Page 27 - November 2022

You probably know how much I like autofeather, but if you are flying a model without it, expect significant windmilling propeller drag until you manually feather.
So what happens to propeller rotation when we feather? Well, duh, it stops, doesn’t it? Not always.
You see, unlike a piston engine, the free-turbine PT6 has almost no resistance to propeller rotation. So unless the feather blade angles are set exactly correct and we are flying without sideslip, it is not uncommon to see slight propeller rotation after feathering a shutdown engine in flight. Sometimes the propeller rotates in the forward direction, sometimes it goes backwards and, yes, sometimes it does completely stop. The Raisbeck-Hartzell “Quiet Turbofan” propellers almost always rotate quite aggressively in the normal direction when feathered ... about 20 RPM. My friend James Raisbeck taught me years ago
that his propellers have such a large amount of twist in them that airframe drag would actually be higher if he designed the feather angle such that they would truly stop.
Once the feathered propeller stops or at least almost stops, the sucking or pulling action of the power turbine that was helping move air through the engine then goes away. Without it, the only factor making the compressor rotate is the ram air supplied by the cowling. It is my experience that the N1 we see now will vary from roughly 3% to 10%. The lower numbers will be seen in level flight at typical single-engine airspeeds. The higher numbers will be seen only in a dive, with IAS above 200 knots. Also, the thicker air at lower altitudes provides more ram effect and tends to rotate the compressor faster. If the propeller had never been feathered, the windmilling compressor speeds
will tend to be about 10% faster due to the sucking action of the power turbine. In other words, instead of a range of 3% to 10%, now we will see probably 13% to 20%.
Pop quiz question: What is the minimum N1 required before introducing fuel during an airstart?
Did you answer 12%? Most people do, but they’re wrong. Yes, 12% is stated in the POH as the minimum compressor speed needed before introducing fuel during a ground start. If we can get that much or more for an airstart, great. However, there is no minimum speed limit listed for an airstart. I believe the King Air 300-series’ POHs “encourage” obtaining at least 10%, but it is not an actual requirement.
Can you guess where I am going with this? Here it is: Can we do a windmilling (no starter assist) airstart with the propeller feathered? The answer is a qualified “Yes.”
NOVEMBER 2022
KING AIR MAGAZINE • 25