|Witch Hawk 500 - Polyspan & Silkspan Covering|
The model airplanes I most enjoy building are built primarily of balsa wood. Balsa is very strong for its weight and great to work with simple hand tools. It does vary in weight and hardness considerably and matching the proper density for the requirements needed in the model construction is critical to build a competitive free flight model. Not paying enough attention to this I have built models that were too heavy or were not strong enough in critical places and broke just from flight loads. Not only is the density of balsa important but also the grain types created by how the balsa was cut from the balsa logs.
|1/2 A Streak Wing Does Not Sag with Polyspan Lite|
Another part of the model construction I have been trying to refine is the proper covering material for the type of model I am building. In the past I most often used a plastic iron-on covering when possible, for smaller models it was tissue paper. For the past couple of years I have had a fascination with the older free flight models. With the older designs the structure was lacking the rigidity needed without using a covering material that gave the structure the strength needed. Yet another aspect of the covering selection that became apparent with the past flying season was what humidity did to the covering. Esaki tissue which is really the only choice for many smaller models starts to sag terribly in early morning or late evening when I often fly. Besides taking on extra weight now that the covering is no longer tight on the model, the structure becomes less rigid.
|Wilbur Center Section Repaired with Tissue over Mylar|
Last winter in my building I tried covering with Polyspan and using mylar under the Esaki tissue. Polyspan does not change with water; to shrink it heat is used. It only comes in white and needs to be filled with nitrate dope before applying the color. I had purchased airbrush equipment and this worked well for finishing the Polyspan. There is also a Polyspan Lite which is considerably lighter but harder to fill. I used this on smaller gas models; it did stay tight but will puncture easily.
|New Gollywock Tissue over Mylar|
Many people recommend covering models with ¼ mylar and then covering over this with tissue. It sounded like a lot of work but I did try it on a New Gollywock model I was building. On the first model I believe I did not get the tissue attached properly resulting in loose spots and wrinkles. In doing a repair on center section of Wilbur rubber model I did a better job of sealing the tissue to the mylar. The covering does not sag in humid conditions but the tissue will still take on weight from the water.
|Eureka e36 Wing Recover with Polyspan Lite - Sagging Tissue on Stab|
For me the challenges of getting the choices closer to optimum in building an outdoor free flight model is what makes it so fun. No doubt with more experience I will become more uniform on the choices of wood and covering material. What I appreciate with the rules for the classes of free flight models I fly is that I am forced to do this and cannot just buy a high-tech composite model that was constructed in molds.