Xuan Feng Pao (旋風砲)

Drawing of a Xuan Feng Pao, from 'Wu Bei Zhi (《武備志》)'.

Xuan Feng Pao (旋風砲, lit. 'Whirlwind trebuchet') is a type of traction trebuchet first invented in China but later spread westwards to Middle East and Europe. It is a "single pole" type trebuchet with its axle and frame mounted on the top of a single pole.

For normal trebuchet, the crews have to pick up entire siege engine just to change its direction. Xuan Feng Pao has no such problem. (Source: Exarc.net)

While less powerful than other trebuchets, the single pole configuration of Xuan Feng Pao offers multiple unique advantages, namely it can pivot to facilitate aiming, can be set up in cramped space, not to mention multiple trebuchets can be packed together to increase density and intensity of bombardment. On top of that, the height of a Xuan Feng Pao can be easily matched to the height of the wall it's intended to defend by simply adjust the length of its pole and arm. This allows the trebuchet to be easily concealed behind the wall, thus improving its survivability.

Variants
Du Jiao Xuan Feng Pao (獨角旋風砲, lit. 'Single-leg whirlwind trebuchet')

Drawing of a Du Jiao Xuan Feng Pao, from 'Wu Bei Zhi (《武備志》)'.

Du Jiao Xuan Feng Pao is a variant of Xuan Feng Pao that is mounted on a four-legged support frame. Unlike normal Xuan Feng Pao that has to be planted into the ground and braced before use, Du Jiao Xuan Feng Pao is always ready for use and can be easily relocated. However, it also takes up more space compared to normal Xuan Feng Pao.


Xuan Feng Wu Pao (旋風五砲, lit. 'Five whirlwind trebuchet')

Drawing of a Xuan Feng Wu Pao, from 'Wu Bei Zhi (《武備志》)'.

Xuan Feng Wu Pao is simply five Xuan Fen Pao mounted on a single platform. It allows the same group of rope-pulling crews to launch up to five projectiles in rapid sucession.


Mobile whirlwind trebuchet

Several variants of wheeled whirlwind trebuchets. From left to right: Pao Che (砲車), Xuan Feng Che Pao (旋風車砲), Wo Che Pao (臥車砲). From 'Wu Bei Zhi (《武備志》)'.

Due to its lightweight and ability to pivot, Xuan Feng Pao is uniquely suitable to be mount on a mobile platform. Cart- or wagon-mounted Xuan Feng Pao can be used as mobile siege defence, offensive siege weapon, and even as field artillery.


Counterweight whirlwind trebuchet?

Drawing of a Lou Chuan (樓舡) or tower ship equipped with a whirlind trebuchet with a spherical fixed counterweight in place of pulling ropes. From a Ming period print of 'Wu Jing Zong Yao (《武經總要》)'.

Although not explicitly specified, some editions of Wu Jing Zong Yao (《武經總要》) contain illustrations that depict what appears to be counterweight version of Xuan Feng Pao mounted on warship (see picture). This suggests that Chinese people may have developed a type of fixed counterweight trebuchet as early as 11th century, well before Mongol invaders introduced them to the more advanced swinging counterweight trebuchet during the 13th century.