Republic RC-3 Seabee Specifications

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Seabee Design Philosophy

As a prelude to the structural analysis of the Seabee, it is interesting to outline the considerations leading to, and the theory underlying, the simplified design.

The prototype Republic RC-1 Thunderbolt Amphibian (NX41816), which was completed in November 1944, was built only to prove the general design. The aircraft was a three-place 175 hp all-metal amphibian monoplane using conventional structure throughout, and was an outgrowth of the original S-12 Air Car amphibian design by Percival H. Spencer, who became a development engineer with RAC.

Manufacturing cost considerations - price would have had to be almost twice the initial price (just under $4,000) based on simplified design - prompted RAC president Alfred Marchev to direct an investigation to determine how airframe structures could be simplified to reduce manufacturing costs sharply.  He believed that extensive simplification could be achieved, with consequent great reduction in number of assembly components, while yet maintaining the high standards required in aircraft construction.

Alfred Z. Boyajian, structures project engineer at Republic, was assigned the task to redesign the Seabee airframe to meet reduced cost requirements.  As a preliminary step, he reviewed: (1) The evolution of conventional design, to ascertain why this type of structure had been adopted; (2) production time studies, made available from wartime experience, to establish what, in general, was causing manufacturing costs to be so high and (3) stress analysis procedures, to determine what conceptions existed which might be altered to justify a vastly simplified structure.

It became apparent that: (1) Despite the progression from the wood and fabric covered wing to the all-metal unit, the latter was still fundamentally similar, in basic pattern, to the former; (2) production complications arose because of the complex "egg box" structure - many internal interconnected members, in turn connected to the outside cover; and (3) the necessity for the retention of numerous rib components (as "irreplaceable" internal members of the conventional metal wing) had not been clearly established.

When engineer Boyajian's simplified - comparatively ribless - structural design was first proposed on paper it was subjected to much discussion in large conferences of engineering personnel. The absence of conventional ribs in the design created much doubt as to structural airworthiness - doubt voiced because it was believed that in accordance with customary methods of stress analysis, the proposed simplified structure was considered to be probably deficient in strength requirements. It was considered, generally, that other than a rib there was no structural member deemed capable of transferring airload shears in a chordwise direction to the bending resistant spars.

Boyajian's theory was that if the cover of a stressed skin wing were sufficiently stiffened, thus creating a heavy torque box, it should be possible to transfer such airload shears with a minimum of internal structure. He arrived at this conclusion after reasoning that the conventional practice for' a stressed skin wing, wherein a section is isolated and analyzed as an independent structure, was not justified, since it was assumed that the other portions of the overall structure did not contribute any vital additional strength characteristics to the isolated section.

True, an isolated ribless section would deflect under airload, because of absence of shear rigidity, and would give a large torsional displacement with respect to the end ribs. But a torque box, for example, comprising the stressed skin leading edge would offer appreciable restraint to such torsional displacement. Further, Boyajian believed that the leading edge cell and the aft cells would serve, in some degree, as beams in bending between end ribs, and that the secondary spars would also act so; He also reasoned that individual beads on leading edge skin would serve partially as truss members pinned at the leading edge.

In effect, Boyajian's proposal was a new application of the theory of stress analysis - a radical departure from conventional practice - which had to be substantiated by static test as the design proceeded, since the stress in the simplified structure could not be adequately calculated.

In the face of doubt and disagreement, but encouraged by Mr. Marchev, the simplified design took shape unit-by-unit, as hand-built specimens, for test purposes.

As an initial step in the development program, the conventional prototype stabilizer -a fair example of a costly all-metal structure consisting of spars, ribs, and stringers - was selected for experimental simplification. Aim was to effect radical manufacturing cost reduction without sacrificing strength-weight characteristics and serviceability.

Below is a comparison table between the conventional prototype construction and the simplified production construction:

Group

Construction

Parts

Manhours

Rivets

Weight

Stabilizer

Conventional

42

14.2

521

13 lbs

Simplified

9

2.7

160

13 lbs

Wing

Conventional

114

280

2627

150 lbs

Simplified

30

10

882

110 lbs

Hull

Conventional

362

590

6500

318 lbs

Simplified

63

20

2400

298 lbs

Technical Description

Landing Gear

The landing gear on the Republic Seabee - an Electrol product designed especially for use on medium and lightweight personal planes - is a straight leg, hydraulic, full cantilever design which can be use on fixed or retractable undercarriages.  The piston does not touch the side of the cylinder because of two bronze bearings located in the lower half of the cylinder.  Elimination of all sliding motion against the interior wall of the cylinder contributes to the efficiency of the unit as well as to economy in its manufacture.  Because of the lack of relative motion between the bearings and the cylinder wall, the cylinder needs only to broached and not honed in the manufacturing process.  In fact, one of the features of the Electrol design is that all work on components is confined to machine operations on either one side or the other of the element, but never on both.

