Ellipticle Curves vs Circular Curves

[troubleScottie]

Sill in the design/thinking stage. Trying to work out a design.

I am looking at peoples' various designs and design discussions and noticed that many of the designs are using circular rather than elliptical curves.

The classic Grumman has elliptical lines.

Is there some technical reason to use circular curves other than easier to cut ie a single pivot. I have found in the wood working sites an elliptical cutting jig which appears to make the task fairly straight forward.

Is it harder to bend plywood along an elliptical edge? It looks like it would be easier except as you approach the crossing of the major axis.

Is it harder to build a hatch that is elliptical ? Or if the hatch spans steepest part of the curve ( where the major axis joins the curve )

Is it harder to get the two roof curves ie the inner roof line and outer roof line assuming you have a template with each curves (two templates).


On a related topic, what thickness of plywood can be easily bent without any cutting. It appears 1/8" works well. Can the same be said for 1/4" ply?

Are there curves that are too small? Obviously very small (3" radius) would be extremely difficult/impossible. But where is the break even point ie 10", 15", 20" ? I suppose one could try them out, but that could be pretty extensive/expensive building so many tests.

[bobhenry]

Here are some real early in my build discussions on the same topic.....

http://tnttt.com/viewtopic.php?f=27&t=13925&p=181475#p181475

Looks like the images didn't survive a couple of crashes.

Luckily I had them tucked away

[bobhenry]

Here is the profile info you may be looking for................

http://www.angib.pwp.blueyonder.co.uk/teardrop/tear55.htm

[aggie79]

My build has circular/radial curves, elliptical curves and compound curves. Following are my thoughts on your questions.

Sill in the design/thinking stage. Trying to work out a design. I am looking at peoples' various designs and design discussions and noticed that many of the designs are using circular rather than elliptical curves. The classic Grumman has elliptical lines. Is there some technical reason to use circular curves other than easier to cut ie a single pivot. I have found in the wood working sites an elliptical cutting jig which appears to make the task fairly straight forward.

My guess is that circular/radial curves are used because they are easier to layout than elliptical or compound curves. My teardrop has all three types of curves. (The upper front quarter of my teardrop profile is the "Trailer For Two". The lower front quarter is flat/vertical with a lower 6" radius". The rear of my teardrop profile is a blend of the "Grumman" and "Cub" profiles.)

I used a template to cut my sidewalls. To layout the profile, I drew a three inch grid and plotted the approximate x-y coordinates. I then placed angled wood stops using double-sided tape at the "hard points" and stretched a wood batten around the hard points. This technique helps blend/transition the curves and lines so that profile was smooth and there were no breaks. (I believe the term is that the curves and lines are tangent.) It did take a few tries with the batten and rearranging some of the hard points to get everything as I wanted.



On a few of my circular/radial curves I used a router with a home-made trammel (compass) to cut arcs. However, the majority of curves were rough cut free-hand and then I used a belt sander to final cut to the layout line.

Is it harder to bend plywood along an elliptical edge? It looks like it would be easier except as you approach the crossing of the major axis.

I don't think this is an issue in most cases. It is easier to bend the plywood on elliptical curve as the radius decreases than it is to bend plywood from a flat to a "tight" (12" or less radius). Going from the flat to a larger radius is not an issue. (This is predicated on using high-quality 1/8"/3mm plywood and not a thicker and/or lesser quality plywood.)

Is it harder to build a hatch that is elliptical ? Or if the hatch spans steepest part of the curve ( where the major axis joins the curve )

My hatch is a compound elliptical and radial curve. It is also longer (as are Grumman and Cub hatches) than most hatches. The lower rear radius of my hatch is 10". That was the most difficult part to "skin" with plywood.

As many others have said, it is best to frame your hatch in place. It will help greatly with maintaining the shape of the hatch and skinning it. If you do this method be sure you leave out part for the galley bulkhead so that you can remove the temporary fasteners to release the hatch.

Is it harder to get the two roof curves ie the inner roof line and outer roof line assuming you have a template with each curves (two templates).

I'm not sure what you mean by this. I don't have a picture of this but I made a jig to hold a pencil 1-1/2" inside of the profile line. I used this to layout the inner profile for the ceiling. (My spar depth is 1-1/2".)

On a related topic, what thickness of plywood can be easily bent without any cutting. It appears 1/8" works well. Can the same be said for 1/4" ply?

1/8" high quality plywood will bend easily to 18" radius. Less than that is more difficult but not impossible. I bent plywood around the lower 6" radius at the front of my teardrop. I would not try to bend 1/4" plywood, especially big box plywood, at any radii tighter than 18".

