Frame Design

The frame of a quadrotor helicopter has to be rugged while being light enough to take flight. When all of the available materials are taken into consideration, carbon fiber proves to be the most effective material for a frame due to its low density and extremely high strength. For example, the carbon fiber chosen for this project has a minimum yield strength of 392 MPa and a density of 1550 kg/m3. 6061-T4 aluminum, on the other hand, has a yield strength of only 240 MPa and a higher density of 2700 kg/m3. In this example, the advantage of carbon fiber is seen in a lighter frame with a much higher strength.

The material chosen for this project is a Standard Modulus Carbon Fiber (33Msi or 227 GPa) /   Epoxy(250 oF)  which is an 8-ply material with a layup of 0,+45,+90,+135 and an epoxy resin matrix. The 12”x12”x0.1” carbon fiber material was bought from Rock West Composites. The final design of the frame is shown in Figure 5 below.

The frame design is intentionally simplistic in nature so that the arms can be detached and reattached without any rewiring. Also the frame utilizes the natural strength of the carbon fiber: Impacts that occur at any angle get absorbed by a secondary, neighboring plate of material.

Carbon fiber is a brittle material which might cause some issues when being cut. Scott McCormic cut the frame with the water jet in the student machine shop at University of California, Berkeley (Figure 6).



While researching methods of cutting carbon fiber, Scott found that the layers of laminate that compose the material might separate when the water jet starts new holes, destroying not only the part being cut but also neighboring parts. To resolve this issue, Scott introduced a sacrificial piece of metal (24S-T86) which was placed on top of the carbon fiber plates. The sacrificial metal acted as a buffer which eased the stress of puncturing starting holes through the carbon fiber. In addition to the sacrificial metals, Scott tuned the water jet for brittle materials and slowed the feed rate. Figure 7 shows both the carbon fiber and aluminum plates after cutting.


Silicon adhesive and an epoxy were used to assemble the frame. The silicon, which is flexible and removable, was used to connect the arm plates to the top and bottom plates. Because quadrotor helicopters are notoriously difficult to control, they are likely to crash and break. The silicon acts as a non-permanent binder that can be removed to replace broken arm plates. The epoxy is used to connect the motor mounts to the arm plates. The Epoxy is a permanent and strong adhesive which is used to create a strong bond, limiting excessive vibrations created by the motors. Now that the frame has been cut, landing gear must be designed and analyzed.

Comments
2 Responses to “Frame Design”
  1. Soham Ray says:

    Sir,
    I am pursuing in B.Tech final year in mechanical engineering, and my final year project is about designing a quad-copter, which can fly (mainly hover) as well as run on the ground. Being a beginner, I am finding your blog very helpful for my studies on this projects.
    Sir, the frame design concept that you have depicted here is really very simple and attractive and strong from design point of view. I would really like to follow this frame design for my project.
    I have a few questions about this design though. They are,
    1) Why is it necessary to provide some curvature to the base and arm plates? What would be the drawbacks if base plates were designed to be simple squares?
    2) What are the exact dimensions(including the radius of curvatures) of the each kind of plates?
    Thanks,
    Raj

    • jedickey says:

      Hi Soham,

      Thanks for checking out my blog, and please feel free to use my design. The curvatures on the arms are for looks only. You could definitely simplify the arms if needed. Also, I would recommend using a thinner carbon fiber sheet. The ones I used are extremely strong, almost to the detriment of weight. I could throw my design out a second story window and it wouldn’t get a scratch. The dimensions can be pulled from the dxf file on my blog.

      Also, I am revamping my cite as we speak. There is a missing piece to the current information. Guidance on motor rated power is missing. Everything else works, but the props might not get to the right speed if the motor is too weak. I should have the blog updated in a few weeks. In the meantime, choose a motor according to my process, then make sure it’s a powerful one. For more guidance on this, check back in a few weeks. Thanks again.

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