01.01 – Direction of Flight and Wind

Aim

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

Part 1 – Lift

Aim

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

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 equation states 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 air is at a lower pressure than slower-moving air (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!

Cambered Aerofoil Section

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 & Levelat altitude, straight and level flight
Ground EffectOne wingspan from the ground
Angle of AttackLanding – slower speed

Aerodynamic Effects

Spanwise Lift Distribution

Wing-Tip Vortices

http://www.ryanwaters.com/wingtip-vortices/ accessed 21 Sep 2013
http://en.wikipedia.org/wiki/File:Airplane_vortex_edit.jpg accessed 21 Sep 2013

Stalling

A stall occurs when the Angle of Attack exceeds the Critical Angle.

https://sites.google.com/site/flightsafetysystems/anti-stall accessed: 20/07/2020

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.
  • G. Stall?
    • Dive recovery – Be careful