Fuselage and Stack

Contents

The Fuselage and the Stack build a body by skinning a surface through a series of cross sections. They share almost everything: the cross section shapes, the skinning controls, the end caps, the engine model and the chevrons. They differ in how the sections are placed. A Fuselage places each section on its own, at a fraction of the Fuselage's length. A Stack places each section relative to the one before it, so moving or turning one section carries every later section with it. Use a Fuselage for a body laid out along a straight axis with a known length; use a Stack for nacelles, ducts, inlets and bodies that bend, where each section is most easily given as a step from the last.

The other tabs are shared with other components. The Gen, XForm, Mass and Sub tabs are described on their own pages. The shape of each section is chosen on the XSec tab; the shapes and their parameters are described in Cross Section Curves, and the tab's common controls (choosing the active section, copying, pasting, converting a section to an editable curve) and the Skinning tab in Cross Section Skinning. The Engine Definition, Engine Representation and Extension groups of the Design tab are described in Engine Model, and the Chevron controls of the Modify tab in Chevrons.

Quick Start

A fuselage

1. Add a Fuselage. A new one has a point nose, three elliptical sections and a point tail, 30 units long.
2. Place it with the XForm tab.
3. Set Length on the Design tab.
4. On the XSec tab choose each section, give it a shape and size, and set X as a fraction of the length. Shape the surface between sections on the Skinning tab.

A nacelle

1. Add a Stack, choose a nacelle under Stack Preset on the Design tab and click Set (see Stack Presets).
2. Place it with the XForm tab.
3. On the XSec tab choose each section, give it a size, and set its Delta X from the section before.
4. Choose how analyses see it under Engine Representation (see Engine Model).

What Differs

Everything else is the same: the section shapes, skinning, per-section tessellation, end caps, the engine model and chevrons.

Building a Body

A Fuselage or Stack can model almost any body, but it is not always the quickest choice. A store, a tank or a placeholder of a standard streamlined shape is a Pod; a round duct or nacelle wall made from one airfoil is a Body of Revolution. Use a Fuselage or a Stack when the body is not round, when its sections change shape along it, or when a nacelle needs the Engine Model.

Build the body in stages. First set its length and the width and height of a few sections, so that its size and proportions are right; then give the sections their shapes; and shape the surface between them on the Skinning tab last.

Use as few sections as the shape needs. Put a section where the shape changes character -- at a corner or a break in the profile, at an inflection, where a constant section begins or ends, where the section's shape changes -- and let the skinning carry the surface between them. Every extra section is one more to keep in step with its neighbors when the body is changed, and each adds a span of Num U points along the body, so the tessellation grows with the count.

Positions along the body given by U count sections, not length: each span between two sections takes the same share of U however long it is (see Attachment Coordinates). A child attached by UW or RST, a sub-surface placed by U and an engine station set by U all keep their U when a section is inserted or cut, and so move along the body. On the default Fuselage, a child attached at U 0.6 moves from $X = 18$ to $X = 15$ when a section is inserted after section 1. Attach by LMN, a fraction of the length, what must stay in place while sections are added or removed.

Placing a Fuselage's Sections

Each Fuselage section has a location X, Y, Z, each a fraction of the Fuselage's Length, and a rotation Rot X, Rot Y, Rot Z about its own center (Figure 1). X runs from 0 at the nose to 1 at the tail; Y and Z may be from -1 to 1. Each slider shows the fraction and, beside it, the distance it gives; typing a distance sets the fraction. Because all three are fractions, changing Length moves every section, sideways and vertically as well as along the body, but does not change the size of any section.

The section's rotation turns it about its center, right-handed about the body's axes: positive Rot X raises its right side ($+Y$), positive Rot Y tilts its top aft, positive Rot Z moves its right side forward. It turns that section only (Figure 2); the sections before and after keep their places and orientations.

Figure 1. One body built both ways. Above, a Fuselage of Length 4: each section's X is its own fraction of the length. Below, a Stack: each section is a step Delta X from the one before.

Placing a Stack's Sections

Each Stack section is placed by a step Delta X, Delta Y, Delta Z and a turn Rot X, Rot Y, Rot Z from the section before it, in that section's own axes. The steps are lengths, not fractions. A turn therefore turns the direction in which every later section is stepped: a Stack is built like a path, each section a step and a turn from the last (Figure 2). Positive Rot Y turns the following sections down ($-Z$), positive Rot Z turns them toward $+Y$. The first section is placed and turned the same way from the Stack's origin, so moving it moves the whole Stack.

Figure 2. Rot Y 30 on section 2 of the body of Figure 1. In the Fuselage (above) only section 2 tilts. In the Stack (below) the sections after it follow the turn. Dashed, the body without the turn.

Coord System chooses, for each section, which set of values drives it (Figure 3).

• Rel (the default): the steps and turns drive the section, and the absolute values are shown greyed.
• Abs: the section's location and orientation in the Stack's own axes drive it, and the steps and turns are shown greyed. The section then stays where it is when earlier sections change, and its steps are worked out from the section before. Later sections are still placed from it.

Use Abs to pin a section that must stay put, such as a lip or a face that another component is placed against, while the sections ahead of it are reshaped.

Figure 3. Section 2 of a new Stack with Rot Y 10 on section 1, in Rel. The steps and turns drive it; the greyed column is its location and orientation in the Stack's axes, which the turn on section 1 has changed.

