Airplane Design For Cargo Transportation

Condensed from an SAE address
by Carlos Wood, Douglas Aircraft Co

Transportation has always had an extremely important effect on world history. Available means of transportation have always tended to control the radius and magnitude of activities, peaceful or military and the effect has been very powerful, as any critical review of history will show.

The type and scope of transportation is divided into two definite classes, both directly linked with man's technological development.

The first era — before the industrial revolution — depended on animal power of some type and was thus limited in radius and. magnitude. Radius of operation depended on the availability of refueling stations — pasture, grain and other food. Magnitude depended on the availability of animals and conveyances of suitable types and numbers. The animals were the results of animal husbandry, the conveyances were the result of technology and artisanship.

The second period — following the industrial revolution — is distinguished by the continually decreasing dependency on animal power and increasing dependence on mechanical power. Radius of operation still depended on the availability of refueling stations, now purveying mineral fuel suitable for use in the available sources of mechanical power. Food was still necessary but was no longer the only necessity. Magnitude of operation was limited by the availability of sources of mechanical power and conveyances of suitable types and numbers. Both the power sources and the conveyances are the result of technology and artisanship.

Of primary interest is the second class of transportation. as it has had the strongest and most immediate impact on personal affairs, military and commercial.

Railway transportation development was tremendously accelerated by the Civil War because of the growing necessity of transporting increasing quantities of men and materiel with speed and dependability. The military lessons were well read by foreign observers with the result of practically immediate military victories in Europe. During the ensuing years commercial railways expanded tremendously, being the basic means of transportation by the time of World War I.

World War I indicated that railroads alone were not sufficient and large numbers of motor trucks were used in transportation of men and materiel. Also, a few airplanes were used for transportation of materiel — bombers began to appear near the end of the war. During the ensuing years commercial trucking expanded tremendously, becoming a major means of transportation by the time of World War II. Air transportation expanded to an extent of being useful by the time of World War II.

World War, II has shown that railroads, trucks and ships are no longer sufficient, and has begun to force the development and construction of large numbers of cargo-carrying airplanes. In the light of past history and experience it is safe to expect a major expansion in the post — war use of airplanes for the transportation of passenger and cargo.

As a result of this trend, it appears that the time has arrived for a dispassionate discussion of the factors involved in the design of airplanes for cargo transportation. It is still not too late for the results of such an analysis to be useful in the design of cargo-carrying airplanes that will go toward winning this war. It is now time to investigate the bases of economical operation of cargo airplanes that will go far toward winning the peace.

Military operations are normally controlled by an economy of scarcity. Some things are always near a minimum of safe quantity in any campaign. The main enemies behind Allied lines are time and distance. For any given unit of transportation the utility can be expressed as the ability to transport a certain amount of pound miles per hour of men or materiel. As a result, the airplane that will transport the largest number of pounds at the highest block-to-block speed for any given range will be the most useful.

The availability in numbers of airplanes comparable with the man-hours to produce those airplanes may be best expressed by the airplane's weight empty as man-hours for production have been found to be essentially a function of the weight empty.

It is suggested that the utility of the airplanes may in many cases be affected by the landing distance or the takeoff distance as these distances will have an effect on the man-hours necessary to build and maintain any usable operating bases. The landing distance varies approximately with the square of the landing speed. The takeoff distance is a function of the landing speed and the power loading. As a result, an additional factor in the cargo utility of an airplane is the landing speed.

The above items should all be considered in the determination of a comparative "Cargo Utility Factor."

    Range CUF = Cargo Load × Flight Time + Handling Time Weight Empty     Landing Speed

It will be noticed that this factor is the product of a weight ratio and a speed ratio, or a more or less conventional means of determining efficiency of operations.

Thus far, only military operation has been considered. Is there any comparison to the requirements of efficient commercial operation? As would be expected the analogy is immediately seen.

It was mentioned that the weight empty gives a figure representing the number of man-hours to produce one airplane or one group of airplanes. Those man-hours represent a cost in dollars and cents. Also, this weight represents an operating cost in dollars per hour — the heavier the airplane the higher the cost of operation per hour.

The landing speed will give an index of the cost of maintaining the running gear portions of the airplane and also of the ground facilities necessary for successful operation of the airplanes.

Thus the two factors in the denominator of the expression for CUF represent a cost in man-hours, or in dollars and cents per hour if desired. With this analogy in mind it is easy to see that the expression for CUF is proportional to pound miles divided by dollars.

In other words, a high value of CUF corresponds to a low cost per pound-mile. This result leads to the belief that common sense design of good military cargo planes will produce efficient airplane types for future use in peacetime.

Cargo Plane Considerations

Any successful airplane must be designed for the job it has to do. Any good cargo plane must be built around the cargo compartment. This is a truism for either military or commercial designs. The compartment must be large enough to contain the cargo and must be properly located with respect to the remainder of the airplane to allow its use to the fullest extent. The compartment should be properly designed internally to allow efficient handling of cargo when inside of the plane, to allow efficient use of the space available, and to allow easy and positive tying down of cargo once loaded.

