Design Considerations for Free Flight Aerobatics
by Wayne Patten
It has long been an observation of mine that many, especially new, model builders are largely influenced by their emotions when it comes to choosing a subject. The airplane that looks really cool or the airplane their grandfather flew in WW II motivates them to proceed. If this is your feeling about model airplanes, then you will be better served by the likes of, say, Guillow’s, Dare Design, Diels Engineering, Dumas, Easy Bilt, etc. or even the plastic scale kits.
While I do have some semi-scale designs in the works, scale authenticity is not conducive to the requirements of free flight aerobatic flying. And, let’s face it, by and far, most real airplanes are not aerobatic. So lacking scale authenticity, what does aerobatic flying have to offer?
Dynamic creative fun. In your back yard or other urban and suburban spaces.
Most of my designs revolve around a six longeron fuselage and a W-truss wing supplemented with a side grain punctured gluing procedure. I did not like the idea of a bunch of square box fuselages, so I added taper to the cross-section profile of most designs. Without the above mentioned gluing procedure, the fuselage cannot be assembled. The slight twist will cause the side frames to separate at the joints. The gluing procedure also improves crash damage resistance. The extra longerons together with the laminated balsa nose block largely eliminated solid object impact damage. The addition of a nylon propeller shaft eliminated bent shafts. I was regularly bending 1/16th, 1.5mm music wire shafts. The stick structure has an added benefit in that a broken component can be stripped of covering and dropped back on the plan to effect sound repairs. This includes the fuselage. A good aerobatic flight does not always allow a gentle landing. These designs shrug off rough landings.
But understand this; while crash damage is a large consideration for a model flying low, fast in the roughest of airs near obstacles, crashes are not the norm. The original prototype has over 100 flights and was retired for posterity, not damage. Free flight aerobatics can be flown as, or more reliably than, traditional endurance free flight. Strong words, no? If we have the intelligence to tune for endurance, it would seem that the same intelligence could be used for other flight modes. Just put assumptions aside.
The stick structure also allows the use of domestic tissue as there are few or no compound curves to deal with. This tissue is said to stronger than the traditional esaki and the color range is ever-changing and quite large. I use and supply each kit with polystyrene sheet to fill in wing tips, cheeks, canopies, etc. It helps round out a basically square design and address those compound curves.
Some may balk at the built-up balsa propeller, but the truth is, once it is glued up, the builder is about 20 or 30 minutes from a finished propeller. An extended snap-blade knife and a curved end sanding block will carve off the “steps” and smooth it into a functional propeller. I let the laminated assembly dry for 2 days due to the deep glue joints. Also, the propeller can be built left or right handed. Islanders are not disregarded! Yes, I tried plastic propellers. The wrong pitch, high speed and rotating mass do not fill the aerobatic bill.
Some designs are from my own imagination, a few will be from scale drawings, and quite a few are coming from old control line (C/L) designs. I am re-interpreting a few of the large gamut of designs from the control line community. They range from cute little puddle-jumpers to sleek jet types and anything in between. For the former think Tiger, Avon Racer, etc, for the latter think USA-1, Sagitta, etc. Nobler, Crossfire, Imitation, and Aristokrat are ready for release. Some of the originals were actually profiles, but good profiles, you know.
Some may balk at the lack of clear canopy. The crash-resistant structure is often not accommodating to a clear canopy and it is an expense that would add considerably to the price with little return. Remember the control line designs? Most of them had simulated canopies, i.e. painted on canopies. The artwork or other simulation is the way to go. Besides, you can add your favorite pilot to the design.