Transcript ss24

Aerodynamic Drag Force
Air resistance (fluid resistance)
motion of the air flowing past projectile
equal to projectile’s velocity BUT in the opposite
direction of projectile’s motion
Headwind
• Vdrag
+
Vheadwind
Tailwind
• Vdrag - Vtailwind
•  flow velocity acting
on body
• body v = 20mps
•  flow velocity acting
on body
• body v = 20mps
• Vheadwind = 5mps
• Vres = -------------
• Vtailwind = 5mps
• Vres = ------------
Skin Friction
most noticed @ low v
rubbing of layers of air
adjacent to projectile
 with: flow v,
surface size, surface
roughness
secondary concern
Profile Drag
 with area exposed to
approaching air flow
 with projectile v
lead side =  pressure
trail side =  pressure
main source of Drag
STREAMLINING
Achieved by:
1. decreasing size of area facing oncoming airflow
2. tapering leading side - air is not abruptly moved
Streamlining results in:
A. more laminar flow past body with less “wake”
B. less turbulence behind body
less difference in pressure zones between front
and tail of body
Mass of Projectile and Drag Effect
a = F
m
a in this case stands for deceleration [negative a]
deceleration = F
m
deceleration inversely proportional to projectile m
Drag Factors
FDrag = ½ CD A ρ v²
Skin Friction and Profile Drag
CD coefficient of drag, indicates how streamlined
a projectile is (low number = very streamlined)
A is the frontal area of projectile facing the flow
ρ (rho) is the air density
(density less in warm air and at higher altitude)
v² means if v doubles, drag quadruples
Profile Drag
increases from
a to c as more
AREA is exposed
to oncoming
airflow
AREA
a: ---------b: ---------c: ---------FIG K.10 pg 424
FLUID LIFT FORCE
FL (Lift Force) always perpendicular to direction of
the oncoming air flow
Lift can be upward, downward, lateral
FL due to difference in pressure zones on opposite
sides of projectile
Bernoulli’s Principle:
high flow velocity creates ------- pressure zone
low flow velocity creates -------- pressure zone
 flow v on top
 p zone on top
 p zone on bottom
upward Flift
 flow v on top
 p zone on top
 p zone on bottom
downward Flift
8-May-2001
National Post
from
“New Scientist”
David Anderson
disputes
Daniel Bernouilli’s
Principle
LIFT : DRAG
• Maximize LIFT FORCE by creating an optimal angle of
attack or shaping projectile like an airfoil
• Minimize DRAG FORCE with a moderate ATTACK 
• FL = ½ CL A ρ v²
CL (lift coefficient)
ρ ( air density)
A (area of pressure)
v² (air flow velocity)
FIG K.9
page 424
http://www.grc.nasa.gov/WWW/K-12/airplane/incline.html
LIFT and DRAG:
Effects of Inclination of an AIRFOIL