P-Nut (oz14325)

About this Plan

P-Nut. Radio control sport model for electric power. Wingspan 13-1/2 in.

Quote: "The P-Nut is the result of a fairly lengthy program of performance improvements in a micro, magnetically controlled model. Several previous models had shown signs of being able to fly in breezy weather or to perform some aerobatic maneuvers, but there were limitations in various areas. With the emergence of high-current Li-poly cells, the situation began to change. Rather than lay out another model that followed full-scale design practices, I decided to build a minimum-size model that was as aerodynamically clean as possible.

Experience from the early days of aerobatic pattern flying had shown that a shoulder-wing model with moderate dihedral could perform the rolling maneuvers quite nicely with rudder and elevator control. Keeping the weight and frontal area to a minimum is a big help in reducing drag. My first efforts were with a sheet-balsa, single-surface wing that was curved to create an airfoil. This showed much promise, but after several variations of size and airfoil, I decided that it was too limited.

Meanwhile, Matt Keennon had demonstrated that the MA409 airfoil could be used to good effect in a micro model. Noting this, I broke out the foam-cutting bow and sliced out wing panels to this airfoil. A little trimming and sanding produced the desired elliptical tip with very modest effort. This provided an immediate improvement in performance, but it made the plane heavier than desired.

Further investigation turned up a medium grade of insulation foam at Home Depot. The wings cut from pink foam were pleasantly light, and a narrow strip of clear packing tape on the top and bottom eliminated flex in flight while maneuvering. Not content to settle for less than the optimum, I fashioned several more wings that had different spans and tip shapes. The one shown here emerged as the best compromise of the bunch. Adding a turbulator of elastic thread to the upper surface improved the performance still more.

The next phase of development was to optimize the propeller and drive. I had identified the motor from the KP-00 drive as the most powerful in this weight range, and my experience with previous models taught me that a fairly large propeller and high gear ratio produced the most thrust with this motor. Although various excellent gear drives are available from hobby sources, most were too bulky to fit in the slender nose I desired. Following a tip from another enthusiast, I dissected a broken, battery-powered clock that was in my junk bin. Sure enough, I found a lovely set of compact gears with a ratio of 7:1. Mounting these on a ball-bearing prop shaft produced a very smooth-running assembly.

The final development phase was figuring out the best propeller for the design, and it was apparent that the P-Nut would do best with a fairly large propeller. Although I enjoy carving my own props, I have discovered other options. The GWS orange propellers come close to working very well in stock form. If changes are desired, heat the blade root with a heat gun and then twist the blade to a different pitch. You can also trim a larger diameter prop to a smaller size. The ideal prop value depends on the model’s final weight and the pilot’s flying style. I settled on props with diameters of 5 to 5.5 inches and pitches of 4.7 to 5.5 inches.

So, what did I end up with? Well, my P-Nut can cruise at nearly 30mph outside and flies with authority in very breezy conditions. As far as aerobatics go, I have done multiple rolls, slow rolls, point rolls, Cuban-8s, square loops, inverted flight and even outside loops. It’s possible that the P-Nut is the first conventional model to perform an outside loop indoors. Even with all this performance, throttled back to a lazy cruise, it is a docile and pleasant model to putt around with.

Construction: Got you interested, did I? Considering the P-Nut’s performance range and the crowded interior of the fuselage, I’ll assume some experience on your part and deal only with the highlights.

The first step is finding suitable material. You aren’t likely to build a light model from heavy wood, right? If you cannot find 1/32-inch-thick balsa that weighs no more than 7 grams for a 3x36-inch sheet at a hobby shop, plan to mail order it. I get very nice wood and good service from Superior Products at superiorproducts.com. As I mentioned, I found the pink foam at Home Depot..."

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