Adapted as standard equipment on the Republic Seabee, this gear has attracted considerable attention as it becomes a main component of an aggressive simplification program.  Claimed for it are ruggedness, light weight, simplicity and interchangeability of parts.  Quick and accurate assembly is another asset.  An entire oleo assembly may be disassembled in one and one-half minutes.

Special attention was paid to reduction of the number of units in each assembly, ease of maintenance and a reduction in unit costs.  The volume of air entrapped in the oleo has been held to a minimum.  A small amount of air is desirable to avoid rebound while the aircraft is being taxied.

The reliable, hydraulically controlled landing gear is maintained in the up or down position by the geometry of the linkage. As noted on the landing gear diagram, the linkage is designed so as to “break” during the transition phase of the gear operation and to “remake” at the up or down position, so that the center pivots of the linkage are past dead center travel. In this manner, positive lock is maintained until hydraulic pressure is applied to the cylinder permitting a “break” in the linkage.

Note that the tail wheel is rotated (to starboard) to the up and down position and that the main gear is retracted and extended. The landing gear position lights (Red and Green) are wired so that when all three wheels are down and locked the green light is illuminated. However, when the Red light (UP) is illuminated it does not necessarily mean the tail wheel is locked up. It may be down or any position including up and locked. Use the wing float mirrors to confirm the tailwheel position.

Main landing gear utilizes Electrol air-oil struts designed to have a relatively low static air pressure to facilitate servicing. Each strut is fully cantilevered from the hull side and through a bolted elbow connects with a shaft extending through the hull to the opposite gear. Bull shaft is in two sections joined at the hull centerline by a welded sleeve. Channel members at each end of the sleeve provide support by extending to the frame at the hull bottom.

Strut torque arms (scissors) are utilized as step means to the cabin, and top torque arm is designed to receive a towing hook.

Retraction and lowering of the main landing gear (equipped with Goodrich wheels and brakes) is accomplished by hydraulic power from an Electrol hand pump located between cabin front seats. Reservoir, thermal expansion valve, and selector valve (flaps are also operated hydraulically) are integral with the hand pump. A bellcrank on the center sleeve connecting the two sections of the hull shaft attaches to the upper arm of a two-arm toggle linkage; lower arm of linkage is attached to a pivot fitting on the hull structure and above this pivot point is also connected the piston of the hydraulic cylinder. Latter is in turn pivoted on a horn attached to the center sleeve. Extension of the piston breaks the toggle linkage from a past-dead-center positive-lock position and rotates the hull shaft to retract the gear. At full-up position of the gear, the linkage again comes together just past dead center to form a positive lock through contact of positioning stops at the break point.

Tail wheel is full swiveling and is looked in the fore-and-aft position by a spring-loaded pin with cable connection to cockpit. The wheel is mechanically raised via a cable connection to the main gear hull shaft. Cable action pulls a lock-pin and then rotates a horizontal shaft on which is pivoted the largely by automatic riveting process. Crown skin is an .040 beaded pressing attached to the crown bow member with the same rivets which fasten the side skins.

 

Fuel System

The fuel system is as simple as it gets. Fuel for the Seabee is contained in one bladder type cell of 75 U.S. gallons capacity located in the hull under the aft baggage compartment. The fuel is piped to the carburetor through a strainer and pumped by two AC diaphragm type engine driven pumps. Either pump can supply sufficient fuel to the engine. Some of the later model Seabees have an electric pump, in addition to an engine driven pump, located close to the fuel tank to provide fuel under pressure to the carburetor.

The fuel quantity gage is electrically operated from a float in the fuel tank. A fuel pressure gage indicates pressure for either the left or right fuel pump (or the electric or engine driven pump) as selected by a fuel pump switch on the instrument panel.

Normally the fuel is shut off by pulling the mixture control to the idle cut-off position; in emergencies fuel may be stopped by pulling on the fuel-flow shut-off control located under the pilot’s seat.

