Re: prop size?
David J. Gall
Ron,toggle quoted messageShow quoted text
The spreadsheet Leon tried to send you is located here:
It is based on NACA Technical Memorandum 212. It should be sufficient to get
you "into the ballpark" for your propeller selection. It can recommend pitch
and diameter for either a two- or a three-bladed prop.
Your reported propeller failures sound like resonance problems. With a 2:1
reduction, the propeller is in lock-step with the vibration of the engine
from the power pulses. This is not due to "some reduction units" but is
characteristic of the gear ratio and the orientation of the propeller. Most
reduciton units use a different, non-integer ratio in an effort to alleviate
this problem. Some use various vibration dampeners, but the real problem is
the ratio and the orientation of the propeller.
For a reduction unit with an integer ratio or for a direct drive engine,
(assuming a horizontally-opposed engine like a Subaru or Continental) the
propeller needs to be installed in such a manner that it is horizontal when
the power pulses occur throughout the crankshaft rotation. This is referred
to as "clocking" the prop. Set the engine at Top Dead Center then install
the prop with the blades horizontal.
If you install the prop with the blades vertical when the engine is at TDC,
then the power pulses impart a bending load on the prop as they move the
prop hub laterally left-to-right while the prop is aligned vertically. This
bends the prop and sets it to oscillating. If the natural oscillating
frequency of the prop is close to a whole-number multiple of the RPM,
resonance may occur. Even absent resonance, large forces may build up in the
prop or in the friction surface where the prop is clamped in the prop
hub/crush plate, leading to prop failure.
See the file "Installing Wood Propellers.rtf" in the files area or try this
link for more info:
David J. Gall