When you need to connect two sections of airframe, a coupler tube is used. Coupler tubes can be found in all the same materials as airframes, which can be read about on the Airframe Materials page. Usually, the airframe and couplers will be made of the same material. 

The coupler tube must have an outer diameter that is very close to the inner diameter of the airframe. For this reason, it is usually recommended to purchase the coupler and airframe from the same manufacturer. 

Coupler Sizing

A coupler must extend at least 1 airframe diameter into each tube it is coupling. For example, coupling two 6-inch sections of body tube will require a 12-inch coupler tube. 

Coupler Attachment

Couplers are usually permanently attached to one side, and temporarily attached to the other. On the permanent attachment side, epoxy is most commonly used. The epoxy must not just cover the ends of the tubes, but also be in-between them. 

On the temporary side, either bolts, friction, or shear pins are used, depending on the application.

For some avionics bay designs, like the one found on 3TTC, the coupler may be temporarily attached to both sides, with a permanent attachment to a small section of tube in-between.