Transcript AAE 241: Aerodynamics - ae440a2009 / FrontPage

Lecture 4: Aerodynamics
Eric Loth
For AE 440 A/C Lecture
Sept 2009
1
Suggested Aerodynamics
Responsibilities
a.
b.
c.
d.
e.
f.
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
2
Wing Design
• Important considerations/constraints:
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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)
3
Wing Layout
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Aspect ratio
Airfoil selection
Wing location
Sweep
Geometric characteristics of the wing planform (Jenkinson).
Taper ratio
Twist (aerodynamic and geometric)
Dihedral
4
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)
5
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:
(CL,max )3 D  0.9(CL,max ) 2 D  cos 
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Effective Lift-Curve Slope
• Helmbolt equation:
AR
CL  Cl
2
2
(Cl /  )  (Cl /  )  AR
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
 DC

L,max 3D
 Swing
  DCL,max 2D 
S
 wing flap

 cos 

Definition of flapped wing area (Roskam).
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Drag Estimation
• Profile drag
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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.
 Ai
CD   CD,i 
S
i
 wing



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Load Distribution
• Start with assuming elliptic distribution
• Refine w/ Schrenk’s 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
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LWT= Low-Wing Tractor
HWT=High-Wing Tractor
LWP=Low-Wing Pusher
Mission Segment Performance Polars
(for configurations person)
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Aero Performance
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