LIFT-FAN LIMITATIONS

It is recommended that a nose-mounted lift-fan NOT be incor­porated into the design of the SSTOVLF for pitch attitude control. XV-5A flight tests demonstrated that although the pitch-fan proved to be effective for pitch attitude control, fan ram drag forces caused adverse handling qualities and reduced the conversion airspeed corridor. It is thus recommended that a reaction control system be incorporated.

The X-14A roll-control lift-fan tests revealed that control of rolling moment by varying fan rpm was unacceptable due to poor fan rpm response characteristics even when closed – loop control techniques were employed. Thus this method should not be considered for the SSTOVLF. However, lift-fan thrust spoiling proved to be successful in the XV-5 and is rec­ommended for the SSTOVLF.

Avoidance of the fan stall boundary placed significant oper­ational limitations on the XV5 and had the potential of doing the same with the SSTOVLF. Fan stall, like wing stall, must be avoided and a well defined safety margin required. Approach to the fan stall boundary proved to be a particular problem in the XV-5B, especially when performing steep terminal area maneu­vers during simulated or real instrument landing approaches. The SSTOVLF preliminary designers must account for antici­pated fan stall limitations and allow for adequate safety mar­gins when determining SSTOVLF configurations and flight profile specifications.