Static Testing the XNQ
By Edwin T. McClanahan

At the Patuxent River Navy Air Test Center, a Fairchild XNQ-1 trainer climbed high above the sun-drenched Maryland countryside and began a series of violent aerobatic maneuvers, culminating in a screaming dive toward the earth at 290 m.p.h.

Fairchild XNQ lifts off for flight testing at Patuxent River Navy Air Test Center. Before flight testing, extensive strength testing was done.

Despite the impression that a casual observer might have gotten, this was no air-show rehearsal or stunt pilot sharpening his skill. Inside the cockpit, special flight test instruments recorded the strains imposed on wings, fuselage and tail surfaces as the plane executed dives, spins, high-speed stalls and rolling pull-outs. With representatives of the Navy’s Bureau of Aeronautics watching intently, the prototype of the XNQ-1 was undergoing its final demonstration flights prior to delivery of the first airplane of this type to the Navy.

But, the successful performance of the XNQ-l at Patuxent was not the first grueling test the sturdy little trainer had faced. Back at Hagerstown, Md., in the Fairchild Engineering Test Laboratories, the structure of another “X” model had been “getting the works” for the past four months. Without leaving the factory area where it was encased in a huge steel fixture, this static test plane had, in effect, been “flown” through turbulent gusts, up-drafts and downdrafts that imposed loads far beyond any condition likely to be encountered in actual flight. Engineers tugged, and twisted and pulled at the plane with complicated devices from every angle to see how much strain it could take before something gave way.

Results of the four-month “inquisition” were detailed and exhaustive, but they merely substantiated a lot of facts about the XNQ-1 its designers already knew. For instance, they had a pretty good idea before it was built just how far, how fast, and how high it would fly, and they had already predicted with remarkable accuracy the strain that could be safely imposed on each part of the planes structure.

Why, then, with such accurate calculations possible, is static testing of a new design necessary? Why cannot a designer simply follow the course of the architect and incorporate a very wide margin of strength into his new aircraft?

The answer to these questions lies in the close relationship between aircraft weight and strength. Since the time of the Wright brothers in the infant days of aviation, designers have grappled with the problem of obtaining maximum strength-weight ratio. To keep weight at a minimum, the designer must also keep the strength of the plane within certain specified limits. He must calculate the loads likely to be imposed on the plane during extreme conditions of flight and provide a safe margin of required strength. Bad design does not necessarily result in a weak airplane; it may result in too much strength and, consequently, too much weight.

It thus falls within the province of the static test engineer to substantiate the calculations previously established. The design engineer knows how much strain each part of the structure ought to with stand; the static test engineer knows, by actual testing, how much it actually will withstand.

The testing of a new type airplane begins early. Before the design drawings are fully completed, tests are made on materials and small structural components to furnish basic strength data for use in selecting the proper material and shape for each structure. Further along, as particular parts of the plane are constructed, such as wing ribs, spars or control surfaces, these are tested to prove their strain-resisting qualities. Finally, the completed prototype is tested to failure for a final verification on the over-all strength.

Drop testing the XNQ to determine the strength of its landing gear. Readings are taken electronically at the impact.

In the case of the XNQ- I, a complete airplane was built under Navy contract for the static test program. The landing gear was tested by dropping the plane from a height to determine shock-absorbing characteristics and by the application of static loads to simulate severe ground loops and other complicated loading conditions. The wings, fuselage, control surfaces, and flight controls were tested functionally and by application of static loads sufficient to produce failure.

For the landing gear drop tests, the fully-loaded airplane was suspended on a flexible cable sling with a pick-up point directly above the fuselage. The sling was adjustable so that the point of suspension could be brought directly over the plane’s center of gravity when it was in the exact attitude desired for the drop. An electrically-operated release mechanism was placed just above the cable sling so that when the plane was raised to the proper level, it could be quickly released from the hoist and dropped freely.

To determine what happens to the landing gear when the plane is ‘dropped in” from various heights, three reaction platforms were used, one under each wheel. Instruments within the platforms recorded the force of the drops on a continuously moving film in a recording oscillograph. The motion of the shock-absorbing struts was also picked up electrically and recorded on the same film. By correlation of all the records on the film at the same instant, it was possible to learn the reaction at each wheel, deflection of the shock absorbers, and critical stresses at each one hundredth of a second intervals. These records revealed the exact efficiency of the landing gear as a shock absorber, and further indicated how the XNQ-1 will stand the gaff when fledgling pilots try to land the plane 15 or 20 feet above the runway.

