Aircraft Design Analysis No 4

Project Engineer, Santa Monica Div, Douglas Aircraft Co

Known to the AAF as the Havoc and to the RAF as the Boston, this fighter-bomber can be used on a dozen different kinds of missions with remarkable effectiveness.

The great versatility of the Douglas attack bomber was designed into it by a scheme of interchange which bids fair to have considerable effect upon future aircraft engineering technique. The interchangeable noses of the A-20 — which convert it from a medium level bomber to a heavily gunned ground strafer, from photographic ship to night fighter, or from a torpedo plane to a day fighter — constitute an original idea in military plane design. And it's an idea offering a basic step toward achieving the "impossible" — creating of the "all-purpose" combat airplane.

Regularly assigned by both British and American Air Forces to many types of missions, the A-20 has piled up a record of versatility unsurpassed by any other airplane. Its routine assignments, all requiring variations in equipment, include medium altitude, short and long range, horizontal, skip, and low level bombing, low level attack, day and night fighting, torpedo launching, smoke screen and chemical warfare operations, reconnaissance, and photographic missions.

As stated, this versatility is not the result of accidental, emergency, or makeshift conversion. Instead, it stems from a specific decision to design a combat plane that would perform the greatest variety of missions without deviating from the basic model.

Paralleling the problem of design was that of production. Obviously, any airplane intended for a variety of uses, would if successful, be required in great numbers, so it must be designed for large-scale manufacture, and the design must be broken down in such manner that volume could be maintained despite the changes all things must undergo in time of war.

Outstanding among many production-design features of the A-20 is the "half shell" or longitudinal half-section method of main fuselage section assembly. This feature allows the concentration of more workers per assembly.

For attack, the A-20 had to have high speed, maneuverability, and armament to destroy ground objectives. The original design still is adequate to meet changing specifications for more and heavier equipment. As a medium bomber, the design had to provide fuel capacity and a bomb bay, plus the stability with the bay open, to permit precision bombing.

For attacks on naval targets, it had to have stability with bomb bay doors removed and with a 2,000 lb torpedo suspended half in and half out of the opening. For smoke screen laying and chemical warfare missions, the wing had to accommodate chemical tank racks and control mechanisms, with but slight changes, easily and quickly made in the field.

As a night fighter or "intruder" the interchangeable nose feature has provided a means of installing detection equipment, including a powerful searchlight with which RAF Bostons point out and momentarily blind the crews of enemy planes in order to get in the first shot. The rugged and roomy fuselage made possible additional armament, including a belly tub with machine guns or cannon, also extra fuel tanks for long range operations.

The strength designed and built into the outer wings permits the installation of wing racks to carry additional bombs. Further, the unusually large bomb bay provides space for additional racks for large numbers of small fragmentation bombs for attacks against personnel.

Carrying a crew of three and equipped with an attack nose in which six .50-caliber machine guns are mounted, with upper, rear, and lower gun positions, and having close to single-seat fighter speed and maneuverability, the A-20 becomes a hard hitting, pugnacious day-fighter.

Such an airplane,of course, could not be designed in entirety back in 1938, when the first model was built. But from that original 11,850-lb twin-engine, 280-mph ship, armed with four fixed, one upper, and one lower gun, all of .30-caliber has come, through two succeeding series, the current A-20. The 7B was followed by the DB7 which, in turn, gave way to the DB-7A for export and the A-20A for the USAAF. Principal differences between the DB-7 and DB-7A were larger and more powerful engines and larger tail surfaces.

The A-20A, which was the immediate forerunner of the present versatile A-20 series, was designed for a gross weight or 19,750 lb, carried a crew of three, and was armed with four forward-firing fixed machine guns, and upper flexible twin-gun installation, and a single gun in the lower rear position, all of .30-caliber.

In its present series (often modified) the A-20 is an all-metal midwing monoplane with over-all wing span of 61' 4". Over-all length is 47' 4", with the bombardier nose, and 48' with the attack nose. And overall height at rest is 18' 1".

Two 14-cylinder 1,600-hp Wright GR-2600 engines drive Hamilton Standard Hydromatic propellers of 11' 3" diameter.

