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Author Topic: Producing cross sections  (Read 974 times)
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SP250
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« on: May 30, 2017, 02:47:51 PM »

Anyone got an accurate way to generate fuselage cross sections from a 3 view and photographs?
Hopefully one which doesn't involve 2D or 3D cad, as I can't drive those yet.
Thanks in advance for any suggestions.

John M
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USch
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« Reply #1 on: May 30, 2017, 03:45:37 PM »

The member strat-o wrote here on HPA about his "Fuselage maker program" during the last winter. I started using it but aborted because of other duty's.
The link
http://www.hippocketaeronautics.com/hpa_forum/index.php?topic=21244.0

Hope that helps,

Urs

Edit: might as well add the link to the online program:
https://github.com/marlin-mixon/FuselageMaker
« Last Edit: May 30, 2017, 03:56:57 PM by USch » Logged

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cavelamb
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« Reply #2 on: May 30, 2017, 11:39:11 PM »

3D cad is the way to go.
But even then there is more to it than one might expect.
"Fairing" a 3D body is what makes the smooth contours.
Otherwise they tend to come out lumpy.




Anyone got an accurate way to generate fuselage cross sections from a 3 view and photographs?
Hopefully one which doesn't involve 2D or 3D cad, as I can't drive those yet.
Thanks in advance for any suggestions.

John M
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SP250
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« Reply #3 on: June 01, 2017, 05:54:46 PM »

Thanks guys for the info but.......

Urs
Thanks for the links, however, I don't have the sections to put into the programme - I need to create them, that's why I asked the question.

CaveLamb
If you read my OP again you will see I can't use cad, but thank you for posting.

John M

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Bredehoft
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« Reply #4 on: June 01, 2017, 06:03:38 PM »

investigate "lofting" techniques.

--george
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USch
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« Reply #5 on: June 01, 2017, 06:19:41 PM »

Ops... I knew there was some flaw in my answer  Sad Sad

If you intend to use FuselageMaker, you have to create yourself 3-4 sections from the 3 view drawing you said above to have. For example behind the spinner the section is certainly a full circle, at the cockpit you can guess the section from the side and top view and at the fin the same. Eventually write a PM to strat-o, he's a helpfull guy  Smiley

Anyway with a good drawing and some photo's it should not be to difficult to establish the width and height of the former and fill in an appropriate curve. FuselageMaker just helps to define further sections.

Urs
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tom arnold
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« Reply #6 on: June 01, 2017, 07:22:12 PM »

When you say "accurate", you have to use that term loosely as no 3 view is truly accurate----even the famous factory 3 views are very suspect. A better term is "so close the eye cannot tell any flaw". If you use definition #2 you can draw your own and tweak things such that it looks dead on to you and that is all that counts. There is, indeed, a way to draw very close (note the wording) cross sections using a couple of photographs but the process is SO complex and math and geometry soaked that even with my engineering degree I could not follow exactly how to do it. The process takes days to do and yields results little better than an artist's eyeball.

Process #1 is to take the 3 views you have and draft up your plane and use the lofting technique mentioned by George B. It helps if you have a  lot of photos to refer to as the lofting process will just give you smooth curves between 2 points and you may need to use that artists eyeball to tweak a section or two. Use any intermediate formers deduced from the 3 view also to help.

Process #2 is to cut a foam block (might have to glue up a couple of sheets), square it up and, using a bandsaw, cut out the side and top views. From the front view you will see the former at the prop location and the widest point on the fuselage. There is 2 sections that now become fixed. If you have any other sections that you can deduce from panel lines, so much the better and they become fixed sections. Cutting out a female template for each fixed section, mark their locations and start sanding away the edges of the foam block. Use very coarse sandpaper like #80 or #60 grit and it goes very fast. Smooth up the final shaping with #100 grit and you really only need to do one side. Don't worry about the dust as you can eliminate 99% of it by just wiping your work with a wet face cloth occasionally. Refer carefully to photos constantly and use your templates often as you sand away and you will get so close that no one could tell any boo-boos. Now mark the locations of the formers you want and with a very sharp knife (or band saw) slice up your fuselage side at each position. Each piece gives you a nice smooth former and you can transfer that to a sheet of grid paper and go from there. This method is a lot faster than even drafting.

I have done this numerous times and it works like a charm. I also compared my efforts against a 3 view with known formers and my foam formers were extremely close....so much so that no observer (even those sharp-eyed sharks in my club) could tell any difference. My carving and sanding skills are very average so take heart that you and your artist's eye and lots of good photos can build anything you want.
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SP250
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« Reply #7 on: June 02, 2017, 04:43:59 AM »

OK - an online search has given me a number of options to read up for lofting techniques - thanks George.

