Page 25 - Volume 14 Issue 3

To “prove” that this in merely a mental imaging exercise, keep in mind that oil pressure from the pump inside the propeller governor provides the force that overcomes the feathering springs’ efforts to send the blade angle to feather. What is the pump doing when rotation stops? Nothing! It’s not turning and hence is creating no additional oil pressure. Therefore, we could never really reach that totally flat blade angle position – the one that gives no rotational tendency – because there’d be no way to keep the blade at that position. It would always start leaking toward feather, and then rotation in the normal CW direction would begin again.
Why have I taken your time to read these past few paragraphs if it is an impossible situation to achieve? Because I want you to consider the effect of windmilling when (1) using reverse thrust after landing, and (2) when conducting LPS run-up tests.
Here we are on a short runway using the Maximum Reverse procedure: High Idle, prop levers fully forward on final, and lifting and pulling back as far as the power levers will go right at touchdown. We are now moving the LPS to its most negative blade angle position and simultaneously asking for about 85% compressor speed. As the blade angle becomes negative, the relative wind is now serving to resist, not aid, propeller rotation. The faster we are moving, the more resistance there is. This
explains why it is common to see the propeller speed slightly increase while in maximum reverse as the airspeed slows and approaches 60 knots, the point at which we should be easing out of the reverse range so as to be at Ground Fine by 40 knots ... to decrease the potential for propeller erosion and FOD (Foreign Object Damage). Proper power lever/engine rigging should be done on a run-up pad that has been swept clean of any debris. Now maximum reverse is obtained with zero airspeed, the only relative wind being what the prop wash itself provides. Do you see that the results observed during this check will not be identical to those observed at 80 knots after touchdown? Namely, the airspeed causes more resistance to propeller rotation and, hence, slightly less propeller speed. Observing different results on the maintenance run-up pad versus on the runway with significant airspeed is to be expected. It’s not cause for concern.
The “Flight Idle Torque Test” is how the pilots and mechanics can determine if the low pitch stop is set at the correct blade angle. It should be labeled the “Low Pitch Stop Test” and the Raisbeck Engineering Maintenance Manual does indeed use that name. For each different propeller – manufacturer, number of blades, designer – a chart exists that presents target torque to be achieved at a specified RPM. The existing