Design Analysis of the
Curtiss Commando

By JOHN FOSTER, JR,
Associate Editor, Aviation

First detailed engineering reference study of the C-46, our noted combination transport-task force warplane now serving 'round the world, tracing development from prototype to current production model.

Designed and built originally as a passenger transport, the Curtiss C-46 Commando has been developed into a task force craft — the largest of its type in the world — designed primarily to move the most material at the lowest possible ton-mile or man-hour cost. The vast operational know-how being swiftly acquired under wartime pressure is providing an unusual back-log of design, operation, and maintenance experience for postwar utilization.

Basic design of the Commando was crystallized in 1936, following three years intensive study aimed at producing a design which would result in an airplane that would make the operator the most money. Curtiss-Wright engineers, under Chief Engineer (now Director of Engineering) George Page, reasoned that the best way to get repeat orders was to give airplane operators just such a plane. On the basis of conversations they planned to build 25 units of the CW-20 — the C-46's predecessor — but tooled for 50, banking on repeat orders. Though 50 planes of its size was a tremendous undertaking in those days, is is but a small part of current monthly production schedules.

One of the major points in the preliminary design study — cabin pressurization for sub-stratosphere flying — resulted in the Commando's unusual fuselage design.

In cross section the fuselage, except for the extreme aft portion, is formed by two intersecting circles with the common chord of intersection as the cabin floor line. A circular section was decided upon, of course, because it is ideal for pressurization. However, one circle for the size airplane involved would have presented excessive frontal area that would greatly reduce aerodynamic efficiency. But by using the two circles, frontal area was reduced without losing the ideal pressure section, and the common chord line serves the dual purpose of tying the circles together and providing the floor structure. Flush riveting in the "drag sensitive" area of the fuselage (as well as leading edges of wing and empennage surfaces) with modified brazier head rivets on other parts of the surfaces.

In the prototype, at will be remembered, the two-circle structure was not apparent because fairing between the outside diameters of the two circles gave the craft an elliptical cross-section appearance. This fairing was added not only for appearance but on the assumption that it was aerodynamically more efficient. But as the craft developed into a warplane, the problem of weight became even more important than before, and the 275-odd pounds, not to mention extra material and production time, caused the fairing to be discarded. Aerodynamically, the "exposed" cross-section is just as efficient as that with the fairing, and many who have worked on both the prototype and the current production model maintain today's plane is better looking.

Three Cargo Compartments

Some 2,300 cu ft of space are available in the cargo compartment, which is 48 ft long and has maximum width and height of 9 ft 10 in and 6 ft 8 in, respectively. In addition to general cargo, main cargo compartment arrangements of the C-46 include provisions for 40 troops, using benches which fold up against the fuselage wall; a maximum of 33 hospital litters or five Wright GR-3350-B5 engines or their weight equivalent. Fittings are also provided on the fuselage bottom to accommodate propeller transportation support hub adapters so propellers can be attached as complete units outside the main compartment.

As a general cargo carrier, standard floor design permits a concentrated load on any one transverse beam of approximately 435 lb. For uniform loadings, wall to wall, the standard floor will accommodate approximately 70 lb per sq ft, and for uniform loading over the middle half of the floor, 100 lb per sq ft can be safely loaded.

Cargo tie-down fittings are set at the juncture of the fuselage wall and floor about 21 in apart for the full length on both sides. Similar fittings are set in the walls 31 in above the floor line directly above those at the floor line, and a third group is set in the roof, 25 in on either side of the center line.

None of the fittings project into the cabin, thus danger is eliminated of chaffing or puncturing fragile packages set close to the wall. All three sets of fittings are designed to take ¾-in rope or 1¾-in webbing straps, and any fitting will take at least 200-lb tension in any direction within the fuselage.

Protection from damage by cargo to fuselage bulkheads and skin is given by rows of metal tubes running up to a height of 31 in above the floor.

For engine loading, two pairs of tracks are set in the floor and are so constructed that the concentrated loads are well distributed. Two rows of tie down fittings are set in the floor 25½ in on each side of the center line. The tie-down rings,of 1½ in inside diameter, are provided with eye bolts to be screwed into the fittings for general cargo or task force equipment.

