LEADING CADET
Principles of Flight Revision Notes
Learning Outcome 1
Know the principles of lift, weight, thrust and drag and how a balance of forces affects an aeroplane in flight
Bernoulli and Newton Laws
The diagram shows air passing over the top surface of a wing in normal flight. The speed of the air at point b is greater than that at point a, and the pressure at point b is lower than at point a. This is Bernoulli’s Principle.
Newton’s Third Law of Motion states that – For every Reaction there is an Equal and Opposite Reaction.
Aerofoils, Lift and Weight
Wing section a is a general purpose aerofoil and wing section b is a ‘high lift’ aerofoil.
Lift on a wing acts through the Centre of Pressure.
If the air density of the airflow over the wings reduces and all other factors remain unchanged then the Lift generated by the wings decreases.
If the air speed over an aerofoil decreases the lift and pressure envelope decreases
Lift operates at 90 degrees to the relative airflow (90 degrees to the flight path).
On a general purpose aerofoil the greatest amount of Lift occurs on the top surface (where it is curved the most). In general about 80% of lift occurs on the wing top surface.
Lift is proportional to the square of the airspeed. For example if an aircraft accelerates from 100 kts to 200 kts, the Lift increases by a factor of 4.
For an aircraft to remain in straight and level flight at a constant speed, the Lift produced by the wing must equal the Weight of the aircraft.
Thrust and Drag
The reaction to the rearward movement of air produced from a jet or propeller engine is called Thrust.
When an aircraft moves through the air it experiences Drag which resists its motion. All parts of the aircraft over which the air flows produce Drag.
Drag is also proportional to the square of the airspeed.
When an aircraft slows down, but maintains the same height, the Drag must exceed the Thrust.
Chord and Angle of Attack
In the diagram the straight yellow line is the Chord drawn between the leading edge to the trailing edge of a wing. Angle a is called the Angle of Attack.
The Angle of Attack at which a wing stalls is called the Critical Angle.
On a general purpose wing, the greatest Lift is produced with an Angle of Attack of about 15 degrees.
Axes of Rotation
The Lateral axis of rotation on an aeroplane runs from wing tip to wing tip and motion around this axis affected by the Elevators is called Pitching.
The Longitudinal axis of rotation on an aeroplane runs from nose to tail and motion around this axis affected by the Ailerons is called Rolling.
The Normal axis of rotation on an aeroplane extends vertically upwards through the Centre of Gravity and motion around this axis affected by the Rudder is called Yawing.
All 3 axes of rotation pass through the Centre of Gravity.
Centre of Gravity|
Yawing / Pitching / Rolling
/ Aircraft movements such as Yawing, Pitching and Rolling are always in relation to the pilot.
Black arrow point to Ailerons
Flaps and Slats
/ a – Slatb – Slot
c – Aerofoil
d - Flap
Types of Flaps
/ Simple Flap
Split Flap
Slotted Flap
Fowler Flap
The use of a single design of a wing for the highest possible flying speed, but the lowest possible landing speed is achieved by the use of Flaps.
In general, a Flap setting of 15 degrees gives the shortest take-off run.
To ensure the steepest possible approach path and also to get maximum Drag from the aircraft’s Flaps, they would be set at 90 degrees.
The effect of selecting full Flap when coming into land increases the angle of approach and improves forward vision.
The purpose of a Slat on the leading edge of the wing is to smooth out the air passing over the wing which will improve slow speed handling.
All types of Slats increase Drag.
Opening Slats increases the critical or stalling angle and reduces stalling speed.
Learning Outcome 2
Understand how the stability of an aeroplane is maintained in flight and how manoeuvrability is controlled
Stability
The tendency of a well-designed aircraft that is disturbed from level flight by turbulence to go back to level flight is called Stability.
The tail plane provides Stability in the pitching plane.
A large or sufficient fin provides the greatest directional Stability.
/ Dihedral Angle increases Stability in the lateral (rolling) plane./ Anhedral Angle reduces Stability in the lateral (rolling) plane
Trim Tabs
/ Arrow points to Rudder Trimming Tab / / Arrow points to Elevator Trimming TabTrim Tabs help cancel out unwanted forces acting on a pilot’s controls.
Learning Outcome 3
Know the principles of stalling
Stalling
Factors affecting Stall
a) Increasing the power will decrease the Stalling speed.
b) Turning tightly will increase the Stalling speed of an aircraft.
c) Decreasing the weight reduces the Stalling speed of an aircraft.
One of the factors which would have the least effect on the Stalling speed of an aircraft is the size of the fin.
The Angle of Attack of the wing does not vary when the wing is at the point of Stalling.
The ‘Pilots Notes give stalling speeds for various flight conditions.
Learning Outcome 4
Be able to apply the principles of flight and control to gliders
Gliders
Forces acting on a glider during balanced flight are Lift, Weight & Drag but NOT Thrust.
Use of airbrakes on approach to landing increases Drag and decreases Lift.
A glider with a gliding angle of 1 in 45 flying in still air over an area of flat ground at a height of 0.5 km will reach the ground in 22.5 km.
A glider with a speed of 35 kts flying into a headwind of 35 kts will appear to an observer on the ground to lose height steadily over one spot.
The gliding angle is least when the lift/drag ratio is at its highest.
The flatter the gliding angle of a glider, the further the glider will travel over the ground.
A glider pilot knows the optimum Angle of Attack by reference to the airspeed indicator.
Learning Outcome 5
Be able to apply the principles of flight and control to rotary wing aircraft
Helicopters
Lift is generated by spinning aerofoil shaped blades.
A helicopters rotor disc is the area swept by the rotor blades
The forward ‘tilting’ of this area or ‘rotor disc’ by the use of the cyclic control will causes the helicopter to travel forward.
The main function of a helicopter’s cyclic control is to control horizontal flight in any direction.
The Collective Pitch lever alters the Pitch angle of each main rotor blade by the same amount at the same time. Increasing the Pitch also increases the Lift produced by the main rotor blades.
The Hand Throttle is located on the end of the Collective Pitch lever.
The opposing force generated when the main rotor blades rotate on a helicopter is called the Torque Reaction. The Torque Reaction will tend to make the fuselage rotate around the main rotor.
The primary purpose of the tail rotor is to oppose the Torque Reaction and stabilise the fuselage.
The Yaw pedals control the tail rotor.
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