NAVWEPS DD-BOT-BD  APPLICATION OF AERODYNAMICS  TO SPECIFIC PROBLEMS OF FLYING  while a negative (down) gust causes a de-  crease  in angle of attack.  Of course, a change  in angle of attack will effect a change in lift  and, if some critical combination of high gust  intensity and high flight speed is encountered,  the change in lift may be large enough to  cause structural damage.  In order to obviate some of the problems  of a deficiency of airspeed at takeoff, usual  result can be an excess  of airspeed at takeoff.  The principal effect of an BXCBJS  takeoff  air@pssd  is the greater takeoff distance which results.  The general effect is that each 1 percent excess  takeoff velocity incurs approximately 2 per-  cent additional takeoff distance. Thus, excess  speed must be compared with the additional  runway required to produce the higher speed.  In addition, the aircraft tires may be subject  to critical loads when the airplane is at very  high rolling speeds and speeds in excess of a  basically high  takeoff speed may produce  damage or failure of the tires.  At low flight speeds  during approach, land-  ing, and takeoff, the effect of the vertical gust  is due to the same mechanism of the change  in angle of attack.  However, at these low  flight speeds, the problem is one of possible  incipient stalling  and sinking rather than  overstress. When the airplane is at high  angle of attack, a further increase in angle of  attack due to a gust may exceed the critical  angle of attack and cause an incipient stalling  of the airplane.  Also, a decrease in angle  of attack due to a gust will cause a loss of lift  and allow the airplane to sink.  For this  reason, any deficiency of airspeed will be quite  critical when operating in gusty conditions.  As with the conditions of landing, excess  velocity or deficiency of velocity at takeoff  is undesirable. The proper takeoff speeds and  angle of attack must be utilized to assure  satisfactory takeoff performance.  The effect of the hori<oonral  gust differs from  the effect of the vertical gust in that the im-  mediate effect is a change of airspeed rather  than a change in angle of attack.  In this  sense, the horizontal gust is of little conse-  quence in the major airplane airloads and  strength limitations.  Of greater significance  is the response of the airplane to horizontal  gusts and wind shear when operating at low  flight  speeds. The possible conditions in  which an airplane may encounter horizontal  gusts and wind shear are illustrated in figure  6.5.  As the airplane traverses  a shear of wind  direction, a change in headwind component  will exist. Also, a climbing or descending  airplane may traverse a shear of wind velocity,  i.e., a wind profile in which the wind velocity  varies with altitude.  The variation of wind velocity and direction  throughout the atmosphere is important be-  cause of its effect on the aerodynamic forces  and moments on an airplane. As the airplane  traverses this variation of wind velocity and  direction during flight, the changes in airflow  direction and velocity create changes in the  aerodynamic forces and moments and produce  a response of the airplane. The variation of  airflow velocity along a given direction exists  with  shear parallel to the flow direction.  Hence, the velocity gradients are often re-  ferred to as the wind “shear.”  The effect of the vertical gust has important  effects on the airplane at high speed because  of the possibility of damaging flight loads.  The mechanism of vertical gust is illustrated  in figure 6.5 where the vertical gust velocity  is added vectorially to the flight velocity to  produce some resultant velocity. The principal  effect  of the vertical gust is to produce a change  in airplane angle of attack, e.g., a positive  (up) gust causes  an increase in angle of attack  The response of an airplane is much de-  pendent upon the airplane characteristics but  certain basic effects are common to all ait-  planes. Suppose that an airplane is estab-  lished in steady, level flight with lift equal to  weight, thrust equal to drag, and trimmedso  367  GUSTS AND WIND SHEAR 