Before the fuel gets to the pumps, it is filtered. This filter should be checked on each pre-flight check and is normally located under the right wing root but some have been located on the other side. Boost pumps and fuel system components may vary from Seabee to Seabee. Check your FAA Approved Flight Manual and STCs for your specific components.

 

Flight Controls

The control surfaces of the Seabee are actuated by the rudder pedals and the control wheel through a series of flexible cables housed under the cabin floor and lead through a series of pulleys to the control surfaces. The dual rudder pedals are synchronized with the pilot’s pedals by mating gears on the connecting torque tubes between the two sets of pedals and the dual control wheel is synchronized with the pilot’s wheel by engaging a split sprocket in the dual control column to a mating sprocket in the pilot’s control wheel column. The dual column is removable and may be stowed under the front seat in the bracket provided.

The water rudder cables are spliced to the air rudder cables so that operation of both surfaces is synchronized and made by the same pedals.

Some models of Seabee have steerable tail wheels that are also part of the rudder cable assembly. The rudder pedals also control the tail wheel steering system.

The elevator trim tabs are controlled through sprockets and chains at the control and the surfaces. The control is by crank located overhead of the pilot. Fixed tabs are provided on the aileron and rudder. An FAA Airworthiness Directive has been issued for the elevator trim system. Each pre-flight should include a thorough check of the trim tabs on the elevators. No more than ⅛” inch play should be evident. If it is more than that, get a mechanic to check it. Steel bushings will negate the AD but it should still be checked prior to flight.

 

Hydraulic System

The flaps, main landing gear and the tail wheel are extended and retracted hydraulically. A single, manually-operated hydraulic pressure system activates both the landing gear and the flaps.

A large lever, extending upward from beneath the floor between the two front seats activates the pump with which hydraulic pressure is built up. Two other arms extending from this unit control the action of the fluid. The right lever directs the section of the landing gear while the one located on the left side determines the position of the flaps. The hydraulic power pack incorporates a series of check valves which prevent the temporary dropping off of pressure when transferring the hydraulic action from wheels to flaps or reverse. Some Seabees have an additional electric hydraulic pump operated by a switch on the forward instrument panel or control wheel button. Momentarily activating the switch or button turns on the pump and pressurizes the system and operates the selected flap or landing gear system until it reaches the up or down position and turns off automatically when the pressure in the system reaches a predetermined value (≈800 psi).

The system has a three and one-half pint capacity and uses a petroleum oil base hydraulic fluid, Specification 3580D or equivalent (Mil-5606).

 

Brakes and Wheels

The main wheels of the Seabee are 7.00 x 8 and the tail wheel is a 10” smooth contour type.

Each main wheel is equipped with bladder-type brake which is fed from its own master brake cylinder at each of the rudder pedals. A brake adjuster and a parking valve are installed in each of the lines between the master brake cylinder and the wheel. The positions of these parking valves are controlled at the instrument panel by a single parking control lever.

A control is provided in the cockpit to engage or disengage a tail wheel lock thus permitting the tail wheel to be locked in the centered position or to be unlocked in order to swivel.

A steerable tail wheel assembly is available on some Seabees. These models use cables attached to the rudder cables that steer the tail wheel up to a certain limit. If forced beyond this limit, the tail wheel unlocks and is free to swivel until forward, straight movement locks it in the steerable position again.

Later modifications have been applied to Seabees that allow for an improved model disc brake system. These are typical disc brakes that are identical to the above system with the exception of the bladder and drum that are replaced by the caliper and disc. There is no adjuster in the system for they are self-adjusting.

 

Wing Structure

Conventional prototype RC-1 wing was a good, typical airfoil structure of 24ST -a tapered, full cantilever unit consisting of ribs, spars, and stringers. Here, again, there were so many interlocking components, all largely inaccessible to automatic machinery, that in the main it had to be assembled almost entirely by hand; hence, it was very costly. Manufacturing of the many detail components was comparatively simple, representing only about 5% of total wing fabrication time; the other 95% was almost all assembly time.

The simplified wing on the RC-3 is a rectangular planform constant-thickness structure, externally braced by a single strut. Reasons underlying the change from tapered to rectangular planform were as follows:

(1) Skin becomes a rectangular sheet, and in bending it to the form of the wing, material losses ordinarily occasioned with a tapered wing are avoided.

(2) A single forming tool can be used for all skin sections on both left & right wing panels, whereas on a tapered wing, each skin section requires a separate forming tool, and a different set of tools is required for the opposite wing.

