It is always nice to see articles that review key basic concepts such as yaw and left-turning tendencies; however, the opportunity to avoid performing the typical gloss-over explanations was lost in Budd Davisson’s “Yaw Killer” (December 2011 Flight Training).
The angle of attack experienced by the propeller is more complex than it may seem, and it is the function of the rotational velocity (rpm) of the propeller and the forward speed of the aircraft. In level flight, the AOA is equal for both ascending and descending propellers. As the aircraft accelerates, the AOA is reduced because the forward velocity helps average the relative wind between the plane of rotation of the propeller and the airspeed of the aircraft. As the aircraft pitches up, however, the descending propeller has a forward moving vector; i.e., that prop is actually partially moving forward, in effect, increasing the rotational velocity of the propeller. This draws the relative wind experienced by that prop closer to the plane of rotation, thus increasing angle of attack. The opposite occurs for the ascending prop, which has a backward movement vector. More thrust is generated on the right side of the prop, yawing the aircraft to the left. This is often misunderstood or simply ignored.
Torque and gyroscopic precession should have been mentioned. Torque actually does affect takeoff. As the aircraft tries to roll to the left, this force pushes on the left tire, creating more friction (normal force) which, in turn, drags that side of the aircraft backwards—resulting in a left-yawing tendency. Also, I was a bit surprised that the fact that gyroscopic precession is actually a right-yawing tendency on rotation for tricycle aircraft was not presented.
For more details, I would suggest readers reference the Pilot’s Handbook of Aeronautical Knowledge pages 4-24 through 4-28.
David C. Ison
St. Pete Beach, Florida
Mr. Davisson says,
“There is no yaw involved. So the control to be used is the aileron. Not the rudder.” Try that on takeoff. You will not keep the airplane on the runway, let alone the centerline. It will be even more exciting with a left crosswind.
Torque is one of the four left-turning tendencies in airplanes. All four are countered with rudder. While torque can be considered a rolling action, in an airplane it translates to a yawing action. Fix it with rudder. Holding a Cessna 172 on a heading, when climbing, with aileron will work. But the ball is out to the right. Center the ball with right rudder and the need for right aileron goes away. Asymmetrical thrust only occurs when the nose of the airplane is above the line of flight. Any discussion of this left-turning tendency should include the rotational velocity of the propeller and the speed of the relative wind on the descending (advancing) blade and the ascending (retreating) blade of that propeller.
David Bradley
New Franklin, Missouri
There was a wasted opportunity in "Technique: Chandelles," December 2011 Flight Training. In the discussion no mention of rudder use was made. The left chandelle was the best teacher of the principles of rudder use I have ever had. As in the left turn, you start with slight left rudder, and at the end are almost standing on the right rudder.
Raul Zambrano
Wellsville, New York
That was a nice illustration of and explanation of how a vertical speed indicator works (“How It Works: Vertical Speed Indicator,” December 2011 Flight Training). A pilot can use the VSI to help adjust his descent rate for let-down and/or visual approach to landing and/or an ILS approach to landing and/or verify his climb rate at takeoff, to clear obstruction. The function of the orifice is the real secret of the VSI.
The orifice (or so-called release valve) is the key component that allows the VSI to display a descent or ascent in feet per minute on a calibrated scale. Whenever the aircraft is moving up or down, there will be a difference in pressure across the inside and outside of the bellows. The faster the ascent or descent, the greater
the differential pressure on the bellows, and therefore the greater the deflection of the needle on the face of the VSI, showing the pilot his or her rate of climb or descent. As you are climbing or descending, the orifice is always trying to equalize this differential pressure. If you stop climbing or descending, it will equalize and show you zero feet per minute.
Rodney Harmon
Nashville, Indiana
Errata
In our “Aviation College Directory” (December 2011 Flight Training), the following errors appeared: Enterprise Ozark Community College should have been listed as Enterprise State Community College Alabama Aviation Center and is VA approved; Wallace State Community College does not offer an aircraft maintenance program; the website for Middle Georgia College in Eastman is www.mgc.edu.
In Rod Machado’s response to Mr. N.W. in “Since You Asked” (December 2011 Flight Training), the second sentence in the last paragraph should have read, “Once you’re established in a right steep turn, you can expect to hold left aileron to sustain that bank, and right rudder to compensate for the airplane’s left-turning tendencies (P-factor and propeller slipstream).”
Flight Training regrets the errors.
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