The Art of the Helicopter John Watkinson - Karatunov.net

186 The Art of the Helicopter
Fig. 5.15 (a) In still air hover, downwash on the tail boom is vertical except for a swirl component. (b) In
crosswind hover, the resultant airflow direction due to downwash and wind may cause significant boom lift.
If this opposes the tail rotor thrust the crosswind hover performance may be impaired. A high-mounted boom
strake may be used to suppress boom lift in crosswinds, whereas a low-mounted boom strake may be used to
define a separation point for downwash in the hover and thereby reduce shaking.
Fig. 5.16 The fenestron is more than a ducted fan because some of the thrust is produced by the low pressure
on the intake side acting on the side of the fin.
5.10 The fenestron
The fenestron is basically a ducted fan built into an opening in the tail fin and was
developed by Aerospatiale. The name comes from the French fenêtre meaning a window.
It is commonly thought that the fenestron works like a small tail rotor, but this
is not quite the case. In fact the fin structure is responsible for a significant amount
of the system thrust. Figure 5.16 shows that the fan in the fenestron is a multi-bladed
variable pitch device of high solidity. However, the fan is set in a duct that is relatively