Design Analysis of the
Curtiss-Wright Helldiver

With a range of over 1000 miles, the SB2C-4 carries one of the greatest loads of destructive power in a single-engine plane

By John A Henderson
Staff Engineer, Columbus Plant
Curtiss-Wriqht Corporation

The only carrier-based dive bomber in the world, the Curtiss-Wright Helldiver also does anti-submarine patrol, skip bombing and strafing. It is effectively used against pill boxes and all fortified strong points in the Pacific, and by sinking and damaging enemy aircraft carriers and other Japanese ships, has caused much havoc among enemy fleets attempting to supply and reenforce their island strongholds.

In addition to the 20-mm guns in the leading edge of the center panel, the SB2C-4, latest version of the Helldiver, can carry bombs on racks under each wing, and more than 1000 lb of bombs in an internal fuselage bomb bay. Four 5" rocket tubes are mounted under each wing. Two flexible rear-firing machine guns are operated by the rear gunner.

Crew consists of a pilot and one gunner. The gunner sits in a manually operated ring, mounting two .30-cal machine guns.

Wing

Wing is of full cantilever, all metal, torsion resisting box construction and is composed of a center section and two folding outer wing panels. The skin, a smooth covering of aluminum alloy sheets, incorporating structural doors, is attached to the structure with flush rivets.

Center wing panel is of the three-spar Wagner-type beam construction composed of front, rear and flap beams. Since the Helldiver carries all its fuselage bombs internally, the front beam is carried over the bomb bay by a Warren truss and the rear beam by an arched bridge truss. The Warren truss is a large fitting installed just aft of the pilot's seat in the manner of a bulkhead. This structure transmits the greater portion of the flight loads from the wing to the fuselage as well as acting as a carry-through structure for the main beam.

Nose section of the center panel (that portion of the structure forward of the main beam) is cut out to provide a recess into which the landing gear retracts. Nose skin of .040 to .051 gage 24ST aluminum alloy is provided to avoid buckling of the leading edge. A fairing retracts with the landing gear to complete the aerodynamic contour of the wing.

Center panel beams incorporate extruded 14ST aluminum alloy "T" shaped capstrips, the caps varying from .199 to .902 gage. Webs are of .040 to .102 gage 24ST aluminum alloy stiffened by vertical bulb angles, with doubler plates at the points of concentrated load. Main wing ribs are spaced approximately 22" apart with intermediate chordwise stiffeners spanwise to the outer panel attachment points.

At the point of outboard wing panel attachment, which is also the wingfold line, there are hinge fittings allowing the outer panels to fold upward over the front canopy. A hydraulic strut mounted spanwise in the outer panel immediately aft of the main beam, folds the outer wing. Hinges are forged from 14ST aluminum alloy and attach to the outer and center panel front and rear beam caps and webs by means of bolts and rivets, respectively. Upper and lower lugs on the forgings have steel bushings pressed into the hinge holes.

Both front and rear hinge locking pins operate from the same hydraulic strut. The rear pin is attached directly to the strut rod, while the front pin is operated by a gear and rack arrangement between the strut rod and pin to reverse its direction.

Each outer wing panel is composed of the main panel, slat, aileron, flap and wing tip, the latter being easily removed for repair or replacement.

There are no stringers in the entire wing, the load being dissipated in the skin. The outer panel, similar to the center panel, has a front and rear Wagner type beam composed of upper and lower caps joined by a single web. Both upper and lower spar caps taper as the tip is approached, while the gage of the shear web drops from .040 to .032. Similar to the center panel, main wing ribs are spaced 212" apart with intermediate chordwise stiffeners. Rib and stiffener gauges run from .025 to .040. The flush riveted skin is from .025 to .051 gage.

To insure smooth airflow and effective aileron control at low speed, high angles of attack, when landing aboard carriers, the outer panel carries a slat on the outboard portion of the leading edge, approximately the same length spanwise as the aileron. These slats are mechanically controlled by a series of cables and drums which allow the slat to extend on tracks when the landing gear is lowered.

