lofting basics help

Hello everyone,

I am new to your forum and new to lofting; I have designed many gadgets but lofting flowing objects is new to me.

I have a lofting question that I need some sound advice on:
I am tinkering with recreating the loft for a model aircraft that I am building (I know… it’s not a boat/ship but the origins of lofting came from boat builders).

I wish to loft it in the traditional way but after reading a few threads within your forum it would appear that I may be spinning my wheels.

I am using AutoCAD and a few folk have commented that it will not fair properly and I will end up with a bumpy/lumpy mess…. do I need to start over with different software?
I started to reverse engineer the lines from the hydrodynamics report table of offsets and a general arrangement drawing at 1-1 scale. Some of the design I will have to just plain use my judgment but that is part of the fun.

Aircraft spec link
http://aerade.cranfield.ac.uk/ara/arc/rm/2834.pdf

I do have a few other questions but I believe the aforementioned question requires a sound answer first.

In training
Dave

 

Dave, briefly... lofting isn't really a software thing. There is no need for AutoCAD at all to do this if you have a lines plan and/or stations and offsets.

I apologize that I couldn't see the PDF, because I'm on a wireless data connection with low bandwidth.

I'm not 100% sure about where to find you a good web link, but a great text book is the following:

http://www.amazon.com/Gougeon-Brothers-Boat-Construction-Materials/dp/0878121668

Get a new version of the book (blue cover). About 1/8-1/4 of the book is on lofting and basically doing what you're trying to do.

I've read enough airplane building sites trying to learn about composites now that I see the clear similarity between the two. I'm not sure if you're using epoxy (or wood) for your aircraft build, but the book takes you through from setting up your shop, to lofting to building molds and jigs, to bonding wood, to cored construction, etc... etc...

Good textbook.

 

Thanks for the book reference Cat builder; I will have to request it from my favorite librarian.

I have read and studied the following books: Ship and Aircraft Fairing and Development by SS Rabl, Building Small Boats by Greg Rossel, and Lofting by Allen H. Vaitses, all very well written books.

My concern is ending up with a lumpy model when all done and will I be able to reconcile the waterlines, buttock lines and stations. AutoCAD will smooth out allot of mistakes but I would sure like to have the classic lofting methods mastered prior to letting the machine smooth out my errors.

Some of what I was having issue with was editing the splines; (lack of experience using splines) adding and removing control points but I believe I have that under control now.

How does the loftsman/designer know how many waterlines buttock lines the craft requires to give a good fair?
I want to have the correct amount not too few and not to many?

My modeling methods require a station approximately every 3 inches for the scale and simplistic planking methods used.

Link to my 1/32 scale Spruce goose model, my next model will be built with the same methods but a bit larger

http://www.rcgroups.com/forums/showthread.php?t=1310701

Dave

 

The number of stations is dependent on the complexity of the form and the construction method. If the form is complex and has a light, quick to bend material, then many stations are needed. If the form has a low rate of curavature change with 4" oak planking then fewer stations are needed. Flying boats are always a mess becuase of the step discontinituities. Looking at the lines in the PDF, the stations look correct based upon the rate of curvature change in the hull. Are you trying to CNC a small model? Or is it hand built?

 

One off build, all hand cut; end goal will be a fiberglass mold (180" span, fuselage aprox 24" in girth)

The faired step is a bit problematic as is interpolating the forward chine radii; I sure wish I had a better grasp on this process.

Allot of modeling is working with incomplete data and solving unknowns.
I will most likely carve the faired step by hand as the bends are too hard for the materials I am using.

I tried bending the forward planks on the Gosse to the FP but they would have no part of that... I inserted a solid block and carved the problematic areas (problem solved).

Dave

 

Sight Reduction Tables.....get off the computer and onto the frames, spacing them as stated above to suit the curvature of the shapes. Close for complex, wide for straighter runs. Splines become battens, laminating them from thin stock will allow greater flexibility. You need to try a few samples of the materials that you intend to use for your model, to see just what you can do with them. Some will break cos they can not be bent that much, so use two thinner pieces instead and glue them when ready to set them. You will find that the shape will become automatic as the battens touch the frames that you have already set, some will have gaps, some will be touching too much, just allow the natural flow of the material to dictate the final shape. You may, or may not be out a few thou, or even parts of inches, do NOT worry, it will become fair as you go if you allow it to be. Most people become too technical, expecting perfection, it is not necessary (unless of course it really is to you to do so, but do not get too anal over it and it will never be noticed or known. Believe me. 99% of boats we build are NOT symmetrical...only boats that are built for racing or some other reason need to be that accurate. Model making really is exactly the same as a real boat, only harder.......