(3) A single tool can be used for fabricating spars in left & right panels.

(4) Exact width strip-stock can be used for the spars, since the flat pattern inboard end of the spar to make attachment to the cabin structure. Attachment for brace strut is accomplished with another forging at the center rib.

Middle spar - essentially a false spar - is an .032 channel member fabricated in a manner similar to the front spar, but has no angle attachments.

Rear spar is a simple .051 channel member having a forging at the inboard end for attachment to the cabin structure.

Between front and middle spars, at about one-quarter the distance from the middle to the tip, is the wing float supporting structure consisting of two pressings forming a socket for the float strut.

All spars are of R-301W material. Lightening holes have simple 45° flanges formed without subsequent heat treating. Because of severe forming, ribs are fabricated of 24SO, and are subsequently heat treated. Rib lightening holes have deep drawn flanges.

Wing skin, with beading similar to that on the stabilizer, is R-301W -.032 on inboard half and .025 on outboard half. Skin sections are pressed on a camel-back draw die, similar to the method used for the stabilizer skin.

In assembly of the wing, skin sections are first spliced on an automatic riveting machine to form a large envelope. Spars are installed progressively, beginning with the front spar, and riveting is done on an automatic riveter afforded access to interior of envelope from rear opening. Rivets are driven through both upper and lower skins and through spar flanges at same time, in about 8 min. Wing tip is quickly installed on outboard rib with sheet metal screws and self-locking sheet metal nuts.

External wing brace strut is a single piece of .091 R-301W turned on itself to provide a streamlined section joined at the trailing edge, on an automatic riveter, along outwardly turned flanges. Extruded fittings on each end of the brace strut carry one bolt for attachment to wing and hull, respectively.

Wing floats are R-301W .051 skins formed in a novel manner. Each float is of fully monocoque construction consisting of left & right pressings (clamshells) joined at the plane of symmetry along outwardly turned flanges adaptable for external automatic riveting. The strut connecting the float to the wing slips into a neck section provided in the pressings. Complete float assembly consists of but five parts - two skin sections, two bearing plates for strut bolts, and a drain plug - and can be fabricated in 15 min. This is in sharp contrast to conventional wing floats with numerous bulkheads which are fastened to the skin by reaching through access holes and driving each rivet by hand.

In this simplified wing design, notable lightness is achieved. Complete with flaps, ailerons, brace strata, and miscellaneous fittings, it weighs but 1.45 lbs per sq ft - unusual, considering that the average wing loading is 16 lbs per sq ft.

In static test, the wing sustained a load of 115%, and in torsional rigidity was four times greater than CAA requirements. Another unusual characteristic of the wing structure was that no skin ripples or buckles appeared up to 100% of design load - a condition rarely achieved in a conventional metal wing structure.

These very satisfactory results obtained with the large wing structure justified the application of this theory of the unit is rectangular. This avoids material losses and additional operations required for the tapered spar.

(5) Rectangular planform wing permits flaps and ailerons, and their hinges and brackets, to be interchangeable left & right, thus eliminating the need for separate tools and material losses attendant with the tapered design.

All of these considerations are extremely important in a simplified structure. In contrast, small differences between two assemblies of a conventional design (such as non-interchangeability of left & right skin sections) were not of much consequence, since this condition required only a new set of parts and tools-representing but a negligible portion of total manufacturing costs which were, largely, consumed in assembly handwork. However, in the simplified design - in which handwork has been greatly eliminated - small differences which require additional tools and prevent the use of components interchangeably, add considerably to the cost of the structure.

Simplified wing framework consists of 3 ribs and 3 spars. Ribs are approximately 8-1/2 ft on centers and spars are approximately 15 in on centers. Inboard rib is a 2-piece member-nose rib and after-portion. Center rib is made up of 3 pieces intercostal between spars. Outboard rib is a single member providing for the attachment of wing tip by incorporating a wide flange.

Front spar, supplying about 90% of wing bending strength, is an .064 channel constant throughout the span and having straight flanges turned on a bending brake. Extruded angles of 14ST are nested in top and bottom flanges and extend from inboard end approximately three-quarters of the span towards the tip. A simple forging is used on the

 

Movable Surfaces

Except for size and shape, the full-slotted flaps, ailerons, elevators, and rudder are fundamentally identical in construction. Each consists of a single beaded skin folded upon itself and joined at the trailing edge, and each has a single stamped channel spar member near the leading edge.

