FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1921
1921 - 0830.PDF
very best "in line" or "Y" type engines. The ratio of connecting rod to crank should not be less than 3.5. Short connecting rods considerably increase the side thrust and mean temperature of the cylinder walls." (3) ." Valve Mechanism.—The valve mechanism of static radial engines has been a constant source of trouble in the past, and the larger the size of the cylinder, the more acute this problem becomes. The author has given considerable time and thought to endeavour to eliminate the generally accepted push-rod mechanism, but is of the opinion that, at any rate for cylinders up to 6-in. bore, this construction is the most satisfactory. When really large cylinders are considered, it will probably be desirable to have individual vertical-bevel drives to each cylinder." >••• ' " M': . ::"'•-- DECEMBER 15, 1921 engine. The results have been poor volumetric efficiencies, high consumptions and burnt exhaust valves on isolated cylinders." (5) " Pistons.—Aluminium is the most suitable metal for pistons. The same alloy should be employed as for the cylinder heads, and the preliminary " growth " carried out before machining. The author's company developed and tested out at great length seven distinct types of piston before arriving at an entirely satisfactory result. The mass of metal in the piston at one's disposal must be very carefully dis- posed. One ounce of metal added to a piston of 5f-in. bore represents only 0.0625 lb. additional reciprocating weight, but represents 78 lbs. additional loading on the big-end of the complete engine. A satisfactory piston for 5^-in. bore Fig. 11.—Mount- ing of "Jupiter" engine in Bristol " Bullet " aero- plane. " Assuming that the push-rod arrangement is the best for cylinders up to 50 h.p. per unit, it is proposed to detail methods for eliminating the weak points of this component. The reciprocating weights of this mechanism must be reduced to the minimum, excessive valve clearance eliminated, and first-class workmanship be put in the manufacture of the- cam-sleeve. The accelerations, and especially the decelera- tions, of the cams must be very carefully watched. The cam- sleeve must therefore be accurately former-ground. Hand- made cams filed to template, although correctly designed on paper, may produce very high momentary loadings." " Excessive valve lifts and large valve clearances must be obviated. The latter is a serious problem with large air- cooled cylinders, when the rocker mechanism is positively fulcrumed on the cylinder head. As the cylinder heats up, m m m cylinder should not exceed 2f lbs. Webs and sliarp corners should be cut out as much as possible, as both are prolific causes of cracks." Present State of Development of Air-Cooled Radials Having outlined methods for dealing with the problems peculiar to static air-cooled radial engines, Mr. Fedden proceeded to deal with the latest type of engine developed by his company—i.e., the Bristol " Jupiter." This last part of the paper was mainly descriptive, and refers to the accom- panying illustrations. He mentioned that the "Jupiter" has recently passed the 1920 type-test of the British Air Ministry, and has obtained its Civil Airworthiness Certificate. The tests extend over 54 hours, and are extremely rigid and strenuous. " Only two other makes of engines," Mr. Fedden Fig. 12. — Mount- ing of "Jupiter" engine in Bristol " Badger " aero- plane. 1 the head carries the rockers away from the push rods, and on a large cylinder of 7|-in. stroke this clearance may be as much as 0.060-0.070 in. With such clearances, broken valve springs and valves breaking off under the head are failures which must be expected. To remedy this trouble, the author is of the opinion that it is absolutely necessary to introduce some method of automatically taking up the valve clearance as the engine warms up. The loading of valve springs is in most instances too high. The shear stress should not be more than 40,000-45,000 lbs. per sq. in." (4) " Uneven Distribution of Mixture.-—On all multi-cylinder engines the question of even distribution is one that has received much thought and attention. On static air-cooled radials this matter has been neglected, because the problem is, if anything, more difficult than on an ordinary " in line " said, " and these water-cooled and of greater weight per b.h.p., have ever passed this test, and the Bristol ' Jupiter ' is the first and only static air-cooled radial to obtain this certificate. It will therefore be realised that the static air- cooled radial is now a really proved proposition. The official weight of the engine complete is 729J lbs., which is equal to 1.92 lbs./b.h.p." " Fig. 1 shows the official power curves of the ' Jupiter' engine. Curve A being the power curve at the commencement of the type test, and Curve B being the power curve at the end of the 54 hours. The average petrol consumption throughout the 54 hours' test was 0.594 pt. per b.h.p. hour ; the average oil consumption, 0.049 pt. per b.h.p. hour. At the end of the test the engine was stripped down for exami- nation. Very few signs of wear were found, and the general 83O
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
