Tail boom for an aircraft provided with a central airframe having at least one lift rotor, said tail boom being provided with an elongate structure which extends in a longitudinal direction over a distance referred to as the "length " along an anteroposterior plane of symmetry of said elongate structure, said structure extending from top to bottom from a vertex to a base which includes a planar surface, characterised in that said tail boom comprises at least one protrusion, each protrusion extending said base downwards in order to reduce the negative lift of the tail boom, each protrusion being rigidly connected to said structure and extending beneath said structure in said longitudinal direction, there being no air passage separating said protrusion from said structure, at least one portion of the tail boom contained in a vertical plane orthogonal to said longitudinal direction having: - an edge surface of said structure which is referred to as the "main edge surface " and extends in elevation downwards from the vertex to said base over a distance reaching a maximum referred to as the "maximum height (Hmax)", and which extends laterally from a second flank to a first flank over a distance reaching a maximum referred to as the "maximum thickness (EPmax)", - an edge surface of each protrusion, referred to as the "extension edge surface ", being contiguous with a lower zone of said structure, said lower zone extending in elevation upwards from said planar surface over a height referred to as the "lower height (Hinf)", which is less than 15% of the maximum height (Hmax), the extension edge surface extending in elevation downwards over a distance reaching a maximum referred to as the "extension height (Hext)" and laterally over a distance reaching a maximum referred to as the "extension thickness (EPext)", said extension height (Hext) being between 0.05 times the maximum thickness (EPmax) inclusive and 0.5 times the maximum thickness (EPmax) inclusive of said portion, said extension thickness (EPext) being between 0 non-inclusive and 0.4 times the maximum thickness (EPmax) inclusive of said portion, said extension height of at least one protrusion being variable along the length of said structure. - an edge surface of said structure which is referred to as the "main edge surface " and extends in elevation downwards from the vertex to said base over a distance reaching a maximum referred to as the "maximum height (Hmax)", and which extends laterally from a second flank to a first flank over a distance reaching a maximum referred to as the "maximum thickness (EPmax)", - an edge surface of each protrusion, referred to as the "extension edge surface ", being contiguous with a lower zone of said structure, said lower zone extending in elevation upwards from said planar surface over a height referred to as the "lower height (Hinf)", which is less than 15% of the maximum height (Hmax), the extension edge surface extending in elevation downwards over a distance reaching a maximum referred to as the "extension height (Hext)" and laterally over a distance reaching a maximum referred to as the "extension thickness (EPext)", said extension height (Hext) being between 0.05 times the maximum thickness (EPmax) inclusive and 0.5 times the maximum thickness (EPmax) inclusive of said portion, said extension thickness (EPext) being between 0 non-inclusive and 0.4 times the maximum thickness (EPmax) inclusive of said portion, said extension height of at least one protrusion being variable along the length of said structure.