Forces, weight, lift, drag, thrust Flashcards
Four forces
Weight, lift, thrust and drag
Equilibrium
All forces are balanced
L/D abbr meaning
Lift / drag ratio
In a cruise flight, what’s the approximate L/D ratio
10 to 1, 10 lift, 1 drag
MTOW
Maximum takeoff weight
MATOW
Maximum allowable takeoff weight
All-up weight
The general weight of the aircraft including fuel, passengers and load. Should be equal or less than MTOW
Wing loading
The load that the wings carry in a straight in level flight
Wing loading formula
Weight of the airplace / Wing area
An aircraft weighs 1220kg and wing area of 20 square meters. what is wing loading
=(1220/20)
=61kg /square meter
also written as: kgm^-2
Venturi effect
High flow velocity = low static pressure
(Like an air flowing through a tube, which is narrower at the center, so at the center the flow will be faster, with lower static pressure)
Boundary layer
The layer of airflow closest to the surface of the aerofoil. It causes friction of air with surface, which can make this layer relative velocity zero. The relative velocity in general is less in the boundary layer. And after it, the layer thickens.
Transition point
The point at which the boundary layer becomes turbulent.
Streamline flow
The desirable flow around aircrafts aerofoil
Separation point
The point at which the boundary layer separates from the surface of an erofil, causing the airflow to break and become turbulent.
Bernoulli principle
Anything in motion has energy.
Static pressure energey; and
dynamic pressure energy (kinetic energy due to motion)
Sum of them will give total energy
a fluid in motion will have
static pressure energy + dynamic pressure energy = constant totsl energy
Dynamic pressure two variables:
- the speed of the body relative to the air
- the density of the air
dynamic pressure equation
1/2 * rho * velocity squared
if velocity increaes, static pressure
decreases. and vice versa
upwash
a stream of air that hits the wing and being forced into a constricted area. its velocity increases above the wing
after an upwash, veloicity is increased above the wing, therefore:
decreasing static pressure above the wing according to bernoulli principle
cambered surface
a curvature causing the airflow over it to accelarate more ans generate more lift at the same AoE
mean camber line
a line drawn halfway between the upper and lower surface (its not straight in a cambered surface)
chord line
a straight line joining the leading edge and tralining edge, in other words, a straight line joining the ends of mean camber lines
camber
the distance between mean camber line and chord line
well cambered wing is associated with
low speed, high lift
total reaction (tr)
consists of the lift and drag components from the center of pressure (the total reaction)
AoA
angle between chord line and relative airflow
Is relative airflow parallel to horizon?
no
Pitch angle
The angle relative to the horizon
IMPORTANT the airflow and pitch angle and AoA,
refer to page 18
Angle of incidence
the angle which the wing is fixed to the airframe relative to the longitudinal axis
Stagnation point
Airflow come to rest at this point. Occurs at the leading edge and trailing edge.
What happens to the coLift and centre of pressure when increasing AoA
- The lifting ability of the wing (coLift) increases
- The centre of pressure moves forward
What happens to CP (center of pressure) when reaching a stalling AoA
Moves backwards
Lift formula
cL 1/2 ρ V^2 S
Explained:
(coefficient of lift) * (1/2*air density) * (velocity sqaured) * (wing surface)
What lift depends on?
Angle of attack and indicated airspeed
A difference in location between center of pressure and center of gravity
Will make the airplane pitch up / down. We can counteract this by using the tailplane controls (elevator controls)
In symmetrical aerofoil, at AoA 0 what is the lift?
0, since the mean camber line and chord line are the at the same position.
What’s called The angle at which laminar flow over the upper wing surface breaks into eddies and sepearates?
Crticial angle
Purpose of tailplane
To balance the MOMENT created by the differing positions of the CP and CG
When AoA increased, where on a low speed aerofoil will flow separation start?