As an ambulance plane — a role in which the Commando is doing yeoman service en route home from war theaters — hospital litters are arranged in two rows, one on each side of the compartment, on special stanchions running from the floor to the roof. If the litters are set up parallel to the floor, 33 can be accommodated. They can, however, be sloped up to 13° fore or aft for special cases, when 24 may be accommodated.

Development of the plane from a peacetime passenger transport to a task force craft brought redesign even to the main cargo loading door through increasing its size to a 96-in width and with depth at the fore edge of 78.5 in and 66.5 in aft. Reason for the greater height at the fore edge is that the floor just inside the door is installed so that it is level when the plane is in three point position. This level portion, created to facilitate loading, extends 18 in ahead of and behind the door to further simplify cargo handling. The cargo door is divided vertically in two sections both of which open up and out. The forward section has an auxiliary door, opening in and up and measuring 55 in high and 30 in wide.

Three extra 20.5 x 26 in auxiliary exits are provided, one on each side of the main cargo compartment just above the wing and one on the right side opposite the main cargo door. They are released inward by pulling two handles.

Additional cargo space (as well as unusual maintenance features to be discussed in detail later) is provided in two compartments fore and aft of the wing center section below the floor line. With a length of 148 in and average center headroom of 44 in, the fore lower compartment has a volume of 197.2 cu ft and capacity of 3,450 lb, and the aft compartment, measuring 143 in in length and of the same average headroom, has a volume of 258.4 cu ft and capacity of 1,750 lb. Both compartments are accessible from the ground through doors of 3 ft 5 in width and 2 ft 8 in height, both on the right side of the fuselage. Adequate compartment lighting is provided by built-in protected lights operating from the plane's electric system. The fore lower compartment is accessible in flight through a trap door in the floor just behind the pilot's seat.

Pilot's Cabin Design

Providing space for two pilots and radio operator, the pilot's cabin follows the basic design, which included visibility and comfort as prime considerations. Both pilot's seats, for example, have fore and aft travel of 9¼ in and can be raised or lowered 5¼ in. The windshield follows the contour of the fuselage, its outer surface being flush with the skin. Downward vision is enhanced by extra side panels, the lower edges of which are about level with the pilot's seat.

A door connects the pilot's cabin with the main compartment, and an emergency exit 20 in wide and 50 in high is set in the left wall. All engine controls are grouped on the pedestal between the pilots. The 25-odd flight, engine and avigation instruments — all "black-lighted" — are grouped in four panels, two on each side of the automatic pilot. For ease and speed of maintenance, each panel may be removed individually, or the panel as a whole can be taken out. Instrument maintenance from the back of the panel is also facilitated by the fact that the nose section of the fuselage can be easily removed, giving plenty of working space on the plumbing and wiring.

Wing design was guided principally by a policy determined to produce a foil with the necessary speed qualifications, yet free of tip stall. Altogether, nine wings with different foils and twist-in were wind tunnel tested at Massachusetts Institute of Technology and California Institute of Technology. The foil selected — NACA 23017 at the root and NACA 4410.5 at the tip — is standard on production models as well as the prototype. No compromise between foil and structure was found necessary: for the engineers made it a point to design the landing gear so that it does not go into the wing structure since, in retracted position, the wheel is between the engine and the front spar.

Wings Follows Basic Design

Of full cantilever construction, the wing is built in three section — an untapered center section and two outer panels with detachable tips. The center section is continuous through the fuselage, with three built-up spars taking landing and flying weights. The fore spar, running only through the center section, was provided to get the maximum moment of inertia and to provide a torque box within a limited area. The landing gear fittings and engine nacelles attach to this spar.