Flaps and ailerons have round-nose end ribs with outwardly turned flanges to afford easy access for automatic riveting.

Elevator has only one rib - inboard end - bolted to the operating torque tube extending between left and right units. No outboard end rib is employed because the tip is formed from the skin as a continuation of the trailing edge. The latter is cut out for a flat stock trim tab at the inboard end, the tab being attached by piano hinge.

Upper and lower tips on rudder are fabricated similar to elevator tip, hence obviating need for end ribs.

On the flap there are three 1/8-in flat stock hinge fittings. Two are mounted on the end ribs, and the third fitting, which includes an operating horn, is mounted on a center nose rib.

On the aileron there are two 1/8-in flat stock hinge fittings and a separate horn, which are attached to intermediate nose ribs.

On the rudder and elevator, the spar supports T-shaped extruded hinge fittings.

Since the cross-section of the aileron is identical to that of the flap, it is possible to use the flap-forming tools to fabricate the aileron skin, spar, and ribs.

Flap, aileron, elevator, and horizontal stabilizer units are interchangeable - with respective opposite - hand installations. Approximate dimensions are: Flap, 9-1/2 ft long by 16 in by 4 in deep at spar; aileron, 7 ft long by 16 in by 4 in at the spar; elevator (tapered in planform), 6 ft long by 20 in at maximum chord, tapering to 10 in at tip, by 3 in average depth at spar; and rudder (double tapered in planform) 8-1/2 ft long by 18 in at maximum chord by 4 in average thickness at spar.

Replacement prices for the various complete assemblies, including attachment mechanisms, are expected to be under these figures: Flap $35; aileron $50; elevator $25; and rudder $25.

Control system is conventional cable installation. Cockpit control is standard wheel arrangement with provision for utilizing removable duplicate wheel for co-pilot, who has rudder pedals but no brake pedals.

 

Hull Structure

The hull, designed so as to permit a major portion of riveting to be done on automatic machinery, consists of three separate assemblies; forebody, afterbody, and stern. Assembled, it has six watertight compartments - 3 in the forebody, 2 in the afterbody, the stern being the last compartment.

Forebody is comprised of four subassembly units; deck (cabin floor), two sides and bottom. Deck is made in three sections - forward, middle and aft. Forward section is a single .051 24SO pressing, subsequently heat treated, and reinforced with 3 hat-sections (12 transverse, 1 longitudinal) supporting cockpit flight controls. Edge of this section has an upwardly turned flange for assembly to the sides by riveting.

Deck middle section is .025 R-301W stiffened by four longitudinal hat-sections supporting the front seats and landing gear brace channels. Margins of middle section are also upwardly flanged for attachment to sides and to front deck section. Attachment to rear deck is by a lap joint. Between the middle section and the sides is a: 1/8 in R-301W Z-section longeron which becomes a simple angle where it overlaps (for about 10 in) the front and aft sections of the deck.

Deck aft section is a single .051 24SO beaded pressing, subsequently heat treated, and having upwardly turned flanges on the sides. At the rear, the aft section of the deck overlaps the forward skin of the afterbody and also the step bulkhead. A cutout is provided in the aft deck section for access to the fuel cell (located between middle and aft bulkheads).

First two watertight bulkheads - .040 24SO beaded pressings, beat treated are sandwiched between the flange junetions of the deck skins.

Sides of forebody are .064 R-301W made up in three sections flanged outwardly at the chine, where it joins the bottom subassembly flange.

At the bow, forward of the front bulkhead, the bottom consists of left & right .072 61SW skins (made in a draw die). From the forward bulkhead to the step, the .051 R-301W skin (made on a bending brake) is reinforced by 7-hat section transverse stiffeners having greatest depth of about 4 in at the keel. On the underside of the chine is an external reinforcing angle extending from the forward bulkhead to the step.

The keel - a T-shaped 14ST extrusion provided with drain plugs for each watertight compartment - runs aft of the first step for splicing to the keel of the afterbody.

Various simple structures within the forebody serve to support the battery, hydraulic pump, and landing gear mechanism, and also provide anchorage for safety belts.

Afterbody - hull section between steps - is comprised of an upper section and the bottom as subassembly units. Upper section is fabricated from three pieces of .051 R-301W, each developable from a flat pattern, joined by simple lap joints. Rivets through the lap joint between first and second skins pick up the flange of the forward watertight beaded bulkhead.

