Aero Engineering 315

Lesson 38

Lateral/Directional Static Stability

Important Safety Tip… Glider project due next lesson! Turn in paper copies of

Cover sheet (name and documentation) Glider Design Project questions Spreadsheet (color version – or highlight which, if

any, blocks are RED) Design AND chart pages

Keep the rest of the info stuff for the fly off portion Remember it is INDIVIDUAL EFFORT ONLY Glider need not be constructed until flyoffs

(beginning Monday, 10 May)Glider Design Spreadsheet

My Glider Design

B-2—Lat/Dir Stability Challenge

Lat/Dir Stability Objectives

Define CN and recognize directional stability

Understand the contributions of the vertical tail, wing and fuselage to directional stability

Define CL and recognize lateral stability

Understand the contributions of the vertical tail, geometric dihedral, wing sweep, and wing placement to lateral stability

Understand coupling effects of directional and lateral stability

Sideslip Angle ()

is the angle between V and the aircraft’s x-axis.

Positive occurs when the aircraft is flying with “wind in the right ear”

V

Directional Static Stability (Weathercock Stability)

Yaw Moment Coefficient:

bSq

NC cg

cg,N

cg,NC

+ Ncg

x

y

V

Dir.-Stat. Stability Derivative:slope of CN,cg vs. curve

For a stable system what sign should this have? (+) positive

Positive slope indicates DIRECTIONAL STATIC STABILITY - a positive generates a positive (restoring) moment - a negative generates a negative (restoring) moment

CN,cg

0C cg,N

ZERO YAW MOMENTAT ZERO

Directional Static Stability (Weathercock Stability)

Vertical Tail Contribution to Directional Static Stability

Design Considerations (main contributor) Vertical tail aft of c.g. is stabilizing To increase directional stability

Top View

x

y

+ Ncg

Lv

V

Vertical tail further aft Vertical tail bigger (or add

another) Increase tail lift curve slope (increase ARvt and/or increase evt) Ventral fin

Design Considerations- Fuselage area forward of the cg

is directionally destabilizing- That’s why aircraft have tails!

Wing/Body Contribution to Directional Static Stability

Top View

x

y

- Ncg

- Ncg

VLw/b

Individual Component Contributions to Directional Static Stability

CN,cg

Tail

Aircraft

Wing/body

Directional Static Stability at High Mach

CN,cg

TAIL LESS EFFECTIVE WITH INCREASING SUPERSONIC MACH

TOTAL ACFT CAN BECOME UNSTABLE AT HIGH MACH(EX.: F-15 & F-16, XB-70)

Tail

Aircraft

Wing/body

THE F-117A IS WEAK IN NEUTRAL DIRECTIONAL STABILITY

HIGH PEAKED CANOPY SMALL RELATIVE VERTICAL STABILIZATION

THE F-117A IS WEAK IN DIRECTIONAL STABILITY

HIGH PEAKED CANOPY SMALL RELATIVE VERTICAL STABILIZATION

WEAK YAW STABILITY TO 0.6 MACH (300 KNOTS) ABOVE 0.6 MACH - UNSTABLE, AND INSTABILITY INCREASES AS

MACH INCREASES AT WEAPONS EMPLOYMENT MACH, AIRCRAFT IS VERY UNSTABLE

YAW INSTABILITY EXAGGERATED BY WEAPON BAY DOORS

THE YAW AXIS. . .

y

z

Rolling Moment Coefficient:

Lat.-Stat. Stability Derivative: slope of CL’,cg vs. curve

Lateral Static Stability (Dihedral Effect)

bSq

'C ,'

cgcgL

L

cgLC ,'

+L’cg

Rear View

y

z

V

Top View

V

y

Negative slope indicates lateral static stability - Positive generates a negative (restoring) moment - Negative generates a positive (restoring) moment

CL’,cg

cg,'LC

Zero roll moment at zero < 0

Lateral Static Stability (Dihedral Effect)

Vertical Tail Contribution toLateral Static Stability

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

Rear View

y

z

-L

V

Vertical tail taller Vertical tail bigger Increase tail lift curve slope

(increase ARvt and/or increase evt)

Wing Sweep Contribution toLateral Static Stability

x

y

+

V

Rear View

y

z

Top View

V

Positive wing sweep isLess lift More lift

stabilizing

Wing Position Contribution toLateral Static Stability

Rear Views

High Wing

Mid Wing

Low Wing

High wing placement is laterally

Low wing placement is laterally

Mid wing placement is

V

V

V

stabilizing

destabilizing

neutral

Geometric Dihedral Contribution toLateral Static Stability

z

y

-V

zy

+V

Positive dihedral is Anhedral is

+L-L

stabilizing destabilizing

y

z

VRear Views

Paper airplane example…

Coupling of Directional and Lateral Static Stability Modes

0C cg,N

cg,'LC

< 0

Directional Static Stability:

Lateral Static Stability:

Good aircraft design provides enough stability for both as well as a reasonable balance between the two:

3/2C

C

3/1cg,'L

cg,N

• Too much directional static stability (compared to lateral)

• Too much lateral static stability (compared to directional)

= bad “spiral mode” (>2/3)

= bad “dutch roll” (<1/3)

Summary Sideslip Directional Static Stability

Vertical Tail Wing/Body

Lateral Static Stability Vertical Tail Geometric Dihedral Wing Sweep Wing Position (high/low)

Lateral/Directional Coupling

Next Lesson (39)…

Prior to class Read dynamic modes of motion handout

In class Discuss dynamic modes and glider

construction