Scratch Foamie

From the ground up, this RC sport design helps beginners go micro

By Bob Aberle

As seen in the May 2010 issue of Model Aviation.

In 2003 and 2004, I authored 10 installments of the 31-part “From the Ground Up” series that was published in MA. The sequence of articles reads like a book and takes a new (beginner) modeler from square one up through his or her first RC model flights.

This is likely an overnight airplane build for the beginner.

This is likely an overnight airplane build for the beginner.

As part of that series, I came up with a design I called the Scratch-One, which was an ultra-simple trainer design that was intended to be a first-time construction project. Shortly after the series was completed, I decided to reduce the Scratch-One to a size and weight that would permit it to be flown indoors in areas as small as school gymnasiums. I shrunk the Scratch-One to 65% of the original size, resulting in a wing area of only 105 square inches. The total weight came out to 2.7 ounces. I called this new version the Scratch-65. A full construction article, with plans, appeared in the February 2005 MA.

The larger of these two models is the original Scratch-One, with 247 sq. in. of area and a total weight of 17 ounces. The smaller version is the Scratch-65 (65% of the original size), which weighs 2.7 ounces.

As originally published, the Scratch-65 was constructed from conventional modeling materials such as balsa sheets and sticks along with iron-on covering. The radio and power systems were basically what was popular five years ago.

The radio, power system, and battery cost roughly $200 back then. The motor was a brushed and geared variety that was not particularly efficient. The battery was a two-cell Li-Poly with a capacity of just 145 mAh. Power loading was a mild 30 watts per pound, but the little airplane flew well, mostly at full throttle.

Now, five years later, I decided that with so much improved technology in micro RC systems, motors, batteries, and even construction materials, this was the time to revisit the Scratch-65. A new design resulted, which I call the Scratch Foamie.

As the name implies, this version is constructed entirely from 2mm Depron foam sheeting. You can build a complete model from a single sheet of this material that measures 15 x 39 inches, at a cost of roughly $3.

With the success of 2.4 GHz spread spectrum technology, it was natural to try a micro RC system. One of the best setups for indoor/micro application is the ParkZone AR6400 “brick” module, which is a receiver with two linear output servos mounted on a single PC board.

You can purchase this unit separately or, as I did, buy a ParkZone Sukhoi RTF for approximately $100. All you need to do is bind the airborne setup to any Spektrum DSM2-type transmitter.

The total weight of all these components is merely 0.63 ounce. The BSD Micro RC special cable (R) is essential if you plan to run a brushless motor from this AR6400 brick.

The total weight of all these components is merely 0.63 ounce. The BSD Micro RC special cable (R) is essential if you plan to run a brushless motor from this AR6400 brick.

After having a lot of fun flying the Sukhoi, I removed the brick and installed it in the new Scratch Foamie. A nice advantage of using 2.4 GHz is that the 34-inch-long receiver antenna that was required five years ago on my 72 MHz RC system has been reduced to 2 or 3 inches.

I decided against a brushed/geared motor and opted for a new 3-gram AP03 brushless outrunner and a companion LCD 1-gram/3-amp micro brushless ESC. I obtained the motor and ESC from Bob Selman at BSD Micro RC.

The best propeller for the AP03 has proved to be a GWS 4.5 x 3 operating on a single Full River 250 mAh LiPo cell. This runs at 2.5 amps and 8.8 watts of input power.

The original Scratch-65 weighed 2.7 ounces, and the motor input power was only 5 watts. The revised design weighs only 1.64 ounces with a motor that operates at 8.8 watts of input power. So the power loading increased from 30 watts per pound (five years ago) to 85 watts per pound. Instead of flying at full throttle, I can fly the Scratch Foamie at less than half throttle. That can provide 12-15 minutes of flying time on a charge. Keep in mind that the brushless motor is more efficient than the brushed variety and certainly has a much longer service life, since there is really nothing to wear out.

The 250 mAh Li-Poly replaced the heavier 145 mAh unit, permitting longer flight times. Still another advantage is that the cost of the power/RC system is less than it was five years ago. The 2.4 GHz spread spectrum radio offers essentially interference-free operation with regard to other modelers.

This new version of the Scratch-65 is a tremendous improvement in every respect.

CONSTRUCTION

I won’t go into every detail of the build sequence, but I will highlight the important areas.

