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

http://www.berkeleyscience.com/airplane.htm

Automatic response to outside

disturbances

› Turbulence

› Wind

Statically unstable

Dynamically unstable

Static & dynamic stability

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

Center of Gravity

(CG)

Neutral Point (NP)

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

Control Surface Used:

• Ailerons

15

Control Surface Used:

• Elevators

16

Control Surface Used:

• Rudder

17