Premature stagnation of the boundary layer 
means that all subsequent airflow will overrun 
this point and the boundary layer will separate 
from the surface. Surface flow which is aft of 
the separation point will indicate a local flow 
direction forward toward theseparation point- 
a flow reversal. If separation occurs the posi- 
tive pressures are not recovered and form drag 
results. The points of separation on any aero- 
dynamic surface may be noted by the reverse 
flow area. Tufts of cloth or string tacked to 
the surface will lie streamlined in an area of 
unseparated flow but will lie forward in an 
area behind the separation point. 
The basic feature of airflow separation is 
stagnation of the lower levels of the boundary 
layer. Airjh ~cparation muh when the lower 
lcvcls of the boundary layer do not have sujicicnt 
kinetic cncrgy in the prwncc of an advcm ps.wrc 
gradient. The most outstanding cases of air- 
flow separation are shown in figure 1.26. An 
airfoil at some high angle of attack creates a 
pressure gradient on the upper surface too 
severe to allow the boundary layer to adhere 
to the surface. When the airflow does not 
adhere to the surface near the leading edge 
the high suction pressures are lost and stall 
occurs. When the shock wave forms on the 
upper surface of a wing at high subsonic speeds, 
the increase of static pressure through the 
shock’ wave creates a very strong obstacle for 
the boundary layer. If the shock wave is 
sufhciently strong, separation will follow and 
“compressibility buffet” will result from the 
turbulent wake or separated flow. 
In order to prevent separation of a boundary 
layer in the presence of an adverse pressure 
gradient, the boundary layer must have the 
highest possible kinetic energy. If a choice is 
available, the turbulent boundary layer would 
be preferable to the laminar boundary layer 
because the turbulent velocity profile shows 
higher local velocities next to the surface. 
The most effective high lift devices (slots, 
slotted flaps, BLC) utilize various techniques 
NAVWEPS OO-SOT-80 
BASIC AERODYNAMICS 
to increase the kinetic energy of the upper sur- 
face boundary layer to withstand the more 
severe pressure gradients common to the higher 
lift coefficients. Extreme surface roughness 
on full scale aircraft (due to surface damage, 
heavy frost, etc.) causes higher skin friction 
and greater energy loss in the boundary layer. 
The lower energy boundary layer may cause a 
noticeable change in C,-” and stall speed. In 
the same sense, vortex generators applied to 
the surfaces of a high speed airplane may allay 
compressibility buffet to some degree. The 
function of the vortex generators is to create a 
strong vortex which introduces high velocity, 
high energy air next to the surface to reduce 
or delay the shock induced separation. These 
examples serve as a reminder that separation is 
the result of premature stagnation of the 
boundary layer-insufficient kinetic energy in 
the presence of an adverse pressure gradient. 
SCALE EFFECT. Since the boundary layer 
friction and kinetic energy are dependent on 
the characteristics of the boundary layer, 
Reynolds Number is important in correlating 
aerodynamic characteristics. The variation of 
the aerodynamic characteristics with Reynolds 
Number is termed “scale effect” and is ex- 
tremely important in correlating wind tunnel 
test data of scale models with the actual flight 
characteristics of the full size aircraft. The 
two most important section characteristics 
affected by scale effects are drag and maximum 
lift-the effect on pitching moments usually 
being negligible. From the known variation 
of boundary layer characteristics with Rey- 
nolds Number, certain general effects may be 
anticipated. With increasing Reynolds Num- 
ber, it may be expected that the section maxi- 
mum lift coefficient will increase (from the 
higher energy turbulent boundary layer) and