Design Analysis of the B-24J Liberator

by F W Fink,
Chief Division Engineer, Consolidated Vultee Aircraft Corporation

Originally designed as a bomber, the B-24's load carrying capacity and great range make it useful for anti-submarine patrol missions; converted, it is also being used as a cargo or transport plane.

The prototype Liberator made its initial test flight on December 29, 1939 — less than nine months after the mockup was completed. At present, the airplane is probably the most numerous heavy bomber on all the battlefronts. And, its raids in the Pacific and on the Ploesti oil fields of Rumania have established distance records that were surpassed only by the B-29.

Although originally designed as a bomber, the Liberator carries such tremendous loads that it has frequently been used as a cargo or transport plane. Its range makes it further useful for anti-submarine patrol missions.

The first Liberators to go into combat were designated the LB-30s and were used by the Royal Air Force. Cargo-transport versions of the airplane are known as the C-87s in the US Army and as RYs in the US Navy. Latest combat models are called B-24Js by the US Army and PBY4s by the US Navy. It is the B-24J that will receive the most attention in the remainder of this article.

Generally speaking, the B-24J is a four-engined mid-wing monoplane with a semi-monocoque fuselage and twin-rudder empennage. It carries a crew of 9. Its landing gear is of the retractable tricycle type, and its power plants comprise Pratt & Whitney R-1830-C4G engines equipped with turbosuperchargers and Hamilton Standard full-feathering propellers. It has a top speed of more than 300 mph, a service ceiling of about 36,000 ft, and a maximum range of about 4000 miles. Carrying 10 to 14 .50-cal machine guns, it has a gross weight of more than 56,000 lb, including a bomb capacity of 10 tons.

Each B-24J consists of approximately 52,959 purchased parts, 49,345 raw material parts, 400,000 rivets, and 85,500 bolts and miscellaneous small parts. Its wing span is 110 ft, its length is 66 ft 4 in, and its height 18 ft with the nose wheel extended on the ground.

Wing

Outstanding feature of any version of the Liberator is its wing, which is an adaptation of the Davis high-lift airfoil. A full-cantilever structure, it comprises a center panel and two outer panels covering an area of 1048 sq ft. Its framework is of aluminum alloy beam-bulkhead and stressed-skin construction.

The wing root chord has a length of 14 ft while the tip chord extends 5 ft 2-13/32 in; this provides a mean aerodynamic chord of 123.72 in. Wing incidence is 3°, and dihedral on the upper 30% of the chord line is 1½°. Aspect ration is 11.55, and the leading edge sweepback is 3½°.

The front wing spar is located at 10% of the chord, while the rear spar is at 66.2% of the chord. The space between spars on each side of the centerline is arranged to hold the main fuel cells, and an access panel is provided on the under surface of the wing on either side of the centerline to facilitate fuel cell installation and removal. Each panel is fastened by screws in self-locking nuts.

Leading edge of the wing consists of readily removable sections which are attached by screws so as to provide immediate access to controls housed therein. Surfaces aft of the rear spar except for the ailerons are of sheet metal, supported by chordwise ribs. All sheet metal and all drawn or rolled sections, except tubing and extrusions, are of the Alclad type aluminum alloys. Few extrusions are used, all stringers, hat sections, and spar flanges being formed from sheet strip. Flush type, retractable, tie-down points are provided on the wing undersurface.

Ailerons are balanced aerodynamically, dynamically and statically. They may be moved either up or down 20°, and the distance from the plane of symmetry to the centroid of the aileron area is approximately 40 ft. Both ailerons are of aluminum alloy torque box and rib construction, fabric covered. In the right aileron is an irreversible trailing edge tab.

The four engines are housed in individual nacelle structures built out from the wing center section. These nacelles are designed so that the engine mount and engine can be removed as a unit. Alclad sheet is used for the nacelle covering, except for that portion near the exhaust system, where stainless steel is used. Each engine mount is a welded steel tube assembly with dynamic mounts for the engine-to-engine-mount attachments. A sheet metal firewall separates the engine accessory section and the wing interior, where the fuel tanks are situated. Also, a ring cowl with controllable cowl flaps is provided for each engine.

Tail Surfaces

Horizontal tail surfaces have an NACA 0015 airfoil section; they have a span of 26 ft, and a 7 ft 8-3/16 in chord.

