So, you built (or bought) an F2A airplane and it's time to test fly it. The ever important question is: How does one take off the ground? That's when Her Majesty Dolly comes to the rescue. Much has been said about various dolly designs and I'm not pretending to invent anything new. This article is a description of a working design with some, hopefully useful, comments.
In general, one can distinguish two types of dollies: lift-off and mouse trap. I have always used the latter but it does not mean the former is bad. The reasons were personal history and view of reliability. My teachers, Alexander Kalmykov and Sergei Pitskalev used the mouse trap and the dolly described below inherited a lot from their design.
The sketch below shows main elements along with most of dimensions. The base of the dolly is comprised of two wires, the thicker 3mm (blue line) making axes of the front wheels, and the thinner 2.4mm (green line) connecting the front part with the rear.
In addition to the two main wires, there are 3 more elements adding rigidity to the structure (see photos below). Also, there is a post at the rear supporting the model's tail boom thus preventing the development of pitch oscillations.
Front wheels have a diameter of 45 mm, rear 30 mm, all turned from 6061 aluminium.
The mouse trap system has two important features probably invented by Carl Dodge (refer to illustrations below).
- The trap wire's axis of rotation is moved forward of the wing. This allows for a very easy release of the dolly. Until I implemented this feature, the dolly would occasionally get stuck on the airplane or get released at an altitude significantly exceeding the trip wire length.
- The trip wire forms a triangle around it's axis of rotation, thus stiffening the whole element. I had a conventional Z-shaped trip wire break twice at the most inappropriate moment, but the triangular design already served two years without failures. Thanks for telling me this, Carl!
Finally, the elements of the dolly contacting the airplane are wrapped with black rubber tubing that has been cut along a spiral - easy to replace and no scratches on your shiny wing! A couple of red flags made of Monocote increase the chances of locating the dolly on the side of the circle but I wish there was something better to improve its visibility - your suggestions are welcome.
November 3, 2010