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Tail Area

CG to tail arm ft
wing area ft2
wing span ft
wing chord ft
horizontal tail area ft2
vertical tail area ft2
total tail area ft2
horz tail coefficient (0.30 to 0.60)
vert tail coefficient (0.02 to 0.05)



Enter arm, area, and span.

Introduction

T he tail size and CG location are the primary factors controlling stability of an aircraft. For most aircraft the CG must remain within 17% to 30% of the wing average chord. With the CG already fixed, the only major factor remaining for stability control is tail size.

Vertical tail

The primary role of the vertical tail is to provide yaw damping and control. Yaw damping is needed to suppress the tenancy to oscillate in side to side motion. The vertical tail also provides yaw stability, though this is usually already achieved once yaw damping is sufficient.

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The rudder stabilizes yaw (left), while dihedral stabilizes spirals (right).

One measure of the vertical tail's effectiveness is the vertical tail volume coefficient:

Vv = (vert area * tail arm) / (wing area * wing chord)

where:
 Vv = vertical tail volume coefficient
 vert tail area (ft2)
 wing area (ft2)
 wing chord (ft)   equal to (wing area / winsg span).
 vert tail arm (ft)  distance from CG to center of the tail.

Typical Vertical Tail Properties

typical values:
 vertical tail volume coefficient values are 0.02 to 0.05
 vertical tail volume is 25-33% of total tail volume.
 rudder volume is 25-33% of vertical tail volume
 rudder chord is 25-33% of vertical tail chord

Note: Too small of rudder area makes crosswind landing control difficult.

The above formula may be rewritten to provide the recommended vertical tail surface. The shape doesn't make any difference, so long as it has the proper area. It is a good idea to keep the aspect ratio of the horizontal stabilizer the same as the main wings. ..

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If Vv is too small, the aircraft will tend to oscillate or "wallow" in yaw as the pilot gives rudder or aileron inputs. This oscillation, is called Dutch Roll, and makes precise directional control difficult. Too small Vv also gives poor rudder roll authority in turns and may cause the allowable CG envelope size to be decreased. And too small Vv can delay spin recovery, or make recovery impossible.

Too large of a vertical tail size can require excessive control forces and can lead to loss of directional control during crosswind landing and taxi.

If Vv is too small, the aircraft will tend to oscillate or "wallow" in yaw as the pilot gives rudder or aileron inputs. This oscillation, is called Dutch Roll, and makes precise directional control difficult. Too small Vv also gives poor rudder roll authority in turns and may cause the allowable CG envelope size to be decreased. And too small Vv can delay spin recovery, or make recovery impossible.

Horizontal tail

The primary role of the horizontal tail is to provide stable pitch control. Too small a surface can cause loss of control at the rear CG limit, diminished altitude control, and porpoising - which is the tendency to have pitching up and down oscillations. A small horizontal tail will decrease the allowable CG envelope size.

If tail volume is too large, then excessive control input may be required by the pilot and excessive drag diminishes the aircraft's performance. A similar formula and coefficient can be found for the horizontal tail surface and will provide the horizontal tail volume effectiveness:

Vh = (horz area * tail arm) / (wing area * wing chord)

where:
 Vh = horizontal tail volume coefficient
 horz tail area (ft2)
 wing area (ft2)
 wing chord (ft)   equal to (wing area / winsg span).
 horz tail arm (ft) distance from CG to center of the tail.

Typical Horizontal Tail Properties

typical values:
 horizontal tail volume coefficient values are 0.30 to 0.60
 horizontal tail volume is 60-75% of total tail volume.
 elevator volume is 25-33% of horizontal tail volume
 elevator chord is 25-33% of horizontal tail chord
 stabulator area is 1/3 smaller than conventional tail

Note: Too small of elevator area makes slow flight and stall recovery difficult.

The above formula may be rewritten to provide the recommended horizontal tail surface. Again, the shape doesn't really matter much, so long as it has the proper area. The vertical and horizontal surfaces do not have to be directly over each other. The important thing is to have the right size.

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