Download AE 2350 Lecture Notes #8

AE 2350 Lecture Notes #8
April 30, 1999
We have looked at..
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Airfoil Nomenclature
Lift and Drag forces
Lift, Drag and Pressure Coefficients
The Three Sources of Drag:
– skin friction drag in laminar and turbulent flow
– form drag
– wave drag
• Read Chapter 8 of text.
Airfoil Drag Polar
Cd vs. Cl
Rough airfoils
have turbulent flow
over them, high drag.
Smooth airfoils have
laminar flow over
at least a portion
of the surface.
Low Drag.
Form Drag
Form drag may be reduced
by proper design, and
streamlining the shape.
Source: http://www.allstar.fiu.edu/aerojava/flight46.htm
Supersonic wave Drag
For a given airfoil or wing or aircraft, as the Mach number
is increased, the drag begins to increase above a
freestream Mach number of 0.8 or so due to shock waves that
form around the configuration.
Shock waves
How can shock waves be
minimized?
• Use wing sweep.
• Use supercritical airfoils, which keep the
flow velocity over the airfoil and the local
Mach number from exceeding Mach 1.1 or
so.
• Use area rule- the practice of making the
aircraft cross section area (from nose to tail,
including the wing) vary as smoothly as
possible.
How can shock waves be
minimized?
Use sweep.
30  sweep
M= 0.8
In your design...
• The Maximum Mach number is 2.0
• Wings for supersonic fighters are designed to
reduce wave drag up to 80% of the
Maximum speed.
• In our case, 80% of 2 is 1.6.
• If we use a wing leading edge sweep angle of
60 degrees or so, the Mach number normal to
the leading edge is 1.6 cos 60°=0.8
Effect of Thickness and Sweep
on Wave Drag
Source:
http://www.hq.nasa.gov/office/pao/History/SP-468/ch10-4.htm
Supercritical Airfoils
Their shape is modified to keep the Mach number on the airfoils
from exceeding 1.1 or so, under cruise conditions.
Conventional vs. Supercritical
Airfoils
Wing Drag
• Since a wing is made up of airfoils, it has
– skin friction drag
– form drag
– wave drag at high speeds,
and
– Induced drag due to tip vortices
TIP VORTICES
Effect of Tip Vortices
Downwash
Downwash changes lift direction
Wind direction
without downwash
V
a
Lift Direction without downwash
New Lift Direction
including downwash
Downwash
New wind direction
including downwash
Induced Drag
Induced drag is caused by the downward rotation of the
freestream velocity, which causes a clockwise rotation of the lift
force.
From AE 2020 theory,
C D ,i
C L2

AR e
e= Oswald efficiency
factor
Parasite Drag and Interference Drag
Parasite Drag is simply Skin Friction Drag+ Form Drag +
Interference Drag + Wave Drag
Variation of Drag with Speed
Induced drag decreases as V
increases, because we need less
values of CL at high speeds.
Other drag forces (form,
skin friction , interference)
increase.
Result: Drag first drops, then rises.
At High Values of a Wings Stall
We need high CL to take-off and land at low speeds.
http://www.zenithair.com/stolch801/design/design.html
Achieving High Lift
One form of flaps, called Fowler
flaps increase the chord length as
the flap is deployed.
How do slats and flaps help?
1. They increase the camber as and when needed- during
take-off and landing.
High energy air from the bottom side of the airfoil
flows through the gap to the upper side, energizes slow speed
molecules, and keeps the flow from stalling.
Leading Edge Slats
Help avoid stall near the leading
edge
High Lift also Causes High Drag
MIG-29
Length: 14.87m
Wingspan: 11.36m
Height: 4.73m
Weight Empty: 10 900kg
Max T/O: 18 500kg
Max Speed: Mach 2.3
Range: 2100km
Ceiling: 17 000m
Powerplant: Two Klimov/Sarkisov RD-33 turbofans
Thrust: 98.8kN