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Aviation History
1942
1942 - 0100.PDF
38 FLIGHT JANUARY 8TH, 1942 boaFd of the machine's axis. I should imagine that this layout would give full scope to a large engine like the Sabre, and would suggest contraprops as there would be plenty of room for their gearing. The undercarriage need not be any higher than usual if the two rear wheels of the tricycle are placed as near the plane of the prop, as possible. " FLT. LT.' CONVOY FIGHTERS Why Not Retractable Floats ? I HAVE been interested recently to read about the catapult ing of fighters for the defence.of convoys, f think that something might be done to develop a specialised fighter sea plane or possibly a small, fast flying boat. To my mind, the type that is required is a fast, well-armed seaplane. The main disadvantage of-the usual seaplane is the drag of the floats and struts; so, as we have retractable landing wheels. CORRESPONDENCE why not a retractable float gear? This could be a main single float retracting backwards and upwards into a recess in the bottom of the fuselage. This fighter would also need retract able wing-tip floats, possibly of a type somewhat like those of the Catalina or, possibly, of the Do.26. A polyhedral similar to that of the Miles Master would mean that these floats would not have to have very long struts. It has a twin-row radial engine for ease of maintenance, and follows conventional lines apart from the above-mentioned points. 1 trust that Flight will continue the good work it has done for so long. VERNON LACEY (Apprentice of the De Havilland Aircraft Co., Ltd.). TWO-ENGINED EFFICIENCY Demonstrated by the C.W.20 THE all-round efficiency of the Curtiss-Wright St. Louis air craft, described in ^he December 18th issue of Flight, is so , far in advance of other four-engined aircraft of similar dis posable load, and the operating costs and complication are reduced to such a degree that one is very much inclined to the belief that two engines are better than four in an aircraft of medium size. Maximum speed of the C.W.20 when mounting Cyclone 14-cyl. engines is now 262 m.p.h. at 13,000ft., which is appre ciably faster than the Boeing four-engined " Stratoliner," although the take-off power of the latter aircraft is 1,000 h.p. more than the St. Louis. Again, the initial rate of climb of the C.W.20 is 1,500 ft./minute with a modest power-loading of 11.76 lb./b.h.p., as against 1,200 ft./min. for the "Strato liner," which has a power loading of 10.23 lb./b.h.p. Disposable load of the St. Louis—a transport version of the C.W.20—at an A.U.W. of 45,000 lb. is no less than 21,455 lb., or considerably more than double that of the Douglas D.C.3 with only 50 per cent, more power. Although the D.C.4 is more than 50 per cent, more powerful than the C.W.20, the maximum speed of the former is 265 m.p.h. at 16,900ft. ; cor rected to an altitude of 13,000ft., the DC.4 is slower and has less disposable load despite its 5,200 h.p. Quite apart from the lower drag of the twin-engined installa tion, the aerodynamic merit of the C.W 20 fuselage has been proved greater "than that of any other streamlined body tested in the N.A.C.A. wind tunnel. " It has beenknown for a long time that the normal-type windscreen with flat panels may add as much as 25 per cent, to the fuselage drag by breaking up the airflow over the latter. Viewed in plan elevation, it is obvious that the acceleration of the airstream over the body of the C.W.20 is much less violent than for the " Stratoliner." Admittedly, the streamline form is a difficult one to mass produce; the Spitfire was no exception to this rule. But we can thank our lucky stars for the idealism of Mitchell that ignored manufacturing difficulties. Had it not been for this designer's idealism, we might have lost the Battle of Britain. Whilst admitting that bomber fuselages are simpler to fabricate in their present non aerodynamic form, the new Martin B-26 bojnber is now bring mass-produced with a beauti fully formed fuselage that must add considerably to its per formance. One cannot help wondering where we will be, after the war, on the score of efficiency if we continue to let power- driven gun turrets massacre our designs aerodynamically. JOHN W. MORRISON. CAMERA CURIOSITY A Matter of Angular Velocity A PUZZLED SCHOOLBOY " asks why, in so many photo graphs of flights oi similar aircraft, the airscrews appear in similar positions. To answer him fully would take several complete issues of Flight. As a painter of aircraft I have come in contact with this problem before and have studied many photographs as well as actual aircrews in an attempt to solve the problem of painting the illusion of airscrews in motion, f will briefly sketch the outline of my conclusions, but I must admit they are by no means final. I am, of course, more interested in the mental image rather than the photographic impression, but both are complementary to each other. It has already been pointed out by Mr. Norman S. Aston that the average exposure is sufficiently long for the airscrew to complete at least one revolution; therefore, for the time being the limitations of exposure can be overlooked. For an image to appear on the plate, that image must remain over any one point for at least a definite minimum perio faj^ This brings us straight to the point "angular velocity. Figure 1 depicts a camera, AB represents a motion from left to right, and a, b the image of that motion. If the speed of that motion from a to b is too great no image will appear. This is simple "angular velocity." Consider the airscrew to be a stick; Figure 2 shows in perspective the disc made by this revolving stick and also the stick itself in two positions at right angles to itself. Thus the disc is cut into four equal segments—AB=BC = CD=DA. In perspective the arc AB is much shorter than DA, yet the period of time from D to A and A to B is the same. Therefon , the " angular velocity " from D to A is greater than from A to B, and points X and Y are roughly the points of lowest angular velocity '' from this viewpoint (these points vary according to the viewpoint). It is obvious that whatever the speed of the airscrew the image will be most powerful in the region of line XY. The images of a row of similar revolving sticks seen from nearly the same viewpoint will be very nearly the same, the error will be less than the untrained eye can see. Now for the complications. Owing to the pitch of an air screw, one blade seen in perspective will have a greater pro jected area than the other (Figure 3). Area has a great deal to do with "angular velocity" and the blade with the wide- image will be magnified in intensity. The curious curved images of blades can be accounted for by this fact in conjunc tion with local "angular velocity.*' The image of a thne-bladed airscrew is more complicated but follows the rule; the blades appear at points of low "angular velocity," which causes great distortion. Whatever the complications, they will be common to all the airscrews in question and, therefore, if one is altered all will be. Finally, if the exposure is less than one airscrew revolution of the slowest airscrew, then it would be a supernatural fluke if the images were-similar, as all the airscrews would have to be perfectly synchronised. C. RUPERT MOORE, A.R.C.A. (Lond.).
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