Chemical Research For the Airplane Industry . . YORK
Head of the Chemical Section, Airplane Division, Research Laboratory, Curtiss-Wriojit Corp., Buffalo 5, . .
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AIRPLANES are compelled to operate * * under extreme weather conditions. Temperatures encountered may range anywhere from 160 F. on the ground in the hot desert sun in North Africa down to 65 F. at an altitude of 40,000 feet. An airplane must be built to withstand these extremes. Temperature changes may be quite rapid, perhaps amounting to as much as 200 F. in the space of a few hours, or even minutes in a dive.
Pressure changes also are important. At 33,000 feet the pressure is one fourth the sea level pressure. At 48,000 feet, the pressure is only one eighth of an at-mosphere. Such low pressures cause vapor lock difficulties in fuel, oil, and hy-draulic systems. At high altitudes the conventional carbon motor brushes wear down within a few minutes.
Structural failure or engine failure in land vehicles rarely results in extensive damage. Similar failure in an airplane usually results in more or less complete destruction of the airplane and frequently loss of life. Materials in airplanes must not f au.
Every part of a plane must be designed to provide the minimum weight and still supply the strength required. For every pound of increase in the weight of the
. . York plane, there is a decrease of a pound in payload. Various estimates ranging from $100 to $500 per pound have been made on the value of that pound, taking the average life of the airplane into consideration. Using the lower figure of $100 per pound, if only 1 per cent of a 45,000 pound gross weight cargo plane could be transferred from empty weight to pay-load, the saving effected during the life of the plane would amount to $45,000.
The most important factors to be remembered for improving airplanes are expressed in the simple axiom: "Make it lighter and make it stronger."
To get the maximum results, the best utilization of the best materials must be made. It is the function of the research chemist and the metallurgist to provide the beet materials, and it is the function of the engineer to utilize these materials in the best possible manner. For real progress in aircraft, the chemist and the engineer are inseparable.
In the discussions of specific items for research which follows, it is recognized that research and development are being conducted on many of these items at the present time. Sonne problems mentioned may have their solutions already completed. In such cases, the solutions are not generally known or have not been generally accepted! by the aircraft industry. However, even where claims might be made that con* pie te solution has been achieved, it must be borne in mind that perfection in a ire rat ft will never be reached.
Metals The production of lighter and stronger
alloys will always present research problems of primary imterest to aircraft manufacturers. Possibilities for improvement are always present t, and trie light alloys used today are no exception to this rule. Magnesium has received a large amount of publicity a s an. attractive construction material for aircraft. At the present time, magnesium* alloys have definite limitations. Stress corrosion cracking is probably the most; important problem yet to be solved. Als*> of great importance is the lack of uniformity of physical properties of magnesium alloys of the same chemical composition. A third important factor which must tne improved before magnesium alloys cam be seriously considered as aircraft structural materials is the low compressive yield strength.
There is no question that aluminum alloys constitute t-he best structural aircraft materials available. However, with these, too, there i s room for improvement. Although greater strength is always desired, a feature which requires more immediate attentiom is the necessity to increase salt water corrosion resistance.
Powder metallimrgy offers interesting research possibilities. The production by means of a briquestting and sintering process of a variety of parts, sometimes involving fairly complicated shapes and cou
t h Research Laboratory of the Curtta-Wright Corp., AitpUnt Division, Buffalo, M. Y.
C H E M I C A L A N D E N O I M E E R I N G N E W S New Text
tours, results in a tremendous time-saving over the conventional machining meth-ods. A considerable saving is also ef-fected in the elimination of machining scrap. One attractive feature of powder metallurgy is the possibility of controlling the physical properties and producing tailormade parts. Oil-less bearings pro-duced from powdered metal have been quite universally accepted. Chemical problems in this field consist in the proper use of the lubricants, the plating of powder parts, and in developing methods for pro-ducing the metal powders.
The various metal-treating processes which include both heat treating and methods for passivating metals offer room for improvement. Chemical treatment to provide passivated surfaces is becoming more important. In the preparation of aluminum surfaces for spot welding, me-chanical means such as wire brushing are being replaced by chemical processes. The advantage of chemical treatment over mechanical surface preparation lies princi-pally in the increased length of time during which the chemically prepared surface re-mains effective.
Improvements in electroplating are an-nounced regularly and such improvements will probably continue and will contribute to better airplanes by helping to control corrosion. The production of self-lubri-cating dies for press forming by suitable electroplating is a distinct possibility.
The improvement of slushing com-pounds to protect machined articles from corrosion during the process of manufac-ture is a suitable subject for research.
From a basic research standpoint, the actual mechanism of corrosion of various metale and alloys, the physical chemistry involved in the surface treatment of alloys for spot welding, a basic study of passi-vated surfaces, physical chemical con-siderations in powder metallurgy, and the chemistry of alloys are all-important sub-jects.
Laminates and plywood have not yet been accepted as general structural air-plane materials. A great many limitations must be removed and a great number of problems must be solved before plastic constructions will be generally accepted.
Whereas for ordinary plastic applica-tions the strength values most often stressed are the tensile strength and impact strength, for aircraft structural design compressive strength is probably of the greatest importance. Physical properties of plastics for aircraft structural design purposes can be rated in approximately the following order of importance:
1. Specific compression yield strength 2. Specific compression ultimate
strength 3. Specific tensile yield strength 4. Specific tensile ultimate strength 5. Compression modulus of elasticity . Shear strength (is always equal to at
least one-half tensue strength)
7. Bearing strength ( 4 per cent defor-mation is the bearing yield)
8. Fatigue strength in bending 9. Forming properties
The impact strength i s of definite im-portance; however, methods of deter-mining impact strength so as to be of value iu aircraft design have n o t yet been well developed.
