This discussion focuses on the design of aircraft tail sections specifically for low Reynolds numbers, relevant for small model-sized cargo planes. Key considerations include the impact of tail length on dutch roll stability, the influence of downwash from the main wing, and the importance of vertical stabilizers for directional stability. Recommended resources include "Understanding Flight" by David F. Anderson and Scott Eberhardt, and "Aircraft Design: A Conceptual Approach" by Daniel P. Raymer, which provide theoretical insights into tail design and stability factors.
PREREQUISITESAerospace engineers, aircraft designers, model aircraft builders, and students studying aerodynamics and aircraft stability will benefit from this discussion.
Extract page 107-8 said:Directional Stability.
In the previous section we discussed only stability in pitch, known as longitudinal stability. In Chapter 1 you were introduced to two other axes, roll and yaw. Roll stability, known as lateral stability, was covered in detail in Chapter 3, on "Wings." The effects of dihedral and sweep were presented and will not be repeated here. Directional stability is the stability in the yaw axis, and gives rise to the vertical stabilizer. The vertical stabilizer and rudder serve the same function as the horizontal stabilizer and elevator, except in yaw, instead of pitch. The rudder is used for control and the vertical stabilizer is for stability. The main function of the vertical stabilizer is to help the airplane weathervane and keep the nose pointed into the direction of flight.
The desire for directional stability is to have the airplane always line itself with the wind. So, if a gust temporarily perturbs the direction the nose is pointed, the tail will have a nonzero angle of attack with the airflow, as shown in Figure 4.6. This causes a restoring force to realign the tail with the direction of travel. The effects of misalignment with the flight path are primarily high drag and poor turn coordination.
The size of the vertical stabilizer depends on several factors. For a single-engine airplane, the requirement that sets the minimum size for the vertical stabilizer is that the vertical area of the airplane aft of the center of gravity be larger than the vertical area forward of the center of gravity. This is the same requirement that puts feathers on arrows for stability. A larger vertical stabilizer is needed to counter propeller rotation effects and adverse yaw in a turn, which was discussed in Chapter 3. A single-engine airplane can get away with the minimum-size vertical stabilizer but will require more work on the pilot's part.
For multiengine airplanes the size of the tail is dictated by the torque caused by the loss of one engine. The net thrust being off center causes the airplane to want to yaw. A large vertical stabilizer, with trim, can compensate for this. That is why twin-engine commercial transports have such large vertical stabilizers.
The FAA dictates limits on directional stability. Modern airplanes now have vertical stabilizers that are so effective as to make the use of the rudder for small corrections almost unnecessary.
Aircraft Design: A Conceptual Approachwahaj said:Anyone know any good resources on airplane tails? I am looking for a tail shape that will work at low reynolds numbers for a small (model sized) cargo plane. I found quite a lot on wings when I was looking for a shape for that but not so much for tails. If anyone knows any good books that explain some of the theory behind tails that would be very helpful.