Punkin II (oz13865)

About this Plan

Punkin II. Radio control sport model for electric power, with Mabuchi N20 motor and 4.2:1 gearing. Wingspan 16-7/8 in, wing area 84 sq in. For 3 channel micro RC.

Quote: "The Punkin II was born of a desire to have a micro biplane that could fly both indoors and out with an extra measure of maneuverability. Previous experience with micro models has shown me that there is a 'weight barrier' at around 2.2 ounces when I use my favorite DC5-2.4 motor with a servo-type control system. With this in mind, I began looking into other readily available motors, especially those that could run on fewer battery cells. The best candidate I found is the Mabuchi N20. This motor is widely available, reasonably light and produces good power on just 4 cells. The Kenway KR-1 is another good candidate; it comes with 4.2:1 gears installed. My motor was a leftover from a turbo twin economy RC system, and I purchased the 4.2:1 gears from Cloud 9 RC. I chose to use 50mAh Ni-Cd batteries because of their higher voltage under load.

Because I wanted to use equipment that was on hand, I went with a Sky Hooks & Rigging Pro receiver and a couple of Wes-Technik servos. The low-voltage ESC came from Bob Selman Designs and is a pleasure to use. Armed with equipment and motor, I next needed to obtain a minimum-weight airframe.

With a model this small and light, all of the balsa should be of the lightest grade. My stock of 4- to 6-pound wood came from Superior Balsa and Hobby Supply. It is often called 'contest balsa' in the hobby trade.

There is a relatively new, lightweight plastic film that’s sold as 'RA Microlite'; it’s very similar to the thick, iron-on coverings. It’s available in a variety of colors and comes complete with adhesive. I decided to give this a try and am very pleased that I did. It’s easily applied and is very light and attractive.

The actual design of the little bird followed my thinking of minimum weight as far as it could. For example, the wings are simple frames covered on one side only, as is the stabilizer. The fuselage has the minimum amount of wood, and the landing gear is quite light. When it all came together, I was pleased with the appearance and the all-up weight of 45 grams. My first choice for the propeller was a blue, plastic 5.8-inch-diameter Kaysun, which ran at about 3,500rpm on a fresh charge.

On the first calm morning, I walked over to my grassy test field, turned things on and gave Punkin a toss. Would you believe that it took off in a brisk climb, trimmed to perfection and proceeded to run through a super maneuver sequence? Not quite! Actually, Punkin headed out nose high and started to stall until I got full down-trim cranked in. Meanwhile, the net altitude gain was maybe 10 feet after the trim had been sorted out—not quite what I had in mind!

I’m no quitter, so I decided to start a modification program to perk up the performance. First, I covered the bottom of the wings. This helped somewhat, but more was needed. The next step was to work with propellers. The Kaysun prop had a rather high pitch, so I decided to try a modified prop that had been twisted to a lower pitch with the same diameter. This was a big help. The climb rate was reasonable, and Punkin would cruise at about 1/2 throttle. Meanwhile, I had fixed the pitch trim by remounting the stabilizer at a new incidence angle.

I could have stopped messing with the design here, but I wanted more performance. My next move was to build a new set of wings with a Benedeck 6356b airfoil, which I covered with ultralight condenser paper and fitted with a turbulator strip to help energize the airflow. This was a very good move. Punkin had become both lively and efficient and could do maneuvers such as loops and rolls as well as cruise for extended periods.

This was a fine stopping point, but a friend suggested that it would do even better with a different gear ratio and a larger prop. Digging deep, I found a 6:1 gear set in an old Futaba servo that I could use to drive a 6-inch-diameter Peck-Polymers plastic prop. Sure enough, the performance improved even more. At this point, I had everything I wanted in performance, but the little sucker was homely! Some cosmetic improvements gave birth to Punkin II, which is aerodynamically the same but, I believe, more attractive.

One lesson I learned through this program of improvements is that we still have a great deal to learn in the fertile field of micro RC. Why don’t you try building a Punkin II yourself and maybe add a few refinements of your own? One obvious option would be to equip Punkin II with one of the ultralight magnetic RC systems for an even lighter model. The plan shows the proper place to cut the controls to add the needed aerodynamic balance for these systems.

CONSTRUCTION

Throughout the process of building a micro RC model, it is essential to keep the weight to a minimum for adequate performance. Perhaps the most valuable tool is a sensitive scale to keep track of weights. Although a modern digital version would be excellent, I am quite satisfied with the triple-beam gram scale I bought on sale at a school-supplies store. Good lighting is a must, and proper visual aids make the going much easier.

You will need very little in the way of actual material, so go ahead and buy the best available. I have mentioned Superior Balsa and Hobby Supply as a balsa source, and the RA Microlite covering and condenser paper is carried by Dave Lewis in Ohio. I do essentially all of my building with medium-grade CA glue. There are plenty of choices here, although I prefer Goldberg Super Jet. Be sure to find a way to dispense the glue in small quantities; you could put a special tip on the bottle or buy one of the small dispensers. Glue is mighty heavy, and we only need enough to hold things together. Please, no filleting with glue here!

I enjoy using the fine-point silk pins that I found at a fabric store, and I keep a fresh blade on my hobby knife; this light balsa is torn very easily by a dull blade. A couple of small sanding blocks with 100- and 150-grit paper should be close at hand as you lay things out. The rest of the tools should be in your box; if they aren’t, check out the model train department in a hobby shop or the Micro-Mark tool company catalog.

The only templates you will need are the two wing ribs. I made mine from thin plywood, but other materials would work fine. The whole job will go more easily if you make a 'kit' of parts before you start sticking things together.

Fuselage: The fuselage construction should go quite rapidly if you make two sides and glue all of the strips to them while they are flat on the board. Cut the various openings after these brace strips are in place. The only 'bulkheads' in the cabin are four 1/16 x 1/8-inch crosspieces that hold the sides together at the front and rear of the cabin. Install these now. Rather than repeat myself, let me just point out that even tiny airplanes need to be well aligned as they are built.

Pull the tail pieces together, and fit the crosspieces as shown. I used the motor plate to hold the nose together initially while I fitted the two bulkheads. The landing gear should go in now. A little extra glue is all the bracing needed for this skinny wire. When you put the sheet planking into place, the job is nearly done. The F-1 bulkhead will need to be trimmed to fit your motor unit, but this should be done at assembly; the same applies to the F-2. Sand the fuselage nice and smooth, and set it aside..."

Direct submission to Outerzone.

Update 17/4/2023: Added article, thanks to theshadow.

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