The horizontal stabilizer is of the nonadjustable type; its setting, relative to the longitudinal axis of the airplane, is 2.5°. Attached to the horizontal stabilizer are the elevators, each of which may be moved 30° up or 20° down; they are aerodynamically, dynamically, and statically balanced; and in the trailing edge of each is a trim tab with irreversible controls. Elevator tabs have a total angular movement of ±10°.

Vertical tail surfaces have an NACA 0007 airfoil section. Each fin is fixed with a normal setting of zero. Rudders may be moved 20° to either right or left. Like the ailerons and elevators, rudders have aerodynamic, dynamic and static balance; and in the trailing edge of each rudder is an irreversible control operated trim tab. Rudder tabs can be moved 10° right or left.

Elevators and rudders are of torque box and rib construction, fabric covered. The stabilizer is a single smooth sheet metal assembly, attached to the fuselage with four fittings so that it can be easily and quickly replaced. Fins are likewise of smooth sheet metal construction, readily replaceable; they are attached directly to the stabilizer ends. Horizontal tail surfaces are reinforced to withstand a force of +25G.

Dual primary air controls are provided in the cockpit. Aileron and elevator controls are of the wheel type; rudder controls are adjustable fore-and-aft pedals. Tension regulators are installed in the aileron, rudder, and elevator control cable systems to keep constant tension on the cables at varying temperatures. Each control surface has non-adjustable stops.

Aileron, elevator, and rudder trim tab controls are situated on a pedestal which is easily accessible to both pilots. They are in the form of wheels or knobs, with their aces parallel to the axes of trimming motion and so connected that movement of the controls produces a trimming movement in the same direction. Indicators show the displacement of the trim tabs with regard to their supporting surfaces.

Elevators, ailerons, and rudders all may be locked in their neutral positions when the airplane is parked on the ground. Locking action for the rudders and elevators is on the control linkage in the tail; for the ailerons, on the control linkage in the wing. The controls may be locked from either pilot position by moving to neutral and setting a latch.

Fowler flaps are situated in the trailing edge of the wing, and extend from the fuselage to the inboard tip of the aileron on each side. Total flap area is 144 sq ft, and range of movement is from 0° to 40° down. When extended approximately 20° down, the flaps appreciably increase the wing lift with only a slight increase in drag. When extended 40° down, the flaps produce an appreciable increase in drag and steepen the landing glide angle.

A hydraulic jack, with an actuating valve in the pilot's cockpit, is provided for lowering or raising wing flaps. There also is an auxiliary hydraulic hand pump for emergency use.

Fuselage

Fuselage maximum cross-section height is 10 ft 5 in, and width is 7 ft 5 in. The structure is of smooth sheet metal skin, reinforced with longitudinal stringers and transverse bulkheads. Sheet and drawn or rolled materials, except for tubing, are of Alclad type aluminum alloys. Stiffeners reinforce the fuselage structure around the wing, which extends continuously through the fuselage.

Arrangement of the fuselage from nose to tail is as follows:

  1. A nose section, containing a power-operated turret.
  2. A compartment at the nose behind the nose turret for the bombardier-navigator. This compartment has provisions for bombsight, chart table, instrument panel, and special navigation equipment; it is accessible through the nose wheel doors, or through a passageway beneath the flight deck.
  3. A flight deck to accommodate the pilot and copilot, seated side-by-side with a control pedestal between.
  4. A radio operator's compartment directly aft of the flight deck. Both radio operator and flight engineer are stationed in this compartment. Between them, and overhead, is a machine gun turret.
  5. Two bomb bays aft of the radio operator's compartment with a catwalk through the center of the bays, leading from the radio compartment to the aft section of the airplane. The bomb day doors, in opening and closing, slide on the sides of the fuselage.
  6. A compartment aft of the rear bomb bay with a power-operated retractable gun turret in the bottom of the fuselage and a flexible gun installation in each side waist position.
  7. A tail section, containing a power-operated machine gun turret.

All transparent panels in the fuselage are either Plexiglas or Lucite, except for the pilot's windshield and certain portions of the machine gun turrets where panels are of bullet-resisting plate glass. Structural openings include pilots' compartment ceiling escape hatch, nose wheel opening with hinged doors, cutout for an observation dome on top centerline of the navigation compartment, two bomb bays with sliding doors, waist gun openings in the aft portion of the fuselage , and main rear bottom entrance door aft of the rear bomb bay. All of these openings are fitted with controls, so that they can be used as emergency exits. Moreover, provision has been made to operate all doors, except for those for the nose wheel, from the fuselage exterior when the airplane is on the ground.

