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12 AIRCRAF T ENGINEERING January, 1931 A Determination of the Limit of Visibility for Use in the Design of Aircraft Lights By A. K. Toulmin-Smith, B.A., A.M.I.E.E., and H. N. Green H E visibility of a light depends, on a clear, shutte r so that the intensity could be varied from th e intensity of the light exhibited and under more dar k night when the effect of atmospheric 0 to 0∙66 candle, giving a range of visibility of favourable weather conditions. T absorption can be neglected, on the inverse from 2∙1 to zero. Th e light viewed, over the same range, consisted squar e of the distance a t which it is observed, while Th e operator, starting with the shutter fully of a small electric lamp with a variable sector disc th e ultimate range at which it becomes invisible opened, slowly closed it until the observer con interposed between the lamp and the observer, depend s on the visual acuity of the observer. sidered that the practical limit had been reached. enabling the intensity of the lamp to be smoothly Under the above conditions an observer with A signal was then sent t o th e operator, who recorded and slowly varied from a maximum of 0∙5 to a average eyesight can detect a light, having an the shutter opening, from which the visibility of minimum of 0∙056 candle. intensit y of about 0∙25 candle power at a distance th e light could be calculated. Tests were also made T o make allowance for the fact that a pilot must of 1 mile, but if it is required to locate the light startin g from a very low visibility and gradually search for a light without knowing its exact position, withou t having any precise idea as to its where th e observer looked away from the light (after each about s or to read its flashing character, a higher intensit y setting had been made) and afterwards visibility must be allowed. verified tha t it could be located wit h certainty when I n this note two members of the Experiment s carried out by the Deutsche allowing th e eye t o travel across or near the position staff of the Royal Aircraft Establish Seewarte, and independently by Paterson & occupied by the light. men t give the results of quantitative Dudding , gave the practical limit for the visibility Durin g the tests the weather conditions were measurement s on the range of visibility for lights at sea as 0∙47 and 0∙41 candle power at favourable, the night was clear and dark and the of lights on the ground seen from air a range of 1 sea mile (1,855 metres) respectively, visibility as given by th e Meteorological Department, craft in comparison with similar ob corresponding to intensities of approximately 0∙35 R.A.E. , was 6 t o 12 miles. servation s for the visibility of lights at an d 0∙31 a t a range of 1 mile (1,609 metres). The Th e following table shows the results obtained :— sea . They arrive at the conclusion conditions of observation from the air are, however, Test No. Conditions Intensity Visibility tha t 0.5 candle power at a range of one considerably more difficult than at sea or on land, 1 Decreasing intensity 0∙185 0∙59 mil e is a practical figure on which to necessitating the adoption of a still higher visibility 2 Increasing 0∙123 0∙39 ,, rat e the visibility of lights provided for for aviation lights. 3 0∙135 0∙43 ,, ,, th e navigation of aircraft. Th e pilot of an aeroplane is hampered by the 4 Decreasing 0∙168 0∙53 ,, wearing of goggles and by cockpit lighting, he is 5 Increasing 0∙118 0∙37 ,, also unable to devote his whole attention for more 6 Decreasing 0∙157 0∙50 ,, tha n a short period a t a time to the task of locating 7 Increasing 0∙157 0∙50 ,, increasing the intensity of the light. Weather an d identifying the signal of a beacon. It was 8 0∙196 0∙62 ,, conditions during the tests were variable, frequent accordingly decided to carry out tests a t the Royal 9 0∙146 0∙46 ,, showers taking place. The actual observations Aircraft Establishment to determine the practical 10 Decreasing ,, 0∙123 0∙39 were made between showers and gave the results limi t of visibility under conditions approximating Fro m these observations the mean value of the shown in the table below. as closely as possible to those obtaining in the air. required visibility is 0∙478. Taking into account the Service goggles were used when viewing the light figure of 0∙537 previously obtained, it is concluded Test No. Intensity of Source Visibility .. .. an d the lighting of the interior from which the tha t 0.5 candle a t 1 mile is a practical figure on which 1 0∙2 7 0∙8 6 2 .. .. observation s were made was approximately the t o rate the visibility of th e lights used in connection 0∙2 3 0∙7 3 .. .. sam e as tha t in the cockpit of an aeroplane. Since wit h aerial navigation. 