Aero Engineering 315
Lesson 39
Dynamic Stability
Glider Construction Tips
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Glider fly-offs Monday, 8 December, 18152015
Use wood glue or fast drying epoxy (flexible)
Make sure dihedral, wing sweep, etc. are
symmetrical
Recommend adjustable tail (rubber band and
wedge)
Clay ballast/c.g. location
Practice flying your glider
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Assess performance (i.e. how straight does it fly)
Perfect your launch techniques
Dynamic Stability Overview
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Dynamic vs. Static Stability
Lateral-Directional Modes
 Spiral
 Dutch Roll
 Roll
Longitudinal Modes
 Short Period
 Phugoid
Objectives
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Know the 5 modes and how aircraft motion varies
with each mode
Dynamic vs. Static Stability
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Static stability describes
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Dynamic stability describes
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Initial tendency once displaced from
equilibrium
Tendency over a period of time once
displaced or excited
Dynamically stable if eventually returns
to equilibrium
Dynamic modes involve different motion
behavior over a period of time
B-747 Dutch Roll
Roll rate
Yaw rate
Lateral-Directional Modes:
Dutch Roll
DUTCH ROLL: High frequency,
lightly damped oscillation.
Frequency near human
response time, so pilot can
aggravate oscillations (PIO).
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Named after Dutch skaters
Typically objectionable when
|CNb / CLb| < 1/3
To correct poor Dutch Roll
characteristics, increase directional
stability and decrease lateral
stability
Glider Hints
- Decrease
dihedral
- Decrease sweep
- Decrease vert.
tail height
Dutch Roll in the Simulator
Lateral-Directional Modes:
Spiral
SPIRAL: Non-oscillatory mode
which primarily consists of a
steady increase in roll angle
(bank) and yaw rate if unstable.
A slightly unstable spiral is
acceptable in many aircraft, BUT
in your glider it will result in it
auguring into the ground…
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Typically objectionable when
|CNb / CLb| > 2/3
To correct an unstable spiral,
increase roll stability and decrease
directional stability
Glider Hints
-Increase dihedral
-Increase sweep back
-Move vert. tail
forward
-Decrease vert. tail
area
Spiral Mode in the Simulator
Lateral-Directional Modes:
Roll
Roll rate response to
pilot’s aileron command
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Measure of how quickly a
steady state roll rate is
reached
Balances the aileron control
power (i.e. how much roll
force is generated) against
the aircraft’s natural roll
damping and roll inertia
Glider Hints
-No ailerons
-No problems
Longitudinal Modes:
Short Period
High frequency, heavily
damped oscillation.
Frequency near human
response time, so pilot can
aggravate oscillations (PIO).
 Typically observed as an
oscillation with a and pitch
attitude, q
Glider Hints
- Not applicable to
glider
Short Period in the Simulator
Longitudinal Modes:
Phugoid
Low frequency, lightly damped
oscillation - exchange of
kinetic and potential energy.
Easily controlled by pilot.
 Motion observed as an
airspeed and altitude
variation, and oscillation of
kinetic and potential energy
 Aggravated by airspeeds
faster and slower than trim
speed
Glider Hints
- Change trim
speed or launch
speed of glider
(i.e. launch your
glider at trim
speed…)
Phugoid (Long Period) Mode in
the Simulator
Angle of Attack vs. Time (e = -1o, t = 0.05 sec)
Well “damped”
C-5A aircraft, W = 579,000 lb, Altitude = 20,000 ft, Mach = 0.6
2
C-5A Short Period
response –
elevator to a
a
Angle of Attack (deg)
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
0
1
U-velocity
(e55 = -1o,6t = 0.05
sec)
2
3 vs. Time
4
7
8
10
10
9
Time
(sec) = 20,000 ft, Mach = 0.6
C-5A aircraft, W = 579,000 lb,
Altitude
Lightly “damped”
630
620
610
U
U-velocity (ft/s)
600
590
580
570
560
550
540
530
0
25
50
75
100
125
150
150
Time (sec)
175
200
225
250
275
300
300
C-5A Phugoid
response –
elevator to
airspeed
Next Lesson (T40)…
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Terazzo Tour—meet at F-4
Look-ahead:
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Lesson 41: quick stability review, Stability
and Control Quiz, then course critiques
Lesson 42: course review