AIAA Team Aircraft Design Competition 2009-2010
Introductory Lectures on Aerospace Design Nov. 6th, 2009
University of Southern California
Lecture-4 Principals of Low Speed & High Speed Vehicle Aerodynamics
Re-iteration: Sizing Charts & Carpet Plots
â€“ Desirable region is hatched, & design point Selected
â€“ Note that this needs to be updated as design progresses
Remarks on Carpet Plots
– Multi Disciplinary (MD) Optimization – They are representations of MD Optimizations. – They are the main tools in performing trade studies b/w concepts or configuration choices (example: Assignment 1) – They are used to observe the behavior of 2 functions of 2 common variables simultaneously. – They can have different forms (i.e. you can have a carpet plot that shows the changes in one function of 2 variables. – Sometimes they are hard to interpret, since they look like a 3-d wire-frame representation. – Team members should understand how they are generated. For further trade studies, the table and graph will not be provided. – We will see more examples of them as we go along.
Aerodynamics: Goals, Methods and Principals
Goals of This Lecture: – Theoretical aerodynamics is a vast field, we will only be looking at few cases and concepts that are most relevant to our project. – Present to you how aerodynamics shapes a flight vehicle, and in what ways performance could be improved by improving vehicle aerodynamics. – Provide you with a comprehensive understanding of aerodynamic forces, and the general estimation methodology used in the industry. – Present to you today’s challenges in terms of aerodynamics of novel concepts which may also be applicable to our design project.
Applied Aerodynamics: â€“ The Science of describing, analyzing, optimizing and predicting the various forces and moments excreted on a traveling object by the flow of air.
Aerodynamics - As the name suggests, Aerospace Engineering has a significant dependency on Aerodynamics. - In no engineering field it is as critical as it is in aerospace engineering.
Applied Aerodynamics: Goals, Methods and Principals
Basics of Lift & Drag Generation:
- Created by any aerodynamic shape that is capable of
creating a pressure difference on it’s upper and lower sides. - Pressure difference is usually achieved by causing a difference in flow speed. - Drag is a consequence of both production of lift and molecular friction with “wetted” surfaces.
- Airfoil shaped geometries try to maximize lift and minimize drag.
Flow Feature Dependencies:
- General Flow features depend on: 1-Flow Regime (Mach Effects) -
0-0.7: Subsonic (almost no Mach effects) 0.7-0.95: Transonic (benign Mach effects) 0.95-2: Super Sonic (significant Mach effects) 2-7: Hyper Sonic (dominated by Mach effects)
2-Reynolds Number (i.e. turbulence level):
Ď Vl Re = Âľ
3-Angle of Attack 4-Significantly influenced by geometry in contact with the flow.
Turbulent vs. Laminar Flow: - A major complication in the process. - Turbulence is still an open question. - Experimental Aspects - Empirical Methods - Numerical Models for turbulence - Using complex mathematics, PDEs & Chaos Statistical techniques + Experiments - Turbulence+Mach effects
Aerodynamics Methods of Analysis
Experimental Aerodynamics: – – – –
Earliest method of approach If performed right possibly the most reliable Time consuming and very expensive Corrections have to be applied
Computational Aerodynamics: â€“ Computers have drastically changed the approaches to aerodynamic analysis, but still have to be used with great deal of caution. â€“ Still, they are a great solution for design problems.
Computational Aerodynamics: â€“ Lattice Based Models (like AVL) are simpler to use & take shorter times to converge â€“ Lowe accuracy but more flexible
Fuselage Aerodynamics: Performed Trade Studies & Selected Design Strategy
Optimization of Fuselage Geometry: - The goal is to increase the Drag Divergent Mach number MD: - This is mainly a function of Length to diameter ratio (L/d) of the nose and aftbody geometry:
Optimization of LN/d to delay Drag-Rise:
- For The nose segment, per ESDU 74013 - Max Cruise Speed is required to be 0.83
Optimization of LA/d to delay Drag-Rise:
- For The aft-body segment, per ESDU 74013 - Max Cruise Speed is required to be 0.83 - The aft body fineness ratio is considerably less than 1. - The aft body is NOT a limiting factor for Drag rise Mach number.
Optimization of Nose Section Geometry : - Fuselage Wave & pressure Drag impacts on nose geometry: - Experimental results have shown a correlation b/w blunting ratio of the nose geometry b=(2r/D) and the wave drag and pressure of the fuselage.
Optimization of Nose Section Geometry : - Minimizing the wave drag using ESDU item 83017 - The average radius of curvature ratio becomes important - The average radius of curvature of the nose compartment :
- Note that it is highly curved in the front and slightly flat at the region of installation of the cockpit windows.
Optimization of Nose Section Geometry : - Minimizing the wave drag using ESDU item 83017 average radii of curvature ratio of 0.7 -
Optimization of Nose Section Geometry : - Minimizing the wave drag using ESDU item 83017 average radii of curvature ratio of 0.4 -
Optimization of Nose Section Geometry : - Minimizing the Pressure drag using ESDU item 89033 for AOA of 0 degrees:
Optimization of Nose Section Geometry : - Minimizing the Pressure drag using ESDU item 89033 for AOA of 5 degrees:
A Case-Study of CFD Application: - CFD tools could be used to streamline the fuselage geometry to improve flow field issues.