Afterbody bottom is a single sheet of .051 R-301W (formed on a bending brake) reinforced by four hat-sections, as in the forebody.

An indication of the sturdiness of the bull bottom structure is that after more than 600 water landings, no dishing or skin ripples were observed.

The bulkhead at the second step supports the tailwheel and is formed as an .040 24SO beaded pressing, subsequently heat treated. At the lower portion of the bulkhead is a bearing plate reinforcement for attachment of the tailwheel. Flanges of the bulkhead face forward and are sufficiently wide to provide a foundation for a butt joint between afterbody rear skin and stern skin. The splice is further reinforced at the bottom by an .051 R-301W structural fairing.

The stern consists of two subassemblies - left & right halves - comprising a clamshell skin structure of .040 R-301W with a cutout at the rear top portion beneath the fin. Each half of the clamshell has a single longitudinal and single diagonal Z-section stiffener, two vertical angle stiffeners for the stabilizer support, and a channel section for stiffening the edge of the cutout and to absorb drag loads from the stabilizer. All of these members are attached to the skin by automatic riveting. The clamshells are assembled with a riveted lap joint at the forward top and on all of the bottom, and at the rear they are attached to the rib-shaped closure bulkhead.

A transverse angle across the top of the cutout serves as a fitting for attachment of front spars of stabilizers and fin. Approximately 15 in. back of this angle member is a deep channel for attachment of rear spars of the stabilizers. Fin rear spar attaches to the closure bulkhead at rear of stern.

A series of tubes, one leading from each watertight compartment in the hull, are grouped in the cockpit, under the rear seat -, for attachment to a bilge pump, and numerous handholes are provided for inspection and servicing.

 

Instrument Panel

Instrument panel is located on left side of cockpit in front of pilot. A package unit in the lower right corner contains Electric Auto-Lite automotive type engine instruments-oil temperature gage, oil pressure gage, fuel quantity gage, fuel pressure gage, tachometer, and ammeter. By removing four nuts which hold four clamps in the rear of the package, the latter may be removed into the cockpit.

Two-way Hallicrafters radio is adjacent to left of engine panel package; and by removal of four screws on underside of support shelf forward of panel, and disconnection of power supply, antenna, and phone plugs, the radio may also be drawn into the cockpit. Microphone is spring-clipped on the instrument panel and the cord passes through the panel, drawn in from behind by spring tension. Optional radio, with broadcast band and loop antenna provisions, fits the standard installation brackets without any alteration.

Flight panel package contains an airspeed indicator, magnetic compass, altimeter, and ball-bank indicator. The package is drawn into the cockpit by removal of false front by prying, then removing eight screws on the face of the panel. Optional flight panel is equipped with sensitive altimeter, bank and turn indicator, clock with sweep-second hand, and the standard-equipment airspeed indicator and magnetic compass.

The instrument panel also carries the following control switches: Cole-Hersee master switch, and Douglas or Cole - Her lower arm of a two-arm yoke on the vertical tail wheel strut. The shaft rotates approximately 132 deg. to place the wheel alongside the boom.

In addition to its function to retract the tail wheel, the horizontal shaft is ingeniously designed to serve as a shock absorber for tail wheel ground loads. It is hollow and surrounds a piston attached to the upper arm of the two-arm tail wheel yoke. In the space between the piston circumference and the interior circumference of the horizontal shaft is a layer of rubber secured to the piston and shaft interior surface. Upon application of ground load to the tail wheel, the piston is displaced inwardly and the surrounding rubber acts in shear to absorb the forces imposed.

Alteration of the tail wheel design is contemplated, to render the unit steerable as well as swivable.

 

Engine Installation

Power plant is a 6-cyl., aircooled, wet sump Franklin engine, mounted as a pusher, located above and aft of. cabin, directly over the firewall decking the baggage compartment. Mounting is essentially a three-point support. Propeller end of power plant is carried by two converging steel tubes with slotted ends welded to heavy attachment plates for bolting to an engine pedestal carrying a rubber shock mount. Attachment plates at bases of supporting tubes are bolted to .049 pressed steel hat-sections, in turn bolted to the firewall. Each of these hat-sections runs forward and up in a longitudinal plane parallel to the engine thrust line and is picked up by a bolt in, another shock mount pedestal on the crankcase. From this pedestal another hat-section on each side runs forward and down and is bolted to the firewall. Components of the mount are, interchangeable for use on either side of engine.

