Radio Aids to Air Navigation.-Radiobeacons using a sound signal to indicate whether a plane is on its proper course have been developed by the bureau and are already used to some extent on the national airways. Such signals require much attention from the pilot. Efforts were, therefore, concentrated upon the perfection of a system using a visual indicator that will show at a glance whether the proper course is being followed. In that system, the beacon transmits two signals modulated at different frequencies, and these actuate two vibrating reeds placed on the instrument board of the plane.

During the year the transmitting equipment for this visual beacon has been much simplified, and a complete beacon equipment has been built for installation at Richmond by the airways division of the department.

Methods have been developed that permit the beacon courses to be placed at any desired angle. This permits the simultaneous fitting of the courses to several airways. In order to serve airports having many converging courses, a 12-course directive beacon has been developed. This requires three modulation frequencies, which are obtained from rotating machines. A beacon of this type has been installed at College Park, Md., and found to be completely satisfactory. The airways division is building seven beacons of this type for use on the airways.

The vibrating reed indicator has been improved in operation and sensitivity. A simple temperature-compensating device that makes the operation of the reed entirely independent of temperature changes has been developed. For use with the 12-course radiobeacon a 3-reed indicator has been developed. This makes it possible to fly on any one of the 12 courses and to identify each course. Specifications have been furnished to manufacturers interested in these instruments.

Receiving sets suitable for both radio beacons and telephony have been developed by several companies, with the cooperation of the bureau. In order to use radio satisfactorily on planes, interference from the engine ignition system must be eliminated. Various commercially available ignition shielding arrangements have been studied. All were found to have inherent defects. In cooperation with the aircraft and radio industries, a shielding assembly that is both satisfactory from an airplane operating standpoint and effective from a radio standpoint has been developed. A conference on ignition shielding was held at the bureau on June 11, 1929, for the purpose of coordinating available information on the subject, and of stimulating the standardization of shielding practice. This conference dealt chiefly with standardization of magneto shielding, the construction of ignition cable, the use of moulded cable or separate wires removable from the shielding, spark plug shielding, and standard tests to determine efficiency of shielding.

Soundproofing of Airplane Cabins. With increasing interest in aviation, comes a demand for an airplane cabin in which a reasonable amount of quiet prevails. The problem is more difficult than that of soundproofing a room because of the necessary limit in weight. It has been known for several years that the opacity of a homogeneous wall to sound is practically proportional to the logarithm of its weight per square foot, and that composite walls are greatly superior to homogeneous walls of the same weight. By a proper choice of materials, a composite wall has been developed which, with

a weight of only 1.24 pounds per square foot, reduced the noise (in flight) within the cabin until it was comparable with that inside a railway coach in motion. For the same reduction, a homogeneous wall would weigh 3.2 pounds per square foot.

Testing of Types of Commercial Airplane Engines.-Members of the bureau's staff have assisted individual engine manufacturers in solving technical problems in connection with engine design and engine testing, and have presented several papers dealing with the objects of, and the lessons that may be drawn from, tests of types of commercial airplane engines. A field representative of the Bureau of Standards has recently been designated as liaison officer between the Department of Commerce and those engaged in the development, testing, production, and inspection of military, as well as commercial, airplane engines.

The number of engines submitted for test has so increased that it has become necessary to supplement the original torque-stand unit at College Park, Md. with a second testing station at Arlington Farms, Va. Future work will be concentrated at the Arlington testing laboratory where an office, shop, and four torque-stands will eventually be available. With three stands in service, it will be possible to test, on the average, one engine a week.

By the end of June, 1929, tests had been undertaken on 29 engines 13 failed, 5 were withdrawn, 9 were approved, and 2 are still under test. Including engines approved on the basis of the Army or Navy tests, 25 engines have received approved-type certificates.

Publications. Laboratory and Service Tests for Engine Safety, H. C. Dickinson, Trans. 17th Annual Safety Conference, October, 1928.

Development and Testing of Commercial Aircraft Engines from the Point of View of Safety and Regulation, H. C. Dickinson, Proc. Int. Civil Aeron. Conference, December 12-14, 1928.

Commercial Aircraft Engines, H. C. Dickinson, Aero Digest, April, 1929. Type Testing of Commercial Airplane Engines of Medium Power, H. K. Cummings, Trans. A. S. M. E., Aeronautical Engineering, 1, p. 45, April-June,

1929.

