Lift
8.4
Lift is the force normal to the fluid flow
Unit # 7
Comparison
of Drags on
a well
designed
Jet plane
Lift
Coefficient
• Lift, L is the force normal to the direction of object’s velocity
• Lift coefficient, CL is defined as
CL =
L
1
ρU 2 A
2
Computation of Lift coefficient in the lab
• Lift is found by integrating the pressure above
and below the object as is done in the Fluids
laboratory
NACA series airfoil
NACA 5-Digit Series: 2 3 0 1 2
2 - max camber in % of chord
30 – position of max. camber in 2/100 of chord (eg 1/2x30 = 15%)
12 – Max. thickness in % chord (eg. 12%)
These sections were generated from a more or less prescribed pressure
distribution and were meant to achieve some laminar flow.
NACA 6-Digit Series: 6 3, 2 - 2 1 2
6- Six series
3 – location of minimum pressure in 1/10 Chord (x/c = .3)
2 – one tenth of max
2 – ideal CL in one tenth (CL= 0.2)
12 – Max thickness in % of chord ( 12%)
After the six-series sections, airfoil design became much more
specialized for the particular application.
Design your airfoil visit
http://www.desktopaero.com/appliedaero/airfoils1/airfoilgeometry.html
Lift coefficient depends on attack angle and flap
Polar Plot (CL vs CD) for NACA 16
Stall speed
• Lift supports the weight of the aircraft
• L = W= Clmax[1/2ρ Vstall2) (A)
• A maximum lift means that there is a
minimum speed, which called stall speed.
• It is the speed that determines the landing
speed of an aircraft
Severe separation causes stalling
Lift augmentation devices
• Slat
It is placed in front of the airfoil to help increase the momentum of
the boundary layer fluid. Thus it delays the separation and enhance
the lift
• Flap
It is placed at the rear of the wing it allows higher momentum fluid
to replace the weaker fluid in the tail of the wing. Thus it avoid
separation. It also increases the drag.
Wing fence
Streamwise vortex is generated to delay separation
Hydrofoil
Airfoil Geometry
Airfoil geometry is often characterized by a few parameters such as:
maximum thickness, maximum camber, position of max thickness,
position of max camber, and nose radius. One can generate a
reasonable airfoil section given these parameters.
The NACA 4 digit and 5 digit airfoils were created by
superimposing a simple meanline shape with a thickness distribution
that was obtained by fitting a couple of popular airfoils of the time:
y = ±(t/0.2) * (.2969*x0.5 - .126*x - .3537*x2 + .2843*x3 - .1015*x4)
where t is maximum thickness
Solved problem-Walter p440
• A hydrofoil boat (111,767 kg) use hydrofoil
section (NACA 16) of average area 16.7 m2. It
engine power is 7600 hp.
What is the power needed to attain this foil borne
speed if maximum lift coefficient for NACA 16 is
0.37
• What is the maximum speed of the boat if the drag
then is 97100 N and propulsive efficiency is 38%.
Problem (Munson 9.57)
• If the takeoff speed of a particular airplane
is 120 mi/hr at sea level, what will be at
Denver (elevation 5000ft) Given air density
at 5000 ft is 2.048 unit and that at sea level
2.377 unit.
Drag/Lift
Coefficients
• Both lift and
drag coefficients
depend on angle
of attack
• Values are
typical for each
type of airfoils
Drag reduction on golf ball
Word problem
• I heard somewhere that a Canadian scientist
who wanted to send objects to the space by
using powerful projectiles developed a
scheme of reducing the drag on the
projectile. The device involved some kind
of heating of air on the projectile surface.
Guess what could be the drag reduction
method
Internet Links (cont.)
• 8.4 - Lift and Drag on Airfoils
– Airfoil - Startup Problem – Animation:
http://www.cfm.brown.edu/people/beskok/small.mpg
– Airfoil Geometry:
http://www.desktopaero.com/appliedaero/airfoils1/airfoil
geometry.html
– FLOW AROUND AN AIRFOIL - Interesting animations
showing many different flow characteristics:
http://www.idra.unige.it/~irro/profilo_e.html
Internet Links (cont.)
• 8.3.2 - Vortex Shedding
– ON-LINE ANIMATIONS:
http://www.city.ac.uk/hydraulics/CFD/animjava.html
– Vortex Shedding – Animation:
http://www.engineering.uiowa.edu/~cfd/gallery/images/ani2.mpg
– Research of Dr. Kevin D. Jones: Unsteady Aerodynamics - Panel
Methods - An interesting animation of vortex shedding from a flapping
wing:
http://www.aa.nps.navy.mil/~jones/research/unsteady/panel_methods/an
im1/
• 8.3.4 - Cavitation
– Cavitation - Some interesting cavitation information:
http://www.rwthaachen.de
/iww/English/Forschung/Kavitation/Kavitation.html
– Cavitation & Bubbly Flows - Much cavitation information and
references to related publications:
http://www1.umn.edu/safl/research/lwr/basic/arndt/