Author: Oscar Weingart
Edition: Model Aviation - 2007/04
Page Numbers: 49,50,51,52,53,54,56
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Plane Talk: Balsa USA 1/3 Scale Ercoupe on Floats

By Oscar Weingart

I enjoy RC float-flying, so most of my models wind up on floats. I recently completed a Balsa USA 1/3 Scale Ercoupe on floats, and according to Balsa USA it was the first of these aircraft to successfully fly off the water. In this article I give an overview of the Balsa USA kit and concentrate on the adaptation of the model to floats.

I had never seen an Ercoupe on floats. My research revealed that the first was the EDO/ERCO prototype, NX86951, in 1946 on the East Coast. It passed the CAA (now FAA) tests, but the government pilot asked for more familiarization time and flipped the airplane on a crosswind takeoff or landing (per an ex-EDO employee). The floats appear to have been standard EDO 1320 "flat-top" models, as were used on the Piper J-3 Cub and other light aircraft of the period with similar sizes and weights. According to Fred Weick, one of the chief designers, rudder pedals were required for crosswind landings and takeoffs with floats. The original Ercoupes were set up for optional rudder pedals, but very few people ordered them that way, so ERCO eliminated the hardware needed for that option in post-war production models. Hence the floatplane Ercoupe was no longer viable.

The second Ercoupe on floats, NX87060, was a private venture by Howard E. Cousins in 1952 near Chicago, Illinois. Todd Westfall's dad Dale was the CAA pilot. The limited elevator travel had to be increased to allow the airplane to get on the step. This made it no longer spin-proof, so they had to do spin tests. In one of those tests the aileron on one wing developed flutter and the wing failed. The airplane crashed and the pilot escaped just in time (per Todd, a newspaper account, and the pilot's log book). Therefore, no certification was forthcoming and no more float-equipped Ercoupes were built.

As on the full-scale Ercoupe on floats, the struts on my model form three triangular trusses connecting the floats to the three tricycle landing-gear hard points on the Ercoupe. When I built my Balsa USA Ercoupe, there were no kits, plans, or ready-made EDO 1320 flat-top floats available in 1/3 scale. All available 1/3-scale floats, including Balsa USA's and Sea Commander's, were the later round-top EDO 2000 style. Therefore, I used the Sea Commander 64-inch EDO 2000-style round-top fiberglass floats on my model. I tried to get the look of the full-scale prototype using the Sea Commander hardware and special adapter brackets I designed and fabricated. These brackets attach the six struts to the tricycle landing-gear hard points on the Balsa USA model. I used two water rudders on the model for positive water handling, driven by cables from the nose-gear servo.

The Balsa USA Ercoupe

I had not built a big model for approximately 10 years; the last was a Stream Schneider Sport 320 floatplane racer with a SuperTigre 3000 engine. Flying ARFs and airplanes that others built had been my usual practice, but when Balsa USA came out with the 1/3 Scale Ercoupe kit I was inspired to take on a more challenging project.

The model looked like a good candidate for the Kavan FK50 four-stroke twin engine I was flying in a ModelTech ARF Cub at the time. The Ercoupe's 2,400 square inches of wing area and ~31-pound weight on wheels were almost identical to that Cub, and the Cub was flying nicely at 40 pounds on floats.

The huge Balsa USA Ercoupe kit is not for the faint of heart—either financially or in terms of building time and skill. The kit, with tax and shipping, will set you back roughly $1,000.

  • The kit came in three packages: a conventional big kit full of lumber; a large box with the giant fiberglass cowl and other goodies; and a big mailing tube containing rolled thin plywood and the seven monstrous sheets of plans. Small parts were nicely and logically packaged in clear plastic bags with labels.
  • The 73-page manual had many photos.
  • The fiberglass cowling was impressive: big, light, flexible, and with difficult trimming already done. It had molded-in fastener heads and panel lines.
  • The entire fuselage top half was built on a table, then inverted in a fixture for completion of the bottom half.
  • The scale operating tricycle landing gear was assembled from massive aluminum castings and pieces of steel. I did a fair amount of filing, drilling, and silver-soldering, but the precision holes and mating surfaces were already drilled and machined. The finished landing gear, with the supplied air wheels, weighed 4 pounds.
  • Wing and tail construction were conventional, except for the scale diagonal wing ribs and complex laminated wingtips. The removable outer wings were stiff and light.

Scale Detailing

My "coupe" was built as a Sport Scale model, not meant for Expert-class competition. A separate big three-view sheet was thoughtfully supplied in the kit; it showed rivet location, pinked-tape placement, and so on. I omitted several of the details shown and/or furnished with the kit, such as the gas gauges, rivets, pinked tape, and decals, but I did build a scale instrument panel based on Ercoupe N87405.

