Page 24 - Volume 15 Number 4

obvious that it was desirable to move the high pitch stop to near 90 degrees: the feathered position that provided the least propeller drag when the engine is shutdown. Thus, with very few exceptions, the propellers on multi-engine airplanes are “constant-speed, full-feathering” propellers in which the maximum blade angle possible is indeed near the 90-degree mark.
How do we convert a “constant- speed, full-feathering” propeller into a “constant-speed, full-feathering, reversing” propeller as installed on most all King Airs? Simple: We move the low pitch limit of blade angle travel from the “don’t fall out of the sky” position where it existed for so many years and now position it to the “back side” of flat pitch.
“Blade angle” is defined as the angle between the chord line of the propeller’s airfoil shape and the plane of propeller rotation ... the disk the prop makes as it rotates. Because propellers have a twist in them, the
The full range of blade angle travel.
Feather 90°
22 • KING AIR MAGAZINE
APRIL 2021
Reverse
Flat Pitch
angle depends on how far out from the propeller’s center (the chord line for measurement) is selected. A bigger angle exists 1 foot from the center than the one that exists at the two-, three- and four-foot locations. The “30-inch station” (30 inches out from the propeller’s center) is the most common choice for blade angle measurement on “smaller”
propellers. As the propeller diameter gets larger, it is common to use a point one-foot further out at the 42-inch station.
Neglecting the twist in the blades for a moment, the highest blade angle capable of being attained – where metal hits metal and the blade cannot move further unless something drastically breaks – the feathered position, is a 90-degree blade angle. The lowest angle where metal hits metal is now a negative angle, reflecting the fact that the blade bite is now pushing air forward instead of backward. Typically, this limit of travel angle is near -10 degrees.
Allowing the blade angle to go to -10 degrees is surely desirable after landing when a short stop is desired yet without excessive brake usage. Allowing the blade angle to reach -10 degrees while still flying? Not so good!
Therefore, what reversing propel- lers contain is a variable, movable, Low-Pitch Stop (LPS). The “how” this movement is achieved will be left out of this simplified discussion. But suf- fice it to say that when the pilot of a King Air lifts up on a power lever and then pulls it aft behind the location of Idle, he or she is indeed repositioning the LPS to lesser and lesser angles, ending up in the most negative blade angle position when the power lever reaches its full-aft, most-rearward po- sition. Vice versa, pushing the power lever forward causes the LPS to come forward.
Beta
Propeller Governing Range
Low Pitch Stop
Maximum Reverse -10°
19 30 15