Ground adjustable Aeromaster propeller (standard equipment) has laminated maple blades chemically sealed in the ferrule. Blade covering is black Aeroloid plastic sheeting, and Monel metal sheathing over the plastic protects the leading edge. Optional Hartzell reversible pitch propeller is pitch-changed by engine oil pressure with manual operation of valve control in cockpit.

Power plant accessories include Electric Auto-Lite starter, generator, regulator, and distributor.

 

Cowling

To eliminate fillets and compound curvature of the rear lower cowling, it has been constructed in two sections, as simple wrapped sheets. Each section is straight at the sides and meets the wing at 90 degrees. Rear of each section wraps around to meet the other section at the centerline of the craft, and attachment to cabin sides is by quick-fasteners. Forward lower cowling on each side is a straight section, also attached by quick-fasteners.

Top cowling, from propeller end forward to engine fan housing, is a single wrap-around sheet pivoted at forward end similar to an automobile hood and is held in the open position by a brace rod on each side. In closed position, top cowling is fastened with quick-latches to lower cowling. Forward of the top cowling is another top section fixed in place.

With top cowling up, and with rear side cowling removed, entire engine accessory section is accessible for servicing. Then, by detaching the top cowling by removal of a few bolts and then unfastening forward side cowling, the entire engine installation is accessible.

Fuel cell is a Goodrich bladder type bag, made of rubber-impregnated fabric, located inside the hull just forward of the main step and between two watertight bulkheads. The unit rests on a plastic sheet over the- hull bottom stiffeners, and is fastened to the deck structure by snap fasteners which have sufficient play to facilitate adjustment in securing the bag to the male fastener components on the structure.

Top of the bag is provided with an opening approximately 5-1/2 by 12 in over which a metal cover plate carrying the filler neck and fuel level gage is installed by bolting to the bag and also to the deck skin, which has a corresponding cutout for removal of the fuel cell. A drain at the bottom of the cell connects to a pipe which runs behind the main step where it is fitted with a drain plug.

 

Cabin Details

Cabin lines have been established by analytic geometry. Mathematical fairing not only greatly reduced tedious lofting time, but made possible the rapid and precise manufacture and inspection of jigs, tools, and dies.

Cabin consists essentially of two main sections: (1) Rear primary structure, tying the wing and engine installation to the hull; and (2) forward section superstructure, enclosing the cabin proper.

Rear primary structure forward bulkhead - consisting of two beaded pressed sections, .040 upper and .015 lower, with a center cutout for access to the baggage compartment - supports the wing front spars and is reinforced by  .064 hat-section uprights at each side. Lower end of each upright is riveted to an external fitting which connects the wing brace strut to the hull. Upper ends of the uprights connect to an inverted hat-section extrusion which serves as a crosstie between front spars of the wing panels Lind also supports the front end of the engine mount. Similarly, over the main step bulkhead there are two other upright hat sections which attach to another inverted transverse hat section connecting the rear spars of the wing panels and also supporting the rear end of the engine mount. Top of rear primary section is an aluminum-coated .019 low carbon steel firewall.

Side skin of the rear primary structure - .040 R-301W stiffened by secondary vertical channels - is comprised of two sections connected along the mating flanges at the trailing end of the cabin structure.

Forward superstructure - cabin enclosure - includes an .091 61SW crown bow member, five door frame uprights, and .040 61SW skins.

Crown bow member is a rolled combination angle-and-Z-section unit. Door frame uprights are identical to crown bow frame but for attachment to latter have gusset portion at the top. Nesting of crown bow and door frame provides effective surfaces for door sealing. The two side doors and the bow doors are .032 618W large single-piece pressings spotwelded to an .025 outer skin.

Cabin skin picks up the rivets; joining the deck to the hull sides, and is also joined to the crown bow and door frames see domelight, instrument light, anchor light, and running light switches. The Pollak or Bendix ignition switch is designed to control the starter by pressing the key in "Both" position. Key for ignition switch also operates cabin door locks.

Other panel controls are pulls for parking brake, carburetor heat, carburetor mixture, and throttle. Signal lights for landing gear position are also installed.

Right half of cockpit panel is omitted to provide free passage to the bow door.

Production installation time for all electric wiring on the craft is approximately 11 min. Wires are furnished in prefabricated terminated lengths. Spring terminal sockets on switches are used to afford push-pull connections, and knife disconnects are used where wing and tail wires join the cockpit connections.

