Mike recorded a 1/2 order vibration that he eliminated by clocking the prop in alignment with the plane of the front crankthrows. I previously mentioned the flexibility of engine components. Les suggested that the mechanics are "something to do with the where the propeller's yaw/pitch polar moment major axis is in relationship to the plane of the front crankpins." Here are some mechanics that might explain the "why" underlying the observations.
Difficult to verbalize, so a drawing is in order:
The first four figures illustrate any clocking that does not place the prop in alignment with the plane of the front crankthrows. For this explanation I've placed the prop 90 degrees to the plane of the crankpins, an exaggeration, but it helps illustrate the principles and the result is the same. The cyan arrows illustrate the direction of the forces applied to the crankthrows by piston inertia and gas pressures. Don't get confused; they do not illustrate the direction of rotation, which is always clockwise. Not shown are the front and second crank bearings, but remember that the 1-2 crankthrows are suspended between them.
In the first figure cyl#1 is on power stroke and cyl#2 is on intake. The power stroke tries the bend the crankthrow set toward the green blade, as does the negative piston inertia and gas pressure of #2. The bending crankthrow set rings the green blade forward.
In the second figure #1 is on exhaust. Since blowdown has released almost all the gas pressure, anything past mid-stroke is a decelerating piston inertia. Cyl#2 is on compression, a positive gas pressure. The combined forces again push the crankthrows toward the green blade, which of course has rotated 180 degrees from the position shown in the first figure.
The third figure has #1 on intake, negative gas pressure and inertia, and #2 on power stroke, a very positive gas pressure. Now crankthrow forces reverse; they try to bend the crank toward the red blade, which moves forward.
The fourth figure continues more of the same; exhaust and compression continue to bend the throw toward the red blade.
Overall, the prop moves through a complete vibratory cycle once every two crank revolutions, a 1/2 order vibration.
When you align the propeller axis to the plane of the crankthrows, the crank bending no longer drives the prop tips fore or aft. Now the prop describes more of a rotation around it's own axis. Actually it would wiggle a bit from side to side because there is a small moment arm from prop hub to somewhere in the center of the front bearing, but no matter.
Mike's data presents an interesting twist that I think illustrates a point; the above is only part of the answer. Note that he got a 66% reduction at 2600 RPM, an 83% reduction at 2400, a 57% reduction at 2200, and very little reduction at 2000. Mike ran the tests at a fixed manifold pressure (about 23 inches). I suspect an additional factor, a quirk of hydrodynamic lubrication in the two main bearings.
A lightly loaded oil lubricated shaft runs close to the center of it's journal. As side load increases, the shaft tries moves closer to side of the journal opposite the load. Viscosity drags a wedge of oil in under the decreasing clearance, and a thin, very high pressure film prevents shaft-journal contact. The quirk? The oil wedge also forces a displacement at an angle to bearing load. The actual displacement increases with load and decreased RPM. See the above illustration; load is along the W axis, but displacement is along the A axis.
Now consider Mike's fixed manifold pressure; bearing load increases with reduced RPM. As RPM is reduced, the displacement angle for axis A becomes greater. The new prop alignment has the prop axis in the plane of the crankthrows, but shaft displacement moves the forces more and more off that axis with decreased RPM. The result is less reduction in vibratory amplitude with decreasing RPM.
These are general theories, nothing more. Some fine tuning would be valuable. Good science requires the development of some experiment to prove or disprove the theory.
One relatively simple experiment would be to strobe the prop tips. If the theory is correct a tip buzz at about 22 hz (a blurry tip) may be visible when they stop a prop with 1-7 clocking. It should be pretty much gone with 3-9 clocking at 2600 and 2400, start to appear again at 2200, and be present at 2000. If Mike and Doug wish to do the work, I will lend them a strobe.
) from a prop efficiency standpoint, shaft HP applied verses thrust made, one blade is gives more thrust for HP applied, but again this is only practical for very low HP engines. The problem is, as you point out, practical factors come into play, like smoothness. Also pilot appeal is low. Visually we like things symmetric. It work on 40 hp cubs, one blade and a counter weight. One blade-ers are also used on model airplanes. Obviously one blade-ers where never used in great numbers and where abandoned, backing-up your "stupid" statement, however its theoretically smart, just not practical.
