How to weigh an airplane

By Bernice K Platt
Engineering Department, Lockheed Aircraft Corporation

Performance requirements in range, payload and structural strength of military aircraft demand a rigid control of weight and balance.

A diminutive red tractor chugs importantly across the expanse of Lockheed Airfield towing a Flying Fortress fresh from the paint hangar, its new paint glossy in the California sun. This Fortress is the first plane of a new series produced at Lockheed Aircraft Corporation's Factory "A" (formerly Vega Aircraft Corporation), and it must be weighed before being released for flight to its Army base.

With a minimum of lost motion the tractor operator cautiously maneuvers the huge bomber through the vast hangar-opening and brings it to a gentle stop with its front landing wheels resting on separate weighing scales.

Immediately a stir of activity surrounds to plane.

"Is the brake set? Okay, boys, lower the chain hoist."

Under the supervision of the weight engineer the chain hoist is attached to the lift lug. As the tail of the bomber is lifted into the air, the engineer uses a spirit level to determine the correct longitudinal and lateral position.

"Hold it!" The airplane is now in a horizontal position approximating that of level flight and for the next two hours the Lockheed weight engineer and his assisting weight computers will be busy with calculators, slide rules, and tabulations.

The dream airplane of the engineer concerned with weighing aircraft is a clean plane fresh from the production line, because it simplifies his job of figuring accurate weight. If the airplane has been flown he must be able to calculate accurately the amount and weight of the residual oil and fuel in the tanks in order to obtain its actual weight and balance.

If the airplane has set out on the field overnight, the condensation on its surface will make a substantial difference in the basic weight. When a Boeing Clipper was weighed some years ago, 60 pounds were allowed for the weight of the dew on the metal surface.

In a rainstorm, an airplane may pick up as much as 100 pounds in weight and, although the water will run off the metal surface in a few hours' time, it will actually take several days for the airplane to dry out completely.

Certain airplanes are "sealed" against rain storms; that is, all of the joints are treated to prevent water soaking. This treating adds extra pounds of weight to the basic total.

Most of the aircraft sent to areas where extreme low temperatures prevail are winterized. Vulnerable portions are wrapped with insulating materials in a process called "lagging." Here again, added weight results.

The actual weighing of an Army bomber is as complicated but fascinating process. First, the hangar doors are closed and air conditioning units shut off so that wind currents will not sway the suspended tail of the plane and disturb the reading of the scales through vibration. In the scale pits, separate readings are taken of the weight bearing on each front wheel. These figures are entered on an official Army basic weight check list by the weight computer. From a catwalk located high in the ceiling girders of the hangar the tail scale reading is taken and recorded.

While the weight computers are obtaining these data, the engineer in charge is establishing the centerline of the bomber so that he may calculate the center of gravity — the point about which the airplane would balance if suspended.

In order to locate the centerline accurately, a cord is stretched between the two front wheels and fastened securely to the center axles of the landing gear. Then a plumb line is dropped from the center of the underside of the airplane between the wheels to the hangar floor and the spot is marked. A second cord is run longitudinally from the reference line to an established point on the tail of the plane. The reference line or datum line is an imaginary vertical line set by the manufacturer at or near the nose of the airplane. Diagrams of each plane show this line as zero.

By taking the weight reaction from the tail and main wheels, and the distance between those main points, the engineer is able to establish the center of gravity. These data are also recorded on the aircraft actual weight and balance form.

In addition to recording the total basic weight of the bomber, the engineer lists all fixed and operating equipment which is in the airplane at all times and weighs five pounds or more. Later, the information entered on the actual weight and balance form is transferred to the running log, which is always in the airplane and serves a purpose similar to that of a ship's log.

The weight of the crew members, oil, gasoline, bombs, and ammunition for each flight is entered on a weight and balance clearance chart by the crew chief at the tactical field of operations. All other items that the bomber would normally carry, such as crew baggage, supplies, spare parts, freight or cargo of any kind, and equipment that may vary with each flight, are itemized on the cargo list. The only concern the weight engineer has with these items is to determine that the gross weight of the bomber is well within the gross weight limits of the airplane design. Gross weight is the basic weight plus the weight of all the items and personnel that will be carried in the airplane. Any loading that exceeds this limit may cause dangerous flight characteristics or structural failure of parts.

Accurate weight control has been the aim of all aviation companies since the industry became weight conscious. Prior to the war, air transport companies were demanding airplanes that gave maximum safety, dependability and comfort at the least cost. Since the cost of an airplane is determined primarily by its weight, this factor has assumed increasing importance in engineering design and shop construction.

A new aircraft is usually designed to fill the specific needs of a customer. For instance, an airplane is desired to carry a certain cargo load or a specified number of passengers and equipment. If, when built, the basic weight of the airplane is too great, it probably can't carry the required load and the customer refuses to accept it.

When the aviation industries turned to the production of military aircraft, the necessity for maintaining rigid weight and balance control was increased by the stringent requirements on performance, range, payload, and structural strength.

The trend is toward aircraft designed to carry more equipment, a larger number of personnel, and to operate over a longer range. With heavier loads being carried, weight and balance limits are, in general, becoming more critical. In addition, there is a greater necessity for an increase in speed, operating efficiency, and safety.

