0perating economy has ever been a major objective for the air transport manufacturer and the airline operator alike. In the new Curtiss-Wright Substratosphere Transport we have incorporated many basic design characteristics assuring operating economies which we believe set a new high standard in air transport maintenance procedure.
This story of our designers' efforts to achieve operating economy does not feature the impressive operating cost figures which already are known, but emphasizes rather that mass of details contributing to the transport's design, the principal consideration of which has been the simplification of maintenance routine with consequent economies. Such factors range from the ease with which a mechanic may replace a complete outer wing panel to the time required for a ship cleaner to clean the ash trays.
Illustrating best the basic design features which reduce direct operating cost, is our choice of two Wright Double Row Cyclone engines to power this transport, rather than a larger number of power plants of less horsepower. The reasons are:
Servicing simplicity has bee achieved in the design of the Curtiss Wright Transport by our adherence to four rules or principles of construction. These rules are:
Numerous examples of our application of these four rules may well be presented as illustrating the careful "design for maintenance" which we ave incorporated in the new transport. While only a few of them will be mentioned here, our solution of the problems involved typifies the manner in which we have solved the general problems maintenance effectively.
With reference to Rule I, which provides for maximum parts interchangeability, all parts and assemblies subject to frequent removal may be exchanged between airplanes in service, quickly and easily. All removable major assemblies and many subassemblies are jig-built to assure interchangeability of installation so that an unusual number of parts may be interchanged between the left and right sides of the airplane.
The extent of interchangeability achieved in the new transport is best shown through a tabulation. All units listed are interchangeable between planes. Items listed under "A" may be interchanged directly with no fitting required and also are interchangeable between the left and the right sides of the airplane; items listed under "B" are interchangeable without fitting, but are not interchangeable left and right; items under "C," otherwise parts which are often designed without consideration of replacement simplicity, are interchangeable between ships but require the alignment of attachments, locating and drilling of attachment holes, etc.
(A) Parts directly interchangeable between airplanes and between left and right sides: complete power plant assemblies forward of and including firewall, engine mounts, exhaust manifold assemblies, removable engine cowlings, firewalls, oil tanks, landing gear assemblies, complete tail assemblies, stabilizers, elevators, elevator tabs, fins, rudders, rudder tabs, stabilizer leading edge.
(B) Parts directly interchangeable between airplanes: outer wing panels, wing tips, wing leading edge, ailerons, aileron tabs, wing flaps, tail wheel assembly, fuel tanks, cabin chairs, instrument panels.
(C) Parts interchangeable between airplanes, but requiring alignment of attachment fittings or holes: engine nacelles, tail cones, fuselage doors and exits, fuselage nose section (pilot compartment), fuselage tail section (aft of ladies' dressing room).
Certain of the most important interchangeable units in this list merit specific comment. The perfection of a power plant installation which may be installed on either nacelle of our Transport, for example, is an outstanding achievement. By removing four engine-mount bolts and disconnecting controls and lines behind the firewall, the entire engine assembly with all accessories and cowling may be detached and installed on the opposite side without modification.
Interchangeable also from one side to the other are such items as the firewall, engine mount, symmetrically shaped cowling, and oil tanks. Fuel units, including both pumps, both strainers, flow meter and check valves are grouped in a separately removable unit mounted on the firewall in the engine compartment; the complete fuel unit assembly is interchangeable right and left and can be removed by detaching only four bolts.
The leading edges of all fixed surfaces, such as the wing outer panels, stabilizer, and fin, are detached by removing a row of flush head screws in each case. This greatly simplifies for the airline maintenance department the problem of repairing the surfaces which are most susceptible to damage. In the case of the stabilizer, removal of the leading edge provides free access to the rudder tab control mechanism. Wing tips are attached by the same method as the leading edge assemblies. All control surfaces are jig-built for interchangeability.
