Forces of Flight and the Forward Slip

The FAA Handbook of Aeronautical Knowledge offers the following guidance to pilots aspiring to master airmanship: “Thrust, drag, lift, and weight are forces that act upon all aircraft in flight. Understanding how these forces work and knowing how to control them with the use of power and flight controls are essential to flight.”
The FAA Handbook of Aeronautical Knowledge offers the following guidance to pilots aspiring to master airmanship: “Thrust, drag, lift, and weight are forces that act upon all aircraft in flight. Understanding how these forces work and knowing how to control them with the use of power and flight controls are essential to flight.”
The laws of motion and how forces work on an aircraft are dramatically demonstrated in the forward slip. Understanding the four forces of flight is akin to my advice that good stick and rudder pilots adhere to Newton’s Laws of Motion. Any pilot who commands an airplane to perform contrary to the laws of physics will have a bad experience.

The forward slip in the context of final approach to landing is a maneuver used to loss altitude; to control the glide path in airplanes without flaps. Before continuing I will address those pilots (and there are many) who adamantly deny any difference between a forward slip and a slid slip. Please read the FAA Flying Handbook, Chapter 8 and see figures 8-11 and 8-12 to learn the difference and to learn to fly the two maneuvers.

Forward slips are confusing to many pilots because of a faulty narrative about drag. It is commonly believed that drag causes an airplane to lose altitude. That mistaken narrative states that in a forward slip a pilot should use rudder to present the side of the airplane to the wind; the resulting drag causes the airplane to lose altitude. That narrative sounds logical; however, the narrative is contrary to the laws of physics, the principles that govern motion, and the mathematics that support those principles.

Reference to the FAA Handbook of Aeronautical Knowledge defines drag: a rearward, retarding force caused by disruption of airflow by the wing, rotor, fuselage, and other protruding objects. As a general rule, drag opposes thrust and acts rearward parallel to the relative wind. In a forward slip, drag is that magical force that makes it possible to control airspeed during the rapid descent. Drag does not cause the airplane to fall; that would be weight, the force of gravity. The Four Forces of Lift diagram illustrates the point that weight (gravity), not drag, opposes lift.

The FAA Handbook of Aeronautical Knowledge defines lift as a force that is produced by the dynamic effect of the air acting on the airfoil, and acts perpendicular to the flight path through the center of lift (CL) and perpendicular to the lateral axis. In level flight, lift opposes the downward force of weight.

When the wings are banked from the level flight attitude, the total force of lift has two components. The vertical component of lift is inverse to bank angle (greater the bank,less vertical lift). In a forward slip the goal is to reduce altitude; therefore it follows that bank angle controls rate of descent.

To do a forward slip, as any other flight maneuver, use the control you need, when you need it and as much as you need it. Use the ailerons to select and maintain bank angle consistent with desired rate of descent; use rudder to control the nose and maintain a flight path forward towards runway; use the elevator (as always) to control airspeed.

Click here to see this month’s feature video Forward Slip verses Side Slip.

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