These requirements must be kept constantly in mind by the aircraft engineers. The process of designing an airplane requires the continuous balancing of the important factors of aerodynamic characteristics, weight and balance control, and structural strength.

It is highly important that the weight engineer consider every way to decrease weight for nothing affects performance so adversely as overweight. It is also his responsibility not to insist on weight savings which are seriously detrimental to the structural strength and safety factors of the airplane.

As the design progresses, he must keep closely in touch with the work to uncover any tendency toward out-of-balance conditions which will give the plane unstable flight characteristics.

At Lockheed, the work of the engineering weight group starts with the preliminary design of a new airplane. Minimum and maximum weights of each airplane component are estimated, as well as the basic weight of the entire plane.

As the first airplane goes into production, the weight group weighs each individual part after fabrication. Later, all assemblies are weighed. These actual weights are all checked against each other an against the estimated weight. When the completed airplane is ready to be weighed the weight group has every part accounted for, from ounces to hundreds of pounds.

Whenever a change is made in structure or equipment, the changed parts are weighed as well as the first completed airplane of any new series. Th design of military aircraft is in an almost constantly changing state to meet the needs of varied tactical missions and loadings. The functions performed by the weight group are an important factor in determining the success or failure of these proposed designs.

Lockheed aircraft are designed to use certain materials, the capabilities and limitations of which have been proven through many years of use. When these materials become critical, substitutes must be analyzed and developed to conform to the company's process specifications and the high standards of the Army and Navy specifications.

This means that a proposed substitute material must be approved by many groups in the engineering department. It will probably be analyzed by the metallurgist to determine the limts of formability, the effect of heat treatment and corrosion; by the materials and standards engineer to ascertain the quantity available on the market; the structures group to test for strength; the weight group to prevent an undesirable weight increase or unstable flight characteristics due to an unbalanced condition.

Any weight that changes the location of the center of gravity beyond the safe limits will make the airplane unsafe to fly. The range of movement which the center of gravity may have without making the plane unstable is determined by the manufacturer in actual test flights.

Actually, weight and balance go hand in hand. When the airplane reaches the field of military operations, constant checking is necessary to ensure proper loading in relation to the center of gravity and to keep the total weight within the gross weight limits. Since all items of weight and load put into the airplane affect the center of gravity, it is essential that the loads be properly distributed fore and aft.

If the balance is too far forward it is difficult for the pilot to lift the nose of the plane for a take-off or a landing. Such a condition will increase the fuel consumption in flight, resulting in less range, and it will tend to increase a dive out of control.

Conversely, with the balance too far aft, the pilot strain in instrument flying is increased and it is difficult to maintain the plane in level flight. The stalling tendency will be increased and speed will be decreased.

To illustrate the importance of balance, a Lockheed Hudson that was participating in a bombing raid on the European continent had the elevators shot away. Immediately, the bomber sarted climbing toward the moon. The pilot ordered the crew members up into the nose compartment. By shifting his load weight he was able to bring the plane back to the airfield and land it safely.

Some years ago an aircraft company built an airplane that was designed for non-stop cross-country flying. It was unique in that the engine was installed on a track. As the fuel stowed in the tail was used up, the pilot would move the engine back along the track to adjust the balance. While this model did not prove to be a commercial success it did demonstrate the importance of load balance.

As recently as 10 years ago separate weight groups were comparatively unknown in aircraft engineering. In fact, it has only been within the past five or six years that the weight engineers have assumed actual control of weight and balance in airplane design.

In the engineering department at Lockheed Factory "A," the weight group maintains for analytical study a file of research data on the weight breakdown of material used in airplanes that have been constructed in the past. The group also keeps records of the current materials available for models that are in the process of engineering design. These are used to forecast material needs and to prevent a production hold-up due to an inadequate supply of a vital metal.

Then, too, there is a constant search for lighter materials that incorporate the same structural strength. Since the cost of materials used in aircraft construction is based on their weight, the substitution of lighter materials having the same tensile strength serves a dual purpose: it assures the customer of an airplane that can transport the maximum payload at high speed, and it helps to guarantee a fair profit to the manufacturer. A pound of material that costs $5 or $6 does not seem like a startling figure when it is used in the construction of a single plane. But when the $5 or $6 is multiplied by the several hundred planes in the contract it can mean the difference between profit and loss.

Today there is a fight against time in this search for lighter materials. It is no longer possible to specify merely that one metal of lighter weight be substituted for another. Because of the critical shortage of raw metals, materials must be tested and developed for airplane construction.

Through all this development and substitution, the weight groups must take an active part. As one of the weight engineers at Lockheed put sit, "We really are the jugglers in airplane engineering, because we constantly have to balance performance, strength, cost, serviceability and weight. Each of those factors plays a vital part in the production of an aircraft that can fulfill the operation for which it was designed."

This article was originally published in the April, 1944 issue of Flying magazine, volume 34, number 4, pp 50-52, 144.
The PDF of this article includes five photos showing a Ventura and a B-17 undergoing the weighing procedures.
Photos credited to Lockheed.