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CAD Exercise: Mustang Outer Mold Line

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Radicaldude1234

Well-Known Member
Joined
May 30, 2009
Messages
430
Location
Front Range, Colorado
One thing I do miss about no longer being an engineer is designing stuff in CAD...so I'm drawing up a Mustang in Solidworks for fun!

This will actually be my second attempt as my first was based on drawings from some Czech scale model magazine. This iteration will be based on some factory drawings I was able to find.

Objectives:
- Create an accurate CAD model -> within 1 inch of original.
- Investigate sheet metal flat-wrapping techniques. The Mustang was designed for flat wrapping and has a remarkable low number of compound-curved surfaces.
- Fine-tune a workflow for drawing aircraft outer mold lines (OML), which is the outer shape of an aerodynamic vehicle.
- Help other members with 3d CAD techniques.

Not Objectives:
- Aerodynamic design; willing to discuss concepts but don't want to talk numbers
- Structural design; only thing I'll say is that the Mustang has a monocoque design. So all you have to do is figure out the loads, size skin thicknesses accordingly, then add stiffeners/longerons/spars to fine tune high stress areas.

I ask that we remain on topic and limit questions to the subject at hand.

Starting off!

My first step is to gather reference material and define reference lines. I usually use the nose as the origin and the thrust line for dimensioning. Front to back profiles (X-axis) are called Fuselage Stations, Centerline to wingtip slices are called Butt Lines, and those vertical are called Waterlines.

My next step is to create master side and top drawings. Pretty much all profiles are dimensioned off of these sketches. The plus is that because Solidworks is parametric, I can merely edit these drawings later on if I want to tweak some dimensions and the entire 3D model will update itself. Beats redrawing everything and/or getting everything perfect the first time.

The Mustang confused me at first. Because the plane originally had an Allison V12 engine rather than the Merlin, adjustments were needed to tweak the aerodynamics. This included angling the thrust line 1.25 degrees down, probably to give the plane more of a pitching moment for stability. The problem is that they didn't angle it from the front of the plane, but rather 41 inches back (probably the center of mass of the engine). Makes sense, but made my original Czech based main drawings worthless.

So this is where I was before I started over. I'm drawing the P-51D model with the bubble canopy, but the D model is remarkably similar to the preceding B/C model. North American tried to reuse as many jigs as they could, so only a couple panels rear of the cockpit are different. Everything below the canopy sill is essentially the same airplane.

2017_1010_OML-3.jpg2017_1010_OML-4.jpg

For the first go, I didn't do much work-flow planning. I basically drew the main drawings, drew some profiles, created some surfaces, and then repeated. While this will create a model in the end, it adds a lot of needless steps that makes the CAD file HUUUUGGEE! Not to mention it's almost impossible to edit later on.

So this time I'm making most of the drawings, then making surfaces, and finally knitting those surfaces to form a model....It's like origami, really!

2017_1013_OML-Accurate.jpg
 
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