Aircraft Structures: Tutorial 2

• Tutorial 2 covers Lectures 5, 6, 7, & 10 and focuses on aircraft structures.

Aircraft Reference Lines

• Fuselage Station Line is a reference datum perpendicular to the longitudinal axis, locating longitudinal points on the aircraft.
• Water Line is a reference datum along the longitudinal axis, locating vertical points.
• Butt Line is a reference datum parallel to the longitudinal axis, locating lateral points across the aircraft.
• Establishing a reference datum allows specification and identification of every area or point on the aircraft.
• Reference datum facilitates locating specific points, is used for weight and balance, and aids in repairs and maintenance.

Fuselage Construction

• Monocoque fuselage construction becomes unsuitable for large aircraft due to the increasing weight-to-strength ratio as the fuselage diameter increases, resulting in thicker skins to manage structural stresses during flight.

Semi-Monocoque Fuselage Components

• Longerons
• Stringers
• Keel beams
• Floor beams
• Shear ties
• Bulkheads
• Frames
• Outer skin.

Stressed-Skin Wing Construction Components

• Ribs
• Stringers
• Spars

Fuselage Components

• Main frame
• Longerons
• Stringers
• Horizontal beam
• Fairing
• Wing root rib
• Floor panel
• Centre wing box
• Seat tracks

Fuselage Construction Types

• The Pratt Truss has longerons connected with rigid vertical and lateral members called struts.
• Pratt Truss uses diagonal members of tubing or solid rods to carry tension/compression loads.
• Warren Truss has longerons connected only with diagonal members.
• All members are capable of carrying both tension and compression loadings.
• Monocoque Design is the construction of a cone or tube without reinforcement from internal structural members.
• Semi-Monocoque Design involves fuselage skin reinforced by additional supporting structures.

Wing Components

• Rear spar
• Front spar
• Ribs
• Stringers

Aircraft Component Functions

• Ailerons are primary control components for the rolling action, attached to the wing's tip.
• Flaps change the camber or increase the wing area to increase lift during takeoff and increase drag for lower flight speeds during landing, usually on the wing's trailing edge.
• Slats energize flow at the boundary layer, delaying flow separation and increasing lift at low speeds, located on the leading edge.
• Spoilers increase drag and reduce lift, reducing aircraft speed during landing and allowing shorter landing distances, usually on the upper wing surfaces.

Safe-Life Criteria

• Designers must estimate the magnitude and frequency of loads a member or structure will encounter during normal operation estimating safe-life.
• Estimation is derived from results obtained over a period of years on other types of aircraft fitted with load measuring and counting devices.

Fail-Safe Philosophy

• Alternate Load Path Construction: One member carries the entire load, with a backup member ready to take over if the primary fails.
• Multiple Load Path Construction: Multiple members resist an applied load, distributing the load among the remaining members if one fails.

Fail-Safe Limitations

• Material thickness must keep stress levels low.
• Low stress levels also need to reduce the chance of fatigue failure.
• Sufficient strength and stiffness is needed in the remaining structure.
• Material must have good resistance to crack propagation.
• Structural design should limit the maximum length of any cracks and facilitates regular structual inspections.

Damage Tolerance steps

• Establish the components which are to be designed as damage tolerant
• Define the loading conditions and extend of damage
• Conduct sufficient representative tests
• Analysis to substantiate design objectives such as life to crack initiation, crack propagation rate and residual strength.
• Establish data for inspection programs to ensure detection of damage

Manufacturing Processes and Aircraft Parts

• Formed Section: Fuselage skin, Wing spar
• Metal Sheets: Rotor hub, Bracket
• Extruded Section: Longerons, Skin stiffener
• Forging: Stringers, Wing rib
• Casting: Wing root fairing, Landing gear wheel hub
• Stamping: Frame, Window frame
• Spinning: Nose radome, Cooling fin