 

What a great little craft - they were such a classic.

The CAD part of the task may not be required as landlubber was saying, unless you want to create a lot of them (which you dont )

My idea is that you dont want to create all the complexity straight away. Build the hull and wings as basic shapes, then use modellers clay to put in the curvy difficult bits. Landlubber made good comments about the perfect symmetry - unless you mean it to fly, if it looks right, it *is* right.


However, if you do one side first, get that correct (like a boat half hull) , you can make small templates for the other side to check for correctness. While you are doing the first side, you may be able to make usefull female templates from any design documentation, as well as the underlying frames males templates.

The other great modellers material is foam. You can carve it into close enough shapes, and then finish it with plaster, epoxy or whatever.

Let us know how it goes - good luck.

 

LandLubber
Sight Reduction Tables
“Tables containing information about the position of the sun, moon, planets and stars. When using celestial navigation these tables help find the position of a boat”
I maybe a little thick headed but I can’t make the connection with sight reduction tables (sorry, can you expand just a little).

I totally agree that the sections do not have to be 100% correct as the Goose had a bit of a fairing issue aft of the step. My solution was stacking the sections in plane, in the vice and filing them fair but this is not ideal.

My end goal is to become very proficient at the classic method of fairing lines with limited/incomplete source data; I consider myself some place near the bottom of the learning curve but close to the rise.

The section spacing recommendation is very helpful and I did not read that in any of the text books I have studied. It would seem that most text is written for folks who already have a table of offsets and they are just going through the motions of cleaning up the fairness of the lines.

How long does it take to become comfortable fairing lines and filling in the unknowns (on average):?:

Summary/recommendations
Add only enough buttocks, waterlines to interpolate the missing/needed sections
Use the provided data/sections and calculate the missing data

Thanks guys,
Dave

 

Carve a half fuselage in foam as large as you can make it (3 feet-5 feet). Trace the profile of the carved foam on a piece of paper. Assign stations into the profile. Cut the foam into sections. Trace the foam sections in paper, aligning the y axis to the profile drawing.

Fair in Autocad. Many experimental aircraft have been done this way.

 

Lofting in the traditional way is done on a painted board with battens and nails and or weights. It is the drawing of lines or sections full size.

 

RX
I considered making a small scale prototype; the act of fairing a tangible object is much easier on my mind and corrections in shape seem to be easier to problem solve. My hope is to become proficient at doing this function in the virtual world using tradition fairing methods.

Gonzo
Please correct me if I am wrong but fairing a spline in a virtual plane should be no different than fairing a batten on the floor of a loft/board. The traditional fairing methods should be the same unless the spline reacts differently than the batten (not trying to make water into wine or confuse anyone):?:

Dave

 

The problem is that if I am lofting a 60' hull, there are no computer screens available of that size. My lofting board used to be 70'X12' (yes feet). The lines full size have two main advantages: firstly a pixel difference on a screen translates to a couple of inches full size and secondly, at full size the effect of perspective becomes evident on the shapes.

 

Gonzo,
Maybe your missing a step in your computer lofting… say lofting the design at 1-1 and periodically scaling down your model/lines so you can see the unfair lines your pixel issue would not be an issue (yes/no) :?:

 

If you have a screen that is 60' wide, then yes, you can see the whole design. Otherwise, you are looking at small sections at a time. In my opinion, that is the main reason for the modern bland and ugly boats. When I look at lines on my lofting board, and walk to follow them, it makes it possible to see if they are "sweet". The computer programs are satisfied with the line following a mathematical formula.

 

I have heard it many times, correct me if I am wrong. Lofting applies to full scale? Fairing applies both to modeling and full scale?

HO229,

Try downloading a free version of HULLFORM. I am not sure if the trial version has an airplane example but it is great for begginers learning to fair. You can do horizontal cuts, sections, and vertical cuts on the model.