ae 1350 lecture notes #8. we have looked at.. airfoil nomenclature lift and drag forces lift, drag...
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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
Airfoil Drag Polar Cd vs. Cl
Rough airfoilshave turbulent flowover them, high drag.
Smooth airfoils havelaminar flow overat least a portionof the surface.Low Drag.
Form Drag
Source: http://www.allstar.fiu.edu/aerojava/flight46.htm
Form drag may be reducedby proper design, andstreamlining the shape.
Supersonic wave Drag
For a given airfoil or wing or aircraft, as the Mach numberis increased, the drag begins to increase above afreestream Mach number of 0.8 or so due to shock waves thatform around the configuration.
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.
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
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 airfoilsfrom exceeding 1.1 or so, under cruise conditions.
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
Induced Drag
Induced drag is caused by the downward rotation of the freestream velocity, which causes a clockwise rotation of the liftforce.
From AE 2020 theory, e
2
, AR
CC L
iD
e= Oswald efficiency factor
Variation of Drag with Speed
Induced drag decreases as Vincreases, because we need lessvalues of CL at high speeds.
Other drag forces (form, skin friction , interference) increase.
Result: Drag first drops, then rises.
At High Values of Wings Stall
We need high CL to take-off and land at low speeds.
http://www.zenithair.com/stolch801/design/design.html
High energy air from the bottom side of the airfoilflows through the gap to the upper side, energizes slow speedmolecules, and keeps the flow from stalling.
How do slats and flaps help?
1. They increase the camber as and when needed- duringtake-off and landing.
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