The A-20 has seven major components: Nose, main fuselage, empennage, right and left inner wings, and right and left outer wing panels. The fuselage is a semi-monocoque all-metal structure comprised of extruded 24S aluminum alloy longitudinals, spaced 4" to 8" apart, and formed of 24ST sheet covering is attached to the frames with flush rivets and is stiffened by the longitudinals. Lap joints are used, with vertical laps either joggled to make a smooth contour or, in the case of heavy sheets, beveled along the outer edge. The semi-monocoque design was chosen because it combines the required strength with the least weight and is simpler to manufacture.

Five major sections comprise the fuselage: Attack or bombardier nose, pilot's cockpit, bomb bay, gunner's compartment, and tail compartment.

Nose sections, keying much of the A-20's adaptability to varied types of missions, are interchangeable, one with the other or from ship to ship. Changing them is merely a "wrench and screwdriver job" that can be done in the field by two men in six hours or less. In general appearance, there are two kinds of noses, bombardier and attack, but actually there are several different attack types, each of which mounts a different combination devices, with the necessary variations in structure.

Only six bolts secure the bombardier nose to the fuselage, while ten are required for the attack noses. Of these ten, eight are spaced around the perimeter of the attach ring or station and two are located inside of the outer line of bolts. Trunnion fittings at the forward end of the nose and swing links toward the after end of the structure form supports for the four center fixed guns. The two side guns are supported directly on castings attached to the nose beams.

In dimensions, all types of noses are identical at the point of junction with the fuselage, but the attack nose is 8" longer than the bombardier version. The latter is manufactured as a single unit, while the attack version is assembled "half shell" and then joined, much the same as in the main fuselage.

Construction of the attack nose calls for building up with formed channel frames, extruded 24ST longitudinals, and 24ST Alclad covering. Provisions are made for carrying the six separate .50-caliber ammunition boxes at the aft section of the nose. The four center guns are fed ammunition from the box over electric boost motors and through flexible ammunition chuting. The two side guns are fed directly from the box. Access to the guns and ammunition boxes is though hinged or removable doors and via the nose cap.

The nose is built up of formed channel sections with the forward third of the structure of formed and extruded magnesium T sections. The latter third of the bombardier nose is covered with Plexiglas, which is retained with strips and flush screws. The interior arrangement provides a bombardier seat under which are ammunition boxes for the side guns. There is an instrument panel, together with brackets, for various types of bomb sights, also a laminated plate glass panel for sighting.

Although the idea of interchangeable noses was part of the original design, the company does not claim, of course, to have foreseen all of the different nose sections called for by combat experience in this war. Basically, there could be very little change in the structure of the several versions. The chief problem has been to find sufficient space for the steadily increasing size and amount of armament without altering the maximum dimensions of the section.

For maximum all around vision, the pilot's compartment is located forward of the wing and engine nacelles. From a normal position in the cockpit the pilot's vision directly forward over the center line of the fuselage exceeds 8° below the horizontal. Access is by means of a door in the top of the cockpit enclosure. This door, extending from the windshield to the leading edge of the wing, is hinged at the right side and opens upward. An assist facilitates entry.

The wind screen is laminated plate glass and molded Plexiglas, the center panel being curved laminated plate and the side and top panels of molded Plexiglas. Behind the laminated plate, and separately mounted in a hinged frame, is a bullet-proof plate glass that can be swung away from the windscreen for cleaning.

Movable transparent side panels extend from the windscreen to a point at the side of the pilot's head. These panels are mounted in a fore and aft slide to permit opening the sides of he cockpit. The corner panels of the windscreen are in two sections, with the upper section hinged and capable of being folded and fastened inward to permit direct vision when desired.

A standard adjustable pilot's seat is provided and a wheel on a cast magnesium column gives control for the ailerons, while the column operates the elevators. The wheel also incorporates the switches for gun firing and bomb release. Rudder pedals are provided with integral brake control.

An instrument panel incorporating all of the necessary flight and engine operation instruments is provided. On the right-hand side near the fuselage skin and just above the floor is the electrical panel for the bombing circuits affording all selections of such circuits. On the left side, directly opposite the bomb selection panel, is the panel which carries the balance of the electrical switches for the operation of the airplane. The pilot has all the basic radio controls, and only interphone positions are provided for the balance of the crew.

Insulation of the cockpit is flameproof cloth-covered “Sepack,” or “Resistohyde” covered kapok felt, to reduce noise.

