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Aviation History
1942
1942 - 0308.PDF
122 FLIGHT FEBRUARY 5TH, 1942 CORRESPONDENCE (Continued) RANGE AND FUEL ECONOMY Mr. Hinings Taken to Task IN his artick in Flight of December nth, 1941, Mr. C. L. Hinings stated that "Flying at maximum lift/drag ratio gives greatest endurance but not greatest range." This should read "Flying at maximum lift/drag gives greatest range but not greatest endurance." Similarly, the paragraphs marked (2) and (3) are incorrect. Range.—The energy of the fuel is used up in overcoming the drag of the aircraft over a certain distance, hence the greatest range is obtained when the drag of the aircraft is a minimum, i.e., when the overall lift/drag ratio is a maximum. Endurance.—The energy of the fuel is used up in overcoming the drag of the aircraft through a certain distance, but the distance flown is equal to speed multiplies1 by the endurance. •0 that the maximum endurance is obtained when the drag multiplied by the speed is a minimum. This occurs at a lower speed than that coi responding to flight at maximum lift/drag ratio. A. H. CRAWFORD, Blackburn Aircraft, Ltd., Assistant Chief Designer. ' Dumbarton An Attempt to Help the Pilot ONE of the most valuable contributions that a technical journal of the standing of Flight can make to the common purpose is to wheedle out some of the accumulated knowledge possessed by us readers in the form of letters in the Correspond ence columns. The article by me which you published in your issue of December nth, 1941, seeking to " marry " the power plant to the aircraft (pilot as best man), begins to serve its pur pose in that comments have been drawn from Mr. Hick and Mi. Tennant, both obviously aerodynamic chaplains. (See issues of December 25th, 1941 and January 1st, 1942.) Incidentally, it is only fair to myself to point out that J iid not seek to write an article, in which case I should have sreferred to tackle the subject from a different standpoint to ' Jinair," and would have avoided using mathematical sym bols such as L/D entirely. My contribution was intended for -.he Correspondence columns, as a modernised addendum to " Jinair's " original article. It should be regarded first and foremost as a plea to cut out the mathematical argument, eliminate the theories of aerodynamics, and attempt to give the pilot something intelligible and helpful in the actual handling of the aircraft in the air. Therefore, although, as Mr. Tennant points out, the point of minimum power on the aircraft " H.P.-required curve does not tally with maximum L/D, but maximum L'/2/D, this does not invalidate my state ment to the pilot that for maximum endurance his practical procedure will be to reduce his A.S.I, to a value at which " his aircraft is just pleasantly manoeuvrable and responsive to con trols above the stalling speeds, and at a fairly low altitude*." Furthermore, this maximum endurance A.S.I, is con siderably lower than the speed which will give him maximum range as recommended invariably by the manufacturers, after confirmation fiom the competent A.M. Research Establish ment. A further point to note is that owing to the flat peak of the range curve, a rather higher cruising A.S.I, may be recommended, as the very small decrease in range is more than compensated for by the operational advantages of the greater speed. A proper comprehension of these two aircraft speeds, each with its own importance, clears the way for the main body of my contribution, which was to indicate how best the modern supercharged engine, with constant-speed airscrew, can be employed to give the power required at each of the aircraft speeds, and, incidentally, to outline very briefly the operational functioning of the power plant. I propose leaving it to Messrs. Hick and Tennant to argue it out between them, as aerodynamic experts, whether, on the one hand, power required varies as L/72/D, or as C, in/Ca, about which they appear to differ. With all due respect, I do not think that the average pilot will be greatly enlightened. Mr. Hick arrhes at a rathei delightful conclusion that "the less petrol you use per mile, the further you can go per gallon," or, in other words, the lower the gallons per mile, the higher the miles per gallon, but it is certainly true that the specific consumptions, as shows by a family of curves over a range of r.p.m with increasing h.p., introduce an engine factor additional to the aerodynamic argument in decid ing where the peak of the range curve comes. •* C. L. HININGS. ELECTRICS versus HYDRAULICS W It. Depends on the Job to be Done I HAVE read with very great interest the article by '' Indi cator '' in the January 8th issue of your paper. This raises a question of very great interest and, indeed, of vital importance. "Indicator" is a hydraulic man, but favours electrical control, whereas I am an electrical man and, in a great number of instances, I favour hydraulic. I am not connected in any way with the aircraft industry, but am a maker of control gear. Now I think it may be stated that, generally, where it is desired to transmit a linear motion, short in length, slow in action and which must stay put in any desired position, hydraulic control should be adopted. Where rotary motion is required, especially continuous run ning, then electric motor operation is to be preferred. There are, of course, a thousand and one considerations which may affect the case for one reason or another, but where hydraulic supply or electrical supply are equally available, then the choice of the method of control should be the one mechanically best suited. Now electrical control may be made to do anything, but it has a number of disadvantages. i. The speed of these motors is usually extremely high in order to reduce weight, so that an immense gear reduction has to be effected to produce a linear movement of, say, iJEttj^. a minute or half a minute. The motor may be running a? anything from 5,000 to 7,000 r.p.m. 2. The starting current of a motor switched direct on is the same as the stalling current. 3. There are a great number of points which require inspec tion and servicing with electrical gear, the brushes of the com mutator, the gearings, lubrication of the gearing, limit switches, protective devices. In the case of hydraulics, practically only one protective device is necessary, and that is a release valve to prevent overloading. The operation of this valve does not stop the operation of the hydraulic device, whatever it may be, whereas if -you have a fuse or an overload circuit breaker for electrical control, the moment either of these operate the whole device is out of action; either a new fuse must be fitted or the circuit-breaker reclosed. In cases where a movement is to stop and stay put, or which comes up against a stop at either end, such as raising and lowering the undercarriage, limit switches have to be provided with a very careful adjustment. Every electrical man knows that these small auxiliary devices give more trouble than the main drive; they have to make and break contact at an exact moment and the contact has to be good and the break has to be clean. Such a device should, therefore, be designed with the greatest possible care. I have seen one instance, on a very well-known plane, . where the electrical control was thoroughly badly designed and unsuitable for the purpose, with the result that the operating motors were continually burning out. Although magnet coils with impregnated insulation and so on have now reached a considerable state of perfection, it nevertheless remains true that the whole operation of any of these controls may depend on a 30 s.w.g. diameter copper wire or finer, and copper is-* always subject to corrosion. Then, again, vibration is most inimical to electrical contacts and, owing to th« low voltage of supply on a plane, any loss of volts at any one place, due to bad contact, is likely to prevent the device working. '' Indicator'' suggests the duplication of electrical control wires, but it is surely just as easy to duplicate the pipe line for hydraulics, and in the case of the latter only two pipes are needed, whereas for reversing a motor, four wires are needed. Motor operation was tried, before the last war, for gun lay ing on H.M. ships and was a complete failure owing to creepage and to the extreme difficulty in arresting the movement at any exact position. An electrical motor has considerable inertia and therefore overruns after the circuit is broken unless you have some form of brake, A good instance of the correct use of electrical and hydraulic transmission is in the case of m#dern machine tools, where the main spindle is motor-operated and various motions, such as traverse, feed and so on, are hydraulically operated, and you may be quite certain that if it had been possible to do the latter satisfactorily by motor drive, it would have been adopted. As regard servicing, it should be possible to train a man to service hydraulic gear in a very short time, whereas it mavv^p take months to train a man to service electrical gear, and there are not half so many people, as "Indicator" seems to think, who are good electrical mechanics. "ELECTRIC CONTROL."
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