V Align moves a section up or down, before it is placed, by its value times half the section's height: at 1 its top lies on the Stack's line through the section centers, at -1 its bottom does. It lets sections of different heights share a flat top or bottom without working out their offsets (Figure 4). The move is the section's alone: the next section is placed from where the section would be at V Align 0.

Figure 4. A Stack of three ellipses of heights 0.8, 1.4 and 0.6 between point ends, every section at V Align 0 (above) and at V Align 1 (below). At 1 each section's center lies half its height H below the Stack's line, and the tops of sections 1 to 3 lie on it.

Spin, in both components, slides the start of the section's curve around the section by a fraction of the curve's parameter, from -1 to 1. It changes no shape; it changes which points of neighboring sections the skinning joins, which matters when sections of different types meet.

Section Order

XSec Order on the Design tab chooses what order the sections may come in.

MONOTONIC is the Fuselage's default and suits a body that runs nose to tail. Its limits hold as the sections are edited, and switching to it from another policy pulls any section that is out of order onto its neighbor, losing its X.

LOOP closes the sections on themselves, for a component whose sections run around a closed path, such as a nacelle whose sections run forward through the flowpath to the inlet lip and back along the outside (Figure 5). The last section is no longer edited: it takes the first section's shape and size, and its place. In a Fuselage it takes the first section's location, rotation and Spin, and both ends lie at X 1, the aft end. In a Stack it is placed exactly on the first section, whatever the steps to it, and takes its Spin and V Align. Its skinning is not copied, so set it to match if the join must be smooth. The Stack's nacelle presets are loops.

Figure 5. The Nacelle Flowthrough Mid Origin preset, cut through its axis. Its sections run in a loop: from section 0 in the middle of the flowpath forward through the inlet to the lip, aft along the outside to the outlet lip, and forward through the outlet back to section 6, which lies on section 0.

Inserting and Cutting Sections

Insert adds a copy of the active section after it and makes the copy active.

• In a Fuselage the copy goes halfway, in X, between the active section and the next one. Nothing else moves. A section cannot be inserted after the last one.
• In a Stack the copy takes the active section's steps and turns, so it lies one more step on, and every later section moves on by that step. A step of zero Delta X becomes 1. In LOOP order a section cannot be inserted after the last one. In FREE order it can, and the Stack grows by the last section's step.

Cut removes the active section and keeps it for pasting; it does nothing when only two sections are left. In a Fuselage nothing else moves. In a Stack the sections after it close up, since each is placed from the one before.

Tessellation Along the Body

The number of points along the body is set section by section. On the XSec tab, Num U of a section is the number of points from the section before it to this one, and Fwd. Cluster and Aft Cluster bunch those points toward the forward and aft ends of that span (values below 1 bunch them). The first section has no span before it, so its controls are greyed, and the Gen tab's Num U is greyed too. Cap Tess on the Design tab sets the points across both end caps.

End Caps

The Tip Treatment group of the Design tab closes an open end of the body: the Nose cap at the first section, the Tail cap at the last. An end whose section is a point needs no cap; an end whose section has area is left open with None.

The caps are the Wing's None, Flat, Round, Edge and Sharp tip caps, with the same Length, Offset, Strength and Sweep Stretch; see Tip Treatment. The extended round caps are the Wing's alone. A cap is built across the section as if it were an airfoil whose chord runs from the section's right side ($+Y$) to its left, so at each point across the section the cap reaches out by half the section's height there (at Length 1). Where the body ends straight, a Round cap on a circular section is therefore a hemisphere, and on an ellipse it reaches out by half the height, whatever the width (Figure 6): a wide, flat section gets a short, blunt cap, and a tall, narrow one a long cap. Where the body narrows toward its end, the cap continues the surface's direction and reaches out less.

Figure 6. A Round tail cap of Length 1 on a straight body, from above (left) and from the side (right). On a circle of diameter D it is a hemisphere. On an ellipse of width W twice its height H it reaches out by H/2 in both views, a half circle from the side and a half ellipse from above.

Stack Presets

Stack Preset and Set replace every section of a Stack with a ready-made layout: a cylinder, or one of many nacelles, each with its XSec Order and its engine definition and representation already set. Set asks first, since the existing sections are lost. The presets are described in Stack Presets.

Converting a Fuselage to a Stack

Convert to Stack on the Fuselage's Design tab replaces the Fuselage with a Stack of the same shape, in the same place in the model, under the same name, with its children, subsurfaces, structures, mesh sources and attributes. Each section is placed where the Fuselage put it, as steps and turns from the section before, so the surface does not change. The Stack keeps the Fuselage's ID and the IDs of its parameters, so links and design variables still find them, but some now mean something else. A section's X, Y and Z, fractions of the length, become its Delta X, Delta Y and Delta Z, lengths from the section before, and its rotations become turns from the section before; check any link or design variable on them after converting. The Fuselage's Length has no counterpart in a Stack. It cannot be undone.

Convert a Fuselage when it must be reshaped section by section, for example to turn its aft body, which a Stack does by turning one section.

Scripting

A script reaches a section's parameters through the XSec it belongs to (GetXSecSurf, GetXSec, GetXSecParm).

The two OrderPolicy enums differ: 0 is FUSE_MONOTONIC in a Fuselage but STACK_FREE in a Stack.