Supercharging of Aircraft Engines.-After extensive alterations to the altitude laboratory, a Curtiss D-12 engine has been tested under conditions corresponding to normal altitudes from sea level up to 30,000 feet. Tests have also been made under ideal supercharging conditions. The results show the gain to be anticipated from supercharging if no power were required to drive the supercharger. They also show the effect of exhaust back pressure on power.

Gaseous Fuels for Aircraft Engines.-Using two different engines, on one of which the compression ratio was increased to 9:1, tests on nine gaseous fuels, ranging from hydrogen to butane and including four mixtures having approximately unit specific gravity, have been made for the Navy. A theoretical analysis based on an ideal Otto cycle has also been made to determine the relative performance to be expected from pure and from mixed gases, as compared with such liquid fuels as octane. The engine tests, though not numerous, agree with the theoretical analysis in showing that the gaseous hydrocarbons compare favorably with gasoline as regards power and efficiency. Tests at the high compression ratio indicate that the gaseous fuels are superior to most liquid fuels as regards antiknock value.

Standard Ailerons for Lateral Control. Since a large proportion of commercial aircraft are now using and will probably continue to use the conventional type of ailerons for lateral control, a study of the properties of such ailerons as affected by their size and shape and the shape of the wing section to which they are attached assumes importance. The Bureau of Standards, in cooperation with the aeronautics branch of the Department of Commerce and with the National Advisory Committee for Aeronautics, has completed a study of the rolling and yawing moments produced by various angular displacement of the ailerons at several angles of pitch, including angles above the stall. The United States Army 27 and Clark Y wing sections have been used. The measurements were made on wings of 5 feet span and 10 inches chord, and at speeds up to 80 feet per second.

By studying the ailerons singly, it has been found that upward displacements are much more effective than downward displacements, and that the characteristics of the control at large angles of pitch would be improved by using single ailerons moving only upward. This type of control introduces some mechanical complication, and some compromises in the selection of size and travel of the ailerons will have to be made. Its possibilities, however, are worthy of further study.

Embrittlement of Sheet Duralumin.-Laboratory and exposure tests continue to support the tentative recommendations based on earlier results. Embrittlement of duralumin by corrosion during service has been shown definitely to be related to heat treatment. Duralumin quenched in cold water and allowed to age at room temperature is very superior to that quenched in hot water or oil, and to that aged at an elevated temperature. This fact is now recognized in the specifications of the Army Air Service. Plain, uncoated duralumin is recommended only for very mild service conditions. A protective coating of some kind is practically always necessary. So far, no thoroughly satisfactory protective coating other than pure aluminum has been found. Aluminum-coated duralumin sheet, in which the coating constitutes an integral part of the sheet, is now commercially available. Such material is extremely resistant to the corrosive influences which cause embrittlement. Even under the combined influence of corrosion and repeated flexure, the most severe set of conditions that can be met, the aluminum coated duralumin is greatly superior to the ordinary product. The commercial development of this material is based on fundamental data obtained by the bureau.

Publications.-National Advisory Committee for Aeronautics, Technical Notes, Nos. 282-285, inclusive, and Nos. 304 and 305.

Fatigue of Alclad Duralumin. In cooperation with the Aluminum Co. of America the flexural fatigue resistance of Alclad (aluminum coated duralumin) is being studied, using the four flexural fatigue machines specially designed for testing sheet and strip material.

Over 118 specimens obtained from both corroded and uncorroded Alclad, and from comparable duralumin sheets have been tested. Some of these specimens have been subjected to over 100,000,000 cycles of stress.

The results indicate that the corroded Alclad specimens have approximately the same life as the uncorroded specimens, while the

life of the corroded uncoated duralumin specimens is less than that of the uncorroded. This is added evidence for the greater durability of aluminum-covered duralumin.

Tests of Magnesium Alloys.-Supplementing the study of the corrosion embrittlement of duralumin, similar weather exposure tests are under way on magnesium and some of its alloys, in sheet and cast form. The work has not progressed very far. The corrosion resistance of magnesium alloys is decidedly lower than that of duralumin. These alloys must be protected by suitable coatings but no thoroughly reliable coating comparable to the use of pure aluminum on duralumin has yet been found.