Tom - thank you for the extensive description of how to "guesstimate" - seems my best option, as my MK 1 eyeball calibration is still good.
Many of the google links describe "lofting" in cad programmes which I know nothing about. 
Pencil and graph paper for me.

Cheers all
John M
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PB_guy
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« Reply #8 on: June 02, 2017, 01:20:47 PM »

Bezier curves are your friend. Every paint program uses them. CAD programs want to make it more difficult and complicated. And you only have to do either the left or right side of a former, then you mirror the best side to match the other.
ian
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« Reply #9 on: June 04, 2017, 08:06:32 PM »

Thanks guys for the info but.......

CaveLamb
If you read my OP again you will see I can't use cad, but thank you for posting.

John M



Sorry, didn't see that.
 I came in on the tail of the thread.

Curious as to WHY you can't use cad tho...
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« Reply #10 on: June 18, 2017, 05:39:47 PM »

John, with all the computers doing the thinking for us these days, pencil drafting is a lost art. We as a civilization are in deep s%$#t if the grid ever breaks down. Our eye/brain hand coordination is now dummied down by the clicks and drags of the mouse/computer age. You will have a hard time finding someone under the age of 60 to show you how its done. Walter Foster publications put out a good "how to" on perspective drawing that covers the subject. I don't intend to pass judgement, just saying if you want to do it you better surrender to the times, or hit the books and buy pencils with good erasers! I will tell you this, its really not that hard to do.
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« Reply #11 on: June 18, 2017, 08:11:21 PM »

I'm coming in a bit late on this topic as well John and you may have already decided on a method, however I would just like to add a few comments.

Toms Arnold's idea of carving a foam plug and taking sections from that is a good way to go and the main instrument you will need is a good eye and care to judge the fairness of the shape.

This approach or very similar has been used to make plugs for small full size aircraft as well - for eg the Arnold AR-6 Goodyear F1 racing aircraft.  it's well worth googling to see his videos on how he achieved this.

The full size plug was produced from sections taken off a 1/12 scale model.

The main difference in his approach is that instead of slicing the master model he made external section shapes at the points he required  and this is probably a better way as it leaves the master intact and the sections can be anywhere.

He did by using  a car flier bog on rough cut templates to reproduce the shape at the station required. The foam of the master was protected with a layer of tape.

It is a bit of work - however.

Manually draughting sections from photographs can be done accurately if the exact perspective of the picture is known but is quite involved.

A reasonable degree of accuracy can be achieved with manual draughting by using fairing lines in elevation and plan with extra fairing lines at an angle - usually 45 deg to the horizontal plan. At least one section and preferably 3 - mid , nose and tail are required to do this. The fairing lines need to be judged "fair" by eye and drawn with French curves or Rail way curves, and the intersection of these curves with the intermediate station lines in plan and elevation translate the point on the master sections to the intermediate sections. The more fairing lines the more accurate the sections will be.

I'm sorry that this is a bit long winded and it will take a couple of hours to actually do. Tom's approach with Arnold modification, should produce better results with a reasonable eye.

North American used a completely mathematical approach(one of the first times this was done I believe) with the design of the  Mustang but I don't know anything about it.

Good luck

John
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« Reply #12 on: June 19, 2017, 02:57:26 PM »

  SP250 (JohnM)

This is probably going to sound a bit rude but it is not meant to be.  The answer to your opening question was ‘NO’.  Since then a lot of people have put a lot of thought into ways to help you but your responses have not given much indication of what you have done with the advice except for regular repeats that CAD is no use to you. I should like to point out that CAD  stands for Computer AIDED draughting. In other words the computer helps if you know something about draughting.  Did you do some technical drawing at school.  If you did then a computer program will help you.  Or were you good at mathematics and are you looking  for a numeric solution?  Or is your manual dexterity such that you can fashion carved blocks to cut up?  If they are I can’t see that that approach can be accurate. Do you see that if we do not know what approach you want to use and how you are progressing it is difficult to help.

Response to #11.
OZPAF(john),
Actually this does not concern anything in your posting except the last line where you mentioned the ‘Mustang’.

Drawing a Conic Curve is not difficult as long as you don’t mix all the lines up.  I have attached a copy of a page of a paper I did for the 2005 Sympo on aerofoil sections.  You will find step by step instructions for drawing a curve.  You may wonder how this fits into a ‘Mustang’.  Well I only know in broad terms, not in detail at all but imagine my sample drawing is the NE quadrant of a ‘Mustang’s’ bulkhead where point A is the top of the fuselage and point B is the side.  I can imagine a conic curve line running tangentially from the pilots cockpit, through point A as a shoulder point on one of the bulkheads and finishing tangentially to the slope down of the fuselage at the rear which has established the height of all the bulkheads.  I can also visualize a side view where all the NE quadrant bulkheads will appear just as vertical straight lines and on each of those lines I could mark the shoulder point with a dot and then I would have seen a line of dots.  However I could have drawn a conic from tail end to pilots cockpit and said where that crossed the quadrants established the shoulder points on the bulkheads.  That is a bit of a gabble so I will stop whilst we are all sane and wait for questions.