From the forward tip of the nose to the bulkhead front end of the bomb bay, the contour of the fuselage represents faired lines. Beginning there and continuing to the bulkhead front end of the rear gunner's compartment and aft end of the bomb bay, a constant section is maintained. In this section, which represents approximately 60% of the fuselage length, is the 14' × 33" bomb bay, considered unusually large for a plane of this type.

It is the constant section, plus those containing the rear gunner's and pilot's compartments, which are assembled in two longitudinal half-sections, joined only after the major portion of assembly and installation work is finished.

The rear gunner's compartment is located aft of the trailing edge of the wing, immediately behind the aft bomb bay. Entrance is by a door in the bottom of the fuselage. The top section of the gunner's compartment is made up of two Plexiglas parts, one fixed and one movable enclosure which slides forward under the fixed section to allow access to the upper .50-caliber flexible gun. The upper gun is stowed in a tunnel immediately aft of the movable enclosure and is accessible through two doors on top of the fuselage operated from the gunner's compartment. The cockpit enclosure can also be used as an exit either in the air or in the event of a landing on the bottom of the fuselage, with wheels retracted. The door and sliding enclosure are operable from either side.

Arrangement of the rear gunner's compartment allows freedom of movement of both gunners. A seat supported on a pedestal-type structure is provided for the upper gunner, whereas the rear gunner's seat is made of a cushion located just forward of the access door on the floor. Safety belts are, of course, provided.

The fuselage is faired upward from the bottom line of the constant section to the aft extremity to provide good aerodynamic design, to raise the empennage control surfaces above wing turbulence and slipstream, and to provide clearance for the tail section during landings with the tail down.

Of conventional design, the fuselage tail cone is constructed as a single unit. Frames are formed 24SOal or 24STal aluminum alloy with extruded 24ST longitudinals to tie them together. Covering is of 24STal sheet. The cone attaches to the main fuselage structure, just aft of the rear gunner's compartment, by means of six internal wrenching bolts.

The wing, of full cantilever aluminum alloy construction, has a single spar and two shear webs. It is the NACA 23018 section tapered to NACA 23010 at the tip, selected because of its very stable center of pressure and high Lift/Drag.

It is built up in six major sections: Left and right inner panels, extending from the fuselage junction to approximately 60" beyond the center-line of the nacelles; left and right outboard panels, similar in construction to the inner ones and extending to a point approximately 30" inboard of the tip; and the left and right wing tip assemblies which complete the wing.

Inboard wing panels are connected to the fuselage structure by aluminum alloy fittings and close fitting steel bolts in shear. The outboard and inboard panels are joined by tension bolts at the center spar and by shear bolts at the forward and aft shear webs. The wing tips are attached to the outboard panels by means of flush screws and anchor nuts.

The single Wagner spar carries all bending loads. Wagner type ribs and arch channel formers, spaced a maximum of 15" apart, are attached to the spar and extend to the leading and trailing edges. Span-wise stiffeners are attached to the ribs and formers, which support the skin. All exposed wing surfaces are flush riveted to provide smooth contours and make the plane as aerodynamically clean as possible.

Ailerons have a main spar and rear shear web with ribs spaced at approximately 12" intervals throughout the span. Sheet 24ST Alclad is used for covering. While the aileron tabs are part of the aileron proper, they may be operated with or independently of the aileron to control the airplane's balance in flight. The aileron is mounted on three hinged brackets at the rear shear web of the outer wing, so arranged that removal and assembly may be performed without removing the wing from the airplane. Ailerons are statically and dynamically balanced by means of lead weights in the leading edge and trim tabs are adjusted from the pilot's cockpit.

Horizontal stabilizer of the craft is of two-piece, all metal, full cantilever design permitting interchangeability, left with right, simplifying both supply and manufacture. It attaches to the fuselage with tension bolts, and is given 10° dihedral to elevate the tips above the wake of the engine nacelles.

Elevators are riveted aluminum alloy frames covered with doped fabric held in place with flush attachments. The framework consists of a deep built-up spar near the leading edge and channel ribs connecting the spar to a metal trailing edge. The nose section of the elevator is covered with light 24ST Alclad to form the leading edge and torque cell attached to the torque tube.

