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Aviation History
1943
1943 - 1525.PDF
JUNE IOTH, 1943 FLIGHT 617 CORRESPONDENCE The Editor docs not hold himself responsible for the views expressed by correspondents. The names and addresses of the witters, not necessarily for publication, must in all cases accompany letters. BENDING BLADES Nut and Bolt AnalogyR EADING in your May 20th edition of Flight D. H. Farley'sletter on the subject of "Bending Blades," I was amazed that he should lor one moment imagine that the airscrewblades should bend forward. The airscrew of a plane does not simply push air backwards;it threads its way through the air with a certain percentage slip, due to the air not being a solid medium. Let us imagine a bolt being threaded into an imperfect nutin which there is an obstruction. So far through the nut the bolt picks up with the obstruction and becomes jambed. Ifthe bolt continues to be screwed into the nut, and the obstruc- tion is immovable, the thread on the bolt will give way (pro-viding, of course, sufficient force is used), and the direction of the " g've" will obviously be backwards or in the reversedirection to the travel of the bolt. In a similar way a plane's airscrew pulling the plane behind it will, on impact with theground at sufficient velocity, be bent in the reverse direction. W. N. ELSON. BLADES AND MANIFOLD PRESSURE Why One Bends and the Other Builds Up T HAVE read with interest letters appearing in your issue-*- of May 20th under the headings "Bending Blades" and "Boost Pressure Variations," and I suggest the followingexplanations for the phenomena mentioned in these letters. With regard to the bending back of airscrew tips: Whenthe airscrew hits the ground its speed is greatly reduced, and since the speed of the aircraft is not proportionately reducedthe airscrew ceases to give thrust, and instead has a braking effect, and if it continues to hit the ground will be bent back-wards. In the case of the normally aspirated engine referred to byanother correspondent, the increase in manifold pressure which follows a coarsening of pitch is, I suggest, due to the slowingdown of the engine without any alteration of throttle setting. When this takes place the effect of the inertia of the mixtureis reduced, and there is more time for the pressure in the mani- fold to build up towards full atmospheric pressure. D. M. RAMSAY. Increasing the Load on Legs and Engines IW ITH reference to Mr. D. H, Farley's letter on the questionof bending airscrew blades I would suggest that he missed the following point. Airscrew pitch and relative movement ofair has no bearing on the subject; it being purely a case of aircraft movement relative to the,ground, and as the airscrewis attached to the machine it will be bent back in exactly the same way as-juy other part, such as an undercarriage leg. Tobend the blade forward by virtue of its screw action would require the aircraft to be stopped absolutely dead at themoment that the blade bit into the ground. Alternatively, if your correspondent cannot accept this as the point on which his "reasoning" breaks down, we can fillour hair with straws and proceed as follows: Let us suppose that the blades are pitched at 25 degrees andare ioin. in diameter. If airscrew speed is 2,000 r.p.m. we then have a theoretical forward speed of 325 m.p.h. approx.Allowing about 75 m.p.h. lost in slip and drag, we have an aircraft speed of 250 m.p.h. If, however, the airscrew beallowed to react against a more substantial medium than air, e.g., solid earth, there would be no slip and the blades wouldimmediately attain the full forward speed of 325 m.p.h., but as the aircraft speed is only 250 m.p.h. the airscrew wouldeither have to accelerate the aircraft to 325 m.p.h. or bend forward. This would only occur if we lightly ignore a numberof unpleasant possibilities, such as shearing, bending chordwise 01 not bending at all. From the foregoing it would seem thatone of four things happen: (1) The blades bend backward: (2) 1 is a mirage, and the bending is in fact forward; (3) theydo not bend at all; (4) something else happens. Incidentally, whose leg do you think is being pulled ? To turn to a decidedly saner letter, Mr. J. Ward states thaton changing the pitch of an airscrew driven by an unblown Gipsy e'ngine the manifold pressure rises to 12 Ib./sq. in.absolute. This is, of course, only natural, as in any engine the manifold pressure must rise if constant r.p.m. are main-tained coupled with an increase in load (such as driving a coarse-pitch airscrew as opposed to one of fine pitch). Thisincrease in manifold pressure will in turn affect the boost gauge of a blown engine owing to the control capsule being subjectto manifold pressure. P. R. BROWN. WHITHER THE FLYING MACHINE ? R.A.F. Riggers are Proud of Their Work IN your publication of Flight ol May 20th 1 read an article,"Whither the Flying Machine?" by 'Indicator." It is very apparent that "Indicator" is what is commonly called"blowing the gaff" towards the end of his article. 1 myself am a rigger charge-hand trained by the Govern-ment in their hour of need in, the early part of the war, and before then I must confess I knew nothing about aircraft. Does"Indicator" think that the Government would allow fools to play around with such expensive things as aircraft, particularlywhere airmen's lives are at stake ? On behalf of all the Government-trained riggers (and, believeme, there are some thousands of us) let me tell " Indicator" that when it comes to a job of rigging or trimming an aircraftwe are the boys who can do it, books or no books, and we are proud of our work, too—as proud as the men who design them. I keep a record of all the machines that pass through myhands, and you may be sure I have a long list of them, and heavy bombers as well as fighters need very accurate trim-ming after they have been equipped. J. HOWARD (Rigger C./H.). POST-WAR TRANSPORT AIRCRAFT - < on<inii< d exact figure of equivalence depending upon prevailing winds, availability of emergency landing places short of the assigned destination, and other factors). On the particular assumptions adopted here, the decrease of relative fuel consumption with increase of size is dis- appointingly slow. An increase of gross weight from 60,000 lb. to 200,000 lb., for example, would decrease the relative consumption over a 5,000-mile distance in still air by only about 1.7 per cent, of the initial gross weight. The optimum distance between refuelling stops, in cases where the operator has some freedom of decision in that respect, tends to increase, with increase of size of the aircraft. I have concluded that the best practical distance between refuelling stops, in cases where their location is not controlled by considerations of traffic and where a given block-to-block speed is required to be maintained over the total length of the route, would be from 700 to 1,200 miles for a 60,000-lb. aircraft. For a machine of 200,000-lb. weight those distances might be increased, by from a third to a half. As a final look into possible future development on this point, I have returned to the true flying wing of 400,000 lb. weight. For such a machine the fuel consumption in a long flight ought to be about 15 per cent, lower than in the conventional aircraft of the same weight as plotted in Fig. 19B. In Fig. 19C fuel consumption, again as a fraction of gross weight at take-ofl, is plotted against length of flight for the three cases of the 6o,ooo-lb. aircraft that seems realis- able in the very near future ; of the 200,000-lb. aircraft similarly realisable; and of the hypothetical flying wing of 400,000 lb. gross weight. These curves represent cruis- ing consumption only, with no allowance for headwinds, no reserves, and no allowance for power used in take-off and climb. (To be concluded next week.)
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