If anyone want the other eleven pages of the article just say and I will attach them somewhere,
John
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Andrew Darby
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« Reply #13 on: June 19, 2017, 04:23:14 PM »

John,

Don't fear the CAD, but don't expect to be able to plot out complicated shapes and fuselages from the off.  CAD packages are like battleships, it's all x's and y's.  Best thing is to download something like Draftsight (for free) and have a play.

I am most definitely under 60, but can plot curves on a drawing board with Pencil,  in 2D CAD and 3D CAD, and believe me the skills are equally difficult to master, they are just different to infer that CAD is just a series of clicks and drags is very insulting.  But when I draw something  in CAD and it goes into a CNC machine, the parts fit perfectly - try and do that with a pencil!

I spend all day using 3D CAD, but do I use it for my models? Hell no! I find carving a lump of balsa and using my French curves much more therapeutic, on that you have me.

I usually carve the shape and feel and eyeball the curves and then literally cut it up into sections and trace them, even when I find 3 views the sections are never where I want formers so this method allows me to get sections where I want the formers to be.  It might not be as scientific, or as efficient, but aren't we model makers?

Andrew
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« Reply #14 on: June 19, 2017, 07:08:30 PM »

Well I hate to admit it but I'm also well past 60 and I would have to agree with all that John (Hepcat ) and Andrew have said. I worked for many years as a manual draughtsman and then had to bite the bullet and learn CAD n my 50's.

First I would strongly advise losing this fear of CAD for the reasons that Andrew mentions. I have found it to be extremely useful and basically do finished working drawings in CAD. I still rough out by hand and find it enjoyable. I can draw up very small components and then print them out accurately or more so than pencil drawings.

I'm not very adept at sculpturing, carving 3d solids - so would always prefer to work from drawings.

I second Andrew's suggestion to download Draftsight and give it a go. There are quite a few who can help you here on HPA with any questions.

As I mentioned in my earlier post - if you wish to see a detailed account of the use of solid masters for producing cross sections I would thoroughly advise you to check this link.

https://www.youtube.com/watch?v=qStFDcZs5Og

It's well worth watching.

John.
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« Reply #15 on: June 19, 2017, 07:13:50 PM »

Quote
If anyone want the other eleven pages of the article just say and I will attach them somewhere,

John(HEPCAT) when you can, would you perhaps post your article on Conic Sections in the Builder's Gallery. I have been intrigued by this method of lofting and have never really checked it out, and would like to know more about it.

John
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« Reply #16 on: June 24, 2017, 01:01:10 PM »

Don't usually lurk on "indoor" threads but occasionally you guys kick around something I can get excited about. So right up front, thanks! I will be checking out several links, listed here.

Observation:
Full size aircraft fuselages are generally simpler that our eyes tell us. Most can be deceptively simple, like say a Ryan STA or PT-22 fuselage.  Observable compound curve usually tell us where the MANUFACTURER spent more of their time, effort, and money! Even the Mustang, with its elegant cowl, soon gives way to simpler construction methods and considerably less curvature, as one looks aft. It is a theme repeated at her radiator fairing, fillets at wing and tail, too.

As modelers, I suppose it's an attempt to adapt a practical shape for our own uses, we tend to operate in reverse practice. We often want to build all smooth and streamlines, for various reasons. Planking in balsa is much like building a boat hull, but is not exactly scale to building an accurate Mustang. In a rather obscene effort to "simplify" a  set of existing basic shapes, we change it back into a more manageable complex shape. Convenience dictated by the build, if you will. Same goes for stringer on former construction. How many Ryan ST models are built with smooth one piece stringer lines, nose to tail, simply because we don't want to deal with that 'sharp break' in the fuselage contours, just aft of rear seat?

Smooth stringer lines mean decidedly more complex lofting, too, when the original is so simple.


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SP250
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« Reply #17 on: July 04, 2017, 05:30:32 AM »

Well I decided to carve the fuselage from blue foam and use the 3 view and pictures that I had for the peanut size model.
Then another mate, who is building the same aircraft, drew up the sections and copied them and sent me a set in the post.

So thanks to all for your suggestions and help - they will be put in the pc memory for future reference.
2D cad would be the real answer, but with such limited spare time, I don't have enough or the will to learn it - prefer to get out and fly the models or ride the motorbike.

John M
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