|
|
Scott:
I can't comment on the aerodynamic efficacy of your cut-down prop, but the speed of your aircraft suggests that it's not too bad.
HOWEVER, it gives me the willies to consider a solid-crank IO-320 driving a CS-prop which had 7 inches chopped off the blades. Consider the following:
a) The IO-320 crankshaft does not have the torsional-absorbing counterweights which the higher-powered 360's have, and in certitfied form, most likely has a yellow band on the tach defining an RPM range best avoided due to harmful interwactions between the engine excitation and one or more natural frequencies of the prop blades;
b) The original 84-inch Hartzell (from which someone lopped off 7" per blade) was, in all probability, designed for and surveyed on a Lyc 540 or Continental 520 / 550, and found to have no potentially harmful interactions between the 6-cylinder engine excitation (3rd order) and any blade natural frequency;
c) The lopping of your blades changed one or more of the blade natural frequencies (most likely causing them to increase), but the magnitude of the change is unknown without a survey. The resulting change could easily cause a serious interaction between a blade frequency and the second-order excitataion of your 4-cylinder engine, which could be stressing your blades to a level they cannot sustain;
d) Metal props on direct-drive piston engines pose a daunting set of vibration problems, the solutions to which are non-trivial, but which include blade profiles, mass distributions, root thicknesses, and a host of other factors, in order to avoid the harmful interactions described above. Consider the fact that a prop which is safe on a particular certified IO-360 (8.7 compression, counterweighted engine) becomes UNSAFE after the STC'd installation of 10:1 compression pistons (more on that and other prop facts at
http://www.epi-eng.com/Prop_TOC.htm)
e) If you're determined to keep on flying this prop, you might consider having Hartzell do a vibration survey on your engine/prop/airframe combination. It's not cheap, but it sure beats the prospect of trying to land after the loss of a couple of inches off one blade tip (probably followed soon thereafter by the entire engine).
Jack Kane
|
|