3 0∙1 3 0∙4 1 .. .. th e practical limit of visibility is in no sense a 0∙6 6 I t was thought possible that fatigue or other 4 0∙2 1 .. .. threshol d limit and therefore depends on th e opinion physiological effects might reduce the visual acuity 5 0∙1 0 0∙3 2 an d judgment of the observer, it is pertinent to . . . . of a pilot and tha t some allowance might be neces 6 0∙0 8 0∙2 5 .. .. ad d that both the authors have had considerable sar y to cover those factors. The question was 7 0∙2 5 0∙7 9 .. .. experience of night flying. therefore referred to the Director of Medical 8 0∙0 9 0∙2 8 Research , Air Ministry, and the opinion given is A preliminary series of tests were made over a Fro m the above the mean value for visibility is rang e of 0∙561 mile. The light viewed consisted tha t the above value allows sufficient margin for 0∙537. In view, however, of the large differences of an electric incandescent lamp enclosed in a box an y such effects as might arise under normal between individual values, it was decided to repeat fitted with a diffusing glass window and a sliding flying conditions. th e tests, using a more accurate method of varying Manchester.— Indicator , Turn: Reid & Meta l Propellers, Ltd., Croydon.—Engines, Air Ministry Contracts Sigrist , Ltd., Kingston-on-Thames.—Lamps, Aircraft , Crankcases : Bristol Aeroplan e Co., Th e following list of contracts placed during Identification : Bulpitt & Sons, Ltd., Bir Ltd. , Bristol.—Engines, Aircraft, Details October is abstracte d from the November issue of mingham.— Masks , Microphone : Telephone for : Rolls Royce, Ltd., Derby.—Engines, th e Ministry of Labour Gazette:— Manufacturin g Co. (1929), Ltd. , London , S.E.— Aircraft , Hubs : Armstrong-Siddeley Motors, Radiator s and Cases: Serck Radiators, Ltd. , Coventry.—Engines, Aircraft, Over Aircraft : A. V. Roc & Co., Ltd., Man haul : Bristol Aeroplane Co., Ltd., Bristol.— Ltd. , Birmingham.—Rags, Old Cotton: chester ; Do Havilland Aircraft Co., Ltd., Engines , Aircraft, Repairs: Armstrong- Wm . C. Jones , Ltd. , Manchester.—Receivers: Edgware , Middlesex; Handley Page, Ltd., Muirhea d & Co., Ltd., Beckenham.—Salvage Siddele y Motors, Ltd. , Coventry ; D. Napie r & London , N.W. ; Supormarine Aviation Works, Crane : Ransomes & Rapiers , Ltd., Ipswich. Son , Ltd., London, W.—Engines, Aircraft, Ltd. , Southampton.—Aircraft, Conversion: — Serge , Blue-Grey : Fox Bros. & Co., Ltd. , Repair s and Spares : Bristol Aeroplane Co., Vicker s (Aviation), Limited, Weybridge.— Wellington ; J. Harper & Sons, Bradford; Ltd. , Bristol.—Engines, Aircraft, Spares: Aircraft , Reconditioning : Blackburn Aero A. W. Hainsworth & Sons, Ltd., Farsley; Roll s Royce , Ltd. , Derby.— Engines , Aircraft, plan e & Moto r Co., Ltd. , Brough , Eas t Yorks. — Watkinso n & Sons, Ltd. , Holmfirth; Reuben Test : Bristol Aeroplane Co., Ltd. , Bristol.— Aircraft , Repair s : H. G. Hawke r Engineer Engines , Aircraft , Tools : D. Napie r & Son, Gaun t & Sons, Ltd., Farsley ; Colbeck Bros., in g Co., Ltd. , Kingston-on-Thames.—Aircraft, Ltd. , London , W.— Ethylen e Glycol : Imperial Ltd. , Wakefield.—Sounders, Relaying: A. Repair s and Spares , etc. : Fairey Aviation Chemica l Industries, Ltd., London, S.W.— W . Hart & Co., London, N.—Sparking Co., Ltd., Hayes, Middlesex.—Aircraft, Plugs : K.L.G. Sparking Plug Co., Ltd., Fabric , Linen : Doag h Fla x Spinnin g Co., Ltd. , Spares : A. V. Roe & Co., Ltd. , Manchester; London , S.W.—Suits, Combination, Blue Belfast ; Stevenson & Son, Ltd. , Dungannon.— Blackbur n Aeroplane & Motor Co., Ltd., Drill : J. B. Hoyle & Co., Hebde n Bridge.— Ga s Starte r Spares : Gillett, Stephe n & Co., Brough , Eas t Yorks ; Fairey Aviatio n Co., Ltd. , Syntonisers : Muirhead & Co., Ltd. , Becken Bookham.— Generatin g Sets : Stuart Turner, Hayes , Middlesex; Gloster Aircraft Co., Ltd., ham.— Tyre s and Tubes , M.T. : Avon India Ltd. , Henley - on - Thames.— Gu n - Control Hucclecote ; Vickers (Aviation), Ltd., Wey Rubbe r Co., Ltd., Melksham.—Voltmeters: Handles : Vickers Armstrongs , Ltd., London, bridge ; Westland Aircraft Works, Yeovil.— S.W.— Hose , Canvas : F. Reddaway & Co., E . Turner, High Wycombe.—Wheels and Airscrews : Bristol Aeroplane Co., Ltd., Ltd. , London , W.C. ; Wm. Ros e Hos e Co., Ltd, . Covers , Aero : Palme r Tyre , Ltd. , London , E.C . Bristol.— Airscrew s and Airscre w Blades :
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Jan 1, 1931
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