NEW VARIABLE-PITCH FAN
Page 24
If you've noticed an error in this article please click here to report it so we can fix it.
NOW in the advanced prototype stage is a new type of engine coolingsystem fan, which has been developed by Dowty Hydraulic Units Ltd., Automotive Components Division, Ashchurch. Tewkesbury, Glos. Known as the Dowty proportional flow fan, its cost (in relation to alternative special types) is low and it takes up no more room than the normal type of fan. It works on the principle of varying the pitch of the blades from coarse to fine as the speed rises. This is done by spigoting each blade into a housing in the central flange so that it is free to turn about its own axis (within limits set by stops which normally provide for a variation in pitch from approximately 11' to 35°), and incorporating torsion springs which tend to retain the blades in coarse pitch. The centrifugal twisting moment acting on the blades as the fan rotates, however, acts against the springs and the resulting tendency for the blades to " straighten " increases as the speed rises. Appropriate choice of torsion springs enables the speed at which pitch reaches a minimum to be chosen to suit individual applications. In this way, both the power absorbed and the noise generated increase to a much lesser extent as the speed rises. Independent comparative tests carried out on a medium-powered car engine showed that whereas the standard fan absorbed 3-4 h.p. at 5,000 r.p.m., the corresponding figure for the Dowty fan was only 1.3 h.p., a saving of more than 60 per cent. The reduction in noise recorded in these tests was even more striking, with a decrease at the recordini point from 10 decibels at 5,000 r.p.m. tc 2.5 decibels. Obviously there will alsc be an improvement in fuel consumption
H.C.H.