Approximately 25 percent of the elevator area is located forward of the hinge line for aerodynamic balance. Elevators are dynamically and statically balanced by the addition of lead weights in the leading edge, left and right being interchangeable. Trim tabs have approximately 5 percent of the elevator area and are adjustable from the pilot's cockpit.

Vertical stabilizer is of full cantilever design and is attached to the fuselage by means of tension bolts.

Like the elevator, the rudder is a riveted aluminum alloy frame, fabric covered, with approximately 22% of its area forward of the hinge line. It, too, is balanced by means of lead weights in the leading edge. Trim tabs have 5% of rudder area and are adjustable in flight from pilot's cockpit.

The single type of vertical fin and rudder was chosen because it was believed to be less subject to flutter in case of combat damage, also because of its greater simplicity of manufacture and its better drag coefficient.

Sealed ball bearing hinges are used for all control surfaces. Carbon steel flexible cables with swaged fittings and ball bearing pulleys connect them to the control wheel and pedals in the pilot's cockpit, the only point of control in the airplane.

Lift and drag increase is achieved by means of conventional flaps, synchronized through constant flow valves in the hydraulic control system that permit a constant and even flow to each operating cylinder. The flaps shorten the takeoff run approximately one-fourth and increase the angle of glide about the same amount, decreasing landing speed approximately 20%. They have an average chord of 22" and a length of 109" each and may be depressed a maximum of 45°. For takeoff under severe conditions, flaps are set at an angle of 22½°.

Nacelles are of semi-monocoque all-metal construction with channel type frames and extruded longitudinal stiffeners. A stainless steel firewall is provided between the engine and the nacelle structure. Inner wings and nacelles are constructed separately and then are joined in the first stations of the inner wing assembly line.

Engine mounts are of welded chrome molybdenum steel tubing, bolted to support fittings at the front shear web of the wing and to a combination engine mount and landing gear support frame near the lower surface of the nacelle. Vibration isolators are installed between the engine and the engine mount. Engine cowling, made of formed sheet, extends to a point immediately aft of the rear row of cylinders. Hydraulically operated cowl flaps control the flow of air. Accessory cowling is attached to the firewall and the diaphragm by quickly detachable fasteners.

The A-20 was one of the first US military planes to be equipped with a tricycle landing gear. US Army authorities had foreseen the need for a gear that would make possible operations from unpaved and otherwise hazardous airfields closest to the fighting fronts, hence it was stipulated that the attack bomber should be so equipped.

Completely retractable and hydraulically operated, the landing gear has a main wheel tread width of 203½ in, a wheel base of 165", and type II smooth contour wheels equipped with disk-type hydraulic brakes. Main wheel and tire size is 44" and nose wheel and tire is 26". Each main wheel is mounted on a single, braced, oleo-pneumatic strut, and the nose wheel is mounted on a single cantilever oleo-pneumatic strut. For normal operation the nose wheel is limited to a movement of 30° each side of center by a hydraulic snubber, but a mechanism is provided to allow 360° caster for ground handling.

Main wheels retract up and aft into the nacelles, while the nose wheel retracts up and aft into the fuselage, and all are completely enclosed when retracted. Retraction and extension of all three wheels is simultaneous and actuated by the same control system. Brakes are pressure operated and individually controlled by pedals integral with the rudder pedals in the pilot's cockpit. A parking brake, operated from the cockpit, is also provided. Positive hydraulically operated mechanical locks hold the gear in the extended position. Positive extension of the landing gear in case of failure of the hydraulic system is accomplished by a bungee system and an emergency pull handle to disengage the gear lock-up mechanism.

A position indicator, red and green warning lights, and warning horn indicate the landing gear position at all times.

Hydraulic system of the A-20 is the "medium pressure" type, consisting of two engine-driven pumps (either of which can furnish the required pressure, one serving as a "stand by"), a hydraulic fluid reservoir, and a pressure accumulator. A hand pump furnishes pressure for ground operation, when the engines are not being run, and for emergency pressure in the air or in landing. The hydraulic systems operate the landing gear, wing flaps, cowl flaps, bomb bay doors, and the main landing gear brakes.

Each engine is provided with a separate lubrication system. Oil tanks have a net capacity of 23 US gal plus a 3-gal air space, and they are of self-sealing material designated as US Rubber Co material No 143. A partial circulation hopper is in each tank. Oil coolers with automatic self-closing shutters are also provided in each nacelle, where the oil tanks are also located, in the upper portion along the center line of each engine. Each lubrication system has electrically controlled oil dilution, and oil tanks incorporate provisions for the installation of an electrically operated immersion type heater, supplied with current from an external source.

