AE 1350 Lecture Notes

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AE 1350 Lecture Notes 8
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We have looked at..

• 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

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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.
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Form Drag
Form drag may be reduced by proper design,
and streamlining the shape.
Source http//www.allstar.fiu.edu/aerojava/flight
46.htm
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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.
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Shock waves
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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.

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How can shock waves be minimized?
Use sweep.
0.8cos30?
30 ? sweep
M 0.8
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In your design...

• The Maximum Mach number is 0.85
• Wings for supersonic fighters are designed to
  reduce wave drag up to 80 of the Maximum speed.
• In our case, 80 of 0.85 is 0.68.
• If we use a wing leading edge sweep angle of 30
  degrees or so, the Mach number normal to the
  leading edge is 0.68 cos 30 0.6

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Effect of Thickness and Sweep on Wave Drag
Source http//www.hq.nasa.gov/office/pao/History
/SP-468/ch10-4.htm
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Supercritical Airfoils
Their shape is modified to keep the Mach number
on the airfoils from exceeding 1.1 or so, under
cruise conditions.
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Conventional vs. Supercritical Airfoils
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Wing Drag

• Since a wing is made up of airfoils, it has
• skin friction drag
• profile drag
• wave drag at high speeds, and
• Induced drag due to tip vortices

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TIP VORTICES
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Effect of Tip Vortices
Downwash
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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,
e Oswald efficiency factor
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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.
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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.h
tml
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Achieving High Lift
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One form of flaps, called Fowler flaps increase
the chord length as the flap is deployed.
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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.
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Leading Edge SlatsHelp avoid stall near the
leading edge
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High Lift also Causes High Drag