Aerodynamics of Lifting Surfaces: Elements, Principals & Design Strategy for Low-Speed & High-Speed Aero
Airfoil Characteristics: Lift Producing Cross-Sections
Lifting Surfaces or Bodies: - General Characteristics:
- Pressure differentials exist b/w top and bottom sides. - Velocity variations exist b/w top and bottom sides.
- Bernoulli's Principal is an analogy to what takes place on an airfoil shaped surface: v2 P + = Constant 2 Ď
Airfoil Categorization & Sources:
- By Designer, Type, or Geometry. - Different types are optimized for different flow regimes: - Thick Airfoils (t/c: 13-17 %) optimized for slow speeds. - Transonic Airfoils (t/c: 7-12 %) optimized for Transonic flight.
- Thin Airfoils (t/c: 3-7 %) optimized for supersonic & transonic flights.
- Among Transonic Airfoils:
- Super Critical Airfoils - Natural Laminar Flow Airfoils - Supercritical Airfoils with Diverted Trailing Edge
Flow Features for Transonic Airfoils:
- Flow around Transonic, Super Critical airfoils
Airfoil Aerodynamic Properties:
- NACA Charts to describe airfoil performance
Flow Features for Transonic Airfoils:
- Flow around, Super Critical airfoils
- Formation of Shock is visible - Analysis is done at very low Re # (Notice the transition)
Low-Speed Aerodynamics: High Lift Devices Purpose, Sizing & Analysis
Principal of High-Lift Devices: - They divert the wingâ€™s local flow downwards, - At the same time they improve the pressure differential b/w top and bottom surfaces near the trailing edge of the wing - Dominantly functional in low Re #s - Dominantly incompressible flows - No Mach effects - Transition occurs towards the TE - Overall: In some aspects analysis are simple: - No Mach effects, more laminar flow - Overall: In some aspects analysis are more complex: Unsteady features, High AOA, High flow path deflections.
Wing Geometry, TE devices:
Main Types of TE High-Lift Devices: - Split Flaps:
- Simple Construction - Light Weight - Simple Analysis
- Single Slotted Flaps:
- More lift - Heavier & more noise - Harder to optimize
- Double Slotted Flaps - ditto
Flow Features of High-Lift Devices: - Function of the slotts:
- Function 1: Diverting the TE flow downwards - Function 2: Amplifying the rear pressure difference via a high speed â€œjetâ€? at the orifice.
Flow Features of High-Lift Devices: - Slotted flaps vs. Simple Flaps: Point of Transition
- Slotted flaps are more powerful per unit area - Slotted flaps are more aerodynamically efficient per unit weight - They are more complicated to design & build
Wing Geometry, LE devices:
Main Types of LE High-Lift Devices: - Simple Slat:
- Powerful - Heavy & Noisy - Complicated deployment
- Upper Surface Kr端ger: - Less Powerful - Lighter & Less Noisy - Simple Deployment
- Drooped LE Device - Obsolete - Ultra Complicated
Combinations & Lift Generation: - Combinations of High lift devices are compared schematically:
Sizing of TE devices: -Three main parameters to solve for: 1- flap chord to wing chord ratio 2- Outer span-wise location of the flaps 3- Deflection Angles They all Depend on each other. One has to be picked to start with.
The flap chord to wing chord ratio is often selected to be 0.30 in commercial aviation design. We will use a single slotted flap for this example.
Optimization of TE devices: Based on ESDU Item 93019: Max. CL
Low-Speed Flow Visualization:
High-Speed Aerodynamics: Transonic Wing Design Elements
Mission Influences on Aerodynamics: - Weight during the cruise determines the required lift coefficient (and consequently Drag coefficient) - CL has to be chosen respectivly
CL to Max. E Cruise CL to Min. Fuel Burn
Process of Preliminary Wing Design: - ESDU 97017 Process: 1-Select a Primary DP 2-Select an airfoil 3-Perform Analysis 4-Perform Optimization 5-Repeat till Satisfied 6-If necessary change airfoils, twist ,etcâ€Ś
Flow Field Around a wing: -
Sweep Effects Reducing the critical Mach number Consequently causing an In-wash flow on the top side Presence of wing causes an up-wash in front and a down wash behind the wing in the flow field.
Flow Field at Wingtips:
- Sweep Effectsďƒ In-wash (upper) & outwashes (lower) - A combination causes vortex at the wing-tips (+ P differential) - This is a large source of waste of kinetic energy in form of vortex drag (AKA induced drag)
Flow Field at the Wingtip:
Winglets to Improve the Aero Efficiency: - Different geometries can be used - Multi disciplinary optimization for: - Wing Oswaldâ€™s Efficiency - Wing Weight - Vs. AR and Taper ratio of the wing
Winglets to Improve the Aero Efficiency: - To isolate the upper side flow fields from the lower side flow field. - Different geometry types for different Methodologies: - Cant Angle could be optimized. - Their AR and Taper ratio could be optimized. - AVL could be used - Trade studies are often performed in form of non-dimensional efficiency factors.
Winglets Optimization: - Vortex Drag Parameter Ratio (V-Drag to minimum theoretical possible value) vs. Flutter Measure of Merit.
High-Speed Surface Flow Visualization: - Turbulent Transition on Leading edge for Airbus A-340
- Turbulent Transition on Leading edge for an experimental BAE concept jet.