*Originally by Kurt Krempetz for AMA Glider*

Most modern glider designs today include the use of carbon fiber, typically in the fuselages and sometimes in the wings. The stiffness/weight of the model can be increased by using carbon fiber. It only adds a little weight, but this is the main reason carbon fiber is used. In gliders, stiffness is needed to obtain good height. Weight needs to be minimized to obtain a good sink rate.

In the design and optimization of any object (like a glider), the first thing one must understand is if the object will break when it is being pushed to the limits, or the object will deflect so much that it will not be able to perform. This translates in engineering terms: is the object stress limited or modulus limited?

In designing a glider, the main problem is deflection limitation. If you are careful and slowly add power to each launch of a well-trimmed out model, you will find that at some point, more power actually reduces the launch height. At this point, you have reached the limit where the model is deflecting so much that the launch height will be compromised. By adding more power, after you reached this point, the model will break or fail.

Now back to basic engineering principles: If the problem is stress limited, then the ultimate strength of the material is important. If it is deflection limited, then Young’s Modulus is important. We are interested in these properties per some weight or density. For example: if a material is very stiff but adds excessive weight, it won't be much help to the stiffness/weight ratio.

Here are some materials and their properties:

What is of most interest to us is stiffness/weight, so one can calculate the ratios of these properties per the density of the material. I also divide by 1,000,000 to get numbers that are not so big. These numbers are only useful to compare "relative goodness". Here we are just trying to get a ratio so we can understand what material is the strongest or stiffest per weight. The larger the number in the table, the better the material is in terms of stress/weight or stiffness/weight.

From this table, one can clearly see that adding carbon fiber to a glider design does make sense. However, there are many other factors to consider and one must really study the details to get the correct answers.

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