HiAlphaBasicsPres - Some High Alpha Aerodynamics W.H Mason...

Slide 1
4/27/10
Some High Alpha Aerodynamics
W.H. Mason
Configuration Aerodynamics Class

Slide 2
4/27/10
Issues in Hi- ± Aero
• General Aviation
– Prevention/recovery from spins
– Beware: Tail Damping Power Factor (TDPF)
» Sometimes advocated for use in design
» Has been shown to be inaccurate!
• Fighters
– Resistance to “departure” from controlled flight
– Carefree hi- ± air combat maneuvering
» Fuselage pointing
» Velocity vector rolls
» Supermaneuverability
» etc.
• Transports
– Control/prevention of pitchup and deep stall (the DC-9 case study)

Slide 3
4/27/10
High Angle of Attack
• Aerodynamics are nonlinear
- complicated component interactions (vortices)
- depends heavily on WT data for analysis
• Motion is highly dynamic - need unsteady aero
• Exact requirements are still being developed
• Keys issues (from a fighter designer’s perspective):
- adequate nose down pitching moment to recover
- roll rate at high alpha
- departure avoidance
- adequate yaw control power
- the role of thrust vectoring

Slide 4
4/27/10
The Hi- ± Story
Longitudinal
Typical unstable modern fighter
±
90°
+
-
Cm
Max nose up moment
Max nose down moment
Cm *
Minimum Cm * allowable is an open question
Issue of including credit for thrust vectoring
Pinch often around ± ~ 30°-40°
0°
Suggested nose down req’t: Pitch accel in 1st sec: -0.25 rad/sec 2

Slide 5
4/27/10
Example: F-16
NASA TP-1538, Dec. 1979
Found in flight, and
also in tests at the
NASA Full Scale
Tunnel, other
tunnels didn’t find
this, and so the
design was made
with this “issue”
An ± -limiter was
put in the control
system to prevent
pilots from going
above 28°

Slide 6
4/27/10
The Hi- ± Story
Directional

Slide 7
4/27/10
Example:
F-5
-0.0050
0.0000
0.0050
0.0100
-10° 0° 10° 20° 30° 40° 50°
C
n ±
² , deg.
Full Config.,
Tail Off
Full Config.,
Vertical Tail On
Forebody alone
F-5 WT data: NASA TN D-7716
Sketch from NASA TN D-7716, 1974
Chine forebodies are
even more effective!
Above 30°, the
directional
stability comes
entirely from
the forebody!

Slide 8
4/27/10
Forebody Vortices
F-5A forebody, ± = 40°, ² = 5°
NASA CR 4465, Aug. 1992
I’m looking for a
better, color, figure
At “high” angles of attack,
vortices form over the
forebody, producing
additional forces, and often
interacting with the rest of
the airplane flowfield

Slide 9
4/27/10
Illustration of Forebody Vortices and a “Fix”
• Laser Light Sheet Video
of the RFC forebody
X-29 forebody
Taken at the USAF Museum,
WPAFB, Ohio
• And Flow Asymmetries:
- an amazing story
- solve with nose strakes

Slide 10
4/27/10
The Hi- ± Story
Lateral
Cl
±
+
-
²
90°
0°
“Stable”
“Unstable”
0
Dihedral Effect
(also due to sweep)
Flow Separates
on wing
Drooping LE devices
helps control
severity

Slide 11
4/27/10
Example: F-4
NASA TN D-7131, July 1973

Slide 12
4/27/10
Example:
F-16 Lateral/
Directional
Control
NASA TP-1538, Dec. 1979
Conventional aero
control surfaces lose
effectiveness at high
angles of attack
Control
Effectiveness
Change with
alpha
± C
n
± C
l
± , deg
²
r
= 30°
²
r
= 30°
²
a
= -20°
²
d
= -5°
²
a
= -20°
²
d
= -5°

Slide 13
4/27/10
A way to get yaw: differential canard