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     Corner ve/ocity turn.    Here, the aircraft touches both the structural and aero-
dynamic limits as represented by n = nUm and CL = CL.n   .We have
                                A
CD = CDO + kC~.,IMX
V-
  T-D
    sin y - -
       w
         V, ,      Y
R- r
co =
V cos y
(2.310)
(2.311)
(2.312)
(2.313)
(2.314)
AIRCRAFT PERFORMANCE
Conventlonal Turn
a)
n
Herbst Manewer
Fig.2.29   Conventionalturn and Herbst maneuver.
139
The time for one complete turn of 27r radians and height lost per turn is given by
t27r -
2rr V cos y
g W- COS2 Y
Ah - (V sin Y)t27r
(2.315)
(2.316)
    Herbst maneuver.    In the conventional turning flight (Fig. 2.29a), the aircraft
that is irutially in a level flight (A) banks in to the turn (B) to have a component
of lift force L sin pr, to generate the requned centripetal acceleration. The aircrafi
will continuously turn (C) until the nose points in the right direction (D), at which
point the pilot will level the wings and return to the le'vel flight condition, The
scenario is quite different in the Herbst maneuver, named after the late German
pilot Wolfgang Herbst, who conceived this poststall maneuver as a quick way
of pointing the aircraft's guns at a target during a close encounter. The Herbst
maneuver is currently explored using the experimental X-31 aircraft. Schematics
of the Herbst maneuver are shown in Fig. 2.29b.
    To start the Herbst maneuver from normal level fiight condition (1), the pilot
decelerates the aircraft by pitching the nose up till the angle of attack goes well
beyond the stall (2), and the aircraft is nearly perpendicular to the flight path (3),
using the entire airframe as a speed brake. The pilot rolls the aircraft about the
velocitjt vector until the nose points in the right direction (4), lowers the nose, and
accelerates back to the desired speed in level fiight in a new direction (5).
   The rolling motion about the velocity vector is often referred to as "coning"
motion. This maneuver is used to mirumize the sideslip excursions during the roll.
If the aircraft roLls about the body axis while operating at high angles of attack,
it will experience significant sideslip buildup. The development of sidesip is not
desirable because it increases drag and affects the turning performance.
                                          Exampfe 2.9
    A propeller airplane weighs 50,000 N and has a wing area of 30 m2, structural
limit load factor of 2.5, and a maximum lift coefficient of 1.75. The drag polar of

 

140             PERFORMANCE, STABILITY, DYNAMICS, AND CONTROL
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