Wing loaded to simulate dive pullout.

Probably the most perplexing problem to be expected in planning a static test program is the difficulty of duplicating with reasonable accuracy the many types of loads—or strains—that are imposed on an aircraft during flight. The test engineer is first concerned with determining the degree and distribution of the loads. Then, since these loads are actually distributed over the entire airplane, he must devise means of duplicating them by an equivalent arrangement of concentrated loads in order to simplify the static loading of the structure. The old method of loading the wings, fuselage and tail surfaces with sand bags made it possible to distribute the rest loads over the entire structure, but this method was cumbersome and time-consuming and had the serious disadvantage of producing complete failure of a test part, once maximum load was reached.

In static testing the XNQ-1, loads were applied almost exclusively by hydraulic jacks and measured on elastic dynamometers. The loads were carried into each part of the structure by sponge rubber tension pads cemented to the loaded surface, 108 being used on each wing panel. Sheet metal plates were cemented to the tops of these pads and loads were applied by means of a series of steel links interconnected with a lever system.

Since loads had to be applied in a number of different directions simultaneously, the airplane was placed in a specially-designed fixture and secured on a four-point flexible support at the outer points of attachment of the center section to the fuselage. At all other points, the plane was entirely free from attachment to the fixture. During the major tests, equilibrium was maintained at all times; that is, when a certain degree of upward load was applied to the wings, an equivalent downward load was applied to the engine mount, fuselage and tail, and so distributed that these opposing loads were in perfect balance, thus creating, artificially a condition encountered in actual flight.

Taking deflection readings as the XNQ’s wing is subjected to flight-simulating loads.

An elaborate system of instrumentation was used by Fairchild test engineers to measure deflections, stresses and strains while loads were imposed on the airplane. The measurement of strain required the use of an electronic device known as a resistance wire strain gauge. Approximately the same size as a postage stamp, a strain gauge is capable of detecting changes in dimensions as slight as 1/50,000th of an inch. These gauges were attached to 150 critical points on the surfaces of the wings and fuselage and recorded the degree of strain at each point on an oscillograph connected to the gauges by miles of wire. Deflections were measured by reading graduated scales suspended from various parts of the structure. Using ordinary engineering levels or transits, technicians sighted the scales—which resemble yardsticks— with the airplane perfectly level and balanced, and again under various load conditions. The difference in readings obtained, indicated how far any part of the plane had been deflected or pulled out of line vertically.

During the course of a static test program, extreme caution must be exercised to preserve the plane intact, even though a large number of tests are scheduled to be performed up to ultimate or failing loads. For example, the wings of the XNQ-1 prototype were tested in five major loading conditions, three of which were carried to ultimate design load and one to failure. Since the airplane was designed to fail as close as possible to ultimate load, it was quite possible that a failure could occur when not desired, thus requiring extensive repairs to make the structure ready for further testing. To reduce this possibility to a minimum, accurate control of applied loads was maintained to such an extent that each failure could be predicted before it occurred. As each test was performed, a team of stress engineers kept close watch on the test instruments for indications of failure, and in some instances, a particular test was stopped prior to completion to add necessary reinforcement to the structure.

Horizontal stabilizer under heavy load begins to wrinkle.

At the conclusion of static tests on the wings, fuselage, landing gear and tail of the XNQ-l, ultimate design load had been applied to all parts of the structure and failing load in a number of areas. The wing failed upon application of the ultimate design load in high angle of attack, as anticipated. The fuselage sustained 115% design load before an indication of imminent failure occurred. All control surfaces withstood ultimate design loads. In the drop tests, the gear took the specified heights of drop with out exceeding the design yield load factor of five Gs.

As is the case in any static test program, a wide range of strength-weight data resulted from the testing of the XNQ-1. But, of more importance from a safety standpoint, this new military training plane was found ready to take the toughest punishment a student pilot can dish our and still come back for more.