Aircraft Design and Performance Essentials

Aircraft Design Fundamentals

• Airplane design is the engineering process of creating a flying machine meeting user specifications or pioneering new ideas/technology.

Mission Profile

• Includes payload type, range, cruise speed, field length, fuel, climb, and maneuvering requirements.

Design Requirements

• Key design factors include range, takeoff distance, stalling velocity, endurance, and maximum velocity.

Structural System

• Shell elements: Curved plates (fuselage, domes, pressure vessels).
• Bar elements: One-dimensional members (rods, trusses) carrying axial, bending, shear, and torsional loads.
• Plate elements: Two-dimensional extensions of bar elements (membranes, panels).

Thermal Loads

• Uniform and non-uniform temperature changes create thermal stresses on a restrained structure.

Flight Vehicle Imposed Loads

• Includes lift, drag, and pitching moments across various angles of attack.

Basic Flight Loading Conditions

• Positive High Angle of Attack (PHAA): Highest possible angle of attack.
• Positive Low Angle of Attack (PLAA): Lowest angle of attack allowing maximum lift.
• Negative High Angle of Attack (NHAA): Intentional maneuvers, or sudden downdrafts.
• Negative Low Angle of Attack (NLAA): Diving-speed limit of an airplane.

Aircraft Structures

• Truss type: rigid framework of members (beams, struts, bars).
• Monocoque: Thin skin carrying all load; strengthened by bulkheads, frames and stringers.
• Semi-Monocoque: Combine skin and internal framing members for load diffusion.

Types of Aircraft Structures

• Truss
• Monocoque
• Semi-Monocoque

Basic Flight Loading Conditions

• Positive High Angle of Attack (PHAA): Highest possible angle of attack.
• Positive Low Angle of Attack (PLAA): Lowest angle of attack allowing maximum lift.
• Negative High Angle of Attack (NHAA): Intentional maneuvers, or sudden downdrafts.
• Negative Low Angle of Attack (NLAA): Diving-speed limit of an airplane.

Basic Flight Loading Conditions

• Positive High Angle of Attack (PHAA): Highest possible angle of attack.
• Positive Low Angle of Attack (PLAA): Lowest angle of attack allowing maximum lift.
• Negative High Angle of Attack (NHAA): Intentional maneuvers, or sudden downdrafts.
• Negative Low Angle of Attack (NLAA): Diving-speed limit of an airplane.

Basic Flight Loading Conditions

• Positive High Angle of Attack (PHAA): Highest possible angle of attack.
• Positive Low Angle of Attack (PLAA): Lowest angle of attack allowing maximum lift.
• Negative High Angle of Attack (NHAA): Intentional maneuvers, or sudden downdrafts.
• Negative Low Angle of Attack (NLAA): Diving-speed limit of an airplane.

Basic Flight Loading Conditions

• Positive high angle of attack (PHAA): Highest possible angle of attack
• Positive low angle of attack (PLAA): Lowest angle of attack allowing maximum lift
• Negative high angle of attack (NHAA): Intentional maneuvers, or sudden downdrafts
• Negative low angle of attack (NLAA): Diving-speed limit of an airplane

Internal Loads/Load Paths - Fuselage

• Fuselage shape influences strength; rectangular, circular, or elliptical are common.

Flight Control Surfaces (Various Types)

• Ailerons: Control roll.
• Elevator: Control pitch.
• Rudder: Control yaw.
• Spoilers: Reduce lift and increase drag.
• Flaps: Increase lift and drag, particularly at low speeds.
• Slats: Increase lift at high angles of attack.
• Leading-edge cuffs: Similar to slats.
• Wingtips: Devices attached to the wingtips intended to improve efficiency of fixed-wing aircraft.

Landing Gears

• Types: Single main, tricycle, bicycle, tail-dragger, etc.
• Considerations: Weight support, impact absorption, ground clearance, pilot visibility, maneuverability.
• Configurations: affect the design.
• Materials: Steel and composite materials are typical.

Propeller Systems

• Types: Fixed pitch, variable pitch, constant-speed.
• Impact on aircraft performance: Torque, gyroscopic precession, and P-factor.
• Design specifications: Clearance, water/structural clearance, pitch considerations (geometric and effective).

Aircraft Stability

• Longitudinal stability (pitch)
• Lateral stability (roll)
• Directional stability (yaw)
• Static stability: System tendency to return to initial condition.
• Dynamic stability: System response with time to disturbances.
• Positive, neutral, and negative static/dynamic stability.
• Factors affecting stability: C.G. position, wing area, horizontal stabilizer, and elevator.
• Dutch Roll: A type of oscillatory motion resulting from a combination of lateral and directional instabilities.
• Spiral divergence: A type of oscillatory motion resulting from a combination of lateral and directional instabilities.