Lofting AE460 Greg Marien Lecturer Background Ship design history Surfaces with CAD Lofting method is independant of CAD system (Solidworks, Creo, NX, etc.) Loft is also know as the Outer Mold Line (OML) Innner Mold Line (IML) is the OML minus the thickness of the skin Loft is the anchor and form factor of the primary structure, i.e skins, frames bulkheads, spars, ribs, stringers, longerons, etc. Lofting uses a combination of: Conics, Splines, Spines, Swepted Blends Surfaces, Multi-Section Surfaces Warning: Lofting is an art, and coordination with aero is required to get the smoothness and accuracy for the loft. Where to Begin?
Start with knowns Wing Planform Tail planform (Vertical and Horiztonal) Airfoils Payloads Avionics Radar Engines Fuel volume Landing Gear/Tire sizing Tip back criteria Create a scale planform and side views sketch and create a lofting plan Use Reference Aircraft Coordinate System (ACS) (0, 0, 0) Ensure every known from above fits Obtain approximate x, y, z (FS, BL, WL) locations from the lofting plan then go to town! Result is the first 3-view FS = Frame or Fuselage Station BL = Butt Line WL = Water Line Scale Sketch Sketch in ACS Obtain X, Y, Z Locations for loft
Dont forget to account for all payloads and systems in the sizing! Submit Sketch(s) for Approval Y X Z Z Y X 0,0,0 0,0,0 ACS Wing Loft 5 Wing Layout LE Sweep
Inputs Apex (x, y, z) Planform Wing Apex Location (x, y, z) Tip and Root Chords Span cr LE Sweep Other Dihedral Incidence Angle Twist (wash-out, wash-in) Airfoil Choices Outputs Wing Loft 6 ct b
Wing Layout Setup in CAD Baseline Planform Chord Geometry Baseline Planform LE and TE Geometry Transformed Planform Geometry Root Chord Z Z Y X WCS (Apex) Y LE line X Tip Chord Tip CS
DZ DX Z Y X ACS 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. Create WCS (Wing Coordinate System) at the Apex Create the Root Chord line at WCS in X direction Create the LE line, based on sweep angle and half span on WCS X-Y plane Create a plane (P1), rotated around the WCS Y-axis using the wing incidence angle Project the Root Chord line to P1 in the Z direction Create a plane (P2), rotated around the WCS X-Axis using the dihedral angle Project the LE line to P2 in the Z direction
Create a CS at the LE of the new tip chord Create the Tip Chord line at the Tip CS in the X direction Create a plane (P3), rotated around the Tip CS Y-axis using the tip incidence angle Project the Tip Chord line to P3 in the Z direction Important Important Notes: Notes: 1. 1. Projected Projected chord chord lines lines are are longer longer than than the the original original chord. chord. The The projected projected chord chord length length is is used used to to scale scale the the airfoil
airfoil points points in in order order to to maintain maintain the the original original planform planform area. area. The The little little bit bit of of angle angle and and length length change change will will not not matter matter much much aerodynamically, aerodynamically, but it is a bookkeeping exercise in maintaining the original planform reference area (S). 7
2. When designing uniform trailing edge thicknesses for manufacturing and aerodynamic purposes, scaling the airfoil may be needed needed to to add add or or subtract subtract from from the the airfoil airfoil trailing trailing edge, edge, or or modification modification of of the the airfoils airfoils trailing trailing edge edge as as required. required. Wing Layout Example (CATIA) WCS WCS-Tip
8 ACS Airfoil Points Airfoil databases Contain NACA and other special purpose/custom airfoil data Points are normalized to unit chord of 1, making scaling them easy ACS C=1 ACS Scaled Airfoils 9 Warning Reminder: Lofting is an art, and coordination with aero is required to get the smoothness and accuracy for the loft. Scaling, Translating, and Rotating Airfoil Points WCS WCS-Tip
Root ACS 10 Tip 1. 2. 3. 4. 5. 6. Import unit (c=1) airfoil points to ACS at 0,0,0 (LE to be at 0,0,0) Add a spline to the points (do not connect the TE upper and lower points) Measure the projected chords for root and tip. Scale the unit chord with the projected chords for root and tip Translate the airfoils from ACS to their respective CS Rotate the airfoils around the respective y axis of the local CS, matching the required incidence angle Add Multi-Section Surfaces Why not just use one curve, and one surface, vs. splitting it in an upper and lower surface? 11
Body Lofting, i.e. Fuselage 12 Example of a Sketch Inputs Scale sketches (planform and side view) with FS, BL, WL locations Output Fuselage or Body Loft in CAD Splines and Conics Background Spline example Spline Can be routed through multiple points in 3-D space Each points curvature can be controlled Allows for tangency and curvature constraints Conic example Conics Feature is required to be on a single plane
Tangent lines need to intersect 14 Process for a Body Loft 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. Create Datums (D1 thru DX) as required to constrain FS and WL curve locations Create planform curve (C1) on WL0 Create Upper Curve (C2) on BL0 Create Lower Curve (C3) on BL0 Create HMBL construction curve (C4) on BL0 Extrude surface (S1) from C1 in +Z direction Extrude surface (S2) from C4 in Y directions Create HMBL curve (C5) using the intersection of S1 and S2 Create intersection points between FS datums and C2, C3, C5 Optional (may not be need) add straight lines at points for conic constraints
Create Conics (CN1 thru CNX) as required (only upper conics shown) Add a multi-section surface using CN1 through CN4 conics, and C2/C5 as guide curves CN4 CN3 C2 D5 CN2 DATUMS should be renamed renamed in in the the CAD CAD system, system, and and should should be be FSXXX, BLXXX, WLXXX where where XXX XXX is is the the distance distance
from ACS, i.e. FS100 Straight Lines C5 C4 C3 CN1 D4 D3 D1 D2 S2 C1 S1 ACS
Nose Loft Shaping Notice the nose and rear surfaces are missing - Do this task last Construction curve development depends on if you want a bulbous nose, or a sharp nose. Use conics, splines, fills as required to complete the task Exercise is left to you 16 Advanced Topic Alternate Method using Angled Datums 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
13. 14. Create Datums (D1 thru DX) as required to constrain FS and WL curve locations Create planform curve (C1) on WL0 Create Upper Curve (C2) on BL0 Create Lower Curve (C3) on BL0 Create HMBL construction curve (C4) on BL0 Extrude surface (S1) from C1 in +Z direction Extrude surface (S2) from C4 in Y directions Create HMBL curve (C5) using the intersection of S1 and S2 Using Datums D1 thru DX, create intersect points on C4 Create DD1 thru DDX, datums normal to C4 and through the points on C4 Using DD1 thru DDX, create intersect points on C2, C3, C5 Optional (may not be need) add straight lines at points for conic constraints Create Conics (CN1 thru CNX) as required (only upper conics shown) Add a multi-section surface using CN1 through CN4 conics, and C2/C5 as guide curves CN4 CN3 C2 D5 CN2 DATUMS should be renamed
renamed in in the the CAD CAD system, system, and and should should be be FSXXX, BLXXX, WLXXX where where XXX XXX is is the the distance distance from ACS, i.e. FS100 Straight Lines C5 C4 C3 CN1
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