Page 20 - Volume 14 Number 8

servo filled with regulated P3 pressure. It is an all-or- nothing system, meaning that when the difference in P3 between the left and right engines gets great enough to trigger the P (Delta P) switch, the rudder force applied is constant, helping apply about 40 pounds of force to the “good” side’s rudder cable.
The first 1,000-plus 200s and B200s and all of the C90As were manufactured with three-blade propellers. These props had no minimum idle speed restrictions since they were not prone to the “reactionless vibration” concerns that came with the four-blade propellers. Thus, the Low Idle prop speed ran a little below 1,000 RPM, making the airplane quieter on the ramp and with less tendency to taxi too fast. Low Idle N1 or Ng speeds were just slightly above 50%. P3 pressure relates to N1 speed but not in a linear fashion. When N1 goes from 50% to 60%, P3 increases more than the 10% you might expect.
Do you understand why it is easier to trigger RB during a ground test when equipped with three-blade, and not four-blade props? It follows this logic: When an engine is actually shut down in flight, P3 becomes basically ambient pressure. To achieve the 60 psi differential pressure that the P switch requires to trigger RB – a little less than 60 psid in the 90-series – requires only moderate power on the operative engine, with its commensurate relatively low N1 speed and with torque well below its maximum limit. For the ground test, the low power engine is usually at Low Idle, putting out a relatively low P3 pressure but a value significantly above ambient pressure. Hence, to achieve the needed P, the “good” engine needs to be turning faster, putting out more P3 and creating more torque. Fortunately, the torque required to trigger RB is still easily attainable.
Using rough numbers for a 200, we might experience RB kicking the good side’s rudder pedal in flight with an engine actually shut down at about 1,400 ft-lbs. of torque. However, we will need perhaps 1,600 to 1,800 ft-lbs. to trigger the kick when doing the ground test with the other engine at Low Idle, near 50% N1. Now put on four-blade propellers and adjust the Low Idle speeds up to near 60%. Since the idling, lower-power, engine is now putting out more P3 pressure because of its elevated Idle speed, the other engine must put out even more power to create enough P3 to trigger the P switch. Now the torque can easily exceed 2,000 ft- lbs. before the rudder kick is felt! In fact, on hot days with the air conditioning operating – requiring an Idle speed of 62% or even more – it can become impossible to check RB without torque exceeding the 2,230 ft-lb. limit. (The chapter in my book discusses ways to still achieve a proper test.)
A slight diversion: The higher Low Idle speeds associated with the four-blade propellers led to another “problem.” The autofeather system originally used 200 ft-lbs. as the setting for the low-pressure switch, the point at which the automatic feathering actually takes place. It was impossible to reach this low torque value in some 200s with the four-blades during the autofeather test, done with the low-power engine at Idle, not actually shut
18 • KING AIR MAGAZINE
AUGUST 2020