Interior trim and upholstery is of Koroseal - waterproof, flameproof, vermin- and mildew-proof - economically utilized in various weights according to degree of service anticipated. Sponge rubber windcord, having a prefabricated edge to simplify attachment, is used for weatherstripping, color-matches the interior, and eliminates necessity of stitching on a separate fabric covering for trimming.

Cabin soundproofing is accomplished by using Fiberglas or similar material.

Retention of each the seven large double-curvature Lucite (Heath) cabin windows is accomplished with a uniquely simple Goodrich rubber S-extrusion, one loop being cemented to the pane and the other loop cemented to the edge of the cabin cutout margin. In addition to serving as a glass retainer, the extrusion functions as a weather seal, a vibration damper, and a decorative trim.

As far as possible, standard automotive type hardware - Cowles door handles, locks (with slight redesign), pullto handles, and dome light - are used, with careful selection in regard to weight, strength, and cost, which factors also dictated the use of Tinnerman sheet metal screws and nuts.

The back of each Reynolds seat is quickly detachable to serve as a life preserver. Front seat tracks and adjustment mechanism are American Forging & Socket standard automotive types.

 

Stabilizer Design

As evolved, the simplified stabilizer structure - approximately 6 ft long, with  average chord of 1 ½ ft consists of front and rear spars and inboard end rib. All material is R-301W, requiring no heat treatment.

Front spar - only internal member of the stabilizer structure - is an .025 channel section with lightening holes having simple flanges for stiffness, and rear spar is an .091 channel section. Both spars have straightline taper and can be made on a mechanical press or on a bending brake. A flat bearing plate is attached at inboard end of each spar for connection to the hull structure.

Inboard end rib -.025 gage - has simple flanged lightening holes. A slot in the rib allows the front spar to pass through without interrupting the rib member, and attachment of latter to spar is made via the metal portion displaced from the slot. It is to be noted that the skin forms the outboard connection for the spars of the stabilizer unit.

On the conventional stabilizer, skin was .020 24ST Alclad, whereas the simplified stabilizer - having the same outline - has skin of .025 R-301W. External stiffening beads, serving to eliminate the internal framework of the conventional structure, are 1/4 in deep by 4 in on centers, and are not considered objectionable as speed-impeders. And it is also felt that the external beading lends a decorative touch to the plane surfaces.  Actual test on the prototype plane, with and without beading (beads were simulated by wooden strips attached to wings and tail surfaces), showed a reduction of but 3 mph at high speed -a loss offset by a reduction of 2 mph in stalling speed.

An example of the type of tooling utilized for fabricating the stabilizer skin - tooling which is typical for fabricating the beaded skins of the wing, other fixed surfaces, and movable surfaces - is the forming die. This is a camel-back draw die which forms the beads with necessary contour and depth. Draw flash is die-trimmed to give the skin its size and outline. Final operation in a bending die forms the camelback into the leading edge of the structure by folding the skin back from the bulge.

In assembly, front spar is attached to inboard end rib as a first operation. This unit is then placed within the skin envelope whose sections have been preassembled on an automatic riveting machine with a single row of rivets. Accessibility from the open end at the rear of the envelope permits automatic riveting of the latter to the front spar and end rib. Next, the rear spar is installed and automatically riveted to the skin to form the rear closure of the structure. And the tip is formed with an external Range, also automatically riveted.

Thus, aside from attachment of stabilizer hinges (bolted) and other minor hand operations, the entire assembly is automatically joined-a fairly typical procedure for all the airfoil structures on the Seabee. This simple method of assembly is in marked contrast to the complicated, manual, slow and costly procedure involved in the fabrication of the conventional structure.

Static test of the simplified stabilizer showed about 10% higher strength over the conventional prototype unit, and it also disclosed very satisfactory rigidity. And most important - since it made possible such fast and cost-saving production methods (replacement price of stabilizer panel complete assembly, including attachment parts, is expected to be under $35) - was the justification of a 'new approach in stress analysis. 

Now that the application of this simplified design bad proved satisfactory for the stabilizer, it was decided to test the construction principles on a much larger unit - the wing panel.

 

SPECIFICATIONS 

Republic RC-3 Seabee
3-view
Drawn by: © Chris Haag

GENERAL

DIMENSIONS

Length (Max)

LANDING GEAR

WINGS

EMPENNAGE

WEIGHTS

SURFACE MOVEMENTS

ENGINE

PROPELLER

PERFORMANCE

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Updated: 2009-06-12

Copyright  © 2006-2009 Steinar Saevdal