The plainjap shown in figure 1.17 is a simple 
hinged portion of the trailing edge. The effect 
of the camber added well aft on the chord 
causes a significant increase in cbr. In addi- 
tion, the zero lift angle changes to a more 
negative value and the drag increases greatly. 
The split flap shown in figure 1.17 consist of 
plate deflected from the lower surface of the 
section and produces a slightly greater change 
in c ImoT than the plain flap. However, a much 
larger change in drag results from the great 
turbulent wake produced by this type flap. 
The greater drag’may not be such a disadvan- 
rage when ir is realized that it may be advan- 
tageous to accomplish steeper landing ap- 
proaches over obstacles or require higher power 
from the engine during approach (to minimize 
engine acceleration time for waveoR). 
The slottedPap is similar to the plain flap but 
the gap between the main section and flap 
leading edge is given specific contours. High 
energy air from the lower surface is ducted to 
the flap upper surface. The high energy air 
from the slot accelerates the upper surface 
boundary layer and delays airflow separation 
to some higher lift coefficient. The slotted 
flap can cause much greater increases in c,,, 
than the plain or split flap and section drags 
are much lower. 
The Fowkr&zp arrangement is similar to the 
slotted flap. The difference is that the de- 
flected flap segment is moved aft along a set of 
tracks which increases the chord and effects 
an increase in wing area. The Fowler flap is 
characterized by large increases in c,,, with 
minimum changes in drag. ,. 
One additional factor requiring consider- 
ation in a comparison of flap types is the aero- 
dynamic twisting moments caused by the 
flap. Positive camber produces a nose down 
twisting moment-especially great when large 
camber is used well aft on the chord (an 
obvious implication is that flaps are not prac- 
tical on a flying wing or tailless airplane). 
The deflection of a flap causes large nose down 
moments which create important twisting 
NAVWEPS OO-BOT-BO 
BASIC AERODYNAMICS 
loads on the structure and pitching moments 
that must be controlled with the horizontal 
tail. Unfortunately, the flap types producing 
the greatest increases in c,,- usually cause the 
greatest twisting moments. The Fowler flap 
causes the greatest change in twisting moment 
while the split flap causes the least. This 
factor-along with mechanical complexity of 
the installation-may complicate the choice 
of a flap configuration. 
The effectiveness of flaps on a wing con- 
figuration depend on many different factors. 
One important factor is the amount of the 
wing area affected by the flaps. Since a 
certain amount of the span is reserved for 
ailerons, the actual wing maximum lift prop- 
erties will be less than that of the flapped 
two-dimensional section. If the basic wing 
has a low thickness, any type of flap will be 
less effective than on a wing of greater thick- 
ness. Sweepback of the wing can cause an 
additional significant reduction in the effec- 
tiveness of flaps. 
High lift devices applied to the leading edge 
of a section consist of slots, slats, and small 
amounts of local camber. The fixed slot in a 
wing conducts flow of high energy air into the 
boundary layer on the upper surface and delays 
airflow separation to some higher angle of 
attack and lift coefficient. Since the slot 
alone effects no change in camber, the higher 
maximum lift coefficient will be obtained at a 
higher angle of attack, i.e., the slot simply 
delays stall to a higher angle of attack. An 
automatic slot arrangement consists of a 
leading edge segment (slat) which is free to 
move on tracks. At low angles of attack the 
slat is held flush against the leading edge by 
the high positive local pressures. When the 
section is at high angles of attack, the high 
local suction pressures at the leading edge 
create a chordwise force forward to actuate