In looking at a humble freight car one sees a very simple enclosed space. Experience has shown that a freight car is a purely utilitarian article. A lot can be learned from it. The floor is flat and generally level. The floor is at a height from the ground that best fits into the economical design of the car and its loading. The car is about 40' long, 8' wide and 9' high. The doors are on the side and large enough to allow the rapid loading of cargo, being about 6' long × 8' high on a normal car. This then gives an idea as to a practical compartment size to work toward.

True, there are special cars used for special purposes. The well car is sometimes used but usually when such things as tunnel and bridge clearances determine the size of the cargo. Cars with openable ends are sometimes used, and so on. But normally, the car design is such as to result in the simplest and most efficient arrangement. This then gives a place to start in the design of an efficient cargo plane. If at all possible, the cargo compartment should have a level floor. adequate width and height throughout its length.

The loading door must be of ample size and should be so arranged as to allow easy loading of the cargo. The compartment must be located with respect to the wing in such a position as to hold the center of gravity of the airplane, loaded and unloaded, between the required limits for the airplane. Generally about 1/3 of the length of the compartment should be ahead of the 25% point of the mean aerodynamic chord of the airplane. This applies for normal design practice. More unique designs may modify the above location.

There continually is expressed either personally or in the press a tremendous amount of misinformation concerning the utility and cargo-carrying abilities of new, larger and different types of cargo planes. Much study has indicated certain general trends to expect in planes with varying size and various basic parameters.

For example, the designer is often told, "Make it bigger — or bigger yet — then we'll really have low-cost transportation (or we'll really be able to carry a lot of pound-miles per hour)." Such a challenge is tough on the designer. He'd like to build them larger and larger, that's the way the imagination runs. But about this time some doubting Thomas breaks in and says, "Yes, but will we really be able to carry more pound-miles per hour per man-hours expended (or be able to carry cargo at a lower cost per pound-mile)?" Questions like this must be answered as they determine any common sense solution of either our military or commercial problems. Comparisons of designs either now in service or soon to go into service bring out the fact that the maximum CUF, or best economy is not apparently a direct function of size or airplane. However, the range at which the utility or economy is available is apparently a function of size only — the larger the airplane the longer the range at which the desired utility or economy is obtained. This is due partly to a slowly increasing percentage of useful load with size and partially due to increasing aerodynamic efficiency of the airplane with increase in size. The increase in percentage of useful load is not automatic but is due primarily to unceasing developments in materials and design.

For a given size of airplane, the range at which maximum cargo utility is available and also the maximum range is naturally a function of the basic layout of the airplane. As a result, design conditions must be set up to define the conditions under which it is intended to use the airplane.

At the beginning of studies of a new plane, consideration must be given to all factors insofar as possible in order to produce an efficient plane for the designed use. The basic factors to be considered for a cargo plane are:

  1. Weight
  2. Drag
  3. Span
  4. Power
  5. Stalling speed
  6. Handling time

As would be expected, the effect of weight is comparatively large. Any increase of weight empty for a given gross weight will produce a corresponding reduction in cargo load, not only reducing the CUF but reducing the maximum range. Under present emergency conditions cargo planes are being used as only visionaries would have normally thought only a short time ago. The present formula is to load the plane as heavy as you can take off — this is the flying weight. This can really increase the CUF — and fast. It must be remarked that the strength factors normally built into the airplane are definitely reduced by this process. If the practice is carried far enough the airplane may have to fly at restricted speeds in order to prevent structural failure in bad weather conditions. The overload will also seriously affect the ceiling.

The designer naturally attempts to design the airplane to be as clean as feasible. Extreme care and sacrifice of weight may reduce the drag to some extent and, likewise, simplified and lighter design will often force a sacrifice in drag. Sometimes, necessity forces cargo or other necessary items to be carried externally, causing extremely large changes in drag. Increase of drag also reduces ceiling, but less severely than weight.

When maximum range is desired, as much span is needed as possible in the state of the art at the time the design is frozen. However, it must be remembered that span costs weight and thus is not an unmixed blessing. The combined effects of span and the weight variations caused by span must be considered together. It will be noticed that a shorter span produces a more efficient airplane for short ranges, while the longer span holds up better at the long ranges. Increase of span tends to increase ceilings.

Power also produces combined effects. It does not affect the operational speed at speed for best range but it does allow the operational speed to be raised, This may lead to a slight increase in CUF at short ranges. Increase in installed power costs weight and as a result reduces the utility in the maximum range condition. This effect is combined with a higher fuel consumption in the maximum cruise power condition.

A large amount of work has been applied toward making the airplane a more versatile machine. Among the many items attacked have been the speed range. The top, or operational, speed has been continually increased by increased aerodynamic cleanliness, improved engines and structure. The stalling speed, by necessity, is being held in bounds by gadgetry, which in many cases is a machine designer's despair. The weight involved in such mechanisms is not a direct function of the stalling speed and thus a combined effect cannot be determined. The stalling speed, serving as a measure for extraneous costs in dollars or man-hours, definitely affects the CUF

Handling time can lead into considerable discussion of both military and commercial types. The question of equipment and the like is not relevant here. However, the effect of handling time on the CUF can be shown. Handling time is defined as the total necessary time to fuel, load, and unload the airplane. Strictly speaking, all layover time between flights due to bad planning, maintenance time and the like should be included but the latter are too unpredictable for a general discussion. The relative importance of the above items must be determined in order to make any logical design decision as to the stress to be put on any item.

