# Interaction of the Vertical Tail. and the Horizontal Tail

The flow conditions at the vertical and horizontal tails are affected not only by the fuselage and wing but also considerably by their mutual interaction. Of special

Figure 7-42 Local efficiency factors of the vertical tail. Curves 90^/90 = const, from [14], bj2R =7.5. Wing of rectangular planfoxm A = 5. (a) High-wing airplane, (b) Low-wing airplane. |

Г {2) Figure 7-43 Interference between vertical and horizontal tails. Circulation distribution and free vortex sheet of a sideslipping vertical and horizontal tail system, from Laschka [19]. |

interest here are the conditions at the tail unit at sideslipping and rolling. A tail unit at which the middle section of the horizontal tail lies over the root of the vertical tail will be considered to demonstrate this fact.

On a vertical tail in an incident flow of sideslip angle j3, a circulation distribution is generated that does not drop to zero at the root section but rather has a finite value because of the end-plate effect of the horizontal tail. A circulation discontinuity results now in the shedding of a single vortex that turns in a direction opposite to that of the rest of the free vortices. This vortex in turn induces at the horizontal tail a downwash exceeding the counteracting induction effect of the continuous free vortex sheet. The resulting circulation distribution at the horizontal tail has, as shown in Fig. 7-43b, a discontinuity in the middle of the horizontal tail; it is antimetric and generates a rolling moment due to sideslip that is reversed from that of the vertical tail (see Fig. 7-43, from Laschka [19]).

To reduce the load induced on the horizontal tail by the sideslipping vertical tail, a positive dihedral may be provided. This increases, however, the total rolling moment due to sideslip. On the other hand, the rolling moment due to sidesUp of the tail unit may be reduced by providing the horizontal tail with a negative dihedral.

By extending and applying a suitable panel method as described in Sec. 6-3-1 for the wing-fuselage system, the pressure distributions, and thus the acting forces and moments, can also be determined for the whole airplane; compare, for example, [15].

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