The complete assembly of wheel, brake, brake flange, tire, tube, and axle on the new transport's landing gear is such that it may be quickly and easily interchanged; an automatic-shutoff coupling in the brake line prevents escape of fluid when the brake unit is detached, making drainage of the line unnecessary, even when the parking brake pressure is applied. Removal of either of the main landing gear wheels may be accomplished by detaching two bolts; the tail wheel may be removed by detaching but four bolts.
In adhering to rule II that all parts of the Curtiss-Wright Transport requiring service should be accessible, we have achieved a new standard of equipment availability. The centralized accessory compartment is located below the wing in the bottom of the fuselage where equipment may be easily reached by a mechanic standing on the ground and still be accessible in flight to a crew member who may descend to it via a hatch in the cockpit floor and the forward cargo compartment. Ground servicing is further simplified by our concentration within the spacious accessory compartment of such items as the hydraulic and electrical equipment, the de-icing and anti-icing equipment, the fire extinguisher and other accessories.
The unusual accessibility of all parts of the power plant installation is directly due to our application of the Curtiss-Wright Cowl, a type which emphasizes such maintenance features as simplicity, ruggedness and accessibility, and which has been thoroughly proved on our airplanes for the last four years. Equally important are this cowling's operating advantages, reduced powerplant drag and improved cooling control.
This cowling is built in three sections, each extending over approximately one-third of the engine periphery. The lower section is bolted to the engine mount and firewall, being designed to offer ample space for servicing the lower cylinders of the engine. Both upper cowl sections are hinged to the long, faired carburetor air intake duct which extends along the top of the engine compartment and attaches to the engine mount and firewall. The cowl is locked by quick-action cam latches. Without being removed, the upper cowls may be swung open, thus making the complete engine and engine compartment accessible for routine servicing. For more extensive servicing, the three cowl sections may very easily be detached.
While twin-row engine installations ordinarily requite a complicated cowl flap of the multiple-vane type consisting of a large number of small flaps around the cowl skirt, which are operated by a somewhat involved mechanism, our engineers have developed a cowl flap arrangement of simple design which eliminates a number of parts ordinarily requiring adjustments. Since the skirts of the upper two cowl sections fit tightly against the nacelle skin, all air is exhausted through an outlet at the bottom of the low cowl. The air flow is controlled by a single large flap attached across the outlet, and is operated by a single control hydraulic jack.
Recognizing that many serious structural repairs result from the development of cracks at points where local concentrated stresses build up, our engineers have followed rule III to avoid local stresses in fittings, at rivets and other attachments. We have thus eliminated for the airline operator a considerable source of wasted time and expense, especially since this type of damage often occurs at inaccessible locations.
Elimination of this cracking is carried into the wing structures. Whereas skin-cracking in the vicinity of the de-icer attachment has presented a problem in the past due to the de-icer imposing load in the skin requiring a special reinforcement, we have installed wing de-icer attachments which are so constructed that the local oscillating stresses produced by the de-icers are distributed through a heavy gauge channel built flush into the nose skin, instead of being carried directly into the wing skin.
Since the usual wrinkling of the skin under load because of its relative thinness or the rather distant spacing of the stiffeners, also contributes to the cracking of stressed skin wings, our engineers have developed a non-wrinkling design which eliminates the possibility of cracking due to stress concentrations and the working of rivets due to resultant tension loads. This has been accomplished by using a heavy-gauge skin reinforced by relatively light rolled Alclad hat sections, formed from the same type of material as the wing covering, providing the effect of a reinforcement of corrugated sheet with alternate corrugations omitted. This combination of heavy-gauge skin and a short free span of skin between stringers makes serious wrinkling remote.
As a further precaution against skin cracking, the skin is reinforced wherever wing or fuselage stiffeners terminate.
