INTRODUCTION TO STABILITY AND CONTROL

STABILITY SUMMARY

Axes, Moments, Velocities – Definitions

Moments and Forces

Static Longitudinal StabilityTail EffectsWing Effects

Static Margin

Directional Static StabilityVertical TailWing/Body

Lateral Static StabilityVertical TailWing Sweep

TRADITIONAL AIRCRAFT CONTROLS

Ailerons

(All moving)Elevators

Rudder

ALTERNATE CONTROL METHODS

Canards

Spoilers (T1) and Speed Brakes

Wing Warping

Center of Gravity Shift

V-tail (combines pitch and yaw control)

Thrust Vectoring or Asymmetric Thrust

Flaperons (flap and aileron)

Elevons (elevator and aileron)

Ruddervators (rudder and elevator)

AIRCRAFT MOTIONS - ROLL

Roll: what is it?

Ailerons

AIRCRAFT MOTIONS - ROLL

Roll: Motion about the longitudinal (X) axis produced by the ailerons (l moment)

Ailerons

AIRCRAFT MOTIONS - PITCH

Pitch:

Elevator

AIRCRAFT MOTIONS - PITCH

Pitch: Motion about the lateral (Y) axis produced by the elevators (m moment)

Elevator

AIRCRAFT MOTIONS - YAW

Yaw:

Rudder

AIRCRAFT MOTIONS - YAW

Yaw: Motion about vertical (Z) axis produced by the rudder(s) (n moment)

Rudder

STABILITY VS.MANEUVERABILITY (CONTROL)

Stable Aircraft—not very easy to move Not very maneuverableC-5, C-17, B-52, Passenger airplanes

Maneuverable Aircraft—very easy to moveNot very stable (unstable in many cases)Require Flight Control Systems to keep aircraft pointy end forward

F-16, F-22

MOMENTS AND FORCES

Trimmed Flight SMcg = 0

Straight and Level, Unaccelerated Flight (S.L.U.F.) SF = 0

L = W T = D

CONVENTIONAL AIRPLANE

cg

Lw

Lt

Ma.c.

xcg

xac

xt

SMcg = 0 = Ma.c + Lw (xcg – xac) – Lt (xt)

CRITERIA FOR LONGITUDINAL STATIC STABILITY

1. CM,0 > 02. ∂CM,cg / ∂ a < 0

Aircraft is not moving in pitch!

LONGITUDINAL STABILITY—TAIL EFFECTSTail aft of cg is Stablizing

Canards are Destabilizing

Increase stability (more negative CMa) by

Lifttail Longer moment arm

St Larger tail

LONGITUDINAL STABILITY—WING EFFECTSWing a.c. forward of c.g. is Unstable

Decrease instability (lower CMa)

↓ (hcg – hac) Shorter Moment Arm or move c.g. forward

LONG.-STATIC STABILITY - TOTAL AIRCRAFT

Most parameters are fixed once the aircraft is built

C.G. can be moved Cargo location Fuel location Weapons, Stores, etc.

Variable Geometry wings—change cg

CONVENTIONAL TAIL - STABILIZING

F-22

F-16

CANARDS I - DESTABILIZING

Su-35

Long-Eze

CANARDS II - EUROFIGHTER

Vertical Tail Contribution to DIRECTIONAL STATIC STABILITY

Design Considerations (Main Contributor)

• Vertical tail aft of c.g. is stabilizing• To increase directional stability

-- Vert. tail further aft -- Vert. tail bigger (or add another)

Top View

x

y

+ Ncg

+b

Lv

V

Wing/Body Contribution to DIRECTIONAL STATIC STABILITY

Design Considerations

- Fuselage area forward of the cg is directionally destabilizing

- That’s why aircraft have tails!

Top View

x

y

- Ncg

- Ncg

+b VLw/b

Vertical Tail Contribution toLATERAL STATIC STABILITY

Design Considerations

- Vertical tail above c.g. is stabilizing - To increase lateral stability

-- Vert. tail taller” -- Vert. tail “bigger” (more area)

-- Increase Vert. tail lift curve slope (Increase ARvt and/or Increase evt)

Rear View

y

z

-L

V

Wing Sweep Contribution toLATERAL STATIC STABILITY

x

y

+ b

V

Rear View

y

z

Top View

V

Positive wing sweep is stabilizing

Less lift More lift