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Aviation History
1945
1945 - 0661.PDF
APRIL 5TH, 1945 FLIGHT 371- AIRSCREW BRAKING I'I. Engine power : 1,000 h.p.A, E, Normal three-bladeairscrew.B. Four-blade airscrewwith wide blades.C, Normal three blade airsciew with extra take-off boost, tem- porary excessive en- gine r.p.m. and increased airsciew diafneter. D. F. Four-blade airscrew with wide blades, with extra take-off boost, temporary excessive r.p.m. and increased airscrew diameter. Fig. 6. During braking the lift of the shaded section of the wing is eliminated by the fact that the airscrew reverses the airflow. This facilitates the land- ing by permitting landing at overspeed without risk, and increases the effec- tiveness of the wheel brakes. such cases the airscrew must be set at braking pitch long before the point of landing is reached, with the engine idling. (At landing speeds common to-day, a minute corresponds to a distance of from 2 to 2.5 miles=3 or 4 kilo- metres.) But in this condition the aircraft is completely helpless against external influence (squalls, cross winds, erroneous estimation of distance by the pilot, etc.); nor can it take off again should there be an unfore- seen obstacle in its way. Braking with this type of air- screw is, therefore, an extremely dangerous proposition and could never be introduced into the commercial air services. It is only by the extremely rapid change of the blade angle to braking pitch and equally rapid return from the braking angle to the normal pitch range that it is possible.to utilise the advantages of airscrew braking without sacrifice of operational safety or manoeuvrability. The change-over of the blades to braking pitch must not be effected earlier than the moment when the success of the landing can be taken for granted, i.e., at the earliest from 300ft. to 150ft. (100 m to 50 m) before the touch- down. Consequently, the time required for the process of pitch change must not exceed one or two seconds. The transition from the finest pitch of the normal range to braking pitch (from about +20 deg. to about —20 deg.) must hence take place at a minimum velocity of at least 20 deg. per second. The same pitch-changing speed must also be made available for returning the blades to positive pitch, in case the pilot is forced to take off again for one reason or another. High pitch-changing speeds are required also because in e course of the change to the braking position the blades raverse pitch angles at which the airscrew may be acceler- ated by the airstream, which easily might produce dan- gerous racing of the engine. However, this windmilling effect disappears just as suddenly as it arises, as soon as the airscrew blades reach a sufficiently coarse negative angle of incidence. For this reason quick pitch change while setting the blades at the braking angle with the aircraft still moving forward is the only means of preventing excessive revolutions: the inertia of the revolving masses will then lbs iooo 8000 4000 2000 B A —r' 0 S I 400 / A 2Co V * / T»/h300 4 a oo kg -3000 — 2000 / Braking force 1000 Forward thr-ist 400 mph Fig. 7. The braking power (E, F) is great and reachesmaximum efficiency at high-speed touch-downs, whilst the wheel-brake efficiency is zero at the moment of groundcontact. Increasing the take-off thrust from A to D results in a shortened take-off run without external aid, correspondingto the shorter landing-run with airscrew braking. prevent the engine speed from rising to inadmissible values. The attainment of sufficiently high velocities of pitch variation has been an insuperable obstacle with airscrew types so far in use, since the latter are incapable of pro- ducing instantly the output required for pitch variation. The power of the pitch control motor amounts, as "an example, to one-fourth or one-half horse power for a pitch- changing speed of between one and two degrees per second in the case of conventional electric airscrews. Thus, if it were desired to raise the pitch-changing velocity to 30 deg. per second, an output of from "]\ to 15 horse power would be required, assuming equal efficiency of the power trans- mission. It will hardly be possible to build electric motors of ten or twenty horse power into the hub of an airscrew, not to mention the reduction gears, worm gears and gear wheels otherwise necessary for the transmission of the pitch changing energy. Hydraulic servo-motors are good sources of direct power, without their energy being first absorbed, after starting, for the acceleration of intermediate com- ponents. However, the change-over from electric or mechanical to hydraulic operation alone is not sufficient. Another step is required in order to realise short period maximum pitch- changing velocities by means of a simple and compact hydraulic power unit. The solution consists of the inser- tion of a pneumatic power accumulator to supply the power to the control mechanism. In this way it is possible to dimension the governor of the airscrew on the basis of the low power involved in normal pitch control (r to 1.5 h.p.). Otherwise the hydraulic oil pump would have to be designed for the output required for high velocity changes (15 to 20 h.p.). AUSTRALIAN FIGHTER WING IN EUROPE A USTRALIA'S first fighter wing in Europe has recently been •£*• formed. One of the R.A.A.F. squadrons forming the new wing was previously employed on precision bombing of V- weapon targets in occupied territory. The Australian fighter wing, at present based in Britain, is to be commanded by Wing Commander Donald George Andrews, D.F.C., of the R.A.A.F., who comes from South- port, Queensland, where in civil life he was a bank official. Wing Commander Andrews enlisted in the R.A.A.F. on November 8, 1940, and readied Britain in August, 1941. He was posted to a Hurricane squadron, served as a night fighter pilot, and took part in the combined operations at Dieppe. PATHFINDING T© PEACE AIR VICE-MARSHAL D. C. T. BENNETT, Chief of BomberCommand Pathfinders, has accepted the invitation of the West Middlesbrough Liberal Association to be their candidate in the by-election caused by the death of Mr. Harcourt John- stone. A.V-M. Bennett lias just published a book, "Freedom from War," which sets forth his plan for establishing in the world the rule of law. "Political inertia is the onlv really serious obstacle," he says, "to the establishment 0/ permanent peace at the end of this war. This must be overcome by the advent of new blood in the political-machine of the world."
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