Crutch Construction

4 April 2024

Building a Simple Scale Model Fuselage: Prioritizing Ease and Visual Appeal

This guide outlines a method for constructing a scale model airplane fuselage that prioritizes ease of construction and visual impact. Here's the breakdown of the steps, keeping the most important aspects (side profile and planform) visually accurate while simplifying the least noticeable (cross-section):

Materials:

• 5mm Depron sheet
• Balsa wood (strips and sheet)
• Glue suitable for Depron and balsa
• Sandpaper (various grits)
• Hobby knife
• Flexible strip (like thin wire)
• Drill (optional)

Steps:

1. Side Profile:

   • Cut the desired side profile of the airplane from 5mm Depron.
   • Mark and cut out a hole for the rubber motor to pass through.

2. Formers (Cross-Sections):

   • Cut rectangular blanks from 5mm Depron for each former location (except for areas needing extra strength like the nose block and motor mount).
   • Ensure the height and width of each blank correspond to the dimensions from the side profile and planform (minus 2.5mm for the central keel piece).
   • On each former blank, sketch an approximation of the actual cross-section.
   • Cut out the center of each former to accommodate the rubber motor.

3. Attaching Formers:

   • Glue the Depron and balsa formers (made from the blanks) onto the side profile cutout.
   • Carefully sand the glued assembly to create a streamlined and symmetrical fuselage shape.

4. Longerons and Stringers:

   • Use a flexible strip (like thin wire) to sight from the nose to the tail of the fuselage. This helps visualize and mark the positions for longerons (vertical supports) and stringers (horizontal supports).

5. Reinforcement:

   • Use a sanding tool (slot tool) to create grooves along the marked positions for longerons and stringers.
   • Glue the balsa longerons and stringers into the grooves.
   • Add additional balsa pieces for reinforcement as needed.

6. Finishing Touches:

   • Drill holes for control linkages or other features if required.
   • Sand the entire fuselage for a smooth finish.

Benefits of this method:

• Simpler Construction: This approach focuses on replicating the most visually impactful aspects (side profile and planform) with simpler methods for the formers.
• Reduced Complexity: By using rectangular blanks for formers and approximating the cross-section, you save time and effort compared to creating detailed formers.
• Visually Appealing: The finished fuselage will maintain a realistic side profile and planform, which are the most noticeable aspects of a model airplane.

Instead of using 4 longerons of 1/16" sq balsa strips to withstand the compression and torque of the rubber motor, it is possible to substitute with 2 strips of 1/2" wide by 1/32" thick balsa. It will also provide support to holding the model plane in your hands. Remember, the rubber motor does not extend throughout the length of the fuselage, neither need the wider balsa sheet. The rest of the stringers can be thinner or lesser. Using 1/8" wide x 1/32" thick balsa for seatings and other strong points.

Modular Power Pod for Easy Swapping

This design incorporates a detachable power pod that screws onto the nose of the fuselage or the inside of one. This pod houses the rubber motor, the primary source of weight and force for your propeller. By making it modular, you can quickly swap between different power pods with varying rubber motor configurations to experiment with different flight characteristics (more power, longer flight times).

Simple Construction:

The pod itself can be built using a lightweight 1/8" x 1/4" balsa motor stick. This stick can be outfitted with a propeller, bearing, and any other necessary components like a hook for launching with elastic. To attach the pod to the fuselage, glue small plastic tabs to the front and rear of the motor stick. These tabs can then be screwed or secured with double-sided tape to the underside of the fuselage.

Benefits of the Modular Pod:

• Fast Motor Changes: The modular design allows for easy swapping of rubber motors, letting you experiment with different flight performances.
• Weight Distribution: The pod concentrates weight in the nose, potentially eliminating the need for additional weight to achieve proper balance.
• Lighter Fuselage: The fuselage itself can be built lighter or even use a simple curled paper skin (like rolling paper) since the pod takes care of the weight and strength requirements. Leaving the bottom of the fuselage uncovered allows for easy pod attachment.

This modular power pod system offers a flexible and customizable solution for your model airplane, letting you experiment with different power configurations and optimize your plane's performance.

26 March 2024

For small rubber powered model aircraft fuselages, it is common to use 2 fuselage sides, whether cut from sheet balsa or constructed into a frame made of 1/16" square balsa longerons and spacers either vertically, diagonally, or horizontally. This method is especially useful for slab sided fuselages. 4 pieces of 1/16" square balsa longerons is strong enough for the rubber strain, and the cavity is generous. An example is the cabin type of  It can also be used for fuselages with non-flat cross sections, by adding on side formers and stringers. If not careful, the fuselage sides are glued skewed and it is not symmetrical on the planform.

A common method for non-flat cross sectioned fuselage is to use crutches, formers and stringers. If the crutch is the vertical keel type, the formers are split into left and right halves, these are more commonly used in WWII fighters and it ensures the side view is correct. Mind, the cockpit discontinues the keel and create stress points. Less common is the horizontal crutch system which ensure that the planform is correct, i.e., no banana fuselage. If the crutch coincides with the passage of the rubber motor, that would be great because then you get a structure to withstand the compression and torque more directly. Now, I have not seen it before, but it ought to be possible to use a double crutch system, one vertical and one horizontal in one fuselage, perhaps because it is double the work!