The aileron, attached to the outer wing panel at four points, is constructed so that all flight loads are transmitted to a rigid D-type nose section consisting of a single flanged 24ST aluminum alloy spar of .032 gage, supplemented by numerous .020 gage ribs and covered with .025 24ST aluminum alloy skin. For access, there are several structural doors along the lower nose section.

Small chord balance tabs located in the center of the trailing edge lighten the aileron loads. The tab is operated by means of a push pull tube attached to a bellcrank at the hinge. The gap between the aileron and the panel is closed with a strip of cloth seal. Aileron differential is 5°, the total throw being 15° down and 10° up. Provisions for trimming out unsymmetrical flight loads are provided by a trim tab on the inboard end of the left aileron. The tab is controllable in flight by the pilot.

Attached to the flap beams of the center and outer panels with piano type hinges are perforated trailing edge double split flaps extending from the sides of the fuselage to the inboard end of the ailerons with a break at the wingfold hinge line. Flaps on the center panel are operated by four hydraulic struts and the outer panel flaps by two struts, making six in all on each panel. With the wing in flight position, a spring loaded lock interconnects the center and outer panel flaps so that they operate as one. A flow divider evenly separates the flow to the six struts on either panel so that the right and left panel flaps operate simultaneously.

Diving and landing flaps are manually selected from the pilot's cockpit by means of a rod operated lock which retains the dive flap in closed position when landing flaps are desired.

Skeleton of the upper and lower flaps are constructed with single full span flanged 24ST aluminum alloy beams of .064 gage with tapered ribs, varying in gage from .032 to .051. Perforated skin is used to allow an increase in the angle of opening of the dive brakes and flaps. This produces a more effective brake in dives as well as a slower landing speed. There is an outer and inner perforated skin of .040 gage 24ST aluminum alloy on the landing flap and the dive brake. A 14ST aluminum alloy extrusion stiffens the landing flap trailing edge while a .250 gage 24ST aluminum alloy comb stiffens the trailing edge of the dive brake. This comb assists in breaking up the airflow when in a dive, inducing still greater drag. Wing tip skeleton consists of two ribs and six spanwise formers which vary from .020 to .040 in gage and are covered with .025 gage 24ST aluminum alloy flush riveted skin. It is attached to the outer panel with a row of screws around the periphery of the tip rib. A navigation light is mounted in the leading edge of the tip.

Fuselage

A deep fuselage is provided to give sufficient area to the bomb bay to allow all fuselage bombs to be carried internally. It consists of the main structure, complemented by a sliding canopy for both the pilot and the gunner, with a fixed cabin between, as well as a collapsible "turtleback" aft of the gunner's canopy.

The structure is semi-monocoque, being built up around the Warren truss. Four heavy 14ST extruded aluminum alloy longerons originating at the engine mount attaching points carry the major loads, supplemented by many rolled formed bulb angle stringers spaced regularly around the fuselage paralleling the longerons. The Warren truss, an integral part of the wing beam structure, attaches to the upper set of longerons by means of bolted fittings and is flush riveted to the skin with sheet metal channels. Although originally designed as a single forging, it now consists of numerous smaller forgings and components which are bolted together. This change became necessary when it appeared that available forging facilities could not produce a single forging that large, in anything near the quantities needed to supply the production demand.

Lower longerons extend aft to the opening which accommodates the center wing panel, and splice to a wing rib capstrip at the leading edge of the outer panel. The loads carry through the ribcaps and ribs back to the trailing edge where they are picked up by two aft longerons.

Pilot's and gunner's canopies consist of aluminum alloy frames with Plexiglas panels and are mounted on trucks whose rollers engage tracks on the fuselage. A cable and pulley arrangement makes it possible to roll the cabins aft or forward to close or open them by a manually operated crank in the cockpit.

The center or fixed cabin covers the area between the pilot and the gunner which houses the life raft, fuselage fuel tank and all radio equipment. For ease of servicing, there are access doors in the fixed cabin to reach the radio equipment.