I made copies of certain portions of the plans and pasted them onto manila folders. The various parts were cut out, and then I traced the outlines onto Depron foam.

Cutting all of the Depron parts takes little time using an X-Acto knife. You will need a foam-friendly, medium-viscosity CA cement and companion accelerator.

Outlines of the various aircraft parts are transferred to the foam sheet and then cut with an X-Acto knife.

Outlines of the various aircraft parts are transferred to the foam sheet and then cut with an X-Acto knife.

Wing: The airfoil shape of the three wing panels is achieved by rolling the foam sheeting over a large-diameter PVC pipe (3 or 4 inches in diameter). Slowly roll the sheeting for a few minutes. It is neither necessary nor desirable to apply heat to gain the airfoil curvature.

A total of only six ribs will help maintain the airfoil shape. Two ribs—located in the center-section—will help attach the wing to the fuselage later. Two more ribs are at the poly joints, and the other two are placed at each wingtip.

Since no spars were employed, I resorted to .007-inch-thick x 1/4-inch-wide carbon-tape reinforcement under both wingtip panels. Cemented in place on the underside, that tape provides a tad extra strength to these outer wing panels.

Airfoil curvature on wing panels results from rolling foam pieces over a large-diameter length of PVC pipe. The outer wing panels have carbon-fiber strips at the point of maximum camber. Polyhedral is added to both wingtip panels; the center-section is fl

Fuselage: I cut a fuselage assembly fixture from 3/16 balsa to the outline of the top view. Then I pinned the thin fuselage sides to this fixture.

Partial formers were inserted above and below the fixture. Once there were enough formers in place, I removed the pins and withdrew the fixture, leaving an aligned fuselage.

A fuselage assembly fixture was made from 3/16 balsa sheet. It follows the top view of the fuselage. Both Depron foam sides are pinned to this fixture. Partial formers are added above and below the fixture.

Then I cut the vertical and horizontal tailpieces. For hinges I used Blenderm tape, which you can obtain at most drug stores. An equivalent Electric Flyer Hinge Tape is available from Du-Bro Products as item 916. I installed Du-Bro Micro Control Horns, item 848, on both the rudder and elevator.

Tail pieces are cut from 2mm Depron sheeting. After spacing the elevator and rudder, hinge tape is pasted in place. Du-Bro Micro Control Horns are used on both control surfaces.

Tail pieces are cut from 2mm Depron sheeting. After spacing the elevator and rudder, hinge tape is pasted in place. Du-Bro Micro Control Horns are used on both control surfaces.

The control rods were made from .025-inch-diameter carbon rods. At each rod end I attached short lengths of .015-inch-diameter wire with Z-bends made on the ends. I attached these wire ends to the carbon rods, using either heat-shrink tubing or thin-diameter aluminum tubing that could be crimped with pliers. This technique allows you to align control surfaces at the neutral positions before the final crimp of the tubing.

The ParkZone AR6400 brick was mounted to a double layer of 2mm Depron using double-stick tape. The LCD 1-gram/3-amp brushless ESC was mounted to the fuselage side, just behind the plywood firewall, also using double-stick tape.

The complete power and RC system is forward of the wing LE with motor installed. An ESC is taped to the side; its wires run under the battery tray. The ParkZone AR6400 brick contains receiver and servos.

When mounting the AP03 motor to the firewall, make sure that you drill a clearance hole for the motor shaft that protrudes out the rear of the power plant.

Your rudder servo will initially operate from the left-hand transmitter control stick. That’s because this brick was intended for four-channel control with the aileron function on the right stick.

To make the rudder work from the right-side transmitter stick, evoke aileron/rudder mix on your transmitter and set it for 100%. As soon as you do that, your rudder will operate from the right-side transmitter stick.

To operate the brushless motor, you will need a special $13 cable that is available from BSD Micro RC. You will also have to reprogram the AR6400 receiver for brushless operation, as outlined in the ParkZone User Guide. It’s a simple process and takes only a few seconds, but it’s necessary when you switch from a brushed to a brushless motor.

Final Assembly: When everything is working, such as the controls and motor, install the wing to the fuselage. The two center ribs were spaced so that they fit just inside the fuselage and press against both sides.

Make sure that the wing is aligned with respect to the tail surfaces. Cement it in place.