All three spars are stiffened shear webs, with extruded flanges next to the skin. Center section ribs are truss type, built up of rolled and extruded sections. Stressed skin of 24ST Alclad is used throughout the wing, with an interesting design for additional rigidity provided by span-wise hat shaped stiffeners of rolled 24ST Alclad sheet stock riveted by the base to the inside of the skin and, in turn, riveted to the ribs by special channel shaped connections fitting over the "crown" of the stiffener. Although in production it has proved as simple to handle as conventional stringers, this construction gives the added strength of a corrugated structure which flight tests and field service have proven minimizes changes in airfoil through wrinkling.

Nacelles Interchangeable

Built as separate assemblies, which may be changed in service if necessary, the engine nacelles are riveted to the center panel. The nacelles, of aluminum alloy, are of semi-monocoque construction with transverse rings and stringers and five longerons, four of which carry fittings to attach the engines and firewalls. The firewalls are of "sandwich" type — light gage stainless steel forward, asbestos sheet for "filling," and aluminum alloy aft. Engine mounts are conventional design chrome molybdenum steel tubing. Fittings are welded to each engine mount ring at six points for attachment, and cushion-type engine support fittings are furnished with the engine. All welded subassemblies parts are normalized.

To facilitate maintenance, this portion of the C-46 has been designed so that the entire power plant, engine mount, and all accessories forward of the firewall, can be removed as a unit, with complete interchangeability between units and between right and left on the same plane.

Designed for Maintenance

Field service has brought refinements to the unusual engine cowl design. Basically, the cowl is 24ST Alclad, reinforced with channel and hat sections, built in four major sections consisting of a top structural section containing the carburetor air scoop, two large side segments, and a bottom section containing the oil cooler scoop. Instead of having venturi type flaps completely encircling the engine, the top and two side segments are flush with the nacelle, giving a much smoother airflow over the top of the wing and reducing drag to a considerable degree.

In the original conception, the side sections of the cowl were designed to be opened by removing fasteners along the bottom edge of each segment, which then swung out and up on a continuous hinge at the top along the carburetor air intake, with the lower section being detached by removing similar fasteners. These are being redesigned, however,so that the top segment hinges at the back, giving quick, complete access to all cylinders within that periphery, while the bottom segment is being broken into smaller subassemblies so that a quickly removable forward-swinging panel gives access to cylinder heads in the lower third area.

The cowling flaps, too, have been modified, resulting not only in excellent cooling control but also increased aerodynamic efficiency. Perhaps no better evidence of this cooling control can be given than the fact that a C-46, with a gross load of 40,000 lb, has successfully towed two loaded CG-4 gliders to 10,000 ft in a shade over 29 min and without overheating. These new flaps are of the expanding fan type, covering the lower 120 ° of the cowling.

Outer Wing Panels

In addition to retaining the hydraulic flap operation, the new cowling retains another prototype feature: The cowling is attached to the engine mount and to the nacelle via a cantilever structure and is not fastened to the engine proper. This greatly reduces vibration which, in turn, means a lighter weight cowl designed to give better field service.

Removable outer wing panels are attached to the center section just outboard of the engine nacelles by special high-strength bolts encircling the wing through a splice plate. The first four ribs of the outer panel are web type, with large irregular hexagonal cutouts to allow space for three fuel tanks. The fifth rib is solid aluminum alloy with extruded flanges, and from there on out to the tips, ribs are web-truss type. These ribs are formed in one hydropress operation, the cutouts being blanked and a bead for added stiffness being put in during the one press stroke. Detachable wingtips are attached by machine screws.

Vital contributions to the Commando's performance are given by the four Curtiss hydraulically-operated flaps which, in the first stage of operation, move 3 in directly to the rear to create a slot and then, via pantograph linkage, pivot downward to a maximum of 35° for landing or any intermediate angle to shorten takeoff run.

There are two pairs of flaps, the inboard extending from the fuselage to the outer panel splice and having a span of 134.63 in and an area of 46.92 sq ft, and the outboard flaps extending to the ailerons with a span of 171 in and an area of 51.79 sq ft each. The flaps themselves are of conventional construction, having a main spar and stamped ribs, metal covered.