Are there curves that are too small? Obviously very small (3" radius) would be extremely difficult/impossible. But where is the break even point ie 10", 15", 20" ? I suppose one could try them out, but that could be pretty extensive/expensive building so many tests.

When you get below 12" radius with 1/8" plywood, things start getting difficult.

[troubleScottie]

Great response.

It answers all the questions (even the vague ones on the compound curve and how to trace/draw the inner roof curve ).

Your images supplies a nice to solution to both drawing and creating an clean, precise template.

PS. I believe the term you were looking for is "spline".

[aggie79]

By the way, when I was trying to decide what profile to use, I overlaid several profiles. You can see this in the (very rough) sketch below:



The profiles are registered to the left side of the drawing (back of the teardrop). If there were profiles for the same design in 4' x 8' and 4' x 10' I used the larger. I had thought there would be greater differences but there were not.

Originally I was thinking about building the Cub/Modernaire profile; however, the extreme curves resulted in quite a loss of storage space. So I "Frankenstein-ed" several designs together to try to achieve a sleek profile but have usable storage space. This is what I ended up building.



To me, at least, the end result was attractive, and has been very functional.

[daveesl77]

I have "cedar strip" curves.

When I made my pattern out of cardboard first I knew the approximate dimensions I wanted for the trailer, 5' tall x 10' long max to curve outside with a 9' trailer bed. I had a bunch of really thin, 10' long cedar strips I'd cut while trying to decide the final thickness of the cedar on the trailer sides. I stuck a couple of finish nails in the bottom of the cardboard, about 8" in from the end of the paper, then began to pull the cedar into the curve it kind of liked best, keeping it inside the max dimensions. Nailed it down, then traced the line. Cut out the cardboard and set it up in my living room to see how it looked. Made a few alterations. When it came time to transfer the pattern to the plywood, did the same thing with the cedar, just to make sure the curve was kind of natural. I have no idea what the actual radii are, but they work.

dave

[KCStudly]

I was able to get my inner skin of 5mm Okoume BS1088 marine ply (3/16 inch thk) to go around 13-1/2 inch radius riblets, but I did have one internal "pop" resulting in a slight bulge about 8 inches long and maybe 1/8 inch deep at the worst. I ended up stabilizing this with a layer of 6oz glass weave and epoxy over the outside to prevent further propagation. Unless you shine a light on it just right from the inside it is hardly noticeable. On the outside I was able to compensate in the foam layer.

Maybe if I had soaked the piece fully/longer (I just sprayed it with water from a spritz bottle during the bending process helping to keep the outer fibers from splitting), and if I had only bent it one time it would have improved my chances (I did a dry fit first... went okay, then installed it after pre-finishing the inside... pop). But in the end I think there may have been a slight flaw or small void in an internal layer at that location, so it really was pot luck at that degree of bend.

To put it into perspective, this is not your garden variety big box ply. Overall I would rate the 5 equal ply sheets I'm using as very good quality, but not pristine or perfect. In 16 sheets I have found a few small internal flaws, and a few places where small areas of the plys have delamed (maybe from the saw blade pulling it apart); but it was less than I can count on one hand, and as per the sellers stated specifications. IIRC, there is higher spec'd stuff available guaranteed to have no internal flaws, but the price goes up exponentially.

Here is part of the BS1088 spec:
"The Face Veneers must have a solid surface without open defects. Face veneers must be free of knots except "sound pin" knots, (no more than six in any one square foot,) the average of pin knots cannot exceed 2 per square foot over the whole surface of the plywood sheet. The grain in the veneers should be reasonably regular. Edge joints are limited, and end joints are not permitted.
Core veneers requirements are similar to face veneers except that slits are allowed as long as they are small. Pin knots and edge joints are also allowed. As in face veneer, end joints are forbidden.
Manufacturing Defects are strictly controlled. Poor bonds, overlaps and pleats, and gaps in faces are not allowed. Gaps may be filled using veneer inserts glued with appropriate adhesive."

[Andrew Herrick]

I shouldn't admit this publicly, perhaps ... but I eyeballed my first elliptical contours. I just located the ellipse circle and the major and minor axes, and sketched a profile until I liked the result. Took about five minutes, a thumb tack and a piece of parachute cord. After I cut out the profile, I used that sheet as the template for the rest of the build.

Certainly not the most robust way to do it! But don't be scared of trying a design with elliptical arcs. They look scary, but as long as you wind up with a template you like, that's what matters!