Page 20 - Volume 10 Number 7

makes sense to complete GUMP and move the prop levers forward early for these operators.
As most of you know, the maximum propeller speed of various King Air models has been decreasing with the passage of time. From a 2,200 RPM maximum in the early 90s and 100s, it went to 2,000 in the 200-series, then to 1,900 in the F90-series and the C90GT variants (and the Blackhawk -135A installations), and finally down to 1,700 in the 300-series. “Great!” you say, “The lower RPM reduces noise and must be good thing!”
Well, it seems to me that the old adage “For every good, there’s a bad,” applies here quite well. Here’s the bad: If other factors are equal, the slower a propeller turns the more difficult it is to utilize Beta and Reverse. Let me try to explain; a graph may be helpful here. Imagine starting at a speed of 140 KIAS or more in your 90-series or 100-series model, pulling power back to Low Idle, and then holding altitude as the airplane slows. There’ll come a time when the propeller speed began to decrease, since the low blade angle limit of the propeller governing range – the LPS, or Low Pitch Stop – had been reached. The RPM would drop below 2,200 well before it would decrease down to 1,900.
Take a look at the graph:
Consider this: If an F90 at maximum weight makes a No Flap landing, it should be at 127 KIAS crossing the 50-foot threshold point on landing. At touchdown, it is probably still going between 110 and 120. That means that no Reverse is available until about another 20 knots is lost while rolling on the runway. Good thing we selected a long enough runway!
Many F90s, F90-1s, C90GTs or Blackhawk- converted 90s, and Raisbeck-converted 90s and 100s, often do their cruising and approach phases with the prop speed pulled back to 1,700 or 1,750 RPM. Do you see what’s coming? My goodness, it will now take probably less than 80 knots to be able to enter Beta and Reverse with the governor set for such a low speed! So I recommend that the prop levers be advanced fully forward – or at least to 1,900 RPM for the Raisbeck systems that still have a higher maximum RPM setting – when flying these models unless you plan to roll a long distance on the runway to make a distant turn-off taxiway, without lifting the power levers until you are quite slow.
“But wait,” you may be thinking, “I’ll just go ahead and run the props forward at touchdown and then I’ll be ready to use Beta and Reverse, right?” The problem with that idea is, since you’ll still be on the governors after touchdown, you will get the resultant prop speed increase, additional drag, and very likely even some asymmetrical drag unless both props are rigged identically. The only way to avoid these annoyances is to wait long enough to observe the RPM decrease before pushing the prop levers forward.
Speaking of asymmetric drag, if you are experiencing some of this while flaring – perhaps it seems the airplane wants to start a little sashaying dance left and right – I think I know the cause and the solution. It only happens with the later style, Type II, prop synchrophaser systems. In this system, there is no fixed master and slave unit, but instead the slower propeller always tries to flatten its pitch to speed up to and match the faster one ... in a very limited RPM range, of course. I believe when this is taking place, yet with the range of blade angle travel being limited by the LPS, we sometimes encounter this left versus right “battle.” Solution? Easy – turn the Prop Sync switch off before landing ... just like you need to do with the older style, Type I system.
As mentioned briefly before, the 300-series airplanes are the exception to the need for a relatively low airspeed before reaching an underspeed condition, before being able to use Beta and Reverse. Their Flight Low Pitch Stop is set at a surprisingly large blade angle, causing them to reach an underspeed condition at a much higher airspeed than other King Airs. Their
As you can see, for the same propeller and the same Low Idle N1 speed, about 110 KIAS is necessary before the propeller speed drops below 2,200 RPM, yet about 95 KIAS is required to get below 1,900 RPM. Realize that Beta and Reverse are achieved by repositioning the movable Low Pitch Stop (LPS). Thus, until the propeller blade angle is being controlled by the LPS – and that only takes place when in an underspeed condition – utilization of Beta and Reverse is impossible.
The F90 made its appearance in 1978 and was the first King Air to have 1,900 RPM as its maximum propeller speed. For the first time, Beech actually added a comment into the POH concerning this fact. It states, “WARNING: Propellers will NOT Reverse at airspeeds in excess of 95 knots IAS.”
18 • KING AIR MAGAZINE
JULY 2016