Page 26 - Volume 14 Issue 3

Pressure Altitude and OAT are the variable parameters that the chart uses (see example below).
For an example, let’s consider the four-blade “Quiet Turbofan” Raisbeck-Hartzell propeller that is widely used on many 90-series airplanes. With the propeller levers fully forward – setting the primary governor to its maximum, takeoff, propeller speed – power is added to bring the propeller speed up to 1,800 RPM. Since we are not yet on the governor, still in an underspeed condition, the propeller blade angle is at the LPS setting. For standard Sea Level conditions, 15°C, the torque should now be 500 ft-lb with a tolerance of +20/-0 ft-lbs and within 20 ft-lbs of the other side.
Seeing torques of, say, 560 on the left side and 480 on the right side, tells us three things: First, the LPS is incorrect on both sides and needs adjustment. Second, the higher torque on the left side indicates that there is more resistance to rotation on that side. Why? Because the blades are taking too big of a bite of air ... the LPS is at a larger blade angle, coarser than it should be. Likewise, the right side’s LPS is too fine, at too small of an angle. (To make this easier to understand, just consider how difficult it would be to spin a feathered propeller at 1,800 RPM. Why, we would likely hit the torque limit first! Vice versa, if the blade angle were totally flat, near zero degrees, it would spin quite rapidly very easily, with
very low torque applied.) The third thing we can learn from this mis-match in “Flight Idle” torques is that we can anticipate a yaw to the right in the flare ... more drag on that side when the propellers finally come off of the governors and hit their respective LPSs.
Quiz time: In addition to Pressure Altitude and OAT, what other variable factor will affect the LPS torque test results?
That’s right ... wind! Based on the whole premise of this article, headwind in the run-up area helps the propeller to rotate. With that help, the flow of exhaust gases across the power turbine blades don’t have to do as much work, don’t have to deliver as much torque. Vice versa – a tailwind in the run-up area leads to that “Reverse Windmilling” tendency and acts to slow the propeller down. Thus, more torque will be required to reach the specified 1,800, RPM value.
And now you know why, when the wind is not calm that the Low Pitch Stop torque value must be the average of one reading taken while facing into the wind and one taken while facing downwind.
OK, OK, I hear you. Next month I will pick a topic that is not so technical! See you then. KA
I am very happy to report that “The King Air Book – Volume II” is now available! This new book, which I finished creating a couple of months ago, is a compilation of all of the articles that I have written for this magazine. They are indexed according to topic, making it easy to find your areas of interest. In addition, a digital copy that is searchable and in color is also available for ordering. Go to www.kingairacademy.com to find the links for both. I appreciate your support!
King Air expert Tom Clements has been flying and instructing in King Airs for over 46 years, and is the author of “The King Air Book.” He is a Gold Seal CFI and has over 23,000 total hours with more than 15,000 in King Airs. For information on ordering his book, contact Tom direct at twcaz@msn.com. Tom is actively mentoring the instructors at King Air Academy in Phoenix.
If you have a question you’d like Tom to answer, please send it to Editor Kim Blonigen at editor@blonigen.net.
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24 • KING AIR MAGAZINE
MARCH 2020