In keeping with the wing design, the nacelle assembly offers many examples of good structural design from the viewpoint of maintenance; being built with unusually heavy skin, thoroughly reinforced so as to eliminate any cracking that might possibly be induced by the slightest residual engine vibration not absorbed by the dynamic suspension mount. Finally the engine cowls are mounted on the engine mount and firewall rather than attached directly to the power plant thus enabling the engine to vibrate freely within the cowling without the cowl oscillations which inevitably result in eventual fatigue failures. This type of cowl mounting incidentally contributes the improved appearance of a steady cowling in flight.
Certain other advantages in structural design also merit description. In the cable control systems, where initial cable wear ordinarily occurs at fairleads, and any angle set in the cable at such points must inevitably aggravate the situation, fairleads are not used to change the cable direction more than one degree. Of lesser importance, but nevertheless a major point in expediting service and repair, is the substitution wherever possible of elastic stop nuts for the use of castellated nuts with cotter keys.
Included also in the cost-reducing category is our use of plastic sheet rather than painted metal at points where there is considerable scuffing as in the control room where interior appearance is preserved. Similarly, the sills or frames of the large cargo door are thoroughly reinforced with heavy-gauge skin formed into a box section sufficiently heavy to prevent denting or damage in loading. Finally, the floor of the cargo compartment is made removable in order to facilitate the repair of any damage resultant from the loading or shifting of high-density cargo.
The results of the engineering department's efforts to adhere to our policy (IV) of designing all individual details so as to reduce time and effort required for maintenance, reduction in operating costs and increase in profits, could be discussed at length. Three major developments, however warrant detailed treatment: The new independent heating unit, the fuel system and the unique instrument installation.
The airline maintenance expert who dreads the approach of winter because of aircraft heating problems will enjoy learning that the Curtiss Wright heater unit operates independently of the main power plant, operates equally well on the ground or in the air, heats the cabin quickly, maintains cabin temperatures on the ground without the aid of outside sources of heat even in the coldest weather, and thus eliminates the customary cold-weather bug-a-boo. Since this system involves no steam lines, the customary ground freezing problem in cold weather is likewise eliminated.
The cold weather troubles that plague air line ground crews have been eliminated in the new transport, wherever economically possible. The traditional wobble pump, heretofore used with difficulty in obtaining fuel pressure during cold starts, has been replaced by an electric fuel pump which assures adequate, controlled fuel pressure for starting and supplements, of course, the usual engine-driven fuel pumps. Another development in the fuel system is the location of all fuel tanks in the wing outer panels where the dihedral gives natural slope to the tanks, assuring positive drainage at all times, complete drainage of water to the large cast aluminum alloy sump at the lower end of each tank, and elimination of the possibility of water remaining in the tanks with consequent damage from corrosion.
Since time-saving is unquestionably the essence of airline maintenance economy, our development of the new Curtiss-Wright "Tell-Tale" system for automatically checking and indicating any malfunctioning of the major controls or instruments of the new transport provides the maintenance department with a new and valuable tool. While the operating advantage of this system have been described many times and are fairly well under stood, the servicing benefits are not so well known.
It is obvious however that a device which instantaneously checks on the correct operation of every engine and air control, and on the operation of the complete powerplant under various conditions, provides the maintenance crews with a rapid bird's-eye view of its problems. Questions that formerly required hours to answer may now be checked in a minute or two.
Our engineering staff has worked hard to achieve a new peak in maintenance perfection; they have concentrated on providing accessibility removability, interchangeability and simplicity on the one hand and to providing local strength and rigidity at every point on the other; they have sought to speed servicing by eliminating parts where many parts would complicate maintenance and by adding equipment which would short-circuit involved routines.
The final result, affirmed by the results obtained during the preliminary test period of the new Curtiss-Wright Substratosphere Transport, I believe is an airliner which will not only be valued by the air transport operator because of its Profit-Ability but will also merit of the praise of the maintenance departments because of its Maintain-Ability.
This article was originally published in the March,1940, issue of Aviation magazine, vol 39, no 1, pp 54-55, 57, 112.
The original article includes 3 photos and a drawing.
Photos are not credited, but are certainly from Curtiss-Wright.