8/13/2019 1938 - 0515

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February 24 1938 Su p p l emen t t o

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 A I R C R A F T

•  E N G I N E E RN o .  1 4 5 .

X V I \ ENGINEERING SECTION F e b r u a r y 2 4 1 9 3 8

W OO DEN BLADES FOR V.P. AIRSCREWSInteresting Developm ents Now Taking Place May Have Far-reaching Effects

(Illustrated by Flight p hotographs)

FOR a great number of years the wooden airscrew held

undisputed sw ay. Du ring the pioneer day s of flyingwood was the ideal material for airscrews as well asfor aerop lanes its low cost and th e ease wi th w hich

i t could be shaped m akin g i t part icula rly at t r act iv e forexperimental work and for construct ion in smal l quant i t ies .In those da ys flying was don e in fine we athe r only an dthus i t did not mat ter that wooden airscrews were apt tosuffer severely from the effects of flying in rain or hail.

As the art of flying developed ai rcraft ven tured o ut inless favourable weather condi t ions and the wooden airscrewwas ap t to suffer da m ag e. Also it was realised th at i tsrelatively thick blade sections were not quite as efficientas would be blades of thinn er sec tion such as could beman ufacture d of m eta l. Successful m eta l airscrews were

introduced and gave excel lent resul ts in service. Thewooden airscrew in spi te of fabric covering an d/ or var-nished surfaces could no t comp ete from the poi nt of viewof durabi l i ty and i ts pop ular i ty began to wane.

The next s tage came when the speed range of ai rcrafthad increased to such an extent that ai rscrews with con-trollable pitch were necessary if a good take-off were to becombined with high efficiency at ope ratio nal speed. Inview of th e good service given by th e fixed-pitch m eta l air-screw i t was natural that when the variable-pi tch ai rscrewcame to be developed i t should have metal blades . Ofvariable-pi tch ai rscrews with metal blades there are nowseveral types in use and giving good service.

In the mean t ime the champ ions of the wooden a i r screwhad not been idle an d they ha d succeeded in discovering

protective coverings which would withstand the effects ofrain and hai l not to ment ion prolonged operat ion in humidatmosp heres frequent spraying by sea-water such as isassociated with seaplane work and so forth. One meth odin part icular was evolved in Germany by the Schwarzfirm of Berl in . The Bri t ish r ights in this meth od w eresecured by the Airscrew Co. Ltd . of We ybridge andfixed-pitch wooden airscrews finished by th e Schw arz pro - cess are now used very extensively in th e ma chines of theRo yal Air Forc e as well as in Britis h civil aircr aft. Th eprocess is well know n and consists in cove ring the woo denblade s with a layer of nitrocellulose th e mate rial be ingapplied in sheet form and forced into the pores of the woodunder ai r pressure in autoclaves .

I t was inevi table th at when the variable -pi tch ai rscrew

The first stage. Imp regna ting a sheet of wood with syn-thetic resin.

Six blocks of wood being compressed under heat in thehydraulic press. Actually eight blocks can be compressed

at a time.