Hall Bulldog 3D Modeling: Refined Details

Part of The Hall Bulldog Project — documenting Bob Hall's 1932 Thompson Trophy racer.

This is the fourth report from Mirco Pecorari of Aircraft Studio Design, documenting continued refinement of the Hall Bulldog 3D model. This report covers the integration of Kevin Kimball’s CAD engineering model with Mirco’s 3D skin model, plus detailed modeling of the vertical and horizontal stabilizers.

Matching CAD Engineering with the 3D Model

With the external 3D model validated against historical photos, the next step was integrating it with Kevin Kimball’s CAD engineering model. The STEP file was exported and Kevin provided screenshots showing how the internal tube structure fits within the skin shell.

CAD engineering model integration overview — Kevin Kimball’s internal structure fitted inside the 3D skin shell, showing how the wing planform was revised

Kevin noted that the wing shape created from photos was “a much more pleasing shape” than the original. The blue line shows the old planform along the leading edge, while the new shape better matches the historical photographs.

Wing planform comparison — Comparison of old and new wing planforms—the photo-derived shape proved more accurate

Gull Wing Attachment Details

The rear spar attachment point in the gull section required careful positioning. Kevin adjusted the location of the tubes to create the slight bumps visible in the skin on top and bottom—exactly as seen in photos of the actual Bulldog.

Skin bumps matching historical photos — The opaque skin view shows how the top tube just breaks through the skin, creating bumps consistent with photos of the actual Bulldog

Internal Structure Views

Kevin’s engineering model shows the complete fuselage structure adjusted to fit inside the skin shell while making room for fairings and other parts.

Internal structure from front — View from the front looking back at the tail—strut locations from the skin model helped determine where the lower longerons needed to be

Skin shell with structure — The skin shell was used to properly place the wing, angle of gull, gear location, struts, and more

Complete internal structure view — Same view showing internal structure—only the right wing is shown (left is a mirror). The mockup landing gear leg helps determine size and angle of the leg attach points.

Modeling the Vertical Stabilizer

With a validated 3D model base that matches the photo collection, the vertical stabilizer and rudder could be modeled directly from photos as shape and dimension references.

Vertical stabilizer modeling — The 3D model overlaid on a historical photograph to verify the vertical stabilizer shape

The overlay method also reveals when the rudder is deflected in photos. The extended rudder version was introduced after stability tests and was adapted on top of the previous version, resulting in a somewhat uneven surface.

Rudder deflection visible in overlay — Multiple photo comparisons showing the vertical stabilizer and rudder alignment

Modeling the Horizontal Stabilizer

The horizontal stabilizer and elevator presented unique challenges. The only available photo showing the planform clearly is very low resolution. Other ground images are useless for understanding the shape due to the viewing angle between the photographer and the surface.

Horizontal stabilizer photo challenges — The challenge: only one usable (but low-res) photo showing the horizontal stabilizer planform

Comparing Historical Drawings

The team collected not only original pictures but also 3-views blueprints made from 1933 to 2006. These drawings vary significantly from each other, making it difficult to find something that precisely matches the photos.

Collection of historical drawings — Collection of 3-views drawings from 1933 (Howard Schmidt), 1957 (Jeffries), 1967 (DW Carter), 1971 (Mendenhall), 1987 (Clements), and 2006 (Gordon J. Rae)

By zooming in on the Pylon 1 bank picture and comparing it with all the drawings, the team determined that the 1987 Clements drawing is the best reference for the horizontal stabilizer planform.

Drawing comparison analysis — Comparing the Pylon 1 bank picture against all available drawings—the 1987 Clements High Res drawing (highlighted in yellow) proved the best match

Building the horizontal stabilizer — Using the Clements drawing as a base, the horizontal stabilizer was modeled and integrated into the complete 3D model

Current 3D Model Status

With the empennage complete, the model now shows the full aircraft from multiple angles.

Four views of the 3D model — Four rendered views of the current 3D model showing the complete airframe

Adding the Engine and Propeller

The team was happy with the model so far, but to judge the overall look it was necessary to have the engine and propeller installed. This would allow adding small details and the paint scheme later.

Comparison with actual aircraft photo — A comparison showing how close the 3D model matches photographs of the actual aircraft

For now, placeholder models were created for the Pratt & Whitney R-1340 radial engine and Hamilton Standard 2D40 propeller. The final versions will include more detail.

Engine and propeller models — Work-in-progress models of the Pratt & Whitney R-1340 engine and Hamilton Standard 2D40 propeller

Complete model with engine and propeller — The Hall Bulldog 3D model with engine and propeller installed—ready for final details and paint scheme

Report 1: Pictures Analysis and 2D Views - Initial analysis and 2D drawings
Report 3: Building and Validating the Model - 3D modeling and photo comparison
Report 5: Final Report - Comprehensive final documentation