01-12-2012, 04:59 PM
Basic Aerodynamics
Aerodynamics.ppt (Size: 591.5 KB / Downloads: 161)
Energy
Definition: Energy is the ability to do work.
Energy cannot be created or destroyed. We can only change its form.
A fluid in motion has (mainly) two forms of energy:
kinetic energy (velocity),
potential energy (pressure).
Lift: Wing Section
Air flows toward the low pressure area above the wing: upwash and downwash.
Newton’s third law of motion: to every action there is an equal and opposite reaction.
“The reaction to downwash is, in fact, that misunderstood force called lift.” Schiff p. 8
Angle of Attack
The angle of attack is the angle between the chord line and the average relative wind.
Greater angle of attack creates more lift (up to a point).
Too Much Lift? Spoilers
Spoilers destroy lift:
to slow down in flight (flight spoilers);
for roll control in flight (flight spoilers);
to slow down on the ground (ground spoilers).
Longitudinal Stability
Static stability (tendency to return after control input)
up elevator increases downward lift, angle of attack increases;
lift increases, drag increases, aircraft slows;
less downward lift, angle of attack decreases (nose drops).
Lateral Stability
If one wing is lowered (e.g. by turbulence), the airplane sideslips.
The lower wing has a greater angle of attack (more lift).
This raises the lower wing.
Adverse Yaw and Frise Aileron
However, more lift on one wing creates more induced drag on that wing: adverse yaw.
Adverse yaw is corrected by rudder application.
Frise ailerons counter adverse yaw:
They create parasite drag on the up aileron.
Preventing Stalls
Slats direct airflow over the wing to avoid boundary layer separation.
Slots are similar but fixed, near the wingtips.
Delays stall near the wingtip (aileron effectiveness).
Stalls and Turns
Greater angles of bank require greater lift so that:
the vertical component of lift equals weight (to maintain altitude),
the horizontal component of lift equals centrifugal force (constant radius, coordinated, turn)
Turns
As bank increases, load factor increases.
But: as airspeed increases, rate of turn decreases.
In order to make a 3 degree per second turn, at 500 Kts the airplane would have to bank more than 50 degrees.
Uncomfortable (unsafe?) load factor.
This is why for jet-powered airplanes, a standard rate turn is 1.5 degrees per second.