Aerodynamics Quiz on Flight Forces

Theory of Flight

• The four forces acting on an airplane are thrust, lift, weight, and drag.
• Thrust is the forward force propelling the aircraft.
• Lift is the upward force opposing weight, created by airflow over and under the wings.
• Weight is the downward force due to gravity.
• Drag is the backward force opposing thrust, reducing speed.

Introduction

• Students will be able to describe the relationship between lift, weight, thrust, and drag; glide ratio; steady state flight and performance; theory of the turn; load factor and its influence on stalling; flight envelope and structural limitations; and methods of lift augmentation.

Four Forces of Flight

• Lift is the upward force created by airflow over and under the wings.
• Weight is the force pulling the aircraft downwards due to gravity.
• Thrust is the forward force propelling the aircraft through the air.
• Drag is the backward force that opposes thrust.

Vectors

• Arrows showing forces acting on an airplane are called vectors.
• The length of the arrow indicates the magnitude of the force.
• The direction of the arrow indicates the direction of the force.
• Multiple forces acting simultaneously combine to create a resultant force.

Lift

• Lift is the key aerodynamic force that opposes weight.
• Air flowing over and under the wings creates a pressure differential.
• This difference in pressure generates lift.

Weight

• Weight is a force caused by gravity acting on the mass of the aircraft.
• Center of Gravity (CG) is the point where the entire weight of the aircraft is considered concentrated.
• CG has a definite relationship with lift.

Centre of Gravity

• CG is the point within an aircraft where the weight acts.
• It is determined by the general design of the aircraft.
• The designer determines how far the center of pressure will travel.
• CG is placed in front of the center of pressure for correct flight speed.
  • This provides restoring moment for flight equilibrium.

Adverse Forward Center of Gravity

• When too much weight is towards the front of the aircraft, the CG is shifted forward.
• Possible flight conditions include: increased tendency to dive, increased difficulty in raising the nose during landing, increased oscillation tendency, increased danger during flap operation, development of dangerous spin characteristics.

Adverse Rear Center of Gravity

• When too much weight is toward the tail of the aircraft, the CG is shifted aft.
• Possible flight conditions include: decreased flying speed, decreased range, increased danger of stall, dangerous spin characteristics, poor stability, and extreme shifts in CG, leading to an accident.

Effect of Stall Speed on Centre of Gravity

• Airspeed and sink rate must both be in the same units.
• Minimum sink speed is the airspeed at which the glider loses altitude at the lowest rate.

Aircraft Glide Ratio

• Glide ratio is the number of horizontal feet a glider travels in still air for every vertical foot it loses.
• This ratio varies with airspeed, as drag varies with airspeed.
• A glide polar is a graph for this relationship, showing a minimum sink rate at a particular airspeed.

Glider Fly

• Gliding flight occurs when thrust is removed from the four forces.
• Lift, drag, and weight must be in equilibrium for a steady glide.
• The angle of attack (AofA) is crucial for maximum lift/drag ratio and a steady glide.

Equations of Motion for Power-Off Glide

• These equations describe the relationship between drag, lift, and weight during a glide, using variables (W-Weight, D-Drag, L - Lift, θ - glide angle).

Aircraft Turn

• Inertia must be overcome for a turn.
• Banking the aircraft tilts the lift direction, generating a horizontal turning component.
• This horizontal component (centripetal force) and lift components are crucial for a turn.
• Maintaining constant altitude requires balanced lift and weight during a level turn.

Sideslip

• Sideslip occurs when the side wind is not coming from ahead of the aircraft.
• If the bank is too great, a sideways force due to gravity will cause a skid.

Skidding

• Skidding is caused by an insufficient bank angle or when a pilot does not correct for centrifugal force.
• The wind will feel like it is coming from the outside of the turning plane.

Sideway Landing

• Sideway landing is typical for crosswind conditions.

Cross Wind Landing

• This is typical for when the wind is landing at an angle.

Balanced Turn

• The pilot does not feel any sideways forces during a balanced turn.

Turning Flight and The V-n Diagram

• A table shows that with no turn, no change in altitude happens, but with a turn there is either an increase or a decrease in altitude.

Level Turn (Coordinated Turn)

• The lift is inclined at a bank angle ($ \phi $) to the vertical.
• The vertical component of the lift exactly balances the weight. (L Cos $ \phi $) = W
• Resultant force (F,)= √ L2 – W2 or F, = L sin $ \phi $

Load Factor

• The load factor is the ratio between lift and weight(L/W).
• Aircraft have load factor limits (5g, for example) for which structural damage may occur.

Load Factor During a Turn

• Lift increases significantly during turns.
• Lift bracing (struts, spars) is required to handle increased loads during turns.

Load Factor Angle of Bank

• The stalling speed is calculated mathematically using the formula W/L = cos e, and L=W/cos e.
• The factors are lift (L), weight (W), and angle of bank (e).
• Stall and maneuvering speed are calculated using these.
• Different values of the bank angle (e.g., 60, 70.5, 75.5, 80.25 degrees) for a given stalling speed can provide values of turning lift.

Wing Loading

• Wing loading is the aircraft's weight divided by the wing area.
• Weight compared to wing area governs the minimum speed.
• Lower Wing loading = lower minimum speed
• Higher Wing loading = higher minimum speed

Wing Loading Stalling Speed

• Lower wing loading = lower stalling speed
• Higher wing loading = higher stalling speed
• Adding weight to the aircraft increases wing loading, minimum speed, and stalling speed.

"g" Limit

• "g" limit is an aircraft's ability to withstand structural stress during turns
• High g-forces lead to structural damage

How to survive high G force?

• High G suits might be needed.

"g" Limit from V-n diagram

• The V-n diagram displays the relationship between airspeed and load factor for various flight conditions.

V-n diagram (Low Speed)

• This diagram shows how stall limits and structural limits affect the maximum load factor, along with speed.

V-n diagram (High Speed)

• The maximum load factor is limited by the aircraft's structural designs.

Maneuver point

• The maneuver point is a point on the curve where the load factor and speed are highest and where the maximum aerodynamic lift and load factor are realized, before stall or structural failure.

Corner velocity (v*)

• The corner velocity is the speed at which the aircraft achieves its maximum turn rate.

V-n diagram (Low speed) and V-n diagram (High speed)

• Stall limits and structural limits affect maximum load factors and speeds.

Additional notes:

• Figures and diagrams will greatly aid understanding.