Page 26 - Volume 15 Number 4

that lead to an underspeed condition – low power – is automatically achieved. The second factor needed to reach an underspeed condition – low airspeed – is one of the remaining factors within our control. The other remaining factor is the governor’s propeller speed setting.
Back to the simple fixed-pitch prop on the trainer: With idle power, the speed of the propeller follows the speed of the airplane. Go into a dive, the prop goes faster; slow into a climb, it goes slower. Sit on the ramp with no wind at idle and it may only turn 600 RPM or so.
As for King Airs, have you noticed the minimum speed allowed for a windmilling airstart? It’s 140 KIAS. Where did this number originate? That is the speed at which the propeller can windmill at takeoff RPM with no engine power whatsoever! The force of the air spinning the propeller at its LPS – alone, with no exhaust gas driving the power turbine – spins the prop right up to redline RPM.
Follow me through on this. Let’s use a PT6A-21- powered C90 as an example. At Low Idle, there are indeed some exhaust gases helping to drive the propeller. It will therefore take less windmilling force to achieve the same RPM. Instead of 140 knots required to reach redline propeller speed, now it may happen at 110 knots. At any speed of 110 KIAS or more, we can reach 2,200 RPM at Low Idle. With the propeller levers fully forward,
setting the propeller governor at 2,200, we will be seeing 2,200 RPM at any airspeed of 110 or higher. If we were to hold altitude with Low Idle power and allow the speed to decrease, the RPM would start dropping as the speed goes below about 110. At 105 knots the RPM may be down to 2,100; 100 knots might yield 2,000 RPM; 95 knots might yield 1,900 RPM.
Now what if we took this same propeller/engine/ airframe combination but installed a new governor that has a maximum speed setting of 1,900 instead of 2,200 RPM? To achieve an underspeed condition – the prerequisite for allowing the propeller to follow the moving LPS into Beta and Reverse – our IAS must be below 95 knots. Do you see where I am going with this? The F90, debuting in 1978, was the first member of the King Air 90-series that used 1,900 instead of 2,200 RPM as its propeller’s normal speed limit. Beech felt it necessary to add a statement into the POH stating, “CAUTION: Propellers will not reverse at speeds above 95 KIAS.”
I often repeat the adage, “For every good, there’s a bad.” The good thing about slower propeller speed is less noise. The bad thing is that it makes reverse harder to obtain. More than one King Air pilot has found out the hard way that touching down with too much excess speed is a sin that use of reverse cannot cure! Of course the reason why is that the propeller blade angle is still being
24 • KING AIR MAGAZINE
APRIL 2021