Bomb bay doors are operable hydraulically from the bombardier's bomb rack control quadrant and from the right side of the fuselage aft of the flight deck. Manual means also is provided, and in an emergency the bomb bay doors can be controlled from the pilot's station. An up-latch prevents the doors from creeping shut. All but the lower third of the skin on the rear bomb bay doors is spotwelded.

Landing gear includes two main wheels mounted under the wing on each side of the fuselage aft of the center of gravity, and a nose wheel mounted beneath the fuselage forepart. These wheels are respectively retractable into wheel wells in the wing and a nose compartment in the fuselage. The retracting mechanism is operated hydraulically. A manually operated mechanical system, independent of the hydraulic system, is provided for emergency use. An interlocking safety system prevents landing gear retraction when the airplane is on the ground.

The two main landing gear wheels are of the smooth-contour type, each 56" in diam and mounted on an oleo-pneumatic shock-absorbing strut. The nose wheel is also of the smooth-contour type; it is 36" in diam and is mounted on an oleo-pneumatic shock-absorbing strut whose assembly includes a shimmy damper to prevent unstable wheel oscillations while allowing free swiveling 45° to either side of neutral for ground maneuvering.

Hydraulic power brakes, individually controlled by auxiliary pedals on the rudder pedals are provided for the main landing gear wheels. On the pilots' pedestal is a parking brake lever. There is a complete dual hydraulic brake system for emergency use.

Lugs on the main landing gear struts enable the Liberator to be towed either forward or backward. Also, there is an attachment on the nose wheel gear for a towing and steering bar. To prevent the fuselage bottom from touching the ground at any time, there is a retractable tail skid near the tail of the airplane; it is extended and retracted with the main landing gear.

Conventional three-bladed Hydromatic propellers were used on the first Liberators, but the B-24Js have special paddle-blade versions to eliminate undesirable compressibility effects which are often encountered in high altitude operations. Propeller controls are situated in the pilots' compartment along with a high and low pitch position setting indicator. Propeller feathering pump oil is obtained from the main oil tanks, and the lines leading thereto are wrapped with suitable lagging material to prevent oil congealing in cold weather.

Engines are equipped with turbosuperchargers and are geared 16:9. Normal horsepower ratings in standard atmosphere are: 1040 bhp @ 2550 rpm at sea level (44" Hg); and 1100 bhp @ 2550 rpm from 6100 ft to 25,000 ft (48½" Hg). Takeoff rating is 1200 bhp @ 2700 rpm (50" Hg) in standard atmosphere.

Engine lubrication system includes a 42-gal self-sealing oil cell, oil cooler and temperature regulator, aluminum alloy oil lines, oil dilution system, and oil cell sump drain cocks.

Main fuel tanks are of the self-sealing type and have a capacity of approximately 2814 US gals. There are provisions for auxiliary tanks in the bomb bays. Each engine has an independent fuel system consisting of an engine-driven pump, a nacelle-mounted strainer and sediment trap, an electric-driven booster pump mounted below the wing to maintain inlet pressure on the engine-driven pump at altitude, and connecting lines. All fuel-carrying lines with an inside diameter of ¾" or more are of self-sealing hose, except for those lines leading from the fuel pumps to the carburetors. Vent lines are of aluminum alloy tubing, except for bellows-type aromatic-resistant synthetic compound tubing used between fuel cells. Drain outlets for the fuel selector valves are connected so as to provide a cross-feed fuel manifold system, which is arranged so that the fuel booster pumps may transfer fuel from any set of main or auxiliary fuel cells to any engine. Two sight-tube fuel quantity gauges are provided for the main fuel tanks and they are equipped with valves which can be used in connecting the cells on which a reading is desired, or to prevent leakage when a gauge tube is broken.

A cable-link control system is used in operating the throttles or mixture controls for the four Liberator engines. The throttles are spring-loaded to open to 65 percent power. The supercharger controls are electrically operated.

Gyroscopic flight instruments obtain suction from vacuum pumps installed on the two left engines. Hydraulic pressure for the flaps, landing gear, and bomb bay doors comes from an engine-driven pump on the right inboard engine.