Fuel is carried in two pairs of wing tanks and two removable fuselage tanks. Wing tanks are equipped with detachable sump plates and finger screens which are removable without disconnecting the fuel lines. The main fuel supply is drawn from the wing tanks through finger screens at each end of the main tanks and interconnected by means of a selective sump valve for each tank. Wing tanks are individually vented, and overflow is discharged at the aft end of each nacelle. Each inboard tank is provided with an emergency dump valve.

The fuselage fuel tank consists of two separate containers installed above the bomb bay and interconnected by lines which equalize the level of fuel in them. An electrically driven booster pump, controlled form the pilot's cockpit, and a pump adapter with a flange to provide a sump, complete the installation. Vents are interconnected and discharge at the high point of the airplane in normal flight or at rest on the ground.

Each power plant is normally supplied by its own independent fuel system, but these may be interconnected by the pilot on either the suction or pressure side of the engine fuel pumps, so that each engine can be supplied from any wing tank, or both engines can be supplied by one fuel pump. The fuselage fuel tank is connected into the left-hand tank selector valve, so that fuel from the fuselage tank may be supplied t either or both engines.

The engine driven fuel pumps are mounted directly on the engines, and fuel is supplied form the suction side of the pump through the selector valves and strainers. Hand fuel pumps and cross-feed valves are located in the fuselage, with fuel valves operated by cable and rod controls and the hand pumps by rod controls. Gauges are of the electric remote indicating type and are located on the instrument panel. The transmitter unit is installed in the tank.

Armor and armament have, of course, changed considerably since the first A-20 was designed, and they will probably continue to change throughout the period of the war, and possibly afterward. But despite the ever increasing weight and amount of both armor and armament, this constant change has not required a major basic alteration in the airplane. Naturally, the switch from .30- to .50-caliber machine guns and the 20-mm cannon, on some models, has required variations in the structures of the nose sections,and a minor change in the fuselage just aft of the bulkhead where the nose attaches, for a distance of approximately 30" aft, because of the greater weight of the nose. However, no changes were required in the main fuselage or wing structures.

The A-20 was designed to carry unusually heavy loads for the size of its wings and power, and it was built to take off from short fields. This basic design decision has made possible operations in which it is not uncommon for A-20's to take off from short, advanced bases with as much as 50 percent overload.

This article was originally published in the January, 1944, issue of Aviation magazine, vol 43, no 1, pp 125-146.
A PDF of this article includes 18 photos, 7 diagrams, a three-view, and 10 detail drawings, plus 15 data tables.
Note that this PDF is preliminary, taken from microfilm and not from original magazine sources, so that the quality is poorer than later versions should be.

Data tables:

Landing gear dimensions
Wheel base165"
Main wheel & tire size44"
Nose wheel & tire size26"

Total weight    1,512 lb
pilot's enclosure  38.6 lb  
bomb bay enclosure  22.6 lb  
radio compartment enclosure  8.72 lb  
nose wheel doors  14.4 lb  
bomb bay doors  92.61 lb  
gunners' access doors  15.3 lb  
Aileronstotal weight    89 lb
including weights  40.8 lb  
Elevatorstotal weight    88 lb
including weights  22.4 lb  
Ruddertotal weight    83 lb
including weights  26.6 lb  
Fuel systemtotal weight    1,278 lb
tanks  810 lb  
piping & equipment  468 lb  
Oil systemtotal weight    344 lb
tanks  131 lb  
piping & equipment  213 lb  
Useful Loads
Fuel  540 gal  3,300 lb
Oil  40 gal  300 lb
Fixed armament  total weight  1,099 lb
  with 2,100 rounds ammunitionweighing654 lb  
Flexible armament  total weight  590 lb
  with 1,200 rounds ammunitionweighing373 lb  
Unit Weights
Fuel systemempty  per gal capacity  2.3 lb
Oil systemempty  per gal capacity  8.5 lb
Wing group464.8 sq ft  5 lb/sq ft
Tail group184.1 sq ft  2.2 lb/sq ft
Balance Factors
Attack condition, gross weight
  CG location, wheels upaft LEMAC30.7% MAC
  CG location, wheels upbelow LEMAC15.6% MAC
  CG location, wheels downaft LEMAC29.4% MAC
  CG location, wheels downbelow LEMAC17.3% MAC
Flying balance range of A-20between 16% MAC and 33% MAC
  (wheels up or down)
Limit Maneuver Load Factors
At Design gross weight (19,750 lb)
  Positive maneuver load factor4.0
  Negative maneuver load factor-2.0
Limit Gust Load factor
  At design gross weightflaps uppositive4.0
  At design gross weightflaps downpositive2.27
Limit Landing factorground landings
  At design gross weight3.33
Limit diving speed
  at design gross weight120% of max level flight speed
Load factor ultimate9G

