Vee have vays of making Das Props
MT, another German company with roots back to those days, does extensive tests with their props to certify them for various applications.
David since you correct me (and you are usually right)
one clarification, MT-Propeller was founded in 1981 by Gerd Muehlbauer, whose prop experience prop design goes back to 1968. But you are right the ideas he uses (natural composite ie wood) go back the the 1930's, pre-WWII Germany.
The engine tends to drive the prop vibration not the other way around. However prop does affect direct drive engines at some "orders" and of course would affect a PSRU. The PSRU as a middle man has a hard act to perform. It has to deal with both the unique harmonics of the engine and prop and it's own. As Jan pointed out, prop balance is important (1st order).
A direct drive crank is a massive thing. The going in position to fight all the pounding from combustion, rods pushing and prop harmonics & inertia, is just make it stout and rigid. If stresses are low, regardless of harmonics it will last. Excitation at higher orders also tend to be less energetic, so they can be tollerated more than lower order harmonics (sometimes). If not, you see "pendulum absorbers" or dampers at critical orders (frequencies). On Lycs you see 3rd, 5th, 6th & 8th order dampers typically. These orders are mostly associated with the torque, inertia and pounding the crank takes from the cyl pressures (via the rods).
None of the orders that typically get crank dampers are prop related, unless you have a three blade prop. Some engines don't need any damper because the crank stresses are low enough to tolerate. The direct drive crank has to tolerate 1/2 and 1-1/2 order vibs by brute strength. That design philosophy works for PSRU's, "when in doubt make it stout".
Below are some significant "Orders" of vibration and the source of that vibration on direct drive engines. (bold = where prop acts on engine or vise verse)
Vibration Orders (from Sacramento Sky Ranch)
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1/2 order
-minor component, single
prop blade aerodynamics.
-major component, Gas pressure (torque).
-ranked relative cylinder influence: Continental 1,2,3,4,5,6; Lycoming 5,6,3,4,1,2.
-notes: Amplitude depends on mount stiffness, crankshaft torsional stiffness, cylinder position and gas pressure.
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1st order
-minor component, slight amounts of inertia and dead weight. Torque increases with order.
-major component, rotating imbalance.
-related orders 2,4,5,7,8. Higher orders are torque orders.
-notes: Amplitude increases with engine speed. Movement of
propeller from rotating imbalance can cause 2nd order vibration on 2-blade
propeller or 3rd order vibration on 3-blade
propeller
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1-1/2 order
-minor component, none.
-major component, gas pressure (torque).
-related orders at 40% power, 1-1/2 and 1/2 order have approximately same amplitude. At higher power settings half order is increasingly stronger than 1-1/2 order.
-ranked relative cylinder influence: Lycoming 5,6,3,4,1,2, Continental 1,2,3,4,5,6,
-note: All 1/2 orders are gas pressure. may be more accurate assessment of gas pressure variation than 1/2 order. Amplitude also depends on mount stiffness, crankshaft torsional stiffness, cylinder position and gas pressure.
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2nd order
-minor component,
propeller dynamics on 2-blade propeller. Rotating balance. Connecting rod couple (slight - subtracts from inertia) and
propeller phase angle.
-major component, inertia.
-related orders 1,4,5,6.
-cylinder influence, none.
-note two blade
propeller phase angle in relationship to forward crank pin may have slight influence. Inertia forces increase with engine speed.
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3rd order
-minor component, gas pressure (counteracts inertia). Phase angle of 3 blade
propeller. Aerodynamic excitation of 3 blade
propeller.
-major component, inertia.
Third order is a major order.
-related orders 6,9.
-cylinder influence, none.
-note: Amplitude increases with rpm. Amplitude decreases with gas pressure. Crankshaft resonant frequency just above red-line rpm.
Propeller phase angle in relationship to phase angle of 3rd order harmonic may increase, decrease, or have a neutral affect on 3rd order amplitude. Inertia forces increase with engine speed.
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4th order
-minor component, inertia 1/3 of gas pressure and counteracts. Slight connecting rod couple adds to gas pressure.
-major component, gas pressure (torque)
-related orders 1,2,5,7,8.
-cylinder influence Continental 1,2,3,4,5,6, Lycoming 5,6,3,4,1,2.
-note: Amplitude increases with manifold pressure on engines without 4th order counterweights (pendulum absorbers).
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4-1/2 order
-minor component, none.
-major component, gas pressure (torque).
-related orders 1-1/2, 7-1/2,
-cylinder influence, may vary if engine has 4-1/2 order counterweight (pendulum absorber): Continental: 1,2,3,4,5,6, Lycoming: 5,6,3,4,1,2.
-note Counterweights (pendulum absorbers) may be used to absorb 4-1/2 order torques. Amplitude increases with manifold pressure on engines without 4-1/2 order counterweights (pendulum absorbers).
4-1/2 order vibrations are especially hard on
propellers.
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5th order
-minor component, 8% inertia.
-major component, gas pressure (torque)
-related orders 1,2,4,6.
-cylinder influence, may vary if engine has 5th order counterweights (pendulum absorbers): Continental 1,2,3,4,5,6, Lycoming 5,6,3,4,1,2.
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6th order
-minor, component, less than 1% inertia, less than 1/2 of 1% connecting rod couple.
-major component, gas pressure (torque).
-related orders, 3,6,9,12.
-cylinder influence, none.
-note: Counterweights (pendulum absorbers) absorb 6th order torque. Vibration measurements may vary depending on manifold pressure and engine rpm. Especially with high manifold pressure at resonant rpm. (This is why some engines have limits on high MAP with very low RPM, in a certain range.)
The above is for direct drive, but it does give us a clue what the PSRU is sitting in the middle of. The PSRU has its own harmonics or major orders as well. This all has to match the Prop and engine. Bolting up a new prop or engine to a PSRU is not trivial. There may be new "interactions" between vibration modes that may not be desirable. Also turbo or supercharging can have affects on all components.
