1 Lecture 4: Aerodynamics Eric Loth For AE 440 A/C Lecture Sept 2009

2 Suggested Aerodynamics Responsibilities a.Aerodynamic design* including external configuration of airfoil, wing geometry and fuselage for cruise, take-off and landing configurations (to configuration person) b.Component and total system lift-and-drag coefficient relationships for wing and aircraft (develop drag polars) c.Develop model for lift & drag for cruise, take-off and landing (to performance person) d.Select optimum airfoil and provide spanwise and even chord-wise wing-loads (to structures person) e.Wing lift-and-moment coefficients (to stability and control person) f.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

3 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)

4 Wing Layout Aspect ratio Airfoil selection Wing location Sweep Taper ratio Twist (aerodynamic and geometric) Dihedral Geometric characteristics of the wing planform (Jenkinson).

5 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)

6 Estimation of C L,max Wing C L,max is always less than the section maximum value. An initial approximation of C L,max for a swept wing is:

7 Effective Lift-Curve Slope Helmbolt equation: Comparison of a NACA airfoil lift curve with that of a wing using the same airfoil (McCormick).

8 Effect of High-Lift Devices Effect of leading edge devices on lift curve (Jenkinson).

9 Estimation of D C L,max due to flaps Definition of flapped wing area (Roskam).

10 Drag Estimation Profile drag –Fuselage –Wing –Tail Surfaces –Engine Nacelles –Landing gear –Flaps

11 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 Drag Components

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

13 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

14 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

15 Induced Effects Geometric tail incidence based on required moment and downwash Approximate model for calculating downwash angle (McCormick).

16 Pre-calculated Charts Downwash angle as a function of the distance of the tail behind and above The wing (McCormick).

17 Effect of sweep back on downwash angle (McCormick).

18 Samples of Aerodynamic Analysis from Previous AE 440 Reports

19 Aircraft Lift Coefficient

20 Airfoil Design and Performance

21 Lift Distribution

22 Aero Performance and Trades LWT= Low-Wing Tractor HWT=High-Wing Tractor LWP=Low-Wing Pusher Mission Segment Performance Polars (for configurations person)

23 Aero Performance