The turtleback structure is aft of the gunner's cockpit, and, in the fully extended position carries the lines of the gunner's canopy back to fair with the vertical stabilizer. It incorporates piano type hinges fore and aft along the sides of the structure. This allows it to collapse when the hydraulic pressure is dumped, thereby increasing the gunner's field of fire. The turtleback is actuated by a separate hydraulic system, including a manual pump operated by the gunner, a hydraulic strut, a reservoir and a dump valve.

Empennage

The entire supporting structure of the empennage is of metal construction.

The vertical stabilizer is of aluminum alloy construction with 24ST .020 to .025 gage aluminum alloy flush-riveted skin. Front, intermediate and rear spars are augmented by bulb angle stringers interconnected by rib sections of .020 to .032 gage, reenforced by gussets at the hinge attachment points. The three spars are tapered channel sections which vary in gage from .025 to .032, with the rear spar stiffened across the web by riveted bulb angles. Rear spar incorporates two 14ST forged aluminum alloy fittings at the hinge attachment points. The fin attaches to the horizontal stabilizer by means of six steel studs, three on either side, through 14ST forged aluminum alloy fittings.

Horizontal stabilizers are of substantially the same construction with flush riveted 24ST aluminum alloy skin. Each stabilizer is bolted to the fuselage at three points with 14ST aluminum alloy forged links attached by steel bolts.

Construction of the rudder and the elevators is similar to that of the aileron in that they have a rigid nose section of sheet metal with light trailing ribs covered by doped fabric. Lead weights are used in the nose section to statically and dynamically balance both the rudder and the elevators. Attachment to their respective stabilizers is made by two permanently lubricated dust sealed bearings attached with steel bolts to the hinge brackets. The rudder has a total travel of ±25° while the elevators are restricted to 35° up and 25° down.

A trim tab is provided on the rudder, as well as on the left elevator. The tabs, hinged at the ends, and controlled by cables from the pilot's cockpit, provide a manual adjustment for trimming the plane in flight.

Alighting Gear

The main landing gear is of the conventional hydraulic type, folding inboard when retracted. In landing position it gives the airplane a tread of 16 ft. Each gear has a single shock absorber strut, braced to take longitudinal and lateral loads by a large built up fitting or drag truss. The strut is of the atmospheric type, ie, when the strut is in fully extended position it is at atmospheric pressure. Attachment to the center panel is made through large forgings bolted to the main beam. Each landing gear is raised and lowered by a hydraulic strut controlled by a valve in the pilot's cockpit. Since in the fully extended position the gear is too long to fit into the center panel wheel wells, a cable is attached between the wheel yoke and a point outboard of the landing gear hinge line. As the gear folds upward, the cable travels through a fore-shortening arc, forcing the piston to retract into the oleo cylinder. This shortens the landing gear stroke, so that it fits into the wells provided. The magnesium wheels mount 32 × 8 high-pressure pneumatic rubber tires and are fitted with internally expanding heavy duty brakes hydraulically operated.

Mechanical locks secure the gear in the up and down positions while a set of visual flags on the panel, and a warning light in the cockpit, indicate landing gear position.

The uplock consists of a spring loaded latch which is activated automatically by a roller bearing on the shock strut as the gear is retracted. The down lock consists of a pin actuated by the last inch of travel of the retracting strut. Visual flags on the wing indicate the locked position of the gear. The locking pins are spring loaded so the gear will automatically lock down in a free fall, when and if the plumbing is shot away or becomes inoperative.

Tail wheel assembly consists of a forged 14ST aluminum alloy drag link, one end of which is hinged at the bulkhead at station 248, while the other end mounts a full swiveling forged steel tail wheel fork supporting an 8½ × 4 solid rubber wheel. A single oleo strut attaches to a fitting on the drag link just aft of the tail wheel attachment. Although a full swiveling tail wheel is used, there is a self-centering cam device which returns the plane of the wheel to a fore-and-aft position for carrier deck landings. A cable-operated tail wheel lock may be operated from the pilot's cockpit.