The first Liberators carried conventional anti-icer and de-icer equipment. However, virtually all recent models utilize Consolidated Vultee's new anti-icer system for preventing or eliminating ice formations on airfoils and for cabin heating. This system consists of a series of suitable air ducts which utilize engine exhaust gases as a heating medium.

A type HRU-28 auxiliary power unit is installed in the aft portion of the fuselage. It is connected with the airplane electrical system and has a complete self-contained fuel and oil system. Electrical energy from the airplane storage batteries is used in starting the unit.

A 200-amp generator is driven by each of the four main engines; it is controlled by a voltage regulator, an ammeter, and a current control relay switch. A resistor is installed in the negative lead of each generator. Meters and generator switches are situated on a generator control panel.

On the left side of the airplane, beneath the flight deck, are two 24-V type G-1 aircraft storage batteries. The battery vent is connected to a glass jar containing a neutralizing agent, which provides a sump for any escaping electrolyte. The batteries are connected in parallel to the distribution circuit, and a battery switch assembly is installed for each battery so that either battery may be isolated in the event of malfunctioning. The battery switches are located near the ignition switches, through which they are safetied. Also, a master battery switch is provided so that the batteries can be completely disconnected from the electrical system.

Signal equipment includes a Type M-8 signal pistol and mount, a Type A-2 pyrotechnic pistol holder, a Type A-7 signal flare container assembly, and 18 Type AN-M-37 to AN-M-42 signal flares.

The oxygen system consists of piping, regulators, outlet, and 22 cylinders mounted in the fuselage. These cylinders are divided into nine groups, and are connected into distribution lines for that group only. However, all cylinders for all groups are connected by a filler line to permit recharging all oxygen cylinders from a filler valve which is aft of the bomb bays and accessible from the exterior. In addition to standard "bail-out" equipment, there is a portable oxygen unit adjacent to each regular outlet for emergency use.

Liberators have complete interphone systems with jack boxes at all flight stations or crew positions. Provisions have been made for the use of either push-button or throat microphones, and there is a relay installed to prevent the pilot and co-pilot from speaking simultaneously.

Special radio equipment includes a radio altimeter, an American recognition radio set, detection units, and a blind approach set. There is also a command set, a dinghy transmitter, RC-32 filter equipment for the pilots, a marker beacon, a liaison set, and an SCR-211 frequency meter.

Generally speaking, good aerodynamic form and cleanness prevail in the design. The only protuberances that tend to spoil a smooth airflow are those absolutely necessary for airplane operation and proper execution of a mission.

Engine nacelles have been located with regard to the wing so that the upper contour line of any nacelle is no more than 1½" higher than the maximum ordinate of the wing at the nacelle location when viewed from the side with the airplane in a level flight attitude.

This Design Analysis article was originally published in the November, 1944, issue of Industrial Aviation magazine, vol 1, no 6, pp 15-16, 18-24, 26-27, 29-30.
The original article includes a thumbnail portrait of the author, 2 photos, a blueprint-style three-view and 13 detail drawings and diagrams, and 4 data tables, plus a ledger-sized foldout with a color phantom rendering and diagrams showing optimum cruise-speed attitude and fuel and oil systems.
Photos are not credited.

Photo and drawing captions:

Data tables:

Loads and Weights (lb)
Crew1,800
Engine oil (128 gal)960
Guns and accessories1,953
Bombs2,007
Empty weight36,562
Gross weight(over) 56,000
Tail Group
Stabilizer
Area (incl elevator balance)140.5 sq ft
Normal setting
  (relative to longitudinal axis)
2.5°
Angular movementfixed
Elevators
Area (total)67.1 sq ft
Area aft of hinge line51.5 sq ft
Angular movement30° up, 20° down
Tab area4.95 sq ft
Tab angular movement±10°
Fins
Area (total)123 sq ft
Normal setting
Angular movementfixed
Rudders
Area (aft of hinge)4.48 sq ft
[48.8 sq ft —JLM]
Angular movement20° left, 20° right
Dynamic balance coefficient0.014 (no greater)
Tab area3.1 sq ft
Tab angular movement10° right, 10° left
Balance Factors
Alternate gross weight CG locations with wheels up — aft of leading edge30.9% MAC
Most forward for adequate control, landing power off23% MAC
Most aft, satisfactory longitudinal stability with power and for satisfactory ground handling34% MAC
Fuselage Dimensions
Length67' 1-3/16"
Height10' 5"
Width7' 5"