I did not use the simple early-Ercoupe panel furnished with the kit. It was fun hunting surplus electronics stores for tiny switches, lights, and circuit breakers. I used JTEC 1/3-scale instruments.

I upholstered the interior with adhesive-backed fabrics I obtained in local plastics and auto-supply stores. The full-body pilot came from H.P. Pilots; he was too tall for the cockpit, so I had to remove about an inch from his midriff. He weighed a whole pound, so I left him out for the initial test flights.

Model Float Installation

My strut design is based on the prototype Ercoupe floatplane and utilizes the three tricycle landing-gear attachment hard points, as does the full-scale version. The struts, spreader bars, and floats form three rigid triangular trusses and make for a sturdy float mounting system. The model's scale bolt-on wheel landing gear made it relatively easy to design suitable adapter brackets to be attached in its place.

Sea Commander furnished the floats along with the material and fasteners to make the spreader bars and struts. I designed and fabricated the adapter brackets, water rudders, and X-bracing from material obtained locally.

The 64-inch Sea Commander EDO 2000 floats are high-quality fiberglass/polyester moldings. The spreader bar saddles are molded in, and there are several internal bulkheads that separate the floats into watertight compartments. A special small bow compartment is provided for balancing weights.

The struts and spreader bars are streamlined 6061-T6 structural aluminum tubing—not typical soft hobby-shop aluminum. These components are bigger and much stronger.

The float assembly weighs a hefty 9 pounds, but the husky Ercoupe landing gear it replaces weighs 4 pounds. Thus we gained only 5 pounds adding the floats, for a 36-pound total weight. With 2,400 square inches of wing area, the wing loading came out a reasonable 35 ounces per square foot. The Ercoupe’s flight characteristics resemble those of my ModelTech ARF Cub.

Spreader Bars

  • The two teardrop aluminum-tubing spreader bars were mounted with the float centerlines 32 inches apart.
  • I match-drilled and installed the furnished aluminum filler bar and three self-tapping screws at the end of each spreader bar after ensuring the floats were parallel and the assembly was square.
  • After initial assembly I disassembled the spreader bars, hardened the screw holes in the floats with thin cyanoacrylate glue, and then reassembled the floats and spreader bars.
  • I trimmed the ends of the spreader bars to clear the covers to be installed later.

Adapter Brackets

  • The three adapter brackets for the model landing-gear attachment points were made from aluminum T-section for the wing main-gear attachment point and aluminum angle for the nose-gear attachment point.
  • The bolt-hole pattern in the adapters matched the hole pattern in the wheel landing-gear legs.
  • I mounted the brackets onto the model using 4-40 hardware—the same as used for the wheel landing gear being replaced.
  • I made four lower float-attachment brackets from aluminum angle and mounted them onto the spreader bars with self-tapping screws. The forward brackets were tricky since they served as attach points for the nose-gear struts and the forward main-gear struts.

Struts

  • I made a simple fixture to hold the floats and the airplane in the correct relative position so the strut lengths could be determined. The float steps were located at the airplane’s CG, and the flat bottom of the wing was kept parallel to the top of the floats so the wing maintained a positive incidence angle to the water surface.
  • I machined the strut end fittings from the furnished aluminum bar. I used a band saw and file to make the slots, a drill press to drill end holes, and a belt sander to round the ends.
  • I match-drilled the struts and end fittings for one end of each strut and installed the furnished hardware. I slid the other end fitting within the strut to get the exact overall end-hole-to-end-hole length, then match-drilled and fastened it. The struts were made in left and right pairs of equal length.
  • I used standard 4-40 hardware to fasten the struts to the airplane and to the floats, with elastic stop nuts on all bolts in the float assembly and Loctite on the main gear adapter bracket bolts.

Finishing

  • The four fiberglass covers that Sea Commander furnished were fastened over the spreader bar saddles using #2 self-tapping screws. These covers had to be individually trimmed and fitted to each saddle.
  • I disassembled everything and painted the floats using Stits Poly-Tone (Rancho Silver) with F&M Enterprises’ Feather Coat primer. The paint gives a scale-looking satin finish; a gloss top coat is available for those preferring a shinier appearance. The paint is prethinned for spraying and contains retarders, flexibilizers, and a self-extinguishing fire retardant.
  • I used the same brand of paint on the rest of the model. Stits has a special dope formula (used instead of primer) that chemically bonds to the polyester wing-covering fabric.

