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Race Engine Technology Magazine

The loadings imposed on the shafts in a PSRU come from a variety of sources. Those sources include:

1. Torsional loads imposed by the engine driving torque, and any multiplications which occur due to gearing and transmissibility;
2. Cyclic bending loads imposed by the gear forces (previous section);
3. Cyclic bending loads imposed by the overhung moment of propeller weight;
4. Cyclic bending loads imposed by propeller gyroscopic moments;
5. Tensile and compressive loads imposed by the propeller thrust.

It is also clear that all the loads imposed on the shafts in a PSRU must be successfully carried by the PSRU housing which supports those shafts, which attaches to the engine, and which supports the PSRU and propeller weight as well as (usually) a substantial portion of the engine weight.

Although experimental aircraft are not required to comply with the vast majority of the requirements defined in the FAR's, it should be obvious that the FAR's provide a rich source of MINIMUM design standards and limits. The applicable FAR's are referenced in the subsequent sections.

SHAFT LOADS

The input and output shafts (and jackshaft if applicable) must be designed for the torsional loads imposed by the engine and the torque increase generated by the gear ratio. The shafting which implements the coupling system between the engine and PSRU must be designed to withstand not only the peak torque values, but also the extreme high-cycle, fully-reversing fatigue environment characteristic of that application.

If the coupling system does not remove the engine torsional excitations, then all the PSRU components, as well as the propeller, must survive the fatigue environment caused by the amplified pulse loadings generated by the engine and multiplied by the PSRU.

All shafts which support gears are subjected to bending loads which vary with the torque the gears are transmitting. Those bending loads are produced by the forces described in the gear design section, and usually can be considered as being applied in the center of the tooth face width.

The propeller imposes a cantilevered load on the nose of the propshaft, which produces a bending moment. As the shaft rotates, that moment causes fully-reversing tensile and compressive stresses on the propshaft. The magnitude of those stresses is a function of the section properties of the shaft, the distance from the prop CG to the PSRU front bearing, and the weight of the propeller.

Gyroscopic moments can impose extreme loads on the propeller shaft, bearings and PSRU housing (not to mention the propeller blade roots and hubs). FAR Part 23 specifies design yaw and pitch rates to determine gyroscopic loads on engine mounts. The gyroscopic loads generated by aerobatic maneuvers (snapping and tumbling maneuvers, the transition from level flight into a 6G pull-up, etc.) can exceed the FAR spec loads by a factor of 2, 3, 4 or more. Those loads can occur from gust loads and severe turbulence as well as from aerobatics.

The propeller bending moments impose large, fully-reversing loads at the propeller flange root and on the shaft itself, as well as on the support bearings. The propshaft must also be stiff enough in bending to minimize the deflection imposed by the gyroscopic moments, and strong enough to sustain the additional fatigue loading imposed by the gyroscopic bending moments.

The tensile and compressive stresses applied to the propeller shaft by the propeller thrust (both forward and reverse directions) must be accounted for in the design. Although the thrust loads can be significant, the stresses are typically low on a propshaft which is adequately designed to withstand the other propshaft loadings.

HOUSING LOADS

FAR 23.371 requires that an engine mount structure be proven to carry, without permanent deformation, the loads applied by the worst-possible combination of:

1. a yaw velocity of 2.5 radians per second,
2. a pitch velocity of 1.0 radian per second,
3. a downward vertical load of 2.5 g, and
4. maximum continuous thrust at 1.25 times the engine torque at maximum continuous power.

Now, consider the fact that, in addition to the INTERNAL loads generated by the power transmission mechanism (gears, chains, belts, whatever),  the combined EXTERNAL loads mentioned above must be carried by the PSRU housing.

Here is an example:

1. 2.5 rps yaw with a common metal prop at 2500 RPM applies over 2250 lb-ft of torque to the propeller flange in the pitch direction..... a 106" metal 3-blade prop at 1900 RPM, such as used on our cropduster conversion, applies nearly 3250 lb-ft to the flange.
2. 1.0 rps pitch with a common metal prop at 2500 RPM applies nearly 915 lb-ft of torque to the propeller flange in the yaw direction;
3. 2.5 times the weight of the PSRU plus Propeller, plus approximately half of the engine weight can amount to well over 1000 pounds of "g-load";
4. anywhere from 800 to 3000 pounds of thrust, plus the PROPELLER torque (gear ratio times engine torque).

The bearing supports in the housing must be capable of carrying all the loads applied by the various shafts. But the housing must also be capable of transmitting the combined loads to (a) the engine and (b) the mount structure without unacceptable levels of BOTH STRESS and DEFLECTION.