NAVWEPS 00-801-80  AIRPLANE PERFORMANCE  the effect of a headwind is to reduce the  takeoff ground velocity by the amount of  the headwind velocity, VW  the effect of these items on takeoff dis-  tance is  or  the effect of wind on acceleration is  negligible,  the effect of these items on takeoff distance  is  (ut 1eaJt this effect because  weight will  alter the net accelerating force)  g+?)x(Z)  J-2  WY2  a  -= -  J-1  (  )  WI  This result approximates the e5ect of gross  weight on takeoff distance for airplanes with  relatively high thrust-to-weight ratios.  In  effect, the takeoff distance will vary at least  as the square of the gross weight.  For ex-  ample, a 10 percent increase ,in takeoff gross  weight would cause:  a 5 percent increase in takeoff velocity  at least a, 9 percent decrease  in acceleration  at least a 21 percent increase in takeoff  distance  where  Xi= zero wind takeoff distance  Sa=takeoff distance into the head-  wind  V,= headwind velocity  VI= takeoff ground velocity with zero  wind,  or, simply, the take05  airspeed  For the airplane with a high thrust-to-weight  ratio, the increase in takeoff distance would  be approximately 21 to 22 percent but, for  the airplane with a relatively low thrust-to-  *eight ratio, the increase in takeoff distance  would be approximately 25 to 30 percent.  Such a powerful effect requires proper con-  sideration of gross weight in predicting takeoff  distance.  As a .result of this relationship, a headwind  wh,ich is 10 percent of the takeoff airspeed will  reduce the takeoff distance 19 percent. How-  ever, a tailwind (or negative headwind) which  is 10 percent of the take05 airspeed will in-  crease the takeoff distance 21 percent. In the  case where the headwind velocity is 50 percent  of the takeoff speed, the takeoff distance would  be approximately 25 percent of the zero wind  takeoff distance (75 percent reduction).  The effect  of wind on takeoff distance is large  and proper consideration also must be provided  when predicting takeoff distance.  The effect  of a headwind is to allow the airplane to reach  the takeoff velocity at a lower ground velocity  while the effect of a tailwind is to require the  airplane to achieve a greater ground velocity  to attain the takeoff velocity.  The effect of  the wind on acceleration is relatively small  and, for the most part, can be neglected. To  evaluate the effect of wind on takeoff distance,  the following relationships are used:  The e5ect of wind on landing distance is  identical to the effect on takeoff distance.  Figure 2.33 illustrates the general dfect of  wind by the percent change in takeoff or land-  ing distance as a function of the ratio of wind  velocity to takeoff or landing speed.  188 