Certain special conditions envisaged for the use of the cargo plane will often affect the basic design. Sometimes such conditions may be of such importance that a special design seems to be called for that is not an optimum arrangement for normal use. However, whenever possible, it is necessary to make any design of such a flexible nature that it will not be useless in every condition but one special type of use. This necessity produces the most headaches in the design room; to produce a design of good general utility that will also meet, or come close to meeting, all the special conditions of operation.

Military operations are a prime example and will be more so for the next few years. Much has been written lately by all of these unknown "experts" about how the cargo airplanes will replace all the shipping and all the railroads. The greater the study of this problem the more it is felt this is not the case. The cargo plane is an invaluable adjunct to present means of transport. Nothing else can deliver comparatively small quantities of critical materiel so rapidly over reasonable ranges. When velocity of replacement is needed the cargo plane is the thing. When mass of replenishment is necessary the ship still stands alone. But all the cargo planes that will be built will be none too many to take care of the "shock" requirements. This is the present field and it should be recognized as such.

It should also be realized that any airplane needs fuel. With the tremendous demands being made on our aviation fuel supply the problem becomes acute. Our western hemisphere is one great source of supply. The United Nations have only one other large source — the Near East. To save transportation of that fuel all over the world it would be most useful if all flights could be round trips, without refueling, from one of these sources.

Military operations sometimes do not have much choice as to cargo destination. If comparatively heavy cargo is carried, suitable handling equipment must be carried within the airplane itself to allow proper cargo loading and unloading. This in many cases will reduce CUF due to the weight involved and present a considerable design problem.

Combat conditions bring a whole new set of troubles. Undoubtedly it will be necessary to fit some defensive protection to our military cargo planes that will be operating in or near the zone of operations. For tactical reasons, glider tugs may be required with all of the structural and cooling troubles that are involved. Certain types of cargo planes for actual tactical use will have to be built with much of the design subordinated to extremely rapid unloading under fire. These are all items that will require much design, thought and ingenuity.

Commercial applications in the future will need more consideration in detail. It is well agreed that the quantity of traffic will increase terrifically in the future. This will require improved means of handling cargo, storing cargo, and dispatching cargo. Suggestions have been made for standard-size packaging. This is a good thought from the operator's standpoint but would undoubtedly cause considerable difficulty from the shipper's standpoint. The thing that appears to be necessary is more available flexibility in design for cargo stowage, not freezing of cargo size.

As cargo traffic increases, the network of places served will also probably increase. At major terminals, suitable facilities for cargo handling will be available, but at smaller stations this will not be the case. Three courses are open:

  1. Design simple ground-based handling devices.
  2. Design suitable airplane-carried handling equipment.
  3. Design the airplane more with cargo loading in mind.

The first course is outside of this province, the second will involve a reduction in utility as we have seen. The probable answer will be to go as far toward designing for cargo handling as possible without suffering from weight and aerodynamic difficulties.

It is reasonable to assume that future passenger planes will be supercharged and it must be remembered that most future airplanes will carry a mixed load. The speeds of operation go up with increase in altitude, increasing the utility. Present indications are that the supercharged cabin cargo airplane will probably pay its way from the efficiency standpoint if the supercharged altitude for the airplane and its power plant is such that large weight penalties are not involved. This fact presages the circular fuselage or something quite close to it.

What are the design trends in cargo planes? Certain trends are evident both from a military and a commercial standpoint:

  1. Operation at higher altitude is apparently on its way. Circular section fuselages can be expected.
  2. The cargo compartment will have a flat floor and will be of general dimensions of a freight car.
  3. The use of airplanes for mixed cargo and passenger loads will call for a re-examination of airplane arrangement for more versatile use.
  4. Every attempt will be made to reduce the distance from the ground to the cargo compartment. This may call for some rather freakish designs at first.

The only really effective way to increase the utility (or decrease the costs of operation) of cargo planes, is to strip them down to a minimum weight empty or overload them above normal gross weights. The installed power must be as low as possible and still meet the legal or practical operating limitations. These things are very important, either when economy is the test or when maximum utility is desired, as the cargo plane always works at a disadvantage from the standpoint of efficiency — it must be remembered that it may have a lift to drag ratio of 20 to 1, but a railway train has a lift to drag ratio of about 100 to 1, and a steamship operates at about 500 to 1.

This article was originally published in the March, 1943, issue of Air News magazine, vol 4, no 3, pp 20-24, 62.
The original article includes 4 drawings and 5 photos of cargo/transport planes.
Drawings credited to Robert Lindgren (Air News, Burnelli. Photos credited to Boeing, Douglas, Curtiss-Wright, Lockheed.