Aero Engineering 315

Lesson 39

Dynamic Stability

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

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). 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

Dutch Roll in the Simulator

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

Spiral Mode in the Simulator

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

Short Period in the Simulator

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

Phugoid (Long Period) Mode in the Simulator

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”

Next Lesson (T40)…

Terazzo Tour—meet at F-4 Look-ahead:

Lesson 41: quick stability review, Stability and Control Quiz, then course critiques

Lesson 42: course review