Calculation/method of decreasing diameter?

[piginapoke]

Can someone point me to the calculations or the method that determines the required curve, angle and length of the adjacent part of a aircraft fuselage as it decreeases in diameter towards the nose?
See attached pic to show the changing angle and length.

[henryk]

Sorry, this is descriptive geometry - either programs like Rhino, "grids"(siatki) or a blender. It's not that easy!

[airdave]

I don't think there can be a specific calculation, since:

the number of concentric parts is determined by the designers.

More connections allow for a transition that better matches the original shape,
but less parts allows for easier construction and less seams.
So its a designer's choice.
Only limited by overall scale.

Also, aircraft fuselages are often not circular (tube cross-section).
And the central axis often changes position as it moves down the length of the fuselage 'tube'.

I don't see how there can be a consistent calculation or geometry to designing a fuselage.

Most fuselage designs just mimic the overall shape and dimensions, based on a three-angle process.
Most designers base the 3D design on a three-view set of plans.
Most designers rely on 3D software to do all the calculations
and extrapolate the individual parts based on the designers input (3 view plan) and designer's preferences (scale and number of parts).

[Phillip Wingrove]

Hello my friend.
I have just started looking into this. Will keep looking.

[cfuruti]

As airdave mentioned, fuselage shapes are much too varied, each requiring a different solution. I once wrote software to generate paper rings for a solid of revolution given a circular texture and a function (for example, y=x^2 generates rings for a paraboloid with orthographically projected texture). It can create a car wheel's hub, a propeller spinner or most jet exhausts. It would not work for an F-16's nose (it's ventrally flattened, and the axis "droops" forward) or a C-119 (the fuselage's cross-section is squarish).

[silverback920]

WARNING: MATH AHEAD
Define the line of symmetry. Divide the object side view into segments with lines perpendicular to the line of symmetry.
On one side only, draw a line through the intersections of adjacent segment lines and the object outline. Extend this line to the line of symmetry. This is the center of the cone. Determine the angle between the previously drawn line and the line of symmetry (angle alpha)
Draw circles from the cone center to each of the intersections, which should yield two concentric circles.
The conical development of this section is the area bounded by the two circles and an angle determined by the formula: sin (alpha) x 360 degrees.
Repeat for each segment.

[Phillip Wingrove]

Hi.
All I know is for circles decreasing in size. This is the Sigatta, the vertical height of the section.
Sigatta = 12 - the square root of ( radius squared - bottom length squared).
I have attached a pictures. Table of Contents - Math Open Reference › sagitta
A technical drawing curve thingy otherwise works for me.
Could always ask Boeing or Airbus about legacy methods....
Will continue to look, regards

[cfuruti]

Originally Posted by cfuruti

... I once wrote software to generate paper rings for a solid of revolution given a circular texture and a function...

Given this test texture,

these are rough, quick-and-dirty builds of four functions:

When looked head-on, all should be identical to the circular portion of the texture:

Essentially the software automates a process similar to silverback920's, plus mapping the texture in "perfect" (= faceless) rings. But again, this is trivial geometry and only works for solids of circular cross-section and normal axis.
Since the OP mentioned Blender, you should stick to it, since it can handle any non-self-intersecting surface of 3/4-sided faces. Or are you planning on automating/extending Blender?