Page 20 - Volume 11 Number 6
P. 20
Ask the Expert Propeller Erosion...
and How to Avoid It
Walk down a ramp where numerous King Airs are parked and look closely at their propeller blades. I wager that you will see quite a variety of conditions. Some will look almost new, whereas others will be badly sandblasted. It may be that the reason for the difference is reasonable and unavoidable. Namely, one airplane operates only on long, paved, well-maintained runways, and the other aircraft spends much of its life operating from a short, dirt strip on the owner’s ranch. But it also may be that the one with the sad-looking props, even though it spends much of its time on good runways, is also suffering from one of two things, or a combination of both: Poor pilot technique and/or power levers that are poorly adjusted in the Beta and Reverse ranges. The goal of this article is to review proper operating techniques with you, as well as, providing a procedure for knowing if your Beta/Reverse rigging is as it should be.
Let’s start with the rigging discussion and first review the three-blade model 200 graph below.
This particular graph presents numbers applicable to a three-blade model 200, and except for the numbers, it applies to all PT6-powered King Airs. The later models have the Ground Fine stop between Beta and Reverse and do not have the red stripes. The (+) and (-) symbols represent the areas where positive or negative thrust occurs, statically on the ramp.
As the graph shows, the position of the power lever controls two different things: Compressor Speed (N1 or Ng) and the position of the propeller’s Low Pitch Stop (LPS). (This stops also goes by the name of “Flight Idle Stop” in some references, including portions of the Maintenance Manuals. I have always believed that “Low Pitch Stop” is a more obvious term that better describes exactly what is being repositioned.)
Notice the flat portion of the upper line, the Beta area. This flat portion, in which N1 should not change, is appropriately known as the “Dead Band” since movement of the power lever within this range causes no response – dead reaction – from the Fuel Control Unit (FCU). By definition, the Beta Range is where the propeller’s LPS is being repositioned to flatter blade angles while N1 is not changing.
Behind Beta is the Reverse Range. By definition, not only is the LPS continuing to be repositioned to lesser blade angles – it is, in fact, going to negative angles, meaning that the propeller is pushing air forward instead of aft – but also N1 is proportionally increasing, getting greater the more aft the power lever is moved. Typically, Maximum Reverse, all the way aft, should yield an N1 speed of about 85 percent. Remember that the relative speed of the compressor is not the same as the engine’s relative power output. In other words, 85 percent N1 does not yield 85 percent power. On the contrary, 85 percent N1 is probably a bit less than 50 percent power!
by Tom Clements
18 • KING AIR MAGAZINE
JUNE 2017
(PHOTO CREDIT: DAN MOORE VMG LLC)