The deflected jet principle is similar to the deflected slipstream, except that the turning is done internally. In hovering flight the exhaust from the turbojet engine, normally expelled in the aft direction, is diverted by a system of vanes to produce a vertical component of thrust.
External Flow, Jet-Augmented Flaps
The question might be asked, “When does a deflected slipstream or jet become an external flow, jet-augmented flap?” This is a little difficult to answer. However, in a deflected slipstream most of the lift is derived from redirecting the jet momentum, whereas in the externally augmented flaps increased lift is produced by the wing by means of circulation and boundary layer controls afforded by blowing over the flap.
Boundary Layer Control (blc)
There are several methods of boundary layer control, each of which has the same purpose of preventing boundary layer separation. One method controls the boundary layer by sucking off the slower-moving air either by a relatively uniform distribution of holes  or in a series of slots running spanwise along the airfoil .
Another method  feeds higher-energy air into the boundary layer by blowing air tangentially to the upper surface of a deflected flap. This scheme is sometimes referred to as a blown flap. It is also possible to accomplish BLC by means of a jet flap, which is simply a sheet of air blown downward from the trailing edge of an airfoil. Its effect on the airfoil is similar to that of the usual flap. In addition to blowing on trailing edge flaps, it is also possible to blow on leading edge flaps to prevent leading edge separation. These schemes are shown in Fig. 1-5.
If the amount of air of a blown or jet flap is increased beyond the value required to prevent boundary layer separation, additional circulation is produced around the airfoil. This increased circulation will produce a lift in excess of that predicted from potential flow or jet reaction.