Page 21 - Volume 11 Number 6

That King Air on the ramp with the badly sandblasted props? I’ll bet its dead band is too small, too narrow. The engines are increasing N1 speed before the propeller blades reach flat pitch.
This not uncommon problem means that the airplane is difficult to slow down while taxiing. Before the blades can reach flat pitch, when the propeller is then acting as a large disk giving neither positive nor negative thrust, power is already being added. In other words, an N1 increase is being encountered before we have reached the bottom of the Beta range. When power is added while the blades are still providing a positive bite of air, we start to go faster, not slower!
What many misguided pilots do in this situation is to pull the power levers back more until finally the taxi speed slows down. What has taken place is that at last the residual thrust has been eliminated by forcing the LPS to flat or even negative pitch but at the expense of a higher-than-needed and higher-than-desired propeller speed (Np), since the increased N1 is creating more exhaust gases that are driving the propeller with more power. This higher prop speed, usually associated with a slightly negative blade angle, causes lots of blade erosion.
I have received this question many times during my King Air training events: “Why don’t we get similar blade erosion when the blade angle is at, say +10°, then when it is at -10°? Even with High Idle selected, we can taxi all day with the power levers
at Idle and not erode the props, yet we chew up the blades at -10° and 70 percent N1. This doesn’t seem to make sense.”
The reason why a blade angle of -10° leads to more erosion than an angle of +10°: Realize that there is a pronounced twist in each propeller blade, such that the inboard areas are taking a significantly larger bite than the outboard areas. So, when +10° is happening at the 30- inch station – the normal location out from the hub where angles are
JUNE 2017
measured – the blade tip near the ground may be almost flat. That flat tip creates very little airflow disturbance so the sand and grit and gravel and dirt on the surface are disturbed little. But when the angle is -10° at the 30-inch station, the tip may be at -20° creating a great little sucking vortex that vacuums the debris off the ground with unfortunate efficiency!
Vice versa, suppose the dead band is too large, like the graph below.
Now it is easy to kill residual thrust without an increase in N1 speed (and I surely like that!), but it is now common to find that propeller speed decreases so much before N1 increases, that Reverse is sluggish and often asymmetric. Also, especially on the Honeywell (née Bendix) FCUs, Maximum Reverse is usually not near the proper 85 percent value. Starting to rotate the FCU’s speed setting shaft too late may not allow it to rotate far enough for the proper amount of Reverse power.
My preference would definitely be to have a bit too much dead band than not enough. So long as Maximum Reverse delivers reasonable stopping power, the wider dead band ensures being able to kill residual thrust for taxi.
By the way, how many readers are pulling the propeller levers all the way back into Feather while taxiing? With some situational awareness, this is a great technique! Not only can we achieve a propeller feathering check, but also with the blades slapping the
KING AIR MAGAZINE • 19