06-04-2012, 03:42 PM
Aircraft Design
Stability_Pt2.pdf (Size: 4.55 MB / Downloads: 116)
Stability
In order of importance:
• Longitudinal stability
» Stability about the pitch axis: horizontal stabilizer
• Lateral stability
» Stability about the roll axis: bi-lateral symmetry, wing design
(dihedral), ailerons, keel effect,…
• Directional stability
» Stability about the yaw
axis: vertical stabilizer
Pitch Stability
• From these we can determine the limits for c.g.
(forward, XF, and aft, XR) - note that a larger tail
provides a larger range of c.g. travel
Pitching Tendencies in Stall
• Low-tail aircraft pitch down in stall; recovery easier
• T-tail aircraft pitch up in stall; tail in stalled wake,
recovery more problematic
Phugoid
• The phugoid is the traditional pitch behavior of an
aircraft responding to a disturbance
Requirements for Direction Control
• Adverse Yaw
» When an airplane is banked to execute a turning maneuver, the ailerons
may create a yawing moment that opposes the turn (adverse yaw). The
rudder must be able to overcome the adverse yaw so that a coordinated
turn can be achieved.
» This usually occurs during slow flight (high CL).
• Crosswind landings
» To maintain alignment with the runway during a crosswind landing the
pilot must fly at a non-zero sideslip angle. The rudder must be powerful
enough to permit the pilot to trim the airplane for specified crosswinds.
» Max. crosswind design value typically 15.5 m/s (51 fps).
• Asymmetric power cond’n
» When one engine fails on a multi-engine plane, a critical asymmetric
power condition occurs. The rudder must be able to overcome the yawing
moment produced by the asymmetric thrust arrangement.
» The farther an engine is away from the centerline, the greater the
asymmetric power control requirements are.
Stick Force or Speed Stability
• Negative stick force gradient provides pilot with
speed stability; once trimmed, the velocity will return
to trimmed speed if perturbed
FBW Philosophy
• The computer should have final authority on the
commands sent to the control system. The pilot’s
inputs should be limited by the computer (hard limits
or protections) to prevent exceeding the physical
design limits of the aircraft (e.g., angle of attack, gloads,
etc.) to protect the integrity and dynamics of
the aircraft.
Landing in Wind Shear
From headwind to tailwind
1. Normal approach
2. Increasing downdraft and
tailwind
3. Airspeed decreases, pitch down
4. Aircraft crashes short of runway
From tailwind to headwind; hard
landing or overshoot