The ailerons are the only fabric covered surfaces on the C-46. The leading edge is a torsion box, and a single spar of flanged sheet metal with lightening holes is employed. Stamped ribs are 24ST. Each aileron — with an area of 39.57 sq ft — is attached by six ball bearing hinges and has an arc of 35° above horizontal to 20° below. Static and dynamic balance is obtained by a lead weight along the leading edge, forward of the hinge line. Each aileron has a trapezoidal shaped trim tab along the inboard trailing edge, the span being 58.5 in, maximum chord 10.30 in, and area 3.97 sq ft.

Production and maintenance simplification are promoted by design in the empennage group, for both stabilizers and elevator panels are interchangeable right and left.

Empennage Units Interchangeable

Stabilizers are full cantilever, all metal stressed skin construction, with three beams extending outboard from the root along constant percentage lines. Additional skin support is given by intermediate bulb-angle stringers. Formed sheet-metal removable tips are attached with machine screws, and the stabilizer itself attaches to the fuselage with bolts through splice angles. Since both panels ate structurally identical, the only difference in their use on right or left side would come from the rigging of the elevator tab motor installations.

Structurally identical elevator panels are all metal, with two beams, stamped ribs, and detachable tips attached with machine screws. Each panel is attached to the stabilizer by four readily removable ball bearing hinges. A steel torque tube, bolted to the inboard end near the hinge line, extends into the fuselage center line, where it attaches to to control horn, on each side of which there is a ball bearing support. Elevators swing over an arc ranging from 33° above horizontal to 17° below, and they have a total area of 98.7 sq ft. Dynamic balance is in the form of a streamlined weight mounted forward of the hinge line near the tip.

Trim tabs, accounting for approximately 15% of the elevator area, are all metal and attach to the elevator by a continuous piano-type hinge. Adjustment is via a push-pull tube extending to the tab motor shaft in the stabilizer. Tab span is 86 in, maximum chord is 17.58 in, and area is 9.35 sq ft.

As are the other empennage units, the 115.4 sq ft area fin is full cantilever and of all metal stressed skin construction with six beams extending up from the root along constant percentage lines. Additional skin support is given by intermediate bulb angle stringers. Like other units it, too, has a detachable tip attached by machine screws, and the whole unit is attached to the fuselage with bolts through splice angles with a normal setting of 0°.

Rudder Design

Like the elevators, the rudder has two beams and stamped metal ribs, 24ST Alclad covered. It attaches to the fin by six readily detachable ball bearing hinges. Its area is 52.8 sq ft , and it swings 20° both ways from the plane's center line. Dynamic balance is obtained by a streamlined weight mounted forward of the hinge line near the top. Actuation is by a steel torque tube bolted to the lower end near the hinge line extending down int the fuselage, where it attaches, hear a ball bearing support, to a push-pull horn.

The rudder tab, accounting for approximately 13 percent of rudder area, is a combination trim and balance type with an area of 7.02 sq ft, its length being 65.12 in and a maximum chord 16.65 in. As is the case with the other tabs, it is all metal and attaches by a continuous piano-type hinge.

Another interesting and important example of design simplification to aid in both peace and wartime operation and maintenance is given in the landing gear. The opportunity to design and tool for large military production has made possible the simplification of the landing gear even though it has been strengthened to carry increased weight, for the Commando is often operated up to 52,000 lb gross weight. On at least one occasion — outside of test flights — it has been flown at 60,000 lb gross.

The main units, having a tread of 25 ft 11 in, consist of single Cleveland Pneumatic Tool Co oleo pneumatic shock absorbers, braced fore and aft by tubular drag struts which are aligned by forged upper and lower drag links. Sidewise bracing is by similar tubes, and all tubular units incorporate forged terminal fittings.

Landing Gear Retraction

Hydraulically operated retraction — taking less than 10 sec — is as follows: The "down" latch at the top of the shock strut is first released and the upper end of the strut moves backward while the wheel moves up and forward, being guided into its position in the nacelle by the rear drag struts and drag links and side brace struts. Retracting action is imparted by the extension of a vertically-mounted hydraulic cylinder fastened at the top to a fitting attached to the front spar and nacelle bulkhead, and at bottom to a bell crank which is part of the outboard side brace strut. When fully retracted the gear is locked in "up" position by a latch. Two fairing doors, hydraulically operated through a sequence valve, follow the wheel up to enclose it completely in retracted position.

