center of gravity aft of the main wheels a bal- 
ancing load on the tail wheel must be produced 
toward the center of turn. When the tail 
wheel is free to swivel, the equilibrium of the 
turn requires a control force opposite to the 
direction of turn-i.e.. control force insta- 
bility. The inherent stability problem exists 
because the center of gravity is aft of the point 
where the main side forces are developed. This 
condition is analogous to the case of static 
longitudinal stability with the center of 
gravity aft of the neutral point. 
The conventional tail wheel configuration 
has this basic instability or ground loop tend- 
ency which must be stabilized by the pilot. 
At high rolling speeds where aerodynamic 
forces are significant, the aerodynamic direc- 
tional stability of the airplane resists the 
ground looping tendency. The most likely 
times for a ground loop exist when rolling 
speeds are not high enough to provide a con- 
tribution of the aerodyhamic forces. When the 
tail wheel is free to swivel or when the normal 
force on the tail wheel is small, lack of pilot 
attention can allow the ground loop to take 
place. 
The tricycle landing gear configuration has 
an inherent stability d,ue to the relative posi- 
tion of the main wheels and the center of 
gravity. Centrifugal force produced by a 
turn is balanced by the side force on the main 
wheels and a side force on the nose wheel in 
the direction of turn. Note that the freeing 
the nose wheel to swivel produces moments 
which bring the aircraft out of the turn. Thus, 
the tricycle configuration has a basic stability 
which.is given evidence by control displace- 
ment and a wheel side force in the direction 
of turn. Because of the contrast in stability, 
the tricycle configuration is much less difficult 
to maneuver than the tail wheel configuration 
and does not provide an inherent ground loop 
tendency. However, a steerable nose wheel 
is usually necessary to provide satisfactory 
maneuvering capabilities. 
NAVWEPS DD-BDT-80 
STABILITY AND CONTROL 
The bicycle configuration of landing gear 
has stability characteristics more like the 
automobile. If directional control is ac- 
complished with the front wheels operated 
by power controls, no stability problem exists 
at low speeds. A problem can exist when the 
airplane is at high speeds because of a distribu- 
tion of normal force being different from the 
ordinary static weight distribution. If the 
airplane is held onto the runway at speeds 
well above the normal takeoff and landing 
speeds, the front wheels carry a greater than 
ordinary amount of normal force and a tend- 
ency for instability exists. However, at these 
same high speeds the rudder is quite powerful 
and the condition is usually well within 
control. 
The basically stable nature of the tricycle 
and bicycle landing gear configurations is best 
appreciated by the ease of control and ground 
maneuvering of the airplane. Operation of 
a conventional tail wheel configuration after 
considerable experience with tricycle cohfigu- 
rations requires careful consideration af the 
stability that must be furnished by the pilot 
during ground maneuvering. 
SPINS AND PROBLEMS OP SPIN 
RECOVERY 
The motion of an airplane in a spin can 
involve many complex aerodynamic and in- 
ertia forces and moments. However, there are 
certain fundamental relationships regarding 
spins and spin recoveries with which all 
aviators should be familiar. The spin differs 
from a spiral dive in that the spin always 
involves flight at high angle of attack while 
the spiral dive involves a spiral motion of 
the airplane at relatively low angle of attack. 
The stall characteristics and stability of 
the airplane at high lift coefficients are im- 
portant in the initial tendencies of the airplane. 
As previously mentioned, it is desirable to 
have the wing initiate stall at the root first 
rather than tip first. Such a stall pattern