Piper Cub Special PA-11 (oz9787)

About this Plan

Piper Cub Special PA-11. Indoor RC sport-scale model for electric power using geared 5:1 GWS LPS B2C-B motor. Wingspan 30in, wing area 131 sq in, weight 5.4 oz.

Direct submission to Outerzone.

Update 8/9/2023: Added article, thanks to theshadow.

Quote: "Who doesn't love a Cub? Piper Cub special model PA-11, by Bob Aberle.

Nearly everyone has heard of the famous Piper Cub (J-3) airplane. Besides its popularity as a full-size aircraft, the Cub has been a favorite of modelers for years and years. Much less well known is a variant of the J-3: the Piper Cub Special, Model PA-11. On close inspection, it is exactly like a J-3 Cub except that it has a fully cowled engine. In other words, the more usual exposed engine-cylinder heads are cowled in.

The PA-11 also had another interesting feature - its color scheme. The J-3 has always been notable for being all yellow with a black lightning bolt on the side of the fuselage. At a time when the Aeronca Champ’s yellow and orange color scheme was popular, it appears that the Piper Aircraft Co decided to depart from the Cub’s all yellow scheme and changed to yellow and dark blue, using the pattern established by Aeronca. In this article, I have attempted to duplicate the Piper Cub Special (PA-11) in that color scheme.

About the model: Gerald Zeigenfuse designed a small version of the Piper Cub (oz14791) and published it in the September 1959 issue of Model Airplane News. To celebrate the magazine's 75th anniversary, Dave Robelen came up with two electric-powered updates of that original Zeigenfuse design . These were published in the January 2004 issue of Model Airplane News with a set of full-size, pullout plans. The smaller model Piper Cub (oz688) had a 22-inch span (71 square inches of wing area). A 1.67 enlargement of those plans would yield a 36-inch-span J-3 (with 198 square inches of wing area).

I wanted a plane specifically for the Sport-Scale event at the next NIRAC Championship, the rules for which state that the wing loading can't exceed 6 ounces per square foot-a considerable limitation on the weight-to-size relationship of the model. After playing with the numbers, I came up with a J-3 that (ultimately) weighed 5.4 ounces with a wing area of 131 square inches. That just made the maximum allowable wing loading at 5.94 ounces per square foot. I modified the cowl and adapted the yellow/blue color scheme; then I had my Piper Cub Special!

I won't give a part-by-part description, but I do want to describe some of the important details. It was not difficult to achieve the 5.4-ounce total weight, and no super-small RC equipment was required. For the motor, I chose the GWS LPS series B2C-B (it has a blue spur gear with a 5:1 reduction ratio). At 0.44 ounce, this motor is lighter than those of the popular IPS series, and it's easily mounted to a 3/16-inchdiameter wooden dowel. Battery power was provided by two FMA/Kokam 340mAh HD Li-poly cells weighing 0.88 ounce. With a GWS 6x5 prop, the motor current at startup was 1.2 amps at 8.06 volts, 8 watts and 4,900rpm. The resulting 1.48 watts per ounce was more than enough to fly the PA-11. Motor run times of at least 15 minutes are normal. My radio equipment included an FMA M5 micro dual-conversion receiver, an ECubed RC reduced size (M-72-I) antenna, 2 FMA PS-20 microservos (for rudder and elevator) and the Castle Creations Pixie-7P ESC (set for a 6V cutoff).

ASSEMBLY NOTES

The all-wood structure is simple to build and requires very little material. Try to select relatively lightweight balsa wood. Avoid heavier wood; it isn't necessary for such a small aircraft. I found it easiest to first assemble the fuselage sides and formers and then cut out and install the stab, vertical fin and their control surfaces. I covered these with Balsa Products' Solite iron-on material. The control surfaces (rudder and elevator) were hinged with SR Batteries' Gapless Hinge Tape. The next step is to cement these covered and hinged tailpieces into the slots at the rear of the fuselage.

I next installed the RC equipment (the receiver and servos), the motor and the ESC along with the two control rods (0.025-diameter wire inside thin plastic tubing). The control rods used Z bends at the control-horn ends and Du-Bro Mini E-Z connectors on the servo-output arms. The remainder of the assembly is continued in a logical manner. Cover the wing at this point, and cement it permanently to the fuselage, making sure that it's properly aligned with the stab. The landing gear is fashioned out of 0.04-inch-diameter wire to which I attached a pair of Du-Bro Mini Lite 11/4-inchdiameter wheels. Install the top and bottom 1/32 balsa sheeting. You can cover the fuselage at this point.

COWL ASSEMBLY I wish I could offer you a vacuum-formed cowl and save you some work, but that isn't possible at this time. I was able to make up a balsa cowl by using pieces of 3/16- and 1/8-inch balsa. The photos explain the process. I cut out side pieces of 3/16 soft balsa and pinned them to the forward fuselage. Then I added a top and bottom piece and, last, a front piece (all made of 1/8 balsa). At this point, I pinned everything into place. I couldn't use CA at this stage because it would easily wick through and attach the cowl permanently to the fuselage. Instead, I used white glue to hold the balsa pieces together. When this glue had cured, I removed the rough cowl and applied thick CA to the interior.

From this point on, you will have to both carve and sand the cowl to achieve the proper shape. It will need a hole in the front for the prop shaft. The cowl is intentionally larger than the firewall because that's how it appeared on the full-size PA-11. I used two Du-Bro no 2x3/8-inch button-head sheet-metal screws (one on each side) to hold the cowl in place. To provide a better anchor for these screws, it's a good idea to add small hardwood inserts on the fuselage side, by the firewall.

The 2-cell, FMA/Kokam 340 HD Li-poly battery pack fits into a small compartment that I crafted out of 1/16-inch balsa sheet at the bottom of the fuselage, directly behind the firewall. As you can see in the photos, the battery is tucked into the compartment; then, when the cowl is in place, it captures the front of the battery. The battery remains mostly exposed, and this makes it easy to access the connectors for charging purposes..."

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