At the end of this lesson, you should be able to correctly:
Express direction of flight:
as a 3-figure group;
in the clock code;
as cardinal and ordinal compass points;
Understand the difference between aircraft heading and track;
Understand wind velocity; and
The relationship between true and magnetic heading.
Express Direction
Direction can be expressed:
As a 3-figure group related to a 360o compass. e.g. 090 is East, 180 is South,120 is East South East.
In a clock code where 12 o’clock is straight ahead of you, 3 o’clock is to your right and 6 o’clock is directly behind you.
As Cardinal and ordinal points:
Cardinal = N, S, E, W.
Ordinal = NE, SE, SW, NW
Aircraft Heading and Track
Aircraft heading is the direction in which the nose of the aircraft is pointing, e.g. North or 360
If the aircraft is affected by wind from the East and is heading North then track will be to the West of North by an amount determined by aircraft speed and wind speed vectors
Wind Velocity
Wind velocity is represented by a direction and speed presented visually as say 180/10; this means it is coming from the South (180o) at 10 knots or nautical miles per hour
True and Magnetic Heading
The relationship between true and magnetic heading:
Magnetic Heading is the direction that the aircraft is pointing in relation to Magnetic North.True Heading is the direction that the aircraft is pointing in relation to True North.
Since True North (directly over the earth’s axis of rotation) and Magnetic North (somewhere over northern Canada) are not at the same place (magnetic North is, in fact, slowly moving over time), both headings often differ, called magnetic variation. Isogonal lines are printed on some maps as dotted purple lines which capture the magnetic variation at the time the map was printed.
To establish True Heading, simply take the reading from a magnetic compass and either add an easterly, or subtract a westerly, magnetic variation
To gain a basic understanding of the principles of aerodynamics, as they apply to unmanned aircraft.
Objectives
At the end of this briefing you will be able to:
Describe the basic principles of Lift
Illustrate an aerodynamic cross-section and show how lift is generated
Examine what is a stall
Forces
A basic question – What Exactly is a Force?
What is a force? ⟹ A force is a push, a pull or a twist. How is a force illustrated? ⟹ A vector (direction, magnitude) How is a force measured? ⟹ Force = Mass × Acceleration What are the unit of force? ⟹ kg.m/s2 (also called Newtons)
Description of a Force
A force is a push or pull upon an object resulting from the object’s interaction with another object.
A force is a vector quantity – it has both magnitude and direction.
It is common to represent a force by an arrow.
Because forces are vectors, the effect of an individual force upon an object is often cancelled by the effect of another force.
A force is represented diagrammatically as an ‘arrow’, the length of the arrow representing magnitude, and the direction the arrow points represents the direction of the force.
Resultant Force
A resultant force is the single force which represents the vector sum of two or more forces.
The ‘start’ of the second force needs to be moved to the ‘end’ of the first force, with the resultant going from the start of the first force directly to the end of the second force (as shown in the diagram).
Components of a Resultant Force
A force acting on a point can be broken up into it’s horizontal (X) and vertical (Y) components:
Rotating a Force
Note: For Straight and Level flight ⟹ Lift = Weight
Principle of Moments
The moment of a force is a measure of its tendency to cause a body to rotate about a specific point or axis.
The magnitude of the moment of a force acting about a point or axis is directly proportional to the distance of the force from the point or axis.It is defined as the product of the force and the moment arm. The moment arm or lever arm is the perpendicular distance between the line of action of the force and the centre of moments.
Principle of Moments
When a system is in equilibrium, the TOTAL anticlockwise moment = TOTAL clockwise moment.
Consider the following system:
Horizontal Stabiliser and its Turning Moment
Most airplanes are designed so that the wing’s centre of lift (CL) is to the rear of the centre of gravity.
Applying simple physics principles, it can be seen that if a bar was suspended at point L with a heavy weight hanging on it at the CG, it would take some downward pressure at point T to keep the ‘lever’ in balance.
Creating Lift
What is Lift?
Lets consider and develop a concept! (Lets not make reality fit our theory, but rather simply consider and apply known fact!)
Consider this!
What if I want to squirt a friend with the garden hose, but the pressure is not quite enough for the water to reach them. Is it possible for me to do this?
A Question?
How does covering the end of the garden hose make the water squirt further?