Emergency lowering of landing gear can be accomplished by an auxiliary manual extension system. A warning horn in the pilot's cabin blows if either throttle is less than one-quarter open when either wheel is not locked in full "down" position. The landing gear has been built so that the plane is not at a sharp angle in three-point position, for the main cargo compartment floor is but 9.5° from horizontal, a feature designed into the plane to make cargo loading and unloading easier. Either wheel or three-point landings can be made with equal facility, depending on the pilot's preference.

Main landing gear wheels are of magnesium alloy, with double expander type brakes operated by conventional rudder toe pedals from the main hydraulic system by means of a brake metering valve. Each wheel brake system is completely independent of the other. The parking brake is applied by pushing the rudder pedals down, pulling the parking brake lever back, then releasing the pedals to lock the parking brake.

Tail Wheel Retraction

The tail wheel is a fully retracting, self-centering, swiveling shimmy-dampened shock strut suspended by a linkage mechanism within the tail cone. The axle is at the rear of a forged aluminum alloy fork which attaches to the shock strut at its mid-point and to a vertical arm at its forward end. The entire shock structure, which embraces the wheel, drag links, shock strut, and vertical arm, swivels about an axis which does not change angular relationship with the ground line during shock absorber travel.

Retraction is back and up by following action of an upper and lower strut which is operated by a hydraulic cylinder. Two fairing doors make a complete enclosure, following the gear up by means of springs released by a trip.

Main landing gear tires are Goodyear 55x19.00-23 heavy duty, 55 lb pressure type, and tail wheel is 24x10.00-7 10-ply heavy duty of the same pressure.

When the Commando's predecessor first went on the boards, design philosophy resulting from the three years' study called for the "fewest number of engines of the greatest horsepower available." In those days, 1,300-hp plants were indicated, but as the plane has grown the power has gone up so that production models now carry two 2,000-hp units — Pratt & Whitney R-2800-51s, swinging four-blade, full-feathering Curtiss electric propellers of 13 ft 6 in diameter.

The engine control system, which is centered on the control pedestal between the pilots, consists of cables from the cockpit to the rear of the engines, with pushpull rods from there for all units except the propeller governor control, which has a cable actuating a flexible push-pull unit. The Curtiss-designed, Solar-built exhaust system is of stainless steel, reinforced at the joints. Collector rings and exhaust ducts have been designed with sufficient flexible joints to prevent undue expansion stresses. Each engine has its own carburetor air intake system which can increase incoming air temperatures at least 110° F above atmospheric temperature when operating at 65% power.

Performance Outstanding

For proof of the soundness of the basic philosophy calling for the "fewest number of engines of the greatest horsepower available" there are these performance figures: Loaded to a gross weight of 45,000 lb, the Commando has a top speed of 265 mph at 13,000 ft and a cruising speed of 227 mph at 10,000 ft at 67% of rated power. Ceiling with one engine out of operation is 12,000 ft, and normal takeoff run to clear a 50-ft obstacle is under 3,250 ft. Perhaps more important, however, are recent service reports from one war theater showing that the Commando, consistently loaded to around 50,000 lb gross, take off from a rough field at 6,200 ft elevation and deliver their cargoes over routes here the ceilings flown reach up to 22,000 ft.

The main hydraulic system is of the accumulator type, comprising two engine-driven pumps, a check valve in the main pressure line to prevent loss of pressure, and two accumulators, one in the accessory compartment under the floor and a reserve for emergency brake operation located above the floor in the pilot's compartment. Check valves are also installed in the main pressure line from the engine pumps to provide normal operation of the system in the event of failure of one engine or pump. A fluid reservoir is located above the floor at the forward left-hand corner of the main cargo compartment and an emergency hand pump is set between the pilots just behind the control pedestal. All hydraulic system fittings are aluminum alloy, except on brake lines and special nacelle swivel and tubing, where they are stainless steel for structural purposes. Normal pressures are 1,200 psi for landing gear, brakes, surface controls, flaps and cowl flaps, and, through a bleeder valve, 150-200 for the automatic pilot.

