templates makes it a lot easier. In the photo you will see both one made for a tapered leading edge and one for a tapered trailing edge, i.e., Ringmaster.

Stack the number of planks of 1/16th or whatever you are going to use between the two template ribs. Drill holes for the screws that will go through to hold it in place while you are working on it. Insert the screws and snug them up, but not too tight. I usually cut away as much of the excess wood away with a razor saw, etc. Then sand away with say 60 grit paper on a sanding block to within close to the edge of the template. Then use something like 220 or 320 to finish sanding down the the final shape. I usually cut the slots for the spars, leading edge and trailing edge planking into the template ribs. Then cut/carve/sand these out. When you are all done loosen the screws and WAHLAA...You have a set of real nice ribs.

The tapered trailing edge is really easier, because the tapered end of the ribs will also have a tapered edge when you are done. You can carefully sand the taper down on each one a little once you are done, but not too much or it will change the shape of the ribs. That's what I've read many don't like about doing this "oldtime" way, but it will still turn out pretty darn good if you are carefull. I usually leave just a hint of the taper on the ribs when the taper is on the leading edge and just plain don't worry about it if it's on the trailing edge (it won't show or affect construction on the trailing edge) and it usually doesn't show anyway on the leading edge once you have the wing covered.

You do....repeat do!!! have to space the ribs out equal distance apart for the entire span. The mehtod is based on this fundamental geometric property and will not turn out right at all if you don't space them all the same.

Give it a try and then see what you think.

Regards,

PS: Should have noted that the finished ribs shown are Not tapered. Just included them so you could get an idea of what the finished ribs will look like.

LAST EDITED ON Jul-20-04 AT 11:07 PM (CDT)
 
I think the "stack and sand" method would be the least labor-intensive. All other methods (save the laser cutting one) requires you to slice out each rib separately, which is a huge pain in the neck. But I do wonder about the rib edges - it seems to me that the angle of the surface at the edge would be too steep, so the whole wing surface will need to be trued up once assembled using a long sanding block.

LAST EDITED ON Jul-20-04 AT 11:47 PM (CDT)
 
Iskandar,

Just offering info on how the stack and sand method works and trying to explain some of it's limitations.

The tapered edge on the ribs is what seems to worry some the most about using this method today. It is a pretty steep angle for a small wing with a small number of ribs. However, my experience is, especially if you are going to sheet the leading edge, it definitely will not make a whole lot difference. There will be a little less surface area for the sheeting to adhere to on the ribs, but most of the strength of the joint at the leading edge will be on the leading edge itself.

I'd also note that it was about the only way people had to build a tapered wing back when a lot of the oldtime and classic designs that we so admire today were built.

I am definitely not the in house expert. This method is good enuogh for what I build and fly. I like to build as accurate as possible, but I have learned over the years not to sweat the really small stuff. If one has the wherewithall to afford the really fancy stuff...go for it...and sometimes I do. I have also learned, as an engineer for the last twenty + years, not to carry a calculation out to 10 places just because your calculator will give you a 10 digit answer, when your data is only good to 2 places. Does the analogy make sense???

Try it, you'll like it...if you don't, that's okay, too

Cheers,

The sandwich method is indeed quite easy. I have minimized the taper which is sanded into each rib by alternating each of the "real" ribs with a piece of 1/8" "junk" balsa (you know, the R/C wood). Throw away the ribs in between the real ones when sanding is finished.

I also improve accuracy by pinning the stack together using 2 short pieces of 1/4" steel rod, and not a bolt. The rib holes are cut using a sharpened piece of 1/4" dia. brass tubing. The fit is perfect with no slop and stays that way. The same 1/4" holes are later used with steel rods to jig the wing for assembly.

LAST EDITED ON Jul-21-04 AT 07:09 AM (CDT)
 
>How do you build a tapered wing when the plans show only the
>root rib and the outboard rib. I saw something recently
>that explained the method but I have misplaced it. As I
>recall all the ribs are drawn using the root rib as a basis.
> Can anyone give me some help?

If you want to get high tech, use any good CAD program and Rhino 3D.

Just scan or plot the rib and tip in a CAD program. Make sure you put the spar notches and whatever cutouts you want. Set them out on the Z-Axis the same distance apart they would be in the real ship. Save as a .dxf. Close the CAD program, open the file with Rhino 3D. Create a surface between the two. Then section where the ribs would go. Save. Bring back up in CAD. If you are using conventional methods, it is simple to bring the ribs back to "0" along the Z-Axis. If you use diagonal ribs, it gets a little trickier, but can be done without too much trouble.

Now, you have a set of rib templates that are as close to perfect as you can get. You can either print them off on your printer and use as templates the .dxf file off to a laser cutter and have them done.

Or, for an easier way, use the template stack method outlined by others. If you actually bolt the templates and rib blanks together, and have sharp tools, you will not get any appreciable distortion. I always make the tip rib templates a little thicker (e.g. fatter) by about 1/32 than shown on the plans to adjust for squaring up the rib for application of the cap strips and sheeting.

Your mileage may vary.

V/r

Thanks for the input, the sandwich method looks fast and close enough. I am building a FW 190 on a Roadrunner wing. We'll see how it goes.