Answer: When you put your finger over the tip of the hose you partially block the end, or in other words: decrease the amount of space the water has to flow through!
…but what if we just PINCH the hose?
When you squeeze the hose, you decrease the amount of space the water has to flow through, and in the pinched area of the hose the water flows faster!
Making the hose squirt!
Since the same amount of water has to flow out of the hose as flows in to the hose, the water must shoot out faster – to keep the amount of water flowing out a constant.
The hose can’t expand to accommodate more water, so the water has to shoot through the ‘reduced’ opening faster. Pressure has to do with how an object will feel as a result of a force exerted on it. Because pressure causes the water to shoot out of the hose faster, it will feel harder, and it will travel farther.
The Principle of Mass Continuity
In fluid dynamics, the continuity equationstates that, in any steady state process, the rate at which mass enters a system is equal to the rate at which mass leaves the system.
The Venturi Effect
Area is inversely proportional to Velocity. Reduced cross sectional area therefore Increased Velocity
Basically: Halve the Area, Double the Velocity
What about a ‘one finger’ pinch?
This still causes a reduced area in the hose for the water to flow through, and in the reduced area of the hose, the water will still flow faster!
Measuring Venturi Effect
Velocity of airflow increases through the restriction. Velocity after restriction is equal to velocity before the restriction.
Airflow “Velocity” through Venturi can also be referred to as the “Dynamic Pressure” of the airflow through the Venturi.
Why Venturi Effect Happens
In fluid dynamics, a fluid’s velocity must increase as it passes through a constriction in accord with the principle of mass continuity, while its static pressure must decrease in accord with the principle of conservation of mechanical energy.
Static Pressure decreases more through the restriction. Static Pressure after restriction is equal to Static Pressure before the restriction.
Bernoulli’s Theorem
Relationship between Velocity & Pressure Total Pressure (H) = Dynamic Pressure (q) + Static Pressure (p) (𝐻 =𝑞 + 𝑝)
Bernoulli’s principle tells us that fast-moving airis at a lower pressurethan slower-movingair (i.e. Faster Air = Lower Pressure)
So how could squirting a garden hose possibly have anything to do with how an aircraft flies?
A similar mechanism to that which causes water to squirt further when a finger is placed over the end of a hose causes one of the “types” of lift experienced by an aircraft wing in straight and level flight!
Aerofoils
The Basic Aerofoil Shape
…its all about the shape!
The Bending of Air
The Flow of Air
The fluid on top of the wing is accelerated and the fluid on the bottom of the wind is slowed down compared to velocity of the aircraft itself because the wing geometry and angle narrows the flow area above the wing and widens the flow area below the wing. (Venturi Effect!)
How Aircraft use this to Fly
Note: For Straight and Level flight ⟹ Lift = Weight
What is Lift with Regards to Flying?
Lift is:
An aerodynamic force
Lift opposes weight
Aerodynamic Forces
Pressure Differential
The Forces
Vector Diagram
Aerofoil Diagram
Ways of Generating Lift
Straight & Level
at altitude, straight and level flight
Ground Effect
One wingspan from the ground
Angle of Attack
Landing – slower speed
Aerodynamic Effects
Spanwise Lift Distribution
Wing-Tip Vortices
Stalling
A stall occurs when the Angle of Attack exceeds the Critical Angle.
There is a significant loss of Lift and increase in Drag. This is caused by the majority of airflow over the upper surface of the wing separating.
Symptoms of Approaching Stall
Level Flight – 1 G Stall:
Low and reducing Airspeed
High nose attitude
Buffeting
Stall warning device activation
Significant aft control column
Sloppy Controls: be very careful with aileron input at low speeds!
Characteristics of Stall
Level Flight – 1 G Stall
At Stall Speed
Pronounced nose drop
Possible wing drop:
Maybe caused by Aileron / Rudder use
Wind Gusts / Turbulence
Flow Strips
Increased Buffeting
Increasing Rate of Descent
Stall Recovery
Simultaneously:
Lower the nose
Full Power
Rudder to stop further wing drop (if any ?)
Do Not use Aileron (Why ?)
When recovered:
Level the wings with Aileron
Regain normal flight
Stall Speed
REMEMBER!
An Aircraft will stall when the CRITICAL ANGLE OF ATTACK is exceeded.
This can occur at any speed within the speed range of the aircraft.