Dolly for WeMoTec Alpha Jet
8 Jul 01 - Design modified to reduce rolling resistance.
2 Sep 00 - This is only a short description of the dolly designed to ease the launching of my WeMoTec Alpha Jet. It was necessary due to the relatively high wing-loading and a number of poor hand launches given to me by friends. I normally hand launch my models myself, but it is almost impossible with a heavier model attached to a bungee.
The pictures above show the prototype now in use, which is simply built from scrap bits of balsa. The base of the fuselage sits on top of the flat central area and the model is held upright by the two vertical arms at the front (as shown in picture 3). There is a pin on the centre line of the dolly that locates in a hole in the bottom of the fuselage, which ensures that the dolly is pulled along by the model. The bungee hook on the fuselage is about 1½" (38mm) behind the axle of the dolly. The rear fuselage currently drags along the ground, which gives a good angle of attack on the ground. I will probably add a simple wire skid to the point in contact with the ground to prevent long term damage. It is important that the axle is square to the fuselage when fitted and that the wheels both run freely.
The bungee lies under the axle holding the dolly in place whilst the line is under tension. This has two advantages, firstly the dolly drops off once the model is airborne minimising the risk of it damaging the model on release. Secondly, the weight of the dolly helps ensure the bungee line detaches cleanly.
Initially the two upright arms broke with almost every launch and a tube of cyanoacrylate was essential. I little more reinforcing of these arms seems to have stopped this happening. Using the dolly is really simple. The dolly is fitted in place, the tensioned bungee line attached and the model is placed on the ground and restrained. When released the model accelerates and tracks straight. Due to the angle of attack, the model lifts off automatically once sufficient speed is attained.
8 Jul 01 - I have been experimenting with launching the Alpha Jet without bungee assistance. In trials it got extremely close to take-off speed before hitting the longer grass. I reduce rolling resistance I have modified the initial design. The first change was to fit bigger, narrow, (100mm) wheels to the main axle. I then added an extended the rear boom with a rear wheel to prevent the back end dragging on the ground.
Model Plans & Designs