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Aviation History
1943
1943 - 2204.PDF
284 FLIGHT 9TH, 1943 Reversible - pitch Airscrews TKeir Effectiveness as Air Brakes, Landptg Brakes and Dive Brakes : " Power 'On" and "Powev&ff" Distinctions By J. MULLIN* rakir fidc- IN recent weeks the correspondence columns of Flighthave included a series of letters discussing thebilities of using reversible-pitch airscrews as airlj and landing brakes for aircraft. Since there appears to.be very little agreement on the manner in which a airscrew functions, and even doubt as to the feasitthis development (which will undoubtedly be of spread future' application), I should like to attempt asimple explanation of this subject. First, I should like to dispel any doubts as to the prac-ticability of using the airscrewyboth as an airbrake, to limit the speed of an aircraft in/a dive or under other con-ditions, and as a landing brake, to reduce the landing run of heavily loaded machines. As far back as September, 1932, a programme of flighttests was carried out by the National Advisory Committee for Aeronautics, the well-known American research insti-tution, in which a series of terminal velocity dives were made with the airscrew set at very small blade anglesand allowed to windmill against the engine friction. These tests showed that the terminal velocity of a diving aircraftcould be very considerably reduced by an air-braking air- screw, and the results have been published in reference 3of the bibliography attached to these notes. The success of the airscrew used as a landing brake, withengine power on and "the airscrew blades set at a large negative angle, is well described in reference 1, in whichthe Escher Wyss Company of Switzerland state that tests of a landing-brake airscrew carried out on a fighter aircrafthave enabled them to reduce the landing run to 28 per " cent, of its value when no brakes of any kind were used.Normal wheel brakes only reduced the run to 61 per cent, of its unbraked value. RESULTANT RATH OF BLADE ELEMENT RESULTANT FORCE- TORQUE- FORCE FORWARD SPEED OF AIRCRAFT •* \ yPT. JB DRAG V-»/A ^ROTATION COMPONENTFORCF V* ^- OF VELOCITYE Fig. 1. A certain amount of data from wind tunnel tests invarious countries lias been published from Jkne to time, giving the characteristics of airscrews when operating atsmall positive and at negative blade angles, a6 they would be during a braking operation. These references are alsoincluded in the attached bibliography. Besides their uses as airbrakes and landing brakes, rever-sible-pitch airscrews are proving extremely useful for manoeuvring large flying boats on the water. In this case,as in the landing brake case, the negative thrust is obtained by turning the blades into negative pitch and using theengine power to drive the airscrew round. Various Ameri- can flying boats have already made use of this feature ofthe Curtiss electric airscrew. j At this point it seems convenient to rn^ke the distinctionbetween " power-on " and "power-off " or " windmilling " conditions of braking.The power-on condition is that used for braking during landing and in flight at low airspeeds, when the airscrewblades are rotated to a large negative angle and the engine power applied. • Technical Assistant, Engineering Department (Airscrews), The De HaviliandAircraft Company, Ltd. RESULTANT PATH OFBLADE ELEMENT DRAGFORCE -ROTATIONAL COMPONENT 0F VEL0CITY c FORWARD SPEED OF AIRCRAFT DRA6 FORCE SLOWING DOWN AIRCRAFT TORQUE FORCE ROTATING AIRSCREW WITHOUT —• AID OF ENGINE Fig. 2. For braking at high airspeeds there is not much increasein braking effect to be gained by using the power-on con- dition compared with the large drag forces that can beobtained by putting the blades into approximately zero pitch and allowing the airscrew to windmill against thefriction of the engine with throttle closed. . Action of Aerodynamic Forces Finally, in order tocsmiplete this discussion, I should liketo 'give &• brief, s>tffple explanation of the aerodynamic forces acting on the airscrew blades during a braking opera-tion. It is usually convenient to consider the performance ofan airscrew as a whole to be represented by the perform- ance of the section at about three-quarters of the tip radiusof the blade, and in this particular problem this will be the simplest way of reaching a clearer understanding ofthe operation of the airscrew. First of all, consider an airscrew operating in the normalway at a positive blade angle and a small positive angle of incidence. In Fig. 1 the three-quarter radius sectionof the blade is travelling sideways along AB due to the rotation of the airscrew^and at the same time is beingmoved forward along the line BC by the forward motion of the aircraft as a whole. Thus the true path of the bladeelement through the air is represented by the line AC. In RESULTANT PATH OF BLADE ELEMENT DRAG FORCE FORWARD SPEEDOF AIRCRAFT ROTATIONAL COMPONENT OF VELOCITY RESULTANT FORCE. LARGE. RETARDING, FORCE ON AIRCRAFT TORQUE FORCE OPPOSING ROTATION OF AWSCREW
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