Water Rudders

  • I cut the rudders from .030-inch aluminum and silver-soldered .030 brass hinge brackets to the 5/32-inch music-wire shafts.
  • Standard hobby-shop nose-gear nylon bearings and steering arms completed the installation. The water rudders were manually retractable and adjusted to kick up if they hit an obstacle.
  • I modified a nylon steering arm to form a bearing and bolted it to the nose-gear adapter bracket. A vertical music-wire shaft was fitted with three more of these steering arms: one for the connection to the servo and the other two to form a tiller bar. Cables connected this tiller bar to the steering arms on the water rudders.
  • The Ercoupe nose-gear-steering servo was used to steer the water rudders.

Balancing

  • I drilled a 1-inch-diameter hole into the top of each float to gain access to the bow ballast compartment.
  • I balanced the assembled floatplane by installing lead weights in modeling clay in the ballast compartments and filled the remaining space with polyurethane soft foam.
  • Scale-looking soft aluminum covers were made for the holes and held in place with small self-tapping screws. The edges of the covers were coated with clear silicone sealer.

Transportation

  • The 1/3-scale Balsa USA Ercoupe has detachable outer wing panels, which make for easier transportation.
  • If you have a large-enough vehicle, the Ercoupe floats can remain attached to the model. With our standard-length Ford Aerostar minivan, the float assembly had to be removed and loaded separately.

Control Setup and Programming

  • The water rudders were permanently linked to the air rudder stick in my Futaba 9VAP transmitter using the programmable mix function. The channel used has a knob on the face of the transmitter, which allows the water rudders to be trimmed independently from the air rudders.
  • Hitec RCD HS-645MG high-torque, metal-gear servos were used throughout.
  • The full-scale Ercoupe had no rudder pedals: ailerons, rudders, and nose wheel were linked to the pilot's control wheel.
  • I programmed a switch-selectable mix for the model. The ailerons and air rudders move together with the aileron stick, but the rudder stick can override the coupled rudder movement. I used the coupled aileron-air rudder mode in flight, but preferred the uncoupled mode for landing and takeoff.

Taxiing and Flying

  • The Ercoupe "turns on a dime" with two large water rudders. I hold full up-elevator when taxiing on water; this keeps the water rudders well under the surface for positive control.
  • The heavy, low-wing Ercoupe is not affected much by crosswinds, but seaplane sailing and taxiing techniques, such as holding aileron against the wind, are useful in stiff crosswinds.
  • Takeoffs and landings are the fun part of flying RC seaplanes. The Ercoupe initially porpoised during takeoff, but a more powerful engine cured this condition. Standard seaplane takeoff and landing techniques work well on the model.

The Balsa USA 1/3 Scale Ercoupe equipped with Sea Commander EDO 2000 64-inch floats makes a unique, attention-grabbing seaplane that looks great and flies beautifully.

Many thanks to Dave Lewis at Balsa USA, Ryan Holm at Sea Commander, and Chip Mull at F&M Enterprises for their help in getting the information and materials needed to build the Ercoupe on floats. Tom Laird-McConnell furnished many fine detail photos of N87405, as well as photos of the prototype Ercoupe floatplane.

The full-scale airplane's background and history were obtained via Google and from several discussion forums on the RC Universe website. Don Lien of the Riverside RC Club (RRCC) was a great partner in painting, fabric covering, and cockpit-glass installation. Initial test pilots were Dale Yaney of RRCC and Dan Egelhoff of Team Artronics, followed by Dave Collis of Vancouver, British Columbia, for test flights at Lake Shuswap, where the Ercoupe won the "Best on the Beach—Scale" award.

—Oscar Weingart [email protected]

Ercoupe Specifications (Model)

  • Model type: Giant Scale kit
  • Pilot skill level: Sport/Scale fliers who are experienced builders
  • Wingspan: 120 inches
  • Wing area: 2,400 square inches
  • Length: 84 inches
  • Weight: 28–32 pounds
  • Wing loading: 30 ounces/square foot (as built)
  • Engine: 45–55cc two-stroke gas or 3.0–3.5 cu. in. glow
  • Radio: Four-channel radio (minimum), six servos
  • Construction: Conventional balsa/plywood/basswood with fiberglass cowling
  • Covering/finish: Fabric on wings, thin fiberglass (furnished) on fuselage and tail
  • Price: $899.95

Test-Model Details

  • Engine used: Zenoah G-45 (on wheels), G-62 (on floats)
  • Propeller: Master Airscrew 20 x 8 (on wheels), APC 22 x 10 (on floats)
  • Fuel: 24-ounce plastic (furnished)
  • Radio system: Futaba 9VAP transmitter with nine-channel PCM receiver
  • Servos: Six Hitec RCD HS-645MG servos
  • Batteries: Two 1500 mAh Cermark 6-volt NiMH packs
  • Cables: Eight Cermark 12-inch servo extensions
  • Finish: Stits Poly-Tone paint
  • Ready-to-fly weight: 31 pounds (wheels), 36 pounds (with floats)
  • Flight duration: Exceeds 15 minutes

