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Aviation History
1943
1943 - 1591.PDF
JUNE 17TH, 1943 FLIGHT 643 CORRESPONDENCE its plane of rotation than perpendicular to it, the bendin« islmost entirely in a backward direction. D Hoping that these remarks are not too far from the truth JAMES R. PALMER. TECHNICAL TRAINING Engineering Cadets and the A.T.C. I HAVE followed with interest the series of fetters and articlesreferring to the formation of an Institute of Aeronautics and advanced education as applied to the aircraft industry. I am training under the " Engineering Cadets " scheme, and I hoped when I volunteered for the course that it would lead to a career in the R.A.F. or in civil aviation after the war. However, I have found that it is impossible for cadets over the age limit of 17 years 9 months to join the A.T.C. Though very interested in aeronautics and aircraft construction, I was unable to join the A.T.C. before owing to lack of spare time from my civilian job Surely, if the Air Ministry want a certain percentage of us cadets, if we cannot leceive a specialised training in aero- nautics, can we not at least be allowed to join the A.T.C. or be attached to them as deferred service men, and receive a training in the elements of aircraft maintenance, law and administration of the R.A.F., and, if possible, some flying instruction? " ENGINEERING CADET." BENDING BLADES The Same Problem in Reverse NOW that your correspondents have cleared away Mr.Farley's worries over the backward-bent airscrew blades, perhaps they would like to tackle the opposite case. At the time of the Battle of Britain the aeronautical Press published a photo of a Ju 88 which had crashed just off our coast on what looked like mud flats or sand uncovered at low tide. The aircraft was not seriously damaged, as far as could be seen, but the airscrew blades were bent forward. How could that happen? J. RIPLEY. [Perhaps it was flying backwards!—ED.] Points That Have Been Neglected HAVING caused the slight controversy over the bendingairscrew blades, I am still convinced that no corre- spondents have yet hit the nail on the head. In Flight of June 3rd each correspondent seems to neglert a vital point. For instance, the first one conveniently forgets that the air- screw blades have pitch Then Mr. Locke does not seem to see that the denser the medium the better the blades will grip. Also, •' Heii O'Naught " made his reasoning easy by neglecting the vital considerations of airscrew efficiency. Only Mr. Hack, in my opinion, gets anywhere near a satis- factory explanation. He says that when they enter the denser medium the blades slow up. Now, when 1,000 horse-power is screwing blades through the denser medium (he takes water foi simplicity), then surely to resist this power the blades must bend at right angles to the major axis, similar to a swastika. Let us imagine an airscrew with 20 per cent. slip. Then i£ it enters a medium which allows no slip, the blades should bend forward, as the rest of the aircraft could not accelerate forward at the same rate. Anyway, in practice the blades are bent back, though the explanation is not so simple. . D. H. FARLE\. Three Simple Illustrations MAY I be allowed to add three more possible explanationsof why propeller blades bend backwards and not forwards when they strike the ground, to those already sent in reply to Mr. Farley's letter appearing in Flight, May 20th, 1943? First—consider the period of time during which the blade tip is in contact with -the ground. The conditions existing during this time are substantially the same as if the propeller we-.- stationary, with one blade pointing vertically downward^ Now if, with the propeller stationary and the aircraft tilted up so that the tip of the blade touched the ground, we trundle the machine forwaid, the propeller blade in question will be bent towards the tail of the aircraft. . Secondly—imagine that you are standing in front of a stationary aircraft equipped with a large heavy hammer. You swing the hammer at a speed of 300 m.p.h. and hit one ofthe propeller blades at its tip (the hammer speed stated is meant to correspond with the speed of the ground relativeto an aircraft in flight travelling at 300 m.p.h.), in which direction will the blade bend? Once again towards the tailof the aircraft. In other words, in the direction of the hammer blow or, if the machine were in flight, in the direction in whichthe ground is moving relative to the aircraft Lastly, imagine that in place of a propeller the aircralt isfitted with a stationary celluloid disc, the same diameter as the propeller. Now tilt the aircraft so that the edge of thecelluloid disc touches the giound, again trundle the machine forward and observe that the section ol the disc in contactwith the ground bends backwards as did the propeller blade in the other two examples. I have used the disc in place of a rotating airscrew in caseyou cannot imagine the small space of time stated in the first example. One more point. The propeller does not tend to move the earth backwards because it does not penetrate to anything like an effective depth; the only direction in which it does tend to move the earth is in the'direction of rotation. I hope that I have made myself clear and also that Mr. Farley is able to derive a little benefit from my efforts to repair the "break-down" in his reasoning. K. A. WORMALD. Resultant Path of Two Velocities ._ "IV/TR. FARLEY has certainly raised an interesting point, •*-"- though I feel the four replies in the issue for June 31 d are all incorrect. The main point that has been ignored—or rather not taken sufficiently into consideration—is that the propeller tip's path along the ground is the result of a forward and a rotational velocity. Shown diagrammatically, we have: Before contact with ground— INCIDENCE OF BLADE NECESSARY ^ \ THRUST FORWARD VELOCITY AND PATH OF AIRCRAFT PERIPHERAL VELOCITY Under these conditions the prop blade would definitely be bent forward on striking the ground. This is obviously what was in Mr. Farley's mind when he propounded his problem. Now when the blade strikes the ground the forward velocity of the aircraft will scarcely alter, but the rotational velocity will. Remember that it is the resultant of these two velocities that determines the path along the ground of the blade tip. The friction of the ground will reduce the rotational velocity to such an extent that we get this: After contact with ground— FORWARD VELOCITY OF" AIRCRAFT PERIPHERAL VELOCITY In other words, the blade's incidence is not _ enough to dealwith the resultant pafti. Consequentlv, the blade will be bent backwards. ' A. M. H. LEWIN. [After these particularly lucid additions to the list of explana- tions already published, 'we feel that Mr. Farley should now be thoroughly satisfied, and this correspondence is, therefore, closed.—ED.]
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