percent of the stall speed for propeller air- 
planes or 90 percent of the stall speed for jet 
airplanes. This feat must be accomplished on 
a smooth runway at all normal service takeoff 
loading conditions. 
Figure 4.19 illustrates the principal forces 
acting on an airplane during takeoff toll. 
When the airplane is in the three point attitude 
at some speed less than the stall speed, the 
wing lift will be less than the weight of the 
airplane. As the elevators must be capable 
of rotating to the takeoff attitude, the critical 
condition will be with zero load on the nose 
wheel and the net of lift and weight supported 
on the main gear. Rolling friction resulting 
from the normal force on the main gear creates 
an adverse nose down moment. Also, the 
center of gravity ahead of the main gear 
contributes a nose down moment and this 
consideration could decide the most aft loca- 
tion of the main landing gear during design. 
The wing may contribute a large nose down 
moment when flaps are deflected but this 
effect may be countered by a slight increase 
in downwash at the tail. To balance these 
nose down moments, the horizontal tail 
should be capable of producing sufficient nose 
up moment to attam the takeoff attitude. at 
the specified speeds. 
The propeller airplane at takeoff power may 
induce considerable slipstream velocity at the 
horizontal tail which can provide an increase 
in the e&iency of the surface. The jet 
airplane does not experience a similar magni- 
tude of this effect since the induced velocities 
from the jet are relatively small compared 
to the slipstream velocities from a propeller. 
LANDING CONTROL REQUIREMENT 
At landing, the airplane must have suthcient 
control power to ensure adequate control at 
specified landing speeds. Adequate landing 
control is usually assured if the elevators are 
capable of holding the airplane just off the 
runway at 105 percent of the stall speed. Of 
course, the most critical requirement will exist 
when the c.g. is in the most forward position, 
NAVWEPS 00-BOT-80 
STABILITY AND CONTROL 
flaps are fully extended, and power is set at 
idle. This configuration will provide the 
most stable condition which is most demand- 
ing of controllability. The full deflection of 
flaps usually provides the greatest wing diving 
moment and idle power will produce the most 
critical (least) dynamic pressure at the hoti- 
zontal tail. 
The landing control requirement has one 
particular difference from the maneuvering 
control requirement of free flight. As the 
airplane approaches the ground surface, there 
will be a change in the three-dimensional flow 
of the airplane due to ground effect. A wing in 
proximity to the ground plane will experience 
a decrease in tip vortices and downwash at 
a given lift coefficient. The decrease in down- 
wash at the tail tends to increase the static 
stability and produce a nosedown moment from 
the reduction in download on the tail. Thus, 
the airplane just off the runway surface will 
requite additional control deflection to trim 
at a given lift coefficient and the landing con- 
trol requirement may be critical in the design 
of longitudinal control power. 
As an example of ground effect, a typical 
propeller powered airplane may requite as 
much as 15” more up elevator to trim at CL- 
in ground effect than in free flight away from 
the ground plane. Because of this effect, many 
aitplaneshavesufIicientcontrolpowertoachieve 
full stall out of ground effect but do not have 
the ability to achieve full stall when in close 
proximity to the ground. 
In some cases the effectiveness of the control 
surface is adversely affected by the use of trim 
tabs. If trim tabs are used to excess in ttim- 
ming stick forces, the effectiveness of the 
elevator.may be reduced to hinder landing or 
takeoff control. 
Each of the three principal conditions re- 
quiting adequate longitudinal control are ctit- 
ical for high static stability. If the forward