Pluses and Minuses

  • Pluses:
  • Beautiful, good-flying scale airplane
  • Complete kit
  • Excellent construction manual
  • Scale functional landing gear
  • Scale sliding cockpit windows
  • Minuses:
  • Complex fuselage construction
  • Balsa quality issues
  • Errors in manual but no errata sheet

EDO 2000 Float Specifications

  • Molded-in fiberglass joiner lip and side stiffener
  • Molded-in fiberglass bottom reinforcements
  • Four fiberglass-reinforced bulkheads
  • Five watertight compartments
  • Composite reinforcements in scale mounting positions
  • Composite reinforced transom for water rudder mounting
  • Available in sizes from 29 to 65 inches
  • Floats pressure-checked for leaks
  • Ready-to-paint and -install fittings
  • Float kit used: 1/3 scale (64 inches)
  • Price: $385.28 (floats only)

Scoop on the "Coupe"

During the 1920s and 1930s many private pilots were injured, killed, or scared off by the terrible ground handling, landing and takeoff characteristics, and poor stall-spin performance of tail-dragger lightplanes. Ground loops during landing were common, and the gentle stall and spin behavior of later designs was unknown.

A group of engineers, including the great Fred Weick, formed the Engineering Research Company (ERCO) to address these problems. The Ercoupe Model 415C was the result.

With performance similar to today's Cessna 150, the Ercoupe could not spin and it was certified as such by the CAA. The Ercoupe was so easy to fly that a special pilot-license category was assigned to it, requiring a minimum of five hours solo and 35 hours total flight time, as opposed to eight hours solo and 40 hours total for conventional lightplanes of the time.

The Ercoupe is a two-seat, low-wing monoplane with twin vertical tail surfaces and a tricycle landing gear. The pilot and passenger sit in a glassed-in cockpit with excellent all-around visibility. Construction is mostly all-metal, except for fabric-covered wings. The monocoque design has no wing or tail struts. Engines ranged from 65 to 90 horsepower.

The rugged Ercoupe tricycle landing gear is configured so the airplane can be landed in a crab in a crosswind, and its geometry causes the aircraft to immediately straighten out on the runway upon touchdown—reducing ground looping and easing ground handling.

With the nose wheel linked to the pilot's yoke, you drive it like a car. A car-like single brake pedal controls both main wheel brakes simultaneously.

The Ercoupe's stall-spin resistance is achieved through several design features: generous wing dihedral, coupled ailerons and rudders (no rudder pedals on early models), and limited elevator travel. These prevented cross-controls, slips, and skids, and aided stall resistance.

More than 5,000 Ercoupes were built, mostly after WWII, and over 2,000 remain registered today. Several companies followed ERCO in building Ercoupes, introducing changes such as a sliding canopy, single tail, rudder pedals, and larger engines. Some of those modifications produced more conventional stall-spin behavior.

The unique color scheme and instrument panel I used on my model are those of N87405. It was owned by Tom Laird-McConnell in the Seattle–Tacoma area and later sold to Harold Gochenour in Lake Havasu City, Arizona. Tom was generous with photos of his airplane; his website (www.ercoupers.com) was very helpful in duplicating the interior and exterior details.

Man Behind the Ercoupe

I recently finished Fred Weick’s autobiography, From the Ground Up (Smithsonian Institution Press, 1988). Weick (born 1899) was an outstanding aeronautical engineer who built and flew model airplanes before World War I. He applied rigorous engineering and scientific approaches to airplane design and testing.

Fred worked for NACA, the Airmail Service, Texas A&M, ERCO, and Piper. He invented the modern tricycle landing gear and led pioneering research into safer aircraft-control systems. He also led the team that developed the NACA low-drag cowling for radial engines.

Fred wrote a paper on simplified propeller design for light aircraft that remains a key reference for home builders. He was active in the EAA and was responsible for the design of the Ercoupe, Piper Cherokee series, and the Ag-1 agricultural aircraft, which influenced modern crop-duster designs like the Piper Pawnee.

For a summary of his career, see: http://en.wikipedia.org/wiki/Fred_Weick

Full-Scale Ercoupe Specifications

  • Length: 20 feet, 9 inches
  • Wingspan: 30 feet
  • Wing area: 142.6 square feet
  • Weight: 1,440 pounds (max.)
  • Wing loading: 10.1 pounds/square foot
  • Engine: Continental C-85

Aircraft Kit Manufacturer

Float Manufacturer

  • Sea Commander
  • 4768 Harvie Ave.
  • Powell River, BC, Canada V8A 2P5
  • (877) 485-2926
  • www.seacommander.com

Items Used in Review

Resources

Transcribed from original scans by AI. Minor OCR errors may remain.