Lecture%204:%20Aerodynamics

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Lecture 4 Aerodynamics
Eric Loth For AE 440 A/C Lecture Sept 2009
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Suggested Aerodynamics Responsibilities

• Aerodynamic design including external
  configuration of airfoil, wing geometry and
  fuselage for cruise, take-off and landing
  configurations (to configuration person)
• Component and total system lift-and-drag
  coefficient relationships for wing and aircraft
  (develop drag polars)
• Develop model for lift drag for cruise,
  take-off and landing (to performance person)
• Select optimum airfoil and provide spanwise and
  even chord-wise wing-loads (to structures person)
• Wing lift-and-moment coefficients (to stability
  and control person)
• Employ CFD Analysis to check and optimize airfoil
  performance at Reynolds number
• Based on historical review, trade studies, and
  logic decisions and other aspects of aircraft
  thus will continually evolve throughout semester

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Wing Design

• Important considerations/constraints
• Performance (cruise, loiter, take-off, landing)
• Flying qualities (handling and stability)
• Structural considerations (spar placement)
• Internal volume (for fuel/payload)
• Stealth characteristics (for military subsonic)
• Airport limitations (wing-span)

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Wing Layout

• Aspect ratio
• Airfoil selection
• Wing location
• Sweep
• Taper ratio
• Twist (aerodynamic and geometric)
• Dihedral

Geometric characteristics of the wing planform
(Jenkinson).
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Airfoil Design

• Designed primarily for best cruise and/or loiter
  for given aircraft/wing configuration (and Mach
  and Reynolds number)
• Maximum section lift coefficient is also
  important but will be a function of flaps for
  landing and take-off
• Designed to have robustness (to icing, roughness,
  damage, etc.) depending on vehicle utility
• Many airfoils have tabled properties from wind
  tunnel data (XFOIL not as accurate but allows
  custom design)
• May include laminar flow design for cruise
  performance (and active flow control for high
  angle performance)

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Estimation of CL,max

• Wing CL,max is always less than the section
  maximum value.
• An initial approximation of CL,max for a swept
  wing is

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Effective Lift-Curve Slope

• Helmbolt equation

Comparison of a NACA 65-210 airfoil lift curve
with that of a wing using the same airfoil
(McCormick).
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Effect of High-Lift Devices
Effect of leading edge devices on lift
curve (Jenkinson).
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Estimation of DCL,max due to flaps
Definition of flapped wing area
(Roskam).
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Drag Estimation

• Profile drag
• Fuselage
• Wing
• Tail Surfaces
• Engine Nacelles
• Landing gear
• Flaps

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Drag Components

• Pressure drag
• Inviscid component often proportional to
  frontal area
• Form drag
• Viscous component often proportional to wetted
  area
• Induced drag
• Inviscid component due to lift distribution
• Interference drag
• Due to components in proximity to each other
• Wave drag
• Present when flying at high Mach numbers
• See Raymer, Roskam, McCormick, etc., on
  estimating each of the drag components

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Component Drag Build-up

• Build-up of parasite drag based on a common area,
  e.g. wing area (Swing)
• Drag Coefficient for individual aircraft
  components (CD,i) may be summed using the
  relevant component area (Swing) which can be a
  wetted area, frontal area, etc.

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Load Distribution

• Start with assuming elliptic distribution
• Refine w/ Schrenks approx., lifting-line or
  advanced analysis
• Design (twist) to avoid stall near outboard
  control surfaces

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Tail Design

• Tail sized and positioned length-wise by
  Stability and Control Person
• Lift and Drag of tail taken into account by
  Aerodynamics Person (using methods similar to
  wing)
• Both select vertical placement to minimize
  interference from wing wake/engine wash

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Induced Effects

• Geometric tail incidence based on required moment
  and downwash

Approximate model for calculating downwash
angle (McCormick).
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Pre-calculated Charts
Downwash angle as a function of the distance of
the tail behind and above The wing (McCormick).

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Effect of sweep back on downwash angle
(McCormick).
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Samples of Aerodynamic Analysis from Previous AE
440 Reports
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Aircraft Lift Coefficient
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Airfoil Design and Performance
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Lift Distribution
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Aero Performance and Trades

• LWT Low-Wing Tractor
• HWTHigh-Wing Tractor
• LWPLow-Wing Pusher

Mission Segment Performance Polars (for
configurations person)
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Aero Performance