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We can imagine it as an upside-down wing. It aims to create a downforce similar to a wing on an F1 car. You might wonder why we need downforce. It is simple. As you see in the first image there 2 forces, lift force created by the wing which acts at the Center of lift and the weight of the aircraft which acts at center mass. Let's do some calculations. We first begin by giving an arbitrary value for the distance between CL and CG to be 1 m. We will also say that lift force is +1000N and weight is -1000N therefore net force is zero. Moment = F x Distance. Let’s calculate the moment at CL which is 1000N x 1m = 1000Nm Counter-clockwise. The Net Moment is not zero resulting in aircraft pitching down.

Author: Thushanth Parameswaran updating Date: 2024/07/17

Background continued…

Vertical Stabilizer

The vertical stabilizer is basically a wing but vertical ( normally symmetrical airfoil unlike a wing of a plane). Vertical stabilizer has a rudder which is responsible for creating Yaw which is movement around vertical axis as seen in the image below.

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Lets see how Yaw is created. Before continue to read please refer to the airfoil section to understand how and why airfoil produce lift. The image below gives you a brief run-through of how yaw in created.

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That is the background info on what empennage is and how it works.

Author: Thushanth Parameswaran updating Date: 2024/07/21

Source: https://www.fzt.haw-hamburg.de/pers/Scholz/HOOU/AircraftDesign_9_EmpennageGeneralDesign.pdf

General Design Rules

The lever arm of the empennage should be as large as possible. (Lever arm is the distance between the aerodynamic center of the wing and the horizontal stabilizer). This will reduce the size of the empennage, saving weight.
The aspect ratio of the horizontal stabilizer should be about half the aspect ratio of the
wing.
Horizontal Stabilizer is fixed, an incidence angle of around 2° to 3° downwards
should be chosen to create a negative lift. (assuming we are using asymmetrical airfoil)
Symmetrical airfoils are exclusively selected for vertical Stabilizer. (NACA 0009 might be a good start)
Horizontal stabilizers can be either symmetrical or slightly cambered to produce negative lift.