Welded Joints in Tubular Members for Aircraft.-The general use, in airplane construction, of steel tubing welded at the joints has created a demand for more complete and reliable data on the strength and other properties of welded joints in structural members. This investigation was undertaken to assist the Aeronautics Branch of the Department of Commerce in its main function of promoting safety in aviation.

The strength of all types of joints in common use and the relative worth of each are being determined, and new types will be developed which may be better than any of the present ones.

Testing of the first group of joints, 165 in all, has been completed. These joints were designed from data supplied by manufacturers, welded under controlled procedure, and tested in special fixtures. Although no quantitative data are available for this report, several conclusions based on the observations may be stated. The most serious problem encountered in designing a welded joint is the formation of cracks in gusset plates after cooling. It is evident that the problem is one of design as well as of welding technique. Best results seem to be obtained by making the thickness of gusset plates somewhat greater than the tube thickness and keeping the plan area of the gusset as small as possible. Whenever possible the design should allow for movement of the members, as the joint cools.

The results of this first series of tests are being prepared as a progress report for publication as a technical note of the National Advisory Committee for Aeronautics.

Aircraft Tubing. An experimental study of the strength of aircraft tubing has been continued. A very careful and time-consuming study of the experimental data has led to a newer and more satisfactory analysis of them.

In this analysis, the experimental data are combined on a semitheoretical basis that enables one to compare directly columns made of materials of markedly different physical properties.

In this way it is possible not only to combine in a single chart data concerning tubes of widely different properties and of different dimensions, but also to set higher safe limits of stress than were possible under the older method for analyzing the data. For certain tubes in the range of high l/r ratios, this increase amounted to over 100 per cent.

Although the experimental results were restricted to a limited range of dimensions, the consistency with which the experimental points fall close to the average curves makes it seem probable that this method will prove generally applicable to tubes of other diameters and wall thicknesses.

A study of the results showed that the strength of the tubing depends greatly upon the yield point of the material. Although the results showed that the ultimate strength of the tubing tested was, probably, as high as could be obtained with commercial tubing, the yield point of the tubing was considerably higher than the specified minimum. If the requirements as to yield point were raised, higher stresses could safely be used in design; that would reduce the weight of tubular aircraft construction.

Publication.-Strength of Tubing Under Combined Axial and Transverse Loading, National Advisory Committee for Aeronautics, Technical Note No. 307, 1929.

Parachute Fabrics.-The important physical properties of parachute fabrics are weight, breaking strength, tear resistance, tension, stretch relation, resilience, and air permeability. Methods for testing resilience and air permeability having been developed, it is now possible to test all of these properties. Thirty-six different silk fabrics of American manufacture were compared with fabrics of foreign origin which had proved by trial and extended use to be suitable for parachute construction. Contrary to the belief that domestic woven fabrics were not suitable for this purpose, it was shown that many of the domestic fabrics are entirely satisfactory, and some are superior to the foreign product. Specifications for the manufacture of silk parachutes were prepared, and service tests carried out by the Navy Department demonstrated the suitability of the material.

AUTOMOTIVE ENGINEERING

Spark Ignition. A special laboratory has been fitted up for a comprehensive study of spark ignition. The following items regarding equipment, methods, and recent observations and activities are worthy of mention: (1) A single-cylinder test engine is available for comparing the effectiveness of different spark plugs and spark generators, under carefully controlled operating conditions. (2) A method, reported to the Navy Department last winter, for ascertaining the physical state of the fuel in the engine cylinder or manifold at any point in the cycle, uses three quartz windows, a stroboscope, and a beam of light. Scattering of the light reveals the presence of unvaporized fuel droplets. (3) If sparks are passed through a glass tube immersed in CO2 snow and containing a mixture of oxygen and hydrogen that is too lean for flame propagation, local burning will produce microscopic ice particles. Scattering of light may be used to determine the volume of charge affected by sparks of different characteristics. (4) By passing sparks through an explosive mixture of oxygen and hydrogen at very low pressure in a discharge tube cooled with liquid air, the region where ignition takes place can be determined from the location of the ice formed and the number of molecules reacting can be determined irom the change in pressure. The relative number of molecules reacting per spark appears to offer a means for comparing the over-all effectiveness of different sparks. (5) A preliminary survey of aircraft ignition shielding, to prevent interference with radio communication, and its relation to reliability of ignition has been made and reported to the National Advisory Committee for Aeronautics. Preparations are being made to undertake the routine testing of "shielded harnesses."