VIBRATION CHARACTERISTICS of Three- and Four-Blade Propellers for HIGH-OUTPUT Engines 410142
ALTHOUGH the problems involved in developing a satisfactory engine-propeller airplane installation using propellers of four or more blades are admittedly more difficult than those encountered with a three-blade propeller, Mr. Guerke points out, a satisfactory installation can be made “if the airplane designer will assume the responsibility of reducing propeller interference effects.” This, of course, may be very difficult if not impossible to accomplish in “pusher” installations, he qualifies. Reporting that two such installations have been made in a normal tractor four-blade installation without recourse to odd blade spacing, he contends that successful installations also can be accomplished in propellers of five or more blades under similar conditions.
Dual-rotation propellers, he believes, will introduce additional propeller design problems of a vibration nature which may require certain compromises in diameter, blade construction, and engine gear ratios before satisfactory installations can be made. Mr. Guerke opines, however, that these compromises can be made without sacrificing airplane performance.
In the technical discussion that precedes the foregoing conclusions, Mr. Guerke takes up engine excitation; aerodynamic excitation; aerodynamically excited three-blade propeller vibration; aerodynamically excited four-blade propeller vibration; methods of reducing or avoiding aerodynamically excited stress; dual rotation; and effects of propeller isolation on aerodynamically excited stress.