- PSRU Load Model -
How To Predict The Life Of Components
A load model is a set of operating scenarios which realistically define the worst-case environment in which a system is intended to operate. It is an important step early in the design process to establish a load model for any system being designed. From that model, you calculate the loads and number of cycles which each scenario imposes on the various parts, then design the components to achieve the desired life under those loads and durations.
The automotive load model deserves mention here, because some products use critical parts taken from automotive systems. Be aware that, depending on the aerodynamics of your vehicle, it only takes between 30 and 60 HP to move your car along the road at 60 MPH. Doubt that? My station wagon gets 21 MPG at a steady 60 MPH. That's a fuel burn of 17.1 pounds per hour. Assuming a BSFC of 0.42, the engine is producing 40.8 HP at that fuel burn. Certainly, more power is required for acceleration and hill-climbing, but if you think about it, most of the operational time in your vehicle is spent in some form of cruise.
Most automotive propulsion systems are designed around a load model in which the maximum rated load occurs during only 5% to 10% of the design life, and 75% or more of the design life is at less than 25% of maximum output. Compare that model to an aircraft load model in which a large portion of the operation (90% or more) is at an engine power output of more than 75% of rated engine power. Contemplation of that fundamental difference should give an insight into why automotive parts are generally unsuitable for use at their rated level in aircraft applications.
The following table is one example of a typical EPI aircraft load model used for propulsion system design, using the torque curve from a normally-aspirated 500 HP engine and a set of flight conditions which reasonably approximate worst case usage.
LOAD MODEL
| Operation | HP | RPM | Torque | % | minutes per hour | |
| 1 | Max Performance Takeoff | 500 | 4600 | 570 | 5% | 3.0 |
| 2 | Vx Climb | 500 | 4600 | 570 | 10% | 6.0 |
| 3 | Vy Climb | 500 | 4600 | 570 | 10% | 6.0 |
| 4 | Max Cruise | 450 | 4000 | 590 | 20% | 12.0 |
| 5 | Normal Cruise | 400 | 3800 | 552 | 53% | 31.8 |
| 6 | Aerobatics (360°/sec yaw & pitch) | 500 | 4600 | 570 | 2% | 1.2 |
| Totals: | 100% | 60.0 |
For each scenario in the load model, one can calculate
- the gear loads from applied torque and tooth dynamics,
- the bearing, shaft and housing loads produced by torque, gear separation, thrust, and gyroscopic forces,
- the cooling load imposed by the power transmitted and
- the lubrication requirements.
For each new design we do at EPI, we generate a Load Model which represents the anticipated usage of the unit. From the loads and durations defined in that model, we design to the following criteria:
- theoretically infinite life in the load model for critical components (shafts, gears and housings) and
- a minimum of 2000 hours life for replaceable components (bearings and seals).
If a PSRU designed for this load model were used for racing or competition aerobatics, the more-severe loads would reduce the life expectancy, just as it does with a certified aerobatics engine such as the Lycoming AEIO-540. However, the life would be adequate considering the maintenance which racing and aerobatic aircraft normally receive.