Considerable improvement in laminated plastics' is anticipated. T h e proper use of re-enforcing materials will result in greater strengths than have so far been achieved. Distinct possibilities are offered by metal-wood and metal-cloth laminates in which the metal takes the outer fiber stresses. The low-pressure laminating resins are es-pecially interesting because of the less specialized and less expensive equipment required to use these materials. The strengths obtained with suitable lami-nated constructions using these resins have been increased to a point where they are receiving serious consideration for structural use in aircraft.
It is well known that in many cases thin films of plastic materials possess certain desirable properties to a much greater ex-tent for a given weight than do greater thicknesses. For example, the electrical properties of cellulose acetate may some-times be utilized to better advantage when the cellulose-acetate film is applied to both sides of a layer of wood which acts as the supporting medium. Similar laminated constructions may serve to utilize to best advantage certain outstanding specific properties in other plastic materials.
Increasing the resistance of synthetic rubbers to high aromatic fuels is an im-portant problem. For applications such as tubing for fuel lines, flexibility a t low temperature should be retained. High water resistance is also a requirement because under conditions prevalent in
service, fuels are generally saturated with moisture.
Plastic bearings are known to be highly resistant to abrasion and also resistant to salt water corrosion. These properties may find a use for plastic bearings in air-craft.
One of the principal advantages of using plastic-molded and laminated constructioo on airplanes is the lower manufacturing cost. In these cases the plastic materials become part of the finished plane. Plas-tics tooling also effects manufacturing economies. Cast resins are being used for forming dies, piercing dies, jigs, and fix-tures. The opinion has been expressed that cast resins are too soft for hydro-press forming, and are too brittle for hand form-ing. There is no question that improve-ments can be made to solve these draw-backs. The development of a clear trans-paient resin which would be hard and tough and which could be cast by simple casting procedures to produce optically clear sheets would be an important con-tribution to better airplanes.
Developments in glues and laminating resins are being followed by airplane manufacturers with great interest. There is considerable room for improvement in all types of adhesives used in aircraft manufacture. The ideal adhesive is stil! a long way from accomplishment. A num-ber of recent developments in adhesives provide materials for the high-strength bonding of metal to metal. While there is no question that such applications will find extensive use in other fields, their general use in aircraft applications may not follow because of the pressures and temperatures required to effect strong bonds, the high temperatures producing a
A view of the Curtis*-Wright carso plane assembly line
V O L U M E SS, N O . S . J A N U A R Y 2 5, 1 9 4 4 87
Fighter plane assembly line in Curtiss-Wright plant
demand which would make large-scale production of complicated aircraft parts prohibitive. The ideal adhesive would be thermosetting at low temperatures and would require only low contact pressures. The adhesive which comes closest to meeting these requirements is a resorcinol-formaldehyde resin which will cure at room temperature. However, so far this material can be used only with wood or other polar materials.
The development of a similar product which will be more universally applicable to all construction materials and still give high bond strengths would be an ideal solution.
One of the objectives of airplane manu-facturers has been the perfection of a true monocoque construction. The fuselage of the deHavilland Mosquito bomber ap-proaches this objective by using a vir-tually all-wood construction in which the plywood skin is wrapped on diagonally. The accomplishments of this particular airplane are fairly well known. Successful monocoque construction might be achieved by the use of a suitable resin and filler, such as wood or fiberglas, together with a style of construction which will utilise the properties of the resin and the filler to best advantage. As improved resins and filler materials become avail-able, they will undoubtedly, require new fabrication techniques.
Surface Coatings Camouflage paints can be improved by
reducing weight, at the same time main-taining .the hiding power required.
After the war, fabric-covered airplanes and helicopters may be produced in greater quantity. For these; cheaper and better finishes will .be required. Among the various possibilities for improvement may be mentioned the water-emulsion paints and improved coatings for fire-
proofing and waterproofing. Coatings which will reduce or eliminate
sunlight reflection from windshields or side panels will make it more difficult for the enemy to spot our planes. Night flying will be made more comfortable by reduc-ing window glare.
Materials are required foi more econom-ical and improved corrosion protection for magnesium and aluminum alloys. Pigments which will extend or replace the chromtes used at present will be of value.
Lubricants As more powerful planes are built,
greater demands will be made on lubri-cants. Oils of higher viscosity index, lower pour point, and increased chemical stability will always be desired. Greases with improved low-temperature charac-teristics, greater load-carrying ability, greater resistance to oxidation, and elim-ination of bleeding are in demand. To help obtain higher rates of production, cutting oils which in themselves inhibit corrosion and which do not develop ran-cidity are required for use in the various machining operations.
Hydraulic Fluids Improved hydraulic fluids will always
be in demand. A minimum increase in viscosity with decrease in temperature, high chemical stability, noncorrosive qual-ities on metal tubing, inertness to gasket and packing materials, ability to with-stand reasonably high temperatures, and nontoxicity are the requirements for a superior hydraulic fluid.
Power The present airplane engine develops
fairly high thermal efficiency, running up to about 30 per cent when operating at best economy. Improvements in the air-
craft power plant may be initiated from one of three general avenues of approach, but these are closely interrelated. De-velopment work for the purpose of in-creasing tbe performance of engines through the use of superior fuels has been producing highly desirable results in re-cent years and will continue to do so. The recently announced new process for the production of triptane is another step for-ward in this direction. The second avenue of approach lies through the use of higher engine operating temperatures, which will require the perfection of greater heat-resisting alloys for cylinder walls, pistons, and bearings. What these alloys will con-sist of is not known. Tellurium, tungsten, and molybdenum alloys may be mentioned as interesting possibilities. The third avenue of approach lies in redesign o...