aero engineering 315 lesson 38 lateral/directional static stability
TRANSCRIPT
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Aero Engineering 315
Lesson 38
Lateral/Directional Static Stability
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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
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B-2—Lat/Dir Stability Challenge
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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
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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
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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
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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)
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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
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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
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Individual Component Contributions to Directional Static Stability
CN,cg
Tail
Aircraft
Wing/body
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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
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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. . .
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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
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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)
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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)
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Wing Sweep Contribution toLateral Static Stability
x
y
+
V
Rear View
y
z
Top View
V
Positive wing sweep isLess lift More lift
stabilizing
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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
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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…
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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)
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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
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Next Lesson (39)…
Prior to class Read dynamic modes of motion handout
In class Discuss dynamic modes and glider
construction