Control Boost System

Designs to make cargo handling easier involve even the hydraulic system, for it supplies power for a loading winch set in the main cargo compartment floor just behind the bulkhead aft of the pilot's cabin.

When the Commando's prototype went into design, it was sought to keep pilot fatigue to a minimum through the use of a hydraulic boost system for all controls. This system, while an integral part of the control set-up, was designed to supplement rather than supplant, for all controls were direct-connected and so designed that failure of any one mechanical unit in either elevator, rudder, or ailerons would not result in loss of action of any other part of the system.

The hydraulic boost set-up, then, was designed to "cut in" on the system, the boost cylinders being located at the control horns of the various surfaces. It was ultimately developed to have a constant 3:1 ratio so that pilot's "feel" of controls was retained.

Since, however, the Commando has proved itself a stable easy-to-fly craft, the theoretical considerations calling for the control boost have been found to be partially unnecessary. Too, war time operation of constantly growing numbers of planes in every war theater embracing all conceivable climatic conditions means that maintenance is far different from peacetime airline operations from well established bases. These conditions, coupled with the fact that skills of vastly increased numbers of hurriedly trained men simply cannot equal the high levels of airline operation, call for elimination of every possible maintenance-requiring unit. Thus, the hydraulic control boost system for the rudder has been eliminated with this control now being straight manually operated.

Safety in Fuel System

The electrical system follows conventional practice and design, consisting of two 24V batteries, two inverters for 400 cycle AC. Components using electricity include: Power plants, radio, warning units, autosyn, instruments and, at present, heater units.

Safety was a prime consideration in original design of the fuel system, for tanks are all located well away from the fuselage in outer wing panels — outboard of the nacelles and the solid splice plate. Each engine has its own fuel system, but the two are interconnected by a cross-feed line.

All fuel tanks are aluminum alloy, with wing tanks being set in padded cutouts in the first and fourth ribs outboard of the splice panel. Ribs and supports are so designed that no flexing or bending can be transmitted to the tanks from surrounding structure. Fuel expansion of 3% of capacity is provided, and filler caps have pressure relief valves. Sump capacity is two pints per 100 gal, so designed that water or foreign matter cannot be taken into the tank outlet.

In the prototype, fuel went to the engine only from the fore tank via transfer pumps having a 7 gal per min capacity, accomplished by the pilot starting transfer pumps.

New Selector System

In C-46 production models, however, this has been changed so that a cable controlled selector valve, operated from dual controls, is accessible to both pilots, enabling them to select and draw from any tank. All fuel system pipes are 52SO aluminum, except for stainless steel cross feed lines and fittings.

Standard Commando fuel capacity totals 1,400 gal — 242 gal in each of two fore tanks, 283 in the center tanks, and 175 in each of two aft tanks — and an additional 800 gal may be carried in 8 fuselage tanks for long range operations.

Separate oil systems are provided for each engine, the tanks having a capacity of 40 gal each, with expansion space or 12½% of rated capacity. Tanks, set just aft of the firewall at the outside of the nacelle, are so arranged that expansion space cannot be filled while the tanks themselves are being filled.

Automatic Oil Cooling

Oil radiators are drum type, of 15 in diameter, mounted in the lower portion of the engine nacelle between the engine and the cowl flap. Each system has an automatic temperature control valve and provisions for oil dilution for cold weather starting. Added oil temperature control is obtained through controllable flaps in each exit duct. To facilitate maintenance, either the tanks or the radiators may be removed with the engine and mount left in place.

This article was originally published in the August, 1943, issue of Aviation magazine, vol 42, no 8, pp 130-153.
The original article includes 24 photos, 24 detail drawings and diagrams, and 13 data tables.
A PDF of this article, including a 3-page article on converting the military C-46 to the civilian CW-20E, both recovered from microfilm, is available on the site.