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What is Flight Stability
› The ability of an aircraft to maintain level
flight without rotating in any of the axes
In flight there are 3 major locations that
are considered for flight stability
› Center of Lift (Neutral Point)- The location
where all of the lifting forces can act
› Center of Gravity- Location where the
weight is considered
› Center of Thrust- Location where the thrust is
considered
Mcg= Moment about the center of gravity
q = dynamic pressure
S = wing area
c = chord length
α = angle of attack
Non-Dimensionalized: M => Cm
SV
LCL 25.0
cSV
MCM 25.0
• Stable:
• Slope must be negative
• Moment Coefficient at 0
angle of attack must be
> 0
• Static Stability Margin:
• The more positive it is, the
more longitudinally stable the
aircraft.
• Determines the maximum aft
C.G. location
Want the nose to naturally drop if a gust causes
the AC to pitch up
› Place the c.g. as far forward as possible
› Place the horizontal tail as far aft as possible
If there is a Canard
A tail upstream of the c.g., reduced static
stability
Move wing aft or c.g. forward to
compensate.
http://etds.studentendansen.nl/45/to-the-airport/by-airplane
Aircraft may become sluggish, hard to maneuver Tail will resist pilot attempts to change the aircraft angle
of attack.
A large tail adds to aircraft weight, and cost.
Tail generates drag!
For improved maneuverability, some aircraft reduce or eliminate the static stability margin. This is called Relaxed Static Stability
Fighter Aircraft are statically unstable: Their nose will continue to pitch up.
the lift will continue to go up after an upward gust is encountered.
Will stall and “flip over”
Actively controlled by the pilot: Onboard computer
Redundant computers systems in case of a failure.
Effective Dihedral
Physical Dihedral
High-Wing, Mid-Wing, Low-Wing
Rearward-Swept, Straight, Forward-
Swept
Too much leads to Dutch Roll
Control is the ability for the pilot to
change orientation of an airplane
› Longitudinal – Elevator
› Lateral – Aileron
› Direction – Rudder
Stability and Control together are
important for flight