Date post: | 21-Dec-2015 |
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Glider Construction Tips Glider fly-offs Monday, 8 December, 1815-
2015 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
Assess performance (i.e. how straight does it fly) Perfect your launch techniques
Dynamic Stability Overview Dynamic vs. Static Stability Lateral-Directional Modes
Spiral Dutch Roll Roll
Longitudinal Modes Short Period Phugoid
Objectives Know the 5 modes and how aircraft motion varies
with each mode
Dynamic vs. Static Stability Static stability describes
Initial tendency once displaced from equilibrium
Dynamic stability describes 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
Lateral-Directional Modes:Dutch Roll
DUTCH ROLL: High frequency, lightly damped oscillation. Frequency near human response time, so pilot can aggravate oscillations (PIO). Named after Dutch skaters Typically objectionable when
|CN / CL| < 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
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… Typically objectionable when |
CN / CL| > 2/3 To correct an unstable spiral,
increase roll stability and decrease directional stability
Glider HintsIncrease dihedralIncrease sweep backMove vert. tail forwardDecrease vert. tail area
Lateral-Directional Modes:Spiral
Roll rate response to pilot’s aileron command 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
Lateral-Directional Modes:Roll
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 and pitch attitude,
Glider Hints Not applicable to
glider
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…)
Longitudinal Modes:Phugoid
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
0 1 2 3 4 5 6 7 8 9 10
Time (sec)
An
gle
of
Att
ack
(deg
)
530
540
550
560
570
580
590
600
610
620
630
0 25 50 75 100 125 150 175 200 225 250 275 300
Time (sec)
U-v
elo
cit
y (
ft/s
)C-5A Short Period response – elevator to
C-5A Phugoid response – elevator to airspeed
150 300
5 10
U
Well “damped”
Lightly “damped”