Two center wing ribs fit snugly inside the two fuselage sides. Align the wing with respect to tail surface, set in the proper incidence angle, and apply CA cement. This is an easy process.

Two center wing ribs fit snugly inside the two fuselage sides. Align the wing with respect to tail surface, set in the proper incidence angle, and apply CA cement. This is an easy process.

Finishing: To decorate the Scratch Foamie, I misted on two colors of paint to dress up the all-white foam sheeting. I used the Krylon-brand H2O Latex spray paint.

I applied the blue on the wing and stabilizer’s LEs. Then I sprayed the red, but it didn’t quite cover the blue. This paint job was quick to do and added little weight.

The forward nose section can be covered and dressed with a simulated cockpit area. Bob made the black windows from material from SR Batteries.

The forward nose section can be covered and dressed with a simulated cockpit area. Bob made the black windows from material from SR Batteries.

Bob decorated his completed Scratch Foamy with a light misting of Krylon H2O Latex spray paint (blue and then red). The process takes little time, but some color is necessary—otherwise you end up with an all-white model.

Bob decorated his completed Scratch Foamie with a light misting of Krylon H2O Latex spray paint (blue and then red). The process takes little time, but some color is necessary—otherwise you end up with an all-white model.

Flying: Final control throw ended up with the rudder moving 3/8 inch either side of the neutral position and 3/16 inch either side for the elevator. Control throws were set using the EPA on my DX7 transmitter. The balance point is 1-1/4 inch back from the wing LE as shown on the plans. My aircraft balanced perfectly, requiring no added weights.

Flying, to date, has taken place in a double-size high school gymnasium located near Lake Ronkonkoma, Long Island, New York. Ceiling height is approximately 20 feet.

A small gymnasium is plenty of room in which to fly a Scratch Foamy. The polyhedral wing helps it turn easily with only rudder input. Joe Cabana photo.

A small gymnasium is plenty of room in which to fly a Scratch Foamie. The polyhedral wing helps it turn easily with only rudder input. Joe Cabana photo.

Most of my flying has been at half throttle, which says a lot for the 85-watt-per-pound power loading. Turns can be made tightly with no resulting stalls. This is a great-flying indoor sport model.

Anyone with basic RC flying skills should be able to handle this airplane. Although the Scratch Foamie is intended primarily for indoor flying, it can also be flown outdoors in extremely calm conditions.

So don’t rule out flying in the early morning or evening during the summer months. In a sense, that makes this design a park flyer as well.

The ParkZone AR6400 brick (receiver with two built-in servos and brushed-motor ESC) has six-channel control capability. You can fly aircraft such as the Sukhoi and P-51 using full four-channel controls, and you could add extra functions such as flaps and retracts.

This particular application required only rudder, elevator, and motor control. But to use a brushless motor, I had to go to this new AR6400 brick. If you use a brushed/gear motor, such as the ones supplied with the Sukhoi and P-51, you could get away with the ParkZone three-channel brick that is used in the Ember and Vapor aircraft.

I’ve proved that this little 3-gram AP03 brushless outrunner, operating at roughly 9 watts of input power, is capable of flying models weighing as much as 2 ounces and possibly 2.5.

As such, it would be a perfect micro power system that is capable of flying many small Rubber Scale designs that companies such as Dumas Products and Dare Designs sell in kit form. You might want to consider this for future indoor/micro projects.

You can get a lot of information about indoor/micro RC from the RC Micro World monthly online magazine. Not to sound commercial, but I have a book/CD, “The World of Indoor/Micro Radio Controlled Model Aircraft,” that covers this entire subject. It is available from the RC Micro World publisher and the AMA museum store.

Specifications

Type: RC sport park flyer

Skill level: Beginner

Wingspan: 29.5 inches

Wing area: 105 sq. in.

Length: 20 inches

Weight: 1.64 ounces

Wing loading: 2.2 ounces/sq. ft.

Power system: Bob Selman AP03 (7,500 Kv) 3-gram brushless outrunner motor,

GWS 4.5 x 3 propeller, LCD 1-gram/3-amp brushless ESC, Full River single-cell, 250 mAh LiPo battery

Radio: Spektrum AR6400 2.4 GHz receiver/ESC/servo module, Spektrum DX7 transmitter

Construction: 2mm Depron