On the upper surface towards teh trailing edge
In a cambered wing surface, when AoA is -4 degrees approx
Lift is 0
Drag is parallel to?
relative airflow. acts opposite of it
Total drag
the total resistance sum of all drag acting opposite of the aircraft movement
Two types of drags
Induced drag
Parasite drag
Parasite drag 3 children
Skin friction
Form drag
Interference drag
Induced drag small explanation
Drag forces associated with the production of lift. I.e. vortices at the trailing edge and wing tips
Parasite drag small explanation
Drag forces not directly associated with lift.
Skin friction drag
Friction between an object the air, depends on:
- Surface area of the aircraft
- If the boundary layer airflow is laminar or turbulent
- Roughness of the surface (like ice)
- Airspeed, more airspeed more skin friction drag
- Aerofil thickness, more thickness more drag
- AoA, more AoA more drag
Form drag
When the airflow spearates from the surface, eddies are formed and streamline is distrubed.
Like landing gear
Difference between form drag and skin friction drag
Imagine a flat plate, at 0 AoA is all skin friction drag and at 90 AoA its all form drag as the air behind it is trubulent
What is streamlining
Making a “shape” (such as landing gear) more streamline friendly to delay the separation point and reduce form drag. Like putting fairings on the landing gear.
Interference drag
The drag generated due to the “junctins” of various surfaces, like the wing junction.
Parasite drag and airspeed
As airspeed increases, parasite drag increases, by V-squared. (Square rule)
Induced drag
By-product of the production of lift and is closely related to angle of attack
Wing tip vortices (induced drag)
As the air flows rearwards, some airflow will “leak” around the wingtip from underneath, joining the eventually both the high pressure below and low preure above. Creating vortices.
When wings are producting high value of cLift, the wingtip vortices
Are stronger, since the pressure difference is greater as well
Newton’s third law
For every action there’s an opposite reaction
Induced downwash
The overall downflow of the air behind the trailing edge behind forced down after a wing
High aspect ratio wings when (span / chord)
Induced drag is reduced
Tapered wings
Wings that at the tip has less “width” and therefore induced drag is less since less vortices are created
Washout
Wing is built such that the AoA at the wing tip is less. It means the wing in general will produce less lift, but also less induced drag
The slower the airspeed
The more vortices, more induced drag. Since the pilot will want to stay on a straight flight, so he increases the AoA.
Induced drag is most significant at
Slow speeds, high AoA
Increased lift efficieny means
More induced drag.
The greater the airspeed
Less induced drag, more parasite drag
Minimum drag speed (Vmd)
The point at which parasite drag and induced drag are balanced, creating the lowest of total drag
Low airspeed is associated with
High angle of attacks. To keep the plane lifted
How to calculate lift/drag ratio
Lift / drag (or the equations)
What approx is the best angle of attack to give the best lift/drag ratio
4 degrees
Blade angle
The angle at which the chord line of apropeller section makes with the plane of rotatio
Name of the cambered side of the blade
Blade back
Name of the flat side of the blade
Blade face
Helical path
When airplane moves forward, the propeller secgtion follows a cockscrew path through the air called helix
Two velocity components affecting a propeller
Rotational velocity
Forward velocity (when aircraft is moving forward)
Propeller torque
The resistance to motion in the plane of rotation
If the aeroplane is into a dive, what can we see on the RPM?
Increase. Even if we didnt open more throttle
Slipstream effect
When propeller is rotating to the right (from the cockpit view), will make a clockwise (right) slipstream around the body of the aircraft, reaching the left side of the fin, causing the tail to be pushed to the right and the nose to yaw left
Propeller torque reaction
If rotates to the right, the torque reaction will tend to rotate the aircraft anti-clockwise and roll it to the left. On the ground the left wheel stops this motion, but makes the airplace yaw to the left
At fixed-pitch propeller the blade angle
is constant
Angle of attack of the propeller blade is
Between the resultant velocity (Down velocity due to rotation, and forward velocity due to motion on the aircraft) and chord line.
Imagine a triangle
More forward velocity with same RPM, the blade AoA is
reduced
More rotational velocity (more RPM) with same forward velocity, the AoA is
higher
