A catapult makes launching aircraft with pusher props safer, removing the possibility of propellers striking the pilot’s hand. It also makes launching more reliable than hand launching when the aircraft is carrying expensive control systems or significant payload. This catapult was designed initially for a Phantom FX-61 flying wing. It is suitable for aircraft up to about 2.5kg.
The frame is 25mm square aluminium with 1.2mm thick walls. The corner pieces are ‘Cubelock’ joiners. 2-way and 3-way joiners are used at the back of the frame. The legs at the upper end are hinged and held in place with folding/locking braces.
The frame supports PVC tubes that act as guide rails. The larger diameter pipe is 34mm O.D. The smaller diameter pipe is 25mm O.D. The smaller diameter pipe at the upper end slides into the larger diameter pipe. Two interlocking tubes are used instead of one long one to make it easier to transport in a medium sized car. These tubes are mounted on six 5-ply wooden supports which snuggly fit into slots in the underside of the tubes. The tube supports are attached using wing nuts and can be folded down for transport.
I use two lengths of exercise tubing. I sourced mine from here, using the green ‘Firm’ type. They are tied to eyelets at the base of the frame and pass around pulleys at the head of the frame.
There are two bungees. One attaches to the release mechanism. The second one attaches to the first one. This gives twice the launch force while the operator only pushes against one bungee at a time to arm the catapult.
Pre-tension and Arming
Each bungee cord is pre-tensioned for launch. Each cord is shortened from about 4.2m to about 3.2m. The first cord is stretched to attach to the catch / release pin. The second cord is stretched and attached to the catch on the first cord. A loop of nylon cord attached to the first tube catch is attached to the hook under the aircraft.
The cords are loosened off for transport and storage.
Catch Release Mechanism
The catch / release mechanism is a pin that slides through holes in three nylon blocks. The pin is moved up and down by a lever. The aircraft sits on the rails just above the catch release mechanism so operating the lever manually is not possible. A remote operating device therefore does this job. It is a simple hydraulic master slave piston system using two syringes, some rubber tube and water as the hydraulic fluid. The master cylinder (syringe) is pressed by a foot lever, pushing water along the tube into the slave cylinder which pushes the piston to move the catch release lever.
A hook is attached to the bottom of the aircraft. The hook is made from aluminium channel and is screwed into three nylon standoffs glued into the foam fuselage. The hook is 100mm in front of the aircraft centre of gravity.
Check that the aircraft is ready for flight, including control surface movement and throttle enabled.
For the launch, the throttle is off. Immediately after launch, the throttle is increased to about 80% and up elevator is applied to climb away.
See a launch video here.
Dismantling for Transport & Storage
All stretch is removed from the bungees. The tubes simply lift off the supports and the supports are folded down. The front legs are folded up. The slave cylinder is removed from the frame using wing nuts so the hydraulic system becomes completely independent.
The frame is 2.02m long. The longer PVC tubes are also 2.02 m long. This length was chosen so that the parts can conveniently fit into my car for transport. The width of the frame is 260mm. The total weight of the catapult is 5.3kg.