Overview
The goal of this cargo plane is to lift a rocket weighing up to 10 lbs to 400ft. This project is for learning composite construction techniques and general model aircraft skills. Given the effort involved in making such a large plane, we decided that we wanted it to have a modular mounting system so that it could be used to lift more than just rockets. A modular mounting system would allow us to build customized mounts for items like a camera gimbal or a smaller deployable plane.
We have opted to use a duel fuselage configuration with the cargo mounted under the center of the wing. This makes for a lightweight plane capable of carrying awkwardly shaped cargo. The tail is an inverted V-tail which reduces the required strength of the booms since they are coupled together and allows us to use just two servos for tail control surfaces.
As of the end of the spring 2023 semester, we have completed the wing, tail, and boom. The fuselage will be molded in early fall 2023. All servos have been installed and the flight controller is in the initial setup phase. Once we have molded the fuselage the plane will be assembled relatively quickly with just general wiring, landing gear, powerplant install, cargo mounts, and final flight controller setup left to complete.
Design
The wingspan is just shy of 12 feet at 140 inches with a root chord of 13 inches and a tip cord of 8.7 inches. The resulting aspect ratio of 12 makes for an unconventionally light wing for a cargo plane. Prior to servo installation, the wing weighed 4.41 pounds. Basic sizing calculations provide confidence that this plane will have no issue lifting 10 lbs of cargo. Using vacuum-bagged composite foam core construction we estimated 15 pounds all up weight without cargo.
Four airfoils were developed using the help of XFLR5 for this specific application. The entire trailing edge of the wing is split into two ailerons, two flaperons, and one flap in the center. This has the added benefit of allowing the wing camber to be adjusted during flight for a more optimal glide ratio or increased lift for takeoff.
Given the large wingspan, the plane was designed with transportation in mind. The wing is easily separated into three wing sections, the V-tail slides off the booms, and the fuselages can be unbolted from the center wing section.
Construction
The lifting surfaces are vacuumed bagged fiberglass and epoxy with a unidirectional carbon fiber spar. The skin is 5 oz fiberglass with a 4.1 oz carbon spar. The core is CNC hotwired 30 psi XPS insulation foam from Pullman Building Supply. The hinges are 1.4 oz/yd^2 bidirectional strips of Kevlar bagged into the surface (known as Kevlar live hinges).
The wing removable section of the spar that transfers load from each section of the wing is made from unidirectional carbon fiber on the top and bottom of a slightly oversized foam core with a layer of 10 oz carbon fiber at +-45 degrees. The oversized core applies pressure when pushed into the mold allowing us to avoid using vacuum bagging. Once removed from the mold, a joiner sleeve can be molded over the joiner for a custom fit. This sleeve is then installed inside the foam core before bagging. This results in a near-perfect fit without relying on expensive one-off tooling.
There are 7 servos used for control surfaces. The three servos used to control the center flap and the two flaperons have internal linkages which prevent damage and reduce drag at the cost of increased installation time. All servos are mounted just in front of the hinge which prevents issues associated with long control rods.
