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PPL Ground School - Theory of Flight

AVI 8810 – 300
Assignment #1
Due: Sept 16th, 8am
Michael Hudson
Assignment 1: Theory of Flight
Thrust, Drag, Lift and Weight are the four forces that act on an airplane in flight. Thrust
is created by the backward airflow cause by the propeller and engines. Drag acts against
thrust, meaning it is the resistance of forward motion. Lift is essentially caused by low
pressure above, and high pressure below the wings. This difference in pressures is what
causes the upward force which is lift. Weight directly opposes lift, meaning if the total
weight of the aircraft is more than the lift force it will not be able to take off. The four
forces and the directions in which they act in straight and level flight are shown below.
Couples, when referring to the forces are when two forces are not only equal and
opposite, but parallel as well. However while being parallel, these forces are not in line.
Weight is placed in front of lift and drag is placed above thrust. They are placed like this
and act together to keep the aircraft airborne, as well to maintain pitch.
3. There are multiple types of drag, however two main ones that affect an aircraft in flight;
Parasite drag, and Induced drag. Parasite drag is composed of form drag and skin
friction. Form drag refers to the resistance of motion caused by the shape of a body.
Skin friction refers to the friction (tendency to cling) of air flowing over a surface. It is
hard to completely reduce parasite drag but two things we can do as pilots is making
sure the landing gear is retracted if possible, and cleaning the airplane from dust, dirt,
ice etc… Induced drag is caused by the parts of the aircraft that are responsible in
AVI 8810 – 300
Assignment #1
Due: Sept 16th, 8am
Michael Hudson
creating lift. As pilots there is nothing we can do to reduce induced drag, however there
are many advancements/designs in aircrafts that are put into place to do just that.
Wings with high aspect ratios produce much less induced drag than ones with a low
aspect ratio.
Angle of Incidence: The angle formed by the wing cord line and the longitudinal axis. A
fixed angle that cannot be changed by the pilot. Measured by the angle where the wing
is attached to the fuselage.
Angle of attack: The angle between the wing cord line and flight path. The angle can
vary depending on the direction of motion of the aircraft.
Chord line: An imaginary line joining the leading and trailing edges of an aero-foil. Is
usually in the direction of airflow.
Relative airflow: The directional movement of the atmosphere relative to an aircraft or
aero-foil. Must be opposite to the direction of movement of aircraft relative to the
Center of pressure: A point along the aero-foil which lift is generated. This point can
change depending on the angle of attack.
Boundary layer: Airflow going over the surface of the aircraft. This air can behave
abnormally depending on the design of the entire aircraft. The air between the space of
the wing and airflow occurring over top of the wing surface.
Critical Angle of Attack: The angle of attack which causes the aircraft to go into a stall.
The airflow over the wing has an abrupt change forcing a decrease in lift.
Aerodynamic Stall: Occurs to an aircraft when the angle of attack is under rapid change
and can no longer produce enough lift to support the aircrafts weight. An aerodynamic
stall occurs most often when the aircraft is travelling at low speeds.
5. There are multiple factors that effect stall speed, to list them: Weight, Centre of Gravity,
Turbulence, Turns, Flaps, snow, frost, ice and Heavy Rain. Weight is a factor because
when an aircraft is heavier, it requires more lift to fly, which is produced by a higher
angle of attack. This means that the stalling speed will be higher. As the centre of gravity
moves forward, more force is applied downward on the horizontal stabilizer due to its
design, therefore increasing the total load factor on the aircraft, and as mentioned
above more weight mean an increase in stalling speed. If flying at a low speed and a
sudden gust upward causes a major increase in angle of attack a stall could occur. When
banking a turn more lift is required to maintain flight, therefore meaning the angle of
AVI 8810 – 300
Assignment #1
Due: Sept 16th, 8am
Michael Hudson
attack must be increased. This in turn causes the stalling speed to increase. Another
factor in turns is the degree of bank. As your turn gets steeper the load factor increase
meaning the stalling speed also increases. Snow, frost, ice and heavy rain can all effect
the lifting capabilities of the airplane, causing the stall to speed to increase and the
critical angle of attack to decrease.
6. The “Manoeuvering speed” is very important for pilots to know as it the speed at which
the specific aircraft can travel before enduring structural damage. It is unsafe to
“maneuver” if approaching or above the designated maneuvering speed.
7. Lift to drag ratio is the amount of lift produced by the wing, divided by the aerodynamic
drag produced by the wing at the same time.
Wingtip vortices are caused by the decreased pressure on top of the wing being less
than the surrounding atmospheric pressure, which causes the air to flow inward. Vice
versa for the bottom of the wing. The inward and outward flowing airflows meet at the
wing tip while flowing contra-wise, then exerting a resistant force, this is what causes
the induced drag that is wingtip vortices.
The four main causes of yaw are asymmetric thrust, gyroscopic precession, slipstream
and aileron drag. Asymmetric thrust is caused at high angles of attack and high power
settings because the descending blade on lookers right has a higher angle of attack than
that of the blade on lookers left. This causes more lift from the right side and
subsequently yaw to the left. Gyroscopic precession happens when an airplane changes
from a nose up to a nose down attitude, like in a takeoff roll in planes that are tail
wheeled. Propellers act like gyroscopes and because of the characteristics of a
gyroscope the raise in tail causes yaw to the right. In this instance the propeller is
spinning towards the right, the air being pushed backwards by it forms a corkscrew
around the aircraft forcing the tail end of that air to hit the fin causing yaw to the left.
Aileron drag is caused by, when in a right bank turn the right aileron will be raised and
the left lowered, this creates drag therefore causing yaw.
Aspect ratio can be defined as the ratio of the wingspan to the mean cord. One can
calculate this ratio by taking the square root of the wingspan divided by the wing area.
The aspect ratio of a wing which spans 24 feet and has a chord of 6 ft is 4:1.
AVI 8810 – 300
Assignment #1
Due: Sept 16th, 8am
Michael Hudson
Higher aspect ratio wings produce less drag and more lift the those of a lower aspect
ration. This combination of less drag and more lift means the higher aspect ratio wing
will have a better glide ratio.
12. The most important difference between a standard wing design and a laminar wing
design is the thickest part of the wing is further back on the laminar design, causing the
transition point where the air turns turbulent to be further back.
13. Maintaining laminar flow for as long as possible is critical because doing so considerably
reduces drag. Turbulent air also increases the stall speed as well as effects the planes
ability to fly in equilibrium, therefore pushing the turbulent air further down the cord
reduces these effects.
14. The main reason for sweptback wing designs in high speed aircraft is the substantial
reduction in drag. This reduction happens because of the onset of supersonic airflow
over the wings.
15. Slots and slats help the wings produce more lift because at higher angles of attack they
essentially allow for more airflow over the wing. Slats also help to provide lateral
16. Wing fences on an aircraft are put in place to stop the whole wing from stalling at once.
The way they are configured it cause the root of the wing to stall before the tip, allowing
for more control to regain correct attitude.
17. When banking a turn to the left or right the load factor on the downward wing is
increased, as well as the amount of drag due to the raised aileron. This combination of
forces means more lift is need so the nose of the plane has to be pitched upwards to
compensate for the increased load factor.
18. Secondary stalls occur when the pilot tries to recover to early, or the opposite doesn’t
recover fast enough by pitching the nose down (trying to use power instead of pitch).
AVI 8810 – 300
Assignment #1
Due: Sept 16th, 8am
Michael Hudson
These are dangerous because of the extreme loss in altitude and prominence of falling
towards the ground.