# - PSRU FAQ's -

## Questions we get asked; Questions you should ask

If you are considering the purchase of a PSRU, here are a few essential questions which you should ask any PSRU vendor you are considering. You, the potential purchaser, should know:

what these questions mean, why each one is significant, and what are the right and wrong answers.You, the purchaser, should be familiar enough with the subjects to evaluate the accuracy and honesty of the answers. These questions contain links to the various places on this site which explain the significance of the question and present the framework for the "correct" answers.

- What is the load model used to evaluate the design of this PSRU?
- Which of the flight loads (high "g" loads and gyroscopic moments) defined by the MINIMUM AIRCRAFT STANDARDS in FAR-Part 23 has this PSRU has been designed to handle, and can you PROVE it?
- Does this PSRU incorporate any methodology to eliminate the torsional excitation applied by the engine?
- If the answer to (3) is YES, then for the SYSTEM consisting of
**your PSRU**with**my specific engine**and**my specific propeller**,

(a) What are the engine-to-gearbox transmissibility values at**Idle**,**Cruise**, and

**Takeoff**power settings?, and

(b) What are the first and second mode torsional resonant frequencies of that specific system? - What are both the TORQUE and HP this PSRU was designed to transmit?
- If the PSRU uses GEARS, what are the Dynamic Stress Levels and Dynamic Load Factors for all gears at Cruise and Max Power?
- If the PSRU has an idler gear subjected to fully-reversing loads, what is the stress level of the idler as a percentage of the input gear stress level?
- If the PSRU uses a chain or a belt, prove that the input TORQUE from my engine does not exceed the belt or chain manufacturer's engineering specifications AT THE RPM I INTEND TO RUN.
- What are the design lives of the following components when evaluated in the load model?

(a) gears;

(b) input shaft;

(c) prop-shaft;

(d) housing. - What is the design life and failure probability of each of the PSRU bearings when evaluated in the load model?
- What is the heat load imposed on this PSRU at the design power level, and how does this PSRU dissipate that heat load?
- Does this PSRU provide for a constant-speed propeller, and if so,

(a) Is the propeller governor drive integral with the PSRU?

(b) What is the load capacity of the governor drive?

(c) What is the design life of the governor drive, and how was that life determined?

(d) What is the is the expected life of the mechanism to transmit propeller governing pressure

(320 psi.) from the (static) housing into the (moving) propshaft? - Is there a
**one-way clutch**anywhere in this PSRU? (This one isn't #13 by accident !)