Locost Nose Cone Page

The word on the net was that no plans existed for the Locost nose cone. So I sat down a few months ago to dash some off. This has turned out be a very laborious task, but having got myself in up to my neck I decided I may as well see it through. Using these tips and the links pages anybody with modest tools and skills should be able to make the tooling necessary to lay up a GRP nosecone for the Locost. (What the hell's a Locost? Yahoo Locost news group/)

Why build your own nose cone? Commercial nose cones are available at not unreasonable prices. True the quality varies, some look bulbous and more exotic materials are quite expensive. The commercial products aren't available to fit all the design modifications Locost builders want to make. And then there are some people with more time than money. Still it's hard to see the economy of building to tooling to do one nose cone, for this reason, I would like folks to build their tooling from my plans and pass it around to other builders. I do not propose to do a primer on GRP fabrication or even provide much detail on finishing the tooling. That has been well done by others. The intent here is to give you a design you can work from to that end.

Disclaimer

In these litigious times I guess this is necessary. I make no representations that I know what I doing here. Follow my advice at your peril. Resins and composite materials may be hazardous to your health. Please read and follow the manufacturer's recommendations. I have no idea as to your skills, experience or intended use, and therefore, will not be responsible for any liability arising from the use of this information. No warranty or assurance is made or implied that anything herein is suitable for any specific purpose. Well, I guess that's enough weasel words.

Lofting a 3-D Shape from 2-D Plans

I'm not an auto designer so when I sat down to design a nose cone for my project I fell back on my boat design/building experience. Thus the terminology used here will be from the world of boat design and construction. In boat building we call drawing the boat's hull to full size "lofting." The lines drawing from which the offsets are derived can be seen at: gif file.

The shape of a boat is traditionally recorded with the help of what's called a table of offsets, and lofting is simply the act of converting the table of offsets back to the original shapes. The offsets, which are measurements from an arbitrary baseline, provide the curved outlines from which you can build the boat, or in our case a nose cone.

The lofting skills and space required for a nose cone are minimal. When you're lofting, you're drawing full-sized slices of the nose. But since the nose isn't that big, the task does not require you to clear out the loft of your workshop. You will be lofting only 15 slices of half the nose cone, which will fit on 14" by 22" piece of paper.

Another reason for lofting is to discover if there are any errors in the offset tables, and to make sure they are fair, that is, free of unsightly bumps and hollows. I have already spent many hours lofting the various lines to assure this outcome. So you shouldn't have to take the time to do this. I worked at 1/100ths of an inch tolerance. By comparison, boat offsets are usually stated in 1/8ths of an inch.

The nose cone was designed with CAD and then the measurements that make up the offsets were made with the dimensioning tools and keyed into a spreadsheet. To understand the offsets you'll need to learn some boat building terms. Or if you'd rather avoid the whole lofting process download a copy of the CAD file of the slices Sections. And print it full size.

If you'll look at the 3D drawing to the right, you'll notice that the lines are of three types, each of which gives us different information about the shape of the nose.

The "Buttocks" describe the surface of the shape as if it were sliced vertically from front to rear. The "Waterlines" describe the shape as if it were sliced horizontally. The "Halfbreaths" describe the surface of the shape as if it were sliced vertically from side to side. Halfbreaths are also called "sections," a term I prefer, if only 'cause my spell checker doesn't like halfbreadths.

To make the nose you only have to loft the sections. So why bother with the other two? Because the measurements of the sections are developed from the others. Also if something on your nose looks amiss they can be used to check your work.

Again looking at the drawing you'll note a series of numbers next to the red buttock line. These are the station numbers of the sections. These start at zero at the rear of the nose cone and go up as they move forward. The nose has 15 stations, which are either 1 or 2 inches apart.

Reading offsets for the nose cone forms (If you have the CAD drawings you can skip ahead to "Putting it Together")

Okay, so we have three types of lines and three main offset tables. The first sets out buttock measurements for the upper surfaces of the nose cone. The second contains waterline measurements for the vertical portions of the nose cone. The last sets out buttock measurements for the lower surfaces of the nose cone. So how does all this help you?

The way the mold for the nose is made is by transferring the section lines to plastic foam forms. The forms are like slices of salami cut at intervals from the sausage that is the nose cone. The chosen interval between forms is 2 inches, except at the front where it is 1 inch. The slicing starts from the rear where the nose cone meets the bonnet, so the designated station is the distance in inches of the form from the rear of the finished nose cone.

Here is part of the upper buttock offsets that describes the section at stations "0 thru 4." The full table is at Offsets .

The station (section) numbers are found in the row to the right of the word "Buttock." The station numbers reflect the section's distance in inches from the rear of the nose cone.  The column of numbers below "Buttock" contains the designators of the buttock lines. Buttock "0" is on the centerline of the nose cone. Buttock "2" is 2 inches to the right of the centerline, and so on.

To loft the first form you have to plot the points found below station "0." These are elevations above the arbitrary baseline. So we draw a horizontal line in the middle of our paper about a foot long and label it "0." This is the buttock baseline. Then draw ten lines perpendicular to and above the baseline, each eleven inches long. Each vertical line should be 2 inches from the next until the fifth (Buttock 8) after which, they should be an inch apart. Label these lines as are the rows in the buttock table of offsets, 0, 2, 4, etc.

To plot the form, measure up each vertical line and mark distance from the "0" column in the table on the line that corresponds to table's row number. For example: the mark on line 0 or cl should be 10.92 inches above the buttock baseline. (Hint: 1/32 is about 3/100ths of an inch.) After entering all the elevations from column "0" you should have something that looks like "Buttocks @ Station "0"."

Just to confuse you, on the waterline table the elevations run down the rows. Using the buttock table you plotted elevations. Now you will plot at the elevations. The stations still run across the columns. Therefore the measurements we need are in column "0" (highlighted in blue). The data provided by this table is the distance a point in the side of the nose cone is from the centerline plane at a specified point on the plane.

In the waterline table (to right) each row represents a waterline. The first column labeled "Elev." gives the elevation where the waterline is located above or below the zero baseline.

Fortunately, this baseline is at the same elevation as the buttock baseline. If you look at our first drawing of the buttock curve, the waterline baseline would be like a wire coming out of, and perpendicular, to the surface of our drawing where the buttock baseline and the centerline meet (the lower "0").

You will note that elevation 1 has no waterline offsets. This is because the top of Locost's frame above the suspension is at elevation 2. Everything below elevation 2 hangs out in front of the frame and this doesn't start to happen until station 12.

Another way to understand the waterline offsets is to look at the 3D again. The waterline offsets are the horizontal distance from the face of grid on the left side of the 3D. This grid is the vertical plane of the centerline. Each cell on the offset table represents a measurement from a point on this plane, where a horizontal and vertical grid line meet, to a point perpendicular to this intersection and on the surface of the side of the nose cone. That made everything clear didn't it? So let's plot these measurements and see what we come up with.

Continued

Now you simply (oh sure, if you have three hands) connect the marks with a batten. This can be a thin strip of
wood, acrylic or steel that will easily bend to the curve described by the marks and run a pencil along the
batten. If working on a not so good drawing board you can drive brads or pins at the points to help hold the
batten. Or you can dispense with connecting the dots on paper and push brads through the paper into your
foam and draw the curve directly on the foam. The curve should look like this (well, without the jiggilies):

"Sure," you say that looks like
something, but not much! True.
As the curve starts to run down
to the baseline and become
more vertical, buttock
elevations lose their accuracy
and we have to refer to the
waterline offsets.