The entire tail wheel assembly is non-retracting, being faired to the fuselage with a formed sheet metal housing.

The arresting gear consists of a forged steel hook which hinges about a pin at station 288. The hook fastens up parallel to the thrust line when not in use and is released by a latch, operated from the pilot's cockpit by means of cables, before landing aboard carriers. The hook is snubbed by an oleo to prevent its bouncing on the deck.

Power Plant

A Wright Cyclone R-2600 engine powers the Helldiver. It is a 14-cylinder twin-row with a single-stage, two-speed supercharger. With the total of more than 1600 hp and normal gross weight, the power loading is 8 and approaches 9 at the overload gross weight. The carburetor is an injection type with an automatic mixture control for various altitudes.

Propeller gear ratio is 16:9. A Curtiss Electric four-blade hollow-steel constant speed propeller is used, 12' 2" in diameter.

Engine Cowling

The engine cowling is NACA modified C type, consisting of upper and lower nose sections, cowl flaps and intermediate panels aft to the fuselage at station O. Skeleton of the upper and lower nose section is of formed aluminum alloy channels joined by extruded bulb angles. Formed aluminum alloy skin panels are flush-riveted to the skeleton. A front and rear support ring attaches to the rocker box studs of the front and rear rows of cylinders by means of small forgings. Upper and lower nose cowling is supported with Lord shock mounts on the above rings. Upper and lower cowl fastens together with steel straps and turnbuckles.

Cowl flaps are mounted on the rear support ring of the nose section cowling. They are divided into two upper sets of four flaps each and two lower sets of three flaps each. The cowl flaps are mechanically actuated from the cockpit by means of a torque tube and universal joint linkages.

The intermediate skin panels are attached to channels by means of Dzus fasteners.

The carburetor air scoop entrance is in the center of the upper section of the nose cowl, covered by a small die-cast grille. A duct carries the air back to the carburetor interrupted by an alternate air door operated for full cold or hot air by a mechanical linkage from the cockpit. The entire scoop, as well as the ducting, is contained within the periphery of the cowling.

An oil cooling air scoop has its entrance in the center of the lower nose section outside the periphery of the cowl. A duct directs the air through the oil cooler, exhausting through a flap on the bottom of the accessory cowling just forward of the firewall.

The built-in fuel provisions of the Helldiver are composed of a self-sealing fuselage tank located just aft of the pilot and two self-sealing wing tanks located in bays on the right and left sides of the center panel. These tanks are easily removed through large stressed doors on the bottom side of the panel. The droppable fuel tank provisions include a tank on the bomb rack of each wing, as well as a tank hung in the bomb bay.

Each of the self-sealing tanks are provided with dual vents with a combination fuel strainer also serving as an outlet to the carburetor. Wing tanks are constructed with a baffled interior. At the tank outlet, a pocket is formed with flapper valves in the baffle so that at low fuel levels the gasoline is trapped in the pocket and held against the fuel outlet.

A fuel selector valve makes it possible to select any tank desired through operation of a handle in the pilot's cockpit.

The engine is provided with a conventional oil system. A 25-gal oil tank mounted on the forward side of the firewall supplies the engine oil requirements. A calibrated stick attached to oil tank cover is used to check oil quantity. Oil temperature is controlled by a bimetallic valve which automatically shunts the oil through or by the 13" diameter by 9" oil cooling radiator as is needed.

One of the outstanding features of the Helldiver is its extensive hydraulic system. Nearly all operating mechanisms are actuated or controlled hydraulically. This system operates the mechanism folding the wing, the wing hinge pins, landing gear, brakes, bomb doors, flaps and dive brakes, the turtleback, and the autopilot.

This Design Analysis article was originally published in the July, 1945, issue of Industrial Aviation magazine, vol 3, no 1, pp 7-8, 10, 12-13, 15-16, 18-23.
The original article includes a thumbnail portrait of the author and 6 photos, 11 detail drawings, and 4 data tables, plus a ledger-sized foldout with a color phantom rendering and detail photos of the landing gear, wingfold and flaps. Photos are not credited.