Helicopter Crutches

Author: Gil Eskayo

Date: 10 December 2023

Designing Blade Crutches for CH-21 Helicopter

In Fall 2020 Autodesk coordinated with the Aerospace Museum of California to sponsor a Fusion 360 design challenge. The goal was to create an innovative and effective new blade crutch design for the CH-21 Flying Banana helicopter in the Museum's Air Park. The aircraft had no rotor blades mounted on the airframe. Due to space constraints, the CH-21 needed its blades to be mounted and folded back along the airframe, then crutched using effective and well designed rotor blade crutches.

Autodesk sponsored the Design Competition, then coordinated 20 student teams from 3 colleges - Los Angeles Pierce College, College of San Mateo, and Cal Poly Pomona. Eight Museum volunteer engineers, Board Members and the Executive Director of the Aerospace Museum of California judged the competition.

My Alpha Team from the College of San Mateo came up with two designs that conform to the criteria.

The-Flying-Banana

Problem Statement

Why does the museum need blade crutches? Without blade crutches, the blades had to be removed as they are not secured and will be moved by wind and other factors. With blade crutches, blades will be displayed to the public again and a better story of this heroic aircraft can be told.

  1. Preference to be removable and resemble design of aircraft, drilling or permanent bolts NOT preferred
  2. Tolerance requirements on pin/attachment point should be loose to accommodate for measurement error
  3. Durability when exposed to outdoor elements/ resistant to pests (i.e. birds)
  4. NOTE: Use of Mounting Points are OPTIONAL

Other factors to consider:

  1. Blades will be displayed in fold position instead of fanned out due to space constraint
  2. This particular plane will not fly. It will only be displayed in the museum
  3. There are 3 blades for each blade rotor. So there are 6 blades total
  4. Museum prefers the crutches to be as invisible as possible to not interfere the viewing of the helicopter self

Judging Criteria

  1. Aesthetics — artistic value of the form, shape, color, texture, finishing, engineering, material used, etc.
  2. Collaboration - how well team collaborated in the project and how team collaborate with stakeholders
  3. Functionality and Utility — ease of use, safety, maintenance; Design meets the intended purpose and needs of the user
  4. Innovation — does the design stand out, provide something new to the market or supplement/improve an existing product or service?
  5. Presentation and Communication - how well the design is presented and communicated out to the audience
  6. Reality of production — how realistic is it, technically and economically, to bring this design to production and broad-scale use
  7. Adjustability — including adjustability in mounting point locations is a bonus, but not a requirement

Final Results

Although we did not win the competition, the experience was extremely valuable and the project was a lot of fun to work on, both of which were hard to come by during this tough year of the pandemic.

DESIGN 1 - TOP MOUNT

Inspiration

The inspiration for the top mount came from our brainstorming about how to meet the aesthetic component of the competition.

Engineering Considerations

We made an engineering assumption that most individuals are not taller than the helicopter so that placing the support on top of the rotor would disguise the rotor lift job from most museum patrons.

Design Path

Our design went through several renditions before what we present to you now. After initially thinking about how a top-mounted design might not provide enough upward force we incorporated a component that would be between the body of the helicopter and the blades. The bottom support would be camouflaged with the blade and rotor built to hide its existence.

Adjustability

Our design allows for not only a displayed state of folded blades as they typically are, but also of the blades fully fanned out as they would be while in flight. This option allows for indoor display or outdoor display depending on the amount of room available.

Holster Design

The holsters are designed from webbing, a weather resistant and durable material generally used in many strength demanding situations that still require a fragile touch. The holster is designed to abide by the “three quarters rule” that is used to balance helicopter blades when being displayed to minimize sagging.

Accounting for Time

The cables that latch onto the blade holsters can be tightened in order to account for any stretch that may happen over time to the cable. This feature will also allow for the tightening of the holster so that the blades can be held firmly and not sag.

DESIGN 2 - SADDLE

Inspiration

The inspiration for our saddle design came from our communication with the Classic Rotors Museum. We talked with them to see what had worked for them in their museum to display their CH-21. By using a design that we already knew would work, we were able to focus on the weight optimization and aesthetics portions of our design.

Ease of Manufacturing & Repair

We also are focusing on renewability and easy manufacturing. We used parts already in existence for the convenience of easy repair if the time was ever to come. We wanted to minimize special made parts as much as possible. We only have two special made parts which are the structure themselves.

Design Decisions

We did the truss frame vs mimicking the solid body from our inspiration because the larger surface area would be more susceptible to moving forces from the wind. The frame allows for more air flow so it could more easily be outside in stronger wind conditions.

Supporting Structure

We can make our design light and structurally sound by utilizing a frame and truss design. We attached the saddle frames with poles that offer 2D motion because you can see the helicopter is slanted at different angles and we wanted to be able to produce one design for both sides.

Materials

The design differs from our Top Mount design in the way that it structurally attaches directly to the helicopter body. This design can be built from lightweight metal with welded crossbeams, or it can be manufactured from lightweight materials like carbon fiber. Recycled tire strips offer a positive environmental impact as they provide scratch protection and a good grip between surfaces.

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