Aero Engineering 315 Lesson 39 Dynamic Stability

Glider Construction Tips  Glider fly-offs Monday, 8 December,  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 |C N / C L | < 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 |C N / C L | > 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 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

C-5A Short Period response – elevator to  C-5A Phugoid response – elevator to airspeed  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