Horizontal tail surfaces
Area(including fuselage area)100 sq ft
Area(less fuselage area)81.7 sq ft
Span21' 1½"
Maximum chord80"
Distance from design gross weight CG
  to 1/3 maximum chord point
  295% MAC
Horizontal Stabilizer
Area (including fuselage and elevator extensions)60.8 sq ft
Area (less fuselage)42.5 sq ft
Normal setting(relative to long axis)+2°
Angular movementnone
Area each39.2 sq ft
Angular movementUp 30°
Down 20°
Vertical Tail Surfaces
  Area, total (less fairing)24.8 sq ft
  Normal setting
  Angular movementnone
Rudder Data
  Area35.1 sq ft
  Angular movementright 22½°
left 22½°
Airfoil sectionrootNACA 23018
tipNACA 23010
Wing areatotal464.8 sq ft
Span61' 4"
Root chord138.5"
Tip chord(projected)44.0"
Taper ratio0.32
Incidenceinboard panel+3°
Outboard panel, root+3°
Outboard panel, tip-1°
DihedralLeading Edge4° 7'
SweepbackLeading Edge
Maximum rib spacing15.5"
Spar location38% chord
Shear web location65.9% chord
Aspect ratio8.07
Mean Aerodynamic Chord length100.23"
Location relative to Leading Edge (Root Chord):
Thickness of wing(root chord)24.93"
Thickness of wing(tip chord)4.4"
Wing area(net)464.8 sq ft
Area (total)42.1 sq ft
Angular movementUp 30°
Down 20°
Differential motion1.5:1
Distance from plane of symmetry
to centroid of aileron area
Instrument Installation
Flight Instruments
  Airspeed Indicator
  Turn and Bank Indicator
  Suction Gauge
  Flight Indicator
  Turn Indicator
  Outside Air Temperature
  Rate of Climb Indicator
Engine Instruments
  Oil Temperature Indicators2
  Manifold Pressure Gauges2
  Engine Temperature Indicators2
  Oil Pressure Gauges2
  Fuel Pressure Gauges2
  Carburetor Mixture Temperature Indicators  2
  Fuel Quantity Indicator
Miscellaneous Instruments
  Hydraulic Pressure Gauge
  Indicator Unit for Landing Wheels and Wing Flaps
Navigation Instruments
  Mark 11 Drift Recorder(rear cockpit, stowed)
  Mark 11 Astro Compass(rear cockpit, stowed)
Radio Units
  SCR-274Command Set1
  SCR-535Identification Set1
  RC-36Interphone System1
  MN-26Radio Compass1

Horsepower Ratings
Wright GR-2600 with 16:9 gear ratio
  and PD-12K1 carburetor
Military ratings:
    1,600 hp at 2,400 rpm to 1,000 ft
    1,400 hp at 2,400 rpm to 10,000 ft
Normal ratings:
    1,350 hp at 2,300 rpm at sea level
    1,350 hp at 2,300 rpm at 5,000 ft
    1,275 hp at 2,300 rpm at 11,500 ft
Propeller Data
Hub manufacturerHamilton Standard
Hub model number23E50
Blade manufacturerHamilton Standard
Blade design number6353A-21
Full Feathering
Diameter11' 3"
Number of blades3
Minimum pitch limits: Constant speed
Maximum pitch limits: Full feathering
Minimum clearances:
  In plane of each propeller disk:
    To ground, level landing9½"
    To fuselage9"
  Normal to plane of propeller disk:
    To cowling at maximum pitch½"
    To leading edge of wing33"