AE 1350LECTURE #3

TOPICS PREVIOUSLY COVERED

• Roadmap of Disciplines

• “English” to “S.I.” units

• Common Aerospace Terminology

• Airplane Axes and Motion

• This information is available at:http://www.ae.gatech.edu/~lsankar/AE1350

TOPICS TO BE COVERED

• Preliminary Thoughts on Aerospace Design

• Specifications (“Specs”) and Standards

• System Integration

• Forces acting on an Aircraft

• The Nature of Aerodynamic Forces

• Lift and Drag Coefficients

PRELIMINARY THOUGHTS ON DESIGN• Design is, in general,

– a team effort– a large system integration activity– done in three stages– iterative– creative, knowledge based.

• The three stages are:– Conceptual design– Preliminary design– Detailed design

Conceptual Design

• What will it do?

• How will it do it?

• What is the general arrangement of parts?

• The end result of conceptual design is an artist’s or engineer’s conception of the vehicle/product.

• Example: Clay model of an automobile.

Preliminary Design

• How big will it be?

• How much will it weigh?

• What engines will it use?

• How much fuel will it use?

• How much will it cost?

• This is what you will do in this course.

Detailed Design

• How many parts will it have?

• What shape will they be?

• What materials?

• How will it be made?

• How will the parts be joined?

• How will technology advancements (e.g. lightweight material, advanced airfoils, improved engines, etc.) impact the design?

SPECIFICATION AND STANDARDS

• The designer needs to satisfy– Customer who will buy and operate the vehicle

(e.g. Delta, TWA)– Government Regulators (U.S. , Military,

European, Japanese…)

CUSTOMER SPECIFICATIONS• Performance:

– Payload weight and volume

– how far and how fast it is to be carried

– how long and at what altitude

– passenger comfort

– flight instruments, ground and flight handling qualities

• Cost

• Prince of system and spares, useful life, maintenance hours per flight hour

• Firm order of units, options, Delivery schedule, payment schedule

TYPICAL GOVERNMENT STANDARDS

• Civil– FAA Civil Aviation Regulations define such things as

required strength, acoustics, effluents, reliability, take-off and landing performance, emergency egress time.

• Military– May play a dual role as customer and regulator

– MIL SPECS (Military specifications)

– May set minimum standards for Mission turn-around time, strength, stability, speed-altitude-maneuver capability, detectability, vulnerability

SYSTEM INTEGRATION

• Aircraft/Spacecraft Design often involves integrating parts, large and small, made by other vendors, into an airframe or spaceframe (also called “the bus.”)

• Parts include– engines, landing gear, shock absorbers, wheels, brakes,

tires– avionics (radios, antennae, flight control computers)– cockpit instruments, actuators that move control surfaces,

retract landing gears, etc...

Examples of integration

• Providing smooth airflow into engine inlets free of debris.

• Insuring that the hot engine does not damage the airframe structure.

• Providing antenna locations with minimum electromagnetic interference.

• providing mountings for power actuators that deflect control surfaces, retract landing gear, etc.

AEROSPACE DESIGN INVOLVES THREE PHASES

• Analyses

• Ground testing and simulation (e.g. wind tunnel tests of model aircraft, flight simulation, drop tests, full scale mock-up, fatigue tests)

• Flight tests

FORCES ACTING ON AN AIRPLANE

THRUST• The engine provides the thrust, using propellers (piston

engines or turboprop), fans (turbofans), or high velocity jets (turbojet).

• Thrust is needed to overcome drag. At steady level flight, thrust equals drag.

• If thrust does not equal drag, the vehicle will accelerate or decelerate.

• Air aircraft without engines providing the thrust is a glider, and will need other energy sources (e.g. thermal currents) to stay aloft.

Lift and Drag•These are the result of the pressure forces and viscous forces exerted by the air molecules on the aircraft.•The component along the flight direction is called drag.•The component normal to the flight direction is called lift.

Viscous forces can act both normal, and tangential to the surface.

Pressure Forces act normalto the aircraft surface.

Lift and Weight

• In steady level flight lift produced by the entire aircraft surface equals weight.

• Much of the lift is produced by the wing.

• The tail surface (stabilizer and elevator) will usually produce a downward directed lift.

• If lift does not equal weight, the aircraft will climb or descend.

Lift Coefficient, CL• Lift is usually non-dimensionalized, so that geometrically similar

configurations (e.g. a 1/10 scale of an aircaft and the full size aircraft) can be easily compared one to one.

• The lift coefficient is defined as:

where L is the lift force, S is the wing planform area, V is the velocity of the aircraft, and is the density of air far upstream of the aircraft.

• Two geometrically scaled models will have identical lift coefficients if they operate at identical Mach number, Reynolds number (definition later) and angle of attack (angular orientation of the aircraft relative to the direction of flight; more precise definition later).

SV

LCL

2

21

Drag Coefficient, CD

• It is defined as:

where D is the drag force, S is the wing planform area, V is the velocity of the aircraft, and is the density of air far upstream of the aircraft.

• Two geometrically scaled configurations will have an identical drag coefficient if other conditions are identical - Mach number, Reynolds number, angle of attack.

SV

DCD

2

21

Mach Number

• Mach number is the ratio between the speed of the aircraft (relative to the ambient air, which itself may be moving), and the speed of sound in the ambient air far upstream of the aircraft.

a

VM

REYNOLDS NUMBER

• It is a measure of how strong the inertial forces (e.g. pressure forces) acting on a fluid particle are, compared to the viscous forces acting on it.

• It is defined as:

• Here is the density, V is the velocity, L is any characteristic length of